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In a dramatic move that reshapes the biopharma landscape, Day One Biopharmaceuticals revealed Thursday that it will acquire Mersana Therapeutics Inc., a clinical-stage oncology innovator, in a deal that could reach $285 million. The transaction, steered by Fenwick & West LLP for Day One and WilmerHale for Mersana, marks a decisive step in expanding the pediatric cancer–focused company’s therapeutic arsenal.

A Take-Private Deal With Big-Ticket Milestones

Under the agreement, Day One will buy Mersana for $25 per share upfront and up to $30.25 per share in additional cash if key clinical, developmental, and regulatory milestones are met. That structure reflects a $129 million equity value at closing, but with room to nearly double as breakthroughs unfold.

Mersana—backed by Bain Capital Life Sciences—specializes in antibody-drug conjugates aimed at cancers with few, if any, effective treatments. Its standout program is Emi-Le, a first-in-class monotherapy targeting a rare, aggressive cancer often originating in the salivary glands.

A Potential “Game-Changer” for the Pipeline

Day One CEO Jeremy Bender said the acquisition represents a strategic expansion of the company’s mission to deliver impactful oncology therapies:

“This acquisition will add a potential game-changing new medicine to the Day One portfolio,” Bender said. “If approved, Emi-Le could significantly broaden our opportunities for patient impact, growth, and long-term value.”

He emphasized that Day One’s research, development, and commercial infrastructure is primed to accelerate therapies for rare and life-threatening cancers affecting patients of all ages.

Shareholder Commitments and Delisting Ahead

A separate statement from Mersana confirmed that its executive team, board members, and shareholders affiliated with Bain—together holding about 8.5% of outstanding shares—have agreed to tender their stock.

Once the deal closes—expected by the end of January—Mersana will become a wholly owned Day One subsidiary, and its shares will be removed from public markets.

Leadership Signals Confidence in the Merger

Mersana President and CEO Marty Huber praised the agreement, calling it a recognition of the innovation behind the Emi-Le program and the company’s ADC technology.

“We believe this proposed acquisition reflects the value created by Mersana,” Huber said. “Combining our programs with Day One’s capabilities may help accelerate the delivery of more medicines to patients waiting for new therapies.”

Details on the full legal teams from Fenwick and WilmerHale were not immediately available Thursday.

As oncology partnerships intensify and innovation races forward, Day One’s purchase of Mersana stands as a bold bid to shape the next frontier of precision cancer treatment.