hERG K+ channel blockade by the antipsychotic drug thioridazine: An obligatory role for the S6 helix residue F656

The phenothiazine antipsychotic agent thioridazine has been linked with prolongation of the QT interval on the electrocardiogram, ventricular arrhythmias, and sudden death. Although thioridazine is known to inhibit cardiac hERG K+ channels there is little mechanistic information on this action. We have investigated in detail hERG K+ channel current (IhERG) blockade by thioridazine and identified a key molecular determinant of blockade. Whole-cell IhERG measurements were made at 37 °C from human embryonic kidney (HEK-293) cells expressing wild-type and mutant hERG channels. Thioridazine inhibited IhERG tails at −40 mV following a 2 s depolarization to +20 mV with an IC50 value of 80 nM. Comparable levels of IhERG inhibition were seen with physiological command waveforms (ventricular and Purkinje fibre action potentials). Thioridazine block of IhERG was only weakly voltage-dependent, though the time dependence of IhERG inhibition indicated contingency of blockade upon channel gating. The S6 helix point mutation F656A almost completely abolished, and the Y652A mutation partially attenuated, IhERG inhibition by thioridazine. In summary, thioridazine is one of the most potent hERG K+ channel blockers amongst antipsychotics, exhibiting characteristics of a preferential open/activated channel blocker and binding at a high affinity site in the hERG channel pore.