Sanofi and Regeneron expanded their long-running immunology partnership, adding four next-generation long-acting antibody programs aimed at the Dupixent (dupilumab) target space. The deal includes a $1 billion upfront payment from Sanofi to Regeneron, with up to $7 billion additional milestone value. Regeneron will lead R&D while Sanofi handles global commercialization, and the companies will share development and profits 50:50. The plan projects pivotal testing to begin in 2029 for the new candidates. The leading asset in the expansion is REGN20423, a long-acting IL-13 monoclonal antibody in Phase I testing (NCT07527923) for atopic dermatitis, alongside a long-acting IL-4xIL-13 bispecific, plus two further long-acting antibodies targeting IL-4 and IL-4 receptor alpha. There is also an option related to lunsekimig once its Phase III study completes. The commercial backdrop is patent timing and competitive differentiation: the partnership is explicitly structured to maintain momentum ahead of Dupixent-related franchise transitions and build a next-generation immunology portfolio.