Adenosine receptor agonists: Synthesis and biological evaluation of the diastereoisomers of 2-(3-hydroxy-3-phenyl-1-propyn-1-yl)NECA

Abstract

Among the recently reported 2-(ar)alkynyl derivatives of 5′- N-ethylcarboxamidoadenosine (NECA), the ( R, S)-2-(3-hydroxy-3-phenyl-1-propyn-1-yl)NECA [( R, S)-PHPNECA or SCH 59761] was found to be a very potent agonist at A 1 and A 2A receptor subtypes, with a K i of 2.5 nM and 0.9 nM, respectively. Furthermore, this compound showed an inhibitory activity on platelet aggregation 16-fold higher than NECA, being the most potent anti-aggregatory nucleoside reported so far. Since this compound bears a chiral carbon in the side chain, the diastereoisomer separation was undertaken both by chiral HPLC and by a stereospecific synthetic method. Binding assays have shown that the ( S)-diastereomer is about fivefold more potent and selective than the ( R)-diastereomer as agonist of the A 2A receptor subtype [( S)-PHPNECA, K iA 2A = 0.5 nM; ( R)-PHPNECA, K iA 2A = 2.6 nM]. Functional studies indicated that ( S)-PHPNECA possesses marked vasodilating activity and produces a relevant decrease in heart rate. Moreover, the ( S)-diastereomer proved to be about ten times more potent than the ( R)-diastereomer in inducing cardiovascular effects, in in vivo hemodynamic studies. However, the greatest difference between these two enantiomers resulted in the platelet aggregation test: in fact, the ( R)-diastereomer displayed an inhibitory activity similar to that of NECA, whereas the ( S)-diastereomer was 37-fold more active than NECA as an inhibitor of rabbit platelet aggregation, induced by ADP. These data suggest that ( S)-PHPNECA could be a useful tool to investigate the mode of binding of agonists to the platelet adenosine receptor subtype.