Pyrazolo[4,3-e]-1,2,4-triazolo[1,5-c]pyrimidine Derivatives:  Potent and Selective A2A Adenosine Antagonists

A series of pyrazolo[4,3-e]-1,2,4-triazolo[1,5-c]pyrimidine derivatives (10a−o,q,r), bearing alkyl and aralkyl chains on positions 7 and 8, were synthesized in the attempt to obtain potent and selective antagonists for the A2A adenosine receptor subtype. The compounds were tested in binding and functional assays to evaluate their potency for the A2A compared with the A1 adenosine receptor subtype. In binding studies in rat brain membranes, most of the compounds showed affinity for A2A receptors in the low nanomolar range with a different degree of A2A versus A1 selectivity. Comparison of N7 (10a−d,h−o)- and N8 (10e−g)-substituted pyrazolo derivatives indicates that N7 substitution decreases the A1 affinity with the concomitant increase of A2A selectivity. Specifically, the introduction of a 3-phenylpropyl group at pyrazolo nitrogen in position 7 (10l) increased significantly the A2A selectivity, being 210-fold, while the A2A receptor affinity remained high (K i = 2.4 nM). With regards to the affinity for A2A receptors, also the compound 10n, bearing in the 7-position a β-morpholin-4-ylethyl group, deserves attention (K i = 5.6 nM) even though the A2A selectivity (84-fold) was not as high as that of 10l. Conversely, the compound 10m (N 7-4-phenylbutyl derivative) showed a remarkable selectivity (A1/A2A ratio = 129) associated with lower A2A affinity (K i = 21 nM). In functional studies, most of the compounds examined reversed 5‘-(N-ethylcarbamoyl)adenosine-induced inhibition of rabbit platelet aggregation inhibition which is a biological response mediated by the A2A receptor subtype. The compounds are potent and selective A2A antagonists which can be useful to elucidate the pathophysiological role of this adenosine receptor subtype. These compounds deserve to be further developed to assess their potential for treatment of neurodegenerative disorders such as Parkinson's disease.