Sanofi agreed to license and obtain exclusive worldwide rights to Kali Therapeutics’ trispecific T-cell engager KT501 in a deal that could generate more than $1.23 billion for the private biotech. Under the terms, Sanofi will pay $180 million in upfront and near-term payments plus up to $1.05 billion tied to development and commercial milestones, with tiered royalties on sales. KT501 is an IgG-like tri-specific TCE designed to bind CD3, CD19, and BCMA with a CD3 masking platform intended to reduce cytokine release while maintaining B-cell depletion potency. Kali reported potent B-cell depletion and “significantly” reduced cytokine production in non-human primate studies. Clinically, KT501 is in a Phase Ia first-in-human trial for rheumatoid arthritis (NCT07234773), with Kali stating it dosed the first patient on March 18. The open-label, dose-escalation study is designed to evaluate safety, tolerability, pharmacokinetics, and pharmacodynamics in up to about 24 participants. The collaboration reflects Sanofi’s broader push into immunology platforms and adds competition to the rapidly expanding trispecific and bispecific TCE landscape, where early translational differentiation—especially around cytokine-related toxicity—can drive partnership confidence.