SEC FORM 3

SEC Form 3

FORM 3

UNITED STATES SECURITIES AND EXCHANGE COMMISSION
Washington, D.C. 20549

INITIAL STATEMENT OF BENEFICIAL OWNERSHIP OF SECURITIES

Filed pursuant to Section 16(a) of the Securities Exchange Act of 1934
or Section 30(h) of the Investment Company Act of 1940

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1. Name and Address of Reporting Person^*

Nielsen Ulrik B.

(Last)

(First)

(Middle)

C/O MERRIMACK PHARMACEUTICALS, INC.

ONE KENDALL SQUARE, SUITE B7201

(Street)

CAMBRIDGE

02139

(City)

(State)

(Zip)

2. Date of Event Requiring Statement (Month/Day/Year)
01/31/2012

3. Issuer Name and Ticker or Trading Symbol
MERRIMACK PHARMACEUTICALS INC [ MACK ]

4. Relationship of Reporting Person(s) to Issuer
(Check all applicable)

	Director	10% Owner
X	Officer (give title below)	Other (specify below)
	SVP and CSO

5. 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 Beneficially Owned
1. Title of Security (Instr. 4)	2. Amount of Securities Beneficially Owned (Instr. 4)	3. Ownership Form: Direct (D) or Indirect (I) (Instr. 5)	4. Nature of Indirect Beneficial Ownership (Instr. 5)
Common Stock	247,443	D

Table II - Derivative Securities Beneficially Owned (e.g., puts, calls, warrants, options, convertible securities)
1. Title of Derivative Security (Instr. 4)	2. Date Exercisable and Expiration Date (Month/Day/Year)		3. Title and Amount of Securities Underlying Derivative Security (Instr. 4)		4. Conversion or Exercise Price of Derivative Security	5. Ownership Form: Direct (D) or Indirect (I) (Instr. 5)	6. Nature of Indirect Beneficial Ownership (Instr. 5)
1. Title of Derivative Security (Instr. 4)	Date Exercisable	Expiration Date	Title	Amount or Number of Shares	4. Conversion or Exercise Price of Derivative Security	5. Ownership Form: Direct (D) or Indirect (I) (Instr. 5)	6. Nature of Indirect Beneficial Ownership (Instr. 5)
Stock Option (right to buy)	⁽¹⁾	08/02/2012	Common Stock	4,368	$2.19	D
Stock Option (right to buy)	⁽¹⁾	05/08/2013	Common Stock	10,483	$2.19	D
Stock Option (right to buy)	⁽¹⁾	08/30/2014	Common Stock	150,000	$1.25	D
Stock Option (right to buy)	⁽¹⁾	08/03/2015	Common Stock	82,977	$1.71	D
Stock Option (right to buy)	⁽¹⁾	08/03/2015	Common Stock	17,023	$1.71	D
Stock Option (right to buy)	⁽¹⁾	10/03/2016	Common Stock	48,175	$2.47	D
Stock Option (right to buy)	⁽¹⁾	10/03/2016	Common Stock	26,825	$2.47	D
Stock Option (right to buy)	⁽¹⁾	10/04/2017	Common Stock	53,378	$2.59	D
Stock Option (right to buy)	⁽¹⁾	10/04/2017	Common Stock	146,622	$2.59	D
Stock Option (right to buy)	⁽¹⁾	09/21/2018	Common Stock	250,000	$1.81	D
Stock Option (right to buy)	⁽²⁾	11/04/2019	Common Stock	180,000	$2.12	D
Stock Option (right to buy)	⁽³⁾	01/31/2020	Common Stock	100,000	$2.12	D
Stock Option (right to buy)	⁽⁴⁾	10/14/2020	Common Stock	60,000	$2.69	D
Stock Option (right to buy)	⁽⁵⁾	12/21/2020	Common Stock	50,000	$2.69	D
Stock Option (right to buy)	⁽⁶⁾	05/02/2021	Common Stock	100,000	$5.54	D

Explanation of Responses:

1. This option is fully vested.

2. This option vested as to 1/12th of the shares on each of November 5, 2009 and February 1, 2010 and vests in equal quarterly installments thereafter until August 1, 2012.

3. This option vested as to 1/12th of the shares on April 1, 2010 and vests in equal quarterly installments thereafter until January 1, 2013.

4. This option vested as to 1/12th of the shares on each of October 15, 2010 and January 1, 2011 and vests in equal quarterly installments thereafter until July 1, 2013.

5. This option vested as to 1/6th of the shares on January 1, 2011 and vests in equal quarterly installments thereafter until July 1, 2013.

6. This option vested as to 1/12th of the shares on August 1, 2011 and vests in equal quarterly installments thereafter until May 1, 2014.

	/s/ Jeffrey A. Munsie, attorney-in-fact	02/01/2012
	** 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 5 (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.