Gossamer Bio’s Phase 3 PROSERA study of seralutinib in pulmonary arterial hypertension failed to meet its primary endpoint on six‑minute walk distance, producing a placebo‑adjusted improvement below the statistical threshold for success. The company cited an outsized placebo response and will analyze data further while engaging the FDA. The negative readout led to a dramatic market reaction with shares collapsing, reflecting investor concern about the drug’s commercial prospects and broader implications for inhaled TKI programs. Reported adverse events included cough and elevated liver enzymes in treated patients. Gossamer paused enrollment in a parallel trial and indicated it will consult regulators on next steps; the result complicates development timelines and raises questions about trial design sensitivity in PAH studies.