On November 13, 2025, Day One Biopharmaceuticals (Day One), a biopharmaceutical company dedicated to developing and commercializing targeted therapies for people of all ages with life-threatening diseases, announced it has signed a definitive merger agreement to acquire Mersana Therapeutics (Mersana). Wilson Sonsini Goodrich & Rosati advised Day One on IP due diligence related to the transaction.

The acquisition will enhance Day One's oncology portfolio with the addition of Emi-Le, a potential first-in-class treatment for ACC, a rare and aggressive cancer. Emi-Le, currently in a Phase 1 study, has shown early anti-tumor activity and targets B7-H4.

Under the terms of the definitive merger agreement, Day One will initiate a tender offer to acquire all outstanding shares of Mersana Therapeutics at $25.00 per share in cash. Additionally, shareholders will receive a non-tradable CVR for potential milestone payments totaling up to $30.25 per share. This brings the total consideration of up to $55.25 per share, representing a total equity value of approximately $129 million at closing and a total deal value of up to approximately $285 million.

The Wilson Sonsini team that advised Day One on IP matters related to the transaction includes:

Julia Minitti
Maya Skubatch
Luis Perla
Ying Chen
Grant Margulieux
Jamie Gleason-Bremer
Tina Xiang

For more information, please see Day One's news release. Additional coverage can be found on Fierce Biotech.