title: hERG K+ channel blockade by the antipsychotic drug thioridazine: An obligatory role for the S6 helix residue F656.
abstract: 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 (I(hERG)) blockade by thioridazine and identified a key molecular determinant of blockade. Whole-cell I(hERG) measurements were made at 37 degrees C from human embryonic kidney (HEK-293) cells expressing wild-type and mutant hERG channels. Thioridazine inhibited I(hERG) tails at -40mV following a 2s depolarization to +20mV with an IC(50) value of 80nM. Comparable levels of I(hERG) inhibition were seen with physiological command waveforms (ventricular and Purkinje fibre action potentials). Thioridazine block of I(hERG) was only weakly voltage-dependent, though the time dependence of I(hERG) inhibition indicated contingency of blockade upon channel gating. The S6 helix point mutation F656A almost completely abolished, and the Y652A mutation partially attenuated, I(hERG) 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.
authors.affiliation: Department of Physiology and Cardiovascular Research Laboratories, School of Medical Sciences, University Walk, Bristol BS8 1TD, UK.
authors.forename: James T
authors.initials: JT
authors.lastname: Milnes
authors.forename: Harry J
authors.initials: HJ
authors.lastname: Witchel
authors.forename: Joanne L
authors.initials: JL
authors.lastname: Leaney
authors.forename: Derek J
authors.initials: DJ
authors.lastname: Leishman
authors.forename: Jules C
authors.initials: JC
authors.lastname: Hancox
mesh_terms: D014150:Antipsychotic Agents / Q000008:administration & dosage; D002460:Cell Line; D002746:Chlorpromazine / Q000008:administration & dosage*; D004305:Dose-Response Relationship, Drug; D000072237:ERG1 Potassium Channel; D051638:Ether-A-Go-Go Potassium Channels / Q000737:chemistry / Q000502:physiology*; D006801:Humans; D015640:Ion Channel Gating / Q000502:physiology*; D007668:Kidney / Q000187:drug effects / Q000502:physiology*; D008564:Membrane Potentials / Q000187:drug effects / Q000502:physiology*; D017434:Protein Structure, Tertiary; D013329:Structure-Activity Relationship
chemical_list: D014150:Antipsychotic Agents; D000072237:ERG1 Potassium Channel; D051638:Ether-A-Go-Go Potassium Channels; C000606913:KCNH2 protein, human; D002746:Chlorpromazine