Phase 3 Trial Misses Prespecified Threshold, but Seralutinib Will Still Seek FDA Review in September

A Phase 3 trial that failed to meet its prespecified statistical threshold has not brought seralutinib’s path to market to an end. After receiving the official minutes from a pre-NDA meeting with the U.S. Food and Drug Administration (FDA), Gossamer Bio decided to submit a new drug application (NDA) for the inhaled pulmonary arterial hypertension drug in September 2026. At the same time, the company regained its worldwide development and commercial rights, consolidating subsequent decision-making within a single company.

The application will use the Phase 3 PROSERA trial as its pivotal study, supplemented by the Phase 2 TORREY trial and other supportive analyses to form independent corroborating evidence. According to the company’s account, the FDA identified the degree of statistical significance and magnitude of treatment effect in PROSERA as issues to be addressed during the formal review, rather than issues that would prevent submission at this stage. This means the company has obtained a pathway to file; it does not mean the FDA has determined that the evidence is sufficient to support approval.

The central point of contention is PROSERA’s primary endpoint. The company previously reported that, after 24 weeks of treatment, the seralutinib group showed an estimated 13.3-meter improvement in six-minute walk distance compared with placebo, with a nominal p-value of 0.0320, missing the trial’s prespecified threshold of 0.025. As a result, the key secondary endpoints also cannot be claimed as statistically significant under the originally planned statistical framework. How the FDA ultimately assesses the clinical significance of this difference, and whether the Phase 2 results and supportive analyses can strengthen the overall evidence, will be central to the review.

Seralutinib is an inhaled inhibitor of PDGFR, CSF1R, and c-KIT, designed to act directly in the lungs and interfere with signaling associated with vascular remodeling and inflammation. Pulmonary arterial hypertension causes the blood vessels in the lungs to progressively narrow, increasing the workload on the right side of the heart. Existing treatments largely focus on dilating blood vessels, while seralutinib seeks to address structural changes in the disease. However, this mechanistic distinction must still be demonstrated through sufficiently reliable clinical efficacy and safety data.

Management said no new safety concerns were discussed at the pre-NDA meeting, nor was a Risk Evaluation and Mitigation Strategy discussed, although these matters will remain subject to the full NDA review. If the FDA accepts the application, the company estimates that a review decision could come as early as the third quarter of 2027. The actual timeline and final outcome could still change because of data completeness, benefit-risk assessments, or additional requirements.

Alongside the regulatory developments, Gossamer and Chiesi terminated their collaboration and licensing agreement. Gossamer will not be required to make an upfront payment; instead, it will receive a one-time payment of $5 million from Chiesi and regain worldwide leadership over the development, manufacturing, and commercialization of seralutinib. The previous 50-50 profit-sharing arrangement for the U.S. market will end, and ex-U.S. rights will also return to Gossamer. Chiesi will retain a capped royalty on worldwide net sales, as well as eligibility for certain regulatory and commercial milestone payments.

This restructuring allows Gossamer to advance seralutinib under a single global strategy while bringing most of the long-term economic benefits and execution risks back in-house. For a resource-constrained biotechnology company whose core value is highly concentrated in a single drug, consolidating the rights could help it pursue partnerships, financing, or launch planning. However, the true inflection point is not whether the company can file in September, but whether the FDA can accept a benefit-risk case centered on a Phase 3 trial that missed its prespecified primary endpoint and reinforced by other evidence.