Dose-dependent pharmacokinetic interaction of clozapine and paroxetine in an extensive metabolizer.

A 53-year-old patient suffering from delusional disorder developed an anticholinergic syndrome 19 days after initiation of paroxetine in addition to a steady dose of clozapine. The clozapine plasma concentration had doubled and was in the toxic range. On re-exposition with a lower clozapine dosage the increase was significantly lower. The importance of a dose-dependent interaction of both drugs is emphasized and a possible pharmacological explanation described. With regard to interactions of SSRIs and clozapine, fluvoxamine, a potent inhibitor of cytochrome P450 1A2, gives rise to higher clozapine levels at an earlier time, compared to other SSRIs (paroxetine, fluoxetine and sertraline), which are potent cytochrome P450 2D6 inhibitors.