PART C Following the conclusion of Part B Phase II, a set of adult subjects diagnosed with CF will participate in Part C. The Part C Phase II treatment group is comprised of 3 cohorts. Part C Cohort 1 will enroll adult subjects with CF who are eligible to take, but not currently taking, ORKAMBI® in accordance with the approved label. Part C Cohort 2 will enroll adult subjects with CF currently on stable KALYDECO® background therapy for a minimum of 3 months at the time of randomization. Part C Cohort 3 will enroll adult subjects with CF who are not currently on any background therapies and are pancreatic sufficient. Each PTI-428 or placebo dose will be administered QD for a total of 28 days.