X0204 3 2012-01-31 0 0001274792 MERRIMACK PHARMACEUTICALS INC MACK 0001538792 McClements William M. C/O MERRIMACK PHARMACEUTICALS, INC. ONE KENDALL SQUARE, SUITE B7201 CAMBRIDGE MA 02139 0 1 0 0 SVP of Corporate Operations Stock Option (right to buy) 6.7800 2021-11-01 Common Stock 150000 D This option vests as to 1/6th of the shares on March 19, 2012 and vests in equal quarterly installments thereafter until September 19, 2014. /s/ William M. McClements 2012-02-01