The novel potentiator GLPG1837 modulates CFTR through different mode of action than Ivacaftor (kalydeco)

The current therapeutic strategy to repair cystic fibrosis-causing defects in the chloride channel CFTR is to develop novel and better correctors (to improve folding) and potentiators (to improve function). Galapagos identified a novel potentiator GLPG1837 that is potent in improving channel activity of temperature-rescued CFTR-F508del as well as CFTR-G551D. Compared to ivacaftor, GLPG1837 does not compromise drug-corrected CFTR-F508del (Peters F, et al. Pediatr Pulmonol. 2015;50(S41):210). GLPG1837 is currently in a Phase 2 study to evaluate safety, tolerability and efficacy in S1251N and G551D CF patients. Using a radiolabeling pulse-chase approach in combination with protease susceptibility we track folding and assembly of each CFTR domain with time, and determine when and where correctors and potentiators work on the newly synthesized protein. Using newly developed antibodies against the membrane-spanning domains of CFTR (TMDs) we investigated mode of action of the clinical drug ivacaftor and found that this potentiator changed protease resistance of TMD1 during late stages of CFTR folding. Ivacaftor mimics the conformational effect that nucleotide-binding domain NBD2 has on TMD1 through the second intracellular loop ICL2 during CFTR folding. These effects occur already during CFTR folding in the early secretory pathway, in apparent contrast to ivacaftor modulating CFTR function at the cell surface. Since the efficacy of GLPG1837 on dysfunctional channel CFTRG551D is greater compared to ivacaftor, we expect to observe the largest difference between GLPG1837 and ivacaftor on this mutant. We therefore pulse-labeled both wild-type and CFTR-G551D in the absence or presence of GLPG1837, chased for 2 h or not (0 h), and determined protease susceptibility of all 5 CFTR domains. The GLPG1837 potentiator had no effect on the late folding events in TMD1 that were changed by ivacaftor, demonstrating that this novel potentiator modulates CFTR through a different mode of action. We are testing additional proteases on all CFTR domains to identify possible mild effects of GLPG1837 on CFTR conformation. These results will provide new and important insights on the impact of potentiators on CFTR's structure and function.