Treprostinil's Mechanistic Leap into Fibrosis: Trial Architecture Sound, Biological Rationale Unproven

United Therapeutics Completes Enrolment for TETON-PPF Trial

United Therapeutics has completed enrolment for its TETON-PPF study, a randomized, multinational, placebo-controlled, double-blind Phase III trial. The study is evaluating the safety and efficacy of nebulised Tyvaso (treprostinil) inhalation solution for progressive pulmonary fibrosis (PPF). A total of 754 participants have been enrolled, and they will be followed for 52 weeks, with top-line results anticipated in the second half of 2027. This milestone underscores the significant unmet need for new PPF treatments.

• Trial Design and Patient Population: The TETON-PPF study is a Phase III, randomized, multinational, placebo-controlled, double-blind registration trial. It successfully enrolled a total of 754 participants, who were randomized in a 1:1 ratio to receive either nebulised Tyvaso or a placebo. The treatment regimen involved starting at three breaths four times daily, with titration up to a target of 12 breaths four times daily or the maximum tolerated dose.
• Primary and Secondary Endpoints: The primary endpoint for the TETON-PPF study is the change in forced vital capacity (FVC) from baseline to week 52. Key secondary measures include the time to first clinical worsening, the incidence of acute exacerbations of interstitial lung disease, 52-week overall survival, and changes in various lung function and quality-of-life parameters, providing a comprehensive assessment of the drug's impact.
• Future Outlook and Regulatory Strategy: Participants will be followed for 52 weeks, with top-line results anticipated in the second half of 2027. If successful, United Therapeutics plans to submit a supplemental new drug application (sNDA) to the US FDA for approval of Tyvaso in PPF. The company is also seeking priority review for a similar application for nebulised Tyvaso in idiopathic pulmonary fibrosis (IPF), building on previous positive TETON-1 and TETON-2 study results.

The Urgent Need for New Progressive Pulmonary Fibrosis Treatments

Progressive pulmonary fibrosis remains a domain of significant unmet medical need, with current approved therapies offering only partial benefit and considerable tolerability challenges. Over the past three years, clinical and research focus has sharpened around specific patient populations and mechanistic gaps that existing treatments fail to adequately address.

• Limited efficacy and tolerability of current anti-fibrotic agents: Approved pharmacotherapies — including glucocorticoids, immunosuppressants, and anti-fibrotics such as nintedanib — slow pulmonary function decline but do not halt disease progression, and are frequently associated with treatment-limiting adverse events. Real-world data highlight that diarrhea alone prompted permanent discontinuation of nintedanib in approximately 20% of treated patients, with a greater proportion maintained on reduced rather than full doses despite limited evidence supporting the efficacy of dose reduction.

• Regulatory and evidence gaps driving off-label prescribing: The scarcity of randomized controlled trial data across CTD-ILD subtypes has resulted in widespread off-label use of pharmacotherapies, with nintedanib representing a notable exception given its regulatory approvals for SSc-ILD and chronic progressive fibrosing ILD across multiple geographies. This gap underscores the need for indication-specific evidence generation across the broader PPF population.

• Autoimmune and connective tissue disease-associated ILD (CTD-ILD): This population — encompassing rheumatoid arthritis-associated ILD, systemic sclerosis-associated ILD, and related subtypes — represents a primary target cohort, comprising approximately 39% of enrolled patients in recent observational studies. The heterogeneity of underlying autoimmune etiologies complicates standardized treatment approaches and necessitates subgroup-specific trial design.

• Non-IPF interstitial lung diseases and hypersensitivity pneumonitis: Non-IPF idiopathic interstitial pneumonias (IIPs) and fibrotic hypersensitivity pneumonitis (FHP) collectively account for a substantial proportion of PPF cases — approximately 37% and 11%, respectively, in recent cohorts — yet remain underrepresented in pivotal trials and lack dedicated approved therapies.

• Interstitial Pneumonia with Autoimmune Features (IPAF): IPAF represents an increasingly recognized high-priority subgroup, with PPF-defined progression observed in 43.3% of IPAF patients versus 19.35% in autoimmune rheumatic disease-associated ILD (p = 0.04). The disproportionate progression rate in this population, combined with the absence of tailored treatment strategies, positions IPAF as a critical area for targeted intervention.

