Striatal and temporal cortical D2/D3 receptor occupancy by olanzapine and sertindole in vivo:: a [123I]epidepride single photon emission tomography (SPET) study

Rationale: Previous work suggests clozapine preferentially targets limbic cortical dopamine systems, which could help account for its lack of extrapyramidal side effects (EPS) and superior therapeutic efficacy. Objectives: To test the hypothesis that olanzapine, a novel atypical antipsychotic drug, occupies temporal cortical D3/D3 receptors to a greater extent than striatal D2/D3 receptors in vivo. Methods: Nine schizophrenic patients taking either olanzapine [(n=5: mean (SD) age: 32.5 (6.5) years; daily dose: 18.3 (2.6) mg)] or sertindole [(n=4: mean (SD) age: 30.3 (7.4) years; daily dose: 16 (5.6) mg] were studied with [I-123]epidepride (S)-N-[(1-ethyl- 2-pyrrolidinyl)methyl]-5-iodo-2,3-dimethoxy-benzamide) and single photon emission tomography (SPET). An estimate of [I-123]epidepride 'specific binding' to D2/D3 receptors was obtained in patients and age-matched healthy volunteers. A summary measure was generated representing striatal and temporal cortical relative %D3/D3 receptor occupancy by antipsychotic drugs. Occupancy data were compared with previously studied groups of patients receiving typical antipsychotic drugs (n=12) and clozapine (n=10). Results: Mean striatal and temporal cortical %D2/D3 receptor occupancy in olanzapine-treated patients was 41.3% (SD 17.9) and 82.8% (SD 4.2), respectively. Unexpectedly low levels of striatal relative %D2/D3 receptor occupancy were seen in two patients with typical antipsychotic-drug-induced movement disorder prior to switching to olanzapine. In the temporal cortex, mean D2/D3 dopamine receptor occupancy levels above 80% were seen for all antipsychotic drugs studied. Conclusions: The atypical antipsychotic drugs olanzapine and sertindole, in common with clozapine, demonstrate higher occupancy of temporal cortical than striatal D2/D3 dopamine receptors in vivo at clinically useful doses. This could help mediate their atypical clinical profile of therapeutic efficacy with few extrapyramidal side effects. Limbic selective blockade of D2/D3 dopamine receptors could be a common action of atypical antipsychotic drugs.