REGN 10-DAY DEADLINE ALERT: Regeneron Pharmaceuticals, Inc. Investors Alerted to September 14, 2026 Lead Plaintiff Deadline in Class Action Lawsuit

Hagens Berman Sobol Shapiro LLP alerts investors in Regeneron Pharmaceuticals (NASDAQ: REGN) that a securities class action lawsuit has been filed after its surprising revelations concerning a Phase 3 clinical trial of a therapy intended to treat patients with melanoma. The news that the trial failed drove the price of Regeneron shares sharply lower and, along with the severe market reaction ($11 billion market cap wipeout), triggered the lawsuit which seeks to represent investors who purchased or otherwise acquired shares of Regeneron common stock between August 1, 2025 and May 15, 2026.

Class Period: Aug. 1, 2025 - May 15, 2026 Lead Plaintiff Deadline: Sept. 14, 2026 Visit: www.hbsslaw.com/regn Contact the Firm Now: REGN@hbsslaw.com 844-916-0895

The litigation is focused on the propriety of Regeneron's repeated optimism about the state of- (and changes to-) its Phase 3 trial of Fianlimab in combination with Libtayo as a first-line treatment for metastatic or locally advanced melanoma (the "Study"). The Study's primary endpoint was progression-free survival ("PFS") and Regeneron has characterized the combination as a "potential blockbuster." "Events" – disease progression or death – determined the timing and statistical power of the primary PFS analysis.

The complaint alleges that Regeneron made false and misleading statements while failing to disclose critical information to investors. In particular, the lawsuit accuses the company and its management of not informing investors that the Study's preliminary statistical assumptions were flawed, the active treatment arm was not achieving meaningful differentiation over standard therapies, and achievement of its primary endpoint was unlikely. Throughout the Class Period, Regeneron and the other defendants assured investors of their confidence in the Trial's achieving its primary endpoint even when events were slowing down. At one point, management said the slowing event rates are "because the test arms are performing well."

The truth began to emerge on April 29, 2026, when Regeneron first revealed that it decided to alter the Trial protocol such that "t]he primary analysis of progression-free survival will now consider all patients enrolled in the study with a minimum follow-up of 6 months." One prominent analyst reportedly questioned whether the decision was made because, in contrast to management's expressed confidence, the "underlying PFS benefit may be insufficient to show statistical significance." Then, on May 12, 2026, Regeneron admitted that the decision to alter the Trial protocol was made in response to "slow event rates," occurred nearly six months ago, and was "submitted it to all the global regulatory authorities in November, December timeframe." Three days later, the final blow came. On May 15, 2026, Regeneron abruptly reported the "trial did not reach statistical significance of the primary endpoint of improvement in progression-free survival (PFS)."

"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 firm's investigation of the pending claims in the suit.