Inwardly rectifying IK1 potassium currents of the heart control the resting membrane potential of ventricular cardiomyocytes during diastole and contribute to their repolarization after each action potential. Mutations in the gene encoding Kir2.1 channels, which primarily conduct ventricular IK1, are associated with inheritable forms of arrhythmias and sudden cardiac death. Therefore, potential iatrogenic inhibition of Kir2.1-mediated IK1 currents is a cardiosafety concern during new drug discovery and development. Kir2.1 channels are part of the panel of cardiac ion channels currently considered for refined early compound risk assessment within the Comprehensive in vitro Proarrhythmia Assay initiative. In this study, we have validated a cell-based assay allowing functional quantification of Kir2.1 inhibitors using whole-cell recordings of Chinese hamster ovary cells stably expressing human Kir2.1 channels. We reproduced key electrophysiological and pharmacological features known for native IK1, including current enhancement by external potassium and voltage- and concentration-dependent blockade by external barium. Furthermore, the Kir inhibitors ML133, PA-6, and chloroquine, as well as the multichannel inhibitors chloroethylclonidine, chlorpromazine, SKF-96365, and the class III antiarrhythmic agent terikalant demonstrated slowly developing inhibitory activity in the low micromolar range. The robustness of this assay authorizes medium throughput screening for cardiosafety purposes and could help to enrich the currently limited Kir2.1 pharmacology.
Keywords: CiPA initiative; K channels; safety pharmacology; sudden death; torsades de pointes; ventricular arrhythmia.