San Diego-based Gossamer Bio released long-awaited Phase 3 data today for its drug seralutinib, revealing a complex set of results that both challenged and encouraged researchers. The PROSERA study, which tested the inhaled medication in patients with pulmonary arterial hypertension (PAH), officially missed its primary goal by a narrow margin. However, the drug appeared to punch above its weight class in patients facing the most severe forms of the progressive heart and lung condition.
The primary measure of the study was the change in how far patients could walk in six minutes after 24 weeks of treatment. Patients taking seralutinib saw a median improvement of 28.2 meters, compared to 13.5 meters for those on a placebo.
While the 13.3-meter difference in favor of the drug was statistically significant by traditional standards (p = 0.0320), it failed to hit the exceptionally high “alpha threshold” of 0.025 that the company had pre-set with regulators. Because that primary domino didn’t fall exactly as planned, other secondary successes in the trial cannot be officially labeled as statistically significant, even though the raw numbers leaned heavily in the drug’s favor.
Despite the technical miss, the data told a more optimistic story for the sickest participants. In a group of 234 patients classified as intermediate-to-high risk, the improvement jumped to 20 meters over the placebo.
This is a critical finding for a disease where the small arteries in the lungs thicken and narrow, often leading to right-sided heart failure. Many of these patients were already on triple or quadruple therapy—the current gold standard of care—suggesting that seralutinib might offer a new layer of help where existing drugs reach their limit.
The drug, which is inhaled as a dry powder, works differently than most current PAH treatments. Instead of just opening up blood vessels, it targets specific proteins like PDGFR and c-KIT to try and stop the underlying “remodeling” or scarring of the lung tissue. This unique approach seemed particularly effective in patients whose PAH is linked to connective tissue diseases; that specific subgroup saw a robust 37-meter boost in their walking distance.
Safety data remained largely in line with what doctors saw in earlier, smaller trials. While about 13% of patients on the drug saw an increase in liver enzymes compared to just 1% in the placebo group, the most common side effect was a cough, affecting 37% of users. Serious adverse events were actually slightly lower in the seralutinib group (16%) than in the placebo group (18.9%).
Looking ahead, Gossamer Bio isn’t throwing in the towel. CEO Faheem Hasnain noted that while the “stringent” threshold wasn’t met, the drug proved it is active and potentially life-changing for those with advanced disease.
The company plans to sit down with the U.S. Food and Drug Administration (FDA) to see if there is a viable path toward approval based on the strength of the high-risk data. For now, they are pausing enrollment in a separate study to analyze why the placebo group performed better in certain regions than expected—a common hurdle in global clinical trials that can sometimes mask a drug’s true effect.
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