Peter van Mourik

142 Chapter 7 leads to opening of the CFTR ion channel and subsequent ion and fluid transport into the organoid lumen in a CFTR-dependent manner. This readout functionally assesses the impact of both CFTR mutations and additional patient-specific genetic factors that act on CFTR function 15 . In previous work we showed that the in vitro response that was measured in rectal organoids correlates with average clinical responses described in patient populations with corresponding genotypes 16 . We also predicted the lack of efficacy of PTC124 (ataluren) in a recent phase 3 clinical trial, by testing of PTC124 in rectal organoids from people carrying nonsense mutations 17,18 . In vitro functional testing in rectal organoids of an individual patient may be a next step to facilitate rapid individual access to treatment for patients with rare CFTR mutations. Currently it is not clear if the in vitro FIS response to CFTR modulating drugs correlates with the in vivo response at the level of the individual patient. Current clinical outcome parameters and in vivo or ex vivo biomarkers of CFTR function are highly valuable for measurement of average treatment effects in clinical trials, but they do not correlate at the individual level. A recent meta-study found a small correlation between the in vivo pulmonary response and the response of an in vivo biomarker of CFTR function (sweat chloride concentration (SCC)), but this study also indicated that individual responses in SCC had a low predictive value for corresponding pulmonary response. Our previous study with rectal organoids showed that two individuals who carried mutations that were not yet characterized, could be successfully selected for a treatment with ivacaftor 16 . We also recently described that FIS measurements of individual patients were related to clinical indicators of CF disease severity, and comparison of FIS and SCC suggested more precise quantification of CFTR function by FIS 19 . We here describe the correlation between the response of FIS of rectal organoids and the in vivo therapeutic response for individual CF patients with multiple CFTR genotypes who were treated with several CFTR modulating drugs, and we study the predictive values of the organoid FIS test for the clinical response. METHODS Forskolin induced swelling of rectal organoids Rectal organoids were cultured according to previously described protocols, and are accessible for study by contacting the Hubrecht Organoid Technology foundation (www.hub4organoids.eu ) 16,20 . FIS of rectal organoids is a fully CFTR-dependent readout and was measured to indicate baseline CFTR function and response to drugs 15,16 . The organoid response to a drug was calculated by subtracting the dimethyl sulfoxide (DMSO) response at the same forskolin concentration. Organoid