Regeneron faces securities suit after $11 billion melanoma trial wipeout

Regeneron Pharmaceuticals faces a securities class action after its Phase 3 melanoma trial failed, wiping out $11 billion in market value.

"We're focused on whether Regeneron altered the Trial protocol without timely telling investors to intentionally mislead them because the defendants knew so-called blockbuster potential for the combination wasn't really there," Reed Kathrein, the Hagens Berman partner leading the firm's investigation, said.

The lawsuit, filed by national shareholder rights firm Hagens Berman, seeks to represent investors who bought Regeneron common stock between Aug. 1, 2025 and May 15, 2026. The complaint alleges Regeneron and its management made false and misleading statements about the trial of fianlimab in combination with Libtayo (cemiplimab) as a first-line treatment for metastatic or locally advanced melanoma, whose primary endpoint was progression-free survival (PFS). Regeneron had characterized the combination as a "potential blockbuster."

The truth emerged in stages. On April 29, 2026, Regeneron disclosed it had altered the trial protocol so the primary PFS analysis would consider all enrolled patients with a minimum follow-up of six months. On May 12, the company admitted the change was made in response to "slow event rates," occurred nearly six months earlier, and was submitted to global regulators in November and December. Three days later, on May 15, Regeneron reported the trial "did not reach statistical significance of the primary endpoint of improvement in progression-free survival (PFS)."

The lead plaintiff deadline is Sept. 14, 2026. The failed readout is a setback for Regeneron's oncology pipeline, which leans on the fianlimab-Libtayo franchise, and the litigation adds financial and reputational risk on top of the $11 billion valuation loss. Investors will watch whether the company settles or contests the claims, and how the case affects sentiment around its commercial portfolio, which posted record Dupixent sales up 38 percent and U.S. EYLEA HD sales up 52 percent to $596 million in the second quarter.

This article is for informational purposes only and does not constitute investment advice.