Sionna Therapeutics reported Monday that its experimental cystic fibrosis drug SION-719 failed to meet the key activity endpoint in a mid-stage trial, sending the company's stock down 92%.
In the PreciSION CF Phase 2a trial, researchers assessed whether adding SION-719 to Trikafta would lower sweat chloride levels, a biomarker reflecting the functional activity of the cystic fibrosis protein. Results showed a mean placebo-adjusted sweat chloride reduction of just -1.0 mmol/L and a p-value of 0.7, the company said. Sionna said it will not advance SION-719 as an add-on to standard-of-care therapy.
The company received the topline data on Friday, Aug. 7, and said it is continuing to analyze the results. Sionna pointed to several factors that may have complicated interpretation of the data, among them unexpectedly wide variation in sweat chloride readings across individual patients and unequal Trikafta blood-level exposure during the SION-719 and placebo treatment periods.
On the safety front, SION-719 showed a clean profile alongside Trikafta across the 14-day treatment window, the company said. Side effects, when they occurred, were predominantly mild or moderate in severity, there were no serious adverse events, and liver function test results showed no concerning trends.
In a separate announcement Monday, Sionna disclosed Phase 1 findings for SION-451 tested in two-drug pairings with either SION-2222 or SION-109 among healthy volunteers. The trial hit its targets for safety, tolerability, and pharmacokinetics, the company said, with the SION-451/SION-2222 pairing emerging as the lead dual-combination candidate. Sionna said it is evaluating next steps for the SION-451 program in light of the SION-719 results.
President and Chief Executive Officer Mike Cloonan called the Phase 2a outcome "unexpected" in a statement. "We are further evaluating the data to decide on next steps for the SION-451 dual combination," he said.
Heading into this period of uncertainty, Sionna reported a second-quarter cash position of roughly $268.3 million across cash, equivalents, and marketable securities, and signaled it would move to protect that runway as it weighs what comes next, according to Barron's . The company hinted at potential workforce reductions, according to STAT News .
Monday's decline put Sionna stock at its lowest close on record, according to Barron's, citing Dow Jones Market Data. The stock is down 89% in 2026.
