Gossamer (GOSS) told investors its Phase 3 PROSERA study was progressing, with executives expressing confidence in the trial’s design and patient selection. A securities complaint now alleges those statements omitted material issues tied to placebo response at Latin American sites.
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A federal securities lawsuit alleges Gossamer concealed material issues concerning the design of its Phase 3 PROSERA trial, including placebo-response issues at Latin American sites, before the stock fell more than 80% after the topline results were disclosed.
Shares of Gossamer Bio (NASDAQ: GOSS) fell from $2.13 to $0.42 on February 23, 2026, after the company announced that its Phase 3 PROSERA study failed to meet its primary endpoint. The lawsuit, filed in the Southern District of California, alleges that defendants made positive statements about the trial’s design while allegedly concealing known risks tied to patient selection at Latin American testing sites. The proposed class period runs from June 16, 2025, through February 20, 2026.
Investors who purchased Gossamer Bio securities during this period may have legal rights. Visit zlk.com to learn more or check your eligibility.
Company Focus and Clinical Pipeline
Gossamer Bio, Inc. is a clinical-stage biopharmaceutical company headquartered in San Diego, California. The company describes its primary focus as the development and commercialization of seralutinib for the treatment of pulmonary hypertension associated with interstitial lung disease.
During the class period, Gossamer’s common stock traded on the NASDAQ under the ticker symbol “GOSS.” The company was working toward establishing seralutinib as a potential first-in-class treatment option for pulmonary arterial hypertension, a serious and progressive form of pulmonary hypertension.
What the Lawsuit Alleges: Trial Design Risks Were Allegedly Concealed
The complaint centers on Gossamer’s Phase 3 PROSERA study, which was evaluating seralutinib for pulmonary arterial hypertension. According to the complaint, defendants made repeated public statements expressing confidence in the study’s design and patient selection while allegedly failing to disclose that patients enrolled at Latin American testing sites were heavily treated and lower risk, a population that the complaint alleges ultimately performed unusually well on placebo.
The plaintiff alleges that this concealment caused investors to purchase Gossamer’s securities at artificially inflated prices during the class period. The lawsuit argues that the defendants knew or recklessly disregarded these trial design issues and nonetheless communicated a positive outlook to investors.
Investors who followed this case from the start and want to stay informed on developments as it moves through the courts can learn more at zlk.com.
What Company Leadership Said During the Class Period
On June 16, 2025, CEO and co-founder Faheem Hasnain stated in a press release announcing enrollment completion that, based on insights from the earlier Phase 2 TORREY Study, the company had focused on selecting a patient population aligned with the study’s objectives and one more likely to exhibit a clinically significant benefit within 24 weeks. He said the company firmly believed it had achieved that patient-selection goal and expressed eagerness to share topline results early the following year.
On August 5, 2025, Hasnain described the upcoming PROSERA readout as the foundation for a potential multi-billion-dollar franchise and stated that the team remained focused on executing the study with discipline and operational excellence, grounded in their conviction in the strength of the science. On November 5, 2025, he called the moment pivotal, praised the team’s focus and professionalism, and again indicated topline results would follow in February.
How the Alleged Truth Emerged
On February 23, 2026, Gossamer published a press release and hosted a Special Call to announce topline results from the PROSERA study. The company disclosed that the trial failed to meet its primary endpoint of an improvement in six-minute walk distance at Week 24, with a placebo-adjusted gain of 13.3 meters that did not meet the prespecified alpha threshold of 0.025.
During the Special Call, Chief Medical Officer Richard Aranda stated that the placebo arm in PROSERA showed a larger improvement than is often seen in other Phase 3 pulmonary hypertension trials, where placebo results typically remain near baseline or decline. He noted that the strong placebo improvement was particularly pronounced in Latin America, where it materially compressed the overall treatment difference. COO and CFO Bryan Giraudo described the Latin American placebo result as “extremely, extremely disturbing” to the team, noting that the company had specifically invested in that geography, expecting it to be its best-performing region, as it had been in prior studies of other drugs. Giraudo acknowledged that the company was still early in its investigation into what happened there.
Following the disclosure, Gossamer’s stock dropped from a closing price of $2.13 per share on February 20, 2026, to $0.42 per share on February 23, 2026, a decline of more than 80% in a single trading session.
Why Investors Are Paying Attention
The complaint alleges that investors who purchased Gossamer shares between June 16, 2025, and February 20, 2026, did so at artificially inflated prices due to statements that omitted material risks related to the trial design. When information about the PROSERA study’s failure emerged, the stock lost the vast majority of its value in a single day, resulting in significant financial losses for shareholders who held positions during the class period.
The case is particularly relevant to investors who were following the PROSERA readout as a catalyst. The complaint contends that management’s repeated expressions of confidence in patient selection and trial design did not reflect what the company allegedly knew about potential issues at Latin American sites.
The Legal Framework Behind the Claims
The lawsuit brings claims under Section 10(b) of the Securities Exchange Act and Rule 10b-5 promulgated by the Securities and Exchange Commission, which prohibit material misstatements and omissions in connection with the purchase or sale of securities. The complaint also brings a claim under Section 20(a) of the Exchange Act against Faheem Hasnain as an alleged controlling person of the company, seeking to hold him liable for the primary violations.
The plaintiff alleges the defendants knew or recklessly disregarded that public statements were materially false or misleading at the time they were made, and that concealed facts would have been important to reasonable investors making decisions about whether to buy or sell Gossamer shares.
Investors who believe they may have been affected by these allegations and want to understand their rights can visit zlk.com for more information.
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