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 (A₁AR) antagonist [¹⁸F]cyclopentyl-3-(3-fluoropropyl)-1-propylxanthine ([¹⁸F]CPFPX), used in imaging human brain A₁ARs by PET, is stable in the brain but rapidly undergoes transformation into one major (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 n.c.a.[¹⁸F]CPFPX contained 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 functionalizations of the N³-fluoropropyl chain.
The putative metabolites displaced the binding of [³H]CPFPX to the A₁AR in pig brain cortex at Kᵢs between 1.9 and 380 nM and the binding of [³H]ZM 241385 to the A₂ᴀAR in pig striatum at Kᵢs greater than 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 A₁AR.