Gossamer Bio Pursues High-Risk NDA for Seralutinib After Phase 3 Failure and Partner Exit

Gossamer Bio Regains Seralutinib Rights, Pursues FDA Filing Post-Phase 3 Fail

Gossamer Bio has reacquired global development and commercial rights for its pulmonary hypertension candidate, seralutinib, from Chiesi. This move follows a Phase 3 trial (PROSERA) where seralutinib failed to significantly improve functional performance, missing its primary endpoint. Despite the trial's outcome, Gossamer plans to submit a New Drug Application (NDA) to the FDA in September 2026, aiming for a regulatory decision by August 2027. The company's decision is bolstered by insights from a pre-NDA meeting where the FDA characterized the statistical significance and treatment effect as "review issues" rather than "filing issues." Chiesi paid Gossamer a one-time $5 million fee as part of the reacquisition agreement.

• Gossamer Bio has successfully reacquired global development and commercial rights for its inhaled tyrosine kinase inhibitor, seralutinib, from Chiesi. This strategic move includes a one-time payment of $5 million from Chiesi to Gossamer to settle all prior obligations. Under the new agreement, Chiesi retains the potential to receive capped royalties on worldwide net sales of seralutinib, along with additional payments if certain regulatory and commercial milestones are achieved.
• Despite the Phase 3 PROSERA trial for seralutinib missing its prespecified statistically significant threshold (p-value of 0.0320 vs. 0.025), Gossamer Bio is proceeding with an FDA New Drug Application submission in September 2026. This decision is informed by minutes from a June pre-NDA Type B meeting, where the FDA characterized the trial's statistical significance and treatment effect as "review issues" rather than "filing issues." The submission will leverage data from both the PROSERA and Phase 2 TORREY trials.
• The PROSERA study demonstrated a placebo-adjusted improvement of 13.3 meters in a six-minute walk distance test at 24 weeks. Financially, Gossamer Bio reported approximately $57 million in cash, cash equivalents, and marketable securities as of June 30. The company plans a reverse stock split in the current or next quarter to improve its capital structure. Following the announcement, Gossamer's stock saw a 25% increase, reflecting renewed investor confidence in the program's future.

Understanding the FDA's Stance on Seralutinib's PROSERA Data

Recent clinical investigations in pulmonary arterial hypertension (PAH) have highlighted both the advancement of novel disease-modifying agents and the refined application of existing therapies. A series of trials including PULSAR, SPECTRA, STELLAR, ZENITH, and HYPERION have established sotatercept as a first-in-class activin signaling inhibitor. This agent acts by restoring balance within the TGF-β/activin-BMPR2 pathway, a key molecular driver of pulmonary vascular remodeling. Across a diverse PAH population, sotatercept has demonstrated significant reductions in pulmonary vascular resistance, improvements in exercise capacity and risk status, and decreased morbidity and mortality when added to background therapy. Its safety profile is characterized by generally mild-to-moderate adverse events, most commonly epistaxis, telangiectasia, and increased hemoglobin. Concurrently, the Phase 2 PHiNO study explored inhaled nitric oxide (iNO) for acute severe right heart failure due to PH. The trial showed a significant reduction in PVR in the iNO group versus control, along with decreased serum BNP and inferior vena cava diameter, with no serious adverse events observed.

Efforts to optimize PAH management also involve generating evidence in specific patient populations and settings. The design of the ALEPH trial, a multicenter, randomized, double-blind, placebo-controlled study, aims to assess ambrisentan monotherapy in early-stage, low-risk PAH patients (mPAP >20 and <25 mm Hg, PVR >2 and ≤3 WUs) who have not previously received PAH-specific therapy. Its primary endpoint is a composite of PAH progression, with secondary endpoints including changes in hemodynamic parameters, NT-proBNP, and functional status. In a different context, a retrospective analysis of post-marketing surveillance data for INOflo (iNO) in Japan evaluated its use in 427 patients with PAH following cardiac surgery. The findings indicated that iNO was associated with significant improvements in hemodynamic parameters, including mean pulmonary artery pressure (mPAP), at 24 and 48 hours post-initiation, irrespective of most baseline patient characteristics.

