Effect of cyamemazine on the steady-state plasma concentrations of risperidone and 9-hydroxyrisperidone: a preliminary retrospective study.

Administration of cyamemazine, an antipsychotic drug with anxiolytic properties, together with other antipsychotic agents is common in patients with schizophrenia. This retrospective study investigated the effects of cyamemazine on the steady-state plasma concentrations of risperidone and 9-hydroxyrisperidone in 47 patients treated with 1 to 12 mg/day of risperidone. Of these 47 patients, 24 were receiving cyamemazine comedication ("cyamemazine" group) and 23 patients were treated with risperidone alone ("control" group). Plasma concentrations were measured using a high-performance liquid chromatographic method with photodiode-array ultraviolet detection. The median plasma concentration of risperidone was significantly higher in the cyamemazine group (31.5 ng/mL) than in the control group (5.0 ng/mL), whereas the 9-hydroxyrisperidone median concentration was significantly lower in the cyamemazine group (16.5 ng/mL versus 39.0 ng/mL in the control group). However, the sum of risperidone plus 9-hydroxyrisperidone (active moiety) plasma concentration was not significantly affected by cyamemazine comedication. A combination with cyamemazine resulted in an inverted metabolic ratio (risperidone/9-hydroxyrisperidone). These findings suggest that cyamemazine inhibits the 9-hydroxylation of risperidone and is probably an inhibitor of cytochrome P450 2D6 as are many other phenothiazine drugs.