Metabolism of Atypical Antipsychotics: Involvement of Cytochrome P450 Enzymes and Relevance for Drug-Drug Interactions

The involvement of cytochrome P450 (CYP) enzymes in the metabolism of the atypical (second-generation) antipsychotics clozapine, risperidone, olanzapine, quetiapine, ziprasidone, aripiprazole, paliperidone and amisulpride is reviewed, and the possible relevance of this metabolism to drug-drug interactions is discussed. Clozapine is metabolized primarily by CYP1A2, with additional contributions by CYP2C19, CYP2D6 and CYP3A4. Risperidone is metabolized primarily by CYP2D6 and to a lesser extent by CYP3A4; the 9-hydroxy metabolite of risperidone (paliperidone) is now marketed as an antipsychotic in its own right. Olanzapine is metabolized primarily by direct glucuronidation and CYP1A2 and to a lesser extent by CYP2D6 and CYP3A4. Quetiapine is metabolized by CYP3A4, as is ziprasidone, although in the latter case aldehyde oxidase is the enzyme responsible for most of the metabolism. CYP2D6 and CYP3A4 are important in the metabolism of aripiprazole, and CYP-catalyzed metabolism of paliperidone and amisulpride appears to be minor. At the usual clinical doses, these drugs appear to not generally affect markedly the metabolism of other coadministered medications. However, as indicated above, several of atypical antipsychotics are metabolized by CYP enzymes, and physicians should be aware of coadministered drugs that may inhibit or induce these CYP enzymes; examples of such possible interactions are presented in this review.