This is a multicenter, randomized, placebo-controlled, dose-escalation study. Enrollment is planned to occur at approximately 14 global sites. Approximately 24 subjects with CF.

This is a multicenter, randomized double-blind, placebo-controlled dose-escalation and parallel-arm, dose-ranging study. Enrollment is planned to occur at approximately 14 global sites. Approximately 24 subjects with CF who are homozygous for the F508del-CFTR mutation will be enrolled in two cohorts.

Inclusion Criteria:

• Male or female subjects with a confirmed diagnosis of CF defined as a sweat chloride value ≥60 mmol/L by quantitative pilocarpine iontophoresis or two CF-causing mutations,documented in the subject's medical record or confirmed at screening.
• Age 18 and above on the date of informed consent.
• Weight ≥40 kg.
• Homozygous for the F508del-CFTR mutation. Genotyping to be confirmed at screening.
• Ability to perform a valid, reproducible spirometry test with demonstration of a forced expiratory volume in 1 sec (FEV1) >40% of predicted normal for age, sex and height.
• Screening laboratory tests with no clinically significant abnormalities that would interfere with the study assessments (as judged by the Investigator).
• Subjects who are sexually active must agree to follow the study's contraception requirements.

Exclusion Criteria:

• An acute upper or lower respiratory tract infection, pulmonary exacerbation, or changes in therapy for pulmonary disease within 4 weeks prior to Day 1.
• Major complications of lung disease (including massive hemoptysis, pneumothorax, or pleural effusion) within 8 weeks prior to screening.
• Impaired renal function or known portal hypertension.
• History of prolonged QT and/or QTcF (Fridericia's correction) interval (>450 msec) or QTcF >450 msec at Screening.
• History of solid organ or hematological transplantation.
• History of alcohol abuse or drug addiction (including cannabis, cocaine and opiates) during the past year, (as judged by the Investigator).
• Use of ivacaftor or lumacaftor, within 4 weeks of Day 1
• Any change (initiation, change in type of drug, dose modification, schedule modification, interruption, discontinuation, or re-initiation) in a chronic treatment/prophylaxis regimen for CF or for CF-related conditions within 4 weeks prior to Day 1.
• Ongoing immunosuppressive therapy (including systemic corticosteroids).
• Hemoglobin <10 g/dL.
• Abnormal liver function, at screening.
• Abnormal renal function at screening.
• Ongoing participation in another clinical study or prior participation without appropriate washout (minimum of 10 half- lives or 30 days, whichever is longer) prior to Screening visit.