Is regionally selective D2/D3 dopamine occupancy sufficient for atypical antipsychotic effect? -: An in vivo quantitative [123I]epidepride SPET study of amisulpride-treated patients

Objective: Atypical antipsychotic drug treatment is clinically effective with a low risk of extrapyramidal symptoms. Explanations for the mechanism underlying this beneficial therapeutic profile of atypical over typical antipsychotic agents include 1) simultaneous antagonism of dopamine D(2) and serotonin 5-HT(2A) receptors or 2) selective action at limbic cortical dopamine D(2)-like receptors with modest striatal D(2) receptor occupancy. Amisulpride is an atypical antipsychotic drug with selective affinity for D(2)/D(3) dopamine receptors and provides a useful pharmacological model for examining these hypotheses. The authors' goal was to evaluate whether treatment with amisulpride results in "limbic selective" D(2)/D(3) receptor blockade in vivo. Method: Five hours of dynamic single photon emission tomography data were acquired after injection of [(123)I]epidepride (approximately 150 MBq). Kinetic modeling was performed by using the simplified reference region model to obtain binding potential values. Estimates of receptor occupancy were made relative to a healthy volunteer comparison group (N=6). Results: Eight amisulpride-treated patients (mean dose=406 mg/day) showed moderate levels of D(2)/D(3) receptor occupancy in the striatum (56%), and significantly higher levels were seen in the thalamus (78%) and temporal cortex (82%). Conclusions: Treatment with amisulpride results in a similar pattern of limbic cortical over striatal D(2)/D(3) receptor blockade to that of other atypical antipsychotic drugs. This finding suggests that modest striatal D(2) receptor occupancy and preferential occupancy of limbic cortical dopamine D(2)/D(3) receptors may be sufficient to explain the therapeutic efficacy and low extrapyramidal symptom profile of atypical antipsychotic drugs, without the need for 5-HT(2A) receptor antagonism.