Comparison of two fluorine-18 labeled benzamide derivatives that bind reversibly to dopamine D₂ receptors: In vitro binding studies and positron emission tomography

The purpose of the present set of studies was to characterize, in vitro and in vivo, two benzamide analogues, 2,3-dimethoxy-N-[1-(4- fluorobenzyl)piperidin-4yl]benzamide (MBP) and 4'-fluoroclebopride (FCP), for studying dopamine D₂ receptors with Positron Emission Tomography (PET). In vitro binding studies were conducted to determine the affinities of MBP and FCP to the three subtypes of dopamine D₂ receptors: D(2(long)), D₃, and D₄ receptors. MBP was found to have a high affinity (K(i) = 1-8 nM) for all three subtypes of the D₂ receptor, whereas FCP had nanomolar affinity (K(i) ~ 5.5 nM) for D(2(long)), and D₃ receptors, and a lower affinity for D₄ receptors (K(i) = 144 nM). In vitro binding studies also revealed that MBP had a relatively high affinity for σ₁ receptors (K(i) = 11 nM) compared to FCP (K(i) = 340 nM). PET imaging studies were conducted in rhesus monkeys with the fluorine-18 labeled analogues of each compound. Both [¹⁸F]MBP and [¹⁸F]FCP displayed reversible binding kinetics during the 3 h time course of PET. [¹⁸F]FCP was found to have a higher basal ganglia:cerebellum ratio and lower variability in the rate of washout from D₂ receptors in vivo relative to [¹⁸F]MBP. Neither radiotracer was found to produce radiolabeled metabolites capable of crossing the blood-brain barrier. The high σ₁ binding affinity and low basal ganglia:cerebellum ratio of [¹⁸F]MBP indicate that this ligand may not be suitable for quantitative studies of D₂ receptors. The results from the in vitro and in vivo studies indicate that [¹⁸F]FCP is a promising ligand for studying D₂ receptors with PET.