Safety, Tolerability, Pharmacokinetics, and Preliminary Efficacy of SPL84 in Patients With Cystic Fibrosis

The goal of this clinical trial is to learn if drug SPL84 is safe for adult patients with cystic fibrosis (CF). It will also learn if the drug works to treat works to treat CF with a specific mutation (3849 +10kb C--\>T). The purpose of this research study is to test the safety and effectiveness of multiple doses of the study drug, SPL84. Researchers will compare drug SPL84 to a placebo (a look-alike substance that contains no drug) to see if drug SPL84 is safe and if it works to treat CF. In cohorts 1-3, SPL84 will be tested as a monotherapy, and in Cohort 4, SPL84 will be tested in participants who are already stable on CFTR modulator therapy. Participants will take drug SPL84 or a placebo by inhalation every week for 9 weeks (cohorts 1-3) or 12 weeks (cohort 4) and visit the clinic approximately weekly for checkups and tests.