Structure-activity relationship studies of pyridine-based ligands and identification of a fluorinated derivative for positron emission tomography imaging of cannabinoid type 2 receptors

Abstract

The cannabinoid type 2 (CB2) receptor has emerged as a valuable target for therapy and imaging of immune-mediated pathologies. With the aim to find a suitable radiofluorinated analogue of the previously reported CB2 positron emission tomography (PET) radioligand [$^{11}$C]RSR-056, 38 fluorinated derivatives were synthesized and tested by in vitro binding assays. With a K$_{i}$ (hCB2) of 6 nM and a selectivity factor of nearly 700 over cannabinoid type 1 receptors, target compound 3 exhibited optimal in vitro properties and was selected for evaluation as a PET radioligand. [$^{18}$F]3 was obtained in an average radiochemical yield of 11 ± 4% and molar activities between 33 and 114 GBq/μmol. Specific binding of [$^{18}$F]3 to CB2 was demonstrated by in vitro autoradiography and in vivo PET experiments using the CB2 ligand GW-405 833. Metabolite analysis revealed only intact [$^{18}$F]3 in the rat brain. [$^{18}$F]3 detected CB2 upregulation in human amyotrophic lateral sclerosis spinal cord tissue and may thus become a candidate for diagnostic use in humans.