8-K: Gossamer Bio Seralutinib Phase 3 PROSERA: Mixed Efficacy

Gossamer Bio announced topline results from its Phase 3 PROSERA study of seralutinib for pulmonary arterial hypertension, which did not meet its primary endpoint but showed compelling signals in high-risk subgroups.

Summary

• The Phase 3 PROSERA study of seralutinib for pulmonary arterial hypertension (PAH) did not meet its prespecified primary endpoint (p-value of 0.0320 vs. α = 0.025) for change in 6-minute walk distance (6MWD) at Week 24 in the overall patient population.
• Patients receiving seralutinib had a median change of +28.2 meters in 6MWD from baseline, while placebo patients had a median change of +13.5 meters, resulting in an estimated Hodges-Lehmann treatment effect of +13.3 meters.
• All four key secondary endpoints favored seralutinib versus placebo in the overall population, though p-values are nominal due to the primary endpoint miss.
• In the prespecified intermediate and high-risk subgroup (REVEAL 2 Lite Risk Score ≥ 6, n = 234), seralutinib demonstrated a +20.0m placebo-adjusted improvement in 6MWD (p = 0.0207).
• Three of four key secondary endpoints in this high-risk subgroup also showed p-values below 0.0125, underscoring seralutinib's activity.
• Change in NT-proBNP at Week 24 showed an estimated location shift of -120.4 ng/L compared with placebo (p=0.0002) in the overall population, and -265.8 ng/L (p=0.0002) in the intermediate/high-risk subgroup.
• Seralutinib was generally well tolerated, with treatment-emergent adverse events (TEAEs) reported in 86.5% of seralutinib patients and 80.5% of placebo patients.
• Treatment-emergent serious adverse events (SAEs) occurred in 16.0% of seralutinib patients and 18.9% of placebo patients.
• Transaminase elevations (three times or greater of the upper limit of normal) were observed in 13% of seralutinib patients compared to 1% of placebo patients.
• Cough was the most frequently reported adverse event in seralutinib-treated patients (37.0%).
• Gossamer Bio expects to meet with the U.S. FDA to discuss the path forward for seralutinib in PAH.
• Enrollment into the SERANATA Study is paused to evaluate the impact of PROSERA results, particularly regional discrepancies in placebo response.
• Results from the CT functional respiratory imaging (FRI) substudy are expected in the coming weeks to provide additional insight into seralutinib's treatment effect.

Sentiment

Score: 5

Explanation: StockSavvy.ai views this as a mixed outcome. While the primary endpoint was missed, strong signals in high-risk subgroups and positive secondary endpoints suggest potential, but the path forward is uncertain and requires FDA consultation.

Positives

• All four key secondary endpoints favored seralutinib in the overall patient population.
• A compelling signal was observed in the prespecified intermediate and high-risk subgroup (REVEAL 2 Lite Risk Score ≥ 6), with a +20.0m placebo-adjusted improvement in 6MWD (p = 0.0207).
• Significant improvement in NT-proBNP was demonstrated, with a location shift of -120.4 ng/L (p=0.0002) in the overall population and -265.8 ng/L (p=0.0002) in the intermediate/high-risk subgroup.
• Seralutinib showed a robust improvement in 6MWD in patients with connective tissue disease-associated PAH (CTDAPAH), achieving a placebo-adjusted gain of +37.0 meters (p=0.0104).
• The safety and tolerability profile was generally well tolerated, with a lower incidence of serious adverse events (SAEs) in the seralutinib arm (16.0%) compared to placebo (18.9%).
• Management believes the overall results demonstrate clear evidence of clinical benefit in a heavily pretreated patient population and support a positive risk-benefit profile for seralutinib.

Negatives

• The Phase 3 PROSERA study did not meet the prespecified threshold on its primary endpoint (p-value of 0.0320 vs. α = 0.025) for change in 6MWD in the overall population.
• Transaminase elevations (three times or greater of the upper limit of normal) were significantly higher in the seralutinib arm (13%) compared to placebo (1%).
• Cough was a frequently reported adverse event in patients treated with seralutinib (37.0%).
• Enrollment into the SERANATA Study is paused to evaluate the impact of PROSERA results and regional discrepancies in placebo response.
• The PROSERA study experienced an elevated placebo 6MWD response compared to other pivotal studies for add-on treatments in PAH, particularly in regions like Latin America and Asia/Middle East, which may have muted the overall treatment effect.

