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

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(3)AR 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(3)AR 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(3)AR 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(1)AR in comparison to the A(3)AR, but fully efficacious, with binding K-i values over 100 nM. The 2-phenylethyl moiety resulted in higher A(3)AR 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-(1-Naphthyl)ethyloxyladenosine (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(3)AR agonists have been identified. Interestingly, although most of these compounds were extremely weak at the A(2B)AR, 2-[2-(2-naphthyl)ethyloxyladenosine (EC50 = 1.4 muM) and 2-[2-(2-thienyl)-ethyloxy]adenosine (EC50 = 1.8 (M) were found to be relatively potent A(2B) agonists, although less potent than NECA (EC50 = 140 nM). (C) 2004 Elsevier Inc. All rights reserved.