X0306 4 2017-12-14 0 0001274792 MERRIMACK PHARMACEUTICALS INC MACK 0001639147 Demetri George C/O MERRIMACK PHARMACEUTICALS, INC. ONE KENDALL SQUARE, SUITE B7201 CAMBRIDGE MA 02139 1 0 0 0 Stock Option (right to buy) 10.18 2017-12-14 4 A 0 4890 0.00 A 2027-12-13 Common Stock 4890 4890 D This option vests as to 1/4th of the total number of shares on 3/14/18 and an additional 1/4th of the total number of shares at the end of each successive three month period thereafter until 12/14/18, 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 12/14/18). /s/ Brian J. Kickham, attorney-in-fact 2017-12-15