Gossamer Bio's Seralutinib Misses Primary Endpoint in Phase 3 PAH Trial

Gossamer Bio announced topline results from the Phase 3 PROSERA study of seralutinib in patients with pulmonary arterial hypertension (PAH) on February 23, 2026. The study narrowly missed its primary endpoint, with seralutinib demonstrating a placebo-adjusted improvement in Six-Minute Walk Distance (6MWD) of +13.3 meters at Week 24 (p = 0.0320), missing the prespecified alpha threshold of 0.025.

At Week 24, patients receiving seralutinib had a median change of +28.2 meters in 6MWD from baseline, while patients receiving placebo had a median change from baseline in 6MWD of +13.5 meters. The estimated Hodges-Lehmann treatment effect was +13.3 meters, with a p-value of 0.0320, which did not meet the prespecified threshold on the primary endpoint (α = 0.025); therefore, p-values for the key secondary endpoints cannot be evaluated for statistical significance. All p-values herein are nominal. All four key secondary endpoints favored seralutinib versus placebo in the overall population.

Consistent with the Phase 2 TORREY Study, seralutinib delivered a compelling signal in the prespecified intermediate- and high-risk subgroup (n = 234), as defined by the REVEAL 2 Lite Risk Score ≥ 6 at screening, with a +20.0m placebo-adjusted improvement in 6MWD (p = 0.0207). Three of four key secondary endpoints also demonstrated a p-value below 0.0125, underscoring seralutinib's activity in patients with higher risk.

The overall treatment effect was most pronounced in North America (n = 75), with a +25.9m placebo-adjusted improvement in 6MWD (p = 0.0573).

A key secondary endpoint, change in NT-proBNP at Week 24, demonstrated an estimated location shift of -120.4 ng/L compared with placebo (p=0.0002) in the overall population, with separation between the arms favoring seralutinib observed starting at Week 4 (-96.0 ng/L; p=0.0002). Key secondary endpoints time-to-clinical worsening (TTCW), clinical improvement and proportion of patients with a one point or greater reduction in REVEAL Lite 2 Risk Score all favored seralutinib as compared to placebo in the overall population.

In a prespecified subgroup analysis of patients with REVEAL Lite 2 score ≥6 at screening, corresponding to intermediate- and high-risk patients, seralutinib demonstrated a pronounced and clinically meaningful response profile across the primary and key secondary endpoints.

The PROSERA population was heavily treated, including 55% of patients on triple or quadruple background PAH therapy and 61% on background prostacyclin therapy. Seralutinib was generally well tolerated, with safety consistent with prior experience.

Gossamer Bio and the Chiesi Group are jointly developing seralutinib under a global collaboration agreement for the treatment of pulmonary arterial hypertension (PAH) and pulmonary hypertension associated with interstitial lung disease (PH-ILD). Gossamer plans to meet with the U.S. FDA to discuss the path forward.