Gossamer Bio Faces Securities Class Action After PROSERA Failure, 80% Stock Plunge

Overview

• - A shareholder class action covering purchases from June 16, 2025 to February 20, 2026 is now on file, with June 1, 2026 set as the deadline to seek the lead plaintiff role.
• - Complaints say the company misled investors about Phase 3 PROSERA’s design, pointing to patient entry rules and site monitoring that left Latin America sites prone to large placebo gains that could hide the drug’s effect.
• - The company said on February 23, 2026 that PROSERA missed its main goal to raise six‑minute walk distance at week 24 in pulmonary arterial hypertension, and it blamed an outsize placebo response and regional differences, especially in Latin America.
• - Shares fell about 80% to roughly $0.42 after the disclosure, and on April 9, 2026 the company reported it had been under Nasdaq’s $1 minimum share price, which puts its listing at risk unless the price recovers.
• - Investor-rights firms are urging investors to submit losses on a contingency basis and inviting whistleblower tips, noting the SEC program can pay up to 30% of funds the agency recovers.