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Torsade de pointes is a rare but potentially fatal ventricular arrhythmia associated with drug-induced delayed repolarization and prolongation of the QT interval. To determine if the arrhythmogenic potential of noncardiac drugs can be assessed in vitro, we evaluated the effects of 12 drugs on the action potential duration (APD) of cardiac Purkinje fibers and compared results with clinical observations. APD changes in canine and porcine fibers were evaluated under physiologic conditions (37°C, [K+]0 = 4 m M) using standard microelectrode techniques. Six of seven drugs associated with QT prolongation or torsade de pointes in man (cisapride, erythromycin, grepafloxacin, moxifloxacin, sertindole, and sotalol) affected concentration-dependent prolongation of the APD in canine fibers during slow stimulation (2-s basic cycle length), attaining greater than 15% prolongation at high concentrations (≥10-fold clinically encountered plasma levels). Each of five drugs not linked clinically to QT prolongation and torsade de pointes (azithromycin, enalaprilat, fluoxetine, indomethacin, and pinacidil) failed to attain 15% prolongation, with fluoxetine, indomethacin, and pinacidil abbreviating the APD. Drugs eliciting the greatest prolongation also demonstrated prominent reverse rate-dependent effects. The antihistamine terfenadine (linked to dose-dependent QT prolongation and torsade de pointes clinically) only minimally prolonged the APD in canine and porcine fibers (and exerted no effect on midmyocardial fibers from left ventricular free wall) at supratherapeutic concentrations. On the basis of concentration-dependent APD prolongation and reverse rate-dependent effects, this Purkinje fiber model detects six of seven drugs linked clinically to acquired long QT syndrome and torsade de pointes, and clears each of five drugs not associated with repolarization abnormalities (overall 92% accuracy), validating the utility of this Purkinje fiber model in the preclinical evaluation of QT prolongation and proarrhythmic risk by noncardiac drugs.