Study Assessing PTI-428 Safety, Tolerability, Pharmacokinetics and Effect in Subjects With Cystic Fibrosis

Inclusion Criteria: * Confirmed diagnosis of CF with the F508del/F508del genotype on record * On tezacaftor/ivacaftor dosing for both label indication and per label dosing for a minimum of 1 month on Day 1 * Forced expiratory volume in 1 second (FEV1) 40-90% predicted, inclusive * Clinically stable with no significant changes in health status within 14 days of Day 1 * Non-smoker and non-tobacco user for a minimum of 28 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 28 days or 5 half-lives, whichever is longer, prior to Study Day 1 * History of cancer within the past 5 years (excluding cervical cancer in situ with curative therapy for at least one year prior to screening and non-melanoma skin cancer) * History of organ transplantation * Hospitalization, sinopulmonary infection, CF exacerbation, or other clinically significant infection or illness (as determined by the investigator) requiring an increase or addition of medication, such as antibiotics or corticosteroids, within 14 days of Day 1 * Initiation of any new chronic therapy (e.g., ibuprofen, hypertonic saline, azithromycin, Pulmozyme®, Cayston®, TOBI®)) or any change in chronic therapy (excluding pancreatic enzyme replacement therapy) within 28 days prior to Day 1 * History or current evidence of alcohol or drug abuse or dependence within 12 months of screening as determined by the investigator * Pregnant or nursing women