In Vivo Occupancy of Dopamine Receptors by Antipsychotic Drugs

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Abstract
To the Editor. — Clozapine hydrochloride, an atypical antipsychotic drug, has been reported to occupy fewer striatal D2dopamine receptors (DRD2) (38% to 63%) than typical neuroleptic drugs (>70%), as determined by positron emission tomography (PET) using raclopride labeled with carbon 11.1-3Furthermore, clozapine has been shown by PET studies using11CSchering 23390 as ligand to occupy more D1dopamine receptors (DRD1) than typical neuroleptics.2,3 See also p 538. DecreasedDRD2and increasedDRD1occupancy has been suggested as the explanation of why clozapine does not produce extrapyramidal symptoms in humans.3We have offered an alternative theory, namely, that clozapine produces fewer extrapyramidal symptoms because of its higher serotonin2(5-HT2) receptor-blocking properties compared with itsDRD2-blocking properties.4We have also suggested that the latter mechanism is relevant to the antipsychotic advantages of clozapine.5Others5,6have suggested that the