Cardiotoxicity of thioridazine and two stereoisomeric forms of thioridazine 5-sulfoxide in the isolated perfused rat heart.

The cardiotoxic potential of the phenothiazine neuroleptic thioridazine and five of its metabolites, including two stereoisomeric forms of thioridazine 5-sulfoxide (ring sulfoxide), was characterized in the isolated perfused rat heart. Hearts from male Sprague-Dawley rats were perfused using a modified Langendorff technique. After a 30-min control period, hearts were perfused for 30 min with 15 or 30 microM thioridazine, racemic and isomeric forms of thioridazine 5-sulfoxide, or thioridazine 2-sulfoxide. Thioridazine disulfoxide, thioridazine 2-sulfone, and desmethylthioridazine 2-sulfoxide were perfused at 15 microM. Thioridazine (15 microM) severely reduced contractile tension and the rate of tension development (dT/dt), transiently increased coronary flow rate but prolonged conductance through the AV node. No arrhythmias were seen. At 30 microM, ventricular premature contractions and irregular rhythms occurred, progressing to asystole. Thioridazine 5-sulfoxide was arrhythmogenic at 15 and 30 microM. Atrial premature contractions and paroxysmal atrial tachycardia progressed to second degree AV block. Contractile tension and dT/dt declined although not as quickly when compared to thioridazine. Each isomeric form of thioridazine 5-sulfoxide was also cardiotoxic at 15 and 30 microM. There were minor differences in the onset and degree of toxicity but the overall results did not suggest a stereoselective effect. Lactic dehydrogenase and aspartate aminotransferase concentrations were unchanged after thioridazine 5-sulfoxide perfusion indicating no direct tissue damage. Thioridazine 5-sulfoxide induced arrhythmias could not be prevented or reversed by treatment with adrenergic agonists. They were reversible upon washout, however. The other metabolites were not arrhythmogenic at 15 microM. Racemic thioridazine 5-sulfoxide appears to be qualitatively and quantitatively more arrhythmogenic than thioridazine, an action that does not appear to be stereoselective.