Mersana Therapeutics (MRSN) Shares Surge 216.46% on Definitive Merger Agreement with Day One Biopharma

The share price rose to its highest level this month today, with an intraday gain of 216.46%.

Mersana Therapeutics (MRSN) announced a definitive merger agreement with Day One Biopharmaceuticals, valued at up to $285 million. The deal includes an upfront cash payment of $25.00 per share and contingent value rights (CVRs) of up to $30.25 per share tied to clinical and regulatory milestones for Mersana’s lead asset, Emi-Le. The transaction, expected to close by late January 2026, triggered a pre-market surge of over 200% in MRSN shares, reflecting investor enthusiasm for the immediate liquidity and long-term upside potential.

The acquisition aligns Day One’s commercial-stage capabilities with Mersana’s antibody-drug conjugate (ADC) platforms, including Dolasynthen and Immunosynthen, which target oncology indications with high unmet needs. Emi-Le, in early trials for adenoid cystic carcinoma, is central to the deal, with CVRs incentivizing Day One to advance its development. The merger also grants Day One access to Mersana’s other assets, such as XMT-2056, expanding its pipeline. While regulatory approvals and shareholder tender conditions remain pending, institutional support and a unanimous board recommendation suggest a high likelihood of completion. The structure balances certainty for Mersana shareholders with performance-based rewards, positioning the combined entity to accelerate next-generation cancer therapies.