PART A The SAD treatment group is comprised of 3 cohorts where subjects will be randomized to either PTI-428 or placebo. Following the conclusion of at least 3 SAD treatment groups, a set of adult subjects diagnosed with CF will participate in an assigned MAD treatment group. The MAD treatment group is comprised of 3 cohorts. MAD Cohort 1 will enroll adult subjects with CF currently on stable ORKAMBI® background therapy for a minimum of 3 months at the time of randomization. MAD Cohorts 2 and 3 will enroll adult subjects with CF who are not currently on any background therapies. Subjects in all MAD cohorts will be randomized to either PTI-428 or placebo. Each dose will be administered once daily (QD) for a total of 7 Days.