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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.

Novel 2,5'-disubstituted adenosine derivatives were synthesized in good overall yields starting from commercially available guanosine. Binding affinities were determined for rat adenosine A(1) and A(2A) receptors and human A(3) receptors. E(max) values were determined for the stimulation or inhibition of cAMP production in CHO cells expressing human adenosine A(2A) (EC(50) values as well) or A(3) receptors, respectively. The compounds displayed affinities in the nanomolar range for both the adenosine A(2A) and A(3) receptor, without substantial preference for either receptor. The derivatives with a 2-(1-hexynyl) group had the highest affinities for both receptors; compound 4 (2-(1-hexynyl)adenosine) had the highest affinity for the adenosine A(2A) receptor with a K(i) value of 6 nM (A(3)/A(2A) selectivity ratio of approximately 3), whereas compound 37 (2-(1-hexynyl)-5'-S-methyl-5'-thioadenosine) had the highest affinity for the adenosine A(3) receptor with a K(i) value of 15 nM (A(2A)/A(3) selectivity ratio of 4). In general, compounds with a relatively small 5'-S-alkyl-5'-thio substituent (methyl-5'-thio) displayed the highest affinities for both the adenosine A(2A) and A(3) receptor; the larger ones (n- or i-propyl-5'-thio) increased the selectivity for the adenosine A(3) receptor. The novel compounds were also evaluated in cAMP assays for their (partial) agonistic behavior. Overall, the disubstituted derivatives behaved as partial agonists for both the adenosine A(2A) and A(3) receptor. The compounds showed somewhat higher intrinsic activities on the adenosine A(2A) receptor than on the A(3) receptor. Compounds 37, 40 and 45, 48, with either a 5'-S-methyl-5'-thio or a 5'-S-i-propyl-5'-thio substituent had the lowest intrinsic activities on the adenosine A(2A) receptor. For the A(3) receptor, compounds 34, 35, 38, 39, and 46, 47, with a 5'-S-ethyl-5'-thio or a 5'-S-n-propyl-5'-thio substituent had the lowest intrinsic activities.

5'-(Alkylthio)-substituted analogues of N6-benzyl- and N6-(3-iodobenzyl)adenosine were synthesized in 37-61% overall yields. The affinities of these compounds for the adenosine A1, A2A, and A3 receptors were determined using rat brain cortex, rat brain striata, and stably transfected human A3 receptors in HEK 293 cells, respectively. The compounds proved to be selective for the adenosine A3 receptor and displayed affinities in the nanomolar range. Compounds 8, 10, and 11 had the highest affinities for the A3 receptor with Ki values ranging from 8.8 to 27.7 nM. In the N6-benzyl series, compound 4 (LUF 5403), with a 5'-methylthio group, maintained a reasonable affinity and had the highest selectivity for the A3 receptor. Compound 12 (LUF 5411), with an N6-(3-iodobenzyl) group and a 5'-(n-propylthio) substituent, had the highest A3 selectivity of all of the compounds and also displayed high affinity for this receptor (Ki = 44.3 nM). The compounds were also evaluated for their ability to stimulate [35S]GTPgamma[S] binding in cell membranes expressing the human adenosine A3 receptor. It appeared that the N6,5'-disubstituted adenosine derivatives behaved as partial agonists. Compounds 2, 4, 8, and 10 had the highest intrinsic activities. Additionally, when tested in a cAMP assay, these compounds also behaved as partial agonists.