Gossamer Bio announced that its Phase III Prosera trial testing inhaled tyrosine kinase inhibitor seralutinib in pulmonary arterial hypertension (PAH) failed to meet its primary endpoint, citing an outsized placebo response. The treatment produced a numerical improvement in six‑minute walk distance but did not reach statistical significance. The study miss triggered a steep sell‑off in Gossamer stock and left the company evaluating next steps with regulators. Gossamer said it will analyze additional trial data and engage the FDA, but the setback complicates the program’s regulatory path and raises questions about design and endpoint selection for inhaled PAH agents.