Study of Cavosonstat (N91115) in CF Patients Who Are Heterozygous for F508del-CFTR and a Gating Mutation and Being Treated With Ivacaftor

Inclusion Criteria: * Confirmed diagnosis of CF, heterozygous for F508del-CFTR and a gating mutation that is approved for treatment with ivacaftor (G551D, G1244E, G1349D, G178R, G551S, S1251N, S1255P, S549N, or S549R) * Have been treated with chronic ivacaftor twice daily for at least 6 months prior to Screening (date of consent) and are currently being treated with commercially available Ivacaftor * Negative serum pregnancy test * Weight ≥ 40 kg at screening * Oxygen saturation by pulse oximetry ≥ 90% breathing ambient air, at screening Exclusion Criteria: * Any acute infection, including acute upper or lower respiratory infections and pulmonary exacerbations that require treatment that has completed within 2 weeks of Study Day 1 or hospitalization discharge within 2 weeks of Study Day 1 * Recent infection (per investigator discretion) with organisms associated with more rapid decline in pulmonary status, for example: Burkholderia cenocepacia, Burkholderia dolosa, and Mycobacterium abscessus * Any change in the regimen for chronic therapies for CF lung disease (e.g., Pulmozyme®, hypertonic saline, Azithromycin, TOBI®, Cayston®) within 4 weeks of Study Day 1 * Blood hemoglobin \< 10 g/dL at screening * Serum albumin \< 2.5 g/dL at screening * Abnormal liver or renal function * History of ventricular tachycardia or other clinically significant ventricular arrhythmias * History, including the screening assessment, of prolonged QT and/or QTcF (Fridericia's correction) interval (\> 450 msec for men; \> 470 msec for women) * History of solid organ or hematological transplantation * History of alcohol abuse or drug abuse (including cannabis, cocaine, and opioids) in the year prior to screening * Use of continuous (24 hr/day) or nocturnal supplemental oxygen