Gossamer Bio, Inc. 8-K

What Happened

• On February 23, 2026 Gossamer Bio announced topline results from its Phase 3 PROSERA study of inhaled seralutinib in pulmonary arterial hypertension (PAH). The trial enrolled 390 patients (197 seralutinib, 193 placebo). At Week 24 median 6‑minute walk distance (6MWD) change was +28.2 m for seralutinib vs +13.5 m for placebo; the Hodges‑Lehmann treatment effect was +13.3 m (p = 0.0320). This p‑value did not meet the prespecified primary endpoint threshold (α = 0.025), so statistical significance cannot be claimed for key secondary endpoints (all p values reported are nominal).
• Key secondary and subgroup signals: NT‑proBNP (a heart‑strain biomarker) showed a location shift of ‑120.4 ng/L vs placebo at Week 24 (p = 0.0002) with separation starting at Week 4. In the prespecified intermediate/high‑risk subgroup (REVEAL 2 Lite ≥6; n = 234), seralutinib showed a placebo‑adjusted +20.0 m improvement in 6MWD (p = 0.0207) and favorable results on several secondaries (e.g., NT‑proBNP ‑265.8 ng/L, p = 0.0002; odds ratio for ≥1‑point REVEAL improvement = 2.033, p = 0.0083). In connective tissue disease–associated PAH (n = 87) seralutinib achieved a +37.0 m placebo‑adjusted gain (p = 0.0104).
• Safety: treatment‑emergent adverse events occurred in 86.5% (seralutinib) vs 80.5% (placebo); serious adverse events 16.0% vs 18.9%. Transaminase elevations ≥3× ULN were reported in 13% of seralutinib patients vs 1% on placebo. The most common adverse event with seralutinib was cough (37.0%).
• Next steps: Gossamer expects to meet with the U.S. FDA to discuss a development path for seralutinib and has paused enrollment in the ongoing SERANATA Phase 3 study to evaluate the PROSERA results and regional placebo response patterns. Slides (Exhibit 99.1) accompany the filing and were presented in a February 23, 2026 webcast.

Key Details

• Trial size: 390 patients (197 seralutinib; 193 placebo).
• Primary endpoint: 6MWD median change at Week 24 — seralutinib +28.2 m vs placebo +13.5 m; treatment effect +13.3 m (p = 0.0320; prespecified α = 0.025 not met).
• Biomarker result: NT‑proBNP Week 24 location shift ‑120.4 ng/L vs placebo (p = 0.0002).
• Safety flag: ALT/AST ≥3× ULN in 13% (seralutinib) vs 1% (placebo).

Why It Matters

• The study produced measurable improvements in exercise capacity and clear biomarker benefits, but the primary endpoint missed the trial’s prespecified statistical threshold. Because of that, key secondary outcomes cannot be formally claimed as statistically significant in the overall population, which affects regulatory interpretation.
• Stronger effects in higher‑risk patients and in the CTD‑APAH subgroup could inform discussions with the FDA about possible regulatory paths, targeted labeling, or additional studies. However, elevated liver enzymes and a higher overall rate of treatment‑emergent AEs will be important safety considerations.
• For investors, the immediate takeaways are: (1) seralutinib shows encouraging signals but did not meet the trial’s predefined primary success criterion, (2) Gossamer is pausing another Phase 3 enrollment and will engage the FDA to define next steps, and (3) future development plans (and potential capital needs) depend on regulatory feedback and fuller data review. The filing also includes the company’s forward‑looking statements and related cautionary language.