Identification of CFTR inhibitors by high-throughput screening. (a) Schematic of screening approach. CFTR was maximally stimulated by multiple agonists in stably transfected epithelial cells coexpressing human CFTR and a yellow fluorescent protein (YFP) with fluorescence sensitive to Cl^–/I^–. After addition of test compound, I^– influx was induced by adding an I^–-containing solution. (b) Representative original fluorescence data from individual wells showing controls (no activators, no test compound) and test wells. (c) Top: Chemical structure of 2-thioxo-4-thiazolidinone CFTR inhibitors. Bottom: Structures of analogs having greatest CFTR inhibitory activity. Relative potencies were 0.2 (CFTR_inh-020), 0.3 (CFTR_inh-029), 1.0 (CFTR_inh-172), 0.2 (CFTR_inh-185), 0.1 (CFTR_inh-214), and 0.1 (CFTR_inh-236).