The trial will consist of a single treatment group enrolling adult subjects with CF on background therapy with KALYDECO®. Approximately 16 subjects will be enrolled.

The single treatment group will enroll adult subjects with CF currently on stable KALYDECO® background therapy for a minimum of 3 months at the time of randomization. Subjects will be randomized to either PTI-428 or placebo. Each dose will be administered once daily (QD) for a total of 14 Days. The subjects will continue treatment with KALYDECO® throughout the study.

Inclusion Criteria:

• Confirmed diagnosis of CF.
• Stable on Kalydeco dosing for both label indication and per label dosing for a minimum of 3 months at the time of randomization
• 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.