8-Bicycloalkyl-CPFPX derivatives as potent and selective tools for in vivo imaging of the A1 adenosine receptor

Imaging of the A₁ Adenosine Receptor (A₁R) by positron emission tomography (PET) with 8-cyclopentyl-3-(3-[¹⁸F]fluoropropyl)-1-propyl-xanthine ([¹⁸F]CPFPX) has been widely used in preclinical and clinical studies. However, this radioligand suffers from rapid peripheral metabolism and subsequent accumulation of radiometabolites in the vascular compartment. In the present work, we prepared four derivatives of CPFPX by replacement of the cyclopentyl group with norbornane moieties. These derivatives were evaluated by competition binding studies, microsomal stability assays and LC-MS analysis of microsomal metabolites. In addition, the ¹⁸F-labeled isotopologue of 8-(1-norbornyl)-3-(3-fluoropropyl)-1-propylxanthine (1-NBX) as the most promising candidate was prepared by radiofluorination of the corresponding tosylate precursor and the resulting radioligand ([¹⁸F]1-NBX) was evaluated by permeability assays with Caco-2 cells and in vitro autoradiography in rat brain slices. Our results demonstrate that 1-NBX exhibits significantly improved A₁R affinity and selectivity when compared to CPFPX and that it does not give rise to lipophilic metabolites expected to cross the blood-brain-barrier in microsomal assays. Furthermore, [¹⁸F]1-NBX showed a high passive permeability (P_c = 6.9 ± 2.9 × 10^-5 cm/s) and in vitro autoradiography with this radioligand resulted in a distribution pattern matching A₁R expression in the brain. Moreover, a low degree of non-specific binding (5%) was observed. Taken together, these findings identify [¹⁸F]1-NBX as a promising candidate for further preclinical evaluation as potential PET tracer for A₁R imaging.

Adenosine A(1) receptor ligand; Autoradiography; Fluorine-18 labelling; Ligand design; Positron emission tomography.