UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

SCHEDULE TO

Tender Offer Statement under Section 14(d)(1) or 13(e)(1)

of the Securities Exchange Act of 1934

Five Prime Therapeutics, Inc.

(Name of Subject Company (Issuer))

Franklin Acquisition Sub, Inc.

a wholly owned subsidiary of

Amgen Inc.

(Name of Filing Persons (Offerors))

Common Stock, $0.001 Par Value

(Title of Class of Securities)

33830X104

(CUSIP Number of Class of Securities)

Jonathan P. Graham, Esq.

Executive Vice President, General Counsel and Secretary

One Amgen Center Drive

Thousand Oaks, California 91320-1799

(805) 447-1000

(Name, Address, and Telephone Number of Person Authorized to Receive Notices and Communications on Behalf

of Filing Persons)

With a copy to:

Francis J. Aquila

Sullivan & Cromwell LLP

125 Broad Street

New York, NY 10004

Telephone: (212) 558-4000

CALCULATION OF FILING FEE

Check the appropriate boxes below to designate any transactions to which the statement relates:

Check the following box if the filing is a final amendment reporting the results of the tender offer. ☐

If applicable, check the appropriate box(es) below to designate the appropriate rule provision(s) relied upon:

This Tender Offer Statement on Schedule TO (together with any amendments and supplements hereto, this “Schedule TO”) is filed by (a) Franklin Acquisition Sub, Inc., a Delaware corporation (“Purchaser”) and a wholly owned subsidiary of Amgen Inc., a Delaware corporation (“Amgen”), and (b) Amgen. This Schedule TO relates to the offer by Purchaser to purchase all of the outstanding shares of common stock, par value $0.001 per share (the “Shares”), of Five Prime Therapeutics, Inc., a Delaware corporation (“Five Prime”), at a purchase price of $38.00 per Share, in cash, minus any applicable withholding taxes and without interest, upon the terms and subject to the conditions set forth in the Offer to Purchase, dated March 18, 2021, a copy of which is filed herewith as Exhibit (a)(1)(A) (together with any amendments, supplements or modifications thereto, the “Offer to Purchase”) and in the related Letter of Transmittal (together with any amendments, supplements or modifications thereto, the “Letter of Transmittal” and, together with the Offer to Purchase, the “Offer”), a copy of which is filed herewith as Exhibit (a)(1)(B). Pursuant to General Instruction F to Schedule TO, the information contained in the Offer to Purchase, including all schedules and annexes to the Offer to Purchase, is hereby expressly incorporated herein by reference in response to Items 1 through 9 and Item 11 of this Schedule TO and is supplemented by the information specifically provided in this Schedule TO. This Schedule TO is intended to satisfy the reporting requirements of Rule 13e-4(c)(2) promulgated under the Securities Exchange Act of 1934, as amended (the “Exchange Act”).

Capitalized terms used and not defined herein shall have the meanings assigned to such terms in the Offer to Purchase.

Item 1.    Summary Term Sheet.

The information set forth in the section of the Offer to Purchase entitled “Summary Term Sheet” is incorporated herein by reference.

Item 2.    Subject Company Information.

(a) The name of the subject company and the issuer of the securities to which this Schedule TO relates is Five Prime Therapeutics, Inc., a Delaware corporation. Five Prime’s principal executive offices are located at 111 Oyster Point Boulevard, South San Francisco, California, 94080. Five Prime’s telephone number at such address is (415) 365-5600.

(b) This Schedule TO relates to the outstanding shares of common stock, par value $0.001 per share, of Five Prime. Five Prime has advised Amgen that, as of the close of business on March 8, 2021, there were (i) 45,941,900 Shares issued and outstanding, (ii) 631,004 Shares underlying Five Prime’s outstanding, unvested, restricted stock awards and (iii) 4,228,899 Shares issuable upon the exercise of Five Prime’s outstanding stock options with an exercise price less than the Offer Price. The information set forth in the sections of the Offer to Purchase entitled “Introduction” and “The Offer—Price Range of the Shares; Dividends on the Shares” is incorporated herein by reference.

(c) The information concerning the principal market in which the Shares are traded, and certain high and low sales prices for the Shares in that principal market, is set forth in the sections of the Offer to Purchase entitled “Summary Term Sheet” and “The Offer—Price Range of the Shares; Dividends on the Shares” and is incorporated herein by reference.

Item 3.    Identity and Background of Filing Person.

(a), (b), (c) This Schedule TO is filed by Amgen and Purchaser. The information set forth in the sections of the Offer to Purchase entitled “Summary Term Sheet,” “Introduction” and “The Offer—Certain Information Concerning Amgen and Purchaser” and in Schedule I to the Offer to Purchase is incorporated herein by reference.

Item 4.    Terms of the Transaction.

(a)(1)(i)—(viii), (x), (xii) The information set forth in the Offer to Purchase is incorporated herein by reference.

(a)(1)(ix), (xi) Not applicable.

(a)(2) Not applicable.

Item 5.    Past Contacts, Transactions, Negotiations and Agreements.

(a), (b) The information set forth in the sections of the Offer to Purchase entitled “Summary Term Sheet,” “The Offer—Certain Information Concerning Amgen and Purchaser,” “The Offer—Background of the Offer; Past Contacts, Negotiations and Transactions,” “The Offer—Purpose of the Offer; Plans for Five Prime” and “The Offer—The Merger Agreement; Other Agreements” is incorporated herein by reference.

Item 6.    Purposes of the Transaction and Plans or Proposals.

(a) The information set forth in the sections of the Offer to Purchase entitled “Summary Term Sheet,” “Introduction” and “The Offer—Purpose of the Offer; Plans for Five Prime” is incorporated herein by reference.

(c) The information set forth in the sections of the Offer to Purchase entitled “Summary Term Sheet,” “Introduction,” “The Offer—Price Range of the Shares; Dividends on the Shares,” “The Offer—Certain Effects of the Offer,” “The Offer—Dividends and Distributions,” “The Offer—Purpose of the Offer; Plans for Five Prime” and “The Offer—The Merger Agreement; Other Agreements” is incorporated herein by reference.

Item 7.    Source and Amount of Funds or Other Consideration.

(a) The information set forth in the sections of the Offer to Purchase entitled “Summary Term Sheet” and “The Offer—Source and Amount of Funds” is incorporated herein by reference.

(b), (d) Not applicable.

Item 8.    Interest in Securities of the Subject Company.

(a), (b) The information set forth in the section of the Offer to Purchase entitled “The Offer—Certain Information Concerning Amgen and Purchaser” is incorporated herein by reference.

Item 9.    Persons/Assets Retained, Employed, Compensated or Used.

(a) The information set forth in the sections of the Offer to Purchase entitled “Summary Term Sheet,” “Introduction” and “The Offer—Fees and Expenses” is incorporated herein by reference.

Item 10.    Financial Statements.

(a), (b) Not applicable.

Item 11.    Additional Information.

(a)(1) The information set forth in the sections of the Offer to Purchase entitled “Summary Term Sheet,” “Introduction,” “The Offer—Certain Information Concerning Amgen and Purchaser,” “The Offer—Background of the Offer; Past Contacts, Negotiations and Transactions,” “The Offer—Purpose of the Offer; Plans for Five Prime” and “The Offer—The Merger Agreement; Other Agreements” is incorporated herein by reference.

(a)(2), (3) The information set forth in the sections of the Offer to Purchase entitled “Summary Term Sheet,” “Introduction,” “The Offer—The Merger Agreement; Other Agreements,” “The Offer—Conditions of the Offer” and “The Offer—Certain Legal Matters; Regulatory Approvals; Litigation” is incorporated herein by reference.

(a)(4) The information set forth in the section of the Offer to Purchase entitled “The Offer—Certain Effects of the Offer” is incorporated herein by reference.

(a)(5) The information set forth in the section of the Offer to Purchase entitled “The Offer—Certain Legal Matters; Regulatory Approvals; Litigation” is incorporated herein by reference.

(c) The information set forth in the Offer to Purchase is incorporated herein by reference.

Item 12.    Exhibits.

See Exhibit Index.

Item 13.    Information Required by Schedule 13E-3.

Not applicable.

SIGNATURES

After due inquiry and to the best of my knowledge and belief, I certify that the information set forth in this statement is true, complete and correct.

Dated: March 18, 2021

EXHIBIT INDEX

Exhibit (a)(1)(A)

Offer to Purchase for Cash

All Outstanding Shares of Common Stock

of

FIVE PRIME THERAPEUTICS, INC.

at

$38.00 Per Share

by

FRANKLIN ACQUISITION SUB, INC.

a wholly owned subsidiary of

AMGEN INC.

Franklin Acquisition Sub, Inc., a Delaware corporation (“Purchaser”) and a wholly owned subsidiary of Amgen Inc., a Delaware corporation (“Amgen”), is making an offer to purchase all outstanding shares of common stock, par value $0.001 per share (the “Shares”), of Five Prime Therapeutics, Inc., a Delaware corporation (“Five Prime”), at a price of $38.00 per Share, (such amount, or any higher amount per Share paid pursuant to the Offer, being the “Offer Price”), in cash, minus any applicable withholding taxes and without interest, upon the terms and subject to the conditions set forth in this Offer to Purchase and the related Letter of Transmittal (which, together with any amendments, supplements or modifications thereto, collectively constitute the “Offer”).

Purchaser is making the Offer pursuant to an Agreement and Plan of Merger, dated as of March 4, 2021 (the “Merger Agreement”), by and among Purchaser, Amgen and Five Prime. As soon as practicable following the consummation of the Offer, Purchaser will be merged with and into Five Prime (the “Merger”) without a vote of the stockholders of Five Prime in accordance with Section 251(h) of the Delaware General Corporation Law, as amended (the “DGCL”), and Five Prime will survive the Merger as a wholly owned subsidiary of Amgen. At the effective time of the Merger (the “Effective Time”), each then issued and outstanding Share not previously purchased in the Offer (other than (a) Shares that at the Effective Time are held by Five Prime, Amgen, Purchaser or any other direct or indirect wholly owned subsidiary of Amgen and (b) Shares outstanding immediately prior to the Effective Time that are held by stockholders of Five Prime who are entitled to appraisal rights under the DGCL and have properly exercised and perfected, and not withdrawn or otherwise lost, such appraisal rights) will be converted into the right to receive the Offer Price, in cash, minus any applicable withholding taxes and without interest. The execution and delivery of the Merger Agreement and all of the transactions contemplated by the Merger Agreement, including the Offer and the Merger, are collectively referred to as the “Transactions.” The date and time at which the Offer expires by its terms (as it may be extended in accordance with the Merger Agreement, as described below) is referred to herein as the “Offer Expiration Time.”

After careful consideration, the board of directors of Five Prime (the “Five Prime Board”) has unanimously (a) determined that the Merger Agreement and the Transactions, including the Offer and the Merger, are fair to, and in the best interest of, Five Prime and its stockholders, (b) declared it advisable to enter into the Merger Agreement, (c) approved the execution, delivery and performance by Five Prime of the Merger Agreement and the consummation of the Transactions, including the Offer and the Merger, (d) resolved that the Merger will be effected under Section 251(h) of the DGCL and (e) resolved to recommend that the stockholders of Five Prime accept the Offer and tender their Shares to Purchaser pursuant to the Offer, subject to the right of the Five Prime Board to withdraw (or modify in a manner adverse to Amgen or Purchaser) or publicly propose to withdraw or withhold (or modify or qualify in a manner adverse to Amgen or Purchaser) its recommendation in accordance with the terms of the Merger Agreement.

THE FIVE PRIME BOARD UNANIMOUSLY RECOMMENDS THAT YOU ACCEPT THE OFFER AND TENDER ALL OF YOUR SHARES TO PURCHASER PURSUANT TO THE OFFER.

The Offer is not subject to any financing condition. The Offer is conditioned on, among other things, there being validly tendered (and not properly withdrawn) prior to the Offer Expiration Time, a number of Shares that (together with any Shares then owned by Purchaser and its “affiliates” (as such term is defined in Section 251(h)(6)(a) of the DGCL), but excluding Shares tendered pursuant to guaranteed delivery procedures that have not yet been “received” by the “depository” (as such terms are defined in Section 251(h)(6) of the DGCL)) represents at least one Share more than 50% of the then issued and outstanding Shares. The Offer is also subject to certain other conditions set forth in this Offer to Purchase, including the expiration or termination of any applicable waiting period under the Hart-Scott-Rodino Antitrust Improvements Act of 1976, as amended, the absence of any legal restraint making the acquisition of or payment for Shares pursuant to the Offer, or the consummation of the Merger, illegal or otherwise prohibited, and other customary conditions as described in Section 15—“Conditions of the Offer.”

A summary of the principal terms of the Offer appears on pages 1-11. You should read this entire Offer to Purchase carefully before deciding whether to tender your Shares.

Questions and requests for assistance may be directed to D.F. King & Co., Inc., our Information Agent, at the address and telephone numbers set forth on the back cover of this Offer to Purchase. Requests for additional copies of this Offer to Purchase, the Letter of Transmittal, the Notice of Guaranteed Delivery or any of the other Offer documents may be directed to the Information Agent. Additionally, copies of this Offer to Purchase, the related Letter of Transmittal and any other materials related to the Offer may be obtained at the website maintained by the Securities and Exchange Commission (the “SEC”) at www.sec.gov. You may also contact your broker, dealer, commercial bank, trust company or other nominee for assistance.

March 18, 2021

IMPORTANT

Stockholders desiring to tender their Shares in the Offer must:

The Letter of Transmittal, the certificates for the Shares and any other required documents must be received by the Depositary at one of its addresses set forth on the back cover of this Offer to Purchase before the Offer Expiration Time, unless the procedures for guaranteed delivery described in Section 3—“Procedure for Tendering Shares” of this Offer to Purchase are followed. The method of delivery of the Shares, the Letter of Transmittal and all other required documents, including delivery through The Depositary Trust Company, is at the election and risk of the tendering stockholder.

This Offer to Purchase and the related Letter of Transmittal contain important information and you should read them carefully and in their entirety before making a decision with respect to the Offer. The Offer has not been approved or disapproved by the SEC or any state securities commission nor has the SEC or any state securities commission passed upon the fairness or merits of the Offer or upon the accuracy or adequacy of the information contained in this Offer to Purchase or the Letter of Transmittal. Any representation to the contrary is unlawful.

TABLE OF CONTENTS

i

SUMMARY TERM SHEET

Franklin Acquisition Sub, Inc., a Delaware corporation (“Purchaser”) and a wholly owned subsidiary of Amgen Inc., a Delaware corporation (“Amgen”), is making an offer to purchase all outstanding shares of common stock, par value $0.001 per share (the “Shares”), of Five Prime Therapeutics, Inc., a Delaware corporation (“Five Prime”), at a price per Share of $38.00 (such amount, or any higher amount per Share paid pursuant to the Offer, being the “Offer Price”), in cash, minus any applicable withholding taxes and without interest, as further described herein, upon the terms and subject to the conditions set forth in this Offer to Purchase and the accompanying Letter of Transmittal and the other exhibits to the Tender Offer Statement on Schedule TO (the “Schedule TO”). The following are some questions you, as a stockholder of Five Prime, may have and answers to those questions. This summary term sheet highlights selected information from this Offer to Purchase, and may not contain all of the information that is important to you and is qualified in its entirety by the more detailed descriptions and explanations contained in this Offer to Purchase and the accompanying Letter of Transmittal and the other exhibits to the Schedule TO. To better understand our Offer to you and for a complete description of the legal terms of the Offer, you should read this Offer to Purchase and the accompanying Letter of Transmittal carefully and in their entirety. Questions or requests for assistance may be directed to D.F. King & Co., Inc., our Information Agent, at its address and telephone numbers set forth on the back cover of this Offer to Purchase. Unless otherwise indicated in this Offer to Purchase or the context otherwise requires, all references in this Offer to Purchase to “we,” “our” or “us” refer to Purchaser or Amgen, as the context requires.

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Who is offering to buy my shares?

Purchaser, a wholly owned subsidiary of Amgen, is offering to purchase for cash, subject to certain conditions, including the satisfaction of the Minimum Condition, any and all of the outstanding Shares. Purchaser is a Delaware corporation that was formed for the sole purpose of engaging in the transactions contemplated by the Agreement and Plan of Merger, dated as of March 4, 2021 (as may be amended, supplemented or otherwise modified from time to time in accordance with its terms, the “Merger Agreement”), by and among Purchaser, Amgen and Five Prime, and actions incidental thereto. See the “Introduction” to this Offer to Purchase and Section 10—“Certain Information Concerning Amgen and Purchaser.”

Is there an agreement governing the Offer?

Yes. Five Prime, Amgen and Purchaser have entered into the Merger Agreement. Pursuant to the Merger Agreement, the parties have agreed on the terms and conditions of the Offer and, following consummation of the Offer, on the subsequent merger of Purchaser with and into Five Prime, with Five Prime as the surviving entity (the “Merger”). If the Minimum Condition and the other conditions to the Offer are satisfied or waived and we consummate the Offer, we intend to effect the Merger as promptly as practicable pursuant to Section 251(h) of the General Corporation Law of the State of Delaware, as amended (the “DGCL”), without a vote on the adoption of the Merger Agreement by the stockholders of Five Prime. See Section 14—“The Merger Agreement; Other Agreements.”

How many Shares are you offering to purchase in the Offer?

We are seeking to purchase, subject to certain conditions, including the satisfaction of the Minimum Condition, any and all of the outstanding Shares on the terms and subject to the conditions set forth in this Offer to Purchase and the Letter of Transmittal. See the “Introduction” to this Offer to Purchase and Section 1—“Terms of the Offer.”

How much are you offering to pay and in what form of payment?

We are offering to pay $38.00, in cash, minus any applicable withholding taxes and without interest, for each Share validly tendered and accepted for payment in the Offer.

Why are you making the Offer?

We are making the Offer because we want to acquire control of, and ultimately all of the equity interests in, Five Prime. If the Offer is consummated, assuming the continued satisfaction of the No Legal Prohibition Condition (as defined below), Amgen intends, as promptly as practicable, to cause Purchaser to consummate the Merger. Upon consummation of the Merger, Five Prime would be a wholly owned subsidiary of Amgen. As used in this Offer to Purchase, the “No Legal Prohibition Condition” means the condition that there will not have been

2

issued by any court of competent jurisdiction or remain in effect any temporary, preliminary or permanent order preventing the acquisition of or payment for the Shares pursuant to the Offer, nor will any action have been taken or any law (other than any antitrust law) promulgated, entered, enforced, enacted, issued or deemed applicable to the Offer or the Merger by any nation, state, commonwealth, province, territory, county, municipality, district or other jurisdiction of any nature, federal, state, local, municipal, foreign or other government or governmental or quasi-governmental authority of any nature, including any governmental division, department, agency, commission, instrumentality, official, ministry, fund, foundation, center, organization, unit, body or entity and any court, arbitrator or other tribunal (“Governmental Body”), which directly or indirectly enjoins, restrains or otherwise prohibits, or makes illegal, the acquisition of or payment for the Shares pursuant to the Offer, or the consummation of the Merger. See Section 13—“Purpose of the Offer; Plans for Five Prime.”

Will I have to pay any fees or commissions?

If you are the record owner of your Shares and you tender your Shares in the Offer, you will not have to pay brokerage fees, commissions or similar expenses to do so. If you own your Shares through a broker, dealer, commercial bank, trust company or other nominee, and your broker or other nominee tenders your Shares on your behalf, your broker or nominee may charge you a fee for doing so. You should consult your broker or other nominee to determine whether any charges will apply. See the “Introduction” to this Offer to Purchase.

What does the Five Prime Board think of the Offer?

After careful consideration, the Five Prime Board has unanimously (a) determined that the Merger Agreement and the Transactions, including the Offer and the Merger, are fair to, and in the best interest of, Five Prime and its stockholders, (b) declared it advisable to enter into the Merger Agreement, (c) approved the execution, delivery and performance by Five Prime of the Merger Agreement and the consummation of the Transactions, including the Offer and the Merger, (d) resolved that the Merger will be effected under Section 251(h) of the DGCL and (e) resolved to recommend that the stockholders of Five Prime accept the Offer and tender their Shares to Purchaser pursuant to the Offer, subject to the right of the Five Prime Board to withdraw (or modify in a manner adverse to Amgen or Purchaser) or publicly propose to withdraw or withhold (or modify or qualify in a manner adverse to Amgen or Purchaser) its recommendation in accordance with the terms of the Merger Agreement.

See the “Introduction” to this Offer to Purchase and Section 12—“Background of the Offer; Past Contacts, Negotiations and Transactions” of this Offer to Purchase, and Five Prime’s Solicitation/Recommendation Statement on Schedule 14D-9 (the “Schedule 14D-9”) that is being filed with the Securities and Exchange Commission (the “SEC”).

Do I have to vote to approve the Offer or the Merger?

No. Your vote is not required to approve the Offer. You only need to tender your Shares if you choose to do so. If, following the completion of the Offer, the Shares accepted for payment pursuant to the Offer together with the Shares otherwise owned by us or our affiliates equal at least a majority of the then-outstanding Shares and the other conditions of the Merger are satisfied or waived, assuming certain statutory requirements are met, we will be able to consummate the Merger pursuant to Section 251(h) of the DGCL without a vote or any further action by the stockholders of Five Prime.

What is the market value of my Shares as of a recent date?

On March 3, 2021, the last full trading day before public announcement of the execution of the Merger Agreement, the closing price of Five Prime’s common stock reported on the Nasdaq Global Select Market (“Nasdaq”) was $21.26 per Share. On March 17, 2021, the last full trading day before commencement of the

3

Offer, the closing price of Five Prime’s common stock reported on Nasdaq was $37.88 per Share. We recommend that you obtain a recent quotation for Five Prime’s common stock in deciding whether to tender your Shares. See Section 6—“Price Range of the Shares; Dividends on the Shares.”

How long do I have to decide whether to tender in the Offer?

You will have until the Offer Expiration Time of 12:00 midnight, New York time, on April 16, 2021 (one minute after 11:59 p.m., New York time, on April 15, 2021), unless the Offer is extended in accordance with the terms of the Merger Agreement, to tender your Shares in the Offer. If you cannot deliver everything that is required to tender your Shares by that time, you may be able to use the guaranteed delivery procedure that is described in this Offer to Purchase.

The date and time at which Purchaser is required to accept for payment all Shares validly tendered (and not properly withdrawn) pursuant to the Offer is referred to as the “Offer Acceptance Time.” The date and time at which the Merger becomes effective is referred to as the “Effective Time.”

See Section 1—“Terms of the Offer” and Section 3—“Procedure for Tendering Shares.”

Can the Offer be extended and under what circumstances?

Yes, the Offer can be extended under certain circumstances. Our ability to extend the Offer is subject to the terms of the Merger Agreement and applicable law.

If, as of the scheduled Expiration Date, any condition of the Offer is not satisfied and has not been waived (to the extent waivable), we may, in our discretion and without the consent of Five Prime or any other person, extend the Offer on one or more occasions, for an additional period of up to ten (10) business days per extension, to permit such condition to be satisfied.

We will extend the Offer from time to time (a) for any period required by any law, any interpretation or position of the SEC, the staff thereof or Nasdaq applicable to the Offer or Merger, and (b) for periods of up to ten (10) business days per extension, until any waiting period (and any extension thereof) applicable to the consummation of the Offer under the Hart-Scott-Rodino Antitrust Improvements Act of 1976, as amended (the “HSR Act”) will have been expired or been terminated.

In addition, if, as of the scheduled Expiration Date, any condition of the Offer is not satisfied and has not been waived (to the extent waivable), we will, at the request of Five Prime, extend the Offer on one or more occasions for an additional period specified by Five Prime of up to ten (10) business days per extension, to permit such condition to be satisfied.

In no event, however, will we be required to, or, without Five Prime’s prior written consent, be permitted to, extend the Offer beyond the earlier to occur of: (a) the valid termination of the Merger Agreement in accordance with its terms or (b) the first business day immediately following December 4, 2021.

See Section 1—“Terms of the Offer” for additional information about our obligations and ability to extend the Offer.

Can the Offer be terminated?

Pursuant to the Merger Agreement, unless the Merger Agreement is terminated in accordance with its terms, Purchaser will not, and Amgen will cause Purchaser not to, terminate or withdraw the Offer prior to any scheduled Expiration Date without the prior written consent of Five Prime. See Section 14—“The Merger Agreement; Other Agreements” for more details on termination of the Offer.

4

What will happen if the Merger Agreement is terminated before the Offer is accepted?

If the Merger Agreement is terminated in accordance with its terms, Purchaser shall, and Amgen shall cause Purchaser to, promptly (and, in any event, within twenty-four (24) hours of such termination), irrevocably and unconditionally terminate the Offer and not acquire any Shares pursuant thereto, and Purchaser shall, and Amgen shall cause Purchaser to, promptly return, and cause any depositary acting on behalf of Purchaser to return, in accordance with applicable laws, all tendered Shares to the registered holders of such Shares. See Section 14—“The Merger Agreement; Other Agreements” for more details on termination of the Offer.

How will I be notified if the Offer is extended?

If we extend the Offer, we will inform American Stock Transfer & Trust Company, LLC (the “Depositary”) and will make a public announcement of the extension before 9:00 a.m., New York time, on the business day after the day on which the Offer was scheduled to expire. See Section 1—“Terms of the Offer.”

Will you provide a subsequent offering period?

No. Pursuant to Section 251(h) of the DGCL and due to the obligations of Purchaser, Amgen and Five Prime pursuant to the Merger Agreement to effect the Merger following the consummation of the Offer, we expect the Merger to occur following the consummation of the Offer without a subsequent offering period. See Section 1—“Terms of the Offer.”

What is the “Minimum Condition” to the Offer?

We are not obligated to purchase any Shares in the Offer unless there has been validly tendered (and not properly withdrawn) prior to the Offer Expiration Time, a number of Shares that (together with any Shares then owned by Purchaser and its “affiliates” (as such term is defined in Section 251(h)(6)(a) of the DGCL), but excluding Shares tendered pursuant to guaranteed delivery procedures that have not yet been “received” by the “depository” (as such terms are defined in Section 251(h)(6) of the DGCL)) represents at least one Share more than 50% of the then issued and outstanding Shares at the Offer Expiration Time. We refer to this condition as the “Minimum Condition.”

If the number of Shares tendered in the Offer is insufficient to cause the Minimum Condition to be satisfied upon the Offer Expiration Time, then (a) neither the Offer nor the Merger will be consummated and (b) Five Prime’s stockholders will not receive the Offer Price or the merger consideration pursuant to the Offer or the Merger, as applicable.

What are the conditions to the Offer other than the Minimum Tender Condition?

The Offer is not subject to any financing condition. In addition to the Minimum Condition, our obligation to purchase Shares tendered in the Offer is subject to the satisfaction, or (to the extent waivable) waiver by Amgen and Purchaser (in accordance with the terms of the Merger Agreement) of the following conditions at the Offer Expiration Time:

5

See Section 15—“Conditions of the Offer.”

Do you have the financial resources to pay for the Shares tendered in the Offer?

Yes. We estimate that the total funds required to complete the Offer and the Merger will be approximately $1.89 billion plus any related transaction fees and expenses. Amgen will provide Purchaser with sufficient funds to make such payments. The Offer is not subject to any financing condition. See Section 11—“Source and Amount of Funds.”

6

Is your financial condition relevant to my decision to tender in the Offer?

No. We do not believe that our financial condition is relevant to your decision to tender Shares in the Offer because:

See Section 11—“Source and Amount of Funds.”

How do I tender my Shares?

To tender certificated Shares, you must deliver the certificates representing your Shares, together with a properly completed and duly executed Letter of Transmittal (or manually executed facsimile thereof) and any other documents required by the Letter of Transmittal, to the Depositary before the Offer expires. If your Shares are held in street name (that is, through a broker, dealer or other nominee), your Shares can be tendered by your nominee through the Depositary.

To tender Shares held in book-entry form, you must deliver confirmation of a book-entry transfer of such Shares into the account of the Depositary at The Depositary Trust Company, together with a properly completed and duly executed Letter of Transmittal (or manually executed facsimile thereof), with any required signature guarantees, or an Agent’s Message (as defined below), and any other documents required by the Letter of Transmittal to the Depositary before the Offer expires pursuant to the book-entry procedures described in Section 3—“Procedure for Tendering Shares—Book-Entry Transfer.”

