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.

(Names of Filing Persons (Offerors))

Common Stock, par value $0.001 per share

(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

(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 relates solely to preliminary communications made before the commencement of a planned tender offer by Franklin Acquisition Sub, Inc. (“Purchaser”), a Delaware corporation and wholly-owned subsidiary of Amgen Inc. (“Amgen”), a Delaware corporation, for any and all of the outstanding shares of common stock, par value $0.001 per share, of Five Prime Therapeutics, Inc. (“Five Prime”), to be commenced pursuant to the Agreement and Plan of Merger (the “Merger Agreement”), dated as of March 4, 2021, among Amgen, Purchaser and Five Prime.

Important Information

This communication is for informational purposes only and is neither an offer to purchase nor a solicitation of an offer to sell securities. The tender offer for the outstanding shares of common stock of Five Prime described in this communication has not commenced. At the time the tender offer is commenced, Amgen and Purchaser will file, or will cause to be filed, tender offer materials on Schedule TO with the U.S. Securities and Exchange Commission (the “SEC”) and Five Prime will file a Solicitation/Recommendation Statement on Schedule 14D-9 with the SEC, in each case with respect to the tender offer. THE TENDER OFFER MATERIALS (INCLUDING AN OFFER TO PURCHASE, A RELATED LETTER OF TRANSMITTAL AND OTHER OFFER DOCUMENTS) AND THE SOLICITATION/RECOMMENDATION STATEMENT WILL CONTAIN IMPORTANT INFORMATION THAT SHOULD BE READ CAREFULLY WHEN THEY BECOME AVAILABLE AND CONSIDERED BEFORE ANY DECISION IS MADE WITH RESPECT TO THE TENDER OFFER. Those materials and all other documents filed by, or caused to be filed by, Amgen and Purchaser and Five Prime with the SEC will be available at no charge on the SEC’s website at www.sec.gov. The tender offer materials and related materials also may be obtained for free (when available) under the “Investors – Financials” section of Amgen’s website at https://investors.amgen.com/financials/sec-filings, and the Solicitation/Recommendation Statement and such other documents also may be obtained for free (when available) from Five Prime under the “Investors & Media – Financial Information” section of Five Prime’s website at https://investor.fiveprime.com/index.php/sec-filings. FIVE PRIME’S SHAREHOLDERS ARE ADVISED TO READ THE TENDER OFFER MATERIALS AND THE SOLICITATION/RECOMMENDATION STATEMENT, AS EACH MAY BE AMENDED OR SUPPLEMENTED FROM TIME TO TIME, AND ANY OTHER RELEVANT DOCUMENTS FILED BY FIVE PRIME OR AMGEN WITH THE SEC WHEN THEY BECOME AVAILABLE BEFORE THEY MAKE ANY DECISION WITH RESPECT TO THE TENDER OFFER. THESE MATERIALS WILL CONTAIN IMPORTANT INFORMATION ABOUT THE TENDER OFFER, FIVE PRIME AND AMGEN.

Forward-Looking Statements

This communication contains forward-looking statements. These forward-looking statements generally include statements that are predictive in nature and depend on or refer to future events or conditions, and include words such as “expect,” “anticipate,” “outlook,” “could,” “target,” “project,” “intend,” “plan,” “believe,” “seek,” “estimate,” “should,” “may,” “assume” and “continue” as well as variations of such words and similar expressions. By their nature, forward-looking statements involve risks and uncertainty because they relate to events and depend on circumstances that will occur in the future, and there are many factors that could cause actual results and developments to differ materially from those expressed or implied by these forward-looking statements. Forward-looking statements include, among other things, statements about the potential benefits of the proposed transaction; the prospective performance and outlook of Five Prime’s business, performance and opportunities; any potential strategic benefits, synergies or opportunities expected as a result of the proposed transaction; the ability of the parties to complete the proposed transaction and the expected timing of completion of the proposed transaction; potential marketing or regulatory approvals for bemarituzumab, or potential future revenues from such product; as well as any assumptions underlying any of the foregoing.

