Gossamer Bio Hit With Investor Class Action After PROSERA Miss, With June 1 Lead‑Plaintiff Deadline

Overview

• The securities case is now in the lead‑plaintiff stage, with multiple shareholder firms urging investors to file motions by June 1, 2026.
• The complaint alleges Gossamer misled investors about the PROSERA trial’s design, including patient entry rules and site monitoring that could have allowed large placebo gains at Latin American sites.
• Shares fell about 80% in a single session after the February 23 disclosure that the Phase 3 study of seralutinib for pulmonary arterial hypertension failed its main goal on six‑minute‑walk distance.
• Company leaders said the treatment effect looked smaller because of an outsized placebo response and regional differences, especially in Latin America.
• Gossamer later disclosed it has been below Nasdaq’s $1 minimum bid price since February 24, 2026, putting its listing at risk while the litigation remains at an early stage.