The purpose of this research study is to assess if the study drug SION-719 is safe and well-tolerated in people with cystic fibrosis who are already taking Trikafta, understand the effect of SION-719 on the body by measuring chloride levels, and to learn how the body absorbs, distributes, and gets rid of the study drug in people with cystic fibrosis. This study is a placebo controlled cross-over study with two study treatment periods, and participants will take part in both study treatment periods. Participants will be assigned either study drug or placebo. Participants will remain on standard dose of physician-prescribed Trikafta throughout the study. This study will be approximately 16 weeks, or 112 days long.