If you cannot deliver a required item to the Depositary prior to the expiration of the Offer, you may be able to obtain additional time to do so by having a broker, bank or other fiduciary that is an eligible institution guarantee, at or prior to the Expiration Date, that the missing items will be received by the Depositary at one of its addresses set forth on the back cover of this Offer to Purchase within two (2) Nasdaq trading days. However, the Depositary must receive the missing items within that two (2)-trading-day period or your Shares will not be validly tendered. See Section 3—“Procedure for Tendering Shares.”

May I withdraw previously tendered Shares?

Yes, you may withdraw previously tendered Shares any time prior to the Expiration Date by following the procedures for withdrawing your Shares in a timely manner. Thereafter, tenders of Shares are irrevocable, except that they may also be withdrawn pursuant to Section 14(d)(5) of the U.S. Securities Exchange Act of 1934, as amended, and the rules and regulations promulgated thereunder (the “Exchange Act”), after May 17, 2021, which is the sixtieth (60^th) day after the commencement of the Offer, unless such Shares have already been accepted for payment by Purchaser pursuant to the Offer. See Section 4—“Withdrawal Rights.”

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How do I withdraw previously tendered Shares?

To withdraw previously tendered Shares, you must deliver a written notice of withdrawal (or a manually signed facsimile thereof) with the required information to the Depositary while you still have the right to withdraw Shares. If you tendered Shares by giving instructions to a broker, dealer, commercial bank, trust company or other nominee, you must instruct the broker, dealer, commercial bank, trust company or other nominee prior to the Expiration Date in a timely manner to arrange for the withdrawal of your Shares. If you hold Shares registered in the name of a broker, dealer, commercial bank, trust company or other nominee, you should be aware that such institutions may establish their own earlier deadline for tendering Shares in the Offer. Accordingly, if you hold Shares registered in the name of a broker, dealer, commercial bank, trust company or other nominee, you should contact such institution as soon as possible in order to determine the times by which you must take action in order to tender Shares in the Offer. See Section 4—“Withdrawal Rights.”

Can holders of Five Prime’s equity-based awards participate in the tender offer?

No. The Offer is being made only for Shares and not for any of Five Prime’s equity-based awards. At the Effective Time, each option to purchase shares of Five Prime common stock that is outstanding and unexercised immediately prior to the Effective Time (each, a “Five Prime Option”) (whether vested or unvested) which has a per Share exercise price that is less than the Offer Price will be cancelled and converted into the right to receive an amount in cash equal to the product of (a) the total number of Shares subject to such Five Prime Option immediately prior to the Effective Time, multiplied by (b) the excess, if any, of (x) the Offer Price over (y) the exercise price payable per Share under such Five Prime Option immediately prior to the Effective Time, and any applicable withholding taxes, such amount to be paid, minus any applicable withholding taxes, in accordance with the Merger Agreement.

At the Effective Time, each Five Prime Option (whether vested or unvested) that has a per share exercise price that is equal to or more than the Offer Price at the Effective Time shall be cancelled without any consideration payable therefor whether before or after the Effective Time, such amount to be paid, minus any applicable withholding taxes, in accordance with the Merger Agreement.

At the Offer Acceptance Time, each Five Prime restricted stock award that is outstanding immediately prior to the Offer Acceptance Time (each, a “Five Prime Restricted Stock Award”) will be deemed vested as of immediately prior to the Offer Acceptance Time and converted into Five Prime common stock with the right to receive the same consideration in the Merger on the same terms and conditions as the other stockholders of Five Prime in respect of each Share subject to such Five Prime Restricted Stock Award.

Have any of Five Prime’s stockholders previously agreed to tender their shares into the Offer?

No stockholders of Five Prime have entered into agreements requiring them to tender their Shares into the Offer. However, Amgen and Purchaser have been advised by Five Prime that all of its directors and executive officers who beneficially own Shares currently intend to tender or cause to be tendered all such Shares into the Offer (other than Shares as to which such holder does not have discretionary authority, Shares which may be retained in order to facilitate estate and tax planning dispositions and any Shares deliverable upon exercise or conversion of Five Prime Options and Five Prime Restricted Stock Awards).

When and how will I be paid for my tendered Shares?

Subject to the terms and conditions of the Offer and the satisfaction or waiver of the conditions of the Offer set forth in Section 15—“Conditions of the Offer,” Purchaser will pay for all Shares validly tendered that have not been validly withdrawn promptly following the Offer Expiration Time. See Section 1—“Terms of the Offer” and Section 2—“Acceptance for Payment and Payment for Shares.”

Purchaser will pay for your Shares by depositing the Offer Price with the Depositary, which will act as our agent for the purpose of receiving payments from us and transmitting such payments to you. In all cases, payment to

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record holders for tendered Shares will be made only after timely receipt by the Depositary of certificates for such Shares, a properly completed and duly executed Letter of Transmittal (or manually executed facsimile thereof) and any other required documents in the case of a record holder of a stock certificate representing Shares or timely compliance with the procedures for book-entry transfer in the case of a record holder of Shares in book-entry form. See Section 2—“Acceptance for Payment and Payment for Shares.”

If Shares are purchased in the Offer, will Five Prime continue as a public company?

No. Following the purchase of Shares in the Offer, we expect to consummate the Merger in accordance with Section 251(h) of the DGCL, and no stockholder vote to adopt the Merger Agreement or any other action by the stockholders of Five Prime will be required in connection with the Merger. We do not expect there to be a significant period of time between the consummation of the Offer and the consummation of the Merger. If you decide not to tender your Shares in the Offer and the Merger occurs as described above, unless you exercise appraisal rights in the manner described below, you will receive as a result of the Merger the right to receive the same amount of cash per Share as if you had tendered your Shares in the Offer. After completion of the Merger, Amgen will own all outstanding capital stock of Five Prime, and Five Prime’s common stock will no longer be publicly traded. The Shares will no longer be eligible to be traded on the Nasdaq or any other securities exchange, there will not be a public trading market for the Shares and, assuming compliance with the SEC’s procedures for deregistration, Five Prime will no longer be required to make filings with the SEC or otherwise comply with the rules of the SEC relating to publicly held companies. See Section 7—“Certain Effects of the Offer.”

Will the Offer be followed by a merger if all Shares are not tendered in the Offer?

If the Offer is completed and we accordingly acquire a number of Shares that satisfies the Minimum Condition and the other conditions to the Merger are satisfied or waived, then, in accordance with the terms of the Merger Agreement, we will complete the Merger without a vote of the stockholders of Five Prime pursuant to Section 251(h) of the DGCL. Pursuant to the Merger Agreement, if the Minimum Condition is not satisfied, we are not required (nor are we permitted) to accept the Shares for purchase in the Offer, nor will we consummate the Merger.

Under the applicable provisions of the Merger Agreement, the Offer and the DGCL, no vote to approve the Merger Agreement by Five Prime stockholders will be required, and if the Merger is consummated, all Five Prime stockholders who did not tender their Shares in the Offer will, if they have not otherwise properly demanded and perfected appraisal rights under the DGCL, have the right to receive an amount in cash in respect of each Share held by them, payable to the holder thereof, without any interest thereon (subject to any tax withholding), equal to the Offer Price upon consummation of the Merger.

See Section 16—“Certain Legal Matters; Regulatory Approvals; Litigation.”

If you do not consummate the Offer, will you nevertheless consummate the Merger?

No. None of Purchaser, Amgen or Five Prime is under any obligation to pursue or consummate the Merger if the Offer has not been earlier consummated.

If I object to the price being offered, will I have appraisal rights?

Appraisal rights are not available as a result of the Offer. However, if the Merger takes place, stockholders who have not tendered their Shares in the Offer and who are entitled to demand and properly exercise and perfect appraisal of such Shares pursuant to, and comply in all respects with, the applicable legal requirements will have appraisal rights under Delaware law. If you choose to exercise your appraisal rights in connection with the Merger and you are entitled to demand, and properly exercise and perfect, appraisal of your Shares pursuant to,

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and comply in all respects with, the applicable provisions of Delaware law, you will be entitled to payment for your Shares based on a judicial determination of the fair value of your Shares, together with interest from the Effective Time through the date of payment of the judgment upon the amount determined to be the fair value.

Notwithstanding the foregoing, at any time before the entry of judgment in the proceedings, the surviving corporation in the Merger may pay to each holder of Shares entitled to appraisal an amount in cash, in which case interest shall accrue thereafter only upon the sum of (a) the difference, if any, between the amount so paid and the fair value of the Shares as determined by the Court of Chancery, and (b) interest theretofore accrued, unless paid at that time. The fair value may be more than, less than or equal to the price that we are offering to pay you for your Shares in the Offer. Section 262 of the DGCL provides that the Court of Chancery shall dismiss the proceedings as to all holders of Shares who are otherwise entitled to appraisal rights unless (a) the total number of Shares entitled to appraisal exceeds one percent (1%) of the outstanding Shares of the class or series entitled to appraisal or (b) the value of the consideration provided in the Merger for such total number of Shares exceeds $1 million. A copy of Section 262 of the DGCL has been filed as Annex B to Five Prime’s Schedule 14D-9.

See Section 16—“Certain Legal Matters; Regulatory Approvals; Litigation.”

If I decide not to tender, how will the Offer affect my Shares?

If you do not tender your Shares in the Offer and the Merger takes place, your Shares will be canceled. Unless you exercise appraisal rights under the DGCL, you will receive the same amount of cash per Share that you would have received had you tendered your Shares in the Offer, minus any applicable withholding taxes and without interest.

Accordingly, if the Merger takes place, the principal difference to you between tendering your Shares and not tendering your Shares in the Offer is that if you tender your Shares in the Offer, you will be paid earlier and you will not have appraisal rights under the DGCL. Because the Merger will be effected under Section 251(h) of the DGCL, assuming the requirements of Section 251(h) of the DGCL are met, no stockholder vote to adopt the Merger Agreement or any other action by the stockholders of Five Prime will be required in connection with the Merger. We do not expect there to be significant time between the consummation of the Offer and the consummation of the Merger. Upon consummation of the Merger, there will no longer be any public trading market for the Shares. Also, assuming compliance with the SEC’s procedures for deregistration, Five Prime will no longer be required to make filings with the SEC or otherwise comply with the rules of the SEC relating to publicly-held companies.

See the “Introduction” to this Offer to Purchase and Section 7—“Certain Effects of the Offer.”

Will I be paid a dividend on my Shares during the pendency of the Offer if such a dividend is declared and payable by Five Prime during such period?

Yes, but only to the extent so declared. The Merger Agreement provides that, from the date of the Merger Agreement until the earlier of the Offer Acceptance Time and the termination of the Agreement pursuant to its terms, except with the prior written consent of Amgen, Five Prime will not establish a record date for, declare, accrue, set aside or pay any dividend or make any other distribution in respect of any of its capital stock. See Section 6—“Price Range of the Shares; Dividends on the Shares.”

Are there any compensation arrangements between Purchaser and Five Prime’s executive officers or other key employees?

As of the date of this Offer to Purchase, no member of Five Prime’s current management has entered into any agreement, arrangement or understanding with Purchaser, Amgen or their affiliates regarding employment with, or the right to participate in the equity of, Amgen or the surviving corporation in the Merger.

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What are the U.S. federal income tax consequences of the Offer and the Merger?

The receipt of cash in exchange for Shares pursuant to the Offer or the Merger will be a taxable transaction for U.S. federal income tax purposes. See the “Introduction” to this Offer to Purchase and Section 5—“Certain U.S. Federal Income Tax Consequences” for a description of certain U.S. federal income tax consequences of the sale of Shares pursuant to the Offer or the Merger. Stockholders are urged to consult with their own tax advisors to determine the particular tax consequences to them (including the application and effect of any state, local or non-U.S. income and other tax laws) of the Offer and the Merger.

Who can I talk to if I have questions about the Offer?

You may call D.F. King & Co., the Information Agent for the Offer, at (212) 269-5550 if you are a bank or broker or (800) 848-3410 if you are a stockholder. See the back cover of this Offer to Purchase for additional information on how to contact our Information Agent. The Information Agent is not making any recommendation with respect to the Offer.

THIS OFFER HAS NOT BEEN APPROVED OR DISAPPROVED BY THE SEC OR ANY STATE SECURITIES COMMISSION, NOR HAS THE SEC OR ANY STATE SECURITIES COMMISSION PASSED UPON THE FAIRNESS OR MERITS OF, OR UPON THE ACCURACY OR ADEQUACY OF THE INFORMATION CONTAINED IN, THIS OFFER TO PURCHASE. ANY REPRESENTATION TO THE CONTRARY IS UNLAWFUL.

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To the Holders of Five Prime’s Common Stock:

INTRODUCTION

Franklin Acquisition Sub, Inc., a Delaware corporation (“Purchaser”) and a wholly owned subsidiary of Amgen Inc., a Delaware corporation (“Amgen”), is making an offer to purchase all outstanding shares of common stock, par value $0.001 per share (the “Shares”), of Five Prime Therapeutics, Inc., a Delaware corporation (“Five Prime”), at a price of $38.00 per Share (such amount, or any higher amount per Share paid pursuant to the Offer, being hereinafter referred to as the “Offer Price”), in cash, minus any applicable withholding taxes and without interest, upon the terms and subject to the conditions set forth in this offer to purchase (this “Offer to Purchase”) and in the related letter of transmittal (the “Letter of Transmittal”) (which, together with any amendments, supplements or other modifications thereto, collectively constitute the “Offer”). The Offer and withdrawal rights will expire at 12:00 midnight, New York time, on April 16, 2021 (one minute after 11:59 p.m., New York time, on April 15, 2021) (the “Offer Expiration Time”). In this Offer to Purchase, we use the term “Expiration Date” to refer to April 15, 2021, unless extended by Purchaser, in which event “Expiration Date” shall mean the latest date at which the Offer, as so extended by Purchaser, shall expire.

Purchaser is making this Offer pursuant to an Agreement and Plan of Merger, dated as of March 4, 2021 (as the same may be amended, supplemented or otherwise modified from time to time in accordance with its terms, the “Merger Agreement”), by and among Amgen, Purchaser and Five Prime. Under the Merger Agreement, as soon as practicable following the consummation of the Offer and the satisfaction or waiver of all of the conditions to the Merger (as defined below), Purchaser will be merged with and into Five Prime without a vote of the stockholders of Five Prime (the “Merger”) in accordance with Section 251(h) of the Delaware General Corporation Law, as amended (the “DGCL”), with Five Prime surviving the Merger as a wholly owned subsidiary of Amgen (the “Surviving Corporation”). At the effective time of the Merger (the “Effective Time”), each then issued and outstanding Share not previously purchased in the Offer (other than (a) Shares that at the Effective Time are held by Five Prime, Amgen, Purchaser or any other direct or indirect wholly owned subsidiary of Amgen and (b) Shares outstanding immediately prior to the Effective Time that are held by stockholders of Five Prime who are entitled to appraisal rights under the DGCL and have properly exercised and perfected, and not withdrawn or otherwise lost, such appraisal rights) will be converted into the right to receive the Offer Price, in cash, minus any applicable withholding taxes and without interest. The execution and delivery of the Merger Agreement and all of the transactions contemplated by the Merger Agreement, including the Offer and the Merger, are collectively referred to as the “Transactions.”

After careful consideration, the board of directors of Five Prime (the “Five Prime Board”) has unanimously (a) determined that the Merger Agreement and the Transactions, including the Offer and the Merger, are fair to, and in the best interest of, Five Prime and its stockholders, (b) declared it advisable to enter into the Merger Agreement, (c) approved the execution, delivery and performance by Five Prime of the Merger Agreement and the consummation of the Transactions, including the Offer and the Merger, (d) resolved that the Merger will be effected under Section 251(h) of the DGCL and (e) resolved to recommend that the stockholders of Five Prime accept the Offer and tender their Shares to Purchaser pursuant to the Offer, subject to the right of the Five Prime Board to withdraw (or modify in a manner adverse to Amgen or Purchaser) or publicly propose to withdraw or withhold (or modify or qualify in a manner adverse to Amgen or Purchaser) its recommendation in accordance with the terms of the Merger Agreement. A more complete description of the Five Prime Board’s reasons for authorizing and approving the Merger Agreement and the Transactions, including the Offer and the Merger, is set forth in Five Prime’s Solicitation/Recommendation Statement on Schedule 14D-9 (the “Schedule 14D-9”) under the U.S. Securities Exchange Act of 1934, as amended, and the rules and regulations promulgated thereunder (the “Exchange Act”), which is being mailed to the stockholders of Five Prime with this Offer to Purchase.

The Offer is not subject to any financing condition. The Offer is conditioned on, among other things, there being validly tendered (and not properly withdrawn) prior to the Offer Expiration Time, a number of Shares that

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(together with any Shares then owned by Purchaser and its “affiliates” (as such term is defined in Section 251(h)(6)(a) of the DGCL), but excluding Shares tendered pursuant to guaranteed delivery procedures that have not yet been “received” by the “depository” (as such terms are defined in Section 251(h)(6) of the DGCL)) represents at least one Share more than 50% of the then issued and outstanding Shares (the “Minimum Condition”). The Offer is also subject to certain other conditions set forth in this Offer to Purchase, including the expiration or termination of any applicable waiting period under the Hart-Scott-Rodino Antitrust Improvements Act of 1976, as amended (the “HSR Act”), the absence of any legal restraint making the acquisition of or payment for Shares pursuant to the Offer, or the consummation of the Merger, illegal or otherwise prohibited, and other customary conditions as described in Section 15—“Conditions of the Offer.”

The conditions to the Offer are for the sole benefit of Amgen and Purchaser and (except for the Minimum Condition, the Termination Condition and the No Legal Prohibition Condition (as defined and described in Section 15—“Conditions of the Offer”), which may be waived by Purchaser only with the prior written consent of Five Prime) may be waived by Amgen and Purchaser, in whole or in part at any time and from time to time, in the sole discretion of Amgen and Purchaser to the extent permitted by applicable law. The foregoing conditions are in addition to, and not in limitation of, the rights and obligations of Amgen and Purchaser with respect to extending, terminating or modifying the Offer pursuant to the terms and conditions of the Merger Agreement or applicable law.

Five Prime has informed Amgen and Purchaser that, as of the close of business on March 8, 2021, there were (i) 45,941,900 Shares issued and outstanding, (ii) 631,004 Shares underlying Five Prime’s outstanding, unvested, restricted stock awards and (iii) 4,228,899 Shares issuable upon the exercise of Five Prime’s outstanding stock options with an exercise price less than the Offer Price. Assuming that no Shares were issued after March 8, 2021, a minimum of 22,970,951 Shares would need to be validly tendered and not properly withdrawn before the Expiration Date or otherwise counted pursuant to Section 251(h) of the DGCL in order to satisfy the Minimum Condition. The actual number of Shares required to be tendered to satisfy the Minimum Condition will depend on the actual number of Shares outstanding on the date Purchaser accepts Shares for payment pursuant to the Offer.

Completion of the Merger is subject to certain conditions, including consummation of the Offer. As soon as practicable following the satisfaction or waiver of these conditions, Purchaser will complete the Merger without a vote of the stockholders of Five Prime in accordance with Section 251(h) of the DGCL, which provides that, following consummation of a successful tender offer for a public corporation, and subject to certain statutory provisions, if the stock irrevocably accepted for purchase or exchange pursuant to such offer and received by the depositary prior to the expiration of such offer, together with stock otherwise owned by the acquirer and its affiliates and any rollover stock, equals at least the amount of shares of each class of stock of the target corporation that would otherwise be required to approve a merger for the target corporation, and the other stockholders receive the same consideration for their stock in the merger as was payable in the tender offer, then the acquirer can effect a merger without the action of the other stockholders of the target corporation.

Under the applicable provisions of the Merger Agreement, the Offer and the DGCL, stockholders of Five Prime who have not tendered their Shares in the Offer will have certain appraisal rights with respect to the Merger. Stockholders must properly perfect their right to seek appraisal under the DGCL in connection with the Merger in order to exercise such appraisal rights. See Section 16—“Certain Legal Matters; Regulatory Approvals; Litigation.” The Merger Agreement is described in Section 14—“The Merger Agreement; Other Agreements.”

The Merger Agreement provides that, from and after the Effective Time, the parties will take all necessary actions so that the board of directors of the Surviving Corporation will consist of the directors of Purchaser as of immediately prior to the Effective Time, each to hold office until their successors are duly elected and qualified, or their earlier death, resignation or removal. From and after the Effective Time, the parties will take all necessary actions so that the officers of Purchaser as of immediately prior to the Effective Time will be the officers of the Surviving Corporation, each to hold office until their successors are duly elected and qualified, or until their earlier death, resignation or removal. See Section 14—“The Merger Agreement; Other Agreements.”

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This Offer to Purchase does not constitute a solicitation of proxies, and Purchaser is not soliciting proxies in connection with the Offer or the Merger. If the Minimum Condition is satisfied, Purchaser would have sufficient voting power after the Offer Acceptance Time to approve the Merger without the affirmative vote of any other stockholder of Five Prime pursuant to Section 251(h) of the DGCL. We do not foresee any reason that would prevent the Merger from being completed pursuant to Section 251(h) of the DGCL following the consummation of the Offer. The date and time at which Purchaser is required to accept for payment all Shares validly tendered (and not properly withdrawn) pursuant to the Offer is referred to as the “Offer Acceptance Time.” See Section 14—“The Merger Agreement; Other Agreements.”

Certain U.S. federal income tax consequences of the sale of Shares pursuant to the Offer and the Merger are described in Section 5—“Certain U.S. Federal Income Tax Consequences.” We recommend that stockholders consult their tax advisors regarding the tax consequences of the sale of Shares.

The Offer is only for Shares and not for any of Five Prime’s equity-based awards. See Section 14—“The Merger Agreement; Other Agreements” for more information about the treatment of equity-based awards.

Tendering stockholders whose Shares are registered in their own names and who tender directly to the Depositary will not be obligated to pay brokerage fees or commissions or, except as set forth in Instruction 6 of the Letter of Transmittal, transfer taxes on the tender of Shares in the Offer. Stockholders who own their Shares through a broker, dealer, commercial bank, trust company or other nominee should consult such institution to determine whether it charges any services fees or commissions. Purchaser or Amgen will pay all fees and expenses incurred in connection with the Offer by American Stock Transfer & Trust Company, LLC, which is acting as the Depositary and paying agent for the Offer (the “Depositary”), and D.F. King & Co., which is acting as the information agent for the Offer (the “Information Agent”). See Section 17—“Fees and Expenses.”

THIS OFFER TO PURCHASE AND THE RELATED LETTER OF TRANSMITTAL AND THE OTHER EXHIBITS TO THE TENDER OFFER STATEMENT ON SCHEDULE TO CONTAIN IMPORTANT INFORMATION. YOU SHOULD READ THESE OFFER DOCUMENTS IN THEIR ENTIRETY BEFORE MAKING ANY DECISION WITH RESPECT TO THE OFFER.

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THE OFFER

Upon the terms and subject to the conditions of the Offer (including, if the Offer is extended or amended, the terms and conditions of such extension or amendment), Purchaser will accept for payment and pay the Offer Price, in cash, minus any applicable withholding taxes and without interest, for all Shares validly tendered before the Offer Expiration Time and not properly withdrawn in accordance with Section 4—“Withdrawal Rights.”

Unless the Merger Agreement has been terminated in accordance with its terms, (a) Purchaser will extend the Offer (i) from time to time for any period required by any law, any interpretation or position of the SEC, the staff thereof or the Nasdaq Global Select Market (“Nasdaq”) applicable to the Offer or Merger, and (ii) for periods of up to ten (10) business days per extension, until any waiting period (and any extension thereof) applicable to the consummation of the Offer under the HSR Act will have been expired or been terminated, (b) if, as of the scheduled Expiration Date, any condition of the Offer is not satisfied and has not been waived (to the extent waivable), Purchaser may, in its discretion and without the consent of Five Prime or any other person, extend the Offer on one or more occasions, for an additional period of up to ten (10) business days per extension, to permit such condition to be satisfied and (c) if, as of the scheduled Expiration Date, any condition of the Offer is not satisfied and has not been waived (to the extent waivable) as of the scheduled Expiration Date, Purchaser will, at the request of Five Prime, extend the Offer on one or more occasions for an additional period specified by Five Prime of up to ten (10) business days per extension, to permit such condition to be satisfied.

In no event, however, will Purchaser be required to, or, without Five Prime’s prior written consent, be permitted to, extend the Offer beyond the earlier to occur of: (a) the valid termination of the Merger Agreement in accordance with its terms or (b) the first business day immediately following December 4, 2021.

Under no circumstances will interest be paid on the Offer Price for tendered Shares, regardless of any extension of or amendment to the Offer or any delay in paying for the Shares.

The Offer is not subject to any financing condition. The Offer is conditioned upon the satisfaction of the Minimum Condition and the other conditions set forth in Section 15—“Conditions of the Offer.”

Subject to the applicable rules and regulations of the SEC and pursuant to the Merger Agreement, Purchaser has expressly reserved the right, at any time, to increase the Offer Price, waive any conditions of the Offer or make any other changes to the terms and conditions of the Offer not inconsistent with the terms of the Merger Agreement. However, pursuant to the Merger Agreement, Purchaser may not, without the prior written consent of Five Prime, (a) decrease the Offer Price, (b) change the form of consideration payable in the Offer, (c) decrease the maximum number of Shares to be purchased in the Offer, (d) impose conditions or requirements to the Offer in addition to the conditions of the Offer under the Merger Agreement, (e) amend or modify any of the conditions of the Offer in a manner that adversely affects, or could reasonably be expected to adversely affect, any holder of Shares or that could, individually or in the aggregate, reasonably be expected to prevent or materially delay the consummation of the Offer or prevent, materially delay or materially impair the ability of Amgen or Purchaser to consummate the Offer, the Merger or the other Transactions, (f) amend, modify, change or waive the Minimum Condition, the Termination Condition or the No Legal Prohibition Condition (each as described in Section 15—“Conditions of the Offer”), (g) terminate the Offer or accelerate, extend or otherwise change the Expiration Date, except as provided by the Merger Agreement or (h) provide any “subsequent offering period” (or any extension thereof) within the meaning of Rule 14d-11 of the Exchange Act.

The rights reserved by Purchaser in the preceding paragraph are in addition to Purchaser’s rights pursuant to Section 15—“Conditions of the Offer.”

Pursuant to Section 251(h) of the DGCL and due to the obligations of Purchaser, Amgen and Five Prime pursuant to the Merger Agreement to effect the Merger following the consummation of the Offer, the Merger is

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expected to occur following the consummation of the Offer without a subsequent offering period. Further, the Merger Agreement does not permit a subsequent offering period for the Offer without Five Prime’s prior written consent.

If Purchaser extends the Offer or if Purchaser (whether before or after its acceptance for payment of Shares) is delayed in its acceptance for payment of, or payment for, Shares or it is unable to pay for Shares pursuant to the Offer for any reason, then, without prejudice to Purchaser’s rights under the Offer and subject to applicable law and the rules and regulations of the SEC, the Depositary may retain tendered Shares on behalf of Purchaser, and such Shares may not be withdrawn except to the extent tendering stockholders are entitled to withdrawal rights as described in Section 4—“Withdrawal Rights.” However, the ability of Purchaser to delay the acceptance for payment of, or payment for, Shares that Purchaser has accepted for payment is limited by Rule 14e-1(c) under the Exchange Act, which requires that a bidder promptly pay the consideration offered. Alternatively, if the Offer is not consummated, the Shares are not accepted for payment or Shares are properly withdrawn, promptly after the termination of the Offer or withdrawal of such Shares, Purchaser shall, and Amgen shall cause Purchaser to, promptly return, and shall cause any depositary acting on behalf of Purchaser to return, in accordance with applicable laws, all tendered Shares to the registered holders of such Shares.

Any extension, delay, amendment or termination of the Offer will be followed promptly by public announcement thereof, such announcement in the case of an extension to be issued no later than 9:00 a.m., New York time, on the next business day after the previously scheduled Expiration Date, in accordance with the public announcement requirements of Rule 14e-1(d) under the Exchange Act. Subject to applicable law (including Rules 14d-4(d), 14d-6(c) and 14e-1(d) under the Exchange Act, which require that material changes be promptly disseminated to stockholders in a manner reasonably designed to inform them of such changes) and without limiting the manner in which Purchaser may choose to make any public announcement, Purchaser shall have no obligation to publish, advertise or otherwise communicate any such public announcement other than by issuing a press release and making any appropriate filing with the SEC. As used in this Offer to Purchase, “business day” means any day ending at 11:59 p.m., New York time, other than a Saturday, a Sunday or other day on which banks in the City of New York are authorized or required by laws to be closed.

