2-Substitution of N6-benzyladenosine-5'-uronamides enhances selectivity for A3 adenosine receptors

Article abstract of DOI:10.1021/jm00047a018

Adenosine derivatives bearing an N⁶-(3-iodobenzyl) group, reported to enhance the affinity of adenosine-5'-uronamide analogues as agonists at A₃ adenosine receptors (J. Med. Chem. 1994, 37, 636-646), were synthesized starting from methyl β-D-ribofuranoside in 10 steps. Binding affinities at A₁ and A(2a) receptors in rat brain membranes and at cloned rat A₃ receptors from stably transfected CHO cells were compared. N⁶-(3- Iodobenzyl)adenosine was 2-fold selective for A₃ vs A₁ or A(2a) receptors; thus it is the first monosubstituted adenosine analogue having any A₃ selectivity. The effects of 2-substitution in combination with modifications at the N⁶- and 5'-positions were explored. 2-Chloro-N⁶-(3- iodobenzyl)adenosine had a K(i) value of 1.4 nM and moderate selectivity for A₃ receptors. 2-Chloro-N⁶-(3-iodobenzyl)adenosine-5'-N-methyluronamide, which displayed a K(i) value of 0.33 nM, was selective for A₃ vs A₁ and A(2a) receptors by 2500- and 1400-fold, respectively. It was 46,000-fold selective for A₃ receptors vs the Na⁺-independent adenosine transporter, as indicated in displacement of [³H]N⁶-(4-nitrobenzyl)-thioinosine binding in rat brain membranes. In a functional assay in CHO cells, it inhibited adenylate cyclase via rat A₃ receptors with an IC₅₀ of 67 nM. 2- (Methylthio)-N⁶-(3-iodobenzyl)-adenosine-5'-N-methyluronamide and 2- (methylamino)-N⁶-(3-iodobenzyl)adenosine-5'-N-methyluronamide were less potent, but nearly as selective for A₃ receptors. Thus, 2-substitution (both small and sterically bulky) is well-tolerated at A₃ receptors, and its A₃ affinity-enhancing effects are additive with effects of uronamides at the 5'- position and a 3-iodobenzyl group at the N⁶-position.