2-SUBSTITUTION OF N-6-BENZYLADENOSINE-5'-URONAMIDES ENHANCES SELECTIVITY FOR A-3 ADENOSINE RECEPTORS

Adenosine derivatives bearing an N-6-(3-iodobenzyl) group, reported to enhance the affinity of adenosine-5'-uronamide analogues as agonists at A(3) adenosine receptors (J. Med. Chem. 1994, 37, 636-646), were synthesized starting from methyl beta-D-ribofuranoside in 10 steps. Binding affinities at A(1) and A(2a) receptors in rat brain membranes and at cloned rat A(3) receptors from stably transfected CHO cells were compared. N-6-(3-Iodobenzyl)adenosine was 2-fold selective for A(3) vs A(1) or A(2a) receptors; thus it is the first monosubstituted adenosine analogue having any A(3) selectivity. The effects of 8-substitution in combination with modifications at the N-6- and 5'-positions were explored. 2-Chloro-N-6-(3-iodobenzyl)adenosine had a K-i value of 1.4 nM and moderate selectivity for A(3) receptors. 2-Chloro-N-6-( 3-iodobenzyl)adenosine-5'-N-methyluronamide, which displayed a K-i value of 0.33 nM, was selective for A(3) vs A(1) and A(2a) receptors by 2500- and 1400-fold, respectively. It was 46,000-fold selective for A(3) receptors vs the Na+-independent adenosine transporter, as indicated in displacement of [H-3]N-6-(4-nitrobenzyl)-thioinosine binding in rat brain membranes. In a functional assay in CHO cells, it inhibited adenylate cyclase via rat A(3) receptors with an IC50 Of 67 nM. 2-(Methylthio)-N-6-(3-iodobenzyl)-adenosine-5'-N-methyluronamide and 2-(methylamino)-N-6-(3-iodobenzyl)-adenosine thyluronamide were less potent, but nearly as selective for A(3) receptors. Thus, 2-substitution (both small and sterically bulky) is well-tolerated at A(3) receptors, and its A(3) affinity-enhancing effects are additive with effects of uronamides at the 5'-position and a 3-iodobenzyl group at the NG-position.