RT Journal Article SR Electronic T1 Blockade of HERG and Kv1.5 by Ketoconazole JF Journal of Pharmacology and Experimental Therapeutics JO J Pharmacol Exp Ther FD American Society for Pharmacology and Experimental Therapeutics SP 727 OP 735 VO 286 IS 2 A1 Robert Dumaine A1 Mary-Louise Roy A1 Arthur M. Brown YR 1998 UL http://jpet.aspetjournals.org/content/286/2/727.abstract AB Ketoconazole, a widely used fungicide in patients, has been associated with Q-T prolongation and torsade de pointes when co-administered with terfenadine (Seldane). Both compounds use the same cytochrome-P450 metabolic pathway, resulting in an increase in plasma concentration of terfenadine. We previously showed that terfenadine blocked HERG (Human Ether-a-Gogo Related Gene), an important component of the repolarizing cardiac delayed rectifier IK with concentration needed to obtain 50% of the block (IC50) in the therapeutic range (300 nM). Another target is Kv1.5 (delayed outward rectifier potassium current), an important component of human atrial ultrarapid delayed rectifier current. Whether Kv1.5 and HERG proteins are direct targets for ketoconazole has yet to be addressed. We heterologously expressed HERG and Kv1.5 in Xenopusoocytes and compared their sensitivities to ketoconazole. HERG and Kv1.5 currents were reduced comparably with apparent IC50values of 49 μM and 107 μM, respectively, when measured using the two-microelectrode recording technique. The differences in the IC50 may help explain the preferential ventricular origin of the ketoconazole-associated arrhythmias during overdose. The mechanism of block was different between Kv1.5 and HERG. Cumulative application of terfenadine and ketoconazole at their respective IC50 concentrations resulted in current reductions that suggest an additive rather than a competitive type of block by the two drugs. We conclude that ketoconazole may potentiate the effects of terfenadine first by an indirect pharmacokinetic action to elevate plasma levels and second by a direct pharmacodynamic action on HERG currents. These potential dual actions on HERG currents suggest that precautions should be taken in long-term ketoconazole treatment, particularly for patients who have decreased liver function or are on a drug regimen requiring simultaneous medications that use cytochrome-P450 for breakdown, such as terfenadine or erythromycin, or Class III antiarrhythmic drugs. The American Society for Pharmacology and Experimental Therapeutics