Peter van Mourik

130 Chapter 6 TO THE EDITOR CFTR biomarker assays in patient-derived model systems in vitro and in patients in vivo enable assessment of response to emerging CFTR-directed therapeutics at the level of the basic CF ion transport defect in individuals with CF and are promising tools to realize the potential of personalized medicine (1, 2). Intestinal organoids represent such a patient-derived in vitro model, in which CFTR function can be determined by the forskolin-induced swelling (FIS) assay that sensitively detects functional consequences of CFTR mutations and response to CFTR-directed therapeutics in vitro , and correlates with clinical outcomes in patients with CF with a broad spectrum of CFTR genotypes (3–5). Traditional outcome measures of CFTR function such as in vivo measurements of sweat chloride concentration (SCC) and nasal potential difference (NPD), and ex vivo intestinal current measurements (ICM) in native tissues cannot be used for preclinical compound testing, but sensitively detect in vivo response to approved CFTR modulator therapy (6, 7). Collectively, these studies support that both organoid swelling and in vivo biomarkers of CFTR function may be promising tools to determine response to CFTR modulators in individual patients, but their relationships have not been studied. The aim of this study was to compare the effects of the approved CFTR modulator lumacaftor-ivacaftor detected by organoid swelling and in vivo CFTR biomarkers, and to determine correlations between paired measurements of organoid FIS with changes in SCC, NPD and ICM in 21 individuals with CF homozygous for the F508del mutation who started treatment with lumacaftor-ivacaftor. METHODS This prospective observational study was approved by the ethics committee of the University of Heidelberg. Written informed consent was obtained from all patients participating in the study, their parents, or legal guardians. Clinical outcomes percentage predicted forced expiratory volume in one second (ppFEV 1 ) and body mass index (BMI), and CFTR biomarkers (SCC, NPD and ICM) of 21 individuals with CF homozygous for the F508del mutation were measured before and 8-16 weeks after starting lumacaftor-ivacaftor treatment. Patient characteristics are described in Table 1. Lung function, SCC, NPD and ICM were performed as previously described (6, 8, 9). Rectal biopsies were sent to the University Medical Center Utrecht for organoid generation and FIS experiments using previously described protocols (4). Organoids were pre-incubated (24h) with lumacaftor (3µM VX-809, Selleck Chemicals LLC) and simultaneously stimulated with forskolin (0.128µM) and ivacaftor (3µM VX-770, Selleck Chemicals LLC). Investigators who performed FIS