GOSS

• Clinical Study Progress

  • The Phase III ProSERA trial of serolutinib for PAH met the traditional 0.05 p-value threshold for statistical significance for its primary endpoint (placebo-adjusted 13.3 meter improvement in 24-week six-minute walk distance) but did not meet the pre-specified 0.025 alpha threshold. All four key secondary endpoints favored serolutinib over placebo, with a stronger treatment effect observed in the pre-specified risk-enriched subgroup.
  • The completed pre-specified CT-FRI sub-study of ProSERA (162 enrolled patients, 125 evaluable paired week-24 scans) found statistically significant multi-compartment reverse remodeling across arterial, venous, and fibrosis-like parenchymal parameters, consistent with serolutinib's mechanism of action (inhibition of PDGFR, CSF1R, and CKIT). All measured changes correlated with improvements in key clinical endpoints including six-minute walk distance, NT-proBNP, and RevealLight2 risk scores, strengthening the totality of evidence for serolutinib's clinical benefit.

• Regulatory Progress

  • The company has secured an in-person Type B pre-NDA meeting with the FDA (the most formal pre-submission meeting type) scheduled for mid-June 2026, after advancing from an initial planned Type C meeting. The pre-NDA briefing book has already been submitted to the FDA.
  • Gossamer plans to submit an NDA under the FDA's "one adequate well-controlled trial plus confirmatory evidence" framework, with ProSERA as the pivotal trial and the Phase 2 TORI study as confirmatory evidence, supported by the new CT-FRI mechanistic data.

• Capital Structure and Cost Reduction

  • The company implemented a 50% reduction in force, paused all non-serolutinib development activities, and implemented broad cross-organizational cost containment measures to extend cash runway while pursuing NDA submission.
  • Gossamer reached a convertible note exchange agreement with noteholders to address upcoming 2027 debt maturity: the exchange reduces outstanding principal from $200 million to $72 million (a $128 million reduction) and extends maturity to 2030. The new notes are secured by substantially all company assets and carry a 7.5% annual cash interest rate paid semiannually, and require a minimum 98% participation rate (which may be waived).

• The NDA submission target for serolutinib remains September 2026, contingent on the outcome of the mid-June Type B pre-NDA meeting with the FDA. If submission proceeds as planned, potential FDA approval is targeted for the third quarter of 2027.
• Based on current operational plans and completed capital structure actions, the company expects its existing $99 million cash balance to provide runway into the first quarter of 2027.
• The company will provide a public update on the outcome of the Type B FDA meeting alongside its Q2 2026 earnings results, scheduled for later in summer 2026.

Gossamer Dio is a clinical-stage biotech focused on the development of serolutinib for pulmonary arterial hypertension (PAH), with no commercial product segments generating revenue as of Q1 2026. As of March 31, 2026, the company held $99 million in cash, cash equivalents, and marketable securities. Q1 2026 operating expenses included one-time charges tied to corporate restructuring; after the restructuring, the company projects lower quarterly cash burn going forward.

• The ProSERA trial did not meet the pre-specified 0.025 alpha threshold for its primary endpoint, creating regulatory uncertainty for the NDA submission. FDA feedback from the pre-NDA meeting could require changes to the submission, trial design, or targeted patient population, which could delay or prevent approval.
• The convertible note exchange transaction requires a minimum 98% participation from existing noteholders; if this threshold is not met (and not waived), the transaction will not close, leaving the company with a 2027 debt maturity that could threaten its ability to complete the NDA process and commercialize serolutinib if it is approved.
• The CT-FRI sub-study is pre-specified exploratory research, with nominal unadjusted p-values that have not been corrected for multiplicity, so the imaging findings could represent chance observations rather than true treatment effects.
• Actual clinical results, regulatory timelines, and cash burn could differ from current expectations due to the inherent uncertainty of clinical development, regulatory review, and capital market conditions.

Q: How does the new CT-FRI data support the upcoming pre-NDA meeting and potential differentiated labeling for serolutinib?

A: The full CT-FRI data set will be included in the NDA submission; it was not complete in time for the pre-NDA briefing book, but will be highlighted during the June meeting. Management notes the CT-FRI data adds important mechanistic confirmatory evidence to support the NDA, and if included in the label, it will appear in the pharmacodynamic section. Management states the demonstration of multi-compartment reverse remodeling is unprecedented for existing PAH therapies, which will make serolutinib's label highly differentiated.

Q: What key milestones and activities are in place between now and the planned September 2026 NDA submission?

A: The NDA dossier is already under active development, with all analyses scheduled for completion by late August to enable mid-September submission. All work is proceeding in parallel with the June Type B meeting, and the company remains on track to meet the September target. The upgrade from a Type C to Type B pre-NDA meeting was recommended by the company's experienced FDA advisors based on the strength of the full serolutinib data set and the high unmet need for new PAH treatments.

Q: Are CT-FRI imaging measures standard clinical practice for PAH, and will additional longer-term imaging data be collected from substudy patients?

A: CT-FRI is not standard routine clinical practice for PAH management, but it provides unique mechanistic insight into structural disease changes that cannot be observed from standard clinical endpoints. CT-FRI confirms serolutinib drives structural reverse remodeling rather than only acute vasodilation, adding credibility to the totality of evidence for regulators and clinicians. The company will explore collecting additional follow-up imaging at 48 and 72 weeks, and will release any new data once available. This level of comprehensive CT analysis is unprecedented for PAH trials and may set a new standard for the field.

Q: How did clinical outcomes look in the CT-FRI substudy cohort, and what patient populations are targeted for commercial use of serolutinib?

A: Clinical outcomes for the substudy cohort matched the broader ProSERA intent-to-treat population, with statistically significant improvements in six-minute walk distance and reductions in NT-proBNP. Management sees a path for serolutinib use across the full spectrum of PAH: it has a pronounced effect in intermediate and high-risk patients, and its favorable safety profile means it could be used earlier in lower-risk patients to slow long-term disease progression. Its novel mechanism of action (with lower toxicity than many existing PAH therapies) creates significant commercial opportunity.