UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

WASHINGTON, D.C. 20549

SCHEDULE 14A

Proxy Statement Pursuant to Section 14(a) of the

Securities Exchange Act of 1934

(Amendment No. 1)

Filed by the Registrant  ☒                             Filed by a Party other than the Registrant  ☐

Check the appropriate box:

Merrimack Pharmaceuticals, Inc.

(Name of Registrant as Specified in its Charter)

(Name of Person(s) Filing Proxy Statement, if Other Than the Registrant)

Payment of Filing Fee (Check the appropriate box):

REVISED PRELIMINARY PROXY MATERIALS—SUBJECT TO COMPLETION

MERRIMACK PHARMACEUTICALS, INC.

One Broadway, 14th Floor

Cambridge, MA 02142

August [    ], 2019

Dear Merrimack Pharmaceuticals, Inc. Stockholder:

You are cordially invited to our Annual Meeting of Stockholders on Thursday, October 17, 2019, beginning at 9:00 a.m., Eastern time, at the offices of Wilmer Cutler Pickering Hale and Dorr LLP, 60 State Street, Boston, Massachusetts 02109. The enclosed notice of annual meeting of stockholders sets forth the proposals that will be presented at the meeting, which are described in more detail in the enclosed proxy statement. Our board of directors recommends that you vote “FOR” each of the director nominees in Proposal 1 (Gary L. Crocker, Ulrik B. Nielsen, Ana Radeljevic and Russell T. Ray), “FOR” Proposal 2 (“say-on-pay”) and “FOR” Proposal 3 (ratification of our independent auditors).

We have received notice from JFL Partners Fund LP, a Delaware limited partnership (“JFL”), which purports to beneficially own approximately 18.7% of the Company’s common stock, expressing the intention of JFL to nominate four (4) nominees for election to our board of directors at the 2019 Annual Meeting in opposition to the four (4) director candidates our board of directors has nominated.

Our board of directors does not endorse any of JFL’s nominees and unanimously recommends that you cause your shares to be voted “FOR” each of the nominees proposed by our board of directors using only the WHITE proxy card or by following the instructions to submit a proxy for your shares over the Internet or by telephone or by voting in person at the 2019 Annual Meeting.

Our board of directors strongly urges you NOT to sign or return any proxy card or voting instruction form that the JFL Group may send to you, even as a protest vote against the JFL Group or any of JFL’s nominees. Even a “WITHHOLD” vote with respect to JFL’s nominees on its proxy card will cancel any previously submitted WHITE proxy card vote. If you do sign a proxy card sent to you by the JFL Group, however, you have the right to change your vote by using the enclosed WHITE proxy card or by submitting a proxy over the Internet or by telephone or voting in person at the 2019 Annual Meeting. Only the latest dated, valid proxy you submit will be counted.

Your vote is important. We look forward to seeing you at the 2019 Annual Meeting.

REVISED PRELIMINARY PROXY MATERIALS—SUBJECT TO COMPLETION

MERRIMACK PHARMACEUTICALS, INC.

One Broadway, 14th Floor

Cambridge, MA 02142

NOTICE OF ANNUAL MEETING OF STOCKHOLDERS

to be held on Thursday, October 17, 2019

The 2019 Annual Meeting of Stockholders (the “2019 Annual Meeting”) of Merrimack Pharmaceuticals, Inc., a Delaware corporation (“Merrimack” or the “Company”), will be held on Thursday, October 17, 2019, at 9:00 a.m., Eastern time, at the offices of Wilmer Cutler Pickering Hale and Dorr LLP, 60 State Street, Boston, Massachusetts 02109, to consider and act upon the following matters:

We have received notice from JFL Partners Fund LP, a Delaware limited partnership (“JFL”), which purports to beneficially own approximately 18.7% of the Company’s common stock, expressing the intention of JFL to nominate four (4) nominees for election to our board of directors at the 2019 Annual Meeting in opposition to the four (4) director candidates our board of directors has nominated. We do not endorse the election of any of the JFL nominees as a director. You may receive proxy solicitation materials from JFL and/or other participants in its proxy solicitation (collectively, the “JFL Group”) or other persons or entities affiliated with the JFL Group, including an opposition proxy statement and proxy card. Please be advised that we are not responsible for the accuracy of any information provided by or relating to the JFL Group contained in any proxy solicitation materials filed or disseminated by the JFL Group or any other statements that the JFL Group may otherwise make.

Our board of directors does not endorse any of JFL’s nominees and unanimously recommends that you vote “FOR” each of the nominees proposed by our board of directors using only the WHITE proxy card or by following the instructions to vote your shares over the Internet, by telephone or in person at the 2019 Annual Meeting.

Our board of directors strongly urges you NOT to sign or return any proxy card or voting instruction form that the JFL Group may send to you, even as a protest vote against the JFL Group or any of JFL’s nominees. Even a “WITHHOLD” vote with respect to JFL’s nominees on its proxy card will cancel any previously submitted WHITE proxy card vote. If you do sign a proxy card sent to you by the JFL Group, however, you have the right to change your vote by using the enclosed WHITE proxy card or by submitting a proxy over the Internet or by telephone or voting in person at the 2019 Annual Meeting. Only the latest dated, valid proxy you submit will be counted.

Your vote is important. Whether or not you expect to be present, please sign, date and return the enclosed WHITE proxy card in the pre-addressed envelope provided for that purpose as promptly as possible. No postage is required if mailed in the United States. In addition to mailing in your proxy card, you may submit a proxy over the Internet or by telephone. The instructions for submitting a proxy over the Internet or by telephone are provided on your proxy card.

Stockholders of record at the close of business on September 16, 2019 will be entitled to notice of and to vote at the 2019 Annual Meeting or any adjournment or postponement thereof. This Notice, the accompanying Proxy Statement and a form of proxy card are being mailed beginning on or about [    ], 2019 to all stockholders entitled to vote at the 2019 Annual Meeting.

If you have any questions or require any assistance with voting your shares, please contact our proxy solicitor: Morrow Sodali, LLC, toll free at (800) 662-5200.

Cambridge, Massachusetts

August [    ], 2019

YOU MAY OBTAIN ADMISSION TO THE 2019 ANNUAL MEETING BY IDENTIFYING YOURSELF AT THE 2019 ANNUAL MEETING AS A STOCKHOLDER AS OF THE RECORD DATE. IF YOU ARE A RECORD OWNER, POSSESSION OF A COPY OF A PROXY CARD WILL BE ADEQUATE IDENTIFICATION. IF YOU ARE A BENEFICIAL (BUT NOT RECORD) OWNER, A COPY OF AN ACCOUNT STATEMENT FROM YOUR BANK, BROKER OR OTHER NOMINEE SHOWING SHARES HELD FOR YOUR BENEFIT ON SEPTEMBER 16, 2019 WILL BE ADEQUATE IDENTIFICATION.

WHETHER OR NOT YOU EXPECT TO ATTEND THE 2019 ANNUAL MEETING, PLEASE COMPLETE, DATE AND SIGN THE ENCLOSED WHITE PROXY CARD AND MAIL IT PROMPTLY IN THE ENCLOSED ENVELOPE IN ORDER TO HELP ENSURE REPRESENTATION OF YOUR SHARES AT THE 2019 ANNUAL MEETING. NO POSTAGE NEED BE AFFIXED IF THE WHITE PROXY CARD IS MAILED IN THE UNITED STATES. ALTERNATIVELY, YOU MAY SUBMIT YOUR PROXY OVER THE INTERNET OR BY TELEPHONE BY FOLLOWING THE INSTRUCTIONS SET FORTH ON THE ENCLOSED WHITE PROXY CARD.

IMPORTANT

Your vote at this year’s Annual Meeting is especially important, no matter how many or how few shares you own. Please sign and date the enclosed WHITE proxy card and return it in the enclosed postage-paid envelope promptly.

THE BOARD STRONGLY URGES YOU NOT TO SIGN OR RETURN ANY PROXY CARD OR VOTING INSTRUCTION FORM THAT YOU MAY RECEIVE FROM THE JFL GROUP OR ANY PERSON OTHER THAN THE COMPANY EVEN AS A PROTEST VOTE AGAINST THE JFL GROUP OR ANY OF JFL’S NOMINEES. Any proxy card you sign and return from the JFL Group for any reason could invalidate previous WHITE proxy cards sent by you to support the Company’s board of directors.

Only your latest dated, signed proxy card or voting instruction form will be counted. Any proxy may be revoked at any time prior to its exercise at the 2019 Annual Meeting as described in this proxy statement.

IMPORTANT!

PLEASE SIGN AND SUBMIT THE WHITE PROXY CARD TODAY!

WE URGE YOU NOT TO SIGN ANY PROXY CARD OR VOTING INSTRUCTION FORM SENT TO YOU BY THE JFL GROUP.

Remember, you can submit a proxy for your shares over the Internet or by telephone. Please follow the easy instructions on the enclosed WHITE proxy card.

If you have any questions or need assistance in voting

your shares, please contact our proxy solicitor:

Table of Contents

REVISED PRELIMINARY PROXY MATERIALS—SUBJECT TO COMPLETION

MERRIMACK PHARMACEUTICALS, INC.

One Broadway, 14th Floor

Cambridge, MA 02142

PROXY STATEMENT FOR THE ANNUAL MEETING OF STOCKHOLDERS

TO BE HELD ON OCTOBER  17, 2019

Information About the Annual Meeting

This proxy statement is furnished in connection with the solicitation of proxies by the board of directors (the “board of directors” or the “board”) of Merrimack Pharmaceuticals, Inc. (“Merrimack,” the “Company,” “we” or “us”) for use at the 2019 Annual Meeting of Stockholders (the “Annual Meeting” or the “2019 Annual Meeting”) to be held on Thursday, October 17, 2019, at 9:00 a.m., Eastern time, at the offices of Wilmer Cutler Pickering Hale and Dorr LLP, 60 State Street, Boston, Massachusetts 02109, and at any adjournment or postponement thereof. On September 16, 2019, the record date for the determination of stockholders entitled to vote at the Annual Meeting, there were outstanding and entitled to vote an aggregate of [                    ] shares of our common stock, $0.01 par value per share (“common stock”). Each share of common stock entitles the record holder thereof to one vote on each of the matters to be voted on at the Annual Meeting.

