Portnoy Law Firm Files Class Action Lawsuit Against Gossamer Bio

Investors allege company misled them about Phase 3 drug trial results

Apr. 15, 2026 at 6:54pm

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A photorealistic studio still life showing a shattered glass beaker or test tube lying on a clean, monochromatic background, symbolizing the failure of a pharmaceutical drug trial and the resulting loss of investor confidence.The collapse of a promising drug trial leaves investors with shattered confidence in the pharmaceutical company's future.Los Angeles Today

The Portnoy Law Firm has filed a class action lawsuit against Gossamer Bio, Inc. on behalf of investors who purchased the company's securities between June 2025 and February 2026. The lawsuit alleges that Gossamer provided investors with overly positive information about the design and results of its Phase 3 PROSERA clinical trial for the drug seralutinib, while concealing material adverse facts that caused the trial to fail to meet its primary endpoint.

Why it matters

This lawsuit highlights the risks investors face when companies allegedly provide misleading information about the progress of their drug development programs. Investors rely on accurate disclosures to make informed decisions, and lawsuits like this aim to hold companies accountable for any deception that leads to financial losses.

The details

According to the complaint, Gossamer told investors it was confident in the PROSERA trial design, but failed to disclose issues with controlling the placebo response at testing sites in Latin America. When Gossamer later announced the trial had failed to meet its primary endpoint, the company's stock price plummeted by over 80% in a single day.

  • The class period covers June 2025 to February 2026.
  • Investors have until June 1, 2026 to file a lead plaintiff motion.

The players

Portnoy Law Firm

A law firm representing investors in pursuing claims caused by corporate wrongdoing.

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 are encouraged to contact attorney Lesley F. Portnoy, by phone 310-692-8883 or email: lesley@portnoylaw.com, to discuss their legal rights, or join the case via https://portnoylaw.com/gossamer-bio-inc.”

— Lesley F. Portnoy, Attorney

What’s next

Investors have until June 1, 2026 to file a lead plaintiff motion in the class action lawsuit against Gossamer Bio.

The takeaway

This case highlights the importance of pharmaceutical companies providing accurate and transparent information to investors about the progress of their drug development programs. Lawsuits like this aim to hold companies accountable and protect investors from potential deception.