The litigation targets Regeneron’s conduct between August 1, 2025, and May 15, 2026, centering on the company’s aggressive promotion of a combination therapy involving Fianlimab and Libtayo. Management had repeatedly characterized the treatment as a potential blockbuster, even as internal data regarding progression-free survival began to falter. The lawsuit alleges that while Regeneron publicly attributed slow event rates to the strong performance of test arms, the company was aware that its statistical assumptions were fundamentally flawed.

Regeneron’s defense began to fracture in late April 2026, when the company disclosed a sudden shift in trial protocol. By May 15, 2026, the company confirmed that the study had failed to achieve statistical significance. Hagens Berman, the firm leading the investigation, argues that Regeneron intentionally withheld information about the trial’s lack of efficacy to maintain market confidence. Investors who suffered losses during this period are now facing a September 14, 2026, deadline to file as lead plaintiffs in the case.