Day One Biopharmaceuticals announced a definitive agreement to acquire Mersana Therapeutics Inc. for an equity value of approximately $129 million, with a potential total transaction value of up to $285 million if all contingent value rights (CVRs) are triggered. The deal will be paid in cash, with an upfront payment of $25 per share and up to $30.25 per share in CVRs tied to clinical, regulatory, and commercial milestones for Mersana’s lead antibody‑drug conjugate (ADC) candidate, Emi‑Le. The parties expect the transaction to close by the end of January 2026.
The acquisition expands Day One’s oncology portfolio beyond its pediatric focus on OJEMDA (tovorafenib) and the PTK7‑targeted ADC DAY301. Emi‑Le targets B7‑H4, a biomarker overexpressed in several solid tumors, and has shown early anti‑tumor activity in a phase‑1 study of adenoid cystic carcinoma. By adding a first‑in‑class ADC that addresses an adult cancer with high unmet need, Day One broadens its therapeutic platform and positions itself to capture a larger share of the oncology market.
Day One’s balance sheet is well‑positioned to fund the acquisition. The company reported $451.6 million in cash and no debt, and its Q3 2025 net product revenue for OJEMDA rose 15% to $38.5 million, up 89% year‑to‑date. In contrast, Mersana’s financials were weaker: the company had a negative EBITDA of $70.56 million in the last twelve months, a market capitalization of $44.27 million, and a projected loss of $1.61 per share for Q3 2025. The premium paid by Day One therefore represents a substantial upside for Mersana shareholders relative to its recent valuation.
Market reaction to the announcement was sharply divided. Mersana’s shares surged more than 200% in pre‑market trading on the announcement day, driven by the 182% premium of the $25 per share cash payment and the potential for additional milestone payments through the CVRs. Day One’s shares dipped about 5.8% on the same day, reflecting investor concern about the large cash outlay and the integration of a new asset, despite the company’s strong liquidity and the strategic fit of the ADC program.
Management emphasized the strategic fit and potential upside. Day One CEO Jeremy Bender said the acquisition “will broaden our opportunities for patient impact and for continued growth and value creation,” highlighting the opportunity to leverage Day One’s development and commercial capabilities. Mersana President and CEO Marty Huber noted that the deal “recognizes the work that Mersana has done to develop Emi‑Le and that the combination of Mersana’s assets and Day One’s expertise has the potential to bring more medicines to patients.”
The transaction signals Day One’s intent to diversify beyond its pediatric niche and to accelerate the development of a promising adult ADC. While the immediate financial impact is a sizable cash outlay, the company’s robust cash position and the high‑potential profile of Emi‑Le suggest that the acquisition could generate significant long‑term value if the program reaches regulatory approval and commercial launch.
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