Clozapine

Clozapine is a neuroleptic agent whose structure consists of a dibenzodiazepine derivative with a piperazinyl side chain. It has been classified as an atypical neuroleptic drug due to its unique neuropharmacologic profile. Clozapine has a weak binding affinity for dopamine D-1 and D-2 receptors by its slightly greater preference for D-1 receptors, as noted with a D-1:D-2 receptor binding ratio of 1.3. Other neuroreceptors are involved, as the drug has potent binding affinity for serotonin receptors 5-HT1A and 5-HT2. Clozapine also has antihistaminic, anticholinergic, and alpha-adrenergic antagonistic properties. Electrophysiologic studies show that it differs from other typical neuroleptics in that its actions appear to be specific for the cortical-limbic dopamine A-10 tract. In animal paradigms, in contrast to typical neuroleptics, clozapine did not produce catalepsy and had only transient effects in antagonizing other dopamine agonists. The drug is rapidly absorbed orally with a bioavailability of 0.27. After a single oral dose the elimination half-life was approximately 8-10 hours, but with several doses it increased to 14.1 hours. The agent is extensively metabolized by hepatic microsomal enzymes that forms the N-desmethyl and N-oxide metabolites. It is an effective neuroleptic that has been studied in short-term and long-term clinical trials, and multicenter trials. Clozapine was superior to chlorpromazine in the treatment of refractory schizophrenia that failed to respond to previous neuroleptic therapy. Reports of extrapyramidal side effects are minimal, and no case reports of tardive dyskinesia have been published. Indeed, clozapine has been used to treat tardive dyskinesia and other movement disorders. Agranulocytosis is the major adverse effect and its prevalence appears to differ among various ethnic groups. Other adverse effects that have been reported include hypersalivation, orthostatic hypotension, and constipation. Clozapine can lower the seizure threshold in a dose-dependent manner. The drug represents a significant advancement in the treatment of mental illness.