Investor Lawsuits Target Gossamer Bio Over Failed PROSERA Trial

Overview

• Investor-rights firms on Friday reminded Gossamer Bio shareholders of a pending class action and invited investors to seek lead plaintiff status by June 1, 2026.
• Filed complaints claim the company misled investors about the Phase 3 PROSERA trial’s design, focusing on how it handled placebo responses at Latin American sites.
• Gossamer disclosed on February 23, 2026 that PROSERA missed its main goal of improving six-minute walk distance, reporting a +13.3 meter placebo-adjusted gain that was not statistically significant under a 0.025 threshold.
• The company said the shortfall stemmed from unusually strong placebo results at Latin American locations, which it linked to enrolling a heavily treated, lower-risk group of patients.
• The suits cite federal securities laws, set a class period of June 16, 2025 through February 20, 2026, and note that the stock fell about 80% to $0.42 after the topline data, with allegations still unproven at this early stage.