Peter van Mourik

182 Chapter 9 function measurements suggested some improvement in CFTR-function 16 (G. Berkers, personal correspondence). High-content drug screening has been aided by the development of organoid screening in 384-wells plates in comparison to 96-wells, thereby significantly improving the throughput. This approach has been applied in the ongoing Rainbow project, where we aim to repurpose drugs for patients with rare mutations whose organoids are available in the biobank. By screening an FDA-approved drug library of >1400 compounds, the goal is to identify non-toxic, clinically available drugs that could improve CFTR function in individual patients. More than 180 organoids have been screened so far. Based on preliminary results, drugs appear not to restore CFTR function directly at a level of efficacy that is within the range of existing CFTR-modulators, but potentially beneficial drugs with different mode-of-action could help to enhance CFTR function or epithelial fluid transport. This could help to develop treatments for patients with rare mutations, who do not have treatment options thus far, and may lead to selections of drugs based on individual profiling. OPTIMIZING CFTR-MODULATOR TREATMENT USING IN- TESTINAL ORGANOIDS The last years have seen the remarkable introduction of CFTR-modulators for patients with CF. Ivacaftor, a CFTR-potentiator, entered clinical trials in 2007 and is very effective in improving lung function, BMI and quality of life in patients with gating mutations 23–25 , while in vivo measurements of CFTR function also markedly improved 26,27 . However, CFTR-function and pulmonary function are not completely normalized through ivacaftor monotherapy, indicating the need for additional therapies. Moreover, ivacaftor did not lead to a clinical benefit in patients homozygous for the F508del mutation 28 which, in addition to gating defects, exhibits misfolding and aberrant trafficking of CFTR 29 . On top of potentiators, correctors were developed to enhance F508del CFTR protein folding. Combination treatment of ivacaftor and the first-generation correctors lumacaftor and tezacaftor results in improved lung function in patients with two F508del alleles, albeit with modest and highly variable effects between individuals 30–32 . Moreover, the treatment does not work sufficiently in F508del heterozygous patients, which suggests that F508del protein folding is not fully corrected 33 . In vitro experiments corroborate these clinical observations, since immature (B-band) CFTR is still detected in lumacaftor corrected F508del cells 34 . In silico predictions in combination with intestinal organoid testing previously indicated that next- generation correctors on top of lumacaftor could enhance the folding correction