The litigation centers on the Phase 3 study of Fianlimab combined with Libtayo, a treatment Regeneron previously touted as a potential blockbuster for metastatic melanoma. While management repeatedly expressed confidence in the trial’s progress, the complaint alleges that the company obscured flaws in its preliminary statistical assumptions and failed to disclose that the treatment arm lacked meaningful differentiation from standard therapies.

Regeneron’s internal narrative began to unravel on April 29, 2026, when the company announced a sudden change to the trial’s protocol. Although management initially attributed slowing event rates to the strong performance of the test arms, the company later admitted the protocol shift was a response to sluggish data collection. By May 15, 2026, Regeneron confirmed the trial had failed to reach the primary endpoint of progression-free survival.

Reed Kathrein, a partner at Hagens Berman Sobol Shapiro LLP, the firm leading the investigation, asserts the suit will examine whether the protocol changes were made to intentionally mislead investors regarding the drug's efficacy. The legal action seeks to recover losses for shareholders who acquired stock during the period when the company maintained its optimistic stance despite internal knowledge of the study's shortcomings.