This proxy statement, the enclosed WHITE proxy card and our 2018 annual report to stockholders are being mailed beginning on or about [    ], 2019 to all stockholders entitled to vote at the 2019 Annual Meeting.

Your vote is important no matter how many shares you own. Please take the time to vote. Take a moment to read the instructions below. Choose the way to vote that is easiest and most convenient for you, and cast your vote as soon as possible.

Proxy Contest at the Annual Meeting

We have received a notice from JFL Partners Fund LP, a Delaware limited partnership (“JFL”), which purports to beneficially own approximately 18.7% of our common stock, expressing its intention to nominate four (4) nominees for election as directors at the 2019 Annual Meeting in opposition to the four (4) director candidates the board has nominated. The board does not endorse the election of any of JFL’s nominees as a director. You may receive proxy solicitation materials from JFL and/or the other members of its proxy solicitation group (collectively, the “JFL Group”), including an opposition proxy statement or proxy card. Please be advised that we are not responsible for the accuracy of any information provided by or relating to the JFL Group contained in any proxy solicitation materials filed or disseminated by the JFL Group or any other statements that they may otherwise make.     

You may receive multiple mailings from the JFL Group. You will also likely receive multiple mailings from the Company prior to the date of the 2019 Annual Meeting, so that our stockholders have our latest proxy information and materials to vote. Proxy cards provided by the Company will be WHITE.

The board is pleased to nominate for election as directors the four (4) persons named in Proposal 1 in this proxy statement and on the enclosed WHITE proxy card, Gary L. Crocker, Ulrik B. Nielsen, Ana Radeljevic and Russell T. Ray. In selecting the director nominees that we are proposing for election in this proxy statement, the board focused on selecting board candidates it believes are highly qualified and best suited to guide the Company at this critical juncture for the Company. As previously disclosed, the Company recently completed an extensive review of its strategic alternatives, following which the board implemented a series of measures designed to extend the Company’s cash runway and preserve its ability to capture the potential milestone payments resulting from the Ipsen transaction. The board of directors has also declared a special cash dividend of an aggregate of $20 million that is payable on September 5, 2019 to stockholders of record as of the close of business on August 28, 2019. During the strategic review, the Company met with JFL on several occasions and carefully considered specific suggestions that JFL made.

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The Company strongly believes that the board’s four nominees collectively have extensive expertise that will help to optimize value for our stockholders. Our three current directors who are being re-nominated were involved in the recent strategic review and have operational familiarity with the Company, its key partnerships in the biotechnology and pharmaceutical industry, and its remaining clinical and preclinical assets. We believe that Ms. Radeljevic is qualified to serve on our board of directors due to her knowledge of the healthcare industry and extensive experience in strategy, finance, operations and corporate development, as well as her management background as an executive in the biotech industry. All our nominees are committed to passing through to our stockholders any milestone payments the Company receives, net of any taxes owed and subject to there being sufficient surplus at that time.

Our board does not endorse any of JFL’s nominees and unanimously recommends that you vote “FOR” each of the nominees proposed by the board using the WHITE proxy card or by following the instructions for submitting a proxy over the Internet or by telephone or voting in person at the 2019 Annual Meeting. Our board strongly urges you NOT to sign or return any proxy card sent to you by the JFL Group. Voting to “WITHHOLD” with respect to any of JFL’s nominees on its proxy card is not the same as voting for our board’s nominees because a vote to “WITHHOLD” with respect to any of JFL’s nominees on its proxy card will revoke any proxy you previously submitted and thus you will cancel any proxy previously given to the Company.

If you previously submitted a proxy card sent to you by the JFL Group, you can change or revoke that proxy and vote for the board’s nominees by (i) visiting the website noted on the WHITE proxy card to submit your proxy over the Internet, (ii) using the telephone number on the WHITE proxy card to submit your proxy telephonically, (iii) signing, dating and returning the WHITE proxy card in the enclosed envelope to submit a proxy by mail, or (iv) attending the 2019 Annual Meeting to vote in person. Only your latest dated, valid proxy or vote will be counted at the 2019 Annual Meeting. Any proxy may be changed or revoked at any time prior to its exercise at the 2019 Annual Meeting.

THE BOARD STRONGLY URGES YOU NOT TO SIGN OR RETURN ANY PROXY CARD OR VOTING INSTRUCTION FORM THAT YOU MAY RECEIVE FROM THE JFL GROUP, EVEN AS A PROTEST VOTE AGAINST THE JFL GROUP OR JFL’S NOMINEES.

If you need assistance changing or revoking your vote, please call the Company’s proxy solicitor, Morrow Sodali, LLC, toll free at (800) 662-5200.

Important Information About Voting

If you are the “record holder” of your shares, meaning that you own your shares in your own name and not through a bank, broker or other nominee, you may cause your shares to be voted in one of four ways:

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All proxies that are executed or are otherwise submitted over the Internet or by telephone will be voted on the matters set forth in the accompanying Notice of Annual Meeting of Stockholders in accordance with the stockholders’ instructions. However, if no choice is specified on a WHITE proxy card as to one or more of the proposals, the proxy will be voted in accordance with our board of directors’ recommendations on such proposals as set forth in this proxy statement.

If you receive more than one WHITE proxy card or voting instruction form, it generally means that your shares are registered differently or are in more than one account. To ensure that all of your shares are voted, please submit each WHITE proxy card or voting instruction form you receive or, if you submit a proxy over the Internet or by telephone, you will need to enter each of your Control Numbers.

After you have submitted a proxy, you may still change your vote and revoke your proxy prior to the Annual Meeting by doing any one of the following things:

Your attendance at the Annual Meeting alone will not revoke your proxy.

THE BOARD STRONGLY URGES YOU TO REVOKE ANY PROXY CARD YOU MAY HAVE RETURNED THAT YOU RECEIVED FROM THE JFL GROUP. Even a “WITHHOLD” vote with respect to JFL’s nominees on their proxy card will cancel any previously submitted WHITE proxy card.

If the shares you own are held in “street name” by a bank, broker or other nominee record holder, which, for convenience, we collectively refer to in this proxy statement as brokerage firms, your brokerage firm, as the record holder of your shares, is required to vote your shares according to your instructions. In order for your shares to be voted, you will need to follow the directions your brokerage firm provides you. Many brokerage firms also offer the option of providing for voting instructions over the Internet or by telephone, directions for which, if available, would be provided by your brokerage firm on the voting instruction form that it delivers to you. As further explained below, under applicable stock exchange rules, if you do not give instructions to your brokerage firm subject to these rules, it will still be able to vote your shares with respect to certain “discretionary” items, but will not be allowed to vote your shares with respect to certain “non-discretionary” items. Because JFL has filed its preliminary proxy statement, the 2019 Annual Meeting will be the subject of a contested solicitation. For brokerage firms that receive proxy materials from or on behalf of both us and JFL, all items listed in the notice for the 2019 Annual Meeting will be considered non-discretionary matters. In that case, if you do not submit any voting instructions to your brokerage firm, your shares will not be counted in determining the outcome of any of the proposals at the 2019 Annual Meeting, nor will your shares be counted for establishing the presence of a quorum. However, if your brokerage firm receives proxy materials only from us, your brokerage firm is entitled to vote shares held for a beneficial holder on discretionary matters, such as the ratification of the selection of Marcum LLP as our independent registered public accounting firm, without instructions from the beneficial holder of those shares. On the other hand, a brokerage firm is not entitled to vote shares held for a beneficial holder on certain non-discretionary items, such as the election of directors or the non-binding, advisory vote on executive compensation, absent instructions from the beneficial holder of such shares. Consequently, if your brokerage firm receives proxy materials only from us and you do not submit any voting instructions to your brokerage firm, your brokerage firm may exercise its discretion to vote your shares on the proposal to ratify the selection of Marcum LLP. If your shares are voted on this discretionary item as directed by your brokerage firm, your shares will constitute “broker non-votes” on each of the non-discretionary items and will have no effect on the non-discretionary items, although they will count for establishing the presence of a quorum.

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Please instruct your brokerage firm how to vote your shares using the voting instruction form provided by your brokerage firm or following any instructions provided by your brokerage firm for submitting a proxy for your shares over the Internet or telephonically, if available.

If your shares are held in street name, you must bring an account statement from your brokerage firm showing that you are the beneficial owner of the shares as of the record date (September 16, 2019) in order to be admitted to the Annual Meeting. To be able to vote your shares held in street name at the Annual Meeting, you will need to obtain a proxy card from the holder of record.

If you have previously signed a proxy card sent to you by the JFL Group, you may change your vote by marking, signing, dating and returning the WHITE proxy card or by submitting a proxy over the Internet or by telephone or voting in person at the 2019 Annual Meeting. Only the latest dated, valid proxy that you submit will be counted. Submitting a proxy card sent to you by the JFL Group will revoke votes you have previously made via the Company’s WHITE proxy card.

Votes Required

The holders of a majority in voting power of the shares of our common stock issued and outstanding and entitled to vote at the Annual Meeting will constitute a quorum for the transaction of business at the Annual Meeting. Shares of common stock represented in person or by proxy (including shares that abstain or vote with respect to one or more but less than all of the matters presented for stockholder approval) will be counted for purposes of determining whether a quorum is present at the Annual Meeting. The following votes are required for approval of the proposals being presented at the Annual Meeting:

Proposal 1: Election of Directors. A nominee will be elected as a director at the Annual Meeting if the nominee receives a plurality of the votes cast “FOR” the applicable seat on our board of directors.

JFL has notified us that it intends to nominate four (4) alternative nominees for election as directors at the Annual Meeting in opposition to the board’s recommended nominees.

Proposal 2: Advisory Vote on Executive Compensation, or “Say-on-Pay.” The affirmative vote of the holders of shares of common stock representing a majority of the votes cast on the matter is required to approve the advisory “say-on-pay” vote on the compensation of our named executive officers. Because the stockholders’ vote on this proposal is advisory, it will not be binding on our board. However, our board, including our organization and compensation committee, values the opinions of our stockholders and, to the extent there are a substantial number of votes cast against the executive officer compensation as disclosed in this proxy statement, we will consider our stockholders’ concerns and evaluate what actions may be appropriate to address those concerns.

Proposal 3: Ratification of Independent Auditors. The affirmative vote of the holders of shares of common stock representing a majority of the votes cast on the matter is required for the ratification of the selection of Marcum as our independent registered public accounting firm for the current fiscal year.

