Structure-activity relationships of 2,N6,5′-substituted adenosine derivatives with potent activity at the A2B adenosine receptor

2, N-6, and 5'-substituted adenosine derivatives were synthesized via alkylation of 2-oxypurine nucleosides leading to 2-arylalkylether derivatives. 2-(3-(Indolyl)ethyloxy)adenosine 17 was examined in both binding and cAMP assays and found to be a potent agonist of the human A(2B)AR. Simplification, altered connectivity, and mimicking of the indole ring of 17 failed to maintain A2BAR potency. Introduction of N-6-ethyl or N-6-guanidino substitution, shown to favor A2BAR potency, failed to enhance potency in the 2-( 3-( indolyl)ethyloxy) adenosine series. Indole 5 ''- or 6 ''-halo substitution was favored at the A(2B)AR, but a 5'-N-ethylcarboxyamide did not further enhance potency. 2-(3 ''-(6 ''-Bromoindolyl)ethyloxy)adenosine 28 displayed an A(2B)AR EC50 value of 128 nM, that is, more potent than the parent 17 (299 nM) and similar to 5'-N-ethylcarboxamidoadenosine (140 nM). Compound 28 was a full agonist at A(2B) and A(2A)ARs and a low efficacy partial agonist at A(1) and A(3)ARs. Thus, we have identified and optimized 2-(2-arylethyl) oxo moieties in AR agonists that enhance A(2B)AR potency and selectivity.