• Emerging pipeline addressing novel mechanistic targets: Several investigational agents are advancing through clinical trials to address these gaps, including nerandomilast (a preferential PDE4B inhibitor with preclinical evidence of anti-fibrotic and immunomodulatory activity, demonstrated through reduction of neutrophils and macrophages and downregulation of pro-fibrotic signaling), admilparant (an LPA1 receptor antagonist), and inhaled treprostinil (a prostacyclin analogue) — each representing mechanistically distinct approaches to PPF beyond the current standard of care.

Unpacking the TETON-PPF Phase III Trial Design

The TETON-PPF Phase III programme represents one of the most rigorously designed efforts to establish a therapeutic standard in progressive pulmonary fibrosis, building on lessons from earlier pivotal trials. Key trials — including INBUILD and FIBRONEER-ILD — have each employed distinct stratification strategies, eligibility criteria, and endpoint hierarchies to capture clinically meaningful outcomes in this heterogeneous population.

Treprostinil's Expanding Role Beyond Progressive Pulmonary Fibrosis

Beyond progressive pulmonary fibrosis, treprostinil is being investigated across several distinct disease areas, with trial designs ranging from randomized controlled studies to real-world observational analyses. The intervention models reflect the heterogeneity of the patient populations and the varying stages of clinical development for each indication.

• Pulmonary Hypertension Associated with Interstitial Lung Disease (PH-ILD): The INCREASE trial evaluated inhaled treprostinil in PH-ILD patients in the United States. A separate Japanese trial assessed inhaled treprostinil in this population using a multicenter, non-randomized, open-label, single-arm design. Additionally, a real-world retrospective single-center study examined inhaled treprostinil efficacy in PH-ILD patients outside the controlled trial setting.

• Pulmonary Arterial Hypertension (PAH): The FREEDOM-EV trial employed an event-driven, randomized controlled design to evaluate oral treprostinil in PAH patients, followed by an open-label extension study to assess longer-term outcomes.

• Systemic Sclerosis-Related Digital Skin Ulcers: A 2-stage, randomized, placebo-controlled, single-ascending-dose study evaluated treprostinil delivered via hydrogel iontophoresis, conducted first in healthy volunteers and then in patients with systemic sclerosis-related digital ulcers using a 3:1 randomization ratio of treprostinil to placebo.

• Summary of Intervention Models Across Indications: Trial designs span open-label single-arm studies, event-driven randomized controlled trials with open-label extensions, randomized placebo-controlled single-ascending-dose studies, and retrospective single-center observational analyses — reflecting treprostinil's progression across different phases and contexts of clinical investigation.

A New Frontier for Inhaled Treprostinil in Fibrotic Lung Disease

The completion of enrollment for the TETON-PPF study marks a pivotal moment for inhaled treprostinil, signaling a strategic move to expand its therapeutic reach beyond pulmonary hypertension associated with interstitial lung disease (PH-ILD) into the broader and highly challenging landscape of progressive pulmonary fibrosis (PPF). This initiative is grounded in compelling evidence, including post-hoc analyses from the INCREASE study that showed improvements in forced vital capacity (FVC) in PH-ILD patients, particularly those with idiopathic pulmonary fibrosis (IPF), a severe form of PPF. Furthermore, preclinical data suggesting treprostinil's potential antifibrotic activity provides a strong scientific rationale for directly investigating its effects on lung fibrosis.

By focusing on FVC as the primary endpoint over 52 weeks, the TETON program aims to demonstrate a direct and sustained impact on the relentless decline in lung function characteristic of PPF. This could position inhaled treprostinil as a critical new option for patients facing limited therapeutic choices. However, several considerations remain. While promising, the efficacy of treprostinil in a broader PPF population, especially those without significant PH, needs to be confirmed. The drug's known airway-related adverse events, such as cough, while generally manageable, could influence long-term adherence in a chronic disease setting. Moreover, with the TETON study allowing concomitant use of existing antifibrotics like pirfenidone or nintedanib, understanding the incremental benefit and safety profile of treprostinil in combination will be crucial for its clinical integration and market positioning. Should the TETON-PPF study yield positive results, it could usher in a new era for managing fibrotic lung diseases, offering a much-needed therapeutic avenue and potentially reshaping treatment algorithms for PPF.