If, subject to the terms of the Merger Agreement, Purchaser makes a material change to the terms of the Offer or the information concerning the Offer, or if it waives a material condition of the Offer, Purchaser will disseminate additional tender offer materials and extend the Offer to the extent required by Rules 14d-4(d), 14d-6(c) and 14e-1 under the Exchange Act. The minimum period during which the Offer must remain open following material changes in the terms of the Offer or information concerning the Offer, other than a change in price, a change in the percentage of securities sought, or inclusion of or changes to a dealer’s soliciting fee, will depend upon the facts and circumstances, including the relative materiality of the changes to the terms or information.

In a public release, the SEC has stated its view that an offer to purchase must remain open for a minimum period of time following a material change in the terms of the offer and that waiver of a material condition is a material change in the terms of the offer. The release states that an offer should remain open for a minimum of five (5) business days from the date that a material change is first published, sent or given to stockholders and that, if material changes are made with respect to information that approaches the significance of price and percentage of securities sought, a minimum of ten (10) business days may be required to allow for adequate dissemination and investor response. Accordingly, if, prior to the Expiration Date, Purchaser changes the number of Shares being sought or the consideration offered pursuant to the Offer, and if the Offer is scheduled to expire at any time earlier than the tenth (10^th) business day from the date that notice of such change is first published, sent or given to stockholders, the Offer will be extended at least until the expiration of such tenth (10^th) business day. The requirement to extend an Offer does not apply to the extent that the number of business days remaining between the occurrence of the change and the then scheduled expiration date equals or exceeds the minimum extension period that would be required because of such amendment.

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If, on or before the Expiration Date, Purchaser increases the consideration being paid for Shares accepted for payment in the Offer, such increased consideration will be paid to all stockholders whose Shares are purchased in the Offer, whether or not such Shares were tendered before the announcement of the increase in consideration.

Purchaser expressly reserves the right (a) not to accept for payment or (subject to any applicable rules and regulations of the SEC, including Rule 14e-1(c) under the Exchange Act) pay for, and (b) to delay the acceptance for payment of, or (subject to any such rules and regulations) the payment for, any tendered Shares, and to the extent permitted by the Merger Agreement, terminate the Offer (x) upon the termination of the Merger Agreement or (y) at any scheduled Expiration Date (subject to any extensions of the Offer pursuant to the Merger Agreement) or amend the Offer if (i) the Minimum Condition has not been satisfied as of one minute following 11:59 p.m. New York time on such Expiration Date or (ii) any of the other conditions to the Offer have not been satisfied or waived (to the extent waivable) in writing by Amgen. See Section 15—“Conditions of the Offer.” Under certain circumstances we may terminate the Merger Agreement and the Offer. See Section 14—“The Merger Agreement; Other Agreements.”

As promptly as practicable following the consummation of the Offer and subject to the satisfaction or, to the extent permitted by applicable law, waiver of certain conditions set forth in the Merger Agreement, Five Prime, Purchaser and Amgen will consummate the Merger without a meeting of the stockholders of Five Prime in accordance with Section 251(h) of the DGCL.

Five Prime has provided Purchaser or its designee with Five Prime’s stockholder lists and security position listings for the purpose of disseminating this Offer to Purchase, the Letter of Transmittal and related materials to holders of Shares. This Offer to Purchase and the related Letter of Transmittal will be mailed by or on behalf of Purchaser to record holders of Shares whose names appear on Five Prime’s stockholder lists and will be furnished by or on behalf of Purchaser to brokers, dealers, commercial banks, trust companies and similar persons whose names, or the names of whose nominees, appear on the stockholder lists or, if applicable, who are listed as participants in a clearing agency’s security position listing, for subsequent transmittal to beneficial owners of Shares.

Upon the terms and subject to the conditions of the Offer (including, if the Offer is extended or amended, the terms and conditions of any such extension or amendment) and provided that the Offer has not been terminated, Purchaser will accept for payment and promptly pay for all Shares validly tendered prior to the Offer Expiration Time and not properly withdrawn in accordance with Section 4—“Withdrawal Rights.” Rule 14e-1(c) under the Exchange Act relates to the obligation of Purchaser to pay for or return tendered Shares promptly after the termination or withdrawal of the Offer. For a description of our rights and obligations to extend or terminate the Offer and not accept for payment or pay for Shares, or to delay acceptance for payment of, or payment for, Shares, see Section 1—“Terms of the Offer.”

In all cases, payment for Shares accepted for payment in the Offer will be made only after timely receipt by the Depositary of:

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Accordingly, tendering stockholders may be paid at different times depending upon when Share Certificates, Letters of Transmittal or Book-Entry Confirmations with respect to Shares are actually received by the Depositary.

For purposes of the Offer, Purchaser will be deemed to have accepted for payment, and thereby purchased, Shares properly tendered to Purchaser and not properly withdrawn as, if and when Purchaser gives notice to the Depositary of Purchaser’s acceptance for payment of the Shares pursuant to the Offer.

Upon the terms and subject to the conditions of the Offer, payment for Shares accepted for payment pursuant to the Offer will be made by deposit of the Offer Price therefor with the Depositary, which will act as agent for tendering stockholders for the purpose of receiving payment from Purchaser and transmitting such payment to tendering stockholders whose Shares have been accepted for payment.

If any tendered Shares are not accepted for payment for any reason pursuant to the terms and conditions of the Offer, or if Share Certificates are submitted evidencing more Shares than are tendered, Share Certificates representing unpurchased or untendered Shares will be returned, without expense, to the tendering stockholder (or, in the case of Shares delivered by book-entry transfer into the Depositary’s account at the Book-Entry Transfer Facility according to the procedures set forth in Section 3—“Procedure for Tendering Shares,” the Depositary will notify the Book-Entry Transfer Facility of Purchaser’s decision not to accept the Shares and such Shares will be credited to an account maintained at the Book-Entry Transfer Facility), promptly after the expiration or termination of the Offer.

If, for any reason whatever, Purchaser is delayed in its acceptance for payment of, or payment for, Shares or is unable to accept for payment or pay for Shares pursuant to the Offer, then, without prejudice to Purchaser’s rights under the Offer (but subject to compliance with Rule 14e-1(c) under the Exchange Act), the Depositary may, nevertheless, on behalf of Purchaser, retain tendered Shares and such Shares may not be withdrawn except to the extent tendering stockholders are entitled to do so as described in Section 4—“Withdrawal Rights.”

Under no circumstances will interest be paid on the Offer Price to be paid by Purchaser for the Shares, regardless of any extension of the Offer or any delay in making payment.

If, prior to the Expiration Date, Purchaser increases the price being paid for Shares, Purchaser will pay the increased consideration for all Shares purchased pursuant to the Offer, whether or not those Shares were tendered prior to the increase in consideration.

Valid Tender. A stockholder must follow one of the following procedures to validly tender Shares in the Offer:

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No alternative, conditional or contingent tenders will be accepted and no fractional Shares will be purchased.

The method of delivery of Share Certificates, the Letter of Transmittal and all other required documents, including delivery through the Book-Entry Transfer Facility, is at the election and risk of the tendering stockholder. Shares will be deemed delivered only when actually received by the Depositary at one of its addresses set forth on the back cover of this Offer to Purchase. If delivery is by mail, registered mail with return receipt requested, properly insured, is recommended. In all cases, sufficient time should be allowed to ensure timely delivery.

Book-Entry Transfer. The Depositary will establish and, subject to such establishments, maintain an account with respect to the Shares at the Book-Entry Transfer Facility for purposes of the Offer. Any financial institution that is a participant in the Book-Entry Transfer Facility’s systems may make book-entry delivery of Shares by causing the Book-Entry Transfer Facility to transfer such Shares into the Depositary’s account at the Book-Entry Transfer Facility in accordance with the Book-Entry Transfer Facility’s procedures for such transfer. However, although delivery of Shares may be effected through book-entry transfer into the Depositary’s account at the Book-Entry Transfer Facility, the properly completed and duly executed Letter of Transmittal (or manually executed facsimile thereof), with any required signature guarantees, or an Agent’s Message in lieu of the Letter of Transmittal, and any other required documents must, in any case, be received by the Depositary at one of its addresses set forth on the back cover of this Offer to Purchase prior to the Offer Expiration Time, or the tendering stockholder must comply with the guaranteed delivery procedures described under “—Guaranteed Delivery” for a valid tender of Shares by book-entry transfer. Delivery of documents to the Book-Entry Transfer Facility does not constitute delivery to the Depositary.

The term “Agent’s Message” means a message, transmitted through electronic means by the Book-Entry Transfer Facility in accordance with the normal procedures of the Book-Entry Transfer Facility and the Depositary, to, and received by, the Depositary and forming a part of a Book-Entry Confirmation, which states that the Book-Entry Transfer Facility has received an express acknowledgment from the participant in the Book-Entry Transfer Facility tendering the Shares that are the subject of such Book-Entry Confirmation, that such participant has received and agrees to be bound by the terms of the Letter of Transmittal and that Purchaser may enforce such agreement against such participant. The term “Agent’s Message” also includes any hard copy printout evidencing such message generated by a computer terminal maintained at the Depositary’s office.

Signature Guarantees. No signature guarantee is required on the Letter of Transmittal:

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In all other cases, all signatures on the Letter of Transmittal must be guaranteed by an Eligible Institution. See Instructions 1 and 5 to the Letter of Transmittal. If a Share Certificate is registered in the name of a person or persons other than the signer of the Letter of Transmittal, or if payment is to be made or delivered to, or a Share Certificate not tendered or not accepted for payment is to be issued in the name of or returned, to a person other than the registered holder(s) of the address of record of the certificates surrendered, then the tendered Share Certificate must be properly endorsed or accompanied by appropriate duly executed stock powers, in either case signed exactly as the name(s) of the registered holders appear(s) on the Share Certificate, with the signature(s) on such Share Certificate or stock powers guaranteed by an Eligible Institution as provided in the Letter of Transmittal. See Instructions 1 and 5 to the Letter of Transmittal.

Guaranteed Delivery. If a stockholder desires to tender Shares in the Offer and the Share Certificates evidencing such stockholder’s Shares are not immediately available, or such stockholder cannot deliver the Share Certificates and all other required documents to the Depositary prior to the Offer Expiration Time, or such stockholder cannot complete the procedure for delivery by book-entry transfer on a timely basis prior to the Offer Expiration Time, or if time will not permit all required documents to reach the Depositary prior to the Offer Expiration Time, the stockholder’s tender may still be effected if all the following conditions are met:

The Notice of Guaranteed Delivery may be delivered by courier or transmitted by facsimile transmission or mail to the Depositary and must include a guarantee by an Eligible Institution in the form set forth in the form of Notice of Guaranteed Delivery made available by Purchaser. In the case of Shares held through the Book-Entry Transfer Facility, the Notice of Guaranteed Delivery must be delivered to the Depositary by a participant by means of the confirmation system of the Book-Entry Transfer Facility. If the notice is sent by the Book-Entry Transfer Facility through electronic means, it must state that the Book-Entry Transfer Facility has received an express acknowledgment from the participant on whose behalf the notice is given that the participant has received and agrees to become bound by the form of the notice to the Depositary and must include a guarantee by an Eligible Institution in the form set forth in such Notice of Guaranteed Delivery made available by Purchaser.

Shares tendered by a Notice of Guaranteed Delivery will not be deemed “received” for the purpose of satisfying the Minimum Condition unless Shares underlying such Notice of Guaranteed Delivery are delivered to the Depositary at one of its addresses set forth on the back cover of this Offer to Purchase prior to the Expiration Date.

Appointment as Proxy. By executing the Letter of Transmittal (or, in the case of a book-entry transfer, delivering an Agent’s Message in lieu of a Letter of Transmittal, as set forth above), the tendering stockholder will irrevocably appoint designees of Purchaser as such stockholder’s attorneys-in-fact and proxies in the manner set forth in the Letter of Transmittal, each with full power of substitution, to the full extent of such stockholder’s rights with respect to the Shares tendered by such stockholder and accepted for payment by Purchaser (and with respect to any and all other Shares or other securities or rights issued or issuable in respect of such Shares on or

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after the date of this Offer to Purchase). All such powers of attorney and proxies will be irrevocable and considered coupled with an interest in the tendered Shares and such other Shares, securities and rights. Such appointment will be effective when, and only to the extent that, Purchaser accepts for payment Shares tendered by such stockholder as provided herein. Upon the effectiveness of such appointment, all prior powers of attorney, proxies and consents given by such stockholder with respect to such Shares or other securities or rights will, without further action, be revoked and no subsequent powers of attorney, proxies, consents or revocations may be given by such stockholder (and, if given, will not be deemed effective). When the appointment of the proxy becomes effective, the designees of Purchaser will thereby be empowered to exercise all voting and other rights with respect to such Shares and other securities or rights, including, without limitation, in respect of any annual, special, adjourned or postponed meeting of Five Prime’s stockholders, by written consent in lieu of any such meeting or otherwise, as such designee in its sole discretion deems proper. Purchaser reserves the right to require that, for Shares to be deemed validly tendered, immediately upon Purchaser’s acceptance for payment of such Shares, Purchaser must be able to exercise full voting, consent and other rights, to the extent permitted under applicable law, with respect to such Shares and other securities or rights, including voting at any meeting of stockholders or executing a written consent concerning any matter. The Offer does not constitute a solicitation of proxies, absent a purchase of Shares, for any meeting of Five Prime stockholders.

Five Prime Options and Five Prime Restricted Stock Awards. The Offer is only for the outstanding Shares and not for any stock options, restricted shares or other rights to acquire Shares. See Section 14—“The Merger Agreement; Other Agreements” for a description of the treatment of Five Prime Options and Five Prime Restricted Stock Awards in the Merger.

Determination of Validity. The Depositary will examine each document received from stockholders for the tender of Shares (including, if applicable, Share Certificates, the Letter of Transmittal and other required documents) and each notice of withdrawal to determine whether such tender or withdrawal may be defective. In the event the Depositary reasonably concludes that any such document or notice has been improperly completed, executed or transmitted or if some other defect or irregularity exists in connection with a tender of Shares or a withdrawal of tender of Shares, if applicable, the Depositary is authorized to notify the person tendering or withdrawing such Shares of the existence of such defect or irregularity and to take such commercially reasonable actions as are necessary to cause such defect or irregularity to be corrected. If such actions prove to be unsuccessful, the Depositary shall consult with Purchaser for instructions as to the number of Shares, if any, it is authorized to accept for tender or withdrawal of tender. All questions as to the validity, form, eligibility (including time of receipt) and acceptance of any tender of Shares, including questions as to the proper completion or execution of any Letter of Transmittal, Notice of Guaranteed Delivery or other required documents and as to the proper form for transfer of any Share Certificates, will be determined by Purchaser, in its sole and absolute discretion, whose determination will be final and binding. Purchaser reserves the absolute right to reject any or all tenders determined by it not to be in proper or complete form, and to determine whether the acceptance for payment of, or payment for, Shares may be unlawful. Purchaser also reserves the absolute right to waive any defect or irregularity in the tender of any Shares of any particular stockholder, whether or not similar defects or irregularities are waived in the case of other stockholders. No tender of Shares will be deemed to have been validly made until all defects or irregularities relating thereto have been cured or waived to the satisfaction of Purchaser. Purchaser’s interpretation of the Offer (including, without limitation, the Offer to Purchase, the Letter of Transmittal and the instructions thereto and the Notice of Guaranteed Delivery) will be final and binding.

Backup Withholding. Under the “backup withholding” provisions of United States federal income tax law, the Depositary may be required to withhold and pay over to the U.S. Internal Revenue Service (“IRS”) a portion of the amount of any payments made to certain stockholders pursuant to the Offer. Backup withholding will apply to payments made in the Offer to each holder of Shares unless (a) the applicable tendering “U.S. holder” completes and returns the IRS Form W-9 included in the Letter of Transmittal certifying, among other things, that such U.S. holder is not subject to backup withholding or (b) the applicable non-U.S. holder completes and submits an IRS Form W-8BEN or W-8BEN-E (or other applicable IRS Form W-8), which can be obtained from the Depositary or from the IRS website at www.irs.gov. For an explanation of the terms “U.S. holder” and

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“non-U.S. holder” and a more detailed discussion of backup withholding, see Section 5—“Certain U.S. Federal Income Tax Consequences.”

Tender Constitutes Binding Agreement. The tender of Shares pursuant to any one of the procedures described above will constitute the tendering stockholder’s acceptance of the Offer, as well as the tendering stockholder’s representation and warranty that such stockholder has the full power and authority to tender, sell, assign and transfer the Shares tendered, as specified in the Letter of Transmittal, and that when Purchaser accepts the Shares for payment, it will acquire good, marketable and unencumbered title, free and clear of all liens, restrictions, charges and encumbrances and not subject to any adverse claims. Purchaser’s acceptance for payment of Shares validly tendered according to any of the procedures described above and in the Instructions to the Letter of Transmittal will constitute a binding agreement between the tendering stockholder and Purchaser upon the terms and subject to the conditions of the Offer (and if the Offer is extended or amended, the terms and conditions of such extension or amendment).

Except as provided in this Section 4 or as provided by applicable law, tenders of Shares made pursuant to the Offer are irrevocable. Shares tendered in the Offer may be withdrawn according to the procedures set forth below at any time prior to the Offer Expiration Time and, unless accepted for payment and paid for by Purchaser in the Offer, may also be withdrawn pursuant to Section 14(d)(5) of the Exchange Act at any time after May 17, 2021, which is the sixtieth (60^th) day after the commencement of the Offer, unless prior to that date such Shares have already been accepted for payment by Purchaser pursuant to the Offer.

For a withdrawal to be effective, a written or facsimile transmission notice of withdrawal must be timely received by the Depositary at one of its addresses set forth on the back cover of this Offer to Purchase. Any such notice of withdrawal must specify the name of the person who tendered the Shares to be withdrawn, the number and type of Shares to be withdrawn and the name of the registered holder of the Shares to be withdrawn, if different from the name of the person who tendered the Shares. If Share Certificates evidencing Shares to be withdrawn have been delivered or otherwise identified to the Depositary, then, before the physical release of such Share Certificates, the serial numbers shown on such Share Certificates must be submitted to the Depositary and the signature(s) on the notice of withdrawal must be guaranteed by an Eligible Institution. If Shares have been tendered according to the procedures for book-entry transfer as set forth in Section 3—“Procedure for Tendering Shares,” any notice of withdrawal must also specify the name and number of the account at the Book-Entry Transfer Facility to be credited with the withdrawn Shares and otherwise comply with the Book-Entry Transfer Facility’s procedures.

If Purchaser extends the Offer, is delayed in its acceptance for payment of Shares or is unable to accept Shares for payment pursuant to the Offer for any reason, then, without prejudice to Purchaser’s rights under the Offer, the Depositary may, nevertheless, on behalf of Purchaser, retain tendered Shares, and such Shares may not be withdrawn except to the extent that tendering stockholders are entitled to withdrawal rights as described herein.

Withdrawals of tenders of Shares may not be rescinded, and any Shares properly withdrawn will no longer be considered validly tendered for purposes of the Offer. However, withdrawn Shares may be retendered by following one of the procedures described in Section 3—“Procedure for Tendering Shares” at any time prior to the Offer Expiration Time.

All questions as to the form and validity (including time of receipt) of notices of withdrawal will be determined by Purchaser, in its sole and absolute discretion, whose determination will be final and binding.

The method for delivery of any documents related to a withdrawal is at the risk of the withdrawing stockholder. Any documents related to a withdrawal will be deemed delivered only when actually received by the Depositary at one of its addresses set forth on the back cover of this Offer to Purchase. If delivery is by mail, registered mail with return receipt requested, properly insured, is recommended. In all cases, sufficient time should be allowed to ensure timely delivery.

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The following discussion summarizes certain U.S. federal income tax consequences expected to result to the holders of Shares upon the tender of Shares for cash pursuant to the Offer or converted to cash in the Merger. This discussion is not a complete analysis of all potential U.S. federal income tax consequences, nor does it address any tax consequences arising under any state, local or non-U.S. tax laws, any U.S. federal non-income tax laws such as estate or gift tax laws or the application of the alternative minimum tax provisions of the Code (except to the limited extent discussed below), the Medicare contribution tax on net investment income, or the special tax accounting rules under Section 451(b) of the Code. This discussion is based on the United States Internal Revenue Code of 1986, as amended, which we refer to as the Code, Treasury Regulations promulgated thereunder, judicial decisions, and published rulings and administrative pronouncements of the Internal Revenue Service, all as in effect as of the date of this Offer to Purchase. These laws and authorities may change, possibly retroactively, resulting in U.S. federal income tax consequences different from those discussed below. No ruling has been or will be sought from the IRS with respect to the matters discussed below, and there can be no assurance that the IRS will not take a contrary position regarding the tax consequences of the Offer and the Merger or that any such contrary position would not be sustained by a court.

This discussion is limited to holders that hold Shares as capital assets within the meaning of Section 1221 of the Code (generally, property held for investment). This discussion does not address all U.S. federal income tax considerations that may be relevant to a holder in light of the holder’s particular circumstances. This discussion also does not consider any specific facts or circumstances that may be relevant to holders subject to special rules under the U.S. federal income tax laws, including, without limitation, certain former citizens or long-term residents of the United States, entities or arrangements treated as partnerships or other pass-throughs for U.S. federal income tax purposes and investors therein, U.S. holders who hold their Shares through “controlled foreign corporations” or “passive foreign investment companies,” corporations that accumulate earnings to avoid U.S. federal income tax, financial institutions, insurance companies, brokers, dealers or traders in securities, commodities or currencies, tax-exempt organizations, tax-qualified retirement plans, U.S. persons whose functional currency is not the U.S. dollar, holders holding Shares as “qualified small business stock” within the meaning of Section 1202 of the Code, and persons holding Shares as part of a hedge, straddle or other risk reduction strategy or as part of a hedging or conversion transaction or other integrated investment. This discussion also does not address the U.S. federal income tax consequences to holders of Shares who acquired their Shares through stock option or stock purchase plan programs, in other compensatory arrangements or otherwise in connection with the performance of services, or those who exercise appraisal rights under the DGCL. Important Note: If you are a citizen or tax resident or subject to the tax laws of more than one country, you should be aware that there might be additional or different tax consequences that may apply to you.

WE RECOMMEND THAT YOU CONSULT YOUR TAX ADVISOR REGARDING THE U.S. FEDERAL TAX CONSEQUENCES OF THE OFFER AND THE MERGER IN RESPECT OF YOUR PARTICULAR CIRCUMSTANCES, AS WELL AS ANY TAX CONSEQUENCES ARISING UNDER ANY STATE, LOCAL OR NON-U.S. TAX LAWS.

As used in this discussion, a “U.S. holder” is any beneficial owner of Shares that is, for U.S. federal income tax purposes:

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A “non-U.S. holder” is any beneficial owner of Shares (i) that is not a partnership (or any other entity or arrangement treated as a partnership for U.S. federal income tax purposes) and (ii) that is not a U.S. holder for U.S. federal income tax purposes.

U.S. Holders

Effect of the Offer and the Merger. The receipt of cash in exchange for Shares in the Offer or the Merger will be a taxable transaction for U.S. federal income tax purposes. In general, a U.S. holder who receives cash in exchange for Shares in the Offer or the Merger will recognize capital gain or loss for U.S. federal income tax purposes equal to the difference, if any, between the amount of cash received and the holder’s adjusted tax basis in the applicable Shares, determined on a per share basis. Any such gain or loss would be long-term capital gain or loss if the holding period for the Shares exceeded one year. Under current law, long-term capital gains of noncorporate taxpayers are generally taxable at a reduced rate. Short-term capital gains are taxed at rates applicable to ordinary income. The deductibility of capital losses is subject to limitations. If a U.S. holder acquired different blocks of Shares at different times or at different prices, such U.S. holder must determine its tax basis, holding period, and gain or loss separately with respect to each block of Shares (i.e., Shares acquired at the same cost in a single transaction) exchanged for cash in the Offer or the Merger. Each U.S. holder should consult such U.S. holder’s tax advisors regarding the manner in which any cash received pursuant to the Offer or the Merger should be allocated among the U.S. holder’s respective different blocks of Shares.

Information Reporting and Backup Withholding. Payments made to U.S. holders in the Offer or the Merger generally will be subject to information reporting and may be subject to backup withholding (currently at a rate of 24%). To avoid backup withholding, U.S. holders that do not otherwise establish an exemption should complete and return the IRS Form W-9 included in the Letter of Transmittal, certifying that such holder is a U.S. person, the taxpayer identification number provided by such holder is correct, and that such holder is not subject to backup withholding. U.S. holders who fail to furnish a taxpayer identification number in the manner required also may be subject to penalties imposed by the IRS. Certain holders (such as corporations) may not be subject to backup withholding. Backup withholding is not an additional tax and may be refunded or credited against the applicable holder’s U.S. federal income tax liability, provided that, such holder timely furnishes the required information to the IRS.

Non-U.S. Holders

Effect of the Offer and the Merger. A non-U.S. holder generally will not be subject to U.S. federal income tax on any gain realized on the receipt of cash for Shares in the Offer or the Merger unless:

Gains described in the first bullet point above generally will be subject to U.S. federal income tax at a flat 30% rate (or applicable lower treaty rate), but may be offset by U.S. source capital losses, even though the individual is not considered a resident of the United States, provided that such non-U.S. holder has timely filed U.S. federal income tax returns with respect to such losses. Unless a tax treaty provides otherwise, gain described in the second bullet point above generally will be subject to U.S. federal income tax on a net income basis in the same manner as if the non-U.S. holder were a resident of the United States. Non-U.S. holders that are corporations also may be subject to a 30% branch profits tax (or applicable lower treaty rate) on such effectively connected gains, as adjusted for certain items. Non-U.S. holders are urged to consult their tax advisors regarding any applicable tax treaties that may provide for different rules.

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A non-U.S. holder should be aware that any gain realized upon the disposition of Shares in the Offer or the Merger also may be subject to U.S. federal income tax if, for such purposes, the Shares constitute a U.S. real property interest because Five Prime was a U.S. real property holding corporation (a “USRPHC”) at any time within the shorter of the five (5)-year period ending on the date of such disposition or such holder’s holding period. In general, a corporation is a USRPHC if the fair market value of its “United States real property interests” (as defined in the Code and applicable Treasury Regulations) equals or exceeds 50% of the sum of the fair market value of its worldwide real property interests and its other assets used or held for use in a trade or business. There can be no assurance that Five Prime has not been, does not currently constitute or will not become a USRPHC. However, since the Shares are regularly traded on an established securities market (within the meaning of applicable Treasury Regulations), if Five Prime is a USRPHC with respect to a non-U.S. holder, that non-U.S. holder’s Shares will be treated as U.S. real property interests only if that non-U.S. holder owned (actually or constructively) during the relevant statutory period more than five percent of the Shares. Non-U.S. holders who have owned (actually or constructively) more than five percent of the Shares should consult their tax advisors regarding the U.S. federal income tax consequences of the Offer and the Merger. Pursuant to Five Prime’s prospectus supplement on Form 424B5, dated as of June 26, 2019, Five Prime did not believe it was a USRPHC as of such date.

Information Reporting and Backup Withholding. Payments made to non-U.S. holders in the Offer and the Merger may be subject to information reporting and backup withholding (currently at a rate of 24%). Non-U.S. holders can avoid backup withholding by providing the Depositary with a properly completed and executed IRS Form W-8BEN or W-8BEN-E (or other applicable IRS Form W-8) certifying the holder’s non-U.S. status or by otherwise establishing an exemption from backup withholding. Backup withholding is not an additional tax and may be refunded or credited against the applicable holder’s U.S. federal income tax liability, if any, provided that such holder timely furnishes the required information to the IRS. IRS Forms W-8 are available for download from the IRS website at: http://www.irs.gov/w8.

The Shares are listed and traded on Nasdaq under the symbol “FPRX.” Five Prime has advised Amgen that, as of the close of business on March 8, 2021, 45,941,900 Shares were issued and outstanding. The following table sets forth, for each of the periods indicated, the high and low reported sales price for the Shares on Nasdaq, based on published financial sources.

