¹⁸F-JHU94620, a high affinity PET radioligand for imaging of cannabinoid subtype 2 receptors (CB2R)

Objectives:

CB2R represents a target with increasing importance for neuroimaging due to its upregulation in various pathological conditions. Encouraged by preliminary results obtained with ¹¹C-A-836339 (Ki = 1.2 nM) in a mouse model of acute neuroinflammation (induced by lipopolysaccharide, LPS), we designed a library of fluorinated analogs to develop a radioligand with improved CB2R binding affinity and selectivity.
Methods:
A series of fifteen analogs of A836339 was synthesized and JHU94620, selected as ligand with the highest CB2R affinity (Ki = 0.38 nM) and selectivity over CB1R (factor 1000). It was labelled from the bromo precursor by standard nucleophilic radiofluorination. For in vivo experiments, control and LPS-treated CD1 mice have been used. Metabolic stability was investigated in plasma samples (30 min p.i) by radio-HPLC. In vitro autoradiography was performed on rat spleen and pig brain using CB1, CB2, and CB1/CB2 specific ligands as competitors.
Results:
¹⁸F-JHU94620 was prepared in ~10% radiochemical yield, >98% radiochemical purity and specific activity of >150 GBq/μmol. Animal PET revealed a brain uptake comparable to 11C-A-836339. 20-30% higher uptake in LPS-treated mice was found (n=3, p<0.05). In plasma, ~10% of total radioactivity accounted for intact tracer at 30 min p.i. Binding of ¹⁸F-JHU94620 on spleen could be displaced by CB2- and CB1/CB2-specific but not by CB1-specific compounds (61%, 44%, and 107% of total binding, respectively, at 1 μM). As expected, in brain slices from healthy pig no specific binding was observed.
Conclusion:
¹⁸F-JHU94620 is a potential candidate for further studies with PET in neuroinflamation and related disorders.