Risperdal

[Risperidone]

Fluoxetine ( 20 mg QD) and paroxetine ( 20 mg QD) have been shown to increase the plasma concentration of risperidone 2.5-2.8 fold and 3-9 fold respectively. Fluoxetine did not affect the plasma concentration of 9-hydroxyrisperidone. Paroxetine lowered the concentration of 9-hydroxyrisperidone an average of 13% ( see PRECAUTIONS Drug Interactions and DOSAGE AND ADMINISTRATION Co--Administration of RISPERDAL ® with Certain Other Medications) .

Repeated oral doses of risperidone ( 3 mg BID) did not affect the exposure ( AUC) or peak plasma concentrations ( C max ) of lithium ( n= 13) ( see PRECAUTIONS Drug Interactions) ) . Repeated oral doses of risperidone ( 4 mg QD) did not affect the pre-dose or average plasma concentrations and exposure ( AUC) of valproate ( 1000 mg/ day in three divided doses) compared to placebo ( n= 21) . However, there was a 20% increase in valproate peak plasma concentration ( C max ) after concomitant administration of risperidone ( see PRECAUTIONS Drug Interactions) ) . There were no significant nteractions between risperidone ( 1 mg QD) and erythromycin ( 500 mg QID) ( see PRECAUTIONS Drug Interactions) .

Excretion

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Risperidone and its metabolites are eliminated via the urine and, to a much lesser extent, via the feces. As illustrated by a mass balance study of a single 1 mg oral dose of 14 C-risperidone administered as solution to three healthy male volunteers, total recovery of radioactivity at 1 week was 84% , including 70% in the urine and 14% in the feces.

The apparent half-life of risperidone was 3 hours ( CV= 30% ) in extensive metabolizers and 20 hours ( CV= 40% ) in poor metabolizers. The apparent half-life of 9-hydroxyrisperidone was about 21 hours ( CV= 20% ) n extensive metabolizers and 30 hours ( CV= 25% ) n poor metabolizers. The pharmacokinetics of the active moiety, after single and multiple doses, were similar in extensive and poor metabolizers, with an overall mean elimination half-life of about 20 hours.

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