Gossamer Bio's Phase 3 PROSERA PAH Trial Misses Primary Endpoint Despite Numerical 6MWD Gain

Company cites unexpectedly strong placebo response and regional heterogeneity as key factors in trial outcome

Feb. 23, 2026 at 6:13pm

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Gossamer Bio reported top-line results from its global Phase 3 PROSERA study evaluating inhaled seralutinib in patients with pulmonary arterial hypertension (PAH). While the trial showed a numerical improvement versus placebo on the primary endpoint of change in six-minute walk distance (6MWD) at week 24, it did not meet the pre-specified statistical threshold. Gossamer attributed the miss largely to an unexpectedly strong placebo response and regional variability in the results.

Why it matters

The failure to meet the primary endpoint is a setback for Gossamer's lead PAH program, which the company had hoped would be a key driver of future growth. The results raise questions about the viability of seralutinib as a potential treatment for PAH and will likely impact Gossamer's strategic planning and resource allocation going forward.

The details

The PROSERA trial randomized 390 PAH patients 1:1 to seralutinib or placebo. At week 24, the seralutinib arm showed a 28.2-meter improvement in 6MWD versus a 13.5-meter improvement in placebo, resulting in a placebo-adjusted gain of 13.3 meters. However, this did not meet the trial's pre-specified statistical threshold of p<0.025. Gossamer cited an 'outsized' placebo response and meaningful regional heterogeneity, with stronger results in North America compared to Latin America, as key factors behind the miss.

  • The PROSERA trial treated patients for up to 48 weeks.
  • Top-line results were reported on February 23, 2026.

The players

Gossamer Bio

A clinical-stage biopharmaceutical company focused on developing oral, once-daily therapies for immune-mediated and inflammatory diseases, as well as oncology indications.

Seralutinib

Gossamer's lead PAH program, an inhaled therapy that was evaluated in the PROSERA Phase 3 trial.

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What they’re saying

“We must continue to investigate the drivers of the regional placebo effect with investigators and our clinical research partners.”

— Gossamer Bio Executives (Marketbeat)

What’s next

Gossamer plans to fully analyze the PROSERA dataset, engage with the FDA, and evaluate strategic options and resource allocation for the seralutinib program. The company will also pause enrollment in the SERANATA Phase 3 study to prioritize resources toward the Group 1 PAH opportunity.

The takeaway

The missed primary endpoint in the PROSERA trial is a significant setback for Gossamer's lead PAH program, raising doubts about seralutinib's viability as a potential treatment. The company will need to carefully assess the full dataset and work with regulators to determine the path forward, while also re-evaluating its overall R&D priorities and resource allocation.