The legal action, captioned Cheathem v. Regeneron Pharm., Inc., et al., claims that company executives violated the Securities Exchange Act of 1934 by misrepresenting the potential of a combination therapy intended for advanced melanoma. While Regeneron publicly described the drug as a potential blockbuster with meaningful clinical differentiation, the complaint alleges the study failed to reach its primary statistical endpoints.

Market confidence evaporated in two distinct waves. On April 29, 2026, the company announced an expansion of study parameters to include all patients with at least six months of follow-up, a move that signaled insufficient positive data. The stock fell 6.2% that day. A second, sharper decline followed on May 15, 2026, after the company formally confirmed that the trial had failed to achieve statistical significance. Shares dropped an additional 9.8% over the next trading session. The law firm Bleichmar Fonti & Auld LLP, which represents the plaintiffs, is currently seeking lead counsel status for investors looking to recover losses incurred during this period.