Exhibit 99.1

JOINT FILING AGREEMENT

In accordance with Rule 13d−1(k)(1) under the Securities Exchange Act of 1934, as amended, the persons named below agree to the joint filing on behalf of each of them of a statement on Schedule 13G (including amendments thereto) with respect to the Common Stock of Mersana Therapeutics, Inc. and further agree that this Joint Filing Agreement be included as an Exhibit to such joint filings. In evidence thereof, the undersigned, being duly authorized, have executed this Joint Filing Agreement this 25^th day of January, 2021.

SARISSA CAPITAL MANAGEMENT LP

By:	/s/ Mark DiPaolo
	Name: Mark DiPaolo
	Title: Senior Partner and General Counsel

/s/Alexander J. Denner                                    
Alexander J. Denner