These statements are not guarantees of future performance and they involve certain risks, uncertainties and assumptions that are difficult to predict. We caution you that actual outcomes and results may differ materially from what is expressed, implied or forecasted by our forward-looking statements. There can be no guarantee that the proposed tender offer or the transaction described in this communication will be completed, or that it will be completed as currently proposed, or at any particular time. Neither can there be any guarantee that Amgen or Five Prime’s product, bemarituzumab, will achieve any particular future financial results, or that Amgen will be able to realize any of the potential strategic benefits, synergies or opportunities as a result of the proposed acquisition. Nor can there be any guarantee that bemarituzumab will be submitted or approved for sale in any market, or at any particular time. Neither can there be any guarantee that such product will be successfully commercialized even if regulatory approvals are obtained. In particular, our expectations could be affected by, among other things: uncertainties as to the timing of the tender offer and the merger; the risk that the proposed transaction may not be completed in a timely manner or at all; uncertainties as to the percentage of Five Prime’s stockholders tendering their shares in the tender offer; the possibility that competing offers or acquisition proposals for Five Prime will be made; the possibility that any or all of the various conditions to the consummation of the tender offer or the merger may not be satisfied or waived, including the failure to receive any required regulatory approvals from any applicable governmental entities (or any conditions, limitations or restrictions placed on such approvals); regulatory actions or delays or government regulation generally, including potential regulatory actions or delays relating to the completion of the potential transaction described in this release, as well as potential regulatory actions or delays with respect to the development of bemarituzumab; the occurrence of any event, change or other circumstance that could give rise to the termination of the merger agreement; the effect of this announcement or pendency of the proposed transaction on Five Prime’s ability to retain and hire key personnel, its ability to maintain relationships with its customers, suppliers and others with whom it does business, its business generally or its stock price; risks related to diverting management’s attention from Five Prime’s ongoing business operations; the risk that stockholder litigation in connection with the proposed transaction may result in significant costs of defense, indemnification and liability; the potential that the strategic benefits, synergies or opportunities expected from the proposed acquisition may not be realized or may take longer to realize than expected; the successful integration of Five Prime into Amgen subsequent to the closing of the transaction and the timing of such integration; and other risks and factors referred to from time to time in Amgen’s and Five Prime’s filings with the SEC, including Amgen’s current Form 10-K and Five Prime’s current Form 10-K on file with the SEC, including those related to the uncertainties inherent in the research and development of new healthcare products, including clinical trial results and additional analysis of existing clinical data; our ability to obtain or maintain proprietary intellectual property protection; safety, quality or manufacturing issues; changes in expected or existing competition; and global trends toward health care cost containment, including government, payor and general public pricing and reimbursement pressures. The effects of the COVID-19 pandemic may give rise to risks that are currently unknown or amplify the risks associated with many of these factors. Amgen and Five Prime are providing the information in this communication as of this date and do not undertake any obligation to update any forward-looking statements as a result of new information, future events or otherwise.

About Amgen

Amgen is committed to unlocking the potential of biology for patients suffering from serious illnesses by discovering, developing, manufacturing and delivering innovative human therapeutics. This approach begins by using tools like advanced human genetics to unravel the complexities of disease and understand the fundamentals of human biology.

Amgen focuses on areas of high unmet medical need and leverages its expertise to strive for solutions that improve health outcomes and dramatically improve people’s lives. A biotechnology pioneer since 1980, Amgen has grown to be 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.

For more information, visit www.amgen.com and follow us on www.twitter.com/amgen.

Item 12. Exhibits.

Exhibit (a)(5)(D)

Message from David Reese, EVP Research and Development, Amgen

Hi everyone -

I am writing to you today for one reason: to convey how delighted we are by yesterday’s announcement and how deeply we respect the expertise that has brought bemarituzumab to this stage of development. We are incredibly excited about the impact bemarituzumab could have for the third most common cause of cancer death worldwide and are committed to advancing this potential therapy as quickly as possible.

