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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 complaint 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 previously emphasized that seralutinib was a “potential first-in-class therapeutic,” describing it as “the possibility of a multi-billion-dollar opportunity across multiple indications.” The company also referenced the Merck Phase 3 STELLAR study of sotatercept for PAH, including management commentary that the best-performing region was Latin America and that the company had patients coming from similar geographies and sites.
In the lawsuit, plaintiffs allege that, unknown to investors, Gossamer knew of or recklessly disregarded trial design issues and instead presented a narrative intended to assure investors that the company would meet its primary endpoint. The complaint further alleges that patients at Latin America sites were largely heavily treated and performed particularly well on placebo.
On February 23, 2026, Gossamer disclosed that PROSERA did not meet its primary endpoint and that efficacy was not statistically significant.
During the conference call that day, management attributed the result to factors including “an outsize placebo response” and “meaningful regional heterogeneity,” stating that these effects “compressed the pool placebo-adjusted difference.” Management specifically said that in “Latin America, outsized placebo improvements materially compressed the pool treatment difference.”
Following the announcement, Gossamer shares fell by 80%.
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.
“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 encouraged investors with substantial losses and individuals with non-public information to contact its attorneys. The article further referenced the SEC Whistleblower program, stating that whistleblowers who provide original information may receive rewards totaling up to 30 percent of any successful recovery made by the SEC.
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