Radiosynthesis & First Preclinical Evaluation of [18F]FE@SNAP - a Potential PET-Tracer for the Melanin Concentration Hormone Receptor 1

AIMChanges in the expression of the Melanin Concentrating Hormone Receptor 1 (MCHR1) have been shown to be involved in a variety of pathologies, especially diabetes, obesity, deregulation of metabolic feedback mechanisms, depression and anxiety disorders. To monitor these pathologies in-vivo, there is an increasing interest in radiolabeled MCHR1-ligands. SNAP-7941 ((+)-methyl (4S)-3-{[(3-{4-[3-(acetylamino)phenyl]-1-piperidinyl}propyl)amino]carbonyl}-4-(3,4-difluorophenyl)-6-(methoxymethyl)-2-oxo-1,2,3,4-tetra-hydro-5¬-pyrimidinecarboxylate) has been described as a very potent antagonist for the MCHR1. After the successful synthesis of [11C]SNAP-7941, we synthesised and evaluated a F-18 fluoroethylated analogue: [18F]FE@SNAP. MATERIALS & METHODSRadiosynthesis: Starting from the precursor Tos@SNAP (3mg/mL) the reaction was carried out in acetonitrile at 170°C using a NanoTek® microfluidic system. Azeotropic dried [18F]fluoride and the precursor solution were pushed through the reactor with an overall flow of 150mL/min. Purification and formulation of [18F]FE@SNAP was done via a Nuclear Interface synthesis module. Cell-binding studies: MCHR1-cell membranes were incubated (25°C) with [125I]MCH and different concentration of FE@SNAP (0.25nM - 2µM). After 2 hours, bound and free radio ligand were separated by centrifugation and analyzed in a Gamma Counter. Plasmastability (in vitro): Human plasma was incubated under physiological conditions with [18F]FE@SNAP over 120 min and analyzed via HPLC. Microsomes (in vitro): Human microsomes were incubated under physiological conditions with [18F]FE@SNAP over 60 min and analyzed via HPLC. Blood brain barrier penetration (in vitro): The passive blood brain barrier penetration of [18F]FE@SNAP was tested via an HPLC assay using an immobilized artificial membrane (IAM) column. RESULTSRadiosynthesis: Starting from 25.6 ± 0.6GBq [18F]fluoride, 595 ± 67MBq of formulated [18F]FE@SNAP (4.3 ± 0.4% EOB) were produced. Cell-binding studies: Ki of FE@SNAP was 22.48nM. Plasmastability (in vitro): A decomposition of 6% of [18F]FE@SNAP was observed after 120 min. Microsomes (in vitro): A decomposition of 7% of [18F]FE@SNAP was observed after 60 min. Blood brain barrier penetration (in vitro): The logkIAMw of [18F]FE@SNAP is 2.0, which is in the same range as for [11C]DASB (logkIAMw=1.8). CONCLUSIONAfter the successful establishment of the radiosynthesis via microfluidics, we were able to produce sufficient amounts of [18F]FE@SNAP for subsequent preclinical studies. [18F]FE@SNAP evinced good binding affinity to the MCHR1 and formidable stability against microsomes and in plasma. As the logkIAMw of [18F]FE@SNAP and [11C]DASB are comparable, the prediction of a blood brain barrier penetration of [18F]FE@SNAP seems reasonable. Further preclinical evaluation steps will include autoradiography and small-animal PET. ACKNOWLEDGEMENTThis research was funded by the Austrian Science Fund (FWF): P20977-B09