Sionna’s SION-719 Fails Phase 2a Sweat Chloride Endpoint in Cystic Fibrosis

Sionna Therapeutics’ SION-719 did not show a meaningful reduction in sweat chloride when added to Trikafta in a Phase 2a proof-of-concept trial in adults with cystic fibrosis (CF), prompting the company to stop development of the drug as an add-on to standard of care. The result also triggered a review of the company’s broader NBD1 stabilizer strategy, including the SION-451 dual-combination program.

SION-719 Misses Key Activity Endpoint

The randomized, double-blind, placebo-controlled PreciSION CF Phase 2a trial (NCT07108153) evaluated SION-719 in 15 adults with CF who were homozygous for the F508del CFTR mutation and receiving a stable dose of physician-prescribed Trikafta (elexacaftor/tezacaftor/ivacaftor).

SION-719 produced a mean placebo-adjusted change in sweat chloride of -1.0 mmol/L (p=0.7), failing to demonstrate the prespecified activity signal. Sweat chloride serves as an important biomarker of CFTR function, making the finding a key test of whether adding the NBD1 stabilizer to existing CFTR modulator therapy could improve functional activity.

Sionna reported potential confounders that could affect interpretation of the result. These included greater-than-expected variability in individual sweat chloride measurements and differences in Trikafta exposure between the SION-719 and placebo treatment periods. The company received the topline results on August 7 and continues to analyze the dataset.

Based on the current findings, Sionna does not plan to advance SION-719 as an add-on to standard of care.

Safety Profile Remained Generally Favorable

SION-719 was administered for 14 days alongside Trikafta and was generally well tolerated. Most treatment-emergent adverse events were mild to moderate, with no serious adverse events reported.

The company also reported no meaningful trends in adverse events associated with liver function tests. However, the limited 15-participant sample and short treatment period restrict the conclusions that can be drawn about longer-term safety.

SION-451 Dual Combination Shows Target Exposure

While the SION-719 result weakens one development path, the Phase 1 healthy-volunteer data provide a separate readout for Sionna’s next-generation CFTR correction strategy.

The Phase 1 trial evaluated SION-451, another NBD1 stabilizer, in combination with either SION-2222 (galicaftor), a transmembrane domain 1-directed CFTR corrector, or SION-109, an intracellular loop 4-directed CFTR corrector. Across both combinations, 120 participants received treatment.

The study met its safety, tolerability, and pharmacokinetic objectives, including the target exposure levels established before the Phase 2a SION-719 results became available. Based on the overall data and observed target coverage, SION-451 plus SION-2222 emerged as the preferred dual combination.

All twice-daily SION-451 plus once-daily SION-2222 doses were generally well tolerated. One participant discontinued because of rash. In the twice-daily SION-451 plus twice-daily SION-2222 cohorts, most treatment-emergent adverse events were mild to moderate, with two discontinuations associated with elevated liver function tests and flu-like symptoms. No serious adverse events occurred in these cohorts.

Capital Preservation Could Shape Development Strategy

Sionna is now evaluating how to proceed with SION-451 and its preferred dual-combination program in light of the SION-719 findings. The company has not yet disclosed whether the SION-451 program will advance into a CF patient study.

The financial position gives the company time to assess the data but also introduces pressure to prioritize capital allocation. Sionna ended the second quarter with approximately $268.3 million in cash, cash equivalents, and marketable securities and said it intends to take actions to preserve capital while determining its next steps.

The SION-719 setback therefore represents both a clinical setback and a strategic decision point for Sionna’s broader CFTR modulator pipeline. The interpretation of the Phase 2a confounders, together with the Phase 1 exposure and safety findings for SION-451 combinations, will determine whether the company can establish a viable path toward testing the dual-combination approach in people with CF.

Reference

Sionna Therapeutics Reports Topline Data from Two Development Programs in Cystic Fibrosis and Provides Corporate Update – August 10, 2026