A PET study of D2 and 5-HT2 receptor occupancy induced by risperidone in poor metabolizers of debrisoquin and risperidone

The hydroxylation of the new antipsychotic drug risperidone to its main, active metabolite 9-hydroxyrisperidone is catalyzed by the hepatic cytochrome P450 enzyme CYP2D6, and cosegregates with the polymorphic hydroxylation of debrisoquin. We have previously examined central D₂ dopamine and 5-HT₂ receptor occupancy after 1 mg risperidone orally in three healthy subjects who were extensive metabolizers (EM) of debrisoquin, using positron emission tomography and the radioligands [¹¹C]raclopride and [¹¹C]NMSP. In this study, the same experimental design was repeated in two healthy poor metabolizers (PM) of debrisoquin to compare the D₂ and 5-HT₂ receptor occupancy induced by risperidone in EM and PM. The two PM had much higher plasma concentrations and longer elimination half-lives of risperidone than the three EM. Plasma concentrations of the sum of risperidone and 9-hydroxyrisperidone partly overlapped among the EM and PM. D₂ receptor occupancy was 50% and 54% in the two PM, as compared to 40%, 43% and 55% in the EM. 5-HT₂ receptor occupancy was 63% and 73%, as compared to 45%, 56% and 68% in the EM. These findings support the view that the active 9-hydroxyl metabolite of risperidone contributes to the in vivo effects of risperidone in humans, and thus partly counterbalances the marked variability in the disposition of risperidone.