N6-substituted D-4'-thioadenosine-5'-methyluronamides: potent and selective agonists at the human A3 adenosine receptor

4'-Thio analogues 3-5 of Cl-IB-MECA (2) (K(i) = 1.0 +/- 0.2 nM at the human A(3) adenosine receptor) were synthesized from d-gulono-gamma-lactone via 4-thioribosyl acetate 14 as the key intermediate. All synthesized 4'-thionucleosides exhibited higher binding affinity to the human A(3) adenosine receptor than Cl-IB-MECA, among which 4 showed the most potent binding affinity (K(i) = 0.28 +/- 0.09 nM). 4 was also selective for A(3) vs human A(1) and human A(2A) receptors by 4800- and 36000-fold, respectively.