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Published Online:https://doi.org/10.1176/ajp.154.11.1525

OBJECTIVE: Loxapine, a dibenzoxazepine antipsychotic, is closely related to clozapine and shares clozapine's high affinity for binding to serotonin 5-HT2 and dopamine D4 receptors. The purpose of this study was to document loxapine's 5-HT2 and D2 receptor occupancy in vivo in patients with psychoses. METHOD: Ten patients who were taking loxapine (10–100 mg/day) had their D2 and 5-HT2 receptors assessed by means of positron emission tomography with [11C]raclopride and [18F]setoperone, respectively. RESULTS: The D2 receptor occupancy ranged from 43% to 90% 5-HT2 occupancy varied from 27% to near saturation. Statistical comparison of the results showed that loxapine was equipotent in blocking 5-HT2 and D2 receptors. CONCLUSIONS: Loxapine differs from typical neuroleptics in demonstrating a high degree of 5-HT2 receptor occupancy. However, it is not “atypical” like clozapine and risperidone, since its 5-HT2 occupancy is not higher than its D2 occupancy. The results demonstrate that a high level of 5-HT2 occupancy is not a sufficient condition for atypicality. If atypical antispychotic action is predicated on a combination of 5-HT2 and D2 effects, then it requires >80% 5-HT2 occupancy in conjunction with <80% D2 occupancy. (Am J Psychiatry 1997; 154:1525–1529)