The litigation targets Regeneron’s conduct between August 1, 2025, and May 15, 2026, centering on the Fianlimab and Libtayo combination therapy. Plaintiffs allege the company misled shareholders by maintaining an optimistic outlook while internal data suggested the study’s statistical assumptions were flawed and the treatment offered no meaningful advantage over standard care. When management reported slowing event rates during the trial, they attributed the trend to the therapy’s efficacy rather than the underlying failure to achieve progression-free survival.

Suspicion intensified on April 29, 2026, when the company retroactively altered trial protocols. By May 15, the firm confirmed the study had failed to reach statistical significance, causing share prices to plummet. Reed Kathrein, a partner at Hagens Berman, the firm leading the class action, stated that the investigation is focused on whether Regeneron intentionally concealed the therapy’s lack of efficacy to preserve its market position. Investors who suffered losses during the designated period have until September 14, 2026, to apply for lead plaintiff status.