Gossamer Bio (GOSS) Study result summary

Study design and patient population

• PROSERA was a global, randomized, double-blind, placebo-controlled phase III trial in PAH patients on background therapy, enrolling 390 patients (197 seralutinib, 193 placebo) for up to 48 weeks.

• The primary endpoint was change in six-minute walk distance (6MWD) at week 24; key secondary endpoints included NT-proBNP, clinical improvement, and REVEAL Lite 2 risk score.

• The study population was heavily pretreated, with 55% on triple/quadruple therapy, 61% on background prostacyclin, and most on two or more background PAH therapies.

• Baseline demographics were balanced, with mean age ~50 years, ~86% female, and WHO Functional Class II or III PAH.

• Enrollment criteria included REVEAL Lite 2 risk score ≥5, with a prespecified subgroup analysis at risk score ≥6 (intermediate/high risk).

Efficacy results

• At week 24, seralutinib showed a 13.3-meter placebo-adjusted improvement in 6MWD (p=0.032), missing the prespecified alpha of 0.025.

• In intermediate/high-risk patients (REVEAL Lite 2 ≥6), seralutinib achieved a 20-meter placebo-adjusted improvement (p=0.0207), with three of four key secondary endpoints statistically significant.

• NT-proBNP reduction at week 24 was -120.4 ng/L overall (p=0.0002), and -265.8 ng/L in higher-risk patients (p=0.0002), indicating a strong biomarker response.

• In PAH associated with connective tissue disease, seralutinib showed a 37-meter improvement (p=0.0104), outperforming other therapies in this group.

• All four key secondary endpoints favored seralutinib over placebo in the overall population.

Placebo response and regional heterogeneity

• The placebo arm showed an unusually high improvement (13.5 meters), especially in Latin America and Asia/Middle East, compressing the treatment effect.

• In North America, the placebo response was minimal (−3.9 meters), and the treatment effect was most pronounced (25.9 meters).

• Over time, placebo effects normalized and drug-placebo separation increased, particularly at week 48.

• The high placebo response was a key factor in not meeting the primary endpoint, especially in lower-risk and certain geographic subgroups.