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On March 3, 2021, the last full trading day before public announcement of the execution of the Merger Agreement, the closing price reported on Nasdaq was $21.26 per Share. On March 17, 2021, the last full trading day before the commencement of the Offer, the closing price reported on Nasdaq was $37.88 per Share. The Offer Price represents an approximate premium of 78.7% of the closing price of the Shares on March 3, 2021 and an approximate premium of 99% over the volume weighted average price of the Shares during the preceding thirty (30)-day trading period.

Purchaser has been advised that Five Prime has never declared or paid a cash dividend with respect to the Shares. The Merger Agreement provides that, from the date of the Merger Agreement until the earlier to occur of the Offer Acceptance Time and the termination of the Merger Agreement pursuant to its terms, except as consented to in writing by Amgen (which consent will not be unreasonably withheld, conditioned or delayed) or otherwise set forth in the Merger Agreement, Five Prime may not establish a record date for, declare, accrue, set aside or pay any dividend or make any other distribution in respect of any of its capital stock, including the Shares.

Stockholders are urged to obtain a current market quotation for the Shares before making a decision with respect to the Offer.

Market for the Shares. If the Offer is completed and we accordingly acquire a number of Shares that satisfies the Minimum Condition and the other conditions to the Merger are satisfied or waived, then, in accordance with the terms of the Merger Agreement, we will effect the Merger as promptly as practicable following the consummation of the Offer without a vote or any further action by the stockholders of Five Prime pursuant to Section 251(h) of the DGCL. As a result of the Merger, there will be no public or other market for the Shares.

Nasdaq Listing. The Shares are listed on Nasdaq. Immediately following the consummation of the Merger, the Shares will no longer meet the requirements for continued listing on Nasdaq because Amgen will be the only stockholder of Five Prime. We intend and will cause the Surviving Corporation to delist the Shares from Nasdaq as promptly as practicable after the Effective Time.

Exchange Act Registration. The Shares are currently registered under the Exchange Act. The purchase of the Shares pursuant to the Offer may result in the Shares becoming eligible for deregistration under the Exchange Act. Registration of the Shares may be terminated by application of Five Prime to the SEC if the Shares are not listed on a national securities exchange and there are fewer than 300 record holders of Shares. We intend to have the Surviving Corporation apply for termination of registration of the Shares under the Exchange Act as promptly as practicable after the Effective Time.

Termination of registration of the Shares under the Exchange Act would reduce the information required to be furnished by Five Prime to its stockholders and to the SEC and would ultimately make certain provisions of the Exchange Act (such as the short-swing profit recovery provisions of Section 16(b), the requirement of furnishing a proxy statement or information statement in connection with stockholders’ meetings or actions in lieu of a stockholders’ meeting pursuant to Sections 14(a) and 14(c) of the Exchange Act and the related requirement of furnishing an annual report to stockholders) no longer applicable with respect to the Shares. In addition, if the Shares are no longer registered under the Exchange Act, the requirements of Rule 13e-3 under the Exchange Act with respect to “going private” transactions would no longer be applicable to Five Prime. Furthermore, the ability of “affiliates” of Five Prime and persons holding “restricted securities” of Five Prime to dispose of such securities pursuant to Rule 144 under the U.S. Securities Act of 1933, as amended, may be impaired or eliminated.

Margin Regulations. The Shares are currently “margin securities” under the regulations of the Board of Governors of the Federal Reserve System (the “Federal Reserve Board”), which has the effect, among other things, of allowing brokers to extend credit using Shares as collateral. Depending upon factors similar to those

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described above regarding listing and market quotations, following the Offer, the Shares may no longer constitute “margin securities” for the purposes of the margin regulations of the Federal Reserve Board, in which event the Shares would be ineligible as collateral for margin loans made by brokers.

As described in Section 14—“The Merger Agreement; Other Agreements—The Merger Agreement—Operating Covenants,” the Merger Agreement provides that, from the date of the Merger Agreement until the earlier of the Offer Acceptance Time or the termination of the Merger Agreement pursuant to its terms, except as consented to in writing by Amgen (which consent will not be unreasonably withheld, conditioned or delayed) or otherwise set forth in the Merger Agreement, Five Prime will not establish a record date for, declare, accrue, set aside or pay any dividend or make any other distribution in respect of any of its capital stock, including the Shares.

Five Prime. The following description of Five Prime and its business has been taken from Five Prime’s Annual Report on Form 10-K for the fiscal year ended December 31, 2019 and is qualified in its entirety by reference to such Annual Report on Form 10-K.

Five Prime was incorporated in December 2001 in Delaware and completed its initial public offering in September 2013. The address of Five Prime’s principal executive office is 111 Oyster Point Boulevard, South San Francisco, California 94080. The telephone number of Five Prime’s principal executive office is (415) 365-5600.

Five Prime is a clinical-stage biotechnology company focused on developing immune modulators and precision therapies to improve the lives of patients with solid tumor cancers. Five Prime’s primary focus is on developing immuno-oncology and targeted cancer therapies. Each of its product candidates has an innovative mechanism of action and addresses patient populations for which better therapies are needed. In addition, Five Prime uses companion diagnostics where appropriate to allow it to select patients most likely to benefit from treatment with its product candidates.

One of the most advanced product candidates that Five Prime is developing is Bemarituzumab (FPA144), an antibody that inhibits fibroblast growth factor receptor 2b, or FGFR2b, and that induces antibody-dependent cellular cytotoxicity that Five Prime is studying in a clinical trial in combination with 5-fluorouracil (5-FU), leucovorin and oxaliplatin, a standard-of-care chemotherapy regimen known as mFOLFOX6, as front-line treatment of patients with gastric (stomach) or gastroesophageal junction, or GEJ, cancer that overexpresses FGFR2b.

Available Information. The Shares are registered under the Exchange Act. Five Prime is subject to the information and reporting requirements of the Exchange Act and in accordance therewith is obligated to file reports, proxy statements and other information with the SEC relating to its business, financial condition and other matters.

Certain information, as of particular dates, concerning Five Prime’s business, capital structure, material pending litigation, operating results, financial condition, directors and officers (including the compensation, options, stock appreciation rights, performance awards, deferred stock and restricted stock granted to them), the principal holders of Five Prime’s securities, any material interests of such persons in transactions with Five Prime and other matters is required to be disclosed in proxy statements and periodic reports distributed to Five Prime’s stockholders and filed with the SEC.

Such reports, proxy statements and other information are available free of charge at the SEC’s website at www.sec.gov. Five Prime also maintains a website at www.fiveprime.com. The information contained in,

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accessible from or connected to Five Prime’s website is not incorporated into, or otherwise a part of, this Offer to Purchase or any of Five Prime’s filings with the SEC. The website addresses referred to in this paragraph are inactive text references and are not intended to be actual links to the websites.

Sources of Information. Except as otherwise set forth herein, the information concerning Five Prime contained in this Offer to Purchase has been furnished by Five Prime or its representatives or taken from or based upon publicly available documents and records on file with the SEC and other public sources. The information concerning Five Prime taken or derived from such documents and records is qualified in its entirety by reference to Five Prime’s public filings with the SEC (which may be obtained and inspected as described above) and should be considered in conjunction with the more comprehensive financial and other information in such reports and other publicly available information.

Amgen. The following description of Amgen and its business has been taken from Amgen’s Annual Report on Form 10-K for the fiscal year ended December 31, 2020 and is qualified in its entirety by reference to such Annual Report on Form 10-K.

Amgen was incorporated in California in 1980 and became a Delaware corporation in 1987. Amgen’s principal executive office is located at One Amgen Center Drive, Thousand Oaks, California 91320. The telephone number at that office is (805) 447-1000. Amgen is one of the world’s leading independent biotechnology companies, has reached millions of patients around the world and is developing a pipeline of medicines with breakaway potential. Amgen focuses solely on human therapeutics and is committed to unlocking the potential of biology for patients suffering from serious illnesses by discovering, developing, manufacturing and delivering innovative human therapeutics.

Amgen’s principal products—those with the most significant annual commercial sales—are ENBREL, Prolia^®, Neulasta^®, Otezla^®, XGEVA^®, Aranesp^®, KYPROLIS^® and Repatha^®. Amgen also markets a number of other products, including Nplate^®, Vectibix^®, MVASI^®, Parsabiv^®, EPOGEN^®, KANJINTI^®, BLINCYTO^®, Aimovig^®, EVENITY^®, AMGEVITA^TM, Sensipar^®/Mimpara^®, NEUPOGEN^®, IMLYGIC^®, Corlanor^® and AVSOLA^®.

Amgen’s strategy includes integrated activities intended to maintain and strengthen its competitive position in the industry. Amgen focuses on six commercial areas: inflammation, oncology/hematology, bone health, CV disease, nephrology and neuroscience, and conducts discovery research primarily in three therapeutic areas: inflammation, oncology/hematology and CV/metabolic diseases. In 2020, Amgen advanced its innovative pipeline, successfully integrated Otezla^®, which was acquired in November 2019, into its inflammation portfolio, advanced its international expansion and provided uninterrupted supply of its medicines globally through the COVID-19 pandemic.

Amgen’s common stock, 1.250% senior notes due 2022 and 2.00% senior notes due 2026 are each listed on The Nasdaq Stock Market LLC under the symbols “AMGN,” “AMGN22” and “AMGN26,” respectively.

Purchaser. Purchaser is a Delaware corporation that was formed by Amgen on March 3, 2021 solely for the purpose of engaging in the Transactions, and has not engaged in any business activities other than as incidental to its formation and in connection with the Transactions, including the Offer and the Merger. Purchaser is a wholly owned subsidiary of Amgen. Upon completion of the Merger, Purchaser will cease to exist and Five Prime will continue as the Surviving Corporation. Purchaser’s principal executive office is located at One Amgen Center Drive, Thousand Oaks, California 91320. The telephone number at that office is (805) 447-1000.

Additional Information. The name, citizenship, business address, current principal occupation or employment and five (5)-year employment history of each of the directors and executive officers of Amgen and Purchaser are set forth in Schedule I hereto.

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During the past five (5) years, none of Amgen, Purchaser or, to their knowledge, any of the persons listed in Schedule I to this Offer to Purchase (a) has been convicted in a criminal proceeding (excluding traffic violations or similar misdemeanors), or (b) was a party to any judicial or administrative proceeding (except for matters that were dismissed without sanction or settlement) that resulted in a judgment, decree or final order enjoining the person from future violations of, or prohibiting activities subject to, federal or state securities laws, or a finding of any violation of such laws.

None of Amgen, Purchaser or, to their knowledge, any of the persons listed in Schedule I to this Offer to Purchase or any associate or other majority-owned subsidiary of Amgen or Purchaser or of any of the persons so listed, (a) beneficially owns or has any right to acquire, directly or indirectly, any Shares or any other equity securities of Five Prime, (b) has effected any transaction in respect of any Shares or any other equity securities of Five Prime during the past sixty (60) days or (c) has any contract, arrangement, understanding or relationship with any other person with respect to any securities of Five Prime (including, but not limited to, any contract, arrangement, understanding or relationship concerning the transfer or the voting of any such securities, joint ventures, loan or option arrangements, puts or calls, guaranties of loans, guaranties against loss or the giving or withholding of proxies, consents or authorizations).

Except as set forth in this Offer to Purchase, as of the date hereof, none of Amgen, Purchaser or, to their knowledge, any of the persons listed in Schedule I to this Offer to Purchase, has had any business relationship or transaction with Five Prime or any of its executive officers, directors or affiliates that is required to be reported under the rules and regulations of the SEC applicable to the Offer. Except as set forth in this Offer to Purchase, as of the date hereof, there have been no negotiations, transactions or material contacts between Amgen or any of its subsidiaries (including Purchaser) or, to their knowledge, any of the persons listed in Schedule I to this Offer to Purchase, on the one hand, and Five Prime or its subsidiaries, on the other hand, concerning a merger, consolidation or acquisition, tender offer or other acquisition of securities, an election of directors or a sale or other transfer of a material amount of assets during the past two (2) years.

Available Information. Pursuant to Rule 14d-3 under the Exchange Act, Amgen and Purchaser filed with the SEC the Schedule TO, of which this Offer to Purchase forms a part, and exhibits to the Schedule TO. The Schedule TO and the exhibits thereto, as well as the other information filed by Amgen and Purchaser with the SEC, are available free of charge at the SEC’s website at www.sec.gov.

The Offer and the Merger are not subject to any financing condition. Amgen and Purchaser estimate that the total funds required to complete the Offer and the Merger will be approximately $1.89 billion plus any related transaction fees and expenses. Purchaser will acquire these funds from Amgen, which intends to provide the funds out of cash on hand. Because the only consideration to be paid in the Offer and the Merger is cash, the Offer involves the purchase of all outstanding Shares and there is no financing condition to the completion of the Offer, we believe that the financial condition of Purchaser and Amgen is not material to a decision by a holder of Shares whether to sell, hold or tender Shares in the Offer.

Background of the Offer.

Amgen regularly evaluates its business and plans and considers a variety of transactions to enhance its business. Amgen has considered a number of alternatives for developing its businesses, including partnerships and acquisitions of other companies and businesses.

In July 2019, representatives of Amgen’s and Five Prime’s respective Business Development teams entered into preliminary discussions regarding Amgen’s potential interest in certain programs in Five Prime’s research and development pipeline.

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On July 29, 2019, Amgen and Five Prime entered into the Confidentiality Disclosure Agreement (as described in Section 14—“The Merger Agreement; Other Agreements—Confidentiality Disclosure Agreement”) with respect to the sharing of confidential information in connection with Amgen’s potential collaboration with Five Prime on the research and development of several Five Prime pipeline programs. Amgen and Five Prime amended the Confidentiality Disclosure Agreement, effective as of August 2, 2019, to expand its scope to cover Five Prime’s bemarituzumab (“Bema”) clinical candidate, a therapeutic antibody targeting FGFR2b, which is overexpressed in approximately 30% of non HER2+ gastric and gastroesophageal junction (“GEJ”) cancers worldwide.

On August 15, 2019, representatives from Amgen and Five Prime held a technical review meeting to discuss Bema. On August 27, 2019, representatives from Amgen and Five Prime held a technical review meeting to discuss Five Prime’s late stage research programs. Thereafter, representatives from Amgen and Five Prime continued to discuss the status of Five Prime’s pipeline from time to time.

On June 12, 2020, in connection with Five Prime’s strategic initiatives planning, Dr. Tarak Mody, Five Prime’s Vice President, Business Development and Alliance Management, contacted a representative of Amgen’s Business Development team regarding a competitive collaboration process that Five Prime intended to launch to advance the development and commercialization of Bema, and FPT155, a soluble CD80 fusion protein (“FPT155”). Amgen and Five Prime amended the Confidentiality Disclosure Agreement, effective as of June 16, 2020, to extend the term and expand its scope to cover the FPT155 program.

On June 18, 2020, representatives of Amgen’s and Five Prime’s respective Business Development teams held a virtual meeting and discussed connecting later in the year when the results of its randomized Phase 2 clinical trial evaluating Bema in combination with chemotherapy in front-line patients with advanced FGFR2b+, non HER2+ gastric and GEJ cancer (the “FIGHT trial”) became available.

Near the end of September 2020, representatives of Amgen’s Business Development team held a telephone conversation with Dr. Mody to begin discussing logistics for management meetings following the release of topline results from the FIGHT trial. Dr. Mody and representatives of Amgen’s Business Development team held several follow-up conversations on such logistics thereafter.

On December 11, 2020, Five Prime provided representatives of Amgen access to a virtual data room for the sharing of certain information with respect to the potential strategic collaboration with respect to Bema (the “Bema Collaboration”). Also on December 11, 2020, representatives of Five Prime delivered a management presentation to representatives of Amgen on the Bema Collaboration.

On December 16, 2020, Dr. Mody sent a process letter regarding bids for a potential Bema Collaboration to representatives of Amgen. The Bema Collaboration process letter asked Amgen to submit preliminary Bema Collaboration proposals to Five Prime by January 27, 2021.

On January 29, 2021, Chris DeRespino, then-Executive Director of Business Development of Amgen, contacted Dr. Mody to inform him that Amgen planned to submit a proposal with respect to a Bema Collaboration over the weekend. On January 30, 2021, Amgen submitted a term sheet to Five Prime with respect to a Bema Collaboration.

On January 31, 2021, at the instruction of the Five Prime Board, representatives of Lazard Freres & Co. LLC (“Lazard”), financial advisor to Five Prime informed representatives of Amgen – Rachna Khosla, Vice President of Business Development, and Mr. DeRespino – that Five Prime had received a third-party offer to acquire Five Prime and inquired as to Amgen’s interest in a potential acquisition transaction. Ms. Khosla and Mr. DeRespino responded that they would discuss internally and revert to Lazard with Amgen’s response.

On February 1, 2021, Ms. Khosla contacted a representative of Lazard and indicated that Amgen was open to considering an acquisition of Five Prime.

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On February 2, 2021, in accordance with the instructions of the Five Prime Board, a representative of Lazard, on behalf of Five Prime, informed representatives of Amgen that the Five Prime Board was considering a sale process for Five Prime and invited Amgen to submit an indication of value for Five Prime.

Between February 8 and February 12, 2021, representatives of Cooley LLP (“Cooley”), Five Prime’s outside legal counsel, and Amgen negotiated the Non-Disclosure Agreement (as described in Section 14—“The Merger Agreement; Other Agreements—Non-Disclosure Agreement”) with respect to a potential acquisition transaction. On February 12, 2021, Amgen and Five Prime executed the Non-Disclosure Agreement, in which Amgen agreed to abide by a customary standstill provision; however, the standstill provision would not prevent Amgen from submitting a private acquisition proposal or making a public acquisition proposal following the announcement of, or entry into, a change-in-control transaction with respect to Five Prime.

Following the entry into Non-Disclosure Agreement, Amgen was provided with access to a virtual data room for the sharing of certain information with respect to a potential acquisition transaction.

On February 11, 2021, a representative of Lazard, on behalf of the Five Prime Board, verbally communicated to Amgen a February 18, 2021 deadline to submit a bid for an acquisition of Five Prime.

On February 13, 2021, Five Prime senior management provided a management presentation to representatives of Amgen.

On February 14, 2021, representatives of Goldman Sachs & Co. LLC (“Goldman Sachs”), Amgen’s financial advisor, and representatives of Lazard discussed the acquisition process and timing, with representatives of Lazard noting that Five Prime had already received a proposal to acquire Five Prime.

On February 15, 2021, Amgen management delivered a memorandum to Amgen’s board of directors via its secure board portal system regarding the potential acquisition opportunity in anticipation of further discussions at the March 3, 2021 regular meeting of the Amgen board of directors.

On February 18, 2021, Amgen submitted a non-binding written proposal to the Five Prime Board for an acquisition of Five Prime for $32.00 per Share.

On February 19, 2021, Amgen received a process letter from representatives of Lazard, noting a “best and final” bid deadline of March 1, 2021, along with an initial auction draft of the Merger Agreement. Later on February 19, 2021, representatives of Goldman Sachs, on behalf of Amgen, and representatives of Lazard, on behalf of Five Prime, held a telephone conversation to discuss next steps in the process, including the timing of a definitive offer. Representatives of Lazard noted that the process was expected to be competitive with Five Prime having already received multiple acquisition proposals.

Between February 22, 2021 and March 1, 2021, representatives of Amgen and their advisors participated in several telephone conversations with representatives of Five Prime and its advisors with respect to due diligence matters.

On February 23, 2021, Sullivan & Cromwell LLP, Amgen’s outside legal counsel (“Sullivan & Cromwell”) and Cooley had an introductory meeting and high-level discussion regarding certain terms of the draft Merger Agreement, including the structure of the transaction as a tender offer, treatment of Five Prime’s outstanding equity awards at closing, regulatory matters and termination rights of the parties, including Five Prime’s ability to terminate the Merger Agreement in order to accept a superior offer.

On February 25, 2021, Sullivan & Cromwell and Cooley had a further discussion regarding Amgen’s position with respect to key issues in the draft Merger Agreement, including with respect to treatment of Five Prime’s outstanding equity awards at closing, gross-ups for tax penalties imposed under Section 280G of the Internal Revenue Code for Five Prime executives, and regulatory matters.

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On February 26, 2021, Sullivan & Cromwell sent a mark-up of the draft Merger Agreement to Cooley.

Also on February 26, 2021, representatives of Goldman Sachs, on behalf of Amgen, and Lazard, on behalf of Five Prime, held another telephone conversation to discuss the transaction process, competitive dynamics and expected next steps following the submission of final bids by all interested potential parties on March 1, 2021. Representatives of Lazard confirmed the competitive nature of the process with Five Prime expecting to receive multiple acquisition proposals on the March 1st deadline.

On February 28, 2021, Cooley and Sullivan & Cromwell further discussed Five Prime’s and Amgen’s respective positions with respect to the draft Merger Agreement, including 280G gross-ups and grants of cash awards (in lieu of annual equity grants) for executives and non-executive employees, and regulatory matters, including efforts required by and restrictions imposed upon the parties between signing and closing.

On March 1, 2021, Amgen submitted to Five Prime a revised proposal of $38.00 per Share. Later on March 1, 2021, representatives of Goldman Sachs, on behalf of Amgen, held a telephone conversation with representatives of Lazard, on behalf of Five Prime, with respect to such bid and to understand the anticipated timing of next steps in the transaction process. Also on March 1, Sullivan & Cromwell shared with Cooley revised language for the Merger Agreement regarding certain regulatory matters.

On March 2, 2021, representatives of Lazard informed representatives of Goldman Sachs that the Five Prime Board had determined to move forward with Amgen, subject to the parties reaching satisfactory terms on the Merger Agreement.

Subsequently on March 2, 2021, Cooley sent a revised draft Merger Agreement and the accompanying confidential disclosure letter to Sullivan & Cromwell. On the evening of March 2, 2021, representatives of Cooley and Sullivan & Cromwell met virtually to discuss the remaining open issues in the draft Merger Agreement, including termination fees and regulatory commitments. Representatives of Sullivan & Cromwell communicated that the revised draft Merger Agreement formed part of and should be considered in conjunction with the “best and final” offer price Amgen had offered.

During the evening of March 2, 2021, Amgen management delivered materials to its board of directors via its secure board portal system regarding its analysis of the Five Prime opportunity at the offer price.

On March 3, 2021, Sullivan & Cromwell and Cooley exchanged multiple drafts of the Merger Agreement and the confidential disclosure letter, and after further feedback and discussions with members of the Five Prime Board and senior management, representatives of Cooley finalized the terms of the proposed Merger Agreement and the exhibits, annexes and schedules thereto with Sullivan & Cromwell.

Later on March 3, 2021, the board of directors of Amgen held its regularly scheduled telephonic board meeting, during which the board discussed the terms of the Merger Agreement and the Transactions. The Amgen board of directors determined that the Merger Agreement and the Transactions were in the best interests of Amgen, and approved the execution, delivery and performance of the Merger Agreement and the consummation of the Transactions, including the Offer and the Merger.

On the morning of March 4, 2021, Five Prime, Amgen and Purchaser executed the Merger Agreement. Also on March 4, 2021, prior to the opening of trading on the Nasdaq stock exchange, Amgen and Five Prime issued a joint press release announcing the execution of the Merger Agreement and the Transactions.

Past Contacts, Negotiations and Transactions.

For more information on the Merger Agreement and the other contacts, transactions, negotiations and agreements among Five Prime, Amgen and Purchaser and their respective related parties, see Section 10—“Certain Information Concerning Amgen and Purchaser” and Section 14—“The Merger Agreement; Other Agreements.”

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Purpose of the Offer. The purpose of the Offer is to enable Amgen, through Purchaser, to acquire control of, and the entire equity interest in, Five Prime. The Offer is being made pursuant to the Merger Agreement and is intended to increase the likelihood that the Merger will be effected and reduce the time required for stockholders to receive the transaction consideration and to complete the acquisition of Five Prime. The purpose of the Merger is to acquire all outstanding Shares not tendered and purchased pursuant to the Offer. The transaction structure includes the Merger to ensure the acquisition of all outstanding Shares.

If the Merger is completed, Amgen will own 100% of the equity interests in Five Prime and will be entitled to all of the benefits resulting from that interest. These benefits include complete control of Five Prime and entitlement to any increase in its value. Similarly, Amgen would also bear the risk of any losses incurred in the operation of Five Prime and any decrease in the value of Five Prime.

The stockholders of Five Prime who tender their Shares in the Offer will cease to have any equity interest in Five Prime and to participate in any future growth in Five Prime. If the Merger is completed, the current stockholders of Five Prime will no longer have an equity interest in Five Prime and instead will have only the right to receive cash consideration according to the Merger Agreement or, to the extent stockholders are entitled to and properly exercise and perfect, and do not withdraw or otherwise lose, appraisal rights under the DGCL, the amounts to which such stockholders are entitled under the DGCL. See Section 14—“The Merger Agreement; Other Agreements.” Similarly, the current stockholders of Five Prime will not bear the risk of any decrease in the value of Five Prime after selling their Shares in the Offer or the Merger.

Merger Without a Vote of the Five Prime Stockholders. If the Offer is consummated, we will not be required to seek, and do not anticipate seeking, the approval of Five Prime’s remaining public stockholders before effecting the Merger. Section 251(h) of the DGCL provides that, following consummation of a successful tender offer for a public corporation, and subject to certain statutory provisions, if the stock irrevocably accepted for purchase or exchange pursuant to such offer and received by the depositary prior to the expiration of such offer, together with stock otherwise owned by the acquirer and its affiliates and any rollover stock, equals at least the amount of shares of each class of stock of the target corporation that would otherwise be required to approve a merger for the target corporation, and the other stockholders receive the same consideration for their stock in the merger as was payable in the tender offer, then the acquirer can effect a merger without the action of the other stockholders of the target corporation. Accordingly, if we consummate the Offer, we intend to effect the closing of the Merger without a vote of Five Prime’s stockholders in accordance with Section 251(h) of the DGCL.

Plans for Five Prime. After completion of the Offer and the Merger, Five Prime will be a wholly owned subsidiary of Amgen. At the Effective Time, the directors of Purchaser immediately prior to the Effective Time and the officers of Purchaser immediately prior to the Effective Time will be the directors and officers, respectively, of the Surviving Corporation pursuant to the Merger Agreement, each to hold office until their successors are duly elected and qualified, or until their earlier death, resignation or removal. Further, at the Effective Time, the Surviving Corporation’s certificate of incorporation as in effect immediately prior to the Effective Time will be amended and restated in its entirety to conform to the form attached as Exhibit A to the Merger Agreement, and the Surviving Corporation’s bylaws will be amended so as to read in their entirety as the bylaws of Purchaser as in effect immediately prior to the Effective Time, except that all references to Purchaser’s name therein will be replaced by references to “Five Prime Therapeutics, Inc.”

Except as disclosed in this Offer to Purchase, we do not have any present plan or proposal that would result in the acquisition by any person of additional securities of Five Prime, the disposition of securities of Five Prime, an extraordinary corporate transaction, such as a merger, reorganization or liquidation, involving Five Prime, or the sale or transfer of a material amount of assets of Five Prime. Based on available information, we are conducting a detailed review of Five Prime and its assets, corporate structure, dividend policy, capitalization, indebtedness, operations, properties, policies, management and personnel, obligations to report under Section 15(d) of the Exchange Act and the delisting of its securities from a registered national securities exchange, and will consider

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what, if any, changes would be desirable in light of the circumstances which exist upon completion of the Offer. We will continue to evaluate the business and operations of Five Prime during the pendency of the Offer and after the consummation of the Offer and will take such actions as we deem appropriate under the circumstances then existing. Thereafter, we intend to review such information as part of a comprehensive review of Five Prime’s business, operations, capitalization and management with a view to optimizing development of Five Prime’s potential. Possible changes could include changes in Five Prime’s business, corporate structure, charter, bylaws, capitalization, board of directors, management, business development opportunities, indebtedness or dividend policy, and although, except as disclosed in this Offer to Purchase, we have no current plans with respect to any of such matters, Amgen, Purchaser and the Surviving Corporation expressly reserve the right to make any changes they deem appropriate in light of such evaluation and review or in light of future developments.

