Therapeutic Drug Monitoring of Quetiapine: Effect of Coadministration with Antiepileptic Drugs in Patients with Psychiatric Disorders

ABSTRACT

Antiepileptic agents, particularly those with mood-stabilizing properties, are increasingly used in the management of psychiatric disorders and may be prescribed in combination with antipsychotics. Aim of the present study was to evaluate the pharmacokinetic interaction between various antiepileptics and quetiapine, a second-generation antipsychotic, in psychiatric patients, by using data from a therapeutic drug monitoring service. Steady-state plasma concentrations of quetiapine were compared in patients treated with quetiapine alone (controls, n=35) and in patients comedicated with valproic acid (n=19), lamotrigine (n=16), carbamazepine (n=6), topiramate (n=6) and oxcarbazepine (n=5). The six groups were matched for sex, age and daily dose of quetiapine. Dose-normalized plasma concentrations of quetiapine were significantly lower in the carbamazepine group than in the control group (p<0.001), while there were no differences in plasma quetiapine concentrations between the valproic acid, lamotrigine, topiramate or oxcarbazepine groups and the control group. In 5 patients assessed on and off carbamazepine comedication, dose-normalized plasma concentrations of quetiapine were significantly lower during combination therapy than on quetiapine alone (p<0.01). By contrast, no appreciable changes in plasma levels of quetiapine were found in the 8 and 6 patients assessed on and off valproic acid or lamotrigine comedication, respectively. These findings confirm that carbamazepine decreases markedly steady-state plasma concentrations of quetiapine, presumably by inducing its CYP3A4-mediated biotransformation. Conversely, concomitant administration of therapeutic doses of valproic acid, lamotrigine, topiramate or oxcarbazepine does not appear to affect significantly plasma levels of quetiapine.

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