The lawsuit, spearheaded by the firm Hagens Berman, alleges that Regeneron misled shareholders regarding the efficacy of Fianlimab in combination with Libtayo. While company management repeatedly touted the regimen as a potential blockbuster, the complaint claims that internal statistical assumptions were flawed and that the treatment failed to show meaningful differentiation against standard therapies. Regeneron’s stock price plummeted when the company finally disclosed that the trial had missed its primary endpoint of progression-free survival.

Management had previously attributed a slowdown in clinical trial events to the high performance of the test arms, but internal disclosures later revealed that the protocol was altered months earlier due to inadequate data. By the time Regeneron admitted on May 15, 2026, that the trial had failed to reach statistical significance, investors had already endured months of optimistic projections. Hagens Berman is now seeking to represent affected shareholders, with a lead plaintiff deadline set for September 14, 2026.