Shares that abstain from voting as to a particular matter on properly executed WHITE proxy cards and shares held in “street name” by brokerage firms who indicate on their proxies that they do not have discretionary authority to vote such shares as to a particular matter will not be counted as votes in favor of such matter, and will also not be counted as shares voting on such matter. Accordingly, abstentions and “broker non-votes” will have no effect on the voting on Proposals 1, 2 and 3 referenced above.

If you have previously signed a proxy card sent to you by the JFL Group, you may change your vote by marking, signing, dating and returning the WHITE proxy card or by submitting a proxy over the Internet or by telephone or voting in person at the 2019 Annual Meeting. Only the latest dated, valid proxy that you submit will be counted. Submitting a proxy card sent to you by the JFL Group will revoke votes you have previously made via the Company’s WHITE proxy card.

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Recommendations of the Board of Directors

Our board unanimously recommends that you vote your shares in accordance with the instructions on the WHITE proxy card as follows:

Our board strongly urges you NOT to sign or return any proxy card sent to you by the JFL Group.

Important Notice Regarding the Availability of Proxy Materials for

the Annual Meeting of Stockholders to Be Held on October 17, 2019:

This proxy statement and our 2018 annual report to stockholders are available at

www.proxyvote.com for viewing, downloading and printing.

A copy of our Annual Report on Form 10-K for the year ended December 31, 2018 as filed with the Securities and Exchange Commission, or SEC, except for exhibits, will be furnished without charge to any stockholder upon written or oral request to Merrimack Pharmaceuticals, Inc., One Broadway, 14th Floor, Cambridge, MA 02142, Attention: Corporate Secretary, Telephone: (617) 441-1000.

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IMPORTANT

The JFL Group may send you solicitation materials in an effort to solicit your vote to elect up to four (4) of JFL’s nominees to the board. THE BOARD STRONGLY URGES YOU NOT TO SIGN OR RETURN ANY PROXY CARD OR VOTING INSTRUCTION FORM THAT YOU MAY RECEIVE FROM THE JFL GROUP OR ANY PERSON OTHER THAN THE COMPANY, EVEN AS A PROTEST VOTE AGAINST THE JFL GROUP OR JFL’S NOMINEES. Any proxy you sign from the JFL Group for any reason could invalidate previous WHITE proxy cards sent by you to support the board.

Your vote at this year’s 2019 Annual Meeting is especially important, no matter how many or how few shares you own. Please sign and date the enclosed WHITE proxy card and return it in the enclosed postage-paid envelope promptly.

Only your latest dated, signed proxy card or voting instruction form will be counted. Any proxy may be revoked at any time prior to its exercise at the 2019 Annual Meeting as described in this proxy statement.

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BACKGROUND OF THE CONTESTED SOLICITATION

The following is a chronology of the material contacts and events in our relationship with the JFL Group leading up to the filing of this proxy statement:

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OUR BOARD STRONGLY URGES YOU NOT TO SIGN OR RETURN ANY PROXY CARD OR VOTING INSTRUCTION FORM THAT YOU MAY RECEIVE FROM THE JFL GROUP, EVEN AS A PROTEST VOTE AGAINST THE JFL GROUP OR ANY OF JFL’S NOMINEES, AS DOING SO WILL INVALIDATE ANY PRIOR VOTE YOU SUBMITTED ON THE WHITE PROXY CARD IN SUPPORT OF THE COMPANY’S DIRECTOR NOMINEES.

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CORPORATE GOVERNANCE

Board of Directors

At the 2019 Annual Meeting, stockholders will vote to elect four (4) directors for a one year term beginning at the 2019 Annual Meeting and ending at our 2020 Annual Meeting of Stockholders.

Our board of directors is currently comprised of six members. However, our board has approved a decrease in its size to four members effective as of the close of the polls at the 2019 Annual Meeting. Our board has nominated Gary L. Crocker, Ulrik B. Nielsen, Ana Radeljevic and Russell T. Ray for election as directors, each to hold office until the 2020 Annual Meeting of Stockholders and until their respective successors have been duly elected and qualified or until their earlier death, resignation or removal. Mr. Crocker, Dr. Nielsen and Mr. Ray are currently members of our board. Ms. Radeljevic, who has been nominated for election as a director for the first time, was initially recommended by our corporate governance and nominating committee, and our board determined to include Ms. Radeljevic among its nominees. The persons named in the enclosed WHITE proxy card will vote to elect each of these nominees as directors, unless and to the extent authority to vote for the election of one or more of the nominees is withheld by marking the proxy to that effect.

Each of our four (4) director nominees was recommended by the board based on the board’s exercise of its judgment and belief that the experience, record and qualifications of each of its nominees make them the best candidates to serve on the board.

The nominees recommended by the board have consented to serving as nominees for election to the board, to being named in this proxy statement and to serving as members of the board if elected by the Company’s stockholders. As of the date of this proxy statement, we have no reason to believe that any nominee will be unable or unwilling to serve if elected as a director. However, if for any reason a nominee becomes unable to serve or for good cause will not serve if elected, the board upon the recommendation of the corporate governance and nominating committee may designate substitute nominees. If any substitute nominees are so designated, we will file an amended proxy statement that, as applicable, identifies the substitute nominees, discloses that such nominees have consented to serving as nominees for election to the board, to being named in the amended proxy statement and to serve as directors if elected, and incudes certain biographical and other information about such nominees required by the applicable rules promulgated by the SEC.

The accompanying WHITE proxy card will not be voted for more than four candidates or for anyone other than the board’s nominees or designated substitutes. Unless otherwise instructed, the persons named in the accompanying WHITE proxy card will vote to elect Mr. Crocker, Dr. Nielsen, Ms. Radeljevic and Mr. Ray to the board, unless, by marking the appropriate space on the WHITE proxy card, the stockholder instructs that he, she or it withholds authority from the proxy holder to vote with respect to a specified candidate(s).

Biographies of the Director Nominees

Set forth below are the names and certain information about each of our director nominees as of July 15, 2019. The information presented includes each individual’s principal occupation and business experience for the past five years and the names of other public companies of which he or she has served as a director during the past five years. We believe that all of our director nominees possess the attributes and characteristics described in “—Board Processes—Director Nomination Process.”

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Gary L. Crocker has served as a member of our board of directors since 2004, as Chairman of the Board since 2005 and as President and Treasurer since June 2019. Mr. Crocker also served as our Interim President and Chief Executive Officer from October 2016 to February 2017. Since 2002, Mr. Crocker has served as President and Managing Director of Crocker Ventures, LLC, a privately-held life science investment firm funding differentiated biotechnology and medical device companies. Mr. Crocker has held senior executive positions or served on the board of directors of several life science companies, including as Chairman of the Board of ARUP Laboratories, co-founder and director of Theratech, Inc. (acquired by Actavis plc) and President, Chief Executive Officer and founder of Research Medical, Inc. (acquired by Baxter International). Mr. Crocker also served on the boards of directors of the publicly traded firms Interleuken Genetics, Inc. and The Med-Design Corporation. Mr. Crocker served as a member of the board of the Federal Reserve Branch of San Francisco from 1999 to 2007, and currently serves as the Chairman of the University of Utah’s Center for Medical Innovation and on the board of the Sorenson Legacy Foundation. Mr. Crocker holds an M.B.A. from Harvard Business School and a B.S. from Harvard College. We believe that Mr. Crocker is qualified to serve on our board of directors due to his experience in the life sciences industry as an entrepreneur, venture capitalist and executive and his service on the boards of directors of a range of public and private companies and government institutions, as well as his ability to provide us with his expertise in diagnostics and therapeutic development.

Ulrik B. Nielsen, Ph.D. has served as a member of our board of directors since January 2015 and is one of our co-founders. Dr. Nielsen led our research and drug discovery in various roles from when he joined us in 2002 to January 2015, including as our Senior Vice President and Chief Scientific Officer from March 2009 until January 2015. Dr. Nielsen has been President and Chief Executive Officer of Tidal Therapeutics, Inc., a biotechnology company, since August 2018. Dr. Nielsen previously served in various capacities at Torque Therapeutics Inc., a biotechnology company, including as President from January 2015 to June 2018, Founder Chairman from November 2017 to June 2018 and Chief Executive Officer from January 2015 to November 2017. Dr. Nielsen also served as Chief Executive Officer of Silver Creek Pharmaceuticals, Inc., a former majority owned subsidiary of ours, from July 2010 to March 2014. Dr. Nielsen also currently serves on the board of directors of Alloy Therapeutics LLC. Dr. Nielsen holds a Ph.D. in molecular biology and an M.S. in biochemistry from the University of Copenhagen. We believe that Dr. Nielsen is qualified to serve on our board of directors due to his extensive knowledge of Merrimack, his leadership and management experience at Merrimack and other biotechnology companies, and his thorough understanding of our business and industry.

Ana Radeljevic is a nominee for election to our board of directors. Ms. Radeljevic has been Head of Business Operations and Corporate Development at Torque Therapeutics Inc., a biotechnology company, since May 2019. Ms. Radeljevic previously served as Chief Executive Officer of Adventus Partners, LLC, a biotech consulting firm, from April 2016 to May 2019, as Vice President of Financial Planning & Analysis at PerkinElmer, Inc., a provider of products, services and solutions for the diagnostics, life sciences and applied markets, from July 2014 to March 2016 and as Global Head of Business Planning and Business Development for the Biosurgery Business of Sanofi Genzyme, a global pharmaceutical company, from September 2009 to July 2014. Ms. Radeljevic holds an M.B.A. from Syracuse University and a B.S.B.A. from Old Dominion University. We believe that Ms. Radeljevic is qualified to serve on our board of directors due to her knowledge of the healthcare industry and extensive experience in strategy, finance, operations and corporate development, as well as her management background as an executive in the biotech industry.

Russell T. Ray has served as a member of our board of directors since January 2015. Mr. Ray has been a Senior Advisor to HLM Venture Partners, a healthcare venture capital firm, since February 2017. Mr. Ray was also a Senior Advisor to HLM Venture Partners from January 2014 to December 2015 and a Partner from 2003 to December 2013. From January 2016 to February 2017, Mr. Ray was a Managing Director and Vice Chairman of Healthcare Investment Banking at Stifel, Nicolaus & Company, Incorporated, an investment banking firm. From 1999 to 2003, Mr. Ray was a Managing Director and Global Co-Head of Healthcare Investment Banking at Credit

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Suisse First Boston. From 1987 to 1999, Mr. Ray was a Managing Director and Global Head of Healthcare Investment Banking at Deutsche Bank and its predecessor entities, BT Alex. Brown and Alex. Brown & Sons. Mr. Ray served on the board of directors of Allergan, Inc. from 2003 to 2015. Mr. Ray holds an M.B.A. from the Wharton School at the University of Pennsylvania, an M.S. in evolutionary biology from the University of Pennsylvania and B.S. degrees from the United States Military Academy and the University of Washington. We believe that Mr. Ray is qualified to serve on our board of directors due to his knowledge of the healthcare industry, his financial expertise and his management background as an executive in the financial services industry.

