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The pharmacokinetics of a novel antipsychotic agent, risperidone, and the prolactin response were studied in 12 dextromethorphan-phenotyped healthy men after administration of 1 mg risperidone intravenously, intramuscularly, and orally. The formation of the equipotent major metabolite, 9-hydroxyrisperidone, exhibited CYP2D6-related polymorphism. The plasma area under the concentration-time curve from time zero to infinity ratio of 9-hydroxyrisperidone to risperidone averaged 3 (intravenous and intramuscular) and 6 (oral administration) in the extensive metabolizers and 0.2 in the poor metabolizers. Risperidone half-life was about 3 hours in extensive metabolizers and 22 hours in poor metabolizers. Risperidone absolute oral bioavailability was 66%. The pharmacokinetics of the active moiety (risperidone plus 9-hydroxyrisperidone) varied little among subjects (mean terminal half-life, 20 +/- 2 1/2 hours; absolute oral and intramuscular bioavailability, 100%). The prolactin response correlated best with the plasma active moiety, which showed little hysteresis. It is concluded that risperidone metabolic polymorphism on increased plasma prolactin is minimal and that the active moiety is clinically relevant.

Hyperprolactinemia is a common clinical disorder that may lead to sexual dysfunction or galactorrhea. It may arise from a variety of etiologies, including the use of antipsychotic agents, presumably because of a dopamine receptor blockade. This analysis was designed to characterize the relationship between risperidone, serum prolactin levels, and possible clinical sequelae. All data from randomized, double-blind studies of risperidone in patients with chronic schizophrenia were analyzed. The two largest studies (the North American and multinational trials) included 841 patients (259 women, 582 men) with paired prolactin level data and 1,884 patients (554 women, 1,330 men) with data on six adverse events possibly associated with increased prolactin levels (amenorrhea, galactorrhea, and decreased libido in women; erectile dysfunction, ejaculatory dysfunction, gynecomastia, and decreased libido in men). Both risperidone and haloperidol produced dose-related increases in plasma prolactin levels in men and women. Among women, the risperidone dose was not correlated with adverse events, nor were the adverse events correlated with endpoint prolactin levels. Among men, the incidence of adverse events was positively correlated with risperidone dose; however, at risperidone doses of 4 to 10 mg/day the incidence of adverse events was not significantly higher than that observed in patients receiving placebo. Furthermore, adverse events in men were unrelated to plasma prolactin levels. Risperidone-associated increase in serum prolactin levels was not significantly correlated to the emergence of possible prolactin-related side effects.