Lead Plaintiff Deadline: September 14, 2026

SAN DIEGO, July 27, 2026 /PRNewswire/ -- Robbins LLP reminds investors that a securities class action has been filed on behalf of investors who purchased or otherwise acquired Regeneron Pharmaceuticals, Inc. (NASDAQ: REGN) securities between August 1, 2025 and May 15, 2026, inclusive (the "Class Period").

The lawsuit alleges that Regeneron Pharmaceuticals misled investors regarding the viability and prospects of its Phase 3 Fianlimab-Libtayo clinical trial for patients with advanced melanoma.

Investors who suffered losses during the Class Period may have legal rights and should be aware of the September 14, 2026 deadline to seek appointment as lead plaintiff.

Why Was Regeneron Sued?

According to the complaint, Regeneron was developing Fianlimab, a human monoclonal antibody targeting the LAG-3 immune checkpoint receptor on T-cells. Fianlimab was being evaluated in combination with Libtayo in a Phase 3 clinical trial investigating whether the combination could serve as a first-line treatment for patients with advanced melanoma.

The Phase 3 Fianlimab-Libtayo study began enrolling patients in mid-2022.

The complaint alleges that Regeneron provided investors with positive statements about the study while failing to disclose material information concerning its prospects.

Specifically, plaintiff alleges that Regeneron failed to disclose that:

• the study's preliminary statistical assumptions were fundamentally flawed;
• the Fianlimab-Libtayo treatment arm was failing to achieve meaningful clinical differentiation from standard therapies; and
• the trial was ultimately unlikely to achieve statistical significance on its primary endpoint, even without unusually strong performance by the control arm.

The complaint alleges that these omissions caused Regeneron's stock to trade at artificially inflated prices.

What Happened to Regeneron Stock?

According to the complaint, information concerning the Phase 3 Fianlimab-Libtayo study emerged through a series of disclosures in April and May 2026.

April 29, 2026: Regeneron Discloses Changes to the Phase 3 Study

On April 29, 2026, during Regeneron's first-quarter earnings call, defendants disclosed that the Phase 3 Fianlimab-Libtayo study had been modified to expand the number of patients eligible for analysis of progression-free survival (PFS).

Following the disclosure, Regeneron's stock price fell from $731.77 per share on April 28, 2026, to $686.36 per share on April 29, 2026, a decline of approximately 6.2% in a single trading day.

May 15, 2026: Fianlimab-Libtayo Trial Fails to Meet Primary Endpoint

After the market closed on May 15, 2026, Regeneron announced that its Phase 3 Fianlimab trial did not achieve statistical significance for the primary endpoint of improvement in progression-free survival.

Following the announcement, Regeneron's stock price declined from $698.25 per share on May 15, 2026, to $629.68 per share on May 18, 2026, a decline of approximately 9.8%.

The complaint alleges that these disclosures revealed information that contradicted or called into question Regeneron's prior statements concerning the clinical trial.

What Is the Regeneron Fianlimab-Libtayo Class Action About?

The lawsuit alleges that Regeneron failed to disclose material information concerning the viability of its Phase 3 Fianlimab-Libtayo study.

According to the complaint, investors were not adequately informed about alleged problems with the study's statistical assumptions, the lack of meaningful clinical differentiation between the treatment and control arms, and the alleged risk that the trial would fail to meet its primary endpoint.

The lawsuit seeks damages on behalf of investors who purchased Regeneron securities during the Class Period.

What Is Fianlimab?

Fianlimab is a monoclonal antibody that targets the LAG-3 immune checkpoint receptor on T-cells. Regeneron was studying Fianlimab in combination with Libtayo in a Phase 3 trial involving patients with advanced melanoma.

The lawsuit centers on allegations concerning the clinical trial's design, statistical assumptions, efficacy results, and ultimate failure to meet its primary progression-free survival endpoint.

What Is Progression-Free Survival?

Progression-free survival (PFS) is a clinical-trial endpoint that measures the length of time patients remain alive without their disease worsening or progressing.

The complaint alleges that the Fianlimab-Libtayo Phase 3 study failed to achieve statistical significance for its primary PFS endpoint.

Who May Be Eligible?

The proposed class includes investors who purchased or otherwise acquired Regeneron Pharmaceuticals, Inc. (NASDAQ: REGN) securities between August 1, 2025 and May 15, 2026.

If you purchased REGN securities during this period and suffered losses, you may have rights under the federal securities laws.

What Is a Lead Plaintiff?

The lead plaintiff is an investor appointed by the court to represent the interests of the proposed class throughout the litigation.

Investors do not have to serve as lead plaintiff to potentially share in any future recovery if the lawsuit is successful.

The deadline to seek appointment as lead plaintiff is September 14, 2026.

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Frequently Asked Questions

What is the Regeneron class action about?

The lawsuit alleges that Regeneron Pharmaceuticals misled investors about the viability and prospects of its Phase 3 Fianlimab-Libtayo clinical trial for advanced melanoma.

Why did Regeneron stock fall in April 2026?

According to the complaint, Regeneron disclosed on April 29, 2026 that the Phase 3 Fianlimab-Libtayo study had been modified to expand the number of patients eligible for analysis of progression-free survival. Regeneron's stock declined approximately 6.2% that day.

Why did Regeneron stock fall in May 2026?

After the market closed on May 15, 2026, Regeneron announced that the Phase 3 Fianlimab trial did not achieve statistical significance for the primary endpoint of improvement in progression-free survival. The stock subsequently declined approximately 9.8% from May 15 to May 18.

Did the Fianlimab-Libtayo trial succeed?

According to Regeneron's May 15, 2026 announcement as described in the complaint, the Phase 3 trial did not reach statistical significance for its primary endpoint of improvement in progression-free survival.

Who can participate in the Regeneron lawsuit?

Investors who purchased or otherwise acquired Regeneron securities between August 1, 2025 and May 15, 2026 may be eligible to participate in the proposed securities class action.

Do I have to become lead plaintiff?

No. Investors do not have to seek appointment as lead plaintiff to potentially share in any future recovery if the litigation is successful.

Does it cost anything to participate?

Robbins LLP represents investors on a contingency fee basis. Investors never pay no attorneys' fees or litigation expenses. If there is a recovery for shareholders, defendants pay attorneys' fees and expenses.

Contact Robbins LLP

Investors seeking additional information about the Regeneron securities class action may submit an inquiry through Robbins LLP's website, email attorney Aaron Dumas, Jr., or call (800) 350-6003.

About Robbins LLP

A recognized leader in shareholder rights litigation, Robbins LLP has helped restore more than $1 billion in value to shareholders, secured some of the largest recoveries in shareholder derivative litigation history, and achieved governance reforms at over 400 Fortune 1000 companies. 

"Behind everything we do is the belief that companies should be governed responsibly, fiduciaries should be held accountable, and shareholders deserve transparency and fairness," said Brian J. Robbins, Founding Partner of Robbins LLP.

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