Functionalized Congeners of A3 Adenosine Receptor-Selective Nucleosides Containing a Bicyclo[3.1.0]hexane Ring System

(N)-Methanocarba nucleosides containing bicyclo[3.1.0]hexane replacement of the ribose ring previously demonstrated selectivity Lis A(3) adenosine receptor (AR) agonists (5'-uronamides) or antagonists (5'-truncated). Here, these two series were modified in parallel at the adenine C2 position. N-6-3-Chlorobenzyl-5'-N-methyluronamides derivatives with functionalized 2-alkynyl chains of varying length terminating in a reactive carboxylate, ester, or amine group were full, potent human A(3)AR agonists. Flexibility of chain substitution allowed the conjugation with a fluorescent cyanine dye (Cy5) and biotin, resulting in binding K-i values of 17 and 36 nM, respectively. The distal end of the chain was predicted by homology modeling to bind at the A(3)AR extracellular regions. Corresponding L-nucleosides were nearly inactive in AR binding. In the 5'-truncated nucleoside series, 2-Cl analogues were more potent at A(3)AR than 2-H and 2-F, functional efficacy in adenylate cyclase inhibition varied, and introduction of a 2-alkynyl chain greatly reduced affinity. SAR parallels between the two series lost stringency at distal positions. The most potent and selective novel compounds were amine congener 15 (K-i = 2.1 nM) and truncated Partial agonist 22 (K-i = 4.9 nM).