Dopamine D-2 receptor occupancy by olanzapine or risperidone in young patients with schizophrenia

A crucial characteristic of antipsychotic medication is the occupancy of th e dopamine (DA) D-2 receptor. We assessed striatal DA D-2 receptor occupanc y by olanzapine and risperidone in 36 young patients [31 males, 5 females; mean age 21.1 years (16-28)] with first episode schizophrenia, using [I-123 ]iodobenzamide (IBZM) SPECT. The occupancy of DA D-2 receptors was not sign ificantly different between olanzapine and risperidone. However, in subgrou ps of most prescribed doses, DA D-2 occupancy was higher in the risperidone 4-mg group (79%) compared to the olanzapine 15-mg group (62%). [I-123]IBZM binding ratios decreased with olanzapine dose (r = -0.551; P < 0.01), indi cating higher DA D-2 receptor occupancy with higher olanzapine dose. Akathi sia and positive symptoms were correlated with [I-123]IBZM binding ratio (r = -0.442; P < 0.01; and r = -0.360; P < 0.05, respectively). Prolactin (PR L) levels were elevated in the risperidone, but not in the olanzapine group , at comparable D-2 receptor occupancy levels. In the olanzapine group, PRL levels were correlated with [I-123]IBZM binding ratio (r = -0.551; P < 0.0 1). In conclusion, both olanzapine and risperidone induce a high striatal D -2 receptor occupancy, dependent on dose and group formation. The lower inc idence of prolactin elevation with olanzapine, compared to risperidone, may not be attributed to a lower D-2 receptor occupancy. (C) 1999 Elsevier Sci ence Ltd. All rights reserved.