As of the date of this Offer to Purchase, no member of Five Prime’s current management has (a) reached an understanding on potential employment or other retention terms with the Surviving Corporation or with Amgen or Purchaser; or (b) entered into any definitive agreements or arrangements regarding employment or other retention with, or the right to participate in the equity of, the Surviving Corporation or Amgen. Moreover, as of the date of this Offer to Purchase, no discussions have been held between members of Five Prime’s current management and Amgen, Purchaser or their affiliates with respect to any such agreement, arrangement or understanding.

THE MERGER AGREEMENT

The following summary description of the Merger Agreement does not purport to be complete and is qualified in its entirety by reference to the Merger Agreement, a copy of which Purchaser has included as Exhibit (d)(1) to the Schedule TO and is incorporated herein by reference. The summary description has been included in this Offer to Purchase to provide you with information regarding the terms of the Merger Agreement and is not intended to modify or supplement any factual disclosures about Amgen, Purchaser, Five Prime or their respective affiliates. The representations, warranties and covenants contained in the Merger Agreement were made only for the purposes of the Merger Agreement, were made as of specific dates, were made solely for the benefit of the parties to the Merger Agreement and may not have been intended to be statements of fact, but rather, as a method of allocating risk and governing the contractual rights and relationships among the parties to the Merger Agreement. In addition, such representations, warranties and covenants may have been qualified by certain disclosures not reflected in the text of the Merger Agreement and may apply standards of materiality and other qualifications and limitations in a way that is different from what may be viewed as material by Amgen, Purchaser, Five Prime or Five Prime’s stockholders. In reviewing the representations, warranties and covenants contained in the Merger Agreement or any descriptions thereof in this summary, it is important to bear in mind that such representations, warranties, covenants or descriptions were not intended by the parties to the Merger Agreement to be characterizations of the actual state of facts or conditions of Amgen, Purchaser, Five Prime or their respective affiliates. Moreover, information concerning the subject matter of the representations and warranties may have changed or may change after the date of the Merger Agreement, which subsequent information may or may not be fully reflected in public disclosures. For the foregoing reasons, the representations, warranties, covenants or descriptions of those provisions should not be read alone and should instead be read in conjunction with the other information contained in the reports, statements and filings that Amgen, its affiliates and Five Prime publicly file.

The Offer

Upon the terms and subject to the conditions set forth in the Merger Agreement, Purchaser has agreed to commence a cash tender offer as promptly as practicable after the date of the Merger Agreement but in no event later than ten (10) business days after the date of the Merger Agreement (i.e., March 18, 2021) to acquire all of the outstanding Shares at a purchase price of $38.00 per Share, minus any applicable withholding taxes and without interest. Purchaser’s obligation to accept for payment and pay for Shares validly tendered and not validly

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withdrawn pursuant to the Offer is subject to the satisfaction of the Minimum Condition and the satisfaction or waiver of the other conditions set forth in Section 15—“Conditions of the Offer.” Purchaser expressly reserves the right to (a) increase the Offer Price, (b) waive any of the conditions to the Offer and (c) make any other changes in the terms and conditions of the Offer not inconsistent with the terms of the Merger Agreement, except that, without the prior written consent of Five Prime, it will not:

Extensions of the Offer

If, as of the scheduled Expiration Date, any condition of the Offer is not satisfied and has not been waived, then to permit such condition to be satisfied: (a) Purchaser may, in its discretion (and without the consent of Five Prime or any other person), extend the Offer on one or more occasions, for an additional period of up to ten (10) business days per extension, and (b) at the request of Five Prime, Purchaser will be required to extend the Offer on one or more occasions, for an additional period specified by Five Prime of up to ten (10) business days per extension. The Merger Agreement further requires that Purchaser will extend the Offer from time to time for (i) any period required by any law or any interpretation or position of the SEC or the staff thereof or Nasdaq, in each case, as applicable to the Offer or Merger, and (ii) periods of up to ten (10) business days per extension, until any waiting period (and any extension thereof) applicable to the consummation of the Offer under the HSR Act will have expired or been terminated. In no event, however, will Purchaser be required to, or, without Five Prime’s prior written consent, be permitted to, extend the Offer beyond the earlier to occur of (x) the valid termination of the Merger Agreement in accordance with its terms or (y) the first business day immediately following December 4, 2021.

As soon as practicable following the Offer Acceptance Time, and subject to the satisfaction or waiver of certain conditions set forth in the Merger Agreement (other than those conditions that by their nature are to be satisfied at the closing of the Merger (the “Closing”), but subject to the satisfaction of such conditions), Purchaser will merge with and into Five Prime, and Five Prime will survive the Merger as a wholly owned subsidiary of Amgen. The date on which the Closing occurs is referred to as the “Closing Date.”

At the Effective Time, the separate corporate existence of Purchaser will cease and Five Prime will continue as the Surviving Corporation. At the Effective Time, each then issued and outstanding Share not previously purchased in the Offer (other than (a) Shares that at the Effective Time are held by Five Prime, Amgen, Purchaser or any other direct or indirect wholly owned subsidiary of Amgen and (b) Shares outstanding

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immediately prior to the Effective Time that are held by stockholders of Five Prime who are entitled to appraisal rights under the DGCL and have properly exercised and perfected, and not withdrawn or otherwise lost, such appraisal rights) will be converted into the right to receive the Offer Price, in cash, minus any applicable withholding taxes and without interest.

At the Effective Time, (a) the certificate of incorporation of the Surviving Corporation as in effect immediately prior to the Effective Time will be amended and restated in its entirety to conform to the form attached as Exhibit A to the Merger Agreement, and (b) the bylaws of the Surviving Corporation as in effect immediately prior to the Effective Time will be amended so as to read in their entirety as the bylaws of Purchaser as in effect immediately prior to the Effective Time, except that all references to Purchaser’s name therein will be replaced by references to “Five Prime Therapeutics, Inc.” The directors of Purchaser immediately prior to the Effective Time and the officers of Purchaser immediately prior to the Effective Time will be the directors and officers, respectively, of the Surviving Corporation pursuant to the Merger Agreement, each to hold office until their successors are duly elected and qualified, or until their earlier death, resignation or removal.

The Merger Agreement provides that the Merger will be effected under Section 251(h) of the DGCL and will be effected without a vote of Five Prime stockholders.

Five Prime Options and Five Prime Restricted Stock Awards

At the Effective Time, each option to purchase shares of Five Prime common stock that is outstanding and unexercised immediately prior to the Effective Time (each, a “Five Prime Option”) (whether vested or unvested) which has a per Share exercise price that is less than the Offer Price will be cancelled and converted into the right to receive an amount in cash equal to the product of (a) the total number of Shares subject to such Five Prime Option immediately prior to the Effective Time, multiplied by (b) the excess, if any, of (x) the Offer Price over (y) the exercise price payable per Share under such Five Prime Option immediately prior to the Effective Time, such amount to be paid, minus any applicable withholding taxes, in accordance with the Merger Agreement.

At the Effective Time, each Five Prime Option (whether vested or unvested) that has a per share exercise price that is equal to or more than the Offer Price at the Effective Time shall be cancelled without any consideration payable therefor whether before or after the Effective Time.

At the Offer Acceptance Time, each Five Prime restricted stock award that is outstanding immediately prior to the Offer Acceptance Time (each, a “Five Prime Restricted Stock Award”) will be deemed vested as of immediately prior to the Offer Acceptance Time and converted into Five Prime common stock with the right to receive the same consideration on the same terms and conditions as the other stockholders of Five Prime in respect of each Share subject to such Five Prime Restricted Stock Award.

Employee Stock Purchase Plan

Pursuant to the Merger Agreement, Five Prime will take all reasonable actions, including adopting any necessary resolution, to (a) terminate Five Prime’s 2013 Employee Stock Purchase Plan (the “ESPP”), as of immediately prior to the Closing Date, (b) ensure that no offering period under the ESPP will commence on or after the date of the Merger Agreement, (c) if the Closing occurs prior to the end of any offering period in existence under the ESPP as of the Closing Date, cause a new exercise date to be set under the ESPP that is ten (10) business days prior to the initial Expiration Date, for the automatic exercise of such options on such date, (d) prohibit participants in the ESPP from increasing their payroll deductions from those in effect on the date of the Merger Agreement and (e) provide that the amount of the accumulated contributions of each participant under the ESPP as of immediately prior to the Effective Time will, to the extent not used to purchase Shares in accordance with the terms and conditions of the ESPP (as amended pursuant to the Merger Agreement), be distributed in cash to such participant as promptly as practicable following the Effective Time.

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Representations and Warranties

In the Merger Agreement, Five Prime has made customary representations and warranties to Amgen and Purchaser that are subject, in some cases, to specified exceptions and qualifications contained in the Merger Agreement or the confidential disclosure letter that Five Prime delivered to Amgen and Purchaser in connection with the execution and delivery of the Merger Agreement. These representations and warranties relate to, among other things:

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In the Merger Agreement, Amgen and Purchaser have made customary representations and warranties to Five Prime that are subject, in some cases, to specified exceptions and qualifications contained in the Merger Agreement. These representations and warranties relate to, among other things:

The representations and warranties of each of Amgen, Purchaser and Five Prime will not survive consummation of the Merger.

Material Adverse Effect

Several of Five Prime’s representations and warranties, as well as certain closing conditions, contained in the Merger Agreement refer to the concept of “Material Adverse Effect.”

For purposes of the Merger Agreement, a “Material Adverse Effect” means any change, effect, circumstance, fact, event or occurrence (each, an “Effect”) that, individually or in the aggregate, has had, or would reasonably be expected to have, a material adverse effect on the business, assets, financial condition or results of operations of Five Prime; provided, that none of the following will be deemed in and of themselves, either alone or in combination, to constitute, and none of the following will be taken into account in determining whether there is, or would reasonably likely to be, a Material Adverse Effect:

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except (a) in the case of the first four bullets above, if and only to the extent that such matter has a materially disproportionate Effect on Five Prime relative to other companies in the industries in which Five Prime operates that are of a similar size to Five Prime (in which case such Effect will be taken into account only to the extent of such materially disproportionate Effect on Five Prime), and (b) in the case of the sixth and seventh bullets above, the underlying causes of any such Effect may be considered in determining whether a Material Adverse Effect occurred to the extent not otherwise excluded by another exception summarized above.

Several of the representations and warranties of Amgen or Purchaser contained in the Merger Agreement refer to the concept of “Amgen Material Adverse Effect.” For purposes of the Merger Agreement, an “Amgen Material Adverse Effect” means any Effect that would, individually or in the aggregate, prevent, materially delay or materially impair the ability of Amgen or Purchaser to consummate the Transactions.

Operating Covenants

Pursuant to the Merger Agreement, from the date of the Merger Agreement until the earlier of the Offer Acceptance Time and the termination of the Merger Agreement pursuant to its terms (the “Pre-Closing Period”), except as (a) required or otherwise contemplated under the Merger Agreement, (b) required by applicable law, (c) required to be taken, or omitted to be taken, pursuant to the COVID-19 Measures, (d) undertaken with the prior written consent of Amgen (which consent will not be unreasonably withheld, conditioned or delayed), or (e) as set forth in the confidential disclosure letter that Five Prime delivered to Amgen and Purchaser in connection with the execution of the Merger Agreement, Five Prime will use its commercially reasonable efforts to (x) conduct in all material respects its business and operations in the ordinary course and (y) preserve intact the material components of its current business organization, including by maintaining its relations and goodwill with all material suppliers, material customers, Governmental Bodies and other material business relations.

In addition, during the Pre-Closing Period, except as (a) required or contemplated under the Merger Agreement, (b) required by applicable laws, (c) required to be taken, or omitted to be taken, pursuant to the COVID-19

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Measures, (d) undertaken with the prior written consent of Amgen (which consent will not be unreasonably withheld, conditioned or delayed), or (e) as set forth in the confidential disclosure letter that Five Prime delivered to Amgen and Purchaser in connection with the execution of the Merger Agreement, Five Prime will not, subject to certain exceptions:

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No Solicitation

Pursuant to the Merger Agreement, during the Pre-Closing Period, Five Prime will not, and will not authorize its directors, officers, employees, attorneys, accountants, investment bankers, consultants, agents, financial advisors, other advisors and other representatives (“Representatives”) to, and will direct them not to, directly or indirectly:

provided, that the Five Prime Board will be permitted to take, or omit to take, any of the actions contemplated by the fourth bullet above in the event that Five Prime determines in good faith, after consultation with its outside legal counsel, that the failure to do so would reasonably be expected to constitute a breach of the fiduciary duties of the Five Prime Board under applicable law.

Five Prime agreed that it and its directors, officers and employees would, and that Five Prime would direct its other Representatives to, within one (1) business day of the date of the Merger Agreement:

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Notwithstanding anything to the contrary in the Merger Agreement, if prior to the Offer Acceptance Time, Five Prime or any of its Representatives receives an unsolicited bona fide written Acquisition Proposal, which was made on or after the date of the Merger Agreement and did not result from any material breach of the non-solicitation provisions in the Merger Agreement, and the Five Prime Board determines in good faith, after consultation with its financial advisors and outside legal counsel, that such Acquisition Proposal constitutes or could reasonably be expected to lead to a Superior Offer (as defined below), then Five Prime and its Representatives may (a) furnish information (including non-public information) with respect to Five Prime pursuant to an Acceptable Confidentiality Agreement (as defined below) to the person(s) who has made such Acquisition Proposal, so long as Five Prime promptly provides to Amgen any non-public information concerning Five Prime that is provided to any person given such access that was not previously provided to Amgen or its Representatives and (b) engage in or otherwise participate in discussions or negotiations with the person(s) making such Acquisition Proposal.

Five Prime will promptly (and in any event within 24 hours after receipt) (a) notify Amgen of any inquiry, proposal or offer received by Five Prime or any of its Representatives that it believes is or may lead to an Acquisition Proposal and provide to Amgen a summary of the material terms and conditions of such Acquisition Proposal, including the identity of the person making such Acquisition Proposal, and (b) provide to Amgen a complete copy of any written proposal, written offer or other written material that constitutes an Acquisition Proposal (or an amendment thereto), including copies of any proposed Specified Agreement (as defined below). Five Prime will keep Amgen reasonably informed of any material developments, discussions or negotiations regarding any Acquisition Proposal on a reasonably prompt basis, and, upon the written request of Amgen, reasonably inform Amgen of the status of any Acquisition Proposal.

Nothing in the Merger Agreement prevents Five Prime from: (a) taking and disclosing to its stockholders a position contemplated by Rule 14e-2(a), Rule 14d-9 or Item 1012(a) of Regulation M-A promulgated under the Exchange Act; (b) making any “stop, look and listen” communication pursuant to Rule 14d-9(f) promulgated under the Exchange Act; or (c) making any legally required disclosure to the stockholders of Five Prime (provided, that such disclosure includes an express reaffirmation of the Five Prime Board Recommendation).

“Acceptable Confidentiality Agreement” means any agreement with Five Prime that is either (a) in effect as of the execution and delivery of the Merger Agreement or (b) executed, delivered and effective after the execution and delivery of the Merger Agreement, in either case containing provisions that require any counterparty thereto (and any of its affiliates and Representatives) that receives material non-public information of, or with respect to, Five Prime to keep such information confidential; provided, however, that, in any case, (x) the provisions contained therein are no less favorable in the aggregate to Five Prime than the terms of the Non-Disclosure Agreement (as described in Section 14—“The Merger Agreement; Other Agreements—Non-Disclosure Agreement”) (it being agreed that such agreement need not contain any “standstill” or similar provisions that prohibit the making of any Acquisition Proposal) and (y) such agreement does not contain any provision that prohibits Five Prime from satisfying its obligations hereunder.

“Acquisition Proposal” means any indication of interest, inquiry, proposal or offer from any person (other than Amgen and its affiliates) or “group,” within the meaning of Section 13(d) of the Exchange Act, relating to, in a single transaction or series of related transactions, any (a) acquisition or license of assets of Five Prime equal to 20% or more of Five Prime’s consolidated assets or to which 20% or more of Five Prime’s revenues or earnings on a consolidated basis are attributable, (b) indirect or direct sale, lease, license, transfer, exchange or other disposition in respect of Five Prime’s FGFR2b antibody product candidate known as bemarituzumab or any intellectual property rights embodied therein, (c) issuance or acquisition of 20% or more of the outstanding Shares or total voting power of Five Prime, (d) recapitalization, tender offer or exchange offer that if consummated would result in any person or group beneficially owning 20% or more of the outstanding Shares or total voting power of Five Prime, or (e) merger, consolidation, amalgamation, share exchange, business combination, recapitalization, liquidation, dissolution or similar transaction involving Five Prime that if consummated would result in any person or group beneficially owning 20% or more of the outstanding Shares or

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total voting power of Five Prime or of the surviving entity or the resulting direct or indirect parent of Five Prime or such surviving entity, in each case other than the Transactions.

“Specified Agreement” means any contract with respect to any Acquisition Proposal (other than an Acceptable Confidentiality Agreement).

“Superior Offer” means a bona fide written Acquisition Proposal on terms that Five Prime Board (or a committee thereof) has determined in good faith, after consultation with its financial advisor and outside legal counsel, (a) is reasonably likely to be consummated in accordance with its terms and (b) would, if consummated, be more favorable, from a financial point of view, to the stockholders of Five Prime (in their capacity as such) than the Transactions (taking into account any legal, regulatory, timing, financing and other aspects of such Acquisition Proposal and any revisions to the Merger Agreement made or proposed in writing by Amgen prior to the time of such determination); provided, that for purposes of the definition of “Superior Offer,” the references to “20%” in the definition of Acquisition Proposal will be deemed to be references to “80%.”

Five Prime Board Recommendation

Five Prime has represented to Amgen and Purchaser in the Merger Agreement that the Five Prime Board adopted resolutions:

Under the Merger Agreement, neither the Five Prime Board nor any committee thereof will:

Notwithstanding the foregoing or anything to the contrary in the Merger Agreement, at any time prior to the Offer Acceptance Time, if Five Prime has received a bona fide written Acquisition Proposal (which was made after the date of the Merger Agreement and did not arise out of a material breach of the non-solicitation provisions of the Merger Agreement summarized above) that has not been withdrawn and constitutes a Superior Offer, (a) the Five Prime Board may make an Adverse Change Recommendation, or (b) Five Prime may terminate the Merger Agreement to enter into a Specified Agreement with respect to such Superior Offer, if and only if:

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Other than in connection with an Acquisition Proposal, the Five Prime Board may make an Adverse Change Recommendation in response to a Change in Circumstance (as defined below), if and only if:

“Change in Circumstance” means any material event or development or material change in circumstances with respect to Five Prime occurring or arising after the date of the Merger Agreement that was (a) not known or reasonably foreseeable to the Five Prime Board as of the date of the Merger Agreement and (b) does not relate to (i) any Acquisition Proposal, (ii) any events, changes or circumstances that are the result of factors generally affecting the industries in which Five Prime operates, the geographic markets in which they operate or where their products or services are sold that have not had or would not reasonably be expected to have a disproportionate effect on Five Prime, (iii) any events, changes or circumstances relating to Amgen, Purchaser or any of their affiliates or (iv) changes in the market price of the Five Prime common stock or the fact that Five

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Prime meets or exceeds any internal or analysts’ expectations or projections (provided that, with respect to this clause (iv), the underlying causes of any such events, changes or circumstances may be considered in determining whether a Change in Circumstance occurred to the extent not otherwise excluded by another exception in this definition).

Efforts to Close the Merger

Each of the parties to the Merger Agreement will use their respective commercially reasonable efforts to take, or cause to be taken, all actions, to file, or cause to be filed, all documents and to do, or cause to be done, and to assist and cooperate with the other parties in doing, all things necessary, proper or advisable under applicable antitrust laws to consummate and make effective the Transactions as soon as reasonably practicable, including:

Notwithstanding the foregoing, under the Merger Agreement, in connection with obtaining any such waivers or consents, Five Prime will not agree to (a) make any payment of a consent fee, “profit sharing” payment or other consideration or concede anything of monetary or economic value or (b) amend, supplement or modify such contract in any manner that would be adverse to the interest of Five Prime or, after the Merger, Purchaser or Amgen, in each case, without the prior written consent of Amgen. Further, Five Prime will only be required to take or commit to take any such action, or agree to any such condition or restriction, if such action, commitment, agreement, condition or restriction is binding on Five Prime only in the event the Closing occurs.

Each of the parties to the Merger Agreement will promptly take, and will cause their affiliates to take, all actions and steps requested or required by any Governmental Body as a condition to granting any consent, permit, authorization, waiver, clearance or approval, and to cause the prompt expiration or termination of any applicable waiting period and to resolve objections, if any, of any Governmental Body for which consents, permits, authorizations, waivers, clearances, approvals and expirations or terminations of waiting periods are sought with respect to the Transactions, so as to obtain such consents, permits, authorizations, waivers, clearances, approvals or termination of the waiting period under the HSR Act or other antitrust laws, and to avoid the commencement of a lawsuit by any Governmental Body under antitrust laws, and to avoid the entry of, or to effect the dissolution of, any order in any legal proceeding that would otherwise have the effect of preventing the Closing or materially delaying the Offer Acceptance Time or the Closing or delaying the Offer Acceptance Time beyond the Expiration Date. However, notwithstanding anything to the contrary in the Merger Agreement, including the “commercially reasonable efforts” requirement described above, in no event will Amgen or Purchaser be required to (a) negotiate, commit to or effect, by consent decree, hold separate order or otherwise, the sale, lease, license, divestiture or disposition of any assets, rights, product lines, or businesses of Five Prime, Amgen or any of their respective subsidiaries, (b) terminate existing relationships, contractual rights or obligations of Five Prime, Amgen or any of their respective subsidiaries, (c) terminate any venture or other arrangement, (d) create any relationship, contractual rights or obligations of Five Prime, Amgen or any of their respective subsidiaries, (e) effectuate any other change or restructuring of Five Prime, Amgen or any of their respective subsidiaries and (f) otherwise take or commit to take any actions with respect to the businesses, product lines or assets of Five Prime, Amgen or any of their respective subsidiaries; provided, further, that Five Prime will only be permitted to take or commit to take any such action, or agree to any such condition or restriction with the prior written consent of Amgen.

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Each of the parties to the Merger Agreement will (and will cause their respective affiliates, if applicable, to): (a) promptly, but in no event later than ten (10) business days after the date of the Merger Agreement unless otherwise agreed to in writing by Amgen and Five Prime, make an appropriate filing of all Notification and Report forms as required by the HSR Act with respect to the Transactions and (b) cooperate with each other in determining whether, and promptly preparing and making, any other filings or notifications or other consents are required to be made with, or obtained from, any other Governmental Bodies in connection with the Transactions.

Access to Information

Other than as prohibited by law and subject to certain conditions and exceptions, Five Prime and its directors, employees and officers will, and will direct its other Representatives to provide Amgen and Amgen’s Representatives, at Amgen’s sole expense, reasonable access during normal business hours to Five Prime’s officers, employees, other personnel, assets and books and records, and to furnish to Amgen all financial, operating and other data and information as Amgen may reasonably request, in each case, so long as it is not detrimental, in the reasonable judgment of Five Prime, to its business or operations.

Employee Matters

Upon the Effective Time, Amgen will assume and honor all severance and employment agreements for all Continuing Employees (as defined below), in each case in accordance with their terms; provided that, nothing in the Merger Agreement will prevent Amgen, the Surviving Corporation or any of their affiliates from amending or terminating any of their benefit plans or, after the Effective Time, any Five Prime employee benefit plan in accordance with their terms or prevent Amgen, the Surviving Corporation or any of their affiliates, after the Effective Time, from terminating the employment of any Continuing Employee.

For a period of one (1) year following the Effective Time, Amgen will provide, or cause to be provided, to each employee of Five Prime who is employed by Five Prime as of immediately prior to the Effective Time and who continues to be employed by the Surviving Corporation (or any affiliate thereof) during such one (1)-year period (each, a “Continuing Employee”) with base salary or base wages and short-term cash incentive compensation opportunities (including, but not limited to, bonuses and commission opportunities) that are no less favorable than those provided to such Continuing Employee immediately prior to the execution of the Merger Agreement, and benefits (including severance benefits and other employee benefits but excluding equity and long-term incentive compensation) that are, in the aggregate and at a minimum, substantially equivalent to the benefits (including severance benefits and other employee benefits) provided to such Continuing Employee immediately prior to the execution of the Merger Agreement.

Each Continuing Employee will be given service credit for all purposes, including for eligibility to participate, benefit levels (including, for the avoidance of doubt, levels of benefits under Amgen’s or the Surviving Corporation’s vacation policy) and eligibility for vesting under Amgen’s or the Surviving Corporation’s employee benefit plans and arrangements with respect to such Continuing Employee’s length of service with Five Prime (and its predecessors) prior to the Closing Date, provided, that the foregoing will not result in the duplication of benefits or apply to any defined benefit pension plan. With respect to any accrued but unused personal, sick or vacation time to which any Continuing Employee is entitled pursuant to the personal, sick or vacation policies applicable to such Continuing Employee immediately prior to the Effective Time, Amgen will, or will cause the Surviving Corporation to and instruct its affiliates to, as applicable (and without duplication of benefits), assume the liability for such accrued personal, sick or vacation time. To the extent that service is relevant for eligibility, vesting or allowances (including paid time off) under any health or welfare benefit plan of Amgen or the Surviving Corporation, then Amgen will use commercially reasonable efforts to (a) cause the waiver of all limitations as to pre-existing conditions, exclusions and waiting periods with respect to participation and coverage requirements applicable to the Continuing Employees, to the extent that such conditions, exclusions and waiting periods would not apply under a similar employee benefit plan in which such employees participated prior to the Effective Time and (b) ensure that such health or welfare benefit plan will, for purposes of eligibility,

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vesting, deductibles, copayments and out-of-pocket maximums and allowances (including paid time off), credit Continuing Employees for service and amounts paid prior to the Effective Time with Five Prime to the same extent that such service and amounts paid were recognized prior to the Effective Time under the corresponding health or welfare benefit plan of Five Prime. For the avoidance of doubt, Amgen will use commercially reasonable efforts to cause any eligible expenses incurred by a Continuing Employee and his or her covered dependents during the portion of the plan year immediately before the Effective Time to be taken into account for purposes of satisfying all deductible, coinsurance and maximum out-of-pocket requirements applicable to such Continuing Employee and his or her covered dependents for the applicable plan year as if such amounts had been paid in accordance with the applicable health or welfare benefit plan of Amgen or the Surviving Corporation.

Prior to the Effective Time, if requested by Amgen in writing, Five Prime will cause its 401(k) plan to be terminated effective immediately prior to the Effective Time and provide Amgen with evidence of such termination (the form and substance of which to be subject to review and approval by Amgen) not later than the day immediately preceding the Effective Time. If the Five Prime 401(k) is terminated in connection with the transactions contemplated by the Merger Agreement, all Five Prime matching contributions will fully vest. In the event that the Five Prime 401(k) plan is terminated based on such a request from Amgen, Five Prime and Amgen will use commercially reasonable efforts to permit Continuing Employees to make rollover contribution of their 401(k) account balances from Five Prime’s 401(k) plan to Amgen’s 401(k) plan.

Other than with respect to confidential communications to or by the Five Prime Board or in connection with an Adverse Change Recommendation, prior to making any broad-based or any written communications to the directors, officers or employees of Five Prime (other than any communications consistent in all material respects with prior communications made by Five Prime or Amgen) pertaining to compensation or benefit matters that are affected by the Transactions, Five Prime will, to the extent not prohibited by applicable law, (a) provide Amgen with a copy of the intended communication, (b) give Amgen a reasonable period of time to review and comment on the communication and (c) consider any such comments in good faith.

Director and Officer Indemnification and Insurance

The Merger Agreement provides that all rights to indemnification, advancement of expenses and exculpation by Five Prime existing in favor of its current and former directors and officers as of the date of the Merger Agreement (the “Indemnified Persons”) for their acts and omissions occurring prior to the Effective Time, as provided in the certificate of incorporation and bylaws of Five Prime (as in effect as of the date of the Merger Agreement) and as provided in the indemnification agreements between Five Prime and such Indemnified Persons in the forms made available by Five Prime to Amgen or Amgen’s Representatives prior to the date of the Merger Agreement, will survive the Merger and will not be amended, repealed or otherwise modified in any manner that would adversely affect the rights thereunder of such Indemnified Persons, and will be observed by Amgen, the Surviving Corporation and their successors and assigns to the fullest extent available under the laws of Delaware for a period of six (6) years from the Effective Time.

