['cystic fibrosis']
['cavosonstat', 'placebo']

        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
      