Sionna Therapeutics Completes Enrollment in PreciSION CF Phase 2a Trial Evaluating NBD1 Stabilizer, SION-719, Added to Standard of Care in Participants with Cystic Fibrosis

Sionna Completes Enrollment for PreciSION CF Phase 2a Trial

Sionna Therapeutics has completed enrollment for its PreciSION CF Phase 2a proof-of-concept trial. This study evaluates SION-719, a first-in-class nucleotide binding domain 1 (NBD1) stabilizer, when added to Trikafta®, the current standard of care for cystic fibrosis (CF). The trial enrolled adult CF patients homozygous for the F508del mutation. SION-719 aims to directly address the underlying defect of this common CF-causing mutation. Sionna anticipates releasing topline data from the PreciSION CF trial in the summer of 2026, marking a significant step in developing novel medicines to normalize CFTR protein function.

• SION-719 represents a first-in-class NBD1 stabilizer, marking the first time such a compound is being clinically evaluated in people with cystic fibrosis. This innovative approach directly targets the fundamental defect caused by the F508del mutation, which is crucial for CFTR protein function, offering a potentially transformative therapeutic strategy beyond existing CFTR modulators.
• The PreciSION CF Phase 2a trial is designed as a randomized, double-blind, placebo-controlled, crossover proof-of-concept study. It specifically enrolled adult CF patients who are homozygous for the F508del genetic mutation and are already receiving a stable dose of physician-prescribed Trikafta, ensuring the evaluation of SION-719 in a highly relevant patient population.
• Key objectives of the trial include evaluating the safety, tolerability, and pharmacokinetics of SION-719 when administered in combination with the standard of care. A critical secondary objective is to assess the change in sweat chloride levels, which is an important and established biomarker for CFTR function, providing a direct measure of the drug's potential efficacy.

Addressing Remaining Challenges in Cystic Fibrosis Treatment

Despite significant advances in cystic fibrosis therapeutics, particularly with CFTR modulators, substantial challenges remain in delivering optimal care to all patients. These limitations span from treatment accessibility and patient variability to emerging complications in an aging CF population.

• Limited therapeutic options for specific patient populations — 10% of people with CF have genetic variants that are not amenable to current modulator treatments, and a significant number remain unresponsive to CFTR modulator drugs or cannot access them due to prohibitive costs, creating significant health equity issues

• Incomplete understanding of long-term outcomes — CFTR modulator treatment effects are merely suspensive rather than curative, and comprehensive data on long-term efficacy, safety during pregnancy, drug-drug interactions, and extra-pulmonary manifestations remain insufficient

• Safety and tolerability concerns — Off-target effects of highly effective modulator therapy include hepatotoxicity, drug-drug interactions, and potential mental health complications that can limit therapeutic use

• Increasing treatment complexity and burden — The growing arsenal of CF therapeutics creates mounting care complexity, leading to adherence challenges that require innovative approaches to reduce treatment burden while maintaining therapeutic efficacy

• Emerging comorbidities in aging populations — The CF demographic is experiencing new metabolic and cardiovascular complications, increased cancer incidence (particularly gastrointestinal), and CF-related diabetes management challenges where insulin therapy efficacy doesn't correlate with glycemic control improvements

• Insufficient evidence base for diabetes management — Randomized controlled trials assessing the efficacy and adverse effects of insulin and other glucose-modulating medications in CF-related diabetes remain lacking

• Global access disparities — Limited worldwide availability of highly effective modulator therapies creates significant treatment inequalities across different healthcare systems and economic regions

Key Endpoints Guiding the PreciSION CF Phase 2a Trial

Cystic fibrosis clinical trials employ a comprehensive array of endpoints to evaluate therapeutic efficacy across multiple disease manifestations. The current "toolbox" encompasses both established clinical measures and emerging biomarkers that capture the multisystem nature of CF pathology. These endpoints are categorized into clinical endpoints (direct patient benefit measures) and surrogate endpoints (predictive indicators), with imaging classified as surrogate endpoints.

