Gossamer Bio reported that its Phase 3 PROSERA study of inhaled tyrosine kinase inhibitor seralutinib in pulmonary arterial hypertension missed the primary endpoint, with an observed placebo‑adjusted improvement below the target threshold. The company attributed part of the outcome to an outsized placebo response. The topline miss sent Gossamer’s stock sharply lower and prompted the company to pause enrollment in a second Phase 3 study while conducting deeper analyses and engaging the FDA on next steps. Reported adverse events included cough and elevated liver enzymes in a subset of treated patients. Gossamer said it will pursue regulatory dialogue and further data review to determine a potential path forward despite the failed primary endpoint.