Merrimack Pharmaceuticals (MACK) 8-K Changes in Registrant's Certifying Accountant

Exhibit 16.1

July 17, 2019

Securities and Exchange Commission

100 F Street, N.E.

Washington, DC 20549

Commissioners:

We have read the statements made by Merrimack Pharmaceuticals, Inc. (copy attached), which we understand will be filed with the Securities and Exchange Commission, pursuant to Item 4.01 of Form8-K of Merrimack Pharmaceuticals, Inc. dated July 15, 2019. We agree with the statements concerning our Firm contained therein.

Very truly yours,

/s/ PricewaterhouseCoopers LLP

Boston, Massachusetts

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