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

Assess the effect of Cavosonstat (N91115) on lung function when added to preexisting treatment with ivacaftor in adult patients with CF who are 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).