Quetiapine has a D_2/3 receptor binding profile similar to clozapine: A [¹⁸F]fallypride PET study in patients with schizophrenia

Abstract

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Objectives: Most antipsychotics exert their antipsychotic effects within a “therapeutic window” between 60-80% striatal D_2/3 receptor occupancy. Clozapine and quetiapine (QUET) occupy striatal D_2/3 receptors to a significantly lesser extent. To further characterize QUET’s striatal and extrastriatal binding characteristics, we performed [¹⁸F]fallypride (FP) PET studies in patients with schizophrenia.

Methods: D_2/3 DA receptors were quantified with FP-PET in 26 patients with schizophrenia. 14 were treated with QUET, 12 age-matched medication-free patients served as controls. After varying time points after the last drug administration (range 3-18 h), subjects underwent dynamic PET scans (180 mins). Binding potentials (BP) were calculated with the simplified reference tissue model. Occupancy (OCC) was calculated as % reduction in BP of treated patients relative to controls. Plasma concentrations and OCC values were fit to a simple one-site ligand binding model by nonlinear regression.

Results: Preliminary data analysis in 8 subjects revealed relatively low D_2/3 receptor OCC in all brain regions investigated, with significantly higher OCC in extrastriatal than striatal regions. Striatal OCC did not exceed 50% even at extremely high plasma concentrations (max. 814 ng/mL), while binding in temporal cortex ranged from 40-80%. OCC declined rapidly with increasing interval between last QUET administration and PET scan.

Conclusions: QUET similar to clozapine occupies a maximum of 30-50% of striatal D_2/3 receptors even at extremely high plasma levels. Extrastriatal OCC is significantly markedly higher. Preferential extrastriatal binding seems to especially characterize compounds with low affinity for D_2/3 receptors.