Board Composition

Our board of directors is currently authorized to have six members. However, our board has approved a decrease in its size to four members effective as of the close of the polls at the Annual Meeting. All of our directors are elected annually for a one year term expiring at the next annual meeting of stockholders. Each director will hold office until his or her successor has been elected and qualified or until his or her earlier death, resignation or removal. Our bylaws provide that the authorized number of directors shall be established by our board of directors. Our bylaws also provide that our directors may be removed with or without cause by the affirmative vote of the holders of at least a majority of the votes that all of our stockholders would be entitled to cast in an annual election of directors, and that any vacancy on our board of directors, including a vacancy resulting from an enlargement of our board of directors, may be filled only by vote of a majority of our directors then in office, and shall not be filled by the stockholders.

Due to the decrease in the size of our board of directors, three of our current directors whose terms are scheduled to expire at the Annual Meeting, Dr. Demetri, Mr. Dineen and Mr. Quigley, are not standing for re-election. In connection with the conclusion of their service as directors, we expect Dr. Demetri, Mr. Dineen and Mr. Quigley to deliver letters confirming that their respective service as directors shall cease as of the close of the polls at the Annual Meeting.

Board Determination of Independence

Rule 5605 of the Nasdaq Listing Rules requires a majority of a listed company’s board of directors to be comprised of independent directors. In addition, the Nasdaq Listing Rules require that, subject to specified exceptions, each member of a listed company’s audit, compensation and corporate governance and nominating committees be independent and that audit committee members also satisfy independence criteria set forth in Rule 10A-3 under the Securities Exchange Act of 1934, as amended (the “Exchange Act”). Under Rule 5605(a)(2) of the Nasdaq Listing Rules, a director will only qualify as an “independent director” if, in the opinion of our board of directors, that person does not have a relationship that would interfere with the exercise of independent judgment in carrying out the responsibilities of a director. In order to be considered independent for purposes of Rule 10A-3, a member of an audit committee of a listed company may not, other than in his or her capacity as a member of the audit committee, the board of directors, or any other board committee: (1) accept, directly or indirectly, any consulting, advisory or other compensatory fee from the listed company or any of its subsidiaries; or (2) be an affiliated person of the listed company or any of its subsidiaries. In addition, in affirmatively determining the independence of any director who will serve on a company’s compensation committee, Rule 10C-1 under the Exchange Act requires that a company’s board of directors consider all factors specifically relevant to determining whether a director has a relationship to such company which is material to that director’s ability to be independent from management in connection with the duties of a compensation committee member, including, but not limited to: (i) the source of compensation of the director, including any consulting, advisory or other compensatory fee paid by such company to the director; and (ii) whether the director is affiliated with the company or any of its subsidiaries or affiliates.

Based upon information requested from and provided by each director concerning his or her background, employment and affiliations, including family relationships, our board of directors has determined that none of Dr. Demetri, Mr. Dineen, Dr. Nielsen, Mr. Quigley and Mr. Ray, representing five of our six current directors, or Ms. Radeljevic, our director nominee, has a relationship that would interfere with the exercise of independent judgment

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in carrying out the responsibilities of a director and that each of these directors is “independent” as that term is defined under Rule 5605(a)(2) of the Nasdaq Listing Rules. Our board of directors had previously made a similar determination of independence with respect to Vivian Lee and Michael Porter, who each served as a director until June 2018, and Mr. Crocker, who was no longer deemed to be independent as of his appointment as President in June 2019. Our board of directors has also determined that Dr. Nielsen, Mr. Quigley and Mr. Ray, who comprise our audit committee, Dr. Nielsen, who comprises our corporate governance and nominating committee, and Dr. Demetri, Mr. Dineen and Mr. Ray, who comprise our organization and compensation committee, each satisfy the independence standards for such committees established by the SEC and the Nasdaq Listing Rules, as applicable. In making such determination, our board of directors considered the relationships that each such non-employee director has with Merrimack, including any transactions of the type described below in “Related Person Transactions,” and all other facts and circumstances our board of directors deemed relevant in determining independence.

How Our Board Is Organized

Board Leadership Structure

Our board of directors, upon the recommendation of our corporate governance and nominating committee, has appointed Mr. Crocker, our President and Principal Executive Officer, as Chairman of the Board. Mr. Crocker also serves as our Treasurer and Principal Financial Officer. Our board has reviewed our current board leadership structure in light of the composition of the board, our company’s size, the nature of our business and other relevant factors, and has determined that a combined Chairman and Principal Executive Officer position is currently the most appropriate board leadership structure for our company. In reaching its determination, the board noted that, given the specific characteristics and circumstances of our company, a combined Chairman and Principal Executive Officer is in the best position to be aware of major issues facing the company, is in the best position to identify key risks and developments facing the company to be brought to the board’s attention and eliminates the potential for confusion and duplication of efforts. We do not have a lead independent director position.

Mr. Crocker’s duties as Chairman of the Board include the following:

Board Committees

Our board of directors has established an audit committee, a corporate governance and nominating committee and an organization and compensation committee, each of which operates under a charter that has been approved by our board. Copies of the committee charters are posted on the Investors section of our website, which is located at investors.merrimack.com. We are not including the information contained on our website as a part of, or incorporating it by reference into, this proxy statement.

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Audit Committee

The members of our audit committee are Dr. Nielsen, Mr. Quigley and Mr. Ray. Mr. Quigley chairs the audit committee. Our audit committee’s responsibilities include:

All audit and non-audit services, other than de minimis non-audit services, to be provided to us by our independent registered public accounting firm must be approved in advance by our audit committee.

Our board of directors has determined that Mr. Quigley is an “audit committee financial expert” as defined in applicable SEC rules. We believe that the composition of our audit committee meets the requirements for independence under the current Nasdaq Listing Rules and SEC rules and regulations.

The audit committee met five times during 2018.

Corporate Governance and Nominating Committee

The sole member of our corporate governance and nominating committee is Dr. Nielsen, who also chairs the committee. Our corporate governance and nominating committee’s responsibilities include:

Mr. Crocker was also a member of our corporate governance and nominating committee during 2018 until his resignation from such committee in June 2019. The corporate governance and nominating committee met once during 2018.

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Organization and Compensation Committee

The members of our organization and compensation committee are Dr. Demetri, Mr. Dineen and Mr. Ray. Mr. Dineen chairs the organization and compensation committee. Our organization and compensation committee’s responsibilities include:

The processes and procedures followed by our organization and compensation committee in considering and determining executive and director compensation are described below under “—Board Processes—Executive and Director Compensation Processes.”

The organization and compensation committee met ten times during 2018.

Board Meetings and Attendance

Our board of directors met eleven times during 2018. During 2018, each director attended at least 75% of the aggregate of the number of board meetings and the number of meetings held by all committees of our board on which he then served.

Our directors are encouraged to attend our annual meetings of stockholders. Four of our nine directors then serving attended our 2018 Annual Meeting of Stockholders.

Board Processes

Oversight of Risk

Our board oversees our risk management processes directly and through its committees. Our management is responsible for risk management on a day-to-day basis. The role of our board and its committees is to oversee the risk management activities of management. They fulfill this duty by discussing with management the policies and practices utilized by management in assessing and managing risks and providing input on those policies and practices. In general, our board oversees risk management activities relating to business strategy, acquisitions, capital allocation, organizational structure and certain operational risks; our audit committee oversees risk management activities related to financial controls, legal and compliance risks and cyber risk; our corporate governance and nominating committee oversees risk management activities relating to board composition; and our organization and compensation committee oversees risk management activities relating to our compensation policies and practices and management succession planning. Each committee reports to the full board on a regular basis, including reports with respect to the committee’s risk oversight activities as appropriate. In addition, since risk issues often overlap, committees from time to time request that that the full board discuss particular risks.

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Director Nomination Process

The process followed by our corporate governance and nominating committee to identify and evaluate director candidates may include requests to board members and others for recommendations, evaluation of the performance on our board and its committees of any existing directors being considered for nomination, consideration of biographical information and background material relating to potential candidates and, particularly in the case of potential candidates who are not then serving on our board, interviews of selected candidates by members of the committee and our board.

In considering whether to recommend any particular candidate for inclusion in our board’s slate of recommended director nominees, our corporate governance and nominating committee applies the criteria set forth in our corporate governance guidelines. Consistent with these criteria, our corporate governance and nominating committee expects every nominee to have the following attributes or characteristics, among others: integrity, honesty, adherence to high ethical standards, business acumen, good judgment and a commitment to understand our business and industry.

The biographies under “—Board of Directors—Biographies of the Director Nominees” indicate the experience, qualifications, attributes and skills of each nominee that led our corporate governance and nominating committee and our board to conclude that he or she is qualified to serve as a director of Merrimack. Our corporate governance and nominating committee and our board believe that each of the nominees has the individual attributes and characteristics required of our directors, and that the nominees as a group possess the skill sets and specific experience desired of our board as a whole. At the Annual Meeting, stockholders will be asked to consider the election of Ms. Radeljevic, who has been nominated for election as a director for the first time. Ms. Radeljevic was initially recommended by our corporate governance and nominating committee, and our board determined to include Ms. Radeljevic among its nominees.

Our corporate governance and nominating committee considers the value of diversity when selecting nominees, and believes that our board, taken as a whole, should embody a diverse set of skills, experiences and backgrounds. The committee does not make any particular weighting of diversity or any other characteristic in evaluating nominees and directors.

