Washington, D.C. 20549


Filed pursuant to Section 16(a) of the Securities Exchange Act of 1934
or Section 30(h) of the Investment Company Act of 1940
OMB Number: 3235-0287
Estimated average burden
hours per response: 0.5
Check this box if no longer subject to Section 16. Form 4 or Form 5 obligations may continue. See Instruction 1(b).
1. Name and Address of Reporting Person*
Santillana Sergio L.

(Last) (First) (Middle)


(City) (State) (Zip)
2. Issuer Name and Ticker or Trading Symbol
5. Relationship of Reporting Person(s) to Issuer
(Check all applicable)
Director 10% Owner
X Officer (give title below) Other (specify below)
Chief Medical Officer
3. Date of Earliest Transaction (Month/Day/Year)
4. If Amendment, Date of Original Filed (Month/Day/Year)
6. Individual or Joint/Group Filing (Check Applicable Line)
X Form filed by One Reporting Person
Form filed by More than One Reporting Person
Table I - Non-Derivative Securities Acquired, Disposed of, or Beneficially Owned
1. Title of Security (Instr. 3) 2. Transaction Date (Month/Day/Year) 2A. Deemed Execution Date, if any (Month/Day/Year) 3. Transaction Code (Instr. 8) 4. Securities Acquired (A) or Disposed Of (D) (Instr. 3, 4 and 5) 5. Amount of Securities Beneficially Owned Following Reported Transaction(s) (Instr. 3 and 4) 6. Ownership Form: Direct (D) or Indirect (I) (Instr. 4) 7. Nature of Indirect Beneficial Ownership (Instr. 4)
Code V Amount (A) or (D) Price
Table II - Derivative Securities Acquired, Disposed of, or Beneficially Owned
(e.g., puts, calls, warrants, options, convertible securities)
1. Title of Derivative Security (Instr. 3) 2. Conversion or Exercise Price of Derivative Security 3. Transaction Date (Month/Day/Year) 3A. Deemed Execution Date, if any (Month/Day/Year) 4. Transaction Code (Instr. 8) 5. Number of Derivative Securities Acquired (A) or Disposed of (D) (Instr. 3, 4 and 5) 6. Date Exercisable and Expiration Date (Month/Day/Year) 7. Title and Amount of Securities Underlying Derivative Security (Instr. 3 and 4) 8. Price of Derivative Security (Instr. 5) 9. Number of derivative Securities Beneficially Owned Following Reported Transaction(s) (Instr. 4) 10. Ownership Form: Direct (D) or Indirect (I) (Instr. 4) 11. Nature of Indirect Beneficial Ownership (Instr. 4)
Code V (A) (D) Date Exercisable Expiration Date Title Amount or Number of Shares
Stock Option (right to buy) $10.74 02/15/2018 A 45,000 (1) 02/14/2028 Common Stock 45,000 $0.00 45,000 D
Explanation of Responses:
1. This option vests as to 1/12th of the total number of shares granted on 5/15/18 and an additional 1/12th of the total number of shares granted at the end of each successive three month period thereafter until 2/15/21.
/s/ Brian J. Kickham, attorney-in-fact 02/16/2018
** Signature of Reporting Person Date
Reminder: Report on a separate line for each class of securities beneficially owned directly or indirectly.
* If the form is filed by more than one reporting person, see Instruction 4 (b)(v).
** Intentional misstatements or omissions of facts constitute Federal Criminal Violations See 18 U.S.C. 1001 and 15 U.S.C. 78ff(a).
Note: File three copies of this Form, one of which must be manually signed. If space is insufficient, see Instruction 6 for procedure.
Persons who respond to the collection of information contained in this form are not required to respond unless the form displays a currently valid OMB Number.

Know all by these presents, that the undersigned 
hereby makes, constitutes and appoints each of 
John L. Green, Brian J. Kickham and Jeffrey A. Munsie, 
signing singly and each acting individually, as the 
undersigned's true and lawful attorney-in-fact with 
full power and authority as hereinafter described to:
(1)   execute for and on behalf of the undersigned, in 
the undersigned's capacity as an Officer of
Merrimack Pharmaceuticals, Inc. (the "Company"), Forms 3,
4, and 5 (including any amendments thereto) in accordance 
with Section 16(a) of the Securities Exchange Act of 
1934 and the rules thereunder (the "Exchange Act");
(2)   do and perform any and all acts for and on 
behalf of the undersigned which may be necessary or 
desirable to prepare, complete and execute any such 
Form 3, 4, or 5, prepare, complete and execute any 
amendment or amendments thereto, and timely deliver 
and file such form with the United States Securities 
and Exchange Commission (the "SEC") and any stock 
exchange or similar authority, including without 
limitation the filing of a Form ID or any other 
application materials to enable the undersigned to 
gain or maintain access to the Electronic Data 
 Analysis and Retrieval system of the SEC;
(3)   seek or obtain, as the undersigned's 
representative and on the undersigned's behalf, 
information regarding transactions in the Company's 
securities from any third party, including brokers, 
employee benefit plan administrators and trustees, and 
the undersigned hereby authorizes any such third party 
to release any such information to the herein 
appointed attorney-in-fact and approves and ratifies 
any such release of information; and
(4)   take any other action of any type whatsoever in 
connection with the foregoing which, in the opinion of 
such attorney-in-fact, may be of benefit to, in the 
best interest of, or legally required by, the 
undersigned, it being understood that the documents 
executed by such attorney-in-fact on behalf of the 
undersigned pursuant to this Power of Attorney shall 
be in such form and shall contain such terms and 
conditions as such attorney-in-fact may approve in 
such attorney-in-fact's discretion.
The undersigned hereby grants to each such 
attorney-in-fact full power and authority to do and 
perform any and every act and thing whatsoever 
requisite, necessary, or proper to be done in the 
exercise of any of the rights and powers herein 
granted, as fully to all intents and purposes as the 
undersigned might or could do if personally present, 
with full power of substitution or revocation, hereby 
ratifying and confirming all that such 
attorney-in-fact, or such attorney-in-fact's 
substitute or substitutes, shall lawfully do or cause 
to be done by virtue of this power of attorney and the 
rights and powers herein granted.  The undersigned 
acknowledges that the foregoing attorneys-in-fact, in 
serving in such capacity at the request of the 
undersigned, are not assuming nor relieving, nor is 
the Company assuming nor relieving, any of the 
undersigned's responsibilities to comply with 
Section 16 of the Exchange Act.  The undersigned 
acknowledges that neither the Company nor the 
foregoing attorneys-in-fact assume (i) any liability 
for the undersigned's responsibility to comply with 
the requirement of the Exchange Act, (ii) any 
liability of the undersigned for any failure to comply 
with such requirements, or (iii) any obligation or 
liability of the undersigned for profit disgorgement 
under Section 16(b) of the Exchange Act.
This Power of Attorney shall remain in full force and 
effect until the undersigned is no longer required to 
file Forms 3, 4, and 5 with the SEC with respect to 
the undersigned's holdings of and transactions in 
securities issued by the Company, unless earlier 
revoked by the undersigned in a signed writing 
delivered to the foregoing attorneys-in-fact.
IN WITNESS WHEREOF, the undersigned has caused this 
Power of Attorney to be executed as of this 6th day of 
June, 2017.

/s/ Sergio L. Santillana
Sergio L. Santillana