Gossamer (GOSS) told investors its Phase three PROSERA study was on track, with executives expressing confidence in the trial design. But the new lawsuit alleges that issues tied to patient enrollment and placebo response at Latin American sites materially undermined the trial’s outcome.
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A federal securities lawsuit filed against Gossamer Bio, Inc. and its chief executive alleges that the company made misleading public statements about a clinical trial for its lead drug candidate, concealing known risks that ultimately caused the trial to miss its primary endpoint and the stock to lose more than 80% of its value in a single trading session.
The complaint, filed March 31, 2026, in the United States District Court for the Southern District of California, covers a class period running from June 16, 2025, through February 20, 2026. According to the complaint, Gossamer’s common stock declined from $2.13 per share on February 20, 2026, to $0.42 per share on February 23, 2026, the day the company disclosed that its Phase 3 PROSERA study had failed to meet its primary endpoint.
Investors who purchased Gossamer securities during the class period and suffered losses may wish to review their rights.
A Clinical-Stage Biopharmaceutical Company Focused on Seralutinib
Gossamer Bio, Inc. is a Delaware corporation headquartered in San Diego, California. The complaint describes it as a clinical-stage biopharmaceutical company focused on the development and commercialization of seralutinib, a drug candidate being studied for pulmonary arterial hypertension and pulmonary hypertension associated with interstitial lung disease. During the class period, the company’s common stock traded on the NASDAQ under the ticker symbol GOSS. As of October 31, 2025, the company had approximately 231.5 million shares of common stock outstanding.
Faheem Hasnain, the company’s Chief Executive Officer, Chairman, and Co-founder, is named individually as a defendant. The complaint alleges he had authority over the company’s public disclosures and was responsible for the statements at issue.
What the Lawsuit Claims Gossamer Concealed from the Market
The lawsuit centers on Gossamer’s Phase 3 PROSERA study evaluating seralutinib as a potential treatment for pulmonary arterial hypertension. According to the complaint, defendants issued a series of positive public statements expressing confidence in the trial design and patient selection while allegedly concealing a material risk: that patients enrolled at Latin American clinical testing sites were allegedly largely heavily treated and lower-risk, and ultimately performed particularly well on placebo. The complaint alleges that this known flaw in the study’s design ultimately caused the trial to fail to meet its primary endpoint of improved six-minute walk distance at week 24.
Investors who followed Gossamer during the class period may want to learn more about this ongoing case.
What Company Executives Said Before the Trial Results Were Released
In a June 16, 2025, press release announcing completion of enrollment in the PROSERA study, Defendant Hasnain stated that the company had focused on selecting a patient population that aligns closely with the study’s objectives and is more likely to exhibit a clinically significant benefit in 24 weeks, and expressed firm belief that this patient selection goal had been accomplished.
On August 5, 2025, in connection with the company’s second-quarter financial results, Hasnain described the team as remaining focused on executing the PROSERA study with discipline and operational excellence, grounded in conviction around the strength of the science and the seriousness of the unmet need in PAH. On November 5, 2025, during Gossamer’s third-quarter update, he stated that the company was progressing through the final stages of the PROSERA Phase 3 Study and expressed pride in the team’s focus, diligence, and professionalism ahead of the anticipated February readout.
How the Alleged Truth Came to Light
On February 23, 2026, Gossamer published a press release and hosted a Special Call announcing topline results from the PROSERA study. The company disclosed that the trial had failed to meet its prespecified primary endpoint, reporting a placebo-adjusted improvement in six-minute walk distance of 13.3 meters with a p-value of 0.0320, which did not clear the required 0.025 alpha threshold.
During the Special Call, Gossamer’s Chief Medical Officer noted that the placebo arm had shown an unusually strong improvement compared to other Phase 3 pulmonary hypertension trials and that outsized placebo improvements in Latin America had materially compressed the treatment difference. The company’s Chief Operating Officer and Chief Financial Officer acknowledged during the call that Latin America had produced what the team described as extremely disturbing results, with near parity between placebo and treatment performance in that geography, and that the team had expected Latin American patients to be among the best-responding based on prior industry data. As a result of these disclosures, Gossamer’s stock price fell from $2.13 on February 20, 2026, to $0.42 on February 23, 2026, a single-day decline of over 80%.
Why This Case Is Relevant to Shareholders
The lawsuit alleges that Gossamer and its CEO expressed repeated confidence in the PROSERA trial while concealing the risk posed by the enrollment of heavily treated, lower-risk patients at Latin American sites who ultimately performed unusually well on placebo. According to the complaint, this alleged omission caused the stock to trade at artificially inflated prices throughout the class period. Investors who purchased Gossamer securities during the class period, including investors who acquired securities through assignments from selling put contracts, and who sustained losses after the trial results were announced, may have been directly affected by the alleged conduct.
The Legal Theories Behind the Case
The complaint asserts claims under Section 10(b) of the Securities Exchange Act of 1934, Rule 10b-5 promulgated thereunder, and Section 20(a) of the Exchange Act. Under Section 10(b) and Rule 10b-5, the plaintiff alleges that defendants made materially false and misleading statements and omitted material facts in connection with the purchase and sale of Gossamer securities. Under Section 20(a), Hasnain is alleged to be liable as a controlling person of the company who directed or permitted the alleged misconduct.
Investors who purchased Gossamer securities during the class period and wish to understand their potential legal rights are encouraged to seek additional information.
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