Development and Evaluation of a Versatile Receptor-Ligand Binding Assay Using Cell Membrane Preparations Embedded in an Agarose Gel Matrix and Evaluation with the Human Adenosine A1Receptor

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 (Pc = 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.

Design, synthesis and biological evaluation of Tozadenant analogues as adenosine A2A receptor ligands

With the aim to obtain potent adenosine A_2A receptor (A_2AR) ligands, a series of eighteen derivatives of 4-hydroxy-N-(4-methoxy-7-morpholin-4-yl-1,3-benzo[d]thiazol-2-yl)-4-methylpiperidine-1-carboxamide (SYN-115, Tozadenant) were designed and synthesized. The target compounds were obtained by a chemical building block principle that involved reaction of the appropriate aminobenzothiazole phenyl carbamates with either commercially available or readily synthesized functionalized piperidines. Their affinity and subtype selectivity with regard to human adenosine A₁-and A_2A receptors were determined using radioligand binding assays. K_i values for human A_2AR ranged from 2.4 to 38 nM, with more than 120-fold selectivity over A₁ receptors for all evaluated compounds except 13k which had a K_i of 361 nM and 18-fold selectivity. The most potent fluorine-containing derivatives 13e, 13g and 13l exhibited K_i values of 4.9 nM, 3.6 nM and 2.8 nM for the human A_2AR. Interestingly, the corresponding values for rat A_2AR were found to be four to five times higher. Their binding to A_2AR was further confirmed by radiolabeling with ¹⁸F and in vitro autoradiography in rat brain slices, which showed almost exclusive striatal binding and complete displacement by the A_2AR antagonist ZM 241385. We conclude that these compounds represent potential candidates for the visualization of the A_2A receptor and open pathways to novel therapeutic treatments of neurodegenerative disorders or cancer.