X0205 3 2012-06-13 1 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 /s/ Jeffrey A. Munsie, attorney-in-fact 2012-06-19