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

Abstract

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)piperidin4yl]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: D2(long), D3, and D4 receptors. MBP was found to have a high affinity (Ki = 1-8 nM) for all three subtypes of the D2 receptor, whereas FCP had nanomolar affinity (Ki approximately 5.5 nM) for D2(long) and D3 receptors, and a lower affinity for D4 receptors (Ki = 144 nM). In vitro binding studies also revealed that MBP had a relatively high affinity for rho1 receptors (Ki = 11 nM) compared to FCP (Ki = 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 rho1 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.

PET Imaging of Dopamine D2 Receptors with [18F]Fluoroclebopride in Monkeys Effects of Isoflurane- and Ketamine-Induced Anesthesia

“…Graphical representation of BG:Cb time activity curve ratios for the isoflurane-and ketamine-induced anesthesia studies is shown in Figure 1 (bottom panel). As reported previously for this ligand (Mach et al 1996), the rapid rate of washout of [ 18 F]FCP from the BG and Cb resulted in a high BG:Cb ratio, reaching a peak value of 3.46 (Ϯ0.31) at approximately 65 min postinjection in Study 1, when anesthesia was induced by isoflurane. In Study 2, the mean peak BG:Cb ratio was 3.42 (Ϯ0.22) at 65 minutes postinjection.…”

“…At the start of the scan, approximately 4 mCi of [ 18 F]FCP was injected, followed by 3 ml of heparinized saline. At appropriate times, arterial blood samples were withdrawn and placed into preheparinized minicentrifuge tubes for analysis (as described in detail in Mach et al 1996). Images were taken for the following time frames: 5 ϫ 1 min, 5 ϫ 2 min, 5 ϫ 5 min, 8 ϫ 10 min, and 3 ϫ 20 min.…”

“…The ligand used was fluoroclebopride (FCP), an F-18 labeled ligand of the D 2 receptor, which we have previously demonstrated to be effective in imaging studies in nonhuman primates (Grant et al 1998;Mach et al 1996Mach et al , 1997. In this study, the within-subject effects of ketamine-vs. isoflurane-induced anesthesia on D 2 receptor density were examined in five monkeys.…”

Effect of Menstrual Cycle Phase on Dopamine D2 Receptor Availability in Female Cynomolgus Monkeys

“…The phase in which each monkey was first scanned was counterbalanced. It should be noted that FCP does not differentiate among subtypes of the D2-like superfamily (ie D 2 , D 3 , and D 4 receptors; see Mach et al, 1996). Prior to each study, monkeys were anesthetized with 10 mg/kg ketamine and transported to the PET Center.…”

“…This dose of ketamine does not affect D2 receptor availability as measured with FCP (Nader et al, 1999). Details regarding the PET data acquisition protocol, blood sampling procedure, and metabolite analysis have been fully described previously (Mach et al, 1996(Mach et al, , 1997Morgan et al, 2002;Nader et al, 1999). An arterial and a venous catheter were inserted by percutaneous stick for blood sampling and tracer injection, respectively.…”