Dopamine D-2 receptor occupancy in vivo by the novel atypical antipsychotic olanzapine - A I-123 IBZM single photon emission tomography (SPET) study

We have studied striatal D-2 dopamine binding in schizophrenic patients treated with the novel atypical antipsychotic drug, olanzapine. I-123 iodobenzamide (IBZM) single photon emission tomography (SPET) was used to estimate striatal dopamine D-2 receptor binding in vivo. Patients were recruited from a prospective, double blind controlled trial of olanzapine versus haloperidol treatment. In vivo striatal D-2 binding data from olanzapine treated patients (n=6) were compared with previously reported data from typical antipsychotic responsive (n=10); clozapine (n=10); and risperidone (n=6) treated patient groups. Mean % Brief Psychiatric Rating Scale score (BPRS) improvement following olanzapine treatment was 49% (SD 44). The hypothesis that clinical improvement in olanzapine treated patients would be associated with higher mean striatal D-2 binding of I-123 IBZM (reflecting lower levels of D-2 occupancy) than typical antipsychotic (1.25+/-0.05) or risperidone (1.24+/-0.04) treatment was confirmed. Olanzapine treated patients had similar levels of striatal D-2 binding in vivo (1.41+/-0.06) as those treated with clozapine (1.49+/-0.04). This preliminary evidence suggests olanzapine is another atypical antipsychotic drug in which therapeutic response is not associated with a high degree of striatal D-2 receptor occupancy in vivo.