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Iodine-123-Iodobenzamide SPECT Assessment of Dopamine D₂ Receptor Occupancy in Risperidone-Treated Schizophrenic Patients

Departments of Nuclear Medicine and Psychiatry, University of Munich, Munich; State Mental Hospital, Haar, Germany

Correspondence: For correspondence or reprints contact: Klaus Tatsch, MD, Department of Nuclear Medicine, University of Munich, Marchioninistr. 15, 81377 Muruch, Germany.

ABSTRACT

The recently introduced neuroleptic, risperidone, was expected to block fewer dopamine D₂ receptors than typical neuroleptics (e.g., haloperidol), but at comparable potency. The aim of this study was to evaluate the degree of dopamine D₂ receptor occupancy in relation to the neuroleptic dosage and to correlate the findings with the presence of extrapyramidal symptoms (EPS). Additionally, the data were compared to previous iodobenzamide (IBZM) SPECT findings in patients treated with other neuroleptics, haloperidol and clozapine. Methods: In 20 patients with schizophrenia [Diagnostic and Statistical Manual of Mental Disorders (Third Edition-Revised)] treated with mean daily doses of risperidone ranging from 0.029 to 0.128 mg/kg body weight, SPECT was performed 2 hr after intravenous injection of 185 MBq ¹²³I-IBZM, a selective dopamine D₂ receptor ligand. Striatal IBZM binding was assessed by calculating a striatal/frontal cortex ratio, expressed as a percentage of the control value. Results: Selective dopamine D₂ receptor binding of the ligand was reduced in all treated patients, with binding values ranging from 7% to 68%. The degree of occupancy displayed an exponential dose-response relationship (r = – 0.86;p < 0.0001). The slope of the curve was between those of haloperidol and clozapine but was closer and more similar in shape to the curve of haloperidol. Extra pyramidal symptoms were observed in 8 of 20 patients with binding values between 7% and 47%. However, there was no clear relationship between the degree of receptor occupancy and the presence of EPS. Conclusion: The findings suggest an exponential dose-response relationship between the daily dosage of risperidone and the dopamine D₂ receptor occupancy. The blockade of specific striatal IBZM binding found under therapy with risperidone is between those of haloperidol and clozapine. The dose-response curve for risperidone, however, shows greater similarity to that of haloperidol.

Key Words: iodine-123-iodobenzamide SPECT • schizophrenia patients • risperidone • extrapyramidal symptoms

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