Development of high-affinity fluorinated ligands for cannabinoid subtype 2 receptor, and in vitro evaluation of a radioactive tracer for imaging

The development of neurodegenerative diseases is associated with cerebral inflammation, which activates resident immune cells of the central nervous system (CNS), namely microglial cells that show an up-regulation of the cannabinoid subtype 2 receptor (CB2R) expression. Therefore our work aimed to design and synthesize a radiotracer for the detection of CB2R expression by positron emission tomography (PET) allowing an early diagnosis of neurodegenerative diseases. For the development of such a PET tracer, N-alkyl-substituted indole-3-yl-tetramethylcyclopropylketones served as lead structures due to their high CB2R potency and selectivity, allowing radiolabeling on the N-alkyl chain. To this end, eight novel fluorinated N-alkyl-indole-3-yl-tetramethylcyclopropylketones were synthesized, investigated in radioligand binding studies, and structure-activity relationships were evaluated. The most promising candidate was (1-(4-fluoropropyl)-1H-indole-3-yl)(2,2,3,3-tetramethyl-cyclopropyl)methanone (Ki: 7.88 nM at the CB2R, 3430 nM at cannabinoid subtype 1 receptor (CB1R)). A precursor was synthesized, radiofluorinated with no-carrier-added [18F]F− by nucleophilic substitution of a tosyl group, and the resulting PET ligand was purified, all being performed on a fully automated synthesis module. The tracer was produced in 34 ± 6% radiochemical yield within 2 h and with molar activities of up to 1500 GBq/μmol. A first preclinical evaluation was carried out including determination of logP, metabolic stability by liver microsomes, and autoradiography. The novel PET tracer for imaging CB2R showed promising results warranting subsequent clinical evaluation.