Risperidone could be subject to two kinds of drug-drug interactions ( see PRECAUTIONS -Drug Interactions) . First, inhibitors of CYP 2D6 interfere with conversion of risperidone to 9-hydroxyrisperidone. This occurs with quinidine, giving essentially all recipients a risperidone pharmacokinetic profile typical of poor metabolizers. The therapeutic benefits and adverse effects of risperidone in patients receiving quinidine have not been evaluated, but observations in a modest number ( n 70) of poor metabolizers given risperidone do not suggest important differences between poor and extensive metabolizers. Second, co-administration of known enzyme inducers ( e. g. , phenytoin, rifampin, and phenobarbital) with risperidone may cause a decrease in the combined plasma concentrations of risperidone and 9-hydroxyrisperidone. It would also be possible for risperidone to nterfere with metabolism of other drugs metabolized by CYP 2D6. Relatively weak binding of risperidone to the enzyme suggests this is unlikely. In a drug interaction study in schizophrenic patients, 11 subjects received risperidone titrated to 6 mg/ day for 3 weeks, followed by concurrent administration of carbamazepine for an additional 3 weeks. During co-administration, the plasma concentrations of risperidone and its pharmacologically active metabolite, 9-hydroxyrisperidone, were decreased by about 50% . Plasma concentrations of carbamazepine did not appear to be affected. Co-administration of other known enzyme inducers ( e. g. , phenytoin, rifampin, and phenobarbital) with risperidone may cause similar decreases in the combined plasma concentrations of risperidone and 9-hydroxyrisperidone, which could lead to decreased efficacy of risperidone treatment ( see PRECAUTIONS Drug Interactions and DOSAGE AND ADMINISTRATION Co--Administration of RISPERDAL ® with Certain Other Medications) . | ||||
Email this article
Printer friendly
Bookmark this pageadvertisement