• Primary pulmonary endpoints include forced expiratory volume in one second (FEV1) and forced vital capacity, which remain the gold standard clinical endpoints despite recognized limitations in capturing early disease progression

• Pulmonary exacerbation metrics encompass exacerbation rates, time to first exacerbation, and treatment-emergent adverse events, providing critical safety and efficacy data for regulatory submissions

• Patient-reported outcome measures (PROMs) utilize validated instruments including the Chronic Respiratory Infection Symptom Score (CRISS) and Cystic Fibrosis Questionnaire-Revised (CFQ-R) to capture health-related quality of life domains not reflected in physiological measures

• Biomarker and inflammatory endpoints include sweat chloride concentration, blood leukocyte count, inflammatory markers, and sputum bacterial load to assess CFTR function and disease activity at the molecular level

• Gastrointestinal clinical endpoints measure intestinal pH, transit time, bile salt malabsorption, intestinal inflammation, exocrine pancreatic function, and fat malabsorption—particularly crucial for evaluating CFTR modulator therapies in pediatric populations

• Growth and nutritional parameters incorporate body mass index (BMI), growth parameters, nutritional status, and markers of pancreatic function as accepted clinical endpoints reflecting systemic disease impact

• Advanced pulmonary assessment tools include lung clearance index (LCI) parameters, respiratory cultures, nasal potential difference, chest radiographs, and computed tomography imaging to provide comprehensive pulmonary function evaluation

• Safety and tolerability measures encompass days on antibiotics, oxidative stress markers, and comprehensive adverse event monitoring during supplementation or treatment periods

Exploring NBD1 Stabilization as a Novel CF Therapeutic Target

Recent research in cystic fibrosis is witnessing a paradigm shift toward genotype-agnostic approaches that promise curative potential for all people with CF. Gene therapy and mRNA therapy represent the most promising emerging strategies, offering hope for patients with rare mutations or genotypes not currently eligible for CFTR modulator therapy. These approaches address fundamental limitations of current treatments, including high costs, accessibility barriers, and variable patient responses, while positioning CF as a model for translational medicine that may inform therapeutic advancements for other genetic diseases.

CFTR modulator therapy, particularly elexacaftor/tezacaftor/ivacaftor (ETI), continues to demonstrate expanding therapeutic applications beyond traditional pulmonary manifestations. Clinical studies reveal substantial improvements in structural lung abnormalities, with 73% reduction in mucous plugging and 51% decrease in bronchial thickening at 12-18 month follow-up. Notably, ETI therapy is showing efficacy in treating CFTR dysfunction presenting primarily as pancreatitis, reducing pancreatic episodes from 0.33 to 0.09 episodes per month. The therapy has demonstrated potential for reversing pancreatic insufficiency, with 10.2% of patients achieving pancreatic sufficiency and another 10.2% reaching moderate insufficiency status, particularly when initiated early in childhood.

Novel antimicrobial strategies are emerging to address Pseudomonas aeruginosa infections in CF patients through phage tail-like bacteriocins (PTLBs). Researchers have identified 34 different PTLBs across 75 P. aeruginosa genomes, with 11 characterized as novel F-type PTLB subtypes. The R1 and F15 PTLB subtypes demonstrate superior efficacy, achieving over 75% survival rates in vivo models while avoiding resistance development due to their absence of genetic material—a significant advantage over traditional phage therapy. This precision medicine approach, combined with functional testing using patient-derived organoids, represents the revolutionary transformation of CF care from exclusively symptomatic treatments to patient-specific therapeutic strategies.

Sionna’s NBD1 Strategy Could Mark the Next Evolution in CF Therapy

The completion of enrollment in Sionna Therapeutics’ PreciSION CF Phase 2a trial signals an important step forward in exploring next-generation approaches to cystic fibrosis treatment. While current therapies like Trikafta have transformed patient outcomes, there remains a gap in fully restoring CFTR protein function, particularly for patients with the F508del mutation. SION-719’s novel mechanism—direct stabilization of the NBD1 domain—aims to address this gap at a more fundamental biological level.

If successful, this approach could complement existing CFTR modulators and potentially unlock higher levels of functional restoration, which may translate into improved long-term outcomes and quality of life. The combination strategy also aligns with a broader trend in CF drug development toward layering therapies to maximize efficacy.

However, the upcoming data readout in summer 2026 will be critical. Demonstrating clear clinical benefit on top of an already highly effective standard of care will be challenging, and the field will be watching closely to see whether this mechanism can deliver meaningful differentiation. Success could open the door to a new class of CF therapies, while failure would reinforce the difficulty of improving upon current best-in-class treatments.