Pyrazolo[4,3-e]-1,2,4-triazolo[1,5-c]pyrimidine derivatives: Potent and selective A(2A) 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 A(2A) adenosine receptor subtype. The compounds were tested in binding and functional assays to evaluate their potency for the A(2A) compared with the A(1) adenosine receptor subtype. In binding studies in rat brain membranes, most of the compounds showed affinity for A(2A) receptors in the low nanomolar range with a different degree of A(2A) versus A(1) selectivity. Comparison of N-7 (10a-d,h-o)- and N-8 (10e-g)-substituted pyrazolo derivatives indicates that N-7 substitution decreases the A(1) affinity with the concomitant increase of A(2A) selectivity. Specifically, the introduction of a 3-phenylpropyl group at pyrazolo nitrogen in position 7 (101) increased significantly the A(2A) selectivity, being 210-fold, while the A(2A) receptor affinity remained high (K-i = 2.4 nM). With regards to the affinity for A(2A) receptors, also the compound 10n, bearing in the 7-position a beta-morpholin-4-ylethyl group, deserves attention (K-i = 5.6 nM) even though the A(2A) selectivity (84-fold) was not as high as that of 101. Conversely, the compound 10m (N-7-4-phenylbutyl derivative) showed a remarkable selectivity (A(1)/A(2A) ratio = 129) associated with lower A(2A) 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 A(2A) receptor subtype. The compounds are potent and selective A(2A) 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.