IBZM-SPECT shows low striatal D2 blockade under therapy with seroquel

It is generally acknowledged that the blockade of D2 receptors may be crucial for the action of most substances with antipsychotic effect. However, the atypical antipsychotic substance clozapine shows a considerably lower striatal D2 receptor occupancy in comparison to typical antipsychotics. The objective of our study was to investigate the striatal antidopaminergic activity of seroquel by means of 123I-IBZM SPECT. SPECT-studies were performed using a three-headed rotating scintillation camera with medium-energy collimators. Two striatal and 2 frontal ROIs were each pooled together and the average counts/pixel were calculated. A ratio was calculated between average count-rates in the striatum (S) and the frontal cortex (F). Data were collected of 3 schizophrenic patients receiving between 600 and 700 mg/d seroquel and of 1 patient receiving 300 mg/d seroquel, each over a minimum period of 14 days. The incidence of extrapyramidal motor side-effects (EPMS) was rated according to the Simpson-Angus scale. Under treatment with the novel antipsychotic substance seroquel the S/F ratios were high, indicating only a minor striatal D2 blockade in 3 patients receiving between 600-700 mg/d seroquel (23% ± 3) and no detectable occupancy in 1 patient receiving 300 mg/d seroquel. These results are in line with the low incidence of EPMS in patients receiving seroquel in comparison to classic antipsychotic substances. Our SPECT data together with preliminary clinical data justify the classification of seroquel as an atypical antipsychotic substance.