Regeneron Faces Class Action Lawsuit After Melanoma Trial Failure

The legal action, initiated by the firm Hagens Berman Sobol Shapiro LLP, represents investors who purchased Regeneron common stock between August 1, 2025, and May 15, 2026. The complaint centers on allegations that management misled shareholders by maintaining an optimistic outlook on the Fianlimab and Libtayo combination trial despite mounting evidence of poor performance. While company leadership attributed slowing disease progression events to the success of the treatment, the lawsuit claims these delays were actually indicators that the therapy was not achieving significant differentiation over standard care.

Regeneron’s internal struggles became public on April 29, 2026, when the company revealed a sudden change to its trial protocol. The firm later admitted that the decision to alter the study was prompted by slow event rates identified as early as November 2025. By May 15, 2026, the company confirmed the trial failed to reach its primary endpoint of progression-free survival. Reed Kathrein, the Hagens Berman partner leading the investigation, stated that the firm is examining whether the protocol was altered to conceal the lack of efficacy in a treatment previously touted for its blockbuster potential. Investors have until September 14, 2026, to file for lead plaintiff status.