Peter van Mourik

151 Personalized treatment of CF using rectal organoids DISCUSSION This study aimed to provide evidence that FIS of rectal organoids can act as a prospective biomarker for in vivo CFTR modulator responses. We demonstrated here that individual in vitro CFTR modulator responses in these patient-derived stem cell cultures correlate with two independent indicators of therapeutic response in vivo . The moderate correlation between FIS and ppFEV 1 and higher correlation between FIS and SCC (an in vivo biomarker of CFTR function) is in agreement with the higher impact of non-CFTR dependent factors on variation in pulmonary function as compared to SCC 6,35 . We did not find a statistically significant correlation between change in SCC and ppFEV 1 , probably because of a weaker correlation between these outcome measurements in combination with a small sample size, as was previously also observed in other studies with comparable sample sizes 36 . These in vivo endpoints are suitable to indicate treatment effects at a group level, but non-CFTR dependent variation in ppFEV 1 and SCC probably limits their precision and accuracy for informing on individual CFTR function modulation. 37 In contrast, in vitro FIS is completely CFTR dependent, has sufficient sensitivity to quantitate CFTR modulator activity and the repeated measurements increase precision. These properties likely facilitate that FIS has sufficient accuracy to inform on both ppFEV 1 and SCC (or their combination), suggesting that FIS is a potent biomarker to quantitate individual CFTR modulator responses. Our dataset provides a first analysis of the predictive potential of the rectal organoids to identify clinical responders and non-responders to treatment. Our data support that FIS can be used to prospectively select responders and non-responders to CFTR modulator treatments but the cut-off value with the highest Youden index still needs to be interpreted carefully as well as the definition of clinical responders. The Youden index selects the most optimal ratio between sensitivity and specificity, but a different threshold with a higher negative predictive value may be preferential to limit the exclusion of treatment responders (e.g. an organoid threshold with a negative predictive value of 100% would have a positive predictive value of 77%). Additionally, it remains unclear how short term treatment responses individually translate into long term clinical response. It could therefore be that the definitions for long term clinical responders are different, leading to other threshold values of predictive tests. We observed that the correlation of the organoid test with response in ppFEV 1 was modified by baseline ppFEV 1 , despite similar correlation in SCC in both groups with differences in baseline ppFEV 1 . This supports that biomarkers of CFTR function such as organoid-based measurements have an important role for assessment of CFTR 7