Shares of Sionna Therapeutics tumbled after the company said a midstage, proof-of-concept trial of its treatment for cystic fibrosis didn't meet its key activity endpoint.

The stock lost nearly 90% of its value in premarket trading Monday, falling to $5.27. Shares ended Friday's regular session at $51.04, up 24% year to date.

The biopharmaceutical company said before the bell that potential confounders were observed in the trial, such as higher-than-anticipated levels of variability in individual sweat chloride levels.

"We are disappointed in the unexpected results from the Phase 2a trial," Chief Executive Mike Cloonan said. "We are further evaluating the data to decide on next steps for the SION-451 dual combination."