San Francisco – September 10, 2026 -- Hagens Berman Sobol Shapiro LLP has filed a securities class action against Regeneron Pharmaceuticals (NASDAQ: REGN) after the failure of a Phase 3 melanoma trial wiped out $11 billion in market capitalization. The suit covers investors who purchased Regeneron shares between August 1, 2025, and May 15, 2026, with a lead plaintiff deadline set for September 14, 2026.
Regeneron's Fianlimab-Libtayo combination failed to hit its primary endpoint
The complaint centers on Regeneron's Phase 3 trial testing Fianlimab combined with Libtayo as a first-line treatment for metastatic or locally advanced melanoma, a therapy management repeatedly called a "potential blockbuster." On May 15, 2026, Regeneron disclosed the trial did not reach statistical significance on its primary endpoint of progression-free survival.
Regeneron altered trial protocol months before disclosing the change to investors
On April 29, 2026, Regeneron revealed it had changed the trial's statistical analysis to include all enrolled patients with a minimum six-month follow-up. By May 12, 2026, the company admitted the protocol change was a response to "slow event rates" and had been submitted to global regulators nearly six months earlier, in November-December 2025.
Lawsuit alleges management concealed weak treatment differentiation
The complaint alleges Regeneron failed to disclose that its preliminary statistical assumptions were flawed and that the active treatment arm was not showing meaningful differentiation from standard therapies. It further claims management attributed slowing event rates to the test arms "performing well," despite allegedly knowing the primary endpoint was unlikely to be met.
"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," said Reed Kathrein, the Hagens Berman partner leading the investigation.
SEC whistleblower program offers rewards up to 30% of recoveries
Hagens Berman is inviting individuals with non-public information about Regeneron's trial disclosures to assist the investigation, noting that SEC whistleblowers providing original information may receive up to 30% of any successful recovery.