5‘-O-Alkyl Ethers of N,2-Substituted Adenosine Derivatives:  Partial Agonists for the Adenosine A₁ and A₃ Receptors

Abstract

New N,5'-di- and N,2,5'-trisubstituted adenosine derivatives were synthesized in good overall yields. Appropriate 5-O-alkyl-substituted ribose moieties were coupled to 6-chloropurine or 2,6-dichloropurine via Vorbrüggen's glycosylation method. Subsequent amination and deprotection of the intermediates yielded compounds 18-35. Binding affinities were determined for rat adenosine A1 and A2A receptors and the human A3 receptor. The ability of compounds 18-35 to inhibit forskolin-induced (10 microM) cyclic AMP (cAMP) production and their ability to stimulate guanosine 5'-O-(3-[35S]thio)triphosphate ([35S]GTPgammaS) binding, via either the adenosine A1 receptor or the adenosine A3 receptor, were assessed. N-Cyclopentyl-substituted adenosine derivatives displayed affinities in the low nanomolar range for the adenosine A1 receptor, whereas N-(3-iodobenzyl)-substituted derivatives had high affinity for the adenosine A3 receptor. Compound 22 had the highest affinity for the adenosine A1 receptor (K(i) value of 16 nM), and compounds 20 and 26 had the highest affinities for the adenosine A3 receptor (K(i) values of 4 and 3 nM, respectively). A chlorine substituent at the 2-position either did not affect or slightly increased the adenosine A1 receptor affinity, whereas the A3 receptor affinity was affected differently, depending on the N-substituent. Furthermore, the introduction of chlorine slightly increased the A3/A1 selectivity ratio. At the 5'-position, an O-methyl substituent induced the highest adenosine A1 receptor affinity, whereas an O-ethyl substituent did so for the A3 receptor. All compounds showed partial agonistic effects in both the cAMP and [35S]GTPgammaS assays, although more marked in the latter assay. In general, the 2-chloro derivatives seemed to have lower intrinsic activities compared to the 2-H-substituted compounds on both the adenosine A1 and the adenosine A3 receptors. The compounds with an N-(3-iodobenzyl) substituent displayed the lowest intrinsic activities. Finally, all compounds also showed partially antagonistic behavior in the [35S]GTPgammaS assay.

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

“…Most of these analogues are 2-ether substituted adenosine derivatives, which have been previously evaluated at the rat A 1 and A 2A ARs but not at the four human subtypes in a systematic manner [15][16][17][18][19][20]. From previous studies [7,13,21] and in greatly expanded form in the present study, it is clear that the intrinsic efficacy of adenosine derivatives at the A 3 AR is dependent on structural changes at both the N 6 -position and the 2-position. The intrinsic efficacy at the A 2A AR tended to be insensitive to the same structural changes.…”

“…The intrinsic efficacy of adenosine derivatives in activation of the A 3 AR is more variable than at other subtypes [7][8][9][10]. Specific groups placed at the N 6 -position and on the ribose moiety have reduced or completely abolished the ability to activate this receptor, while maintaining high binding affinity.…”

Structural determinants of efficacy at A3 adenosine receptors: modification of the ribose moiety

“…In previous studies it has been demonstrated that the intrinsic efficacy of adenosine derivatives in human A 3 AR activation depended upon both N 6 -and 2-substitutions at the adenine moiety [8][9][10][11][12]. For example, N 6 -substituents consisting of a large cycloalkyl group (>5 carbons) reduced efficacy compared to smaller rings in the activation of the human A 3 AR.…”

“…1) was synthesized [14,15,20,21] and compared in binding and functional assays ( Table 1). The set of analogues included nucleosides having a 5′-uronamide modification (1, 2, 8, 9, and 13-16), modification of a hydroxy group either through chiral inversion (3) or through fluorosubstitution (4-9), or a 4′-thio modification (10)(11)(12)(13)(14)(15). 3′-Fluoro (7-9) and 5′-uronamide-4′-thionucleoside (13-15) analogues were reported previously and partially characterized pharmacologically [15].…”

“…Thus, the efficacy of adenosine derivatives appears to be more dependent on small structural changes at the A 3 AR than at other subtypes. Structural determinants critically involved in A 3 AR activation have been identified [8][9][10][11][12][13], leading to the conclusion that the ability of an adenosine derivative to activate the A 3 AR tends to be highly structure-sensitive. 3′-Fluoro has recently been found to interfere with A 3 AR binding [14], and certain 4′-thio nucleosides have been found to be potent and selective human A 3 AR agonists [15].…”