Effects of terfenadine and its metabolites on a delayed rectifier K+ channel cloned from human heart.

Use of the nonsedating antihistamine terfenadine has been associated with altered cardiac repolarization in certain clinical settings. For this reason we examined the effects of terfenadine, and its metabolites, on a rapidly activating delayed rectifier K+ channel (fHK) cloned from human heart. fHK was stably expressed in human embryonic kidney cells, and both whole-cell currents and currents from excised inside-out patches were recorded. Terfenadine (3 microM) blocked whole-cell fHK current by 72 +/- 6%. In inside-out patches, terfenadine applied to the cytoplasmic surface blocked fHK with an IC50 value of 367 nM. The main effect of terfenadine was to enhance the rate of inactivation of fHK current and thereby reduce the current at the end of a prolonged voltage-clamp pulse. The blockade displayed a weak voltage dependence, increasing at more positive potentials. The mechanism of action of terfenadine is therefore consistent with blockade of open channels. In contrast, the metabolites of terfenadine were weakly active on fHK. IC50 values for all of the metabolites tested ranged from 27-fold to 583-fold higher than that obtained for terfenadine. It is concluded that terfenadine, but not its metabolites, blocks at least one type of human cardiac K+ channel at clinically relevant concentrations and that this activity may underlie the cardiac arrhythmias that have been associated with the use of this drug.