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Vol. 302, Issue 1, 320-327, July 2002
Department of General Pharmacology, Groton Laboratories, Pfizer
Global Research and Development, Groton, Connecticut
Several macrolides have been reported to cause QT prolongation and
ventricular arrhythmias such as torsades de pointes. To clarify
the underlying ionic mechanisms, we examined the effects of six
macrolides on the human ether-a-go-go-related gene
(HERG)-encoded potassium current stably expressed in
human embryonic kidney-293 cells. All six drugs showed a
concentration-dependent inhibition of the current with the following
IC50 values: clarithromycin, 32.9 µM; roxithromycin, 36.5 µM; erythromycin, 72.2 µM; josamycin, 102.4 µM;
erythromycylamine, 273.9 µM; and oleandomycin, 339.6 µM. A
metabolite of erythromycin, des-methyl erythromycin, was also found to
inhibit HERG current with an IC50 of 147.1 µM. These findings imply that the blockade of HERG may
be a common feature of macrolides and may contribute to the QT
prolongation observed clinically with some of these compounds.
Mechanistic studies showed that inhibition of HERG
current by clarithromycin did not require activation of the channel and
was both voltage- and time-dependent. The blocking time course could be
described by a first-order reaction between the drug and the channel.
Both binding and unbinding processes appeared to speed up as the
membrane was more depolarized, indicating that the drug-channel
interaction may be affected by electrostatic responses.
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