VX-809 corrects folding defects in cystic fibrosis transmembrane conductance regulator protein through action on membrane-spanning domain 1

Cystic fibrosis (CF) is a fatal genetic disorder associated with defective hydration of lung airways due to the loss of chloride transport through the CF transmembrane conductance regulator protein (CFTR). CFTR contains two membrane-spanning domains (MSDs), two nucleotide-binding domains (NBDs), and a regulatory domain, and its channel assembly requires multiple interdomain contacts. The most common CF-causing mutation, F508del, occurs in NBD1 and results in misfolding and premature degradation of F508del-CFTR. VX-809 is an investigational CFTR corrector that partially restores CFTR function in people who are homozygous for F508del-CFTR. To identify the folding defect(s) in F508del-CFTR that must be repaired to treat CF, we explored the mechanism of VX-809 action. VX-809 stabilized an N-terminal domain in CFTR that contains only MSD1 and efficaciously restored function to CFTR forms that have missense mutations in MSD1. The action of VX-809 on MSD1 appears to suppress folding defects in F508del-CFTR by enhancing interactions among the NBD1, MSD1, and MSD2 domains. The ability of VX-809 to correct F508del-CFTR is enhanced when combined with mutations that improve F508del-NBD1 interaction with MSD2. These data suggest that the use of VX-809 in combination with an additional CFTR corrector that suppresses folding defects downstream of MSD1 may further enhance CFTR function in people with F508del-CFTR

Structure-guided combination therapy to potently improve the function of mutant CFTRs

Available drugs are unable to effectively rescue the folding defects in vitro and ameliorate the clinical-phenotype of cystic fibrosis (CF), caused by deletion of F508 (ΔF508 or F508del) and some point mutations in the CF transmembrane conductance regulator (CFTR), a plasma membrane (PM) anion channel. To overcome the corrector efficacy ceiling, here we show that compounds targeting distinct structural defects of CFTR can synergistically rescue mutants expression and function at the PM. High throughput cell-based screens and mechanistic analysis identified three small-molecule series that target defects at the nucleotide binding domain (NBD1), NBD2 and their membrane spanning domains (MSDs) interfaces. While individually these compounds marginally improve ΔF508-CFTR folding efficiency, function, and stability, their combinations lead to ~50-100% of wild type-level correction in immortalized and primary human airway epithelia, and in mouse nasal epithelia. Likewise, corrector combinations were effective for rare missense mutations in various CFTR domains, probably acting via structural allostery, suggesting a mechanistic framework for their broad application