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Gossamer Bio’s investigational tyrosine kinase inhibitor seralutinib failed to significantly improve functional performance in a Phase 3 pulmonary arterial hypertension study, casting considerable doubt on the program’s prospects moving forward.
The biotech ended Monday’s trading session at $0.42 per share, 80% lower than its closing price of $2.13 on Friday.
Results, revealed in anews releaseon Monday, come from the Phase 3 PROSERA trial, which randomly assigned 390 patients with pulmonary arterial hypertension (PAH) to receive either inhalable seralutinib or placebo. After 24 weeks, patients in the active treatment arm saw a 28.2-meter improvement in the six-minute walk distance test, versus 13.5 meters in placebo counterparts.
The placebo-adjusted treatment effect came out to +13.3 meters, which was in favor of seralutinib but fell short of a prespecified statistical threshold.
Aside from missing the significance bar, PROSERA’s findings fell “well below what we believe could be considered clinically meaningful results,” Guggenheim Partners wrote in a Monday note to investors. “It is difficult for us to see how [Gossamer] can bring seralutinib to the market with their current dataset (without possibly doing another more targeted trial) and given their current financial position.” As of Sept. 30, 2025, the biotech had$180 millionin cash, equivalents and marketable securities, enough to keep it going into 2027.
Despite the topline miss, Gossamer pointed to what it called a “compelling signal” in patients with intermediate- and high-risk PAH, in whom seralutinib treatment led to stronger functional improvements. The investigational drug also led to a significant decrease in levels of NT-proBNP, a key disease biomarker.
These positive signals, according to analysts at H.C. Wainwright & Co., “demonstrate evidence of clinical activity,” particularly in patients who have been heavily pretreated. “A path forward may yet emerge” for seralutinib, they added, “but uncertainty surrounding regulatory strategy and capital implications is likely to remain an overhang until greater clarity emerges.”
Gossamer, for its part, plans to meet with the FDA to determine the best way forward for seralutinib. Pending the results of those discussions, however, the biotech is suspending enrollment into its Phase 3 SERANATA study, which is testing the drug in pulmonary hypertension associated with interstitial lung disease.
Monday’s late-stage stumble for Gossamer comes as Merck’s Winrevair continues to entrench its leadership status in the space. The drug was first approved in March 2024 in a decision that experts said was a “game changer” for the disease. In November last year, Phase 2 data pointed to Winrevair’spotential to expandinto combined post- and precapillary pulmonary hypertension, for which the pharma now plans to run late-stage trials.