From and after the Effective Time until its sixth (6^th) anniversary, Amgen and the Surviving Corporation (together with their successors and assigns, the “Indemnifying Parties”) will, to the fullest extent permitted under applicable laws and the certificate of incorporation and bylaws of Five Prime (as in effect as of the date of the Merger Agreement), indemnify and hold harmless each Indemnified Person in their capacity as an officer or director of Five Prime against all losses, claims, damages, liabilities, fees, expenses, judgments or fines incurred by such Indemnified Person in connection with any pending or threatened legal proceeding based on or arising out of, in whole or in part, the fact that such Indemnified Person is or was a director or officer of Five Prime at or prior to the Effective Time and pertaining to any and all matters pending, existing or occurring at or prior to the Effective Time, whether asserted or claimed prior to, at or after the Effective Time, including the Transactions.

Without limiting the foregoing, from the Effective Time until its sixth (6^th) anniversary, the Indemnifying Parties will also, to the fullest extent permitted under applicable laws and the certificate of incorporation and bylaws of

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Five Prime (as in effect as of the date of the Merger Agreement), advance reasonable and documented out-of-pocket costs and expenses (including reasonable and documented attorneys’ fees) incurred by the Indemnified Persons in connection with matters for which such Indemnified Persons are eligible to be indemnified pursuant to the Merger Agreement, subject to the execution by such Indemnified Persons of appropriate undertakings to repay such advanced costs and expenses if it is ultimately determined in a final and non-appealable judgment of a court of competent jurisdiction that such Indemnified Person is not entitled to be indemnified.

The Merger Agreement requires that either (a) the Surviving Corporation maintain in effect, from the Effective Time until its sixth (6^th) anniversary, the current policy of directors’ and officers’ liability insurance maintained by Five Prime as of the date of the Merger Agreement for the benefit of the Indemnified Persons who are currently covered by such existing policy with respect to their acts and omissions occurring prior to the Effective Time in their capacities as directors and officers of Five Prime (as applicable), on terms with respect to coverage, deductibles and amounts no less favorable than the existing policy, or (b) at or prior to the Effective Time, Amgen or Five Prime (through a nationally recognized insurance broker approved by Amgen (such approval not to be unreasonably withheld, conditioned or delayed)) purchase a six (6)-year “tail” policy for the existing policy effective as of the Effective Time. However, in no event will the Surviving Corporation be required to expend in any one year an amount in excess of 300% of the annual premium currently payable by Five Prime with respect to its current policy, it being understood that if the annual premiums payable for such insurance coverage exceed such amount, Amgen shall be obligated to cause the Surviving Corporation to obtain a policy with the greatest coverage available for a cost equal to such amount.

Conditions of the Offer

See Section 15—“Conditions of the Offer.”

Conditions to the Merger

The obligation of each party to consummate the Merger is subject to the satisfaction of the following conditions:

Termination

The Merger Agreement may be terminated, and the Offer and the Merger may be abandoned:

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In the event of the termination of the Merger Agreement in accordance with its terms, the Merger Agreement will become of no further force or effect (except for certain customary obligations and certain provisions related to termination and the payment obligations described under the sections below entitled “—Termination Fee,” and “—Fees and Expenses”), without any liability on the part of Amgen, Purchaser or Five Prime or their respective directors, officers and affiliates following such termination, except that the termination of the Merger Agreement will not relieve any party from any claim, liability or damages arising out of its willful breach of the Merger Agreement.

Termination Fee

Five Prime will pay, or cause to be paid, to Amgen a termination fee of $76,000,000 in cash (the “Termination Fee”) in the event that:

In no event will Five Prime be required to pay the Termination Fee on more than one occasion.

Payment of the Termination Fee will constitute the sole and exclusive remedy of Amgen, Purchaser or any of their respective former, current or future officers, directors, partners, stockholders, optionholders, managers, members, affiliates or representatives against Five Prime and any of its former, current or future officers, directors, partners, stockholders, optionholders, managers, members, affiliates or representatives (collectively, the “Five Prime Related Parties”) in any circumstance in which the Termination Fee becomes due and payable, and upon payment of the Termination Fee, none of the Five Prime Related Parties will have any further liability or obligation relating to, arising out of or in connection with the Merger Agreement or the Transactions. Amgen or Purchaser may seek specific performance to cause Five Prime to consummate the Transactions in accordance with the Merger Agreement or the payment of the Termination Fee, but in no event will either party be entitled to both such equitable relief and the payment of the Termination Fee.

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Rule 14d-10(d) Matters

Prior to the consummation of the Offer, to the extent permitted by applicable laws, the compensation committee of the Five Prime Board will approve as an “employment compensation, severance or other employee benefit arrangement” within the meaning of Rule 14d-10(d)(2) under the Exchange Act, each agreement, arrangement or understanding between Five Prime or its affiliates and any officer, director or employee of Five Prime pursuant to which compensation is paid to such officer, director or employee, and will take all other actions reasonably necessary to satisfy the requirements of the non-exclusive safe harbor set forth in Rule 14d-10(d) of the Exchange Act.

Section 16 Matters

Prior to the consummation of the Offer, Five Prime and the Five Prime Board will take appropriate action, to the extent necessary, to approve the disposition or cancellation of Shares and Five Prime’s stock awards held by each individual who is subject to the reporting requirements of Section 16(a) of the Exchange Act with respect to Five Prime, and to cause such dispositions or cancellations to be exempt under Rule 16b-3 under the Exchange Act.

Fees and Expenses

Subject to certain exceptions and subject to the provisions described in “—Termination Fee,” all fees and expenses incurred in connection with the Merger Agreement, the Offer, the Merger and the other Transactions will be paid by the party incurring such fees or expenses, whether or not the Offer or the Merger is consummated.

Amendment; Waiver

Prior to the Offer Acceptance Time and subject to the indemnification and insurance provisions of the Merger Agreement, the Merger Agreement may be amended only with the approval of the Five Prime Board and the board of directors of Amgen (or a duly authorized committee thereof). The Merger Agreement may not be amended except by an instrument in writing signed on behalf of each of the parties to the Merger Agreement.

Any failure or delay in exercising any power, right, privilege or remedy under the Merger Agreement will not operate as a waiver of such power, right, privilege or remedy, and no single or partial exercise thereof will preclude any other or further exercise thereof or the exercise of any other power, right, privilege or remedy. No party to the Merger Agreement will be deemed to have waived any claim arising out of the Merger Agreement, or any power, right, privilege or remedy under the Merger Agreement, unless the waiver is expressly set forth in a written instrument duly executed and delivered on behalf of such party.

Governing Law and Venue

The Merger Agreement is governed by Delaware law. The venue for any action or proceeding relating to the Merger Agreement or any of the Transactions is the Court of Chancery of the State of Delaware and any state appellate court therefrom or, to the extent that such courts do not have subject matter jurisdiction, the United States District Court sitting in New Castle County in the State of Delaware.

Specific Performance

The parties to the Merger Agreement acknowledged and agreed that irreparable damage for which monetary damages, even if available, would not be an adequate remedy, would occur in the event that the parties do not perform their obligations under the provisions of the Merger Agreement in accordance with its specified terms or otherwise breach such provisions, and that none of the parties would have entered into the Merger Agreement without the right of specific performance. Each party will be entitled to an injunction, specific performance or other equitable relief to prevent breaches of the Merger Agreement and to enforce specifically the terms and provisions thereof in the courts described above under “—Governing Law and Venue” without proof of damages or otherwise.

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Waiver of Jury Trial

Each of the parties to the Merger Agreement irrevocably and unconditionally waived any right it may have to a trial by jury in any litigation arising out of, relating to or in connection with the Merger Agreement.

OTHER AGREEMENTS

Confidentiality Disclosure Agreement

On July 29, 2019, Five Prime and Amgen entered into a confidentiality disclosure agreement (as the same may be amended, supplemented or otherwise modified from time to time, the “Confidentiality Disclosure Agreement”) with respect to the sharing of confidential information in connection with Amgen and Five Prime’s potential collaboration on the research and development of several Five Prime pipeline programs. Amgen and Five Prime amended the Confidentiality Disclosure Agreement, effective as of August 2, 2019 (the “CDA Amendment No. 1”), to expand its scope to cover the Bema program. Amgen and Five Prime subsequently amended the Confidentiality Disclosure Agreement, effective as of June 16, 2020 (the “CDA Amendment No. 2”), to extend the term from one year to two years after the initial date of the Confidentiality Disclosure Agreement and expand its scope to cover another Five Prime pipeline program.

This summary does not purport to be complete and is qualified in its entirety by reference to the full text of the Confidentiality Disclosure Agreement, CDA Amendment No. 1 and CDA Amendment No. 2, copies of which have been filed as Exhibits (d)(2), (d)(3) and (d)(4) to the Schedule TO and which are incorporated herein by reference.

Non-Disclosure Agreement

On February 12, 2021, Five Prime and Amgen entered into a letter agreement (as the same may be amended, supplemented or otherwise modified from time to time, the “Non-Disclosure Agreement”), pursuant to which, among other things, Amgen agreed, subject to certain exceptions, to keep confidential certain non-public information disclosed to it or its representatives by Five Prime for a period of three (3) years. Amgen also agreed to abide by a standstill provision expiring on May 12, 2022, which included the right for Amgen to confidentially make proposals to the chief executive officer of Five Prime or the Five Prime Board during the standstill period. The Non-Disclosure Agreement included a fall-away of such standstill provisions upon (a) entry by Five Prime into a definitive agreement pursuant to which a person or group would acquire more than 40% of Five Prime’s issued and outstanding capital stock or more than 40% of Five Prime’s consolidated assets, (b) the commencement of an exchange offer for more than 40% of the issued and outstanding capital stock of Five Prime or (c) the consummation of any business combination, restructuring, recapitalization, tender or exchange offer or other extraordinary transaction with respect to more than 40% of Five Prime’s issued and outstanding capital stock or assets. The Non-Disclosure Agreement also included a fifteen (15)-month non-solicitation provision.

This summary does not purport to be complete and is qualified in its entirety by reference to the full text of the Non-Disclosure Agreement, a copy of which has been filed as Exhibit (d)(5) to the Schedule TO and which is incorporated herein by reference.

Notwithstanding any other term of the Offer or the Merger Agreement, Purchaser will not be required to accept for payment or, subject to any applicable rules and regulations of the SEC, including Rule 14e-1(c) under the Exchange Act, pay for, and may delay the acceptance for payment of or, subject to such rules and regulations, the payment for, any tendered Shares, and to the extent permitted by the Merger Agreement, terminate the Offer (a) upon the termination of the Merger Agreement and (b) at any scheduled Offer Expiration Time (subject to

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any extensions of the Offer pursuant to the Merger Agreement) or amend the Offer if (i) the Minimum Condition has not been satisfied as of one minute following 11:59 p.m., New York time, on such Expiration Date or (ii) any of the additional conditions to the Offer set forth below has not been satisfied or waived (to the extent waivable) in writing by Amgen:

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The foregoing conditions will be in addition to, and not a limitation of, the rights and obligations of Amgen and Purchaser to extend, terminate and/or modify the Offer in accordance with the terms and conditions of the Merger Agreement or applicable law. The foregoing conditions are for the sole benefit of Amgen and Purchaser and, subject to the terms and conditions of the Merger Agreement and applicable law, may be waived by Amgen and Purchaser in whole or in part at any time and from time to time in their sole discretion.

General. Except as described in this Section 16, none of Five Prime, Purchaser or Amgen is aware of any license or regulatory permit that appears to be material to the business of Five Prime that might be adversely affected by Purchaser’s acquisition of the Shares in the Offer, or of any approval or other action by a domestic or foreign governmental, administrative or regulatory agency or authority (other than under the HSR Act) that would be required for the acquisition and ownership of the Shares by Purchaser in the Offer. Should any such approval or other action be required, we presently intend to seek such approval or other action, except as described below under “Business Combination Statutes.” Except as otherwise described in this Offer to Purchase, although Purchaser does not presently intend to delay the acceptance for payment of, or payment for, Shares tendered in the Offer pending the outcome of any such matter, there can be no assurance that any such approval or other action, if needed, would be obtained or would be obtained without substantial conditions or that the failure to obtain any such approval or other action might not result in consequences adverse to Five Prime’s business or that certain parts of Five Prime’s business might not have to be disposed of or other substantial conditions complied with in the event that such approvals were not obtained or such other actions were not taken in order to obtain any such approval or other action. If certain types of adverse action are taken with respect to the matters discussed below, Purchaser could decline to accept for payment or pay for any Shares tendered. See Section 15—“Conditions of the Offer.”

Litigation. Except as described in this Section 16, neither Purchaser nor Amgen is aware of any pending legal proceeding relating to the Transactions, but legal proceedings arising out of or relating to the Transactions may be filed in the future.

Antitrust Matters. Under the HSR Act, and the rules and regulations promulgated thereunder, certain transactions may not be consummated until certain information and documentary materials have been furnished to the Antitrust Division of the U.S. Department of Justice (“DOJ”) and the Federal Trade Commission (“FTC”), and the applicable HSR Act waiting period has expired or been terminated. The requirements of the HSR Act apply to the acquisition of Shares in the Offer.

Under the HSR Act, the purchase of shares in a cash tender offer may not be completed until the expiration of a fifteen (15)-day waiting period following the filing by the acquiring person of a Premerger Notification and Report Form with the DOJ and the FTC, but this period may be (a) shortened if the reviewing agency grants “early termination” of the waiting period (provided, however, that the FTC announced on February 4, 2021 that the FTC and DOJ will not grant early termination during the transition to the new Biden Administration while the FTC is reviewing the processes and procedures used to grant early termination, stating that the FTC anticipated that the suspension will be temporary and brief, but that suspension continues), (b) restarted if the acquiring person voluntarily withdraws and refiles to allow an additional fifteen (15)-day waiting period, or (c) continued if the reviewing agency issues a request for additional information and documentary material (a “Second Request”). The parties agreed in the Merger Agreement to file such Premerger Notification and Report Forms under the HSR Act with the DOJ and the FTC in connection with the purchase of the Shares in the Offer promptly, and no later than ten (10) business days from the date of the Merger Agreement unless otherwise agreed to in writing between the parties. If a Second Request is issued, the waiting period with respect to the Offer would be continued until ten (10) days following the date of Amgen’s compliance with that request, unless earlier terminated. Thereafter, the reviewing agency may take action to preliminarily or permanently enjoin the

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transaction or the parties may agree not to consummate the transaction for some period of time. If either the fifteen (15)-day initial waiting period or the ten (10)-day post-substantial compliance waiting period expires on a Saturday, Sunday or federal holiday, then the period is extended until 11:59 p.m. Eastern Time on the next day that is not a Saturday, Sunday or federal holiday.

The DOJ and the FTC may scrutinize the legality under the antitrust laws of Amgen’s proposed acquisition of Shares pursuant to the Offer. At any time before or after Amgen’s acceptance for payment of Shares pursuant to the Offer, if the DOJ or the FTC concludes that Amgen’s acquisition of Five Prime would violate antitrust law by substantially lessening competition in any line of commerce affecting U.S. consumers, the DOJ and the FTC have the authority to challenge the acquisition by seeking a federal court order and/or administrative order enjoining the acquisition or, if Shares have already been acquired, requiring disposition of such Shares, or the divestiture of assets of Amgen, Five Prime, or any of their respective subsidiaries or affiliates, or requiring other relief. United States state attorneys general and private persons may also bring legal action under U.S. antitrust law seeking similar relief or conditions to the completion of the Offer. Competition authorities in other countries may also investigate the acquisition, seek to block it, or seek relief after it is consummated, under foreign competition law. While Amgen and Purchaser believe that consummation of the Offer and the Merger would not violate any antitrust or competition laws, there can be no assurance that a challenge to the Offer or the Merger on antitrust or competition law grounds will not be made or, if a challenge is made, what the result will be.

See Section 15—“Conditions of the Offer” for certain conditions of the Offer, including conditions with respect to litigation and certain government actions.

As used in this Offer to Purchase, “antitrust laws” means the Sherman Act, as amended, the Clayton Act, as amended, the HSR Act, the Federal Trade Commission Act, as amended, state antitrust laws and all other applicable laws and regulations (including non-U.S. laws and regulations) issued by a government authority that are designed or intended to preserve or protect competition, prohibit and restrict agreements in restraint of trade or monopolization, attempted monopolization, restraints of trade and abuse of a dominant position, or to prevent acquisitions, mergers or other business combinations and similar transactions, the effect of which may be to lessen or impede competition or to tend to create or strengthen a dominant position or to create a monopoly.

Business Combination Statutes. Five Prime is incorporated under the laws of the State of Delaware and is subject to the provisions of Section 203 of the DGCL (the “Business Combination Provisions”), which imposes certain restrictions upon business combinations involving Five Prime. These restrictions will not be applicable to Purchaser or Amgen because the Five Prime Board has approved the Offer, the Merger, the Merger Agreement and the other transactions contemplated thereby, including for purposes of Section 203. The following description is not complete and is qualified in its entirety by reference to the provisions of the Business Combination Provisions. In general, the Business Combination Provisions prevent a Delaware corporation from engaging in a “business combination” (which is defined to include a variety of transactions, including mergers) with an “interested stockholder” for a period of three (3) years following the time such person became an interested stockholder unless:

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For purposes of the Business Combination Provisions, the term “interested stockholder” generally means any person (other than the corporation and any direct or indirect majority-owned subsidiary of the corporation) that (i) is the owner of 15% or more of the outstanding voting stock of the corporation or (ii) is an affiliate or associate of the corporation and was the owner of 15% or more of the outstanding voting stock of the corporation at any time within the three (3)-year period immediately prior to the date on which it is sought to be determined whether such person is an interested stockholder, and the affiliates and associates of such person.

Upon consummation of the Offer, Amgen and Purchaser could collectively be deemed to be an “interested stockholder” for purposes of the Business Combination Provisions and, absent the prior approval of the Five Prime Board, the Business Combination Provisions could prohibit consummation of the Merger for a period of three (3) years following consummation of the Offer. However, the Five Prime Board has approved the execution of the Merger Agreement, the Offer, the Merger and the transactions contemplated thereby. Accordingly, Amgen and Purchaser do not believe that the Business Combination Provisions, or any similar business combination laws or regulations of any other state, will be an impediment to the consummation of the Offer or the Merger.

A number of states have adopted laws and regulations that purport to apply to attempts to acquire corporations that are incorporated in such states, or whose business operations have substantial economic effects in such states, or which have substantial assets, security holders, employees, principal executive offices or principal places of business in such states. We are not aware of any other state takeover laws or regulations that are applicable to the Offer or the Merger and have not attempted to comply with any state takeover statutes in connection with the Offer or the Merger, other than the Business Combination Provisions. We reserve the right to challenge the validity or applicability of any state law or regulation allegedly applicable to the Offer or the Merger, and nothing in this Offer to Purchase or any action that we take in connection with the Offer is intended as a waiver of that right. In the event that it is asserted that one or more takeover or business combination statutes applies to the Offer or the Merger, and it is not determined by an appropriate court that the statutes in question do not apply or are invalid as applied to the Offer or the Merger, as applicable, we may be required to file certain documents with, or receive approvals from, the relevant state authorities, and if such a governmental authority sought or obtained an injunction seeking to prevent our purchase of Shares in the Offer, we might be unable to accept for payment or purchase Shares tendered in the Offer or be delayed in completing the Offer. In that case, we may not be obligated to accept for purchase, or pay for, any Shares tendered. See Section 15— “Conditions to the Offer.”

Stockholder Approval Not Required. Section 251(h) of the DGCL provides that stockholder approval of a merger is not required if certain requirements are met, including that (i) the acquiring company consummates a tender offer for all of the outstanding stock of the company to be acquired that, absent Section 251(h) of the DGCL, would be entitled to vote on the Merger Agreement and (ii) immediately following the consummation of such tender offer, the stock irrevocably accepted for purchase pursuant to the offer, together with the stock otherwise owned by the acquiring company or its affiliates and any “rollover stock” (as defined in Section 251(h) of the DGCL), equals at least such percentage of the stock of the company to be acquired that, absent Section 251(h) of the DGCL, would be required to adopt the Merger Agreement. If the Minimum Condition is satisfied and we accept Shares for payment pursuant to the Offer, we will have received a sufficient number of Shares to ensure that Five Prime will not be required to submit the adoption of the Merger Agreement to a vote of the stockholders of Five Prime. Following the consummation of the Offer and subject to the satisfaction of the remaining conditions set forth in the Merger Agreement, Amgen, Purchaser and Five Prime will effect the Merger as soon as practicable, without a vote of stockholders of Five Prime in accordance with Section 251(h) of the DGCL.

Going Private Transactions. The SEC has adopted Rule 13e-3 under the Exchange Act, which is applicable to certain “going private” transactions and which may under certain circumstances be applicable to the Merger or other business combination following the purchase of Shares pursuant to the Offer in which Purchaser seeks to acquire the remaining Shares not then held by it. Purchaser believes that Rule 13e-3 will not be applicable to the Merger because it is anticipated that the Merger will be effected within one year following completion of the Offer and, in the Merger, stockholders will receive the same price per Share as paid in the Offer. Rule 13e-3 would otherwise require, among other things, that certain financial information concerning Five Prime and certain information relating to the fairness of the proposed transaction and the consideration offered to minority stockholders be filed with the SEC and disclosed to stockholders before completion of a transaction.

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Appraisal Rights. Holders of the Shares do not have appraisal rights in connection with the Offer. However, if the Merger is consummated, holders of the Shares at the Effective Time will have certain rights under the provisions of Section 262 of the DGCL, including the right to demand appraisal of, and to receive payment in cash of the fair value of, their Shares. Five Prime stockholders who (a) did not tender their Shares in the Offer; (b) duly demand appraisal pursuant to Section 262 of the DGCL; and (c) do not effectively withdraw or whose appraisal rights are not otherwise waived or lost pursuant to Section 262 of the DGCL, will be entitled to receive a judicial determination of the fair value of their Shares (excluding any appreciation or depreciation in anticipation of the Merger) and to receive payment of such fair value in cash, together with a statutory rate of interest thereon, unless the court in its discretion determines otherwise for good cause shown. Any such judicial determination of the “fair value” of the Shares could be based upon factors other than, or in addition to, the price per Share to be paid in the Offer or the market value of the Shares. Holders of Shares should recognize that the value so determined could be more or less than, or the same as, the price per Share to be paid in the Merger. Moreover, we may argue in an appraisal proceeding that, for purposes of such proceeding, the fair value of such Shares is less than such amount.

Under Section 262 of the DGCL, where a merger is approved under Section 251(h), either a constituent corporation before the effective date of the merger, or the surviving corporation within ten (10) days thereafter, must notify each of the holders of any class or series of stock of such constituent corporation who are entitled to appraisal rights of the approval of the merger or consolidation and that appraisal rights are available for any or all shares of such class or series of stock of such constituent corporation, and must include in such notice a copy of Section 262 of the DGCL. The Schedule 14D-9 will constitute the formal notice of appraisal rights under Section 262 of the DGCL.

As will be described more fully in the Schedule 14D-9, if a stockholder elects to exercise appraisal rights under Section 262 of the DGCL, such stockholder must do all of the following:

The foregoing summary of the rights of stockholders seeking appraisal rights under the DGCL does not purport to be a complete statement of the procedures to be followed by stockholders desiring to exercise any appraisal rights available under the DGCL and is qualified in its entirety by reference to Section 262 of the DGCL. The preservation and exercise of appraisal rights require strict adherence to the applicable provisions of the DGCL. If a stockholder withdraws or loses his right to appraisal, such holder’s Shares will be automatically converted in the Merger into, and represent only the right to receive, the price per Share to be paid in the Merger, in cash, without interest. A copy of Section 262 of the DGCL is included as Annex B to the Schedule 14D-9.

The information provided above is for informational purposes only with respect to your alternatives if the Merger is consummated. If you tender your Shares pursuant to the Offer, you will not be entitled to exercise appraisal rights with respect to your Shares but, instead, subject to the satisfaction or waiver of the conditions of the Offer and Purchaser’s acceptance for purchase of Shares, you will receive the Offer Price for your Shares.

Except as set forth below, we will not pay any fees or commissions to any broker, dealer or other person for soliciting tenders of Shares in the Offer.

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Purchaser and Amgen have retained Goldman Sachs to act as our financial advisors in connection with the Transactions. Goldman Sachs will receive fees for its services in connection with the Transactions. Amgen and Purchaser have also agreed to reimburse Goldman Sachs for certain reasonable out-of-pocket expenses and to indemnify Goldman Sachs against certain liabilities in connection with its services, including certain liabilities under the federal securities laws.

Purchaser and Amgen have retained American Stock Transfer & Trust Company, LLC to act as the Depositary and paying agent in connection with the Offer. The Depositary and paying agent will receive reasonable and customary compensation for its services. Purchaser and Amgen have also agreed to reimburse the Depositary and paying agent for certain reasonable out-of-pocket expenses and to indemnify such firm against certain liabilities in connection with its services, including certain liabilities under the federal securities laws.

Purchaser and Amgen have retained D.F. King & Co., Inc. to act as the Information Agent in connection with the Offer. The Information Agent will receive reasonable and customary compensation for its services. Purchaser and Amgen have also agreed to reimburse the Information Agent for certain reasonable out-of-pocket expenses and to indemnify the Information Agent against certain liabilities in connection with its services, including certain liabilities under the federal securities laws. In connection with its engagement, the Information Agent may contact holders of Shares by personal interview, mail, electronic mail, telephone, telex or other methods of electronic communication and may request that brokers, dealers, commercial banks, trust companies and other nominees forward the Offer materials to beneficial holders of Shares.

Except as set forth above, Purchaser will not pay any fees or commissions to any broker or dealer or other person for making solicitations or recommendations in connection with the Offer. Purchaser will, upon request, reimburse brokers, dealers, commercial banks, trust companies and other nominees for customary mailing and handling expenses incurred by them in forwarding material to their customers.

Purchaser is not aware of any jurisdiction in which the making of the Offer or the tender of Shares in connection therewith would not be in compliance with the laws of such jurisdiction. If Purchaser becomes aware of any jurisdiction in which the making of the Offer would not be in compliance with applicable law, Purchaser will make a good faith effort to comply with any such law. If, after such good faith effort, Purchaser cannot comply with any such law, the Offer will not be made to (nor will tenders be accepted from or on behalf of) the holders of Shares residing in such jurisdiction. In any jurisdiction where the securities, blue sky or other laws require the Offer to be made by a licensed broker or dealer, the Offer will be deemed to be made on behalf of Purchaser by one or more registered brokers or dealers licensed under the laws of such jurisdiction to be designated by Purchaser.

No person has been authorized to give any information or to make any representation on our behalf not contained in this Offer to Purchase, the Letter of Transmittal or the Notice of Guaranteed Delivery and, if given or made, such information or representation must not be relied upon as having been authorized.

Amgen filed with the SEC a Tender Offer Statement on Schedule TO pursuant to Rule 14d-3 under the Exchange Act, together with the exhibits thereto, furnishing certain additional information with respect to the Offer, and may file amendments thereto. In addition, Five Prime has filed a Solicitation/Recommendation Statement on Schedule 14D-9 pursuant to Rule 14d-9 under the Exchange Act, together with exhibits thereto, setting forth its recommendation and furnishing certain additional related information. Such Schedules and any amendments thereto, including exhibits, may be examined and copies may be obtained in the manner set forth in Section 9—“Certain Information Concerning Five Prime” and Section 10—“Certain Information Concerning Amgen and Purchaser.”

Franklin Acquisition Sub, Inc.

March 18, 2021

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SCHEDULE I—DIRECTORS AND EXECUTIVE OFFICERS OF AMGEN AND PURCHASER

Directors and Executive Officers of Amgen. The following table sets forth the name, present principal occupation or employment and material occupations, positions, offices or employment during at least the past five (5) years for each of Amgen’s directors and executive officers. The current business address of each person identified below is One Amgen Center Drive, Thousand Oaks, California 91320-1799, and the current business phone number of each such person is (805) 447-1000, and unless otherwise stated herein, each individual listed below is a citizen of the United States.

