On October 1, Regeneron Pharmaceuticals fell 3.08% in regular trading, trading at $735.77/share, with turnover of $1.36 billion. The decline came despite the company announcing an expanded global immunology alliance with Sanofi and receiving an analyst upgrade.

Regeneron and Sanofi announced the addition of four next-generation, long-acting antibody programs to their partnership, including REGN20423, a clinical-stage long-acting IL-13 monoclonal antibody, a long-acting IL-4xIL-13 bispecific, and two additional preclinical antibodies. Under the deal, Regeneron will receive $1 billion upfront and up to $7 billion in potential milestone payments, with both companies sharing development costs and future profits. Separately, Argus upgraded the stock from Hold to Buy with an $850 price target.

However, ahead of the announcement, RBC Capital Markets had flagged that Regeneron needed to secure better economic terms. A buyside poll revealed an almost even split between investors who viewed the Dupixent follow-on economics as sufficient and those seeking additional sweeteners. This divergence in expectations likely contributed to a sell-the-news reaction, as the market weighed whether the renegotiated terms adequately reflected Regeneron's pipeline contributions.

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