The lawsuit centers on Regeneron’s repeated public optimism regarding a combination therapy of Fianlimab and Libtayo, which the company previously touted as a potential blockbuster treatment for metastatic melanoma. Investors who purchased common stock between August 1, 2025, and May 15, 2026, claim that management failed to disclose that the trial’s preliminary statistical assumptions were flawed and that the treatment was failing to differentiate itself from standard therapies.

Regeneron management maintained confidence in the study’s primary endpoint, progression-free survival, even as clinical event rates slowed. The company attributed this deceleration to the successful performance of the test arms. However, the narrative shifted on April 29, 2026, when the firm announced an alteration to the trial protocol. By May 15, Regeneron confirmed the study had failed to reach statistical significance, contradicting earlier assurances provided to the market.

Reed Kathrein, a partner at Hagens Berman Sobol Shapiro LLP, the firm leading the investigation, stated that the legal challenge focuses on whether the company altered the trial protocol without timely disclosure to conceal the lack of efficacy. Shareholders seeking to participate as lead plaintiffs must meet a September 14, 2026, deadline.