XNAS:MACK Merrimack Pharmaceuticals Inc Insider Activity 4 Filing - 4/3/2012

Effective Date 4/3/2012

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FORM 4
Check this box if no longer subject to Section 16. Form 4 or Form 5 obligations may continue. See Instruction 1(b).
UNITED STATES SECURITIES AND EXCHANGE COMMISSION
Washington, D.C. 20549

STATEMENT OF CHANGES IN BENEFICIAL OWNERSHIP OF SECURITIES

Filed pursuant to Section 16(a) of the Securities Exchange Act of 1934, Section 17(a) of the Public Utility Holding Company Act of 1935 or Section 30(h) of the Investment Company Act of 1940
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(Print or Type Responses)
1. Name and Address of Reporting Person *
Sinskey Anthony J
  2. Issuer Name and Ticker or Trading Symbol
MERRIMACK PHARMACEUTICALS INC [MACK]
5. Relationship of Reporting Person(s) to Issuer
(Check all applicable)
__X__ Director _____ 10% Owner
_____ Officer (give title below) _____ Other (specify below)
(Last)
(First)
(Middle)
C/O MERRIMACK PHARMACEUTICALS, INC., ONE KENDALL SQUARE, SUITE B7201
3. Date of Earliest Transaction (Month/Day/Year)
04/03/2012
(Street)

CAMBRIDGE, MA 02139
4. If Amendment, Date 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
(City)
(State)
(Zip)
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
Common Stock 04/03/2012   P   7,142 A $7 326,309 D  
Common Stock 04/03/2012   C   36,723 A (1) 363,032 D  
Common Stock 04/03/2012   C   18,024 A (2) 381,056 I By Wife

Reminder: Report on a separate line for each class of securities beneficially owned directly or indirectly.
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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 Underlying Securities
(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 of Derivative Security: 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
Series B Convertible Preferred Stock (1) 04/03/2012   C     23,793   (1)   (1) Common Stock 36,723(1) $ 0 0 D  
Series C Convertible Preferred Stock (2) 04/03/2012   C     18,024   (2)   (2) Common Stock 18,024 $ 0 0 I By Wife

Reporting Owners

Reporting Owner Name / Address Relationships
 Director  10% Owner  Officer  Other
Sinskey Anthony J
C/O MERRIMACK PHARMACEUTICALS, INC.
ONE KENDALL SQUARE, SUITE B7201
CAMBRIDGE, MA 02139
  X      

Signatures

 /s/ Jeffrey A. Munsie, attorney-in-fact   04/05/2012
** Signature of Reporting Person Date

Explanation of Responses:

* 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).
(1) Each share of Series B Convertible Preferred Stock automatically converted into approximately 1.5435 shares of Common Stock upon the closing of the issuer's initial public offering and had no expiration date.
(2) Each share of Series C Convertible Preferred Stock automatically converted into one share of Common Stock upon the closing of the issuer's initial public offering and had no expiration date.

Note: File three copies of this Form, one of which must be manually signed. If space is insufficient, see Instruction 6 for procedure.

Potential persons who are to respond to the collection of information contained in this form are not required to respond unless the form displays a currently valid OMB number.

XNAS:MACK Merrimack Pharmaceuticals Inc Insider Activity 4 Filling

Merrimack Pharmaceuticals Inc XNAS:MACK Stock - Get Insider Activity SEC Filing of Merrimack Pharmaceuticals Inc XNAS:MACK stocks, including company profile, shares outstanding, strategy, business segments, operations, officers, consolidated financial statements, financial notes and ownership information.

XNAS:MACK Merrimack Pharmaceuticals Inc Insider Activity 4 Filing - 4/3/2012
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