In vivo 123I IBZM SPECT imaging of striatal dopamine-2 receptor occupancy in schizophrenic patients treated with olanzapine in comparison to clozapine and haloperidol

Dopamine D2 receptor occupancy by olanzapine or risperidone in young patients with schizophrenia

A crucial characteristic of antipsychotic medication is the occupancy of the dopamine (DA) D2 receptor. We assessed striatal DA D2 receptor occupancy by olanzapine and risperidone in 36 young patients [31 males, 5 females; mean age 21.1 years (16-28)] with first episode schizophrenia, using [123I]iodobenzamide (IBZM) SPECT. The occupancy of DA D2 receptors was not significantly different between olanzapine and risperidone. However, in subgroups of most prescribed doses, DA D2 occupancy was higher in the risperidone 4-mg group (79%) compared to the olanzapine 15-mg group (62%). [123I]IBZM binding ratios decreased with olanzapine dose (r = -0.551; P < 0.01), indicating higher DA D2 receptor occupancy with higher olanzapine dose. Akathisia and positive symptoms were correlated with [123I]IBZM binding ratio (r = -0.442; P < 0.01; and r = -0.360; P < 0.05, respectively). Prolactin (PRL) levels were elevated in the risperidone, but not in the olanzapine group, at comparable D2 receptor occupancy levels. In the olanzapine group, PRL levels were correlated with [123I]IBZM binding ratio (r = -0.551; P < 0.01). In conclusion, both olanzapine and risperidone induce a high striatal D2 receptor occupancy, dependent on dose and group formation. The lower incidence of prolactin elevation with olanzapine, compared to risperidone, may not be attributed to a lower D2 receptor occupancy.

Pharmacokinetics and tissue distribution of olanzapine in rats

The single dose pharmacokinetics of olanzapine in rats, following an oral dose and its distribution in the brain and other tissues after repeated oral and intra-peritoneal (i.p.) administration, were studied. Olanzapine in plasma, brain, liver, lung, kidney, spleen and fat was assayed at predose, 0.25, 0.5, 1, 2, 5, 12, 24, 36, 48 h postoral dose of 6 mg/kg and after daily oral and i.p. doses of 0.25, 1, 3, and 6 mg/kg/day of olanzapine for 15 consecutive days by a sensitive and specific HPLC method with electrochemical detection. Olanzapine was readily absorbed and distributed in plasma and tissues as the peak concentrations were reached within approximately 45 min after the oral dose. The terminal half-life of olanzapine in plasma was 2.5 h and in tissues it ranged from 3 to 5.2 h. The area under the concentration-time curve (AUC(last)) was lowest in plasma and largest in liver and lung. The AUC(last) of olanzapine was eight times larger in brain and three to 32 times larger in other tissues than that in plasma. After repeated oral doses, the plasma and tissue concentrations of olanzapine were generally higher than those after repeated i.p. doses. The liver and spleen had the highest concentrations after oral and i.p doses, respectively. In both cases, the tissue concentrations were four- to 46-fold higher than that in plasma and correlated with administered doses. Likewise, plasma concentrations strongly correlated with the simultaneous brain and tissue concentrations (r(2)>0.908, p<0.0001). On average, the brain levels were 6.3-13.1 and 5.4-17.6 times higher than the corresponding plasma level after oral and i.p. doses, respectively. The tissue to plasma level ratio of olanzapine was higher in other tissues. The data indicated that olanzapine is rapidly absorbed and widely distributed in the tissues of rats after oral and i.p. administration. The plasma concentration appears to predict the simultaneous concentration in brain and other tissues. There was no marked localized accumulation of olanzapine in any of the regions of the rat brain.