Copper-mediated automated radiofluorination and biological evaluation of a highly affine cannabinoid receptors type 2 ligand with PET

Objectives: The development of CB2R PET radioligands has been intensively explored due to the pronounced CB2R upregulation in various pathological conditions. Herein we report on the development of a series of highly affine fluorinated indole-carbamate ligands targeting the CB2R. Starting from a pinacol-ester precursor, cooper-mediated automated radiofluorination and preliminary biological evaluation were also performed for the most promising ligand.
Methods: A series of fifteen indole-carbamate derivatives was synthesized and their binding affinities (Ki) towards CB2R were determined. Compound RM365 was further selected for PET development due to its high CB2R affinity (KiCB2 = 2.1 nM) and pronounced selectivity over CB1R (factor >300). A fully automated copper-mediated radiofluorination of [18F]RM365 was established starting with the corresponding arylboronic ester precursor. The metabolic stability of [18F]RM365 was investigated in plasma and brain samples (30 min p.i) by radio HPLC. PET studies with [18F]RM365 were performed under baseline and blocking conditions (60 min scan).

Results: [18F]RM365 was obtained with moderate radiochemical yield (5%), high radiochemical purity (>98%) and molar activities of about 35 GBq/µmol. PET studies revealed that [18F]RM365 readily crossed the blood-brain barrier and accumulated in the spleen, a CB2R-rich organ. Metabolite studies at 30 min p.i. showed that 55% and 90% of the total extracted activity accounted for the percentage of parent tracer, in plasma and brain samples, respectively.

Conclusion: A fully automated cooper-mediated radiosynthesis was established for [18F]RM365. Further blocking experiments will demonstrate the CB2R specificity of [18F]RM365 in vivo and will be use as pass-fail criterion for further application of the radiotracer in CB2R-related animal models.