Development of selective blockers for Ca²(+)-activated Cl channel using Xenopus laevis oocytes with an improved drug screening strategy.

Ca²(+)-activated Cl channels (CaCCs) participate in many important physiological processes. However, the lack of effective and selective blockers has hindered the study of these channels, mostly due to the lack of good assay system. Here, we have developed a reliable drug screening method for better blockers of CaCCs, using the endogeneous CaCCs in Xenopus laevis oocytes and two-electrode voltage-clamp (TEVC) technique. Oocytes were prepared with a treatment of Ca²(+) ionophore, which was followed by a treatment of thapsigargin which depletes Ca²(+) stores to eliminate any contribution of Ca²(+) release. TEVC was performed with micropipette containing chelerythrine to prevent PKC dependent run-up or run-down. Under these conditions, Ca²(+)-activated Cl currents induced by bath application of Ca²(+) to oocytes showed stable peak amplitude when repetitively activated, allowing us to test several concentrations of a test compound from one oocyte. Inhibitory activities of commercially available blockers and synthesized anthranilic acid derivatives were tested using this method. As a result, newly synthesized N-(4-trifluoromethylphenyl)anthranilic acid with trifluoromethyl group (-CF) at para position on the benzene ring showed the lowest IC. Our results provide an optimal drug screening strategy suitable for high throughput screening, and propose N-(4-trifluoromethylphenyl)anthranilic acid as an improved CaCC blocker.