Regeneron Faces Lawsuit Over Misleading Phase III Study Claims

Regeneron Pharmaceuticals, Inc., a leading biotechnology company, faces legal scrutiny over alleged misrepresentations regarding its Phase III Fianlimab-Libtayo clinical trial. According to a lawsuit filed on April 29, 2026, during the class period from August 1, 2025, to May 15, 2026, the company and certain executives are accused of making false or misleading statements and failing to disclose critical information.

The lawsuit, titled Cheatham v. Regeneron Pharmaceuticals, Inc., alleges that Regeneron created a false impression that its Phase III Fianlimab-Libtayo study was well-positioned for success, while downplaying the risks. The case further claims that the company’s preliminary statistical assumptions were flawed, and that the active treatment arm was failing to achieve meaningful clinical differentiation from standard therapies. Additionally, it asserts that the trial would ultimately fail to reach statistical significance, even without overperformance of the control arm.

On April 29, 2026, during the company’s first quarter earnings call, Regeneron disclosed that the Phase III Fianlimab-Libtayo study had been altered, expanding the number of patients eligible for "analysis of progression-free survival." This announcement caused a significant drop in the company’s stock price, with shares declining over 6% in the following trading session. Further, on May 15, 2026, Regeneron issued a press release announcing that the "Phase 3 Trial of Fianlimab . . . did not reach statistical significance for the primary endpoint of improvement in progression-free survival (PFS)," leading to a nearly 10% drop in the stock price.

Investors who purchased or acquired Regeneron common stock during the class period have until September 14, 2026, to seek appointment as lead plaintiff in the lawsuit. The Private Securities Litigation Reform Act of 1995 allows any investor who suffered substantial losses to take part in the case. A lead plaintiff is typically the investor with the greatest financial interest in the relief sought by the putative class, who is also typical and adequate of the putative class. A lead plaintiff acts on behalf of all other class members in directing the lawsuit.

Robbins Geller Rudman & Dowd LLP, one of the world’s leading law firms representing investors in securities fraud and shareholder rights litigation, is handling the case. The firm ranked first in the most recent ISS Securities Class Action Services Top 50 Report, recovering over $916 million for investors in 2025. With offices across the country and a team of 200 lawyers, Robbins Geller has a proven track record of obtaining significant recoveries for investors, including the largest ever, $7.2 billion, in In re Enron Corp. Sec. Litig.

The lawsuit charges Regeneron and its executives with violations of the Securities Exchange Act of 1934. Regeneron is a pharmaceutical company that discovers, invents, develops, manufactures, tests, and commercializes medicines to treat various disorders worldwide. The case highlights the potential consequences for companies that fail to disclose critical information to investors, emphasizing the importance of transparency and accurate communication in the pharmaceutical industry.

As the deadline for investors to seek appointment as lead plaintiff approaches, the outcome of this lawsuit could have significant implications for Regeneron and the broader pharmaceutical sector. Investors are urged to act promptly to ensure their voices are heard in the case.