Class action filed against Gossamer Bio over PROSERA trial disclosures

Robbins LLP reminded Gossamer Bio, Inc. (NASDAQ: GOSS) stockholders that a class action has been filed on behalf of investors who purchased or otherwise acquired Gossamer Bio securities between June 16, 2025 and February 20, 2026.

Gossamer Bio is a clinical stage biopharmaceutical company focused on the development and commercialization of seralutinib for the treatment of pulmonary hypertension (PH) associated with interstitial lung disease.

Class period and allegations

The class period is June 16, 2025 to February 20, 2026.

Robbins LLP said it is investigating allegations that Gossamer Bio misled investors regarding the viability of its PROSERA study.

According to the complaint, during the class period, defendants provided investors with material information about Gossamer’s Phase 3 PROSERA study evaluating seralutinib for the treatment of pulmonary arterial hypertension (PAH). The statements included, among other things, confidence in the trial design.

The complaint alleges that these positive statements were made while defendants allegedly disseminated false and misleading statements and/or concealed material adverse facts concerning the Phase 3 PROSERA study design, particularly regarding controlling for the placebo response at the Latin American testing sites. Robbins LLP said the alleged conduct caused investors to purchase Gossamer securities at artificially inflated prices.

Key development and study outcome

Robbins LLP said the “truth emerged” on February 23, 2026, when Gossamer published a press release and hosted a special call announcing topline results for the Phase 3 PROSERA study.

The company’s results, according to the complaint, failed to meet the primary endpoint of improved six-minute walk distance (6MWD) at Week 24. The placebo-adjusted gain was reported as +13.3 meters, with a p-value of 0.0320, which the complaint said did not meet the required 0.025 alpha threshold.

Robbins LLP also noted that Gossamer attributed the miss to patients at Latin American sites performing particularly well on placebo, citing enrollment of a heavily-treated lower-risk population.

Market reaction

On the news, Robbins LLP said the price of Gossamer’s common stock declined from a closing market price of $2.13 per share on February 20, 2026 to $0.42 per share on February 23, 2026, a decline of over 80% in one day.

What shareholders can do

Robbins LLP said shareholders may be eligible to participate in the class action against Gossamer Bio, Inc.

Shareholders who wish to serve as lead plaintiff must submit their papers to the court by June 1, 2026. The lead plaintiff is described as a representative party that directs the litigation on behalf of other class members. Robbins LLP said shareholders do not have to participate to be eligible for recovery, and those who take no action can remain absent class members.

Fees and representation

Robbins LLP said all representation is on a contingency fee basis, and shareholders pay no fees or expenses.

About Robbins LLP

Robbins LLP described itself as a shareholder rights litigation firm that has been dedicated to helping shareholders recover losses, improve corporate governance structures, and hold company executives accountable since 2002.

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