Gossamer Bio ($GOSS) Phase 3 Trial Design Issues Case

Investors can submit applications for the lead plaintiff role.

S.D. California

Court

3:26-cv-02016

Case number

06/16/2025

Class period Start

02/20/2026

Class period End

06/01/2026

Lead Plaintiff motion deadline

$GOSS investors filed a claim against Gossamer Bio for allegedly hiding problems in its Phase 3 PROSERA study design, especially placebo response issues at Latin American sites.
After Gossamer disclosed that PROSERA missed its main goal and tied the miss to unusually strong placebo performance at Latin American sites, $GOSS fell 80.3% on February 23, 2026.
$GOSS investors can join this case to be notified about potential recovery.
Case Details:

Between June 16, 2025, and February 20, 2026, Gossamer Bio told investors its Phase 3 PROSERA study for seralutinib in pulmonary arterial hypertension was well designed and on track. Executives emphasized patient selection, confidence in the study’s setup, operational execution, and their expectation of sharing topline results in February.

However, during this period, investors allege the study had design and execution problems that undercut the reported outlook. Gossamer Bio allegedly failed to disclose that its patients at Latin American sites were largely heavily treated and lower risk. Those patients performed unusually well on placebo, reducing the measured treatment benefit, and those issues created a serious risk that PROSERA would miss its main six-minute walk distance endpoint at Week 24.

Then, on February 23, 2026, the company revealed topline results showing PROSERA failed to meet its primary endpoint, with a +13.3 meter placebo-adjusted gain that did not clear the required statistical threshold. $GOSS fell 80.3%, closing at $0.42. 

Additional revelations followed on March 5, 2026, including an analyst report stating the Latin America issue appeared to be a trial execution problem and that Gossamer was conducting a data review investigation. 

By February 23, 2026, shares had dropped to $0.42, representing a total decline of 80.3% over the correction period.

Based on these events, $GOSS investors filed a claim against Gossamer Bio, alleging the company:
It hid problems in the PROSERA study design.
It gave investors a confident picture of the trial while not disclosing that placebo results in Latin America could skew the outcome.
It misled investors about the study’s reliability and chances of success, leading to losses when the results were disclosed.
Investors argue Gossamer Bio misled the market about the reliability of the PROSERA trial and its chances of meeting the main endpoint, causing losses when the truth emerged.

Case Type

US Securities Class Action

Case Status

Lead Plaintiff Submission

Alleged Offence

Misleading Statements,

Fraud,

Failure to Disclose,

Omissions

Suspected Party

Directors,

Management

Security Type

Stocks

Trade Direction

Long

Shock Event Date

02/23/2026

Filing date

03/31/2026

Lead Plaintiff Deadline

06/01/2026

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