Robbins LLP said investors who bought Regeneron shares between Aug. 1, 2025 and May 15, 2026 may be eligible to participate in a lawsuit tied to the melanoma trial’s alleged shortcomings and subsequent stock declines.
Robbins LLP said a class action lawsuit has been filed on behalf of investors who purchased or otherwise acquired Regeneron Therapeutics, Inc. shares between August 1, 2025 and May 15, 2026, alleging the company misled investors about the viability of its Phase III Fianlimab-Libtayo Study in advanced melanoma. The complaint centers on Fianlimab, a human monoclonal antibody targeting the LAG-3 immune checkpoint receptor on T-cells, which was being tested with Libtayo as a potential first-line treatment for advanced melanoma in a phase 3 trial that began enrollment in mid-2022. Plaintiff alleges Regeneron made positive statements while concealing that the study’s preliminary statistical assumptions were flawed, the active treatment arm was not achieving meaningful clinical differentiation over standard therapies, and the trial would ultimately fail to reach statistical significance on its primary endpoint. The filing points to two disclosures. On April 29, 2026, during Regeneron’s first-quarter earnings call, the company disclosed changes to the study that expanded the number of patients eligible for analysis of progression-free survival, after which the stock fell about 6.2% to $686.36 from $731.77 a day earlier. Then after the market closed on May 15, 2026, Regeneron said the phase 3 trial did not reach statistical significance for the primary endpoint of improvement in progression-free survival, and the stock fell about 9.8% to $629.68 on May 18 from $698.25 on May 15.