Sanofi licensed Kali Therapeutics’ lead trispecific T-cell engager KT501 under a collaboration valued at up to $1.23 billion. Kali reported dosing the first patient in a phase Ia trial for KT501 in rheumatoid arthritis, and the companies framed KT501 around a CD3 masking approach intended to separate potency from cytokine release. KT501 is designed to bind CD3, CD19, and BCMA with an IgG-like tri-specific architecture. Kali said non-human primate studies demonstrated broad B-cell depletion alongside reduced cytokine production, supporting exploration across B-cell-mediated autoimmune diseases. Sanofi will obtain exclusive worldwide rights to KT501 and will support clinical development, while Kali will receive $180 million in upfront and near-term payments plus up to $1.05 billion in development and commercial milestones and tiered royalties. The first-in-human start adds momentum for Sanofi’s push into immunology via next-generation T-cell engaging formats that aim to reduce toxicity while maintaining deep immune modulation.