Lawsuit Filed Against Gossamer Bio Over Failed Drug Trial

Gainey McKenna & Egleston announces a securities class action lawsuit on behalf of Gossamer Bio investors.

Apr. 1, 2026 at 5:38pm

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A securities class action lawsuit has been filed against Gossamer Bio, Inc. (NASDAQ: GOSS) on behalf of investors who purchased the company's securities between June 16, 2025, and February 20, 2026. The lawsuit alleges that Gossamer provided investors with misleading information about its Phase 3 PROSERA study evaluating the drug seralutinib for pulmonary arterial hypertension, which ultimately failed to meet its primary endpoint.

Why it matters

This lawsuit highlights the risks investors face when companies provide overly optimistic information about their drug development pipelines. The failure of Gossamer's PROSERA study led to an 80% drop in the company's stock price, causing significant losses for shareholders who relied on the company's positive statements.

The details

The lawsuit alleges that Gossamer made misleading statements about the PROSERA study design, particularly regarding how the company was controlling for the placebo response at Latin American testing sites. When Gossamer announced the study's failure to meet its primary endpoint on February 23, 2026, the company's stock price plummeted from $2.13 per share to $0.42 per share.

  • The class period covers June 16, 2025, to February 20, 2026.
  • Gossamer announced the PROSERA study results on February 23, 2026.
  • The lead plaintiff motion deadline is set for June 1, 2026.

The players

Gainey McKenna & Egleston

A law firm that filed the securities class action lawsuit against Gossamer Bio on behalf of investors.

Gossamer Bio, Inc.

A biopharmaceutical company that was developing the drug seralutinib for the treatment of pulmonary arterial hypertension.

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What they’re saying

“Investors who purchased or otherwise acquired shares of Gossamer should contact the Firm prior to the June 1, 2026 lead plaintiff motion deadline.”

— Thomas J. McKenna, Attorney, Gainey McKenna & Egleston

“A lead plaintiff is a representative party acting on behalf of other class members in directing the litigation.”

— Gregory M. Egleston, Attorney, Gainey McKenna & Egleston

What’s next

The judge will set a deadline for investors to join the class action lawsuit as lead plaintiffs.

The takeaway

This case highlights the importance of pharmaceutical companies providing accurate and transparent information to investors about their drug development programs. The significant stock price drop following Gossamer's failed trial demonstrates the financial risks investors face when relying on overly optimistic company disclosures.