Introduction of alkynyl chains on C-8 of adenosine led to very selective antagonists of the A3 adenosine receptor

Some 8-alkynyladenosines were synthesized and evaluated for their adenosine receptor activity, utilizing radioligand binding studies (A1, A2A, A3) or adenylyl cyclase activity assays (A2B). Furthermore, the maximal induction of guanosine 5′-(γ-thio)triphosphate ([35S]GTPγS) binding to G proteins and the inhibition of NECA-stimulated binding, in membranes of CHO cells which express the human A3 receptor, were used to determine the intrinsic activity of these nucleosides at the A3 adenosine receptor. The results showed that these new adenosine derivatives are very selective ligands for the A3 receptor subtype and behave as adenosine antagonists, since they do not stimulate basal [35S]GTPγS binding, but inhibit NECA-stimulated binding. This is the first report that adenosine derivatives, with unmodified ribose moiety, are adenosine receptor antagonists.