X0306 4 2014-05-27 0 0001274792 MERRIMACK PHARMACEUTICALS INC MACK 0001531564 Stewart Edward J. C/O MERRIMACK PHARMACEUTICALS, INC. ONE KENDALL SQUARE, SUITE B7201 CAMBRIDGE MA 02139 0 1 0 0 SVP & Pres, Healthcare Sol Common Stock 2014-05-27 4 M 0 16000 1.25 A 37385 D Common Stock 2014-05-27 4 S 0 16000 8.00 D 21385 D Stock Option (right to buy) 1.25 2014-05-27 4 M 0 16000 0 D 2014-08-30 Common Stock 16000 24000 D This transaction was effected pursuant to a Rule 10b5-1 trading plan. The price reported in Column 4 is a weighted average price. These shares were sold in multiple transactions at prices ranging from $8.00 to $8.04, inclusive. The reporting person undertakes to provide to the Issuer, any security holder of the Issuer or the Staff of the Securities and Exchange Commission, upon request, full information regarding the number of shares sold at each separate price within such range. This option is fully vested. /s/ Jeffrey A. Munsie, attorney-in-fact 2014-05-29