Block of HERG potassium channels by the antihistamine astemizole and its metabolites desmethylastemizole and norastemizole.
BACKGROUND
The selective H1-receptor antagonist astemizole (Hismanal) causes acquired long QT syndrome. Astemizole blocks the rapidly activating delayed rectifier K+ current I(Kr) and the human ether-a go-go-related gene (HERG) K+ channels that underlie it. Astemizole also is rapidly metabolized. The principal metabolite is desmethylastemizole, which retains H1-receptor antagonist properties, has a long elimination time of 9 to 13 days, and its steady-state serum concentration exceeds that of astemizole by more than 30-fold. A second metabolite is norastemizole, which appears in serum in low concentrations following astemizole ingestion and has undergone development as a new antihistamine drug. Our objective in the present work was to study the effects of desmethylastemizole, norastemizole, and astemizole on HERG K+ channels.


RESULTS
HERG channels were expressed in a mammalian (HEK 293) cell line and studied using the patch clamp technique. Desmethylastemizole and astemizole blocked HERG current with similar concentration dependence (half-maximal block of 1.0 and 0.9 nM, respectively) and block was use dependent. Norastemizole also blocked HERG current; however, block was incomplete and required higher drug concentrations (half-maximal block of 27.7 nM).


CONCLUSIONS
Desmethylastemizole and astemizole cause equipotent block of HERG channels, and these are among the most potent HERG channel antagonists yet studied. Because desmethylastemizole becomes the dominant compound in serum, these findings support the postulate that it becomes the principal cause of long QT syndrome observed in patients following astemizole ingestion. Norastemizole block of HERG channels is weaker; thus, the risk of producing ventricular arrhythmias may be lower. These findings underscore the potential roles of some H1-receptor antagonist metabolites as K+ channel antagonists.
Zhou
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Z
Section of Cardiology, University of Wisconsin, Madison 53792, USA.
Vorperian
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VR
Gong
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Zhang
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January
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D016589:Astemizole / Q000031:analogs & derivatives* / Q000494:pharmacology*; D001562:Benzimidazoles / Q000494:pharmacology*; D027682:Cation Transport Proteins; D002460:Cell Line; D004268:DNA-Binding Proteins; D004305:Dose-Response Relationship, Drug; D000072237:ERG1 Potassium Channel; D051638:Ether-A-Go-Go Potassium Channels; D006634:Histamine H1 Antagonists / Q000494:pharmacology*; D006801:Humans; D010880:Piperidines / Q000494:pharmacology*; D026902:Potassium Channel Blockers; D015221:Potassium Channels; D024642:Potassium Channels, Voltage-Gated; D016593:Terfenadine / Q000494:pharmacology; D015534:Trans-Activators; D000071230:Transcriptional Regulator ERG
D001562:Benzimidazoles; D027682:Cation Transport Proteins; D004268:DNA-Binding Proteins; C072312:ERG protein, human; D000072237:ERG1 Potassium Channel; D051638:Ether-A-Go-Go Potassium Channels; D006634:Histamine H1 Antagonists; C000606913:KCNH2 protein, human; C092509:KCNH6 protein, human; D010880:Piperidines; D026902:Potassium Channel Blockers; D015221:Potassium Channels; D024642:Potassium Channels, Voltage-Gated; D015534:Trans-Activators; D000071230:Transcriptional Regulator ERG; D016593:Terfenadine; D016589:Astemizole; C040463:desmethylastemizole; C106448:tecastemizole