On November 13, 2025, Mersana Therapeutics, Inc. (Nasdaq: MRSN), a clinical-stage biopharmaceutical company focused on developing antibody-drug conjugates targeting cancers in areas of high unmet medical need, announced that it had entered into a definitive merger agreement with Day One Biopharmaceuticals, Inc., under which Day One will acquire Mersana. Under the terms of the agreement, Day One will commence a tender offer to acquire all outstanding shares of Mersana common stock at a price of $25 per share in cash at closing, plus one non-tradable contingent value right per share to receive certain milestone payments of up to an aggregate of $30.25 per CVR in cash. This represents total consideration of up to $55.25 per share in cash, a total equity value of approximately $129 million at closing, and a potential total deal value of up to approximately $285 million.

WilmerHale is representing Mersana in the transaction. 

The WilmerHale team is led by Partners Chris Barnstable-Brown and Stuart Falber, and includes Counsels Andrew Langworthy, Andrea Sorrentino, and Sarah Matchett; Senior Associate Michaela Rosen; and Associate Sophie Bonadeo on M&A, corporate, and securities matters. Additional team members include Partners Steve Barrett, Caroline Dotolo, Ciara Baker, David Strong, Bruce Manheim, Dominic Vote, and Kirk Nahra; Counsels Mat Trachok, Ben Kelsey, and Ali Jessani; and Attorney Carla Gilbertson on licensing, intellectual property, equity compensation, tax, public company counseling, regulatory, antitrust, and privacy matters.