COMPARATIVE EFFECTS OF AZIMILIDE AND AMBASILIDE ON THE HUMAN ETHER-A-GO-GO-RELATED GENE (HERG) POTASSIUM CHANNEL EXPRESSED IN CHINESE HAMSTER OVARY (CHO-K1) CELLS
B.D. Walker*, C.B. Singleton, H. Tie, J.A. Bursill, K. Wyse, M.R. Qiu, S. Valenzuela, S.N. Breit, T.J. Campbell.
Department of Medicine, Centre for Immunology, and Victor Chang Cardiac Research Institute, St. Vincent's Hospital, Sydney, New South Wales.
Background: Azimilide is a new class III antiarrhythmic drug which has potential efficacy in the treatment of atrial and ventricular tachyarrhythmias, and acts via blockade of 1Kr (encoded by HERG) and 1Ks. Ambasilide, a non-selective inhibitor of both components of 1K, causes action potential prolongation in the human and canine myocardium. We compared the effects of azimilide and ambasilide on HERG, stably transfected into CHO-K1 cells using a whole cell voltage clamp technique.
Results: Azimilide potently blocked HERG tail currents following a step to +30mV (IC50 700 nM, 95% CI 540-890 nM; human plasma concentration ` 1(M). Azimilide had a dual effect, inhibiting current at voltage steps above -30mV and augmenting current at voltages negative to -30mV. Fast inactivation was accelerated in the presence of azimilide 1 (M at most potentials (eg. 14.2±1.1 ms vs 11.2±0.7 ms at -40 mV, p<0.05). the voltage of half maximal inactivation markedly shifted from -61.0 + 1.5 mV to -79.0 + 2.3 mV (p<0.001). No effect was observed on deactivation or recovery from inactivation. Amasilide also inhibited the HERG channel (IC50 3.0 (M, 95% CI 2.7 - 3.4 (M). Inhibition displayed reverse use-dependence and was not voltage dependent. In contrast to azimilide, channel inhibition was independent of pulse duration using an envelope of tails protocol. Moreover, there was no change in the voltage dependence of steady state inactivation or channel kinetics.
Conclusion: Both azimilide and ambasilide are potent inhibitors of HERG channels with different mechanisms of action. Azimilde binds with greatest affinity to open or inactivated states. In contrast, amabasilide either binds very rapidly to the open state or more likely to a closed channel state.
