Peter van Mourik

184 Chapter 9 Exploring CFTR-potentiator effects The effect of CFTR-modulators on many CFTR variants other than F508del has not been thoroughly characterized, while it is known that some variants could be highly responsive 40 . Therefore, more research is needed to further study the effects of these drugs on (rare) CFTR variants. In this thesis, we explored the effects of CFTR- modulators on a range of CFTR variants. In chapter 2 , we study organoids with the R117H allele in combination with a 7T poly-T tract and their response to ivacaftor 41 . A large variability in CFTR function was observed, consistent with literature on clinical presentations of individuals harbouring R117H-7T 42–45 . Interestingly, we found a correlation between CFTR-mRNA expression and ivacaftor response, with some organoids having low CFTR-mRNA expression and very limited responsiveness to ivacaftor. In addition, clinical trial data for patients harbouring R117H indicate large differences in response to treatment 46 . When combining these two observations, could it be that differences in mRNA expression explain response variability? So far, no data is available on whether mRNA expression could influence CFTR function and drug response of other mutations. If that should be the case, several interesting questions arise: could treatment be optimized by stratifying treatment according to in vivo CFTR-mRNA expression? Could patients with decreased mRNA expression especially benefit from additional treatment with an amplifier? These questions warrant further research into the origins of variability in CFTR-modulator response between individuals with similar genotypes, especially when regarding the high costs of these therapies and potential efficiency gains. Since ivacaftor monotherapy does not normalize CFTR function in patients with gating mutations, chapter 3 examines whether combination treatment of ivacaftor and the natural food supplements curcumin and genistein could boost CFTR- function of gating mutations 47 . All compound combinations showed a synergistic effect on organoid swelling as compared to monotherapy, most markedly for the combination of ivacaftor and genistein, which led us to trial these combinations in several subsequent clinical studies 48 . First, genistein/curcumin treatment was tested in a group of patients with the S1251N mutation, which did not result in a clear clinical benefit although an improvement in sweat chloride concentration was observed. Next, genistein was added on top of ivacaftor treatment, but did not lead to apparent improvements in clinical parameters. These somewhat disappointing results could be explained by the limited bioavailability of both genistein and curcumin 49,50 , which is corroborated by the very low plasma concentrations of genistein and curcumin in patients participating in the trials. Further supporting this explanation is the fact that plasma from patients on active treatment with genistein and curcumin (which should thus contain these compounds) did not induce organoid swelling in vitro, while