Radiolabeling and in vitro and in vivo evaluation of [18F]-FE-DABP688 as a PET radioligand for the metabotropic glutamate receptor subtype 5

IntroductionFluoroethyl-desmethyl-ABP688 (FE-DABP688) is a novel derivative of the previously described positron emission tomography (PET) ligand 3-(6-methyl-pyridin-2-ylethynyl)-cyclohex-2-enone-O-[11C]-methyl-oxime. FE-DABP688 was radiolabeled with fluorine-18 and characterized as a PET imaging agent for the metabotropic glutamate receptor subtype 5 (mGluR5).MethodsFE-DABP688 was radiolabeled by reacting 2-[18F]-fluoroethyl tosylate with the sodium salt of 3-(pyridin-2-ylethynyl)-cyclohex-2-enone-oxime in dry DMF. The in vitro affinity of [18F]-FE-DABP688 for mGluR5 was determined by Scatchard analysis of saturation binding data using rat whole-brain membranes (without cerebellum). Further in vitro characterization of the tracer involved plasma stability and lipophilicity testing. In vivo evaluation of [18F]-FE-DABP688 was performed by postmortem biodistribution experiments and PET studies in rats using the dedicated small-animal PET tomograph quad-HIDAC.ResultsThe radiotracer was obtained in good radiochemical yields in an overall synthesis time of 150 min. The radiochemical yield after semipreparative HPLC was 25±8% (n>7, decay corrected), and specific activity was 30±5 GBq/μmol (n>7). [18F]-FE-DABP688 exhibited optimal lipophilicity with a logD value of 2.1±0.1 and high plasma stability. Saturation assays of [18F]-FE-DABP688 revealed a single high-affinity binding site with a dissociation constant (Kd) of 1.6±0.4 nM and a Bmax value of 119±24 fmol/mg protein. PET scanning indicated radioactivity uptake in mGluR5-rich regions such as the hippocampus, striatum and cortex, while radioactivity accumulation in the cerebellum, a region with negligible mGluR5 density, was significantly lower. Biodistribution studies showed a similar distribution pattern of [18F]-FE-DABP688 binding in the brain. The hippocampus-to-cerebellum and striatum-to-cerebellum ratios were 1.81±0.16 and 1.93±0.36, respectively. Blocking studies using coinjection of [18F]-FE-DABP688 and unlabeled 2-methyl-6-((3-methoxyphenyl)ethynyl)-pyridine (1 mg/kg) revealed more than 45% specific binding in the hippocampus and striatum, thus demonstrating the in vivo specificity of tracer binding.Conclusions[18F]-FE-DABP688 may be a useful PET tracer for imaging mGluR5 in rodents.