The litigation centers on the development of Fianlimab in combination with Libtayo, a treatment Regeneron touted as a potential blockbuster for metastatic melanoma. The complaint alleges that management misled shareholders by repeatedly expressing confidence in the trial’s progress while failing to disclose that preliminary statistical assumptions were flawed and that the treatment arm showed no meaningful improvement over standard therapies.

Regeneron’s narrative began to unravel in late April 2026, when the company revealed a sudden change to the trial protocol regarding progression-free survival analysis. While management initially attributed slower event rates to the strong performance of the test arms, subsequent disclosures in May 2026 confirmed the protocol shift was a response to insufficient data. On May 15, the company officially reported that the study failed to reach its primary endpoint. Hagens Berman partner Reed Kathrein stated the firm is investigating whether the company intentionally concealed the lack of efficacy to maintain the stock's valuation.