Novel Voltage Protocols for Determining hERG Channel Kinetics

Abstract

Drug interactions with cardiac ion channels can cause disturbances to normal electrical activity in the heart. In particular, hERG channel block has been linked with QT prolongation and increased pro-arrhythmic risk. Consequently, hERG channel screening, quantifying drug block at different drug concentrations, forms a routine part of cardiovascular safety assessment. Recent initiatives led by the US Food and Drug Administration (FDA) aim to reform the current safety assessment guidelines and in silico approaches are envisaged to form an integral part of the new regime (Sager et al., Am Heart J., 2014).Mathematical ion channel representations, embedded within action potential models, are typically constructed through fitting to patch clamp data that are acquired by applying a series of voltage-step protocols. But different models exhibit disparate behaviour under non-standard protocols; including physiologically relevant action potential clamps, or clamps mimicking arrhythmogenic behaviour. We demonstrate the influence the choice of hERG channel representation can have on action potentials simulated from a mathematical ventricular myocyte model when predicting drug-induced effects; indicating the necessity of including accurate descriptions of hERG channel kinetics.We present novel rapidly fluctuating voltage-clamp protocols, designed mathematically to enable a robust model of hERG channel kinetics to be determined, on a cell-specific basis. The protocols have been tested in manual patch clamp experiments, using hERG-1a transfected HEK 293 cells. The approach will be extended to determine compound-specific models describing hERG channel kinetics in the presence of different drugs, which may enhance the predictive ability of in silico approaches used for cardiovascular safety assessment.