Day One Biopharmaceuticals announced that it will acquire Mersana Therapeutics in a transaction valued at up to $285 million. Mersana shareholders will receive $25.00 in cash per share and up to $30.25 per share in contingent value rights (CVRs) that will be paid if certain clinical, regulatory, and commercial milestones are achieved. The deal is expected to close by the end of January 2026.
The acquisition expands Day One’s oncology portfolio beyond its core pediatric focus. With $451.6 million in cash at the end of Q3 2025, Day One can fund the purchase without raising additional capital. The company sees the addition of Emi‑Le, its lead antibody‑drug conjugate (ADC) targeting B7‑H4, as a strategic fit that will broaden its pipeline to include rare and adult cancers.
For Mersana, the deal provides an immediate liquidity event and a path to overcome a “going concern” warning that stemmed from a $29.3 million net cash burn in Q1 2025 and a $22.6 million burn in Q2 2025. The company had $102.3 million in cash as of March 31 2025, but the acquisition will extend its runway and remove the uncertainty that has weighed on its valuation, which was approximately $44 million at the time of the announcement.
Emi‑Le, the ADC that drives the transaction, has shown early anti‑tumor activity in phase 1 studies for adenoid cystic carcinoma and triple‑negative breast cancer and has received two Fast Track designations from the FDA. The CVRs in the deal are tied to the achievement of key milestones in Emi‑Le’s development, regulatory approval, and commercial launch, as well as a milestone under an existing collaboration.
Management comments underscore the strategic fit. Marty Huber, Mersana CEO, said the acquisition “recognizes the work that Mersana has done to develop Emi‑Le and that the combination of Mersana’s assets and Day One’s research, development and commercial capabilities has the potential to bring more medicines to patients waiting for new therapies.” Jeremy Bender, Day One CEO, added that the deal “will add a potential game‑changing new medicine to the Day One portfolio and, if approved, will broaden our opportunities for patient impact and for continued growth and value creation.”
The announcement was met with strong investor interest, reflecting the premium offered to Mersana shareholders and the resolution of the company’s cash burn concerns. The deal is considered material to both companies, reshaping Mersana’s corporate structure and providing Day One with a new therapeutic platform that extends beyond its pediatric focus.
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