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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 case follows Gossamer’s announcement on February 23, 2026 that top-line results for its Phase 3 PROSERA study did not meet the primary endpoint—change from baseline in six-minute-walk distance at week 24. The study evaluated seralutinib for the treatment of pulmonary arterial hypertension (PAH).
The litigation centers on the propriety of Gossamer’s disclosures regarding the Phase 3 PROSERA trial design, including its patient recruitment protocol and site-level monitoring.
According to the complaint, Gossamer allegedly knew of, or recklessly disregarded, trial design issues with PROSERA and instead presented a narrative intended to assure investors that the company would meet the primary endpoint.
The complaint further alleges that, unknown to investors, patients at the study’s Latin America sites were largely heavily treated and were performing particularly well on placebo.
In prior communications, Gossamer emphasized that seralutinib is a “potential first-in-class therapeutic,” describing it as representing “the possibility of a multi-billion-dollar opportunity across multiple indications.”
In mid-November 2025, management pointed to the Merck Phase 3 STELLAR study of sotatercept for treating PAH. Management said that the best performing region was Latin America and that the company had more patients coming from the same geographies and sites. Management also stated that it had gone to locations where precedent studies showed the greatest efficacy and that entry criteria were intended to ensure patients would show improvement based on background disease at week 24.
On February 23, 2026, Gossamer announced that PROSERA did not meet its primary endpoint and that efficacy was not statistically significant.
During the conference call that day, management said the 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.” Management also stated that in Latin America, “outsized placebo improvements materially compressed the pool treatment difference.”
Following the announcement, the market reaction included an 80% decline in Gossamer’s share price.
After the Class Period, on April 9, 2026, Gossamer disclosed that since February 24, 2026 it had not met the minimum share bid price of $1 required for continued listing on the Nasdaq Global Select Market.
The proposed class period is June 16, 2025 through February 20, 2026. The lead plaintiff deadline is June 1, 2026.
“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.
The firm also referenced the SEC Whistleblower program, stating that whistleblowers who provide original information may receive rewards totaling up to 30% of any successful recovery made by the SEC.
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