Key Facts About Regeneron Pharmaceuticals, Inc. (NASDAQ: REGN)

●        The Allegation: The complaint alleges Regeneron misled investors about the Phase III Fianlimab-Libtayo Study in first-line advanced melanoma by portraying slowed event accrual as a likely sign of durable efficacy. Plaintiffs allege defendants concealed that the slowdown increased the risk of clinical failure, required a late protocol amendment, and reflected flawed assumptions about the study’s ability to reach statistical significance.

●        The Stock Drop: REGN fell $68.57, about 9.8%, to $629.68 per share on May 18, 2026, after Regeneron announced the Phase III Fianlimab-Libtayo Study did not reach statistical significance on its primary endpoint; REGN earlier fell $45.41, about 6.2%, to $686.36 per share on April 29, 2026, after Regeneron disclosed the study’s primary analysis would consider all enrolled patients with at least 6 months of follow-up.

●        Class Period & Defendants: The class period runs from August 1, 2025 through May 15, 2026, inclusive. Defendants are Regeneron Pharmaceuticals, Inc., George D. Yancopoulos (Co-Founder, President, Chief Executive Officer, and Co-Chairman), Israel Lowy (Senior Vice President and Clinical Development Unit Head of Oncology), and Ryan Crowe (Senior Vice President of Investor Relations and Strategic Analysis).

●        Lead Plaintiff Deadline: September 14, 2026. Investors who wish to seek appointment as lead plaintiff must apply by the deadline. No action is required before the deadline to remain a potential class member.

Introduction

A securities fraud class action has been filed against Regeneron Pharmaceuticals, Inc. The lawsuit concerns investors who purchased Regeneron common stock from August 1, 2025 through May 15, 2026.

The complaint alleges defendants made materially false and misleading statements about Regeneron’s Phase III Fianlimab-Libtayo Study. Defendants allegedly described slowed event accrual as a likely positive signal, but the trial later needed a major protocol change.

Regeneron disclosed on May 15, 2026 that the trial missed its primary endpoint. REGN fell about 9.8% on May 18, 2026, after an earlier 6.2% decline on April 29, 2026.

Company Profile

Regeneron Pharmaceuticals, Inc. is a pharmaceutical company that discovers, invents, develops, manufactures, tests, and commercializes medicines to treat various disorders worldwide. During the relevant period, Regeneron was testing fianlimab, a human monoclonal antibody targeting the LAG-3 immune checkpoint receptor on T-cells, in combination with Libtayo as a first-line treatment for advanced melanoma.

Class Period

August 1, 2025-May 15, 2026

Investors who purchased or acquired Regeneron Pharmaceuticals, Inc. (REGN) common stock during the Class Period may be eligible to seek recovery under the federal securities laws.

Allegations

The complaint alleges that Regeneron and the individual defendants provided investors with material information about the Phase III Fianlimab-Libtayo Study, a trial testing fianlimab in combination with cemiplimab, marketed as Libtayo, as a first-line treatment for metastatic or locally advanced melanoma. According to plaintiffs, defendants repeatedly expressed confidence in the drug combination’s potential and mischaracterized the risk created by a prolonged slowdown in progression-free survival event accrual.

On August 1, 2025, Regeneron released second quarter fiscal 2025 results and held a shareholder call. George D. Yancopoulos stated that enrollment for the progression-free survival cohort had been completed in January as expected, but results were delayed because blinded event accrual had slowed. The complaint alleges defendants did not disclose that the delay could require a protocol change or increase the risk of a negative study outcome.

The alleged reassuring statements continued through investor conferences. On November 17, 2025, Israel Lowy told attendees at the 7th Annual Wolfe Research Healthcare Conference that Regeneron believed slow event accrual reflected strong active-arm performance and that the company was in as strong a position as possible for clinical trial success. On December 2 and December 3, 2025, Ryan Crowe cited Phase I data showing a 57% response rate and a pooled median progression-free survival of 24 months, and said Regeneron had hope and confidence that fianlimab plus Libtayo could generate meaningful differentiation against current standards of care.

Plaintiffs allege these statements were materially false and misleading because defendants knew or recklessly disregarded that the slowdown created structural and statistical risks. The complaint claims Regeneron’s preliminary assumptions were fundamentally flawed, the active treatment arm was failing to achieve meaningful clinical differentiation over standard therapies, and the trial would fail to reach statistical significance on its primary endpoint even without overperformance by the control arm.

The Truth Emerges

The first alleged corrective disclosure came during Regeneron’s April 29, 2026 earnings call. Yancopoulos disclosed that the Phase III Fianlimab-Libtayo Study’s primary progression-free survival analysis would now consider all patients enrolled in the study with a minimum follow-up of 6 months. According to the complaint, that disclosure revealed a late-stage alteration to the study protocol after months of slow event accrual.

Analysts immediately focused on the amendment. Wells Fargo reportedly stated that Regeneron’s shares were down primarily because the expanded progression-free survival cohort raised concerns that the underlying benefit might be insufficient to show statistical significance. Evercore ISI noted that the protocol amendment changed the statistical analysis plan to essentially ensure the timing of the readout, and said the need for the change after a roughly 1.5-year delay made the study more unpredictable.

The full truth allegedly emerged after market close on May 15, 2026, when Regeneron issued a press release announcing that the Phase III trial of fianlimab did not reach statistical significance for the primary endpoint of improvement in progression-free survival. Citi downgraded Regeneron to Neutral and cut its price target more than 22%, stating that the dataset did not support a regulatory filing or commercial value in its model. Bernstein also highlighted that the issue was underwhelming performance of the investigational arm rather than pembrolizumab overperformance.

Market Reaction

Following the April 29, 2026 disclosure, Regeneron’s common stock fell from $731.77 per share on April 28, 2026 to $686.36 per share on April 29, 2026. That was a decline of $45.41 per share, or about 6.2%, in one trading day.

After Regeneron announced the trial’s primary endpoint failure on May 15, 2026, REGN fell again. The stock declined from $698.25 per share on May 15, 2026 to $629.68 per share on May 18, 2026, a decline of $68.57 per share, or about 9.8%.

Next Steps

●        Lead Plaintiff Deadline: September 14, 2026

●        After the lead plaintiff deadline, the Court will consider any lead plaintiff motions.

●        Defendants may file a motion to dismiss.

●        If the case proceeds, the Court may later consider class certification.

Disclaimer: This shareholder alert is for informational purposes only and does not constitute legal advice. Consult a qualified attorney for personalized guidance. No specific outcomes are guaranteed.