A securities class action has been filed against Regeneron Pharmaceuticals, Inc. over alleged misstatements tied to the Phase III Fianlimab-Libtayo study, with shareholders who bought between August 1, 2025 and May 15, 2026 told they have until September 14, 2026 to seek lead plaintiff status. The complaint says Regeneron portrayed slowed progression-free survival, or PFS (time before cancer worsens), event accrual as potentially consistent with strong treatment performance, but later disclosed on April 29, 2026 that the PFS analysis would be expanded to all patients with at least six months of follow-up and then said on May 15, 2026 that the Phase 3 trial failed to reach statistical significance on its primary endpoint (main trial goal). REGN shares fell from $731.77 on April 28, 2026 to $629.68 on May 18, 2026, including a $45.41 drop after the April disclosure and a $68.57 decline after the May announcement, and the case was filed in the United States District Court for the Southern District of New York under federal securities laws.