X0306 4 2014-05-07 0 0001274792 MERRIMACK PHARMACEUTICALS INC MACK 0001108494 PORTER MICHAEL E C/O MERRIMACK PHARMACEUTICALS, INC. ONE KENDALL SQUARE, SUITE B7201 CAMBRIDGE MA 02139 1 0 0 0 Common Stock 2014-05-07 4 P 0 5000 6.70 A 495865 D Common Stock 2014-05-08 4 M 0 10000 1.71 A 505865 D Common Stock 2014-05-08 4 M 0 2500 2.59 A 508365 D Common Stock 2014-05-08 4 M 0 2500 1.81 A 510865 D Common Stock 2014-05-08 4 M 0 5000 2.12 A 515865 D Common Stock 2014-05-08 4 P 0 5000 6.48 A 520865 D Stock Option (right to buy) 1.71 2014-05-08 4 M 0 10000 0 D 2015-08-03 Common Stock 10000 0 D Stock Option (right to buy) 2.59 2014-05-08 4 M 0 2500 0 D 2017-10-04 Common Stock 2500 0 D Stock Option (right to buy) 1.81 2014-05-08 4 M 0 2500 0 D 2018-09-21 Common Stock 2500 0 D Stock Option (right to buy) 2.12 2014-05-08 4 M 0 5000 0 D 2019-11-04 Common Stock 5000 0 D The price reported in Column 4 is a weighted average price. These shares were purchased in multiple transactions at prices ranging from $6.6999 to $6.70, 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 purchased at each separate price within such range. The price reported in Column 4 is a weighted average price. These shares were purchased in multiple transactions at prices ranging from $6.4699 to $6.50, 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 purchased at each separate price within such range. This option is fully vested. /s/ Jeffrey A. Munsie, attorney-in-fact 2014-05-09