Comparative effects of azimilide and ambasilide on the human ether-a-go-go-related gene (HERG) potassium channel

Objective: To evaluate the effects of azimilide and ambasilide on the biophysical properties of the human-ether-a-go-go-related (HERG) channel. Methods: HERG was stably transfected into Chinese hamster ovary (CHO-K1) cells and currents were measured using a whole cell, voltage-clamp technique. Results: Azimilide had a ‘dual effect’, inhibiting current at voltage steps above −40 mV and augmenting current at −40 and −50 mV. Tail current inhibition following a step to +30 mV did not vary with temperature (IC₅₀ 610 nM at 22°C and 560 nM at 37°C). The agonist effect at −50 mV was concentration-dependent and correlated with a hyperpolarizing shift in the V_1/2 of activation (r = 0.98, P<0.05). Time constants of inactivation were faster and there was a −10 mV shift in the V_1/2 of steady state inactivation suggestive of open and inactivated state binding. By comparison, ambasilide inhibited HERG channels with lower potency (IC₅₀ 3.6 μM), in a voltage- and time-dependent but frequency-independent manner (0.03–1 Hz). Ambasilide had no effect on activation or inactivation gating but prolonged both fast and slow components of deactivation consistent with unbinding from the open state. The net effect of both drugs was similar during a voltage ramp which simulated a cardiac action potential. Conclusions: Inhibition of HERG channels by azimilide and ambasilide exhibits a similar time and voltage-dependence. While both exhibit affinity for the open state, azimilide also binds to inactivated channels.