The study population is comprised of adult subjects with cystic fibrosis (CF) who are homozygous for the F508del mutation and are currently receiving background treatment with tezacaftor/ivacaftor for a minimum of 1 month prior to Day 1. The planned sample size is approximately 40 subjects. 20 subjects will be assigned to PTI-428 dose level 1 or placebo and 20 subjects will be assigned to PTI-428 dose level 2 or placebo. At each dose level, subjects will be randomized at a 3:1 randomization ratio. Subjects will receive once daily oral doses of PTI-428 or placebo for 28 days, while the subjects continue to receive background treatment with tezacaftor/ivacaftor per product label. The study drug administration period will be followed by a 14-day safety follow-up period.

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