Peter van Mourik

166 Chapter 8 (genet.sickkids.on.ca and cftr2.org) 4,5 . Moreover, patients carry a combination of two CFTR variants which further complicates the prediction of drug efficacy in individual patients. The role of current CFTR-modulators in these patients is hardly described and clinical efficacy can not be tested in classical clinical trials due to extremely low patient numbers. In this population, there is a clear unmet need to match the best CFTR-modulator (combination) to each individual. Recent stem cell technologies enable the long-term culture of patient-derived tissues in the laboratory as organoids 20,21 . Organoids are three-dimensional cell cultures that recapitulate features of the tissue from which they originate 20 . Dekkers et al developed a functional CFTR assay in intestinal organoids that were grown from easily obtainable rectal biopsies which provides a way to rapidly collect and biobank tissues from individual patients 22 . CFTR function can be measured upon stimulation of organoids with forskolin, which leads to CFTR phosphorylation, CFTR channel opening and subsequent transport of chloride and water into the organoid lumen. Although forskolin-induced swelling (FIS) of intestinal organoids is an indirect measurement of CFTR function as it quantifies fluid rather than ion transport, FIS is fully CFTR dependent and provides a rapid, phenotypic assessment of CFTR function. Residual CFTR function in organoids correlates with several clinical disease parameters 23,24 and biomarkers of CFTR function 25 . Importantly, the effect of CFTR modulators on CFTR function can be robustly measured by the FIS assay through quantification of increased swelling upon drug incubation in vitro 26 . Using this assay, correlations were observed between organoid drug-induced swelling and drug responses in clinical trials at the population level 26 , as well as a high-to-excellent accuracy predicting individual in vivo responses to CFTR modulators 27 . This has led to the identification of successful treatment of several patients with ultra-rare genotypes that showed high FIS to CFTR-modulating drugs in their organoids 26,27 . Moreover, drugs that showed potential in other pre- clinical model systems but not in organoids subsequently failed to produce clinical benefit, highlighting the specificity of organoids in finding meaningful clinical changes 28 . By providing an individual assessment of CFTR modulator efficacy without a priori knowledge of the CFTR mutations, organoid CFTR function might be exceptionally suited to stratify people with ultra-rare mutations for effective treatments. To further implement the FIS assay as a personalized medicine approach for patients with rare CFTR-mutations, we developed the HIT-CF project (www.hitcf.org) 29 .