The litigation centers on a protocol amendment for the Fianlimab-Libtayo study, which analysts argue signaled a significant risk to the trial's success. On April 29, 2026, Regeneron revealed it would expand the progression-free survival cohort, a move that prompted Wells Fargo to cut its price target. Analysts expressed concern that the adjustment indicated the underlying clinical benefit might be insufficient to achieve statistical significance, contradicting earlier, more optimistic projections from company executives.

Regeneron shares suffered sharp declines throughout the spring of 2026. After peaking during the class period, the stock dropped $45.41 on April 29 and faced further downward pressure following the May 15 announcement that the trial failed to meet its primary endpoint. The lawsuit contends that these events caused substantial financial harm to investors who bought shares based on incomplete or inaccurate data. Lead plaintiff applications in the case must be submitted by September 14, 2026.