Stockholders may recommend individuals for consideration as potential director candidates by submitting their names, together with appropriate biographical information and background materials, and information with respect to the stockholder or group of stockholders making the recommendation, including the number of shares of common stock owned by such stockholder or group of stockholders, to our Corporate Secretary at Merrimack Pharmaceuticals, Inc., One Broadway, 14th Floor, Cambridge, MA 02142, Attention: Corporate Secretary. The specific requirements for the information that is required to be provided for such recommendations to be considered are specified in our bylaws and must be received by us no later than the date referenced below in “Other Matters—Deadline for Submission of Stockholder Proposals for 2020.” Assuming that appropriate biographical and background material has been provided on a timely basis, the corporate governance and nominating committee will evaluate stockholder-recommended candidates by following substantially the same process, and applying substantially the same criteria, as it follows for candidates submitted by others.

Executive and Director Compensation Processes

Currently, we do not have any employees and instead use a limited number of consultants for the operation of our company. However, historically, our executive compensation program has been administered by the organization and compensation committee of our board of directors, subject to the oversight and approval of our full board of directors. In that role, our organization and compensation committee reviewed our executive compensation practices on an annual basis and based on this review made recommendations to our board of directors for approval, which had full discretion to approve or modify the recommendations of the organization and compensation committee.

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In designing our executive compensation program, our organization and compensation committee considered publicly available compensation data for national and regional companies in the biotechnology/pharmaceutical industry to help guide its executive compensation decisions at the time of hiring and for subsequent adjustments in compensation. The organization and compensation committee is directly responsible for the appointment and oversight of any compensation consultants and other advisors it retains.

Our director compensation program is administered by our board of directors with the assistance of the organization and compensation committee. The organization and compensation committee periodically reviews director compensation and makes recommendations to our board with respect thereto.

Communications with Stockholders

Our management will give appropriate attention to written communications that are submitted by stockholders, and will respond if and as appropriate. Stockholders may communicate with our management by writing to our Corporate Secretary at Merrimack Pharmaceuticals, Inc., One Broadway, 14th Floor, Cambridge, MA 02142, Attention: Corporate Secretary, or by calling (617) 441-1000. Additional information about contacting Merrimack is available on the Investors section of our website, which is located at investors.merrimack.com.

In addition, stockholders who wish to communicate with our entire board may do so by writing to Gary L. Crocker, Chairman of the Board, Merrimack Pharmaceuticals, Inc., One Broadway, 14th Floor, Cambridge, MA 02142. Communications will be forwarded to other directors if they relate to substantive matters that the Chairman of the Board, in consultation with our General Counsel, if any, considers appropriate for attention by the other directors. In general, communications relating to corporate governance and long-term corporate strategy are more likely to be forwarded than communications relating to ordinary business affairs, personal grievances or matters as to which we tend to receive repetitive or duplicative communications.

Corporate Governance Guidelines

Our board of directors has adopted corporate governance guidelines to assist in the exercise of its duties and responsibilities and to serve the best interests of Merrimack and our stockholders. The guidelines provide that:

A copy of the corporate governance guidelines is posted on the Investors section of our website, which is located at investors.merrimack.com.

Board Policies

Related Person Transactions

Our board of directors has adopted written policies and procedures for the review of any transaction, arrangement or relationship in which Merrimack is a participant, the amount involved exceeds $120,000 and one of our executive officers, directors, director nominees or 5% stockholders, or their immediate family members, each of whom we refer to as a “related person,” has a direct or indirect material interest.

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Since January 1, 2018, we have not engaged in any related person transactions.

Code of Business Conduct and Ethics

Our board of directors has adopted a written code of business conduct and ethics that applies to our directors, officers and employees, including our principal executive officer, principal financial officer, principal accounting officer or controller, or persons performing similar functions. A copy of the code of business conduct and ethics is posted on the Investors section of our website, which is located at investors.merrimack.com. In addition, we intend to post on our website all disclosures that are required by law or the Nasdaq Listing Rules concerning any amendments to, or waivers from, any provision of our code of business conduct and ethics.

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EXECUTIVE COMPENSATION

Executive Officers

Our sole executive officer, his current position and his age as of July 15, 2019 is set forth below and his biography is set forth above in “Corporate Governance—Board of Directors—Biographies of the Director Nominees”:

Mr. Crocker is not receiving any additional compensation for his service as President and Treasurer of the Company. Mr. Crocker is compensated for his service on the board of directors pursuant to the existing terms of the Company’s non-employee director compensation policy, as described below in “Director Compensation Arrangements.” Our “named executive officers” for 2018 are Richard Peters, M.D., Ph.D., our former President and Chief Executive Officer, Jean M. Franchi, our former Chief Financial Officer and Treasurer, and Sergio L. Santillana, M.D., our former Chief Medical Officer. The biographies of our named executive officers are as follows:

Richard Peters, M.D., Ph.D. served as our President and Chief Executive Officer and a member of our board of directors from February 2017 to June 2019. Prior to joining us, Dr. Peters served in various capacities at Sanofi Genzyme, a global pharmaceutical company, since 2008, including as Senior Vice President, Head of Global Rare Diseases Business Unit since January 2015, Vice President, Strategy Development Officer, U.S. Rare Disease Unit from May 2014 to December 2014, Vice President, Division Medical Officer, Global Oncology Division from 2011 to May 2014, and Vice President, Head of Global and U.S. Medical Affairs, Hematology and Transplant from 2008 to 2011. Prior to Sanofi Genzyme, Dr. Peters held medical affairs roles at Onyx Pharmaceuticals, Inc. and Amgen Inc., both pharmaceutical companies, and was a co-founder and Chief Executive Officer of Mednav, Inc., a healthcare information technology company. Dr. Peters has also served on the faculty at Harvard Medical School/Massachusetts General Hospital. Dr. Peters holds an M.D. and a Ph.D. in pharmacology from the Medical University of South Carolina and a B.S. from the College of Charleston.

Jean M. Franchi served as our Chief Financial Officer and Treasurer from August 2017 to June 2019. Ms. Franchi previously served as Chief Financial Officer, Treasurer and Secretary at Dimension Therapeutics, Inc., a biotechnology company, from August 2015 to July 2017. From February 2012 to July 2015, Ms. Franchi served as Chief Financial Officer at Good Start Genetics, Inc., a molecular genetics information company. From 1995 to 2011, Ms. Franchi held various positions at Sanofi Genzyme, a global pharmaceutical company, including Senior Vice President of Corporate Finance, Senior Vice President of Business Unit Finance, Vice President of Finance and Controller, Product Line and International Group. Ms. Franchi holds a B.B.A. from Hofstra University and successfully completed the Uniform CPA Examination.

Sergio L. Santillana, M.D. served as our Chief Medical Officer from June 2017 to April 2019. Dr. Santillana previously served as Vice President of Clinical Research at ARIAD Pharmaceuticals, Inc., a pharmaceutical company, from March 2016 to June 2017, and as Chief Medical Officer at ARIAD from May 2016 to June 2017. From August 2014 to March 2016, Dr. Santillana served as Senior Medical Director and Global Clinical Lead of Oncology Clinical Research at Takeda Pharmaceuticals International, Inc., a global pharmaceutical company. From 2006 to August 2014, Dr. Santillana served in various capacities at GlaxoSmithKline plc, a global pharmaceutical company, including as Global Project Physician Leader and Director of Clinical Development from February 2012 to August 2014. Dr. Santillana also served as a Clinical Research Physician for Eli Lilly and Company, a global pharmaceutical company, from 2002 to 2006. Dr. Santillana was also a practicing medical oncologist for 15 years, first at the National Cancer Institute of Peru (INEN) from 1994 to 2002, and then in private practice from 2002 to 2006. Dr. Santillana holds an M.S. in experimental therapeutics from Kellogg College at the University of Oxford and an M.D. and B.S. from the Universidad Nacional Federico Villarreal School of Medicine.

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Our board of directors has declared a special cash dividend (the “special dividend”) of an aggregate of $20 million that is payable on September 5, 2019 to stockholders of record as of the close of business on August 28, 2019. The ex-dividend date for the special dividend will be September 6, 2019, the first trading day following the payment date. As required by our 2008 Stock Incentive Plan, as amended, and our 2011 Stock Incentive Plan, equitable adjustments will be made to all awards outstanding under such plans at the time of the special dividend. As a result, the exercise prices of outstanding stock options will be reduced and the number of shares subject to such options will be increased. The number of shares issuable and the exercise prices for stock option awards noted in this proxy statement do not reflect the equitable adjustment described above since the amount of the adjustment will not be calculable until after the special dividend.

Summary Compensation Table

The following table sets forth the total compensation awarded to, earned by or paid to our named executive officers during 2017 and 2018.

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Narrative to Summary Compensation Table

We used base salaries to recognize the experience, skills, knowledge and responsibilities of our named executive officers. None of our named executive officers was party to an employment agreement that provided for automatic or scheduled increases in base salary.

For 2018, each named executive officer was eligible to receive an annual performance-based cash bonus (“annual cash bonus”) with a target amount equal to a fixed percentage of his or her base salary (“bonus percentage”). Dr. Peters’ target annual cash bonus for 2018 was 65% of his 2018 base salary and was based solely on the achievement of our corporate objectives. Ms. Franchi’s and Dr. Santillana’s target annual cash bonuses for 2018 were 35% of her or his respective 2018 base salary and were based 80% on the achievement of our corporate objectives and 20% on the achievement of specified individual objectives. The board of directors determined retroactively at what level we had achieved our corporate objectives for 2018, and the organization and compensation committee determined retroactively at what level Ms. Franchi and Dr. Santillana had achieved her or his respective individual objectives for 2018.

Our equity award program was the primary vehicle for offering long-term incentives to our named executive officers. While we did not have any equity ownership guidelines for our named executive officers, we believe that equity grants provided our named executive officers with a strong link to our long-term performance, created an ownership culture and helped to align the interests of our named executive officers and our stockholders. Typically, the stock options we granted to our named executive officers vested quarterly over a three year period. Vesting and exercise rights cease shortly after termination of an employment or consulting relationship, except in the case of death or disability. The exercise price of all stock options granted since the closing of our initial public offering was equal to the fair market value of shares of our common stock on the date of grant, which generally is determined by reference to the closing market price of our common stock on the date of grant.

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Outstanding Equity Awards at 2018 Year End

The following table sets forth information regarding outstanding stock options held by our named executive officers as of December 31, 2018.

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Securities Authorized for Issuance under Equity Compensation Plans

The following table contains information about our equity compensation plans as of December 31, 2018. As of December 31, 2018, we had two equity compensation plans, both of which were approved by our stockholders: our 2008 Stock Incentive Plan, as amended, and our 2011 Stock Incentive Plan.

