2-Phenylimidazo[2,1-i]purin-5-ones: Structure–Activity relationships and characterization of potent and selective inverse agonists at Human A3 adenosine receptors Original Research Article

Structure–activity relationships of 2-phenyl-imidazo[2,1-i]purin-5-ones as ligands for human A3 adenosine receptors (ARs) were investigated. An ethyl group in the 8-position of the imidazoline ring of 4-methyl-2-phenyl-imidazopurinone leading to chiral compounds was found to increase affinity for human A3 ARs by several thousand-fold. Propyl substitution instead of methyl at N4 decreased A3 affinity but increased A1 affinity leading to potent A1-selective AR antagonists. The most potent A1 antagonist of the present series was (S)-8-ethyl-2-phenyl-4-propyl-4,5,7,8-tetrahydro-1H-imidazo[2,1-i]purin-5-one (S-3) exhibiting a Ki value of 7.4 nM at rat A1 ARs and greater than 100-fold selectivity versus rat A2A and human A3 ARs. At human A1 ARs 2-phenylimidazo[2,1-i]purin-5-ones were generally less potent and therefore less A1-selective (S-3: Ki=98 nM). 2-, 3-, or 4-Mono-chlorination of the 2-phenyl ring reduced A3 affinity but led to an increase in affinity for A1 ARs, whereas di- (3,4-dichloro) or polychlorination (2,3,5-trichloro) increased A3 affinity. The most potent and selective A3 antagonist of the present series was the trichlorophenyl derivative (R)-8-ethyl-4-methyl-2-(2,3,5-trichlorophenyl)-4,5,7,8-tetrahydro-1H-imidazo[2,1-i]purin-5-one (R-8) exhibiting a subnanomolar Ki value at human A3 ARs and greater than 800-fold selectivity versus the other AR subtypes. Methylation of 4-alkyl-2-phenyl-substituted imidazo[2,1-i]purin-5-ones led exclusively to the N9-methyl derivatives, which exhibited largely reduced AR affinities as compared to the unmethylated compounds. [35S]GTPγS binding studies of the most potent 2-phenyl-imidazo[2,1-i]purin-5-ones at membranes of Chinese hamster ovary cells expressing the human A3 AR revealed that the compounds were inverse agonists at A3 receptors under standard test conditions. Due to their high A3 affinity, selectivity, and relatively high water-solubility, 2-phenyl-imidazo[2,1-i]purin-5-ones may become useful research tools.