Clozapine is the prototype atypical antipsychotic drug. The term atypical antipsychotic refers to drugs that do not cause catalepsy in animals, have a minor or no effect on plasma prolactin, do not cause extrapyramidal symptoms in humans, do not cause tardive dyskinesia, are effective in the treatment of negative symptoms, and may be effective in nonresponders to classical neuroleptics. We have previously demonstrated in positron emission tomography (PET) studies that clozapine is atypical also with regard to the degree of striatal D2 dopamine receptor occupancy during clinical treatment. While all classical antipsychotic drugs occupied 70% to 90% of the striatal D2 receptors, the D2 occupancy during clozapine treatment was significantly lower (20%-67%).1 The maximal D2 occupancy during clinical treatment with clozapine at very high plasma concentrations was estimated to be about 60%.2
Nordström A, Nyberg S, Olsson H, Farde L. Positron Emission Tomography Finding of a High Striatal D2 Receptor Occupancy in Olanzapine-Treated Patients. Arch Gen Psychiatry. 1998;55(3):283–284. doi:
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