Sanofi agreed to license Kali Therapeutics’ lead tri-specific T-cell engager KT501, gaining exclusive worldwide rights to the CD3-masked antibody program with upfront and milestone payments potentially totaling more than $1.23 billion. Kali said KT501 is designed to bind CD3, CD19, and BCMA to drive broad B-cell depletion while aiming to limit cytokine release via a CD3 masking platform. KT501 is now in a Phase 1a first-in-human trial (NCT07234773) in rheumatoid arthritis. The open-label dose-escalation study began dosing the first patient in March and targets up to about 24 participants, with primary completion estimated for February 2027. Sanofi’s payment structure includes $180 million upfront and near-term, plus up to $1.05 billion tied to development and commercial milestones and tiered royalties. The deal underscores a continued shift toward T-cell engager formats tailored for autoimmune indications, where safety and patient selection remain key development constraints.