On January 15, 2015, Ulrik B. Nielsen provided notice of his resignation as Senior Vice President and Chief Scientific Officer of Merrimack Pharmaceuticals, Inc. (the Company), effective as of January 30, 2015. On January 19, 2015, Anthony J. Sinskey provided notice of his resignation from the Company's Board of Directors (the Board), effective as of January 30, 2015. Dr. Sinskey will continue to serve the Company as a scientific advisor.

On January 20, 2015, the Board elected Dr. Nielsen as a director of the Company, effective as of January 30, 2015, to fill the vacancy created by the resignation of Dr. Sinskey.