Sanofi and Regeneron broaden their long-standing immunology alliance with a deal that adds four next-generation long-acting antibody programs tied to IL-4/IL-13 signaling. Sanofi will pay Regeneron $1 billion upfront, with up to $7 billion more tied to development, regulatory and commercial milestones, and the companies will split development and profits on a 50:50 basis. The expanded portfolio is designed to create follow-ons after Dupixent (dupilumab), which continues to drive revenue across multiple indications. The lead new asset, REGN20423 (long-acting IL-13 monoclonal antibody), is in Phase I for atopic dermatitis, with additional programs including an IL-4xIL-13 bispecific and further IL-4/IL-4Rα-directed antibodies. Industry impact centers on how big pharma is repositioning around long-acting biologics for chronic inflammatory diseases—extending the franchise beyond patent cliffs while maintaining platform leverage from existing clinical and commercial expertise. For biotech professionals, the transaction is also a reminder that pipeline expansion in immunology increasingly hinges on antibody longevity, differentiated pharmacology, and clear development timelines that can translate into earlier pivotal study readouts.