Equity Compensation Plan Information

Employment Agreements

We were previously a party to employment agreements with Dr. Peters prior to his resignation on June 28, 2019, Ms. Franchi prior to her resignation on June 28, 2019 and Dr. Santillana prior to his resignation on April 12, 2019. The following is a description of the employment agreements that we had with Dr. Peters, Ms. Franchi and Dr. Santillana prior to their resignations. See “Potential Payments Upon Termination or Change in Control” for a discussion of the Transition, Separation and Release of Claims Agreements and Consulting Agreements that we entered into with each of Dr. Peters, Ms. Franchi and Dr. Santillana in connection with their resignations.

The employment agreements, and the non-disclosure, developments, non-competition and non-solicitation agreements that are incorporated therein, prohibit our named executive officers, during the term of employment and for a period of one year thereafter, from competing with us and soliciting or hiring our employees.

Pursuant to the terms of the employment agreements, our named executive officers received the following base salaries, which have been adjusted by our organization and compensation committee and board of directors since our named executive officers originally entered into the employment agreements, and were eligible for the following bonus percentages.

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Upon execution and effectiveness of a severance agreement and release of claims, each named executive officer is entitled to severance payments if we terminate the named executive officer’s employment without cause, as defined in the employment agreement, including our decision not to renew the named executive officer’s term of employment, or the named executive officer terminates employment with us for good reason, as defined in the employment agreement.

If a named executive officer’s employment terminates under these circumstances, in each case prior to a change in control, as defined in the employment agreement, we are obligated for a period of 12 months to pay such named executive officer his or her base salary and pay for coverage for such named executive officer under any company sponsored insurance and benefit programs available to our senior management employees. In addition, we would be obligated to pay to each of our named executive officers a pro-rata bonus for the portion of the year in which such named executive officer was employed by us based on his or her average annual bonus payments over each of the three years prior to the year of termination, or such lesser period during which such named executive officer served as one of our executive officers.

If a named executive officer’s employment terminates under these circumstances, in each case within 18 months following a change in control, we are obligated to pay such named executive officer a lump sum amount equal to 36 months of his or her base salary plus a bonus equal to three times the average of his or her annual bonus payments over each of the three years prior to the year of termination, or such lesser period during which such named executive officer served as one of our executive officers, accelerate the vesting of all outstanding stock options, restricted stock or other equity awards granted to the named executive officer and pay for coverage for such named executive officer under any company sponsored insurance and benefit programs available to our senior management employees for a period of 18 months.

In the event of termination of the named executive officer’s employment due to death or disability, the named executive officer (or his or her estate in the event of death) will be eligible to receive a pro-rata bonus for the portion of the year in which such named executive officer was employed by us based on his or her average annual bonus payments over each of the three years prior to the year of termination, or such lesser period during which such named executive officer served as one of our executive officers.

Potential Payments Upon Termination or Change in Control

The following tables set forth information regarding potential payments that each named executive officer who was serving as an executive officer as of December 31, 2018 would have received if the named executive officer’s employment had terminated as of December 31, 2018 under the circumstances set forth below. Dr. Peters, who resigned on June 28, 2019, Ms. Franchi, who resigned on June 28, 2019, and Dr. Santillana, who resigned on April 12, 2019, each entered into a Transition, Separation and Release of Claims Agreement and a Consulting Agreement with us as described below.

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We entered into a Transition, Separation and Release of Claims Agreement (the “Peters Separation Agreement”) with Dr. Peters on June 25, 2019. Pursuant to the Peters Separation Agreement, in connection with Dr. Peters’ resignation from his positions as President, Chief Executive Officer and a member of our board of directors, we agreed to:

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The Peters Separation Agreement also includes a release of claims by Dr. Peters against us.

In addition, on June 25, 2019, we entered into a Consulting Agreement (the “Peters Consulting Agreement”) with Dr. Peters, pursuant to which Dr. Peters will assist us with our on-going reporting obligations and the administration of our remaining assets. Dr. Peters will be compensated at a rate of $30,989.43 per month for his services under the Peters Consulting Agreement, which will be paid quarterly. The term of the Peters Consulting Agreement continues until June 29, 2021. Either we or Dr. Peters may terminate the Peters Consulting Agreement for any reason upon 30 days written notice.

We entered into a Transition, Separation and Release of Claims Agreement (the “Franchi Separation Agreement”) with Ms. Franchi on June 25, 2019. Pursuant to the Franchi Separation Agreement, in connection with Ms. Franchi’s resignation from her positions as Chief Financial Officer and Treasurer, we agreed to:

The Franchi Separation Agreement also includes a release of claims by Ms. Franchi against us.

In addition, on June 25, 2019, we entered into a Consulting Agreement (the “Franchi Consulting Agreement”) with Ms. Franchi, pursuant to which Ms. Franchi will assist us with our on-going reporting obligations and the administration of our remaining assets. Ms. Franchi will be compensated at a rate of $17,291.78 per month for her services under the Franchi Consulting Agreement, which will be paid quarterly. The term of the Franchi Consulting Agreement continues until June 29, 2021. Either we or Ms. Franchi may terminate the Franchi Consulting Agreement for any reason upon 30 days written notice.

We entered into a Separation and Release of Claims Agreement (the “Santillana Separation Agreement”) with Dr. Santillana on April 15, 2019. Pursuant to the Santillana Separation Agreement, in connection with Dr. Santillana’s resignation from his position as Chief Medical Officer, we agreed to:

The Santillana Separation Agreement also includes a release of claims by Dr. Santillana against us.

The election of a majority of the JFL nominees would not constitute a change in control within the meaning of such term as it is used in the Transition, Separation and Release of Claims Agreements described above or a reorganization event within the meaning of such term as it is used in our 2008 Stock Incentive Plan, as amended, and our 2011 Stock Incentive Plan, as amended.

401(k) Plan

We maintain a defined contribution employee retirement plan for our employees. Our 401(k) plan is intended to qualify as a tax-qualified plan under Section 401 of the Internal Revenue Code of 1986, as amended, so that

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contributions to our 401(k) plan, and income earned on such contributions, are not taxable to participants until withdrawn or distributed from the 401(k) plan. Our 401(k) plan provides that each participant may contribute up to 100% of his or her pre-tax compensation, up to a statutory limit, which was $18,500 for 2018. Participants who are at least 50 years old can also make “catch-up” contributions, which in 2018 was up to an additional $6,000 above the statutory limit. Under our 401(k) plan, each employee is fully vested in his or her deferred salary contributions. Employee contributions are held and invested by the plan’s trustee. Our 401(k) plan also permits us to make discretionary contributions and matching contributions, subject to established limits and a vesting schedule. For 2018, we generally matched 50% of employee contributions up to a maximum contribution by us of 3% of the employee’s deferrable income, subject to employer match limitations by the Internal Revenue Service. The match and any earnings thereon generally vest at 25% per year over the first four years of an employee’s employment, after which, any match that is contributed is 100% vested.

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DIRECTOR COMPENSATION

Compensation for 2018

The following table sets forth information regarding the total compensation awarded to, earned by or paid to each of our non-employee directors during the year ended December 31, 2018 for their service on our board of directors. The compensation amounts presented in the table below are historical and are not indicative of the amounts we may pay our directors in the future. Richard Peters, our former President and Chief Executive Officer, did not receive any compensation for his service as a director. The compensation that we paid to Dr. Peters is discussed under “Executive Compensation” above.

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As of December 31, 2018, the aggregate number of shares of our common stock subject to each non-employee director’s outstanding option awards was as follows: Mr. Crocker, 73,665; Dr. Demetri, 11,890; Mr. Dineen, 25,555; Dr. Nielsen, 291,564; Mr. Quigley, 41,759; and Mr. Ray, 26,685. As of December 31, 2018, neither Dr. Lee nor Dr. Porter had outstanding option awards.

Director Compensation Arrangements

Our non-employee directors were compensated for their services to our board during 2018, and will continue to be compensated for their services to our board during 2019, as follows:

In addition, we have reimbursed, and will continue to reimburse, our non-employee directors for their travel, lodging and other reasonable expenses incurred in attending meetings of our board and committees of our board.

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AUDIT-RELATED MATTERS

Audit Committee Report

The audit committee of the board of directors of Merrimack Pharmaceuticals, Inc. has reviewed Merrimack’s audited financial statements for the fiscal year ended December 31, 2018 and discussed them with Merrimack’s management and PricewaterhouseCoopers LLP, Merrimack’s independent registered public accounting firm for the fiscal year ended December 31, 2018.

The audit committee has received from, and discussed with, PricewaterhouseCoopers LLP various communications that PricewaterhouseCoopers LLP is required to provide to the audit committee, including the matters required to be discussed by the applicable requirements of the Public Company Accounting Oversight Board.

The audit committee has received the written disclosures and the letter from PricewaterhouseCoopers LLP required by applicable requirements of the Public Company Accounting Oversight Board regarding the independent registered public accounting firm’s communications with the audit committee concerning independence, and has discussed with Merrimack’s independent registered public accounting firm its independence.

Based on the review and discussions referred to above, the audit committee recommended to Merrimack’s board of directors that the audited financial statements referred to above be included in Merrimack’s Annual Report on Form 10-K for the year ended December 31, 2018.

By the audit committee of the board of directors of Merrimack Pharmaceuticals, Inc.

Ulrik B. Nielsen, Ph.D.

James H. Quigley

Russell T. Ray

Audit Fees and Services

The following table summarizes the fees that PricewaterhouseCoopers LLP, our independent registered public accounting firm for the fiscal year ended December 31, 2018, billed to us for each of the last two fiscal years.

All such accountant services and fees were pre-approved by our audit committee in accordance with the “Pre-Approval Policies and Procedures” described below.

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Pre-Approval Policies and Procedures

Our audit committee has adopted policies and procedures relating to the approval of all audit and non-audit services that are to be performed by our independent registered public accounting firm. This policy generally provides that we will not engage our independent registered public accounting firm to render audit or non-audit services unless the service is specifically approved in advance by our audit committee or the engagement is entered into pursuant to a de minimis exception in accordance with applicable SEC rules.

Change in our Independent Registered Public Accounting Firm

On July 15, 2019, our audit committee dismissed PricewaterhouseCoopers LLP (“PwC”) as our independent registered public accounting firm, effective following the filing, on July 17, 2019, of our Quarterly Report on Form 10-Q for the three months ended June 30, 2019.

