X0306 4 2012-08-23 0 0001274792 MERRIMACK PHARMACEUTICALS INC MACK 0001234382 MENDELSOHN JOHN C/O MERRIMACK PHARMACEUTICALS, INC. ONE KENDALL SQUARE, SUITE B7201 CAMBRIDGE MA 02139 1 0 0 0 Stock Option (right to buy) 7.53 2012-08-23 4 A 0 21300 0 A 2022-08-22 Common Stock 21300 21300 D This option vests as to 1/4th of the total number of shares on 11/23/12 and an additional 1/4th of the total number of shares at the end of each successive three month period thereafter until 8/23/13, provided that such option shall vest in full on the date that is one business day prior to the date of the issuer's next annual meeting of stockholders (if earlier than 8/23/13) or in the event of a change in control (as determined by the issuer's board of directors). /s/ Jeffrey A. Munsie, attorney-in-fact 2012-08-27