Day One Biopharmaceuticals has signed a definitive merger agreement to acquire Mersana Therapeutics for up to $285 million, gaining access to emiltatug ledadotin (Emi-Le), a novel antibody-drug conjugate (ADC) showing promise in treating adenoid cystic carcinoma type-1 (ACC-1). The transaction represents a strategic expansion of Day One's oncology portfolio into a rare cancer with significant unmet medical need.
Novel ADC Targets Rare Cancer with No Approved Therapies
Emi-Le is a potential first-in-class monotherapy directed against B7-H4, a well-characterized target highly expressed in ACC and other adult and pediatric cancers. The investigational ADC has demonstrated early anti-tumor activity in an ongoing Phase 1 study for ACC-1, an aggressive cancer that most often arises from the salivary gland and currently has no approved therapies or clear standard of care.
"This acquisition 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," said Jeremy Bender, Ph.D., chief executive officer of Day One. "The addition of the Emi-Le program to our portfolio allows us to leverage the research and development expertise, and the commercial capabilities, that already exist within Day One to address underserved, rare and life-threatening cancers in patients of all ages."
Strategic Transaction Structure with Performance-Based Milestones
Under the merger agreement terms, Day One will commence a tender offer to acquire all outstanding Mersana shares at $25 per share in cash, plus one non-tradable contingent value right (CVR) per share. The CVRs provide potential milestone payments of up to $30.25 per share, bringing total consideration to $55.25 per share and representing a total equity value of approximately $129 million at closing.
The milestone structure includes clinical achievements such as Breakthrough Therapy Designation ($1.00 per share) and first dosing in a registrational trial ($4.00 per share), regulatory milestones including FDA approval ($9.00 per share), and commercial targets with annual net sales thresholds of $100 million by 2032 ($2.00 per share), $200 million by 2035 ($4.00 per share), and $300 million by 2037 ($6.00 per share).
Clear Development Pathway for Underserved Population
The acquisition positions Day One to apply its existing scientific and commercial capabilities to advance Emi-Le through what the company believes could be a fast path to registration. ACC represents a well-defined patient population with high unmet need, and the early anti-tumor activity observed in the ongoing Phase 1 study may support accelerated development timelines.
Mersana Therapeutics developed Emi-Le using its proprietary Dolasynthen ADC platform. The company's pipeline also includes XMT-2056, an Immunosynthen ADC targeting a novel epitope of human epidermal growth factor receptor 2 (HER2).
Financial and Timeline Details
Day One expects to finance the acquisition with existing cash resources, with the company's strong financial profile expected to enable development of Emi-Le through potential approval without requiring additional financing. The transaction is expected to close by the end of January 2026, subject to regulatory approvals and other customary conditions.
Gordon Dyal & Co., LLC serves as exclusive financial advisor to Day One, with Fenwick & West LLP as legal counsel. TD Cowen acts as financial advisor to Mersana, with Wilmer Cutler Pickering Hale and Dorr LLP serving as legal counsel.
