Peter van Mourik

186 Chapter 9 published an open-access organoid culturing and FIS-assay protocol ( chapter 4) . The protocol has been endorsed by the ECFS Standardization committee and the European Reference Networks for lung diseases, which will greatly help in the adoption of the protocol by other research laboratories. Moreover, to study whether results produced across laboratories are comparable, we have performed validation studies using a selection of six organoid lines distributed to each laboratory. Although small differences in swelling were observed between laboratories, agreement was generally high, and each separate organoid line behaved similar across laboratories (manuscript in preparation). The observed differences highlight the feasibility of widespread implementation of this technology, but stress the need for experienced technicians and strict protocols, since the experiments were performed in laboratories with several years of experience in organoid culturing and FIS assays 4,52,53 . In chapters 6 and 7 , we explore the relationship between organoid response to drugs and clinical benefit. Chapter 6 explores improvements in several in vivo parameters in F508del homozygous patients treated with lumacaftor/ivacaftor and correlations with organoid measurements. No clear correlations could be identified on the individual level, although almost all patients showed improvement in at least one parameter and all organoids were responsive to lumacaftor/ivacaftor, albeit with variability in the extent of responsiveness. In chapter 7 we studied the correlation between organoid response and clinical response to different CFTR-modulating compounds in patients with a broad range of genotypes, ranging from severe to mild mutations. Here, we found a strong correlation between organoid response and clinical response, and a large area under the Receiver Operations Characteristics curve, indicating high-to-excelled predictive value of organoids in this population 54 . How can we explain the discrepancy between the absent correlation in the F508del homozygous population and the high correlation in the wider range of genotypes? Several mechanisms could have had an impact on these results. An important difference between the studies in chapters 7 and 8 is the choice of patient population. Chapter 7 explores differences between patients with identical CFTR-genotypes, while chapter 8 comprises patients with a wide range of genotypes but does include a significant proportion of patients with the S1251N mutation. Based on the results of chapter 8, two potential hypotheses could be generated: (i) organoids can be used to predict CFTR-modulator response on the individual level, and (ii) organoids can predict CFTR-modulator responsiveness of CFTR-genotypes, while chapter 7 seems to imply that prediction of individual responsiveness to lumacaftor/ivacaftor within the