Synthesis of heterocycles and nucleosides as ligands of adenosine receptors

Adenosine (Ado) is involved in the regulation of many physiological and pathophysiological processes through the activation of four cell membrane receptors termed A1, A2A, A2B and A3, which belong to the family of G-proteins coupled receptors and are ubiquitously expressed in the body. Adenosine receptors (ARs) are present on virtually every tissue and are often co-expressed in the same cell type, and their activation is predominantly responsible for the wide variety of effects produced by adenosine throughout several organ systems. Research on adenosine receptors during the past 20 years has resulted in considerable progress in identifying selective agonists and antagonists and an increased understanding of the particular roles adenosine receptor subtypes play in physiological processes. With this research work new ligands for the ARs have been synthesized and tested on A1, A2A, A2B and A3 subtypes. Hence, in the search for agonists for the A2B AR subtypes, 2-phenylhydroxypropynyl-5'-N-alkylcarboxamidoadenosine derivatives and N6-substituted 2-phenylhyroxypropynyladenosine derivatives were synthesized on the basis that the introduction of a phenylhydroxypropynyl chains in 2-position of Ado derivatives resulted in compounds with reasonably good A2B potency. Adenylyl cyclase assay, performed with cloned human A2B ARs, showed that these modifications did not produce the expected increase of potency at A2B receptors. Furthermore, in the search for A3 AR agonists, the synthesis of 2-phenylethynylAdo derivatives, bearing different substituents in the phenyl ring of the alkynyl chain and a methoxy group in the N6-position, was undertaken on the bases that these modifications are reported to favour the interaction of Ado derivatives with the A3 AR subtype. Selective compounds were also modified in the 4' position of the sugar moiety by introduction of a methyl or an ethyl carboxamido substituent. The new 2-aralkynyl-N6-methoxyAdo derivatives were found to possess high affinity and in some cases very high selectivity for the human A3 receptor subtype. Finally, based from the observation that the replacement of the sugar moiety in adenosine derivatives with alkyl groups led to adenosine receptor antagonists, in the last part of this work, the ribose moiety of Ado has been substituted with a ethyl group and alkynyl chains were introduced in the 2- and 8- positions, respectively. Binding studies (A1, A2A, A3) and adenylyl cyclase assay (A2B) performed at both human and rat ARs showed that all the synthesized compounds bound the ARs with good affinity. It is worthwhile to note that, in the series of the 2-substituted derivatives, the 2-cyclohexylethynyl-9-ethyladenine and 2-phenylhydrxypropynyl-9-ethyladenine are the ones endowed with the highest affinity at both human and rat A1 and A2A receptors. The same trend has been observed with the 8-substituted derivatives: in fact the 8-phenylethynyl-9-ethyladenine resulted the most active and selective compound at both human and rat A3 subtype. From these results it is possible to conclude that the selectivity of these two series of derivatives for the A1, A2A, and A3 receptor subtypes is strictly correlated to the nature of the alkynyl chain, and that there is a good correlation between the affinity at human and rat receptors.