Sanofi will pay Regeneron Pharmaceuticals $1 billion upfront to expand an immunology alliance that has already produced the blockbuster Dupixent (dupilumab), with up to $7 billion in additional milestone payments tied to the deal announced Thursday.

The agreement covers four new long-acting, Regeneron-invented antibodies targeting interleukin-13 (IL-13), interleukin-4 (IL-4), IL-4 receptor alpha, and an IL-4xIL-13 bispecific. The targets sit squarely on the cytokine axis that made dupilumab — a fully human monoclonal antibody against IL-4 receptor alpha — the standard-setting biologic across atopic dermatitis and a growing list of type 2 inflammatory diseases.

The clinical starting point differs across the four molecules. The IL-13 antibody REGN20423 is already in Phase I testing for atopic dermatitis. The other three antibodies are expected to enter clinical studies in 2027, according to the companies.

Under the expanded terms, Regeneron will lead R&D activities and Sanofi will lead global commercialization. The two companies will share development and commercialization costs, with profits split 50:50 on a global basis.

Leonard Schleifer, Regeneron's CEO, called the alliance — which dates to 2003 — "one of the most productive in biopharma history." He said the expanded agreement will position "promising new long-acting antibodies for the kind of rapid, global impact that our shared work on Dupixent has already achieved."

The deal also carries an option structure beyond the four named programs. Regeneron will have the option to bring Sanofi's lunsekimig, an investigational bispecific targeting TSLP and IL-13, into the collaboration following the completion of Phase III trials in chronic obstructive pulmonary disease.

For Sanofi, the agreement carries strategic weight beyond the pipeline itself. Belén Garijo, the French drugmaker's new CEO — brought in with a mandate to boost R&D productivity — framed the payment as part of a broader reset. "This is a deliberate first step in igniting Sanofi's innovation engine," she said. "By deepening our collaboration, we will make rapid progress in creating the next generation of meaningful therapies for patients with immune-mediated diseases, while contributing to Sanofi's long-term future."

The choice of targets signals where the partners see the next competitive window in dermatology and immunology. All four programs concentrate on the IL-4 and IL-13 signaling pathways — the mechanistic territory where dupilumab built its franchise — but with a long-acting design brief that suggests the companies are aiming at dosing intervals as a differentiator against the current generation of injected biologics.

Dermatologists will watch REGN20423 most closely in the near term. A long-acting IL-13 inhibitor entering Phase I in atopic dermatitis would, if it advances, compete in a space already occupied by targeted biologics and oral JAK inhibitors, where dosing convenience and durability of response are active battlegrounds.

For now, the data are early. One molecule is in first-in-human testing; three have yet to reach the clinic. What is concrete is the financial commitment: $1 billion upfront, up to $7 billion contingent on milestones, a 50:50 profit split, and a partnership structure that keeps Regeneron at the discovery bench and Sanofi at the commercial helm — the same division of labor that carried Dupixent from the lab to blockbuster status.

With three more antibodies slated to enter clinical studies in 2027 and a lunsekimig option pending Phase III COPD readouts, the expanded alliance gives both companies a sustained pipeline position in type 2 immunology for the decade ahead.