Gossamer Bio, Inc. (GOSS) Faces Securities Class Action Amid 80% Drop On Phase 3 PROSERA Trial Failure- Hagens Berman

SAN FRANCISCO, April 27, 2026 /PRNewswire/ -- A securities class action lawsuit has been filed against Gossamer Bio, Inc. (NASDAQ: GOSS) and an executive, seeking to represent investors who purchased or otherwise acquired Gossamer securities between June 16, 2025 and February 20, 2026.

The lawsuit follows Gossamer's bombshell announcement on February 23, 2026 that top-line results for its Phase 3 PROSERA study did not meet the primary endpoint (the change from baseline in six-minute-walk distance at week 24). The study evaluated seralutinib for the treatment of pulmonary arterial hypertension ("PAH").

The developments, including the trial failure and 80% stock drop, prompted national shareholder rights firm Hagens Berman to commence an investigation into the alleged pending claims that Gossamer violated federal securities laws.  The firm encourages Gossamer investors who suffered substantial losses on Class Period GOSS investments to submit your losses now.

The firm also encourages persons with knowledge who may be able to assist the investigation to contact its attorneys.

Class Period: June 16, 2025 – Feb. 20, 2026
Lead Plaintiff Deadline: June 1, 2026
Visit: www.hbsslaw.com/investor-fraud/goss
Contact the Firm Now: [email protected]
                                       844-916-0895

Gossamer Bio, Inc. (GOSS) Securities Class Action:

The litigation is focused on the propriety of Gossamer's disclosures about the Phase 3 PROSERA trial design, including its patient recruitment protocol and site-level monitoring.

In the past, Gossamer has emphasized that serlutinib is a "potential first-in-class therapeutic[,]" which "represents the possibility of a multi-billion-dollar opportunity across multiple indications[.]"

As recently as mid-November 2025, the company's management cited the highly successful Merck Phase 3 STELLAR study of sotatercept for treating PAH. Gossamer's management said, "if you look at their data, the best performing region was Latin America, and we have actually more patients coming from those same geographies and same sites." Management also assured investors that "we have gone to the places where precedent studies have shown the greatest amount of efficacy, as well as having an entry criteria that is ensuring that we have patients who, we believe, will really show an improvement based upon, again background disease at week 24."

The complaint alleges that, unknown to investors, Gossamer knew of or recklessly disregarded the trial design issues with the Phase 3 PROSERA study and, instead, crafted a narrative assuring investors that it would meet its primary endpoint. Also unknown to investors, patients at the study's Latin America sites were largely heavily-treated and performing particularly well on placebo.

Investors' expectations were dashed on February 23, 2026. That day, Gossamer announced that PROSERA did not meet its primary endpoint and therefore efficacy was not statistically significant.

Management said during the conference call that day, "[t]he overall treatment effect and statistical parameters were materially diluted by an outsize placebo response and meaningful regional heterogeneity, which compressed the pool placebo-adjusted difference." More specifically, management revealed that in "Latin America, outsized placebo improvements materially compressed the pool treatment difference."

The market swiftly reacted, sending the price of Gossamer shares down by 80%.

After the Class Period, on April 9, 2026, the company revealed that since February 24, 2026 it has not met the minimum share bid price ($1) required for continued listing on the Nasdaq Global Select Market.

"We're focused on whether Gossamer may have misled investors about the PROSERA trial design, including patient entry criteria, as alleged in the pending lawsuit," said Reed Kathrein, the Hagens Berman partner leading the firm's investigation.

If you invested in Gossamer Bio and have substantial losses, or have knowledge that will assist the firm's investigation, submit your losses now.

If you'd like more information and answers to additional frequently asked questions about the Gossamer case and the firm's investigation, read more »

Whistleblowers: Persons with non-public information regarding Gossamer Bio should consider their options to help in the investigation or take advantage of the SEC Whistleblower program. Under the new program, whistleblowers who provide original information may receive rewards totaling up to 30 percent of any successful recovery made by the SEC. For more information, call Reed Kathrein at 844-916-0895 or email [email protected].

About Hagens Berman
Hagens Berman is a global plaintiffs' rights complex litigation firm focusing on corporate accountability. The firm is home to a robust practice and represents investors as well as whistleblowers, workers, consumers and others in cases achieving real results for those harmed by corporate negligence and other wrongdoings. Hagens Berman's team has secured more than $2.9 billion in this area of law. More about the firm and its successes can be found at hbsslaw.com. Follow the firm for updates and news at @ClassActionLaw.

SOURCE Hagens Berman Sobol Shapiro LLP