Risks

• Topline results are based on preliminary analysis of key efficacy and safety data, and such data may change following a more comprehensive review.
• Topline data may not accurately reflect the complete results of a clinical trial.
• Gossamer may not be able to identify a development path forward for seralutinib, whether as a result of FDA feedback or otherwise.
• Any identified path forward may require additional capital and other resources or may limit the commercial opportunity for seralutinib.
• Gossamer may need to evaluate its current workforce in light of potential development paths for seralutinib.
• Potential delays in the commencement, enrollment, and completion of clinical trials.
• Comparative safety information is not based on a head-to-head comparison, and differences exist between study designs and subject characteristics which could confound the results.
• The Company's dependence on third parties in connection with product manufacturing, research, and preclinical and clinical testing.
• The results of preclinical studies and early clinical trials with seralutinib are not necessarily predictive of future results.
• The success of any future Gossamer's clinical trials and preclinical studies for seralutinib is not guaranteed.
• Regulatory developments in the United States and foreign countries could impact seralutinib's development.
• Unexpected adverse side effects or inadequate efficacy of seralutinib may limit its development, regulatory approval, and/or commercialization, or may result in clinical holds, recalls, or product liability claims.
• Gossamer's ability to obtain and maintain intellectual property protection for seralutinib.
• Gossamer's ability to comply with its obligations in collaboration agreements with third parties or intellectual property licensing agreements.
• Gossamer may use its capital resources sooner than it expects.

Future Outlook

Gossamer Bio expects to meet with the U.S. FDA to discuss the path forward for seralutinib in pulmonary arterial hypertension. The company is pausing enrollment into the SERANATA Study to evaluate the impact of PROSERA results, particularly regional discrepancies in placebo response. Additionally, PROSERA results from the CT functional respiratory imaging (FRI) substudy are expected in the coming weeks and are anticipated to provide additional insight into seralutinib's treatment effect.

Management Comments

• "While study narrowly missed primary endpoint, we believe the overall results demonstrate clear evidence of clinical benefit in a heavily pretreated patient population."
• "PROSERA confirmed observation from TORREY Phase 2 of enhanced separation from placebo in patients with more severe baseline disease."
• "Safety and tolerability profile appear acceptable as on add-on therapy in PAH, with primary safety observations (cough and liver enzyme elevations) well understood by PAH treaters given profiles of existing therapies."
• "We believe the PROSERA and TORREY results together support a positive risk-benefit profile for seralutinib, potentially offering a new mechanism of action for a progressive disease with significant unmet need."

Industry Context

StockSavvy.ai notes that the elevated placebo response observed in the PROSERA study, particularly in certain regions like Latin America and Asia/Middle East, is a known challenge in clinical trials for PAH and can complicate the interpretation of drug efficacy. The strong performance in the intermediate and high-risk subgroup, consistent with the Phase 2 TORREY study, suggests seralutinib may address a critical unmet need for patients with more severe disease, where existing therapies may be less effective.

Comparison to Industry Standards

• The placebo-adjusted 6MWD improvement of +13.3m in the overall PROSERA population is comparable to or better than several approved PAH add-on therapies, such as oral Treprostinil (FREEDOM-EV: 8m, FREEDOM-C2: 10m, 11m) and Selexipag (GRIPHON: 12m).
• The +20.0m placebo-adjusted improvement in the intermediate and high-risk subgroup for seralutinib is particularly strong, exceeding most listed comparators except for Sotatercept (STELLAR: 41m).
• The reported placebo 6MWD change of +13.5m in PROSERA is notably higher than in many other pivotal PAH studies (e.g., TRIUMPH: 3.0m, FREEDOM-C2: 4.8m, STELLAR: 1.0m), which may have diluted the overall treatment effect and impacted the primary endpoint.

Stakeholder Impact

• Shareholders: Potential for volatility due to mixed results; uncertainty regarding future development and commercialization of seralutinib.
• Patients (PAH): Hope for a new treatment option, especially for intermediate-to-high risk patients, but the regulatory approval path is now less clear.
• Employees: Potential for workforce evaluation depending on identified development paths for seralutinib.

Next Steps

• Complete in-depth analyses of the PROSERA data across endpoints and subgroups, pending the results from the CT FRI substudy, expected in the coming weeks.
• Engage with the FDA to discuss the results to understand their perspective on the path forward for seralutinib.
• Assess ramifications for seralutinib and Gossamer's portfolio, including impacts on capital allocation.

Key Dates

Recommendation

The primary endpoint miss is a significant setback, but the strong performance in high-risk subgroups and positive secondary endpoints suggest potential value. The company's next steps, particularly FDA discussions and further data analysis, are crucial. An investor should hold to await clarity on the regulatory path and further strategic decisions before making a definitive buy or sell decision.

Keywords

Pulmonary Arterial Hypertension, PAH, Seralutinib, PROSERA Study, Phase 3 Clinical Trial, Tyrosine Kinase Inhibitor, Gossamer Bio, Drug Development, Clinical Results, Biotechnology, FDA, 6MWD, NT-proBNP, REVEAL Lite 2 Risk Score, CTDAPAH