2-Substituted adenosine derivatives: affinity and efficacy at four subtypes of human adenosine receptors

Abstract

The affinity and efficacy at four subtypes (A 1, A 2A, A 2B and A 3) of human adenosine receptors (ARs) of a wide range of 2-substituted adenosine derivatives were evaluated using radioligand binding assays and a cyclic AMP functional assay in intact CHO cells stably expressing these receptors. Similar to previous studies of the N 6-position, several 2-substituents were found to be critical structural determinants for the A 3AR activation. The following adenosine 2-ethers were moderately potent partial agonists ( K i, nM): benzyl (117), 3-chlorobenzyl (72), 2-(3-chlorophenyl)ethyl (41), and 2-(2-naphthyl)ethyl (130). The following adenosine 2-ethers were A 3AR antagonists: 2,2-diphenylethyl, 2-(2-norbornan)ethyl, R- and S-2-phenylbutyl, and 2-(2-chlorophenyl)ethyl. 2-( S-2-Phenylbutyloxy)adenosine as an A 3AR antagonist right-shifted the concentration–response curve for the inhibition by NECA of cyclic AMP accumulation with a K B value of 212 nM, which is similar to its binding affinity ( K i = 175 nM). These 2-substituted adenosine derivatives were generally less potent at the A 1AR in comparison to the A 3AR, but fully efficacious, with binding K i values over 100 nM. The 2-phenylethyl moiety resulted in higher A 3AR affinity ( K i in nM) when linked to the 2-position of adenosine through an ether group (54), than when linked through an amine (310) or thioether (1960). 2-[2-(l-Naphthyl)ethyloxy]adenosine ( K i = 3.8 nM) was found to be the most potent and selective (>50-fold) A 2A agonist in this series. Mixed A 2A/A 3AR agonists have been identified. Interestingly, although most of these compounds were extremely weak at the A 2BAR, 2-[2-(2-naphthyl)ethyloxy]adenosine (EC 50 = 1.4 μM) and 2-[2-(2-thienyl)-ethyloxy]adenosine (EC 50 = 1.8 μM) were found to be relatively potent A 2B agonists, although less potent than NECA (EC 50 = 140 nM).