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1 Because of serious side-eects of thioridazine and tricyclic antidepressants (cardiotoxicity), a possible in¯uence of imipramine and amitriptyline on the pharmacokinetics and metabolism of thioridazine was investigated in a steady state (2-week treatment) in rats. 2 Imipramine and amitriptyline (5 and 10 mg kg 71 i.p., respectively) elevated 30 and 20 fold, respectively, the concentration of thioridazine (10 mg kg 71 i.p.) and its metabolites (Ndesmethylthioridazine, 2-sulphoxide, 2-sulphone, 5-sulphoxide) in blood plasma. Similar, yet weaker increases in the thioridazine concentration were found in the brain. Moreover, an elevation of thioridazine/metabolite ratios was observed. 3 Imipramine and amitriptyline added to control liver microsomes in vitro inhibited the metabolism of thioridazine via N-demethylation (an increase in K m ), mono-2-sulphoxidation (an increase in K m and a decrease in V max ) and 5-sulphoxidation (mainly a decrease in V max ). Amitriptyline was a more potent inhibitor than imipramine of the thioridazine metabolism. 4 The varying concentration ratios of antidepressant/thioridazine in vivo appear to be more important to the ®nal result of the pharmacokinetic interactions than are relative direct inhibitory eects of the antidepressants on thioridazine metabolism observed in vitro. 5 Besides direct inhibition of the thioridazine metabolism, the decreased activity of cytochrome P-450 towards 5-sulphoxidation, produced by chronic joint administration of thioridazine and the antidepressants, seems to be relevant to the observed in vivo interaction. 6 The obtained results may also point to inhibition of another, not yet investigated, metabolic pathway of thioridazine, which may be inferred from the simultaneous elevation of concentrations of both thioridazine and the measured metabolites.

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