A securities class action has been filed against Regeneron Pharmaceuticals after the company disclosed that a Phase 3 trial of fianlimab plus Libtayo in first-line metastatic or locally advanced melanoma failed to meet its primary endpoint of progression-free survival, or PFS. The disclosure triggered a sharp drop in Regeneron shares and erased about $11 billion in market value. The proposed class covers investors who purchased or otherwise acquired Regeneron securities between Aug. 1, 2025 and May 15, 2026, with Sept. 14, 2026 set as the deadline for lead plaintiff motions. The complaint alleges Regeneron and certain officers misled investors about the study by failing to disclose allegedly flawed statistical assumptions, limited clinical differentiation versus standard therapies, and a low likelihood that the trial would achieve statistical significance on its primary endpoint. The case also focuses on Regeneron's later disclosure that it had changed the trial protocol months earlier because of slow event rates.