PwC’s reports on our financial statements as of and for the fiscal years ended December 31, 2017 and 2018 did not contain an adverse opinion or a disclaimer of opinion, and were not qualified or modified as to uncertainty, audit scope or accounting principles, except that PwC’s report on our financial statements as of and for the fiscal years ended December 31, 2017 and 2018 included an emphasis of matter paragraph indicating that we will require additional financing to fund future operations.

During our fiscal years ended December 31, 2017 and 2018 and the subsequent interim period through July 15, 2019, there were:

We provided PwC with a copy of this disclosure and requested PwC to furnish to us a letter addressed to the SEC stating that it agrees with the statements made above. A copy of PwC’s letter dated July 17, 2019 is attached as Exhibit 16.1 to our Current Report on Form 8-K filed with the SEC on July 17, 2019.

In addition, on July 24, 2019, our audit committee engaged Marcum as our independent registered public accounting firm for the fiscal year ending December 31, 2019. We have authorized PwC to respond fully to the inquiries of Marcum.

During our fiscal years ended December 31, 2017 and 2018 and the subsequent interim period through July 24, 2019, neither we nor anyone on our behalf has consulted with Marcum on any matter that:

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MATTERS TO BE VOTED ON

Proposal 1: Election of Directors

At the Annual Meeting, stockholders will vote to elect four directors for a one year term beginning at the Annual Meeting and ending at our 2020 Annual Meeting of Stockholders.

Our board of directors is currently comprised of six members. However, our board has approved a decrease in its size to four members effective as of the close of the polls at the Annual Meeting. Our board has nominated Mr. Crocker, Dr. Nielsen, Ms. Radeljevic and Mr. Ray for election as directors, each to hold office until the 2020 Annual Meeting of Stockholders and until their respective successors have been duly elected and qualified or until their earlier death, resignation or removal. Mr. Crocker, Dr. Nielsen and Mr. Ray are currently members of our board. Ms. Radeljevic, who has been nominated for election as a director for the first time, was initially recommended by our corporate governance and nominating committee, and our board determined to include Ms. Radeljevic among its nominees.

JFL has notified us that it intends to nominate four nominees for election to our board at the Annual Meeting in opposition to the four nominees recommended by our board of directors above.

Our board does not endorse any JFL nominee and urges you NOT to sign or return any proxy card that may be sent to you by the JFL Group. Voting to “WITHHOLD” with respect to any of JFL’s nominees on its proxy card is not the same as voting for the Company’s nominees because a vote to “WITHHOLD” with respect to any of JFL’s nominees on its proxy card will revoke any WHITE proxy you previously submitted. If you have already voted using the JFL Group’s proxy card, you have every right to change your vote by using the WHITE proxy card or by submitting a proxy over the Internet or by telephone or voting in person at the Annual Meeting. Only the latest dated, valid proxy that you submit will be counted—any proxy may be revoked at any time prior to its exercise at the Annual Meeting by following the instructions under the section above titled “Important Information About Voting”. If you have any questions or require any assistance with voting your shares, please contact our proxy solicitor, Morrow Sodali, LLC, toll free at (800) 662-5200.

Unless contrary instructions are provided on the WHITE proxy card, the persons named as proxies will, upon receipt of a properly executed proxy, vote for the election of Mr. Crocker, Dr. Nielsen, Ms. Radeljevic and Mr. Ray as directors for a term expiring at the 2020 Annual Meeting of Stockholders. Each of the nominees has consented to serving as nominees for election to the board, to being named in this proxy statement and to serve on our board, if elected. If any nominee should be unable to serve, the person acting under the proxy may vote the proxy for a substitute nominee designated by our board. We do not contemplate that any of the nominees will be unable to serve if elected.

OUR BOARD OF DIRECTORS UNANIMOUSLY RECOMMENDS THAT YOU VOTE FOR THE ELECTION OF EACH OF OUR DIRECTOR NOMINEES NAMED ON THE ENCLOSED WHITE PROXY CARD, GARY L. CROCKER, ULRIK B. NIELSEN, ANA RADELJEVIC AND RUSSELL T. RAY. OUR BOARD URGES YOU NOT TO SIGN OR RETURN ANY PROXY CARD SENT TO YOU BY OR ON BEHALF OF THE JFL GROUP.

Proposal 2: Advisory Vote on Executive Compensation

We are providing our stockholders the opportunity to vote to approve, on an advisory, non-binding basis, the compensation of our named executive officers as disclosed in this proxy statement in accordance with the SEC’s rules. This proposal, which is commonly referred to as “say-on-pay,” is required by the Dodd-Frank Wall Street Reform and Consumer Protection Act of 2010, which added Section 14A to the Exchange Act. Consistent with the preference expressed by our stockholders at our 2015 Annual Meeting of Stockholders, we have determined to hold an advisory vote on executive compensation annually.

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Currently, we do not have any employees and instead use a limited number of consultants for the operation of our company. Historically, our executive compensation programs have been designed to attract, motivate and retain our executive officers, and we anticipate any future executive compensation programs would be similarly designed. Under these programs, our named executive officers were rewarded for the achievement of our short-term and longer-term financial and strategic objectives and for driving corporate financial performance and stability. The programs contained elements of cash and equity-based compensation and were designed to align the interests of our executives with those of our stockholders.

The “Executive Compensation” section of this proxy statement describes our executive compensation programs and the decisions made by the organization and compensation committee and our board with respect to the year ended December 31, 2018.

Our board is asking stockholders to approve a non-binding advisory vote on the following resolution:

RESOLVED, that the compensation paid to Merrimack’s named executive officers, as disclosed pursuant to the compensation disclosure rules of the Securities and Exchange Commission, the compensation tables and any related material disclosed in this proxy statement, is hereby approved.

The affirmative vote of the holders of shares of common stock representing a majority of the votes cast on the matter is required to approve the advisory “say-on-pay” vote on the compensation of our named executive officers. As an advisory vote, this proposal is not binding. The outcome of this advisory vote does not overrule any decision by us or our board (or any committee thereof), create or imply any change to the fiduciary duties of us or our board (or any committee thereof), or create or imply any additional fiduciary duties for us or our board (or any committee thereof). However, our organization and compensation committee and our board value the opinions expressed by our stockholders in their vote on this proposal and will consider the outcome of the vote when making future compensation decisions for our named executive officers.

OUR BOARD OF DIRECTORS UNANIMOUSLY RECOMMENDS THAT YOU VOTE FOR APPROVAL OF THE COMPENSATION OF OUR NAMED EXECUTIVE OFFICERS.

Proposal 3: Ratification of Independent Auditors

The audit committee of our board of directors has selected the firm of Marcum as our independent registered public accounting firm for the fiscal year ending December 31, 2019. This is the first year that Marcum will serve as our independent registered public accounting firm. PricewaterhouseCoopers LLP previously served as our independent registered public accounting firm since the fiscal year ended December 31, 2001, but was dismissed by our audit committee on July 17, 2019. Although stockholder approval of the selection of Marcum is not required by law or Nasdaq Listing Rules, our audit committee believes that it is advisable and has decided to give our stockholders the opportunity to ratify this selection. If this proposal is not approved at the Annual Meeting, our audit committee may reconsider this selection.

Representatives of Marcum are expected to be present at the Annual Meeting and will have the opportunity to make a statement if they desire to do so. It is also expected that they will be available to respond to appropriate questions from stockholders.

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The affirmative vote of the holders of shares of common stock representing a majority of the votes cast on the matter is required for the ratification of the selection of Marcum as our independent registered public accounting firm for the current fiscal year.

OUR BOARD OF DIRECTORS UNANIMOUSLY RECOMMENDS THAT YOU VOTE FOR THE RATIFICATION OF THE SELECTION OF MARCUM AS OUR INDEPENDENT REGISTERED PUBLIC ACCOUNTING FIRM FOR THE FISCAL YEAR ENDING DECEMBER 31, 2019.

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STOCK OWNERSHIP AND REPORTING

Security Ownership of Certain Beneficial Owners and Management

The following table sets forth information with respect to the beneficial ownership of our common stock as of July 31, 2019 by:

Beneficial ownership is determined in accordance with the rules and regulations of the SEC and includes voting or investment power with respect to our common stock. Shares of our common stock subject to options that are currently exercisable or exercisable within 60 days after July 31, 2019 are considered outstanding and beneficially owned by the person holding the options for the purpose of calculating the percentage ownership of that person, but not for the purpose of calculating the percentage ownership of any other person. Except as otherwise noted, to our knowledge, the persons and entities in this table have sole voting and investing power with respect to all of the shares of our common stock beneficially owned by them, subject to community property laws, where applicable. The inclusion herein of any shares as beneficially owned does not constitute an admission of beneficial ownership.

The column entitled “Percentage of Shares Beneficially Owned” is based on a total of 13,353,786 shares of our common stock outstanding as of July 31, 2019. Except as otherwise set forth below, the address of the beneficial owner is c/o Merrimack Pharmaceuticals, Inc., One Broadway, 14th Floor, Cambridge, MA 02142. Beneficial ownership representing less than one percent of our outstanding common stock as of July 31, 2019 is denoted with an “*.”

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OTHER MATTERS

Our board of directors does not know of any other matters that may come before the Annual Meeting. However, if any other matters are properly presented to the Annual Meeting, it is the intention of the persons named in the accompanying proxy to vote, or otherwise act, in accordance with their judgment on such matters.

Solicitation of Proxies

This proxy is solicited on behalf of our board of directors. We will bear the expenses connected with this proxy solicitation. We expect to pay banks, brokers and other nominees their reasonable expenses for forwarding proxy materials and annual reports to principals and obtaining their voting instructions. In addition to the use of the mails, our directors and our sole executive officer may, without additional remuneration, solicit proxies in person or by telephone, facsimile, electronic mail, Internet and text messaging. Such persons are listed on Appendix A to this proxy statement.

Due to the possibility of a proxy contest, we have engaged Morrow Sodali, LLC to solicit proxies from stockholders in connection with the 2019 Annual Meeting. Morrow Sodali, LLC expects that approximately [    ] of its employees will assist in the solicitation of proxies. We will pay Morrow Sodali, LLC a fee not to exceed $[            ], plus costs and expenses.

