Day One Biopharmaceuticals Surges 11.23% on Mersana Therapeutics Acquisition Completion

Day One Biopharmaceuticals surged 11.23% in pre-market trading on January 8, 2026, driven by strategic developments that bolstered investor confidence. The stock’s sharp rise followed the completion of its $25-per-share cash acquisition of Mersana Therapeutics, including non-tradable contingent value rights offering upside potential.

The deal, finalized after meeting all tender offer conditions, added Emi-Le—a B7-H4-targeting antibody-drug conjugate—to Day One’s pipeline. This ADC addresses rare cancers like adenoid cystic carcinoma, with early clinical data positioning it as a transformative therapy. The acquisition, valued at up to $285 million, expands Day One’s foothold in adult oncology while reinforcing its focus on unmet medical needs.

CEO Jeremy Bender highlighted the strategic alignment with Day One’s mission, emphasizing rapid development of the asset and leveraging existing strengths in oncology. The move follows a 2024 licensing agreement for another ADC, signaling a deliberate expansion into targeted therapeutics. Analysts view the transaction as a catalyst for long-term growth, combining immediate portfolio enhancement with potential milestone-driven value from the CVR structure.

With a robust R&D pipeline and growing therapeutic focus, Day One is well-positioned to capitalize on emerging opportunities in oncology. The integration of Mersana’s assets is expected to be a key differentiator in its competitive landscape, especially in the niche but high-growth rare cancer space.