During the past five (5) years, to the best of our knowledge, none of the persons listed below have been convicted in a criminal proceeding (excluding traffic violations or similar misdemeanors) or been a party to any judicial or administrative proceeding (except for matters that were dismissed without sanction or settlement) that resulted in a judgment, decree or final order enjoining any such person from future violations of, or prohibiting activities subject to, U.S. federal or state securities laws, or a finding of any violation of U.S. federal or state securities laws.

SC-1

SC-2

SC-3

SC-4

SC-5

SC-6

SC-7

SC-8

SC-9

SC-10

SC-11

Directors and Executive Officers of Purchaser. Purchaser was formed by Amgen on March 3, 2021 solely for the purpose of engaging in the Transactions, and has not engaged in any business activities other than as incidental to its formation and in connection with the Transactions, including the Offer and the Merger. Purchaser is a wholly owned subsidiary of Amgen. Purchaser’s principal executive office is located at: One Amgen Center Drive, Thousand Oaks, California 91320. The telephone number at that office is (805) 447-1000.

The directors and executive officers of Purchaser are as follows:

Certain information regarding Dr. Reese and Mr. Griffith is set forth above.

SC-12

The Letter of Transmittal, certificates for Shares and any other required documents should be sent or delivered by each stockholder of Five Prime or his or her broker, dealer, commercial bank, trust company or other nominee to the Depositary, at one of the addresses set forth below.

The Depositary for the Offer is:

Deliver by mail, hand, express mail, courier

or other expedited service to:

American Stock Transfer & Trust Company, LLC Operations Center

Attn: Reorganization Department

6201 15th Avenue

Brooklyn, New York 11219

Questions and requests for assistance or additional copies of this Offer to Purchase, the Letter of Transmittal, the Notice of Guaranteed Delivery and the IRS Form W-9 may be directed to the Information Agent at the location and telephone numbers set forth below. Stockholders may also contact their broker, dealer, commercial bank or trust company for assistance concerning the Offer.

The Information Agent for the Offer is:

D.F. King & Co., Inc.

48 Wall Street, 22nd Floor

New York, New York 10005

Stockholders may call toll-free:

(800) 848-3410

Banks and Brokers may call collect:

(212) 269-5550

Email: fiveprime@dfking.com

Exhibit (a)(1)(B)

Letter of Transmittal to Tender Shares of Common Stock

of

FIVE PRIME THERAPEUTICS, INC.

at $38.00 Per Share in Cash Pursuant to the Offer to Purchase dated March 18, 2021 by

FRANKLIN ACQUISITION SUB, INC., a wholly owned subsidiary of AMGEN INC.

The undersigned represents that the undersigned has full authority to tender without restriction the certificate(s) listed below. American Stock Transfer & Trust Company is hereby authorized and instructed to deliver to the address indicated below (unless otherwise instructed in the boxes in the following page) a check representing a cash payment for shares of common stock, par value $0.001 per share (the “Shares”), of Five Prime Therapeutics, Inc. (“Five Prime”) tendered pursuant to this Letter of Transmittal, at a price of $38.00 per share, in cash, minus any applicable withholding taxes and without interest, upon the terms and subject to the conditions set forth in the Offer to Purchase, dated March 18, 2021 (together with any amendments, supplements or other modifications thereto, the “Offer to Purchase” and, together with this Letter of Transmittal, together with any amendments, supplements or other modifications hereto, the “Offer”).

Method of delivery of the certificate(s) is at the option and risk of the owner thereof. See Instruction 2.

Mail or deliver this Letter of Transmittal, together with the certificate(s) representing your shares, to the following address whether by mail, hand, express mail, courier, or other expedited service:

American Stock Transfer & Trust Company, LLC

Operations Center

Attn: Reorganization Department

6201 15th Avenue

Brooklyn, New York 11219

Pursuant to the offer of Franklin Acquisition Sub, Inc. to purchase all outstanding Shares of Five Prime, the undersigned encloses herewith and tenders the following Shares (and, in the case of certificated Shares, certificates representing such Shares):

PLEASE READ THE INSTRUCTIONS ACCOMPANYING THIS LETTER OF TRANSMITTAL CAREFULLY BEFORE COMPLETING THIS LETTER OF TRANSMITTAL.

IF YOU WOULD LIKE ADDITIONAL COPIES OF THIS LETTER OF TRANSMITTAL OR ANY OF THE OTHER OFFERING DOCUMENTS, YOU SHOULD CONTACT THE INFORMATION AGENT, D.F. KING & CO., INC., AT (800) 848-3410 IF YOU ARE A STOCKHOLDER OR (212) 269-5550 IF YOU ARE A BANKER OR BROKER.

You have received this Letter of Transmittal in connection with the offer of Franklin Acquisition Sub, Inc., a Delaware corporation (“Purchaser”) and a wholly owned subsidiary of Amgen Inc., a Delaware corporation (“Amgen”), to purchase all outstanding shares of common stock, par value $0.001 per share (the “Shares”), of Five Prime Therapeutics, Inc., a Delaware corporation (“Five Prime”), at a price of $38.00 per Share, in cash, minus any applicable withholding taxes and without interest, as described in the Offer to Purchase, dated March 18, 2021 (together with any amendments, supplements or other modifications thereto, the “Offer to Purchase” and, together with this Letter of Transmittal, together with any amendments, supplements or other modifications hereto, the “Offer”).

You should use this Letter of Transmittal to deliver to American Stock Transfer & Trust Company (the “Depositary”) Shares represented by stock certificates, or held in book-entry form on the books of Five Prime, for tender pursuant to the Offer. If you are delivering your Shares by book-entry transfer to an account maintained by the Depositary at The Depository Trust Company (“DTC”), you must use an Agent’s Message (as defined in Instruction 2 below).

If certificates for your Shares are not immediately available or you cannot deliver your certificates and all other required documents to the Depositary prior to the Expiration Date (as defined in the Offer to Purchase) or you cannot complete the book-entry transfer procedures prior to the Expiration Date, you may nevertheless tender your Shares according to the guaranteed delivery procedures set forth in Section 3 of the Offer to Purchase. See Instruction 2 below. Delivery of documents to DTC will not constitute delivery to the Depositary.

NOTE: SIGNATURES MUST BE PROVIDED BELOW.

PLEASE READ THE ACCOMPANYING INSTRUCTIONS CAREFULLY.

2

Ladies and Gentlemen:

The undersigned hereby tenders to Franklin Acquisition Sub, Inc., a Delaware corporation (“Purchaser”) and a wholly owned subsidiary of Amgen Inc., a Delaware corporation (“Amgen”), the above-described shares of common stock, par value $0.001 per share (collectively, the “Shares”), of Five Prime Therapeutics, Inc., a Delaware corporation (“Five Prime”), at a price of $38.00 per Share, in cash, minus any applicable withholding taxes and without interest, on the terms and subject to the conditions set forth in the Offer to Purchase, dated March 18, 2021, receipt of which is hereby acknowledged, and this Letter of Transmittal (together with any amendments, supplements or modifications thereto, the “Offer to Purchase” and, together with this Letter of Transmittal, together with any amendments, supplements or modifications hereto, the “Offer”). The undersigned understands that Purchaser reserves the right to transfer or assign, from time to time, in whole or in part, to one or more of its affiliates the right to purchase the Shares tendered herewith.

On the terms and subject to the conditions of the Offer (including, if the Offer is extended or amended, the terms and conditions of such extension or amendment), subject to, and effective upon, acceptance for payment and payment for the Shares validly tendered herewith, and not properly withdrawn, prior to the Expiration Date (as defined in the Offer to Purchase) in accordance with the terms of the Offer, the undersigned hereby sells, assigns and transfers to, or upon the order of, Purchaser, all right, title and interest in and to all of the Shares being tendered hereby and any and all cash dividends, distributions, rights, other Shares or other securities issued or issuable in respect of such Shares on or after the date hereof (collectively, “Distributions”). In addition, the undersigned hereby irrevocably appoints American Stock Transfer & Trust Company, LLC (the “Depositary”) the true and lawful agent and attorney-in-fact and proxy of the undersigned with respect to such Shares and any Distributions with full power of substitution (such proxies and power of attorney being deemed to be an irrevocable power coupled with an interest in the tendered Shares) to the full extent of such stockholder’s rights with respect to such Shares and any Distributions (a) to deliver certificates representing Shares (the “Share Certificates”) and any Distributions, or transfer of ownership of such Shares and any Distributions on the account books maintained by The Depository Trust Company (“DTC”), together, in either such case, with all accompanying evidence of transfer and authenticity, to or upon the order of Purchaser, (b) to present such Shares and any Distributions for transfer on the books of Five Prime and (c) to receive all benefits and otherwise exercise all rights of beneficial ownership of such Shares and any Distributions, all in accordance with the terms and subject to the conditions of the Offer.

The undersigned hereby irrevocably appoints each of the designees of Purchaser the attorneys-in-fact and proxies of the undersigned, each with full power of substitution, to the full extent of such stockholder’s rights with respect to the Shares tendered hereby which have been accepted for payment and with respect to any Distributions. The designees of Purchaser will, with respect to the Shares and any associated Distributions for which the appointment is effective, be empowered to exercise all voting and any other rights of such stockholder, as they, in their sole discretion, may deem proper at any annual, special, adjourned or postponed meeting of Five Prime’s stockholders, by written consent in lieu of any such meeting or otherwise. This proxy and power of attorney shall be irrevocable and coupled with an interest in the tendered Shares. Such appointment is effective when, and only to the extent that, Purchaser accepts the Shares tendered with this Letter of Transmittal for payment pursuant to the Offer. Upon the effectiveness of such appointment, without further action, all prior powers of attorney, proxies and consents given by the undersigned with respect to such Shares and any associated Distributions will be revoked and no subsequent powers of attorney, proxies, consents or revocations may be given (and, if given, will not be deemed effective). Purchaser reserves the right to require that, in order for Shares to be deemed validly tendered, immediately upon Purchaser’s acceptance for payment of such Shares, Purchaser must be able to exercise full voting, consent and other rights, to the extent permitted under applicable law, with respect to such Shares and any associated Distributions, including voting at any meeting of stockholders or executing a written consent concerning any matter.

The undersigned hereby represents and warrants that the undersigned has full power and authority to tender, sell, assign and transfer the Shares and any Distributions tendered hereby and, when the same are accepted for payment by Purchaser, Purchaser will acquire good, marketable and unencumbered title thereto, free and clear of all liens, restrictions, charges and encumbrances and the same will not be subject to any adverse claim. The undersigned hereby represents and warrants that the undersigned is the registered owner of the Shares, or the Share Certificate(s) have been endorsed to the undersigned in blank, or the undersigned is a participant in DTC whose name appears on a security position listing as the owner of the Shares. The undersigned will, upon request, execute and deliver any additional documents deemed by the Depositary or

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Purchaser to be necessary or desirable to complete the sale, assignment and transfer of the Shares and any Distributions tendered hereby. In addition, the undersigned shall promptly remit and transfer to the Depositary for the account of Purchaser any and all Distributions in respect of the Shares tendered hereby, accompanied by appropriate documentation of transfer and, pending such remittance or appropriate assurance thereof, Purchaser shall be entitled to all rights and privileges as owner of any such Distributions and may withhold the entire purchase price or deduct from the purchase price the amount or value thereof, as determined by Purchaser in its sole discretion.

It is understood that the undersigned will not receive payment for the Shares unless and until the Shares are accepted for payment and until the Share Certificate(s) owned by the undersigned are received by the Depositary at the address set forth above, together with such additional documents as the Depositary may require, or, in the case of Shares held in book-entry form, ownership of Shares is validly transferred on the account books maintained by DTC, and until the same are processed for payment by the Depositary.

IT IS UNDERSTOOD THAT THE METHOD OF DELIVERY OF THE SHARES, THE SHARE CERTIFICATE(S) AND ALL OTHER REQUIRED DOCUMENTS (INCLUDING DELIVERY THROUGH DTC) IS AT THE OPTION AND RISK OF THE UNDERSIGNED AND THAT THE RISK OF LOSS OF SUCH SHARES, SHARE CERTIFICATE(S) AND OTHER DOCUMENTS SHALL PASS ONLY AFTER THE SHARES OR SHARE CERTIFICATE(S) HAVE BEEN DELIVERED TO THE DEPOSITARY (INCLUDING, IN THE CASE OF A BOOK-ENTRY TRANSFER, BY BOOK-ENTRY CONFIRMATION (AS DEFINED BELOW)). IF DELIVERY IS BY MAIL, IT IS RECOMMENDED THAT ALL SUCH DOCUMENTS BE SENT BY PROPERLY INSURED REGISTERED MAIL WITH RETURN RECEIPT REQUESTED. DELIVERY WILL BE DEEMED EFFECTIVE AND RISK OF LOSS AND TITLE WILL PASS FROM THE OWNER ONLY WHEN DELIVERED TO THE DEPOSITARY. IN ALL CASES, SUFFICIENT TIME SHOULD BE ALLOWED TO ENSURE TIMELY DELIVERY.

All authority conferred or agreed to be conferred pursuant to this Letter of Transmittal shall not be affected by, and shall survive, the death or incapacity of the undersigned and any obligation of the undersigned hereunder shall be binding upon the heirs, executors, administrators, trustees in bankruptcy, personal representatives, successors and assigns of the undersigned. Except as stated in the Offer to Purchase, this tender is irrevocable.

The undersigned understands that the acceptance for payment by Purchaser of Shares tendered pursuant to one of the procedures described in Section 3 of the Offer to Purchase and in the instructions hereto will constitute a binding agreement between the undersigned and Purchaser upon the terms and subject to the conditions of the Offer.

Unless otherwise indicated herein under “Special Payment Instructions,” the check for the purchase price will be issued in the name(s) of, and/or any Share Certificates representing Shares not tendered or accepted for payment will be returned to, the registered owner(s) appearing under “Description of Shares Tendered.” Similarly, unless otherwise indicated under “Special Delivery Instructions,” the check for the purchase price will be issued to, and/or any Share Certificates representing Shares not tendered or accepted for payment (and accompanying documents, as appropriate) will be returned to, the address(es) of the registered owner(s) appearing under “Description of Shares Tendered.” In the event that both the Special Delivery Instructions and the Special Payment Instructions are completed, the check for the purchase price and/or any Share Certificates representing Shares not tendered or accepted for payment (and any accompanying documents, as appropriate) will be issued in the name of, and such check will be delivered and/or such Share Certificates (and any accompanying documents, as appropriate) will be returned to, the person or persons so indicated. Unless otherwise indicated herein in the box titled “Special Payment Instructions,” any Shares tendered hereby or by an Agent’s Message and delivered by book-entry transfer, but which are not purchased, will be credited to the account at DTC designated above. The undersigned recognizes that Purchaser has no obligation pursuant to the Special Payment Instructions to transfer any Shares from the name of the registered owner thereof if Purchaser does not accept for payment any of the Shares so tendered.

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SPECIAL PAYMENT INSTRUCTIONS

(See Instructions 1, 4, 5 and 7)

To be completed ONLY if Share Certificate(s) not tendered or not accepted for payment and/or the check for the purchase price in consideration of Shares accepted for payment are to be issued in the name of someone other than the undersigned or if Shares tendered by book-entry transfer which are not accepted for payment are to be returned by credit to an account maintained at DTC other than that designated above.

Issue:     ☐        Check and/or     ☐        Share Certificates to:

Also complete the Internal Revenue Service (“IRS”) Form W-9 included herein, or the appropriate version of IRS Form W-8, as applicable.

SPECIAL DELIVERY INSTRUCTIONS

(See Instructions 1, 4, 5 and 7)

To be completed ONLY if Share Certificate(s) not tendered or not accepted for payment and/or the check for the purchase price of Shares accepted for payment are to be sent to someone other than the undersigned or to the undersigned at an address other than that shown in the box titled “Description of Shares Tendered” above.

Deliver:     ☐        Check(s) and/or     ☐        Share Certificates to:

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IMPORTANT—SIGN HERE

(U.S. Holders Please Also Complete the Enclosed IRS Form W-9 (“IRS Form W-9”))

(Non-U.S. Holders Please Obtain and Complete Appropriate IRS Form W-8 (“IRS Form W-8”))

Dated:                  , 2021

(Must be signed by registered owner(s) exactly as name(s) appear(s) on Share Certificate(s) or on a security position listing or by person(s) authorized to become registered owner(s) by certificates and documents transmitted herewith. If signature is by trustees, executors, administrators, guardians, attorneys-in-fact, officers of corporations or others acting in a fiduciary or representative capacity, please set forth full title and see Instruction 5. For information concerning signature guarantees, see Instruction 1.)

GUARANTEE OF SIGNATURE(S)

(For use by Eligible Institutions only;

see Instructions 1 and 5)

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INSTRUCTIONS

Forming Part of the Terms and Conditions of the Offer

1. Guarantee of Signatures. Except as otherwise provided below, all signatures on this Letter of Transmittal must be guaranteed by a financial institution (including most commercial banks, savings and loan associations and brokerage houses) that is a member in good standing of a recognized Medallion Program approved by the Securities Transfer Association, Inc., including the Security Transfer Agents Medallion Program, the New York Stock Exchange Medallion Signature Program and the Stock Exchanges Medallion Program (each, an “Eligible Institution”). Signatures on this Letter of Transmittal need not be guaranteed (a) if this Letter of Transmittal is signed by the registered owner(s) (which term, for purposes of this document, includes any participant in any of DTC’s systems whose name appears on a security position listing as the owner of the Shares) of Shares tendered herewith and such registered owner has not completed the box titled “Special Payment Instructions” or the box titled “Special Delivery Instructions” on this Letter of Transmittal or (b) if such Shares are tendered for the account of an Eligible Institution. See Instruction 5.

2. Delivery of Letter of Transmittal and Certificates or Book-Entry Confirmations. This Letter of Transmittal is to be completed by stockholders if Share Certificates are to be forwarded herewith. If tenders are to be made pursuant to the procedures for tender by book-entry transfer set forth in Section 3 of the Offer to Purchase, an Agent’s Message must be utilized. A manually executed facsimile of this document may be used in lieu of the original. Share Certificates representing all physically tendered Shares, or confirmation of any book-entry transfer into the Depositary’s account at DTC of Shares tendered by book-entry transfer (“Book-Entry Confirmation”), as well as this Letter of Transmittal properly completed and duly executed with any required signature guarantees, or an Agent’s Message in the case of a book-entry transfer, and any other documents required by this Letter of Transmittal, must be received by the Depositary at its address set forth herein prior to the Expiration Date. Please do not send your Share Certificates directly to Purchaser, Amgen, or Five Prime.

Stockholders whose Share Certificates are not immediately available or who cannot deliver all other required documents to the Depositary prior to the Expiration Date or who cannot complete the procedures for book-entry transfer prior to the Expiration Date may nevertheless tender their Shares by properly completing and duly executing a Notice of Guaranteed Delivery pursuant to the guaranteed delivery procedure set forth in Section 3 of the Offer to Purchase. Pursuant to such procedure: (a) such tender must be made by or through an Eligible Institution, (b) a properly completed and duly executed Notice of Guaranteed Delivery substantially in the form provided by Purchaser with the Offer to Purchase must be received by the Depositary prior to the Expiration Date and (c) Share Certificates representing all tendered Shares, in proper form for transfer (or a Book-Entry Confirmation with respect to such Shares), this Letter of Transmittal (or facsimile thereof), properly completed and duly executed with any required signature guarantees (or, in the case of a book-entry transfer, an Agent’s Message), and all other documents required by this Letter of Transmittal, if any, must be received by the Depositary within two Nasdaq Global Select Market trading days after the date of execution of such Notice of Guaranteed Delivery.

A properly completed and duly executed Letter of Transmittal (or facsimile thereof) must accompany each such delivery of Share Certificates to the Depositary.

The term “Agent’s Message” means a message, transmitted through electronic means by DTC to, and received by, the Depositary and forming part of a Book-Entry Confirmation, which states that DTC has received an express acknowledgment from the participant in DTC tendering the Shares which are the subject of such Book-Entry Confirmation that such participant has received and agrees to be bound by the terms of this Letter of Transmittal and that Purchaser may enforce such agreement against the participant. The term “Agent’s Message” also includes any hard copy printout evidencing such message generated by a computer terminal maintained at the Depositary’s office.

THE METHOD OF DELIVERY OF THE SHARES, THIS LETTER OF TRANSMITTAL AND ALL OTHER REQUIRED DOCUMENTS, INCLUDING DELIVERY THROUGH DTC, IS AT THE ELECTION AND RISK OF THE TENDERING STOCKHOLDER. DELIVERY OF ALL SUCH DOCUMENTS WILL BE DEEMED MADE AND RISK OF LOSS OF THE SHARE CERTIFICATES SHALL PASS ONLY WHEN DELIVERED TO THE DEPOSITARY (INCLUDING, IN THE CASE OF A BOOK-ENTRY TRANSFER, BY BOOK-ENTRY CONFIRMATION). IF SUCH DELIVERY IS BY MAIL, IT IS RECOMMENDED THAT ALL SUCH DOCUMENTS BE SENT BY PROPERLY INSURED REGISTERED MAIL WITH RETURN RECEIPT REQUESTED. IN ALL CASES, SUFFICIENT TIME SHOULD BE ALLOWED TO ENSURE TIMELY DELIVERY PRIOR TO THE EXPIRATION DATE.

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No alternative, conditional or contingent tenders will be accepted and no fractional Shares will be purchased. All tendering stockholders, by execution of this Letter of Transmittal (or facsimile thereof), waive any right to receive any notice of the acceptance of their Shares for payment.

All questions as to validity, form and eligibility (including time of receipt) of the tender of any Share Certificate hereunder, including questions as to the proper completion or execution of any Letter of Transmittal, Notice of Guaranteed Delivery or other required documents and as to the proper form for transfer of any certificate of Shares, will be determined by the Purchaser, in its sole and absolute discretion, whose determination will be final and binding. Purchaser reserves the absolute right to reject any and all tenders determined by it not to be in proper or complete form, and to determine whether the acceptance for payment of, or payment for, Shares may be unlawful. Purchaser also reserves the absolute right to waive any defect or irregularity in the tender of any Shares or Share Certificate(s) of any particular stockholder, whether or not similar defects or irregularities are waived in the case of any other stockholder. A tender of Shares will not be deemed to have been validly made until all defects and irregularities have been cured or waived to the satisfaction of Purchaser.

3. Inadequate Space. If the space provided herein is inadequate, the certificate numbers and/or the number of Shares should be listed on a separate schedule attached hereto and separately signed on each page thereof in the same manner as this Letter of Transmittal is signed.

4. Partial Tenders (Applicable to Stockholders Tendering Share Certificates Only). If fewer than all the Shares evidenced by any Share Certificate delivered to the Depositary are to be tendered, fill in the number of Shares which are to be tendered in the column titled “Number of Shares Tendered” in the box titled “Description of Shares Tendered.” In such cases, new certificate(s) for the remainder of the Shares that were evidenced by the old certificate(s) but not tendered will be sent to the registered owner, unless otherwise provided in the appropriate box on this Letter of Transmittal, as soon as practicable after the Expiration Date. All Shares represented by Share Certificates delivered to the Depositary will be deemed to have been tendered unless otherwise indicated.

5. Signatures on Letter of Transmittal; Stock Powers and Endorsements. If this Letter of Transmittal is signed by the registered owner(s) of the Shares tendered hereby, the signature(s) must correspond with the name(s) as written on the face of the Share Certificate(s) without alteration or any other change whatsoever.

If any Shares tendered hereby are owned of record by two or more joint owners, all such owners must sign this Letter of Transmittal.

If any tendered Shares are registered in the names of different holder(s), it will be necessary to complete, sign and submit as many separate Letters of Transmittal (or facsimiles thereof) as there are different registrations of such Shares.

If this Letter of Transmittal or any certificates or stock powers are signed by trustees, executors, administrators, guardians, attorneys-in-fact, officers of corporations or others acting in a fiduciary or representative capacity, such persons should so indicate when signing, and proper evidence satisfactory to Purchaser of their authority so to act must be submitted.

If this Letter of Transmittal is signed by the registered owner(s) of the Shares listed and transmitted hereby, no endorsements of Share Certificates or separate stock powers are required unless payment is to be made to, or Share Certificates representing Shares not tendered or accepted for payment are to be issued in the name of, a person other than the registered owner(s), in which case the Share Certificates representing the Shares tendered by this Letter of Transmittal must be endorsed or accompanied by appropriate stock powers, in either case, signed exactly as the name(s) of the registered owner(s) or holder(s) appear(s) on the Share Certificates. Signatures on such Share Certificates or stock powers must be guaranteed by an Eligible Institution.

If this Letter of Transmittal is signed by a person other than the registered owner(s) of the Share(s) listed, the Share Certificate(s) must be endorsed or accompanied by the appropriate stock powers, in either case, signed exactly as the name or names of the registered owner(s) or holder(s) appear(s) on the Share Certificate(s). Signatures on such Share Certificates or stock powers must be guaranteed by an Eligible Institution.

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6. Transfer Taxes. Purchaser will pay any transfer taxes with respect to the transfer and sale of Shares to it or to its order pursuant to the Offer (for the avoidance of doubt, transfer taxes do not include United States federal income or backup withholding taxes). If, however, payment of the purchase price is to be made to, or (in the circumstances permitted hereby) if Share Certificates not tendered or accepted for payment are to be registered in the name of, any person other than the registered owner(s), or if tendered Share Certificates are registered in the name of any person other than the person signing this Letter of Transmittal, the amount of any transfer taxes (whether imposed on the registered owner(s) or such person) payable on account of the transfer to such person will be deducted from the purchase price unless satisfactory evidence of the payment of such taxes, or exemption therefrom, is submitted.

Except as provided in this Instruction 6, it will not be necessary for transfer tax stamps to be affixed to the Share Certificates listed in this Letter of Transmittal.

7. Special Payment and Delivery Instructions. If a check for the purchase price is to be issued, and/or Share Certificates representing Shares not tendered or accepted for payment are to be issued or returned to, a person other than the signer(s) of this Letter of Transmittal or to an address other than that shown in the box titled “Description of Shares Tendered” above, the appropriate boxes on this Letter of Transmittal should be completed. Stockholders delivering Shares tendered hereby or by Agent’s Message by book-entry transfer may request that Shares not purchased be credited to an account maintained at DTC as such stockholder may designate in the box titled “Special Payment Instructions” herein. If no such instructions are given, all such Shares not purchased will be returned by crediting the same account at DTC as the account from which such Shares were delivered.

8. Requests for Assistance or Additional Copies. Questions or requests for assistance may be directed to the Information Agent at its address and telephone numbers set forth below or to your broker, dealer, commercial bank, trust company or other nominee. Additional copies of the Offer to Purchase, this Letter of Transmittal, the Notice of Guaranteed Delivery and other tender offer materials may be obtained from the Information Agent as set forth below, and will be furnished at Purchaser’s expense.

9. Backup Withholding. Under U.S. federal income tax laws, the Depositary will be required to withhold a portion of the amount of any payments made to certain stockholders pursuant to the Offer or the Merger (as defined in the Offer to Purchase), as applicable. In order to avoid such backup withholding, each tendering stockholder or payee that is a United States person (as defined in the instruction to IRS From W-9), must provide the Depositary with such stockholder’s or payee’s correct taxpayer identification number (“TIN”) and certify that such stockholder or payee is not subject to such backup withholding by completing the attached IRS Form W-9. Certain stockholders or payees (including, among others, certain corporations, non-resident foreign individuals and foreign entities) are not subject to these backup withholding and reporting requirements. A tendering stockholder who is not a U.S. person (as defined in the instructions to IRS From W-9) should complete, sign, and submit to the Depositary the appropriate IRS Form W-8. An applicable IRS Form W-8 may be obtained from the Depositary or downloaded from the Internal Revenue Service’s website at the following address: http://www.irs.gov. Failure to complete the IRS Form W-9 or IRS Form W-8 will not, by itself, cause Shares to be deemed invalidly tendered, but may require the Depositary to withhold a portion of the amount of any payments made of the Offer Price (as defined in the Offer to Purchase) pursuant to the Offer. If the correct TIN is not provided, then the stockholder may be subject to a penalty imposed by the IRS.

NOTE: FAILURE TO COMPLETE AND RETURN THE IRS FORM W-9 OR THE APPROPRIATE IRS FORM W-8, AS APPLICABLE, MAY RESULT IN BACKUP WITHHOLDING OF A PORTION OF ANY PAYMENTS MADE TO YOU PURSUANT TO THE OFFER. PLEASE REVIEW THE “IMPORTANT TAX INFORMATION” SECTION BELOW.