Addressing Unmet Needs in Pulmonary Arterial Hypertension Treatment

Despite substantial therapeutic progress over the past two decades—with more than 10 novel agents approved—pulmonary arterial hypertension (PAH) remains an incurable disease with a persistently disappointing prognosis. Current standard-of-care therapies target vasodilation via the prostacyclin, endothelin-1, and nitric oxide pathways, but they address symptoms rather than the underlying vascular remodeling that drives disease progression. This gap has spurred growing interest in anti-remodeling and precision-medicine approaches.

• Symptomatic rather than disease-modifying therapy: Prostacyclin analogues, endothelin receptor antagonists, and phosphodiesterase-5 inhibitors normalize vascular tone and relieve symptoms but do not reverse the underlying pathology, effectively delaying rather than curing disease progression.

• Incomplete understanding of pathogenesis: The molecular drivers of PAH remain only partially elucidated, complicating efforts to design therapies that target root causes rather than downstream vasodilatory effects.

• Formulation and pharmacokinetic limitations: Existing pharmacotherapies are constrained by short half-lives, stability challenges, and suboptimal formulations, which reduce efficacy and increase the risk of systemic side effects; inhaled anti-PAH drug options remain particularly limited in application.

• Unmet need in PAH associated with pulmonary disease: Despite approved therapies for idiopathic pulmonary fibrosis (IPF) and pulmonary hypertension (PH) independently, no agents are currently approved specifically for PH occurring in the context of significant underlying pulmonary disease.

• Failure to address vascular remodeling and right heart involvement: Current treatments do not target the pro-proliferative, apoptosis-resistant phenotype of pulmonary vascular cells that underlies remodeling, nor do they directly address right heart dysfunction—both central contributors to disease progression and mortality.

• Complexity of treatment optimization: With an expanded therapeutic armamentarium, clinicians face increasingly complex decisions around agent selection, balancing efficacy, side-effect profiles, cost, and convenience of administration; further studies are needed to define optimal dosing and combination strategies.

• Emerging directions to close these gaps: Novel approaches are shifting focus toward anti-remodeling mechanisms—such as modulation of BMP/TGF-β signaling (e.g., sotatercept), metabolic and epigenetic pathways, and precision-medicine strategies targeting BMPR2 mutations (e.g., hydroxychloroquine, phenylbutyrate)—alongside more rigorous trial designs aimed at validating disease-modifying potential rather than symptomatic relief alone.

Seralutinib's Potential Role in the Evolving PAH Landscape

Over the past five years, the treatment landscape for pulmonary arterial hypertension (PAH) has continued to rely on therapies targeting the endothelin, nitric oxide, and prostacyclin pathways. While effective at improving exercise capacity, these foundational treatments are primarily vasodilatory and have only modestly altered long-term mortality, with median survival remaining around seven years. A 2024 network meta-analysis of over 6,800 patients confirmed the benefits of these pathways on 6-minute walk distance (6MWD), with combination therapy (endothelin + nitric oxide) showing the greatest improvement (43.7 m vs. placebo). However, the analysis also highlighted significant heterogeneity in treatment effects, underscoring the persistent unmet need for novel therapies that can address the underlying proliferative vascular remodeling of PAH, rather than just its hemodynamic consequences.

The therapeutic paradigm is now shifting decisively toward disease modification, exemplified by the recent success of sotatercept, a first-in-class activin signaling inhibitor. By trapping pro-proliferative ligands, sotatercept directly addresses a key molecular driver of the disease. Pivotal trial data demonstrated its profound clinical impact; in high-risk patients, sotatercept reduced the risk of death, lung transplantation, or PAH-related hospitalization by 76% (HR 0.24) versus placebo. This evolution is also reflected in a robust pipeline of agents with novel mechanisms, including inhaled tyrosine kinase inhibitors like seralutinib (targeting PDGFR, CSF1R, and c-KIT), repurposed SGLT2 inhibitors showing improvements in cardiac indices, and precision-medicine candidates such as satralizumab. This new wave of development is accompanied by an evolution in clinical trial design, moving beyond 6MWD to include event-driven primary endpoints and incorporating advanced tools like noninvasive pulmonary artery pressure monitoring to better capture clinically meaningful outcomes.