Peter van Mourik

187 General discussion F508del population is not possible. Nevertheless, we were able to find differences between individual organoids with identical genotypes in chapter 2 , and a previous study into residual CFTR-function differences within the F508del homozygous population suggests that differences measured in organoids do correspond to clinical differences 55 . Another potential confounding factor is the study duration, which might have impacted on the ability to detect potential clinical improvements. While 8-16 weeks of lumacaftor/ivacaftor might not improve clinical outcomes such as ppFEV 1 or BMI, increased CFTR function was detected in all F508del homozygous patients using either FIS, Intestinal Current Measurement (ICM), Nasal Potential Difference (NPD) or Sweat Chloride Concentration (SCC). This suggests that more sensitive clinical outcome measures (i.e. endpoints that are able to detect small differences with more certainty), or longer clinical follow-up, might be necessary to unequivocally determine if individual patients benefit from lumacaftor/ivacaftor treatment. However, chapter 7 mainly studies effects of ivacaftor with significantly better improvements in all clinical parameters in the responding population. The differences in results could possibly be explained by the intrinsic variability of the studied endpoints and biomarkers. ppFEV1 is the most commonly used endpoint in clinical trials, and is sensitive in detecting small average treatment effects in large populations, such as the average treatment response of ~3-4% to lumacaftor/ivacaftor in F508del homozygous subjects 31 . However, when it is used in a personalized setting, the measurement variability becomes problematic. Inter-measurement variability in stable subjects approximates 5% 56 , which is larger than the expected treatment effect. Therefore, determining whether an observed change in ppFEV1 is due to a treatment effect or measurement variability is impossible, unless the change is far greater than could be accounted to variability. Many commonly used biomarkers in CFTR-modulator research such as sweat chloride concentration (SCC) and NPD show high inter-measurement variability 57,58 , and therefore finding correlations is challenging if the estimated effect size is small. Further impacting on the results found in these studies are factors that cannot be modeled in an in vitro system. Organoids harbor the unique genetic make-up of the individual donor, but are not exposed to environmental factors that impact on clinical disease severity, while environmental factors are thought to produce ~50% of pulmonary function variation 59 . These factors include, but are not limited to, air pollution 60 , second-hand smoke exposure 61,62 and socio-economic status 63 . Since clinical disease severity could impact on drug responsiveness and experienced 9