X0204 3 2012-01-31 0 0001274792 MERRIMACK PHARMACEUTICALS INC MACK 0001032566 GAY ROBERT C C/O MERRIMACK PHARMACEUTICALS, INC. ONE KENDALL SQUARE, SUITE B7201 CAMBRIDGE MA 02139 1 0 0 0 Series B Convertible Preferred Stock Common Stock 113586 D Series D Convertible Preferred Stock Common Stock 142857 D Series B Convertible Preferred Stock Common Stock 113586 I See footnote Series D Convertible Preferred Stock Common Stock 142857 I See footnote Stock Option (right to buy) 2.5900 2017-10-22 Common Stock 60000 D Stock Option (right to buy) 1.8100 2018-09-21 Common Stock 20000 D Stock Option (right to buy) 2.1200 2019-11-04 Common Stock 20000 D Stock Option (right to buy) 5.5400 2021-05-02 Common Stock 53000 D Common Stock Warrant (right to buy) 3.0000 2015-12-17 Common Stock 8766 D Common Stock Warrant (right to buy) 3.0000 2015-12-17 Common Stock 8766 I See footnote Each share of Series B Convertible Preferred Stock will automatically convert into approximately 1.5435 shares of Common Stock upon the closing of the issuer's initial public offering. The shares have no expiration date. Each share of Series D Convertible Preferred Stock will automatically convert into one share of Common Stock upon the closing of the issuer's initial public offering. The shares have no expiration date. These shares are held by the Robert C. Gay 1998 Family Trust, of which Dr. Gay is the grantor. Dr. Gay disclaims beneficial ownership of these shares, except to the extent of his pecuniary interest therein, and the inclusion of these shares in this report shall not be deemed an admission of beneficial ownership of any such shares for purposes of Section 16 of the Securities Exchange Act of 1934, as amended, or for any other purpose. This security is fully vested. This warrant is held by the Robert C. Gay 1998 Family Trust, of which Dr. Gay is the grantor. Dr. Gay disclaims beneficial ownership of this warrant and the shares underlying this warrant, except to the extent of his pecuniary interest therein, and the inclusion of this warrant in this report shall not be deemed an admission of beneficial ownership of this warrant or any shares underlying this warrant for purposes of Section 16 of the Securities Exchange Act of 1934, as amended, or for any other purpose. /s/ Jeffrey A. Munsie, attorney-in-fact 2012-02-01