This trial will consist of three arms: Part A, Part B, and Part C. Part A has two groups. The first group will enroll adult subjects with cystic fibrosis (CF) into a single ascending dose (SAD) treatment group. The second group will enroll adult subjects with CF, including those on background treatment with ORKAMBI® and those not on a cystic fibrosis transmembrane conductance regulator (CFTR) modulator, into a multiple ascending dose (MAD) treatment group. Part B will enroll adult subjects with CF currently on stable ORKAMBI® background therapy for a minimum of 3 months into a Phase II treatment group consisting of two cohorts. Part C will enroll adult subjects with CF, including those on background treatment with KALYDECO® and those not on a CFTR modulator, into a Phase II treatment group consisting of three cohorts. Approximately 136 subjects will be enrolled.

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.

PART B Following the conclusion of MAD Cohort 1, a set of adult subjects diagnosed with CF currently on stable ORKAMBI® background therapy for a minimum of 3 months will participate in Part B. The Part B Phase II treatment group is comprised of 2 cohorts where subjects will be randomized to either PTI-428 or placebo. Each dose will be administered QD for a total of 28 days.

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.

Inclusion Criteria:

• Confirmed diagnosis of CF.
• Forced expiratory volume in 1 second (FEV1) 40-90% predicted.
• Non-smoker and non-tobacco user for a minimum of 30 days prior to screening and for the duration of the study.

Exclusion Criteria:

• Participation in another clinical trial or treatment with an investigational agent within 30 days or 5 half-lives, whichever is longer, prior to Study Day 1.
• History of cancer within the past five years (excluding cervical CIS with curative therapy for at least one year prior to screening and non-melanoma skin cancer).
• History of organ transplantation.
• Any sinopulmonary infection or CF exacerbation requiring a change or addition of medication (including antibiotics) within 1 month of Study Day 1 or any other clinically significant infection as determined by the investigator within 1 month of Day 1.
• History of alcohol or drug abuse or dependence within 12 months of screening as determined by the Investigator.
• Male and female of child-bearing potential, unless they are using highly effective methods of contraception during participation in the clinical study and for 4 weeks after termination from study.
• Pregnant or nursing women.