Item 1.01. Entry into a Material Definitive Agreement.

On August 24, 2012, Merrimack Pharmaceuticals, Inc. (the "Company") and RB Kendall Fee, LLC (the "Landlord") entered into an Indenture of Lease (the "Amended Lease"), which amends and restates the Indenture of Lease between the Company and the Landlord dated as of May 12, 2006, as previously amended on March 23, 2007, July 1, 2007, April 1, 2008, November 17, 2008, July 6, 2009, January 27, 2010, June 29, 2010, March 31, 2011 and March 8, 2012 (the "Original Lease"), under which the Company will continue to lease research, manufacturing and office space at One Kendall Square in Cambridge, Massachusetts (the "Facility").

The Amended Lease provides that, among other things, as of various times no later than April 1, 2013, the Company will lease from the Landlord an additional 23,250 square feet of the Facility, for a total of 109,132 square feet (the "Leased Space"), all of which will be leased until June 30, 2019. The rent for the Leased Space will be an average of $40.11 per rentable square foot per year and will increase to an average of $46.89 per rentable square foot per year by the end of the lease term. Under the terms of the Amended Lease, the Landlord will provide the Company with a finish work allowance of up to approximately $6,585,000.

The Amended Lease expires on June 30, 2019. The Company retains an option to renew the Amended Lease with respect to all of the Leased Space for an additional period of either one or five years.

The foregoing description of the Amended Lease does not purport to be complete and is qualified in its entirety by reference to the Amended Lease, which the Company expects to file as an exhibit to its Form 10-Q for the quarter ending September 30, 2012.

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