N6,5′-disubstituted adenosine derivatives as partial agonists for the human adenosine A3 receptor

5'-(Alkylthio)-substituted analogues of N-6-benzyl- and N-6-(3-iodobenzyl)adenosine were synthesized in 37-61% overall yields. The affinities of these compounds for the adenosine A(1), A(2A), and A(3) receptors were determined using rat brain cortex, rat brain striata, and stably transfected human A(3) receptors in HEK 293 cells, respectively. The compounds proved to be selective for the adenosine A(3) receptor and displayed affinities in the nanomolar range. Compounds 8, 10, and 11 had the highest affinities for the A(3) receptor with K-i values ranging from 8.8 to 27.7 nM. In the N-6-benzyl series, compound 4 (LUF 5403), with a 5'-methylthio group, maintained a reasonable affinity and had the highest selectivity for the A(3) receptor. Compound 12 (LUF 5411), with an N-6-(3-iodobenzyl) group and a 5'-(n-propylthio) substituent, had the highest A(3) selectivity of all of the compounds and also displayed high affinity for this receptor (K-i = 44.3 nM). The compounds were also evaluated for their ability to stimulate [S-35]GTP gamma[S] binding in cell membranes expressing the human adenosine A(3) receptor. It appeared that the N-6,5'-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.