We not only believe that bemarituzumab is a strong strategic fit with Amgen’s innovative oncology portfolio, but also in your mission and values we have observed. Amgen’s mission is simple — to serve patients. We strive to serve them by transforming the promise of science and biotechnology into therapies that have the power to restore health or save lives. We too tackle the tough scientific challenges to deliver truly novel, breakthrough treatments. For example, Amgen has taken on one of the toughest challenges of the last 40 years in cancer research by developing sotorasib, a KRAS^G12C inhibitor. Sotorasib was the first KRAS^G12C inhibitor to enter the clinic and is under priority review with the FDA for the treatment of KRAS^G12C mutated locally advanced or metastatic non-small cell lung cancer.

We look forward to welcoming you to the Amgen family and working with you to realize the full potential of bemarituzumab for patients around the world as quickly as possible. We will be arranging a meeting next week where we can begin the journey of getting to know one another better and attempt to address some of the questions you no doubt have. We cannot promise that we will have all the answers just yet, but you have our commitment that we will work diligently to address your questions because we view the entire Five Prime team as critical to the continued successful progression of the molecule.

I will end by reiterating the primary importance of our shared mission to bring hope and cutting-edge cancer treatments to patients in need.

I look forward to meeting many of you next week.

Regards

Dave

Important Information

This communication is for informational purposes only and is neither an offer to purchase nor a solicitation of an offer to sell securities. The tender offer for the outstanding shares of common stock of Five Prime Therapeutics, Inc. (“Five Prime”) described in this communication has not commenced. At the time the tender offer is commenced, Amgen Inc. (“Amgen”) and its acquisition subsidiary, Franklin Acquisition Sub, Inc. (“Purchaser”), will file, or will cause to be filed, tender offer materials on Schedule TO with the U.S. Securities and Exchange Commission (the “SEC”)

and Five Prime will file a Solicitation/Recommendation Statement on Schedule 14D-9 with the SEC, in each case with respect to the tender offer. THE TENDER OFFER MATERIALS (INCLUDING AN OFFER TO PURCHASE, A RELATED LETTER OF TRANSMITTAL AND OTHER OFFER DOCUMENTS) AND THE SOLICITATION/RECOMMENDATION STATEMENT WILL CONTAIN IMPORTANT INFORMATION THAT SHOULD BE READ CAREFULLY WHEN THEY BECOME AVAILABLE AND CONSIDERED BEFORE ANY DECISION IS MADE WITH RESPECT TO THE TENDER OFFER. Those materials and all other documents filed by, or caused to be filed by, Amgen and Purchaser and Five Prime with the SEC will be available at no charge on the SEC’s website at www.sec.gov. The tender offer materials and related materials also may be obtained for free (when available) under the “Investors – Financials” section of Amgen’s website at https://investors.amgen.com/financials/sec-filings, and the Solicitation/Recommendation Statement and such other documents also may be obtained for free (when available) from Five Prime under the “Investors & Media – Financial Information” section of Five Prime’s website at https://investor.fiveprime.com/index.php/sec-filings. FIVE PRIME’S SHAREHOLDERS ARE ADVISED TO READ THE TENDER OFFER MATERIALS AND THE SOLICITATION/RECOMMENDATION STATEMENT, AS EACH MAY BE AMENDED OR SUPPLEMENTED FROM TIME TO TIME, AND ANY OTHER RELEVANT DOCUMENTS FILED BY FIVE PRIME OR AMGEN WITH THE SEC WHEN THEY BECOME AVAILABLE BEFORE THEY MAKE ANY DECISION WITH RESPECT TO THE TENDER OFFER. THESE MATERIALS WILL CONTAIN IMPORTANT INFORMATION ABOUT THE TENDER OFFER, FIVE PRIME AND AMGEN.

