Ultra-rapid kinetic interactions of ranolazine with Herg K+ current

Abstract

Human-ether-a-go-go related gene (HERG) encodes the cardiac rapidly activating delayed rectifier K+ current (IKr). Inhibition of HERG K+ current (IHERG) is a mechanism for drug-induced long QT syndrome. Ranolazine, an anti-ischemic agent inhibits IHERG, and causes a small QT interval prolongation. We investigated the kinetics of ranolazine block of IHERG at 23 °C using voltage-clamp analysis of HERG channels expressed in HEK293 cells. Block of IHERG by ranolazine was reversible and voltage-dependent, but frequency-independent. Block developed rapidly following channel activation, suggesting state-dependence. At 0 mV, the time constants for development of block were 76.6±1.6, 35.8±2.4, and 19.4±1.7 ms with 10, 30 and 100 μM ranolazine (n=4), respectively. The time course of ranolazine-induced IHERG decay was used to estimate the apparent dissociation constant (14.2 μM). Following repolarization, ranolazine-induced block of IHERG reversed rapidly. At −80 and −100 mV, recovery from block followed a monophasic time course with τ values of 204.3±51.5 and 155.0±31.9 ms (n=5), respectively. Intracellular but not extracellular application of a membrane-impermeable (permanently charged) ranolazine analog caused rapid block of IHERG. Ranolazine antagonized E-4031 block of IHERG, suggesting that both compounds compete for a common binding site. Taken together, the unique ultra-rapid kinetics of block (at positive potentials) and unblock (upon hyperpolarization) of IHERG by ranolazine may explain the observations that: (1) ranolazine causes minimal QT interval prolongation with no reverse use dependence, and (2) pro-arrhythmic events have not been observed during exposures of cardiac myocytes or whole hearts to ranolazine.