A Homogeneous Cell-Based Halide-Sensitive Yellow Fluorescence Protein Assay to Identify Modulators of the Cystic...

Cystic fibrosis (CF), an inherited genetic disease, is caused by mutation of the Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) gene, which encodes an ion channel involved in hydration maintenance by anion homeostasis. Ninety percent of CF patients possess one or more copies of the F508del CFTR mutation. This mutation disrupts trafficking of the protein to the plasma membrane and diminishes function of mature CFTR. Identifying small molecule modulators of mutant CFTR activity or biosynthesis may yield new tools for discovering novel CF treatments. One strategy utilizes a 384-well, cell-based fluorescence-quenching assay, which requires extensive wash steps, but reports sensitive changes in fluorescence-quenching kinetic rates. In this study, we describe the methods of adapting the protocol to a homogeneous, miniaturized 1,536-well format and further optimization of this functional F508del CFTR assay. The assay utilizes a cystic fibrosis bronchial epithelial (CFBE41o^-) cell line, which was engineered to report CFTR-mediated intracellular flux of iodide by a halide-sensitive yellow fluorescence protein (YFP) reporter. We also describe the limitations of quench rate analysis and the subsequent incorporation of a novel, kinetic data analysis modality to quickly and efficiently find active CFTR modulators. This format yields a Z' value interval of 0.61 ± 0.05. As further evidence of high-throughput screen suitability, we subsequently completed a screening campaign of >645,000 compounds, identifying 2,811 initial hits. After completing secondary and tertiary follow-up assays, we identified 187 potential CFTR modulators, which EC₅₀'s < 5 μM. Thus, the assay has integrated the advantages of a phenotypic screen with high-throughput scalability to discover new small-molecule CFTR modulators.