Peter van Mourik

194 Chapter 9 control CFTR function. Moreover, although both intestinal and airway cells are derived from the endoderm, it is unclear whether intestinal epithelial CFTR function is directly related to airway epithelial CFTR function, which might influence treatment response 77,78 . Intestinal organoids are cultured as three-dimensional structures with the basolateral cell surface to the outside and the apical membrane facing the lumen of the organoid 17 . Thus, drugs added in vitro need to cross the basolateral membrane of the cell, which could impact absorption efficacy, especially for drugs that are designed to enter the cell via the apical membrane (such as inhaled drugs aimed at airway cells). Furthermore, the tissue most severely affected by CF is the airway epithelium, which differently expresses CFTR, differs with regards to cell turnover and protein homeostasis, and contains a range of other cell types than present in the intestinal epithelium 18–20 . Therefore, results cannot be directly extrapolated to airway cells. In contrast, airway epithelial cells can be used to study the mucus layer which is present in vivo and creates an additional barrier for compounds 21,22 , and could thus provide useful information on the uptake of drugs and their effects on mucus (composition). A combination of primary cell models including AECs and two-dimensional cultures could therefore remain necessary to confidently assess potential of drugs for clinical application. Nevertheless, the potential to biobank and repeatedly test intestinal organoids facilitates robust personalized medicine approaches. Future studies are needed to elucidate which model is superior in terms of feasibility and predictive capacity. CONCLUSION AND FUTURE DIRECTIONS In this thesis, I describe the use of intestinal organoids for the development and tailoring of CFTR-modulating treatments. Available data indicate that intestinal organoids can be a very helpful tool to preclinically assess novel CFTR-modulating treatments and can serve as a useful model for developing new approaches such as gene editing. Moreover, currently available CFTR-modulator treatment can be repurposed and optimized through testing in intestinal organoids, while pilot studies indicate that we can predict clinical treatment effect in individual subjects using their intestinal organoids as a surrogate. Although drug screening on organoids is currently possible in 384 wells format, this approach is time-consuming and expertise in organoid culturing is necessary. Moreover, data analysis has not been fully automated, limiting widespread implementation. In the future, screening platforms utilizing robots to culture organoids and artificial intelligence to analyse the data could increase the throughput and effectiveness of drug screening in organoids. In addition, studies directly comparing