We estimate that our additional out-of-pocket expenses beyond those normally associated with soliciting proxies for the 2019 Annual Meeting as a result of the potential proxy contest will be $[            ] in the aggregate, of which approximately $[            ] has been spent to date. Such additional solicitation costs are expected to include the fees incurred to retain Morrow Sodali, LLC as the Company’s proxy solicitor, as discussed above, fees of outside counsel, and fees of public relation advisors to advise the Company in connection with a possible contested solicitation of proxies, increased mailing costs, such as the costs of additional mailings of solicitation materials to stockholders, including printing costs, mailing costs and the reimbursement of reasonable expenses of banks, brokerage houses and other agents incurred in forwarding solicitation materials to beneficial owners, as described above, and the costs of retaining an independent inspector of election.

Householding of Annual Meeting Materials

Some banks, brokers and other nominee record holders may be participating in the practice of “householding” proxy statements and annual reports. This means that only one copy of our proxy statement or annual report may have been sent to multiple stockholders in the same household. We will promptly deliver a separate copy of either document to any stockholder upon request submitted in writing to us at Merrimack Pharmaceuticals, Inc., One Broadway, 14th Floor, Cambridge, MA 02142, Attention: Corporate Secretary, or by calling (617) 441-1000. Any stockholder who wants to receive separate copies of the annual report and proxy statement in the future, or who is currently receiving multiple copies and would like to receive only one copy for his or her household, should contact his or her bank, broker or other nominee record holder, or contact us at the above address or phone number.

Deadline for Submission of Stockholder Proposals for 2020 Annual Meeting of Stockholders

Proposals of stockholders intended to be presented at our 2020 Annual Meeting of Stockholders pursuant to Rule 14a-8 promulgated under the Exchange Act must be received by us at our principal executive offices, One Broadway, 14th Floor, Cambridge, MA 02142, no later than [            ] in order to be included in the proxy statement and proxy card relating to that meeting.

If a stockholder wishes to present a proposal at our 2020 Annual Meeting of Stockholders, but does not wish to have the proposal considered for inclusion in our proxy statement and proxy card, pursuant to the advance notice provision in our bylaws, such stockholder must give written notice to our Corporate Secretary at our principal executive offices at the address noted above. Our Corporate Secretary must receive such notice no earlier than June 19, 2020 and no later than July 19, 2020, provided that if the date of the 2020 Annual Meeting of Stockholders is advanced by more than 20 days, or delayed by more than 60 days, from the first anniversary of the Annual Meeting,

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such notice must instead be received by our Corporate Secretary no earlier than the 120^th day prior to the 2020 Annual Meeting of Stockholders and not later than the close of business on the later of (i) the 90^th day prior to the 2020 Annual Meeting of Stockholders and (ii) the tenth day following the day on which notice of the date of the 2020 Annual Meeting of Stockholders was mailed or public disclosure of the date of the 2020 Annual Meeting of Stockholders was made, whichever occurs first.

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IMPORTANT!

PLEASE SIGN AND SUBMIT THE WHITE PROXY CARD TODAY!

WE URGE YOU NOT TO SIGN ANY PROXY CARD OR VOTING

INSTRUCTION FORM SENT TO YOU BY THE JFL GROUP.

Remember, you can submit a proxy for your shares over the Internet or by telephone. Please follow the

easy instructions on the enclosed WHITE proxy card.

If you have any questions or need assistance in voting

your shares, please contact our proxy solicitor:

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Appendix A

Supplemental Information Concerning Participants in the Company’s Solicitation of Proxies

The following tables (“Directors and Nominees” and “Officers”) list the name and business address of the directors and nominees of Merrimack Pharmaceuticals, Inc. (the “Company”) and the name, present principal occupation and business address of the Company’s officers who, under SEC rules, are considered to be participants in the Company’s solicitation of proxies from its stockholders in connection with the Company’s 2019 Annual Meeting of Stockholders (collectively, the “Participants”).

Directors and Nominees

The principal occupations of the Company’s directors and nominees are included in the following table and additional information regarding the Company’s nominees is included in the biographies under the section above titled “Corporate Governance—Board of Directors—Biographies of the Director Nominees.” The names of each director and nominee are listed below, and the business addresses for all the directors and nominees is c/o Merrimack Pharmaceuticals, Inc., One Broadway, 14th Floor, Cambridge, MA 02142.

Officers

The Company’s sole executive officer is Gary L. Crocker, who is considered a Participant, and his principal occupation is included in his biography under the section titled “Corporate Governance—Board of Directors—Biographies of the Director Nominees.” Tim Surgenor is an advisor to the Company and the Company’s secretary, and is considered a Participant. Mr. Surgenor’s principal occupation is Partner at Red Sky Partners, LLC. The business address for Mr. Crocker and Mr. Surgenor is Merrimack Pharmaceuticals, Inc., One Broadway, 14th Floor, Cambridge, MA 02142.

Information Regarding Ownership of Company Securities By Participants

The number of shares of the Company’s common stock beneficially held as of July 31, 2019 by its directors and nominees and its sole executive officer who is a Participant appears in the “Security Ownership of Certain Beneficial Owners and Management” section of this proxy statement. Except as described in the proxy statement or this Appendix A, none of the persons listed above in “Directors and Nominees” and “Officers” owns any debt or equity security issued by us of record that he or she does not also own beneficially.

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Transactions in the Company’s Securities by Participants — Last Two Years

The following table sets forth information regarding purchases and sales of the Company’s securities by each Participant between July 31, 2017 and July 31, 2019, other than Ms. Radeljevic who did not have any such transactions. Unless otherwise indicated, all transactions were in the public market or pursuant to the Company’s equity compensation plans, and no part of the purchase price or market value of those shares is represented by funds borrowed or otherwise obtained for the purpose of acquiring or holding such securities.

Miscellaneous Information Regarding Participants

Except as described in the proxy statement or this Appendix A, to the Company’s knowledge: none of the Participants (i) beneficially owns (within the meaning of Rule 13d-3 under the Exchange Act), directly or indirectly, any shares or other securities of the Company or any of the Company’s subsidiaries, (ii) has purchased or sold any of such securities within the past two years, or (iii) is, or within the past year was, a party to any contract, arrangement or understanding with any person with respect to any such securities. Except as disclosed in the proxy statement or this Appendix A, no associates of a “participant” beneficially owns, directly or indirectly, any of our securities. Other than as disclosed in the proxy statement or this Appendix A, neither we nor any of the “participants” have a substantial interest, direct or indirect, by security holdings or otherwise, in any matter to be acted upon at the Annual Meeting. In addition, neither the Company nor any of the Participants has been within the past year party to any contract, arrangement or understanding with any person with respect to any of our securities, including, but not limited to, joint ventures, loan or option arrangements, puts or calls, guarantees against loss or guarantees of profit, division of losses or profits or the giving or withholding of proxies.

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Other than as set forth in the proxy statement or this Appendix A, none of the Participants or any of their associates have (i) any arrangements or understandings with any person with respect to any future employment by the Company or the Company’s affiliates or with respect to any future transactions to which the Company or any of its affiliates will or may be a party or (ii) a direct or indirect material interest in any transaction or series of similar transactions since the beginning of the fiscal year preceding the Company’s last fiscal year or any currently proposed transactions, to which the Company or any of its subsidiaries was or is to be a party in which the amount involved exceeded the lesser of (a) $120,000 or (b) one percent of the average of the Company’s total assets at year end for the Company’s last two completed fiscal years.

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IMPORTANT!

PLEASE SIGN AND SUBMIT THE WHITE PROXY CARD TODAY!

WE URGE YOU NOT TO SIGN ANY PROXY CARD OR VOTING

INSTRUCTION FORM SENT TO YOU BY THE JFL GROUP.

Remember, you can submit a proxy for your shares over the Internet or by telephone. Please follow the

easy instructions on the enclosed WHITE proxy card.

If you have any questions or need assistance in voting

your shares, please contact our proxy solicitor:

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REVISED PRELIMINARY COPY SUBJECT TO COMPLETION

MERRIMACK PHARMACEUTICALS, INC.

Annual Meeting of Stockholders

October 17, 2019 9:00 a.m., Eastern time

This proxy is solicited by the Board of Directors

		The stockholder(s) hereby appoint(s) Gary L. Crocker and Ulrik B. Nielsen, or either of them, as proxies, each with the power to appoint his substitute, and hereby authorize(s) them to represent and to vote, as designated on the reverse side of this proxy, all of the shares of common stock of MERRIMACK PHARMACEUTICALS, INC. that the stockholder(s) is/are entitled to vote at the Annual Meeting of Stockholders to be held at 9:00 a.m., Eastern time, on October 17, 2019, at the offices of Wilmer Cutler Pickering Hale and Dorr LLP, 60 State Street, Boston, MA 02109 or any adjournment or postponement thereof. The proxies are further authorized to vote, in their discretion, for the election of a person to the Board of Directors if any nominee named herein becomes unable to serve or for good cause will not serve, upon all matters incident to the conduct of the meeting, and upon such other business as may properly come before the meeting or any adjournment or postponement thereof, to the extent permitted by Rule 14a-4(c) of the Securities Exchange Act of 1934, as amended.   This proxy, when properly executed, will be voted in the manner directed herein. If no such direction is made, this proxy will be voted in accordance with the Board of Directors’ recommendations.   Please mark, sign, date and return this proxy card promptly using the enclosed reply envelope. If you vote your proxy by Internet or Telephone, you do NOT need to mail back your proxy card. This proxy revokes all previous proxies.

PLEASE SIGN, DATE AND RETURN THIS PROXY CARD PROMPTLY USING THE ENCLOSED POSTAGE PAID ENVELOPE

p TO VOTE BY MAIL, PLEASE DETACH HERE p

Your telephone or internet proxy authorizes the named proxies to vote your shares in the same manner as if you marked, signed and returned your proxy card.

AUTHORIZE YOUR PROXY BY INTERNET: THE WEB ADDRESS IS www.proxyvoting.com/[    ]. You will be asked to enter a CONTROL NUMBER which is located in the lower right hand corner of this form.

AUTHORIZE YOUR PROXY BY PHONE: You will be asked to enter a CONTROL NUMBER which is located in the lower right hand corner of this form.

Call  ê  ê  Toll Free  ê  ê  

1-800-[    ]

There is NO CHARGE to you for this call

Internet and Telephone voting is available through 11:59 P.M.		CONTROL NUMBER
Eastern Time on October 16, 2019.		for Telephone/Internet Proxy Authorization