Pharmacokinetics and Pharmacodynamics of Clozapine

The introduction of clozapine has given clinicians a unique agent for treating patients with schizophrenia that is refractory to other neuroleptics. Despite its efficacy, the drug continues to be prescribed with trepidation due to the incidence of agranulocytosis. This article reviews the pharmacokinetic and pharmacological properties of clozapine and the clinical implications for monitoring plasma concentrations. Various assays have been developed for clozapine that include gas-liquid chromatography, radioimmunoassay and high performance liquid chromatography. Only a few studies have examined the pharmacokinetics of clozapine in patients with schizophrenia. These studies have revealed a wide interpatient variability in pharmacokinetic parameters that include: time to reach peak plasma concentrations 1.1 to 3.6h; elimination half-life 9.1 to 17.4h; clearance 8.7 to 53.3 L/h; and a volume of distribution of 1.6 to 7.3 L/kg. Clozapine is metabolised via the hepatic microsomal enzyme system into 2 principle metabolites: demethyl-clozapine and clozapine N-oxide. Urine samples have reported the ratio of clozapine: demethyl: N-oxide to be 1: 1: 2. The Clozapine N-oxide binding affinity with ³H-haloperidol was 4 times lower than clozapine and its conversion back to clozapine is hypothesised. Although the exact pharmacological mechanism of action of clozapine is not fully understood, the drug does possess significant binding affinity for different dopamine receptors, with recent evidence supporting binding to the D₄ receptor subtype. Clozapine transiently increases serum prolactin levels with minimal changes in homovanillic acid plasma levels. Limited studies investigating the relationship between clinical response and plasma clozapine concentrations have investigated the range between 100 and 800 µg/L. In the treatment of patients with refractory schizophrenia, a minimum concentration of 350 µg/L was suggested as needed. The occurrence of agranulocytosis could have a genetic basis and patients should be rigorously monitored during treatment. The incidence of tardive dyskinesia and extrapyramidal side effects is minimal. Clozapine can lower the seizure threshold in a dose- and time-dependent manner. Careful patient selection and monitoring are required when clozapine therapy is used in patients with schizophrenia.