10. Lost, Destroyed, Mutilated or Stolen Share Certificates. If any Share Certificate has been lost, destroyed, mutilated or stolen, the stockholder should promptly notify Five Prime’s stock transfer agent, Computershare Trust Company, N.A at (312) 588-4738 or (312) 735-5264. The stockholder will then be instructed as to the steps that must be taken in order to replace the Share Certificate. This Letter of Transmittal and related documents cannot be processed until the procedures for replacing lost, mutilated, destroyed or stolen Share Certificates have been followed.

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11. Waiver of Conditions. Purchaser expressly reserves the right, in its sole discretion, to, upon the terms and subject to the conditions of the Offer, increase the Offer Price, waive any condition of the Offer set forth in the Merger Agreement (other than the Minimum Condition, Termination Condition or No Legal Prohibition Condition (each as defined in the Offer to Purchase), which may only be waived with the prior written consent of Five Prime) or make any other changes to the terms and conditions of the Offer not inconsistent with the terms of the Merger Agreement (as defined in the Offer to Purchase).

IMPORTANT: THIS LETTER OF TRANSMITTAL (OR A MANUALLY EXECUTED FACSIMILE COPY THEREOF) OR AN AGENT’S MESSAGE, TOGETHER WITH SHARE CERTIFICATE(S) OR BOOK-ENTRY CONFIRMATION OR A PROPERLY COMPLETED AND DULY EXECUTED NOTICE OF GUARANTEED DELIVERY AND ALL OTHER REQUIRED DOCUMENTS, MUST BE RECEIVED BY THE DEPOSITARY PRIOR TO THE EXPIRATION DATE.

IMPORTANT TAX INFORMATION

Under United States federal income tax law, a stockholder that is a non-exempt United States person (as defined in the instructions to IRS Form W-9) whose tendered Shares are accepted for payment in the Offer, or whose Shares are exchanged for cash payment in the Merger, is required by law to provide the Depositary (as payer) with such stockholder’s correct TIN on IRS Form W-9 attached hereto. If such stockholder is an individual, the TIN is generally such stockholder’s social security number. If the Depositary is not provided with the correct TIN, the stockholder may be subject to penalties imposed by the IRS and payments that are made to such stockholder with respect to Shares purchased pursuant to the Offer, or purchased in the Merger, may be subject to U.S. federal backup withholding.

If backup withholding applies, the Depositary is required to withhold at a rate currently equal to 24% of any payments of the purchase price made to the stockholder. Backup withholding is not an additional tax. Rather, the tax liability of persons subject to backup withholding may be reduced by the amount of tax withheld. If withholding results in an overpayment of taxes, a refund or credit may be obtained from the IRS provided that the required information is timely furnished to the IRS.

To prevent backup withholding on payments that are made to a United States stockholder with respect to Shares purchased pursuant to the Offer or purchased in the Merger, as applicable, the stockholder is required to notify the Depositary of such stockholder’s correct TIN by completing a IRS Form W-9 certifying, under penalties of perjury, (a) that the TIN provided on the IRS Form W-9 is correct (or that such stockholder is awaiting a TIN), (b) that such stockholder is not subject to backup withholding because (i) such stockholder has not been notified by the IRS that such stockholder is subject to backup withholding as a result of a failure to report all interest or dividends, (ii) the IRS has notified such stockholder that such stockholder is no longer subject to backup withholding or (iii) such stockholder is exempt from backup withholding, and (c) that such stockholder is a U.S. person.

Certain stockholders (including, among others, certain corporations and certain foreign individuals and entities) may not be subject to backup withholding and reporting requirements. In order for an exempt stockholder who is not a U.S. person (as defined in the instructions to IRS Form W-9) to avoid backup withholding, such person should complete, sign and submit an appropriate IRS Form W-8 signed under penalties of perjury, attesting to his, her or its exempt status. IRS Forms W-8 can be obtained from the Depositary, or from the IRS website at: http://www.irs.gov. Such stockholders should consult a tax advisor to determine which version of IRS Form W-8 is appropriate. Exempt stockholders who are U.S. persons should furnish their TIN, check the “Exempt payee” box on the IRS Form W-9 and sign, date and return the IRS Form W-9 to the Depositary in order to avoid erroneous backup withholding.

What Number to Give the Depositary

Each United States stockholder is generally required to give the Depositary its social security number or employer identification number. If the tendering United States stockholder has not been issued a TIN and has applied for a number or intends to apply for a number in the near future, the stockholder should write “Applied For” in Part I, sign and date the IRS Form W-9. Please refer to IRS Form W-9 instructions and consult your tax advisor regarding the application of backup

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withholding in such case. We note that your IRS Form W-9, including your TIN, may be transferred from American Stock Transfer & Trust Company, acting in its capacity as the Depositary, to American Stock Transfer & Trust Company, acting in its capacity as the Paying Agent under the Merger Agreement, in certain circumstances.

Please consult your accountant or tax advisor for further guidance regarding the completion of IRS Form W-9 or the appropriate IRS Form W-8, as applicable, to claim exemption from backup withholding, or contact the Depositary.

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The Depositary for the Offer to Purchase is:

Deliver by mail, hand, express mail, courier,

or other expedited service to:

American Stock Transfer & Trust Company, LLC

Operations Center

Attn: Reorganization Department

6201 15th Avenue

Brooklyn, New York 11219

DELIVERY OF THIS LETTER OF TRANSMITTAL TO AN ADDRESS OTHER THAN AS SET FORTH ABOVE WILL NOT CONSTITUTE A VALID DELIVERY TO THE DEPOSITARY.

Questions and requests for assistance or additional copies of this Offer to Purchase, the Letter of Transmittal and IRS Form W-9 may be directed to the Information Agent at the address and telephone numbers set forth below. Stockholders may also contact their broker, dealer, commercial bank, trust company or other nominee for assistance concerning the Offer.

The Information Agent for the Offer is:

D.F. King & Co., Inc.

48 Wall Street, 22nd Floor

New York, New York 10005

Stockholders (toll-free): (800) 848-3410

Banks and Brokers: (212) 269-5550

Email: fiveprime@dfking.com

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Exhibit (a)(1)(C)

Notice of Guaranteed Delivery

for

Offer to Purchase for Cash

All Outstanding Shares of Common Stock

of

FIVE PRIME THERAPEUTICS, INC.

at

$38.00 Per Share

Pursuant to the Offer to

Purchase Dated March 18, 2021

by

FRANKLIN ACQUISITION SUB, INC.

a wholly owned subsidiary of

AMGEN INC.

(Not to be used for Signature Guarantees)

This Notice of Guaranteed Delivery, or a substantially equivalent form, must be used to accept the Offer (as defined below) if certificates representing shares of common stock, par value $0.001 per share (the “Shares”), of Five Prime Therapeutics, Inc., a Delaware corporation (“Five Prime”), and all other required documents cannot be delivered to American Stock Transfer & Trust Company, LLC (the “Depositary”) prior to the Offer Expiration Time (as defined in the Offer to Purchase), if the procedure for delivery by book-entry transfer cannot be completed prior to the Offer Expiration Time, or if time will not permit all required documents to reach the Depositary prior to the Offer Expiration Time. This form may be delivered by courier or transmitted by hand delivery, facsimile transmission or mail to the Depositary and must include a guarantee by an Eligible Institution (as defined in the Offer to Purchase) in the form set forth herein. See Section 3 of the Offer to Purchase for more information regarding the guaranteed delivery procedures.

The Depositary for the Offer is:

Deliver by mail, hand, express mail, courier

or other expedited service to:

American Stock Transfer & Trust Company, LLC

Operations Center

Attn: Reorganization Department

6201 15th Avenue

Brooklyn, New York 11219

By Facsimile (Eligible Institutions only):

(718) 234-5001

DELIVERY OF THIS NOTICE OF GUARANTEED DELIVERY TO AN ADDRESS OTHER THAN ONE SET FORTH ABOVE WILL NOT CONSTITUTE A VALID DELIVERY.

THIS NOTICE OF GUARANTEED DELIVERY IS NOT TO BE USED TO GUARANTEE SIGNATURES. IF A SIGNATURE ON A LETTER OF TRANSMITTAL IS REQUIRED TO BE GUARANTEED BY AN “ELIGIBLE INSTITUTION” UNDER THE INSTRUCTIONS THERETO, SUCH SIGNATURE GUARANTEE MUST APPEAR IN THE APPLICABLE SPACE PROVIDED IN THE SIGNATURE BOX ON THE APPROPRIATE LETTER OF TRANSMITTAL.

The Eligible Institution that completes this form must communicate the guarantee to the Depositary and must deliver the Letter of Transmittal or an Agent’s Message (as defined in the Offer to Purchase), certificates for Shares and/or any other required documents to the Depositary within the time period shown herein. Failure to do so could result in a financial loss to such Eligible Institution.

The Guarantee included herein must be completed.

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3

4

Exhibit (a)(1)(D)

Offer to Purchase for Cash

All Outstanding Shares of Common Stock

of

FIVE PRIME THERAPEUTICS, INC.

at

$38.00 Per Share

Pursuant to the Offer to Purchase

Dated March 18, 2021

by

FRANKLIN ACQUISITION SUB, INC.

a wholly owned subsidiary of

AMGEN INC.

March 18, 2021

To Brokers, Dealers, Banks, Trust Companies and other Nominees:

Franklin Acquisition Sub, Inc., a Delaware corporation (“Purchaser”) and a wholly owned subsidiary of Amgen Inc., a Delaware corporation (“Amgen”), and Amgen have appointed D.F. King & Co., Inc. to act as the information agent in connection with Purchaser’s offer to purchase all outstanding shares of common stock, par value $0.001 per share (the “Shares”), of Five Prime Therapeutics, Inc., a Delaware corporation (“Five Prime”), at a price of $38.00 per Share (the “Offer Price”), in cash, minus any applicable withholding taxes and without interest, upon the terms and subject to the conditions set forth in the Offer to Purchase, dated March 18, 2021 (as it may be amended, supplemented or otherwise modified from time to time, the “Offer to Purchase”) and in the related Letter of Transmittal (as it may be amended, supplemented or otherwise modified from time to time, the “Letter of Transmittal,” which, together with the Offer to Purchase, constitutes the “Offer”) enclosed herewith.

Please furnish copies of the enclosed materials to those of your clients for whom you hold Shares registered in your name or in the name of your nominee.

THE BOARD OF DIRECTORS OF FIVE PRIME UNANIMOUSLY RECOMMENDS THAT STOCKHOLDERS OF FIVE PRIME ACCEPT THE OFFER AND TENDER ALL OF THEIR SHARES TO PURCHASER PURSUANT TO THE OFFER.

The Offer is not subject to any financing condition. The conditions of the Offer are described in Section 15 of the Offer to Purchase.

For your information and for forwarding to your clients for whom you hold Shares registered in your name or in the name of your nominee, enclosed herewith are copies of the following documents:

We urge you to contact your clients as promptly as possible. Please note that the Offer and any withdrawal rights will expire at 12:00 midnight, New York time, on April 16, 2021 (one minute following 11:59 P.M., New York time, on April 15, 2021), unless the Offer is extended.

The Offer is being made pursuant to an Agreement and Plan of Merger, dated as of March 4, 2021 (the “Merger Agreement”), by and among Purchaser, Amgen and Five Prime. Pursuant to the Merger Agreement, as soon as practicable following the consummation of the Offer and on the terms and subject to the conditions set forth in the Merger Agreement, Purchaser will be merged with and into Five Prime (the “Merger”) without a vote of the stockholders of Five Prime in accordance with Section 251(h) of the Delaware General Corporation Law, as amended (the “DGCL”) and Five Prime will survive the Merger as a wholly owned subsidiary of Amgen. At the effective time of the Merger (the “Effective Time”), each then issued and outstanding Share not previously purchased in the Offer (other than (a) Shares that at the Effective Time are held by Five Prime, Amgen, Purchaser or any other direct or indirect wholly owned subsidiary of Amgen and (b) Shares outstanding immediately prior to the Effective Time that are held by stockholders of Five Prime who are entitled to appraisal rights under the DGCL and have properly exercised and perfected, and not withdrawn or otherwise lost, such appraisal rights) will be converted into the right to receive the Offer Price, in cash, minus any applicable withholding taxes and without interest.

After careful consideration, the board of directors of Five Prime has unanimously (a) determined that the Merger Agreement and the Transactions, including the Offer and the Merger, are fair to, and in the best interest of, Five Prime and its stockholders, (b) declared it advisable to enter into the Merger Agreement, (c) approved the execution, delivery and performance by Five Prime of the Merger Agreement and the consummation of the Transactions, including the Offer and the Merger, (d) resolved that the Merger will be effected under Section 251(h) of the DGCL and (e) resolved to recommend that the stockholders of Five Prime accept the Offer and tender their Shares to Purchaser pursuant to the Offer, subject to the right of the Five Prime Board to withdraw (or modify in a manner adverse to Amgen or Purchaser) or publicly propose to withdraw or withhold (or modify or qualify in a manner adverse to Amgen or Purchaser) its recommendation in accordance with the terms of the Merger Agreement. The execution and delivery of the Merger Agreement and all of the transactions contemplated by the Merger Agreement, including the Offer and the Merger, are collectively referred to as the “Transactions.”

The Offer is not subject to any financing condition. The Offer is conditioned on, among other things, there being validly tendered (and not properly withdrawn) prior to the Offer Expiration Time, a number of Shares that (together with any Shares then owned by Purchaser and its “affiliates” (as such term is defined in Section 251(h)(6)(a) of the DGCL), but excluding Shares tendered pursuant to guaranteed delivery procedures that have not yet been “received” by the “depository” (as such terms are defined in Section 251(h)(6) of the DGCL)) represents at least one Share more than 50% of the then issued and outstanding Shares. The Offer is also subject to certain other conditions set forth in the Offer to Purchase, including the

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expiration or termination of any applicable waiting period under the Hart-Scott-Rodino Antitrust Improvements Act of 1976, as amended, the absence of a legal restraint making the acquisition of or payment for Shares pursuant to the Offer, or the consummation of the Merger, illegal or otherwise prohibited, and other customary conditions as described in Section 15—“Conditions of the Offer” of the Offer to Purchase. A summary of the principal terms of the Offer appears on pages 1-11 of the Offer to Purchase.

Purchaser is not aware of any jurisdiction in which the making of the Offer or the tender of Shares in connection therewith would not be in compliance with the laws of such jurisdiction. If Purchaser becomes aware of any jurisdiction in which the making of the Offer would not be in compliance with applicable law, Purchaser will make a good faith effort to comply with any such law. If, after such good faith effort, Purchaser cannot comply with any such law, the Offer will not be made to (nor will tenders be accepted from or on behalf of) the holders of Shares residing in such jurisdiction. In any jurisdiction where applicable laws require the Offer to be made by a licensed broker or dealer, the Offer will be deemed to be made on behalf of Purchaser by one or more registered brokers or dealers licensed under the laws of such jurisdiction to be designated by Purchaser.

For purposes of the Offer, Purchaser will be deemed to have accepted for payment, and thereby purchased, Shares properly tendered to Purchaser and not properly withdrawn as, if and when Purchaser gives notice to the Depositary of Purchaser’s acceptance for payment of the Shares in the Offer. In all cases, payment for Shares accepted for payment in the Offer will be made only after timely receipt by the Depositary of (a) the certificates evidencing such Shares or, if the Shares are held via book entry at The Depository Trust Company (the “Book-Entry Transfer Facility”), confirmation of a book-entry transfer of such Shares into the Depositary’s account at the Book-Entry Transfer Facility pursuant to the procedures set forth in Section 3—“Procedure for Tendering Shares” of the Offer to Purchase, (b) a Letter of Transmittal, properly completely and duly executed (or a manually executed facsimile thereof), with any required signature guarantees or, in the case of a book-entry transfer at the Book-Entry Transfer Facility, an Agent’s Message in lieu of the Letter of Transmittal, and (c) any other documents required by the Letter of Transmittal. Accordingly, tendering stockholders may be paid at different times depending upon when Share certificates, Letters of Transmittal or book-entry confirmations with respect to Shares are actually received by the Depositary.

UNDER NO CIRCUMSTANCES WILL INTEREST BE PAID ON THE OFFER PRICE FOR THE SHARES, REGARDLESS OF ANY EXTENSION OF THE OFFER OR ANY DELAY IN MAKING PAYMENT.

To tender certificated Shares, the certificates representing such Shares, together with a properly completed and duly executed Letter of Transmittal (or manually executed facsimile thereof) and any required signature guarantees, must be timely received by the Depositary prior to the Offer Expiration Time. If Shares are held in street name, such Shares can be tendered by a nominee through the Depositary. To tender Shares held in book-entry form, either a properly completed and duly executed Letter of Transmittal (or manually executed facsimile thereof), together with any required signature guarantees, or an Agent’s Message (as defined in the Offer to Purchase) in lieu of such Letter of Transmittal, and any other documents required by the Letter of Transmittal, must be timely received by the Depositary prior to the Offer Expiration Time pursuant to the book-entry procedures described in Section 3—“Procedure for Tendering Shares—Book-Entry Transfer” of the Offer to Purchase.

If a stockholder desires to tender Shares in the Offer and the Share certificates and all other required documents cannot be delivered to the Depositary prior to the Offer Expiration Time, if the procedure for delivery by book-entry transfer cannot be completed prior to the Offer Expiration Time, or if time will not permit all required documents to reach the Depositary prior to the Offer Expiration Time, the stockholder’s tender may still be effected by following the guaranteed delivery procedures set forth in the Offer to Purchase and the Letter of Transmittal.

Neither Purchaser nor Amgen will pay any fees or commissions to any broker or dealer or other person (other than to D.F. King & Co., Inc. in its capacity as Information Agent and American Stock Transfer & Trust Company, LLC in its capacity as the Depositary, as described in the Offer to Purchase) for making solicitations or recommendations in connection with the Offer. Purchaser will, however, upon request, reimburse you for customary mailing and handling expenses incurred by you in forwarding the enclosed materials to your customers. Purchaser will pay all stock transfer taxes applicable to its purchase of Shares pursuant to the Offer, subject to Instruction 6 of the Letter of Transmittal.

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Questions and requests for additional copies of the enclosed materials may be directed to the Information Agent at the address and telephone numbers set forth on the back cover of the enclosed Offer to Purchase.

Very truly yours,

D.F. King & Co., Inc.

NOTHING CONTAINED HEREIN OR IN THE ENCLOSED DOCUMENTS SHALL RENDER YOU OR ANY OTHER PERSON THE AGENT OF PURCHASER, AMGEN, THE DEPOSITARY OR THE INFORMATION AGENT OR ANY AFFILIATE OF ANY OF THEM, OR AUTHORIZE YOU OR ANY OTHER PERSON TO GIVE ANY INFORMATION OR MAKE ANY REPRESENTATION ON BEHALF OF ANY OF THEM WITH RESPECT TO THE OFFER NOT CONTAINED IN THE ENCLOSED DOCUMENTS.

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Exhibit (a)(1)(E)

Offer to Purchase for Cash

All Outstanding Shares of Common Stock

of

FIVE PRIME THERAPEUTICS, INC.

at

$38.00 Per Share

Pursuant to the Offer to Purchase

Dated March 18, 2021

by

FRANKLIN ACQUISITION SUB, INC.

a wholly owned subsidiary of

AMGEN INC.

March 18, 2021

To Our Clients:

Enclosed for your consideration are the Offer to Purchase, dated March 18, 2021 (as it may be amended, supplemented or otherwise modified from time to time, the “Offer to Purchase”), and the related Letter of Transmittal (as it may be amended, supplemented or otherwise modified from time to time, the “Letter of Transmittal,” which, together with the Offer to Purchase, constitutes the “Offer”) relating to the offer by Franklin Acquisition Sub, Inc., a Delaware corporation (“Purchaser”) and a wholly owned subsidiary of Amgen Inc., a Delaware corporation (“Amgen”), to purchase all outstanding shares of common stock, par value $0.001 per share (the “Shares”), of Five Prime Therapeutics, Inc., a Delaware corporation (“Five Prime”), at a price of $38.00 per Share (the “Offer Price”), in cash, minus any applicable withholding taxes and without interest, upon the terms and subject to the conditions set forth in the Offer to Purchase and in the related Letter of Transmittal.

Also enclosed is Five Prime’s Solicitation/Recommendation Statement on Schedule 14D-9.

THE BOARD OF DIRECTORS OF FIVE PRIME UNANIMOUSLY RECOMMENDS THAT YOU ACCEPT THE OFFER AND TENDER ALL OF YOUR SHARES TO PURCHASER PURSUANT TO THE OFFER.

WE (OR OUR NOMINEES) ARE THE HOLDER OF RECORD OF SHARES HELD BY US FOR YOUR ACCOUNT. A TENDER OF SUCH SHARES CAN BE MADE ONLY BY US AS THE HOLDER OF RECORD AND PURSUANT TO YOUR INSTRUCTIONS. THE LETTER OF TRANSMITTAL IS FURNISHED TO YOU FOR YOUR INFORMATION ONLY AND CANNOT BE USED TO TENDER SHARES FOR YOUR ACCOUNT.

We request instructions as to whether you wish to tender any or all of the Shares held by us for your account according to the terms and conditions set forth in the enclosed Offer.

Your attention is directed to the following:

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If you wish to have us tender any or all of the Shares held by us for your account, please so instruct us by completing, executing, detaching and returning to us the instruction form on the detachable part hereof. An envelope to return your instructions to us is enclosed. If you instruct us to tender your Shares, all such Shares will be tendered unless otherwise specified on the detachable part hereof.

YOUR PROMPT ACTION IS REQUESTED. YOUR INSTRUCTIONS SHOULD BE FORWARDED TO US IN AMPLE TIME TO PERMIT US TO SUBMIT A TENDER ON YOUR BEHALF BEFORE THE OFFER EXPIRATION TIME.

Payment for Shares accepted for payment in the Offer will be made only after timely receipt by American Stock Transfer & Trust Company, LLC (the “Depositary”) of (a) the certificates evidencing such Shares or, if the Shares are held via book entry at The Depository Trust Company (the “Book-Entry Transfer Facility”), confirmation of a book-entry transfer of such Shares into the Depositary’s account at the Book-Entry Transfer Facility pursuant to the procedures set forth in Section 3—“Procedure for Tendering Shares” of the Offer to Purchase, (b) a Letter of Transmittal, properly completely and duly executed (or a manually executed facsimile thereof), with any required signature guarantees or, in the case of a book-entry transfer at the Book-Entry Transfer Facility, an Agent’s Message in lieu of the Letter of Transmittal, and (c) any other documents required by the Letter of Transmittal. Accordingly, tendering stockholders may be paid at different times depending upon when Share certificates, Letters of Transmittal or book-entry confirmations with respect to Shares are actually received by the Depositary.

UNDER NO CIRCUMSTANCES WILL INTEREST BE PAID ON THE OFFER PRICE FOR THE SHARES, REGARDLESS OF ANY EXTENSION OF THE OFFER OR ANY DELAY IN MAKING PAYMENT.

Purchaser is not aware of any jurisdiction in which the making of the Offer or the tender of Shares in connection therewith would not be in compliance with the laws of such jurisdiction. If Purchaser becomes aware of any jurisdiction in which the making of the Offer would not be in compliance with applicable law, Purchaser will make a good faith effort to comply with any such law. If, after such good faith effort, Purchaser cannot comply with any such law, the Offer will not be made to (nor will tenders be accepted from or on behalf of) the holders of Shares residing in such jurisdiction. In any jurisdiction where applicable laws require the Offer to be made by a licensed broker or dealer, the Offer will be deemed to be made on behalf of Purchaser by one or more registered brokers or dealers licensed under the laws of such jurisdiction to be designated by Purchaser.

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Instructions with Respect to the

Offer to Purchase for Cash

All Outstanding Shares of Common Stock

of

FIVE PRIME THERAPEUTICS, INC.

at

$38.00 Per Share

Pursuant to the Offer to Purchase

Dated March 18, 2021

by

FRANKLIN ACQUISITION SUB, INC.

a wholly owned subsidiary of

AMGEN INC.

The undersigned acknowledge(s) receipt of your letter, the Offer to Purchase, dated March 18, 2021 (as it may be amended, supplemented or otherwise modified from time to time, the “Offer to Purchase”), and the related Letter of Transmittal (as it may be amended, supplemented or otherwise modified from time to time, the “Letter of Transmittal”), in connection with the offer by Franklin Acquisition Sub, Inc., a Delaware corporation (“Purchaser”) and a wholly owned subsidiary of Amgen Inc., a Delaware corporation, to purchase all outstanding shares of common stock, par value $0.001 per share (the “Shares”), of Five Prime Therapeutics, Inc., a Delaware corporation, at a price of $38.00 per Share, in cash, minus any applicable withholding taxes and without interest, upon the terms and subject to the conditions set forth in the Offer to Purchase and the Letter of Transmittal.

The undersigned hereby instruct(s) you to tender to Purchaser the number of Shares indicated below or, if no number is indicated, all Shares held by you for the account of the undersigned, upon the terms and subject to the conditions set forth in the Offer to Purchase and the Letter of Transmittal.

The undersigned understands and acknowledges that all questions as to validity, form and eligibility of the surrender of any certificate representing Shares submitted on my behalf to American Stock Transfer & Trust Company, LLC (the “Depositary”) will be determined by the Purchaser, in its sole and absolute discretion, whose determination shall be final and binding.

The method of delivery of this document is at the election and risk of the tendering stockholder. If delivery is by mail, then registered mail with return receipt requested, properly insured, is recommended. In all cases, sufficient time should be allowed to ensure timely delivery.

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Dated:

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Exhibit (a)(1)(F)

This announcement is neither an offer to purchase nor a solicitation of an offer to sell Shares (as defined below). The Offer (as defined below) is made solely pursuant to the Offer to Purchase (as defined below), the related Letter of Transmittal, and any amendments or supplements to such Offer to Purchase or Letter of Transmittal, and is being made to all holders of Shares. The Offer is not being made to (and no tender will be accepted from or on behalf of) holders of Shares in any jurisdiction in which the making of the Offer or the acceptance thereof would not be in compliance with the securities, “blue sky” or other laws of such jurisdiction. Purchaser (as defined below) may, in its discretion, take such action as it deems necessary to make the Offer to holders of Shares in any such jurisdiction in compliance with such laws. In those jurisdictions where applicable laws require the Offer to be made by a licensed broker or dealer, the Offer will be deemed to be made on behalf of Purchaser by one or more registered brokers or dealers that are licensed under the laws of such jurisdiction to be designated by Purchaser.

Notice of Offer to Purchase for Cash

All Outstanding Shares of Common Stock of

FIVE PRIME THERAPEUTICS, INC.

at

$38.00 Per Share by

FRANKLIN ACQUISITION SUB, INC.

a wholly owned subsidiary of

AMGEN INC.

Franklin Acquisition Sub, Inc., a Delaware corporation (“Purchaser”) and a wholly owned subsidiary of Amgen Inc., a Delaware corporation (“Amgen”), is offering to purchase all the outstanding shares of common stock, par value $0.001 per share (the “Shares”), of Five Prime Therapeutics, Inc., a Delaware corporation (“Five Prime”), at a price of $38.00 per share (the “Offer Price”), in cash, minus any applicable withholding taxes and without interest, upon the terms and subject to the conditions set forth in the Offer to Purchase dated March 18, 2021 (as it may be amended, supplemented or otherwise modified from time to time, the “Offer to Purchase”) and the related Letter of Transmittal (as it may be amended, supplemented or otherwise modified from time to time, the “Letter of Transmittal,” which, together with the Offer to Purchase, constitutes the “Offer”). Tendering stockholders whose Shares are registered in their own names and who tender directly to the Depositary (as defined below) will not be obligated to pay brokerage fees or commissions or, except as set forth in the Letter of Transmittal, transfer taxes on the tender of Shares in the Offer. Purchaser will pay all fees and expenses incurred in connection with the Offer by American Stock Transfer & Trust Company, LLC, which is acting as the Depositary for the Offer (the “Depositary”), and D.F. King & Co., Inc., which is acting as the information agent for the Offer (the “Information Agent”). Stockholders with Shares held through a broker, dealer, commercial bank, trust company or other nominee should consult with their broker or other nominee to determine whether any charges will apply.