Thioridazineinteractions

Reduced cytochrome P450 2D6 isozyme activity, drugs which inhibit this isozyme (e.g., fluoxetine and paroxetine), and certain other drugs (e.g., fluvoxamine, propranolol, and pindolol) appear to appreciably inhibit the metabolism of thioridazine. The resulting elevated levels of thioridazine would be expected to augment the prolongation of the QTc interval associated with thioridazine and may increase the risk of serious, potentially fatal, cardiac arrhythmias, such as Torsades de pointes type arrhythmias. Such an increased risk may result also from the additive effect of coadministering thioridazine with other agents that prolong the QTc interval. Therefore, thioridazine is contraindicated with these drugs as well as in patients, comprising about 7% of the normal population, who are known to have a genetic defect leading to reduced levels of activity of P450 2D6 (see WARNINGS and CONTRAINDICATIONS ). Drugs That Inhibit Cytochrome P450 2D6 In a study of 19 healthy male subjects, which included 6 slow and 13 rapid hydroxylators of debrisoquin, a single 25 mg oral dose of thioridazine produced a 2.4-fold higher C max and a 4.5-fold higher AUC for thioridazine in the slow hydroxylators compared to rapid hydroxylators. The rate of debrisoquin hydroxylation is felt to depend on the level of cytochrome P450 2D6 isozyme activity. Thus, this study suggests that drugs that inhibit P450 2D6 or the presence of reduced activity levels of this isozyme will produce elevated plasma levels of thioridazine. Therefore, the coadministration of drugs that inhibit P450 2D6 with thioridazine and the use of thioridazine in patients known to have reduced activity of P450 2D6 are contraindicated. Drugs That Reduce the Clearance of Thioridazine Through Other Mechanisms Fluvoxamine The effect of fluvoxamine (25 mg b.i.d. for one week) on thioridazine steady-state concentration was evaluated in ten male inpatients with schizophrenia. Concentrations of thioridazine and its two active metabolites, mesoridazine and sulforidazine, increased 3-fold following coadministration of fluvoxamine. Fluvoxamine and thioridazine should not be coadministered. Propranolol Concurrent administration of propranolol (100 mg to 800 mg daily) has been reported to produce increases in plasma levels of thioridazine (approximately 50% to 400%) and its metabolites (approximately 80% to 300%). Propranolol and thioridazine should not be coadministered. Pindolol Concurrent administration of pindolol and thioridazine have resulted in moderate, dose related increases in the serum levels of thioridazine and two of its metabolites, as well as higher than expected serum pindolol levels. Pindolol and thioridazine should not be coadministered. Drugs That Prolong the QTc Interval There are no studies of the coadministration of thioridazine and other drugs that prolong the QTc interval. However, it is expected that such coadministration would produce additive prolongation of the QTc interval and, thus, such use is contraindicated.