Regeneron Faces Class Action Lawsuit Following Failed Melanoma Trial

The litigation focuses on the development of Fianlimab paired with Libtayo, a treatment Regeneron previously touted as a potential blockbuster for metastatic melanoma. According to the complaint, the company repeatedly signaled confidence in the trial’s primary endpoint—progression-free survival—even as internal data suggested the study was failing to differentiate itself from standard care.

Investors who purchased shares between August 1, 2025, and May 15, 2026, allege that Regeneron obscured critical weaknesses in the trial’s design. While management attributed slowing event rates to the superior performance of the treatment arms, the lawsuit claims these delays were evidence of flawed statistical assumptions. The deception allegedly culminated in a mid-trial protocol change, which the company delayed disclosing to the public for months.

On May 15, 2026, the company confirmed that the trial did not reach statistical significance, triggering a sharp decline in share price. Reed Kathrein, a partner at Hagens Berman, the firm leading the investigation, argues that the company’s silence during the protocol adjustment process effectively misled shareholders about the drug's actual viability. The firm is now seeking lead plaintiffs to participate in the action ahead of the September 14, 2026 deadline.