Drug Trapping in hERG Channels does not Require Closure of the Activation Gate

Abstract

Human ether-a-go-go related gene (hERG) channel inhibitors can be trapped in the channels at rest. The structural peculiarities of hERG blockers that enable trapping or alternatively resting state dissociation are currently unknown. Propafenone (small molecule, MW 341 g/mol) is efficiently trapped in the closed hERG channel pore (1). To investigate whether the size of the blocking molecule plays a role in trapping we synthesized bulky propafenone derivatives containing benzoyl and trimethylphenyl side chains, attached by piperazine linkers, with molecular weights of 500 (Fba212) and 650 g/mol (Fba213) respectively. hERG channels were expressed in Xenopus laevis oocyte and potassium current inhibition was studied using the two - microelectrode voltage clamp technique.It was found: first, both compounds are potent hERG blockers with IC50 3.7μM (Fba212) and 52μM (Fba213). Secondly, channel block by Fba212 and 213 was prevented by mutations Y652A and F656A as previously shown for propafenone. Third, both propafenone derivatives were trapped at rest. To obtain insights into the molecular mechanism of channel block docking experiments with Fba212 and Fba213 in closed and open conformation were performed. Both compounds interact with the propafenone binding site (Y652A and F656A). Fba213 was found to exceed the size of the closed channel cavity of our hERG homology model. We conclude that drug trapping in hERG channels does not necessarily require full closure of the activation gate.1. Witchel HJ, Dempsey CE, Sessions RB, Perry M, Milnes JT, Hancox JC, Mitcheson JS.Mol Pharmacol. 2004 Nov; 66(5):1201-12.