Purine nucleosides bearing 1-alkynyl chains as adenosine receptor agonists

The synthesis and the pharmacological activity of alkynyl derivatives of adenosine (Ado) and N-ethylcarboxamidoadenosine (NECA), that have been tested on adenosine receptors from different sources, have been reviewed. Most of compounds have been characterized in the last ten years by using radioligand binding assays on rat brain membranes and functional studies on different animal models. More recently, the four human adenosine receptor subtypes have been stably transfected into Chinese hamster ovary (CHO) cells allowing for comparative studies in a similar cellular background, utilizing radioligand binding studies (A(1), A(2)A, A(3)) or adenylate cyclase activity assays (A(2B)). From the whole pattern of studies the following structure-activity relationships have been drown: The activities of 2-alkynylAdos resulted slightly higher at A(1) and lower at A(3) and A(2B) subtypes than the corresponding NECA derivatives, whereas the affinities at A(2A) subtype are similar for the two series of nucleosides. The presence of a methyl group on N-6 of the 2-alkynyladenosines, inducing a contemporary increase in affinity at the human A(3) receptor and a decrease at the other subtypes, resulted in a relevant increase in A(3) selectivity. In particular, 2-phenylethynyl-N-6-methylAdo showed an A(3) affinity in the low nanomolar range (K-i A(3) = 3.4 nM), and about 500 fold A(1)/A(3) and about 2500 fold A(2A)/A(3) selectivity. The presence of a hydroxyl group in some alkynyl side chains led to potent inhibitors of platelet aggegation induced by ADP. Introduction of particular substituents, such as the racemic 2-phenylhydroxypropynyl group, both in adenosine and in NECA analogues, led to highly potent, non selective agonists at all the four subtypes. For the potency at A(2B) receptor it seems to be very important the type of alkynyl chain in 2-position and the presence of the carboxyamido group on the sugar; in fact, the (S)-2-phenylhydroxypropynylNECA [(S)PHPNECA, EC50 A(2B) = 220 nM] proved to be one of the most potent A(2B) agonist reported so far. The introduction of alkynyl chain in 8-position of adenosine led to very selective ligands for the A(3) receptor subtype. These nucleosides behave as adenosine antagonists, since they do not stimulate basal [S-35]GTPgammaS binding, but inhibit NECA-stimulated binding.