The atypical antipsychotic agents, clozapine and risperidone, have a similar pharmacological profile of D2 dopamine receptor affinity, α2, and serotonin2 antagonism.1 Despite this similarity, the clinical profile of these 2 drugs are quite different. For instance, risperidone produces dose-dependent extrapyramidal side effects, whereas clozapine does not.2 Moreover, there may be differences in the antipsychotic efficacy of these 2 drugs n treatment-resistant patients. One possibility for this difference in clinical profile is that these drugs produce different levels of D2 receptor occupancy. Previous estimates of D2 receptor occupancy of these atypical agents have examined different individuals and do not account for interindividual variation.3, 4 We are, therefore, conducting a double-blind, randomized, crossover sodium iodide I123-benzamide—single photon emission computed tomographic study with schizophrenic patients treated with fixed doses of clozapine and risperidone to investigate the degree of D2 receptor occupancy by these drugs.
To date, we