Gossamer Bio (NASDAQ:GOSS) shares dropped roughly 80% on Monday after the company announced that its investigational therapy seralutinib failed to achieve the primary endpoint in the Phase 3 PROSERA study targeting pulmonary arterial hypertension (PAH).
The trial reported a placebo-adjusted improvement in Six-Minute Walk Distance (6MWD) of 13.3 meters at Week 24, with a p-value of 0.0320, falling short of the study’s predefined statistical significance threshold of 0.025. Patients treated with seralutinib recorded a median improvement of 28.2 meters from baseline, compared with a 13.5-meter median improvement among those receiving placebo.
Although the primary endpoint was not met, the therapy demonstrated positive signals in certain subgroups. Among the prespecified intermediate- and high-risk patient group, comprising 234 participants, seralutinib achieved a placebo-adjusted improvement of 20.0 meters in 6MWD with a p-value of 0.0207. The strongest treatment effect was observed in North America, where the drug showed a 25.9-meter placebo-adjusted improvement, though this result did not reach statistical significance.
Results for a key secondary endpoint indicated that seralutinib reduced NT-proBNP levels by 120.4 ng/L relative to placebo at Week 24. In patients with connective tissue disease-associated PAH, the treatment delivered a placebo-adjusted gain of 37.0 meters in six-minute walk distance.
The drug was generally well tolerated, with treatment-emergent adverse events reported in 86.5% of patients receiving seralutinib compared with 80.5% in the placebo group. Cough was the most commonly reported side effect, affecting 37.0% of patients treated with the drug.
Gossamer Bio said it plans to engage with the U.S. Food and Drug Administration to determine next steps for the development program. The company has also paused enrollment in its SERANATA study while it assesses the implications of the PROSERA findings, including regional differences in placebo response. The trial population included 55% of patients receiving triple or quadruple background PAH therapies and 61% on prostacyclin-based treatment.