Forward-Looking Statements

This communication contains forward-looking statements. These forward-looking statements generally include statements that are predictive in nature and depend on or refer to future events or conditions, and include words such as “expect,” “anticipate,” “outlook,” “could,” “target,” “project,” “intend,” “plan,” “believe,” “seek,” “estimate,” “should,” “may,” “assume” and “continue” as well as variations of such words and similar expressions. By their nature, forward-looking statements involve risks and uncertainty because they relate to events and depend on circumstances that will occur in the future, and there are many factors that could cause actual results and developments to differ materially from those expressed or implied by these forward-looking statements. Forward-looking statements include, among other things, statements about the potential benefits of the proposed transaction; the prospective performance and outlook of Five Prime’s business, performance and opportunities; any potential strategic benefits, synergies or opportunities expected as a result of the proposed transaction; the ability of the parties to complete the proposed transaction and the expected timing of completion of the proposed transaction; potential marketing or regulatory approvals for bemarituzumab, or potential future revenues from such product; as well as any assumptions underlying any of the foregoing.

These statements are not guarantees of future performance and they involve certain risks, uncertainties and assumptions that are difficult to predict. We caution you that actual outcomes and results may differ materially from what is expressed, implied or forecasted by our forward-looking statements. There can be no guarantee that the proposed tender offer or the transaction described in this communication will be completed, or that it will be completed as currently proposed, or at any particular time. Neither can there be any guarantee that Amgen or Five Prime’s product, bemarituzumab, will achieve any particular future financial results, or that Amgen will be able to realize any of the potential strategic benefits, synergies or opportunities as a result of the proposed acquisition. Nor can there be any guarantee that bemarituzumab will be submitted or approved for sale in any market, or at any particular time. Neither can there be any guarantee that such product will be successfully commercialized even if regulatory approvals are obtained. In particular, our expectations could be affected by, among other things: uncertainties as to the timing of the tender offer and the merger; the risk that the proposed transaction may not be completed in a timely manner or at all; uncertainties as to the percentage of Five Prime’s stockholders tendering their shares in the tender offer; the possibility that competing offers or acquisition proposals for Five Prime will be made; the possibility that any or all of the various conditions to the consummation of the tender offer or the merger may not be satisfied or waived, including the failure to receive any required regulatory approvals from any applicable governmental entities (or any conditions, limitations or restrictions placed on such approvals); regulatory actions or delays or government regulation generally, including potential regulatory actions or delays relating to the completion of the potential transaction described in this release, as well as potential regulatory actions or delays with respect to the development of bemarituzumab; the occurrence of any event, change or other circumstance that could give rise to the termination of the merger agreement; the effect of this announcement or pendency of the proposed transaction on Five Prime’s ability to retain and hire key personnel, its ability to maintain relationships with its customers, suppliers and others with whom it does business, its business generally or its stock price; risks related to diverting management’s attention from Five Prime’s ongoing business operations; the risk that stockholder litigation in connection with the proposed transaction may result in significant costs of defense, indemnification and liability; the potential that the strategic benefits, synergies or opportunities expected from the proposed acquisition may not be realized or may take longer to realize than expected; the successful integration of Five Prime into Amgen subsequent to the closing of the transaction and the timing of such integration; and other risks and factors referred to from time to time in Amgen’s and Five Prime’s filings with the SEC, including Amgen’s current Form 10-K and Five Prime’s current Form 10-K on file with the SEC, including those related to the uncertainties inherent in the research and development of new healthcare products, including clinical trial results and additional analysis of existing clinical data; our ability to obtain or maintain proprietary intellectual property protection; safety, quality or manufacturing issues; changes in expected or existing competition; and global trends toward health care cost containment, including government, payor and general public pricing and reimbursement pressures. The effects of the COVID-19 pandemic may give rise to risks that are currently unknown or amplify the risks associated with many of these factors. Amgen and Five Prime are providing the information in this communication as of this date and do not undertake any obligation to update any forward-looking statements as a result of new information, future events or otherwise.