Synthesis and Pharmacological Evaluation of Identified and Putative Metabolites of the A1 Adenosine Receptor Antagonist 8-Cyclopentyl-3-(3-fluoropropyl)-1-propylxanthine (CPFPX).

The A1 adenosine receptor (A1 AR) antagonist [18 F]8-cyclopentyl-3-(3-fluoropropyl)-1-propylxanthine ([18 F]CPFPX), used in imaging human brain A1 ARs by positron emission tomography (PET), is stable in the brain, but rapidly undergoes transformation into one major (3-(3-fluoropropyl)-8-(3-oxocyclopenten-1-yl)-1-propylxanthine, M1) and several minor metabolites in blood. This report describes the synthesis of putative metabolites of CPFPX as standards for the identification of those metabolites. Analysis by (radio)HPLC revealed that extracts of human liver microsomes incubated with no-carrier-added (n.c.a.)[18 F]CPFPX contain the major metabolite, M1, as well as radioactive metabolites corresponding to derivatives functionalized at the cyclopentyl moiety, but no N1-despropyl species or metabolites resulting from functionalization of the N3-fluoropropyl chain. The putative metabolites were found to displace the binding of [3 H]CPFPX to the A1 AR in pig brain cortex at Ki values between 1.9 and 380 nm and the binding of [3 H]ZM241385 to the A2A AR in pig striatum at Ki values >180 nm. One metabolite, a derivative functionalized at the ω-position of the N1-propyl chain, showed high affinity (Ki 2 nm) to and very good selectivity (>9000) for the A1 AR.