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Merrimack Pharmaceuticals, Inc.

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*** Exercise Your Right to Vote *** Important Notice Regarding the Availability of Proxy Materials for the Stockholder Meeting to Be Held on June 12, 2018. Meeting Information MERRIMACK PHARMACEUTICALS, INC.  Meeting Type: Annual Meeting For holders as of: April 17, 2018 Date: June 12, 2018 Time: 4:00 PM ET Location: Merrimack Pharmaceuticals, Inc.  One Kendall Square, Suite B7201 Cambridge, Massachusetts 02139 You are receiving this communication because you hold shares in the company named above. This is not a ballot. You cannot use this notice to vote these shares. This communication presents only an overview of the more complete proxy materials that are available to you on the Internet. You may view the proxy materials online at or easily request a paper copy (see reverse side).  We encourage you to access and review all of the important information contained in the proxy materials before voting.  E46396-P08670 MERRIMACK PHARMACEUTICALS, INC.  ONE KENDALL SQUARE SUITE B7201 CAMBRIDGE, MA 02139See the reverse side of this notice to obtain proxy materials and voting instructions.


Before You VoteHow to Access the Proxy MaterialsProxy Materials Available to VIEW or RECEIVE:1. NOTICE, PROXY STATEMENT AND PROXY CARD 2. ANNUAL REPORT TO STOCKHOLDERSHow to View Online:Have the information that is printed in the box marked by the arrow (located on the following page) and visit: to Request and Receive a PAPER or E-MAIL Copy:If you want to receive a paper or e-mail copy of these documents, you must request one. There is NO charge for requesting a copy. Please choose one of the following methods to make your request:  1) BY INTERNET: 2) BY TELEPHONE: 1-800-579-1639 3) BY E-MAIL*:* If requesting materials by e-mail, please send a blank e-mail with the information that is printed in the box marked by the arrow (located on the following page) in the subject line..XXXX XXXX XXXX XXXXE46397-P08670.XXXX XXXX XXXX XXXXRequests, instructions and other inquiries sent to this e-mail address will NOT be forwarded to your investment advisor. Please make the request as instructed above on or before May 29, 2018 to facilitate timely delivery.How To VotePlease Choose One of the Following Voting MethodsVote In Person: Many stockholder meetings have attendance requirements including, but not limited to, the possession of an attendance ticket issued by the entity holding the meeting. Please check the meeting materials for any special requirements for meeting attendance. At the meeting, you will need to request a ballot to vote these shares. Vote By Internet: To vote now by Internet, go to Have the information that is printed in the boxmarked by the arrow (located on the following page) available and follow the instructions. Vote By Mail: You can vote by mail by requesting a paper copy of the materials, which will include a proxy card..XXXX XXXX XXXX XXXX


Voting ItemsThe Board of Directors recommends you vote FOR each director nominee:1. Election of Directors Nominees: 01) Richard Peters, M.D., Ph.D.  02) Gary L. Crocker 03) George D. Demetri, M.D. 04) John M. Dineen 05) Ulrik B. Nielsen, Ph.D. 06) James H. Quigley 07) Russell T. RayThe Board of Directors recommends you vote FOR the following proposals:2. To approve, on an advisory basis, our executive compensation.3. To ratify the selection of PricewaterhouseCoopers LLP as our independent registered public accounting firm for the fiscal year ending December 31, 2018.4. To approve an amendment to our certificate of incorporation to increase the number of authorized shares of common stock from20,000,000 to 30,000,000.E46398-P08670NOTE: The proxies are authorized to vote, in their discretion, upon such other business as may properly come before the meeting or any adjournment or postponement thereof.