Background: A decrease in the lumacaftor-mediated increase in F508del-CFTR function and expression upon prolonged exposure to ivacaftor (VX-770) has previously been described. However, the efficacy observed with ivacaftor-containing CFTR modulator therapies in vivo is in conflict with these reports. We hypothesized that a portion of the apparent decrease in CFTR function observed after prolonged ivacaftor exposure in vitro was due to an increase in constitutive CFTR-mediated ion transport.
Methods: Human nasal epithelial (HNE) cells were obtained by brushings from three CF individuals homozygous for the F508del CFTR mutation. Differentiated epithelia were pre-treated with prolonged (24 h) exposure to either lumacaftor (VX-809; 3 µM), tezacaftor (VX-661; 3 µM), elexacaftor (VX-445; 3 µM), and/or ivacaftor (0.1-6.4 µM) or DMSO (vehicle control), and CFTR function was assayed by Ussing chamber electrophysiology.
Results: In cells treated with lumacaftor, constitutive CFTR activity was not increased at any concentration of co-treatment with ivacaftor. Constitutive CFTR activity was also unchanged in cells treated with the combination of tezacaftor and elexacaftor. An increase in constitutive CFTR activity above the DMSO controls was only observed in cells treated with the combination of tezacaftor and elexacaftor and co-treated with at least 0.1 µM ivacaftor.
Conclusions: These results demonstrate that ivacaftor is a critical component in the triple combination therapy along with tezacaftor and elexacaftor to increase constitutive CFTR function. This work further elucidates the mechanism of action of the effective triple combination therapeutic that is now the primary clinical tool in treating CF.
Keywords: CFTR modulators; Cystic fibrosis; Elexacaftor; Ivacaftor; Orkambi; Symdeko; Tezacaftor; Trikafta.
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