IBZM SPECT imaging of striatal dopamine-2 receptors in psychotic patients treated with the novel antipsychotic substance quetiapine in comparison to clozapine and haloperidol

We investigated the striatal dopamine-2 (D-2) receptor occupancy caused by different antipsychotic substances in 18 psychotic patients (16 with schizophrenic and two with schizoaffective disorder according to DSM-TV) with single photon emission computed tomography (SPECT) using I-123-iodobenzamide (IBZM) as tracer substance. Four patients were treated with the novel antipsychotic compound quetiapine (300-700 mg/day), six with clozapine (300-600 mg/ day) and eight with haloperidol (10-20 mg/day). They were compared with eight healthy controls. Measurement of S/F ratios and consecutive calculation of D-2 receptor occupancy revealed a significantly lower striatal D-2 occupancy rate with quetiapine and clozapine in comparison to haloperidol. In correspondence with the low striatal D-2 receptor occupancy rates and again in contrast to the haloperidol treatment group, there were no extrapyramidal motor side-effects (EPS) in the quetiapine and clozapine treatment groups. Therefore, the reported data support the position that quetiapine can be considered to be an atypical antipsychotic substance due to its relatively weak striatal D-2 receptor blocking property and therefore its low propensity to induce EPS.