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 derivative s (10a-o,q,r), bearing alkyl and aralkyl chains on positions 7 and 8, were synthesized in the attempt to obtain potent and selective antagon ists for the A(2A) adenosine receptor subtype. The compounds were test ed in binding and functional assays to evaluate their potency for the A(2A) compared with the A(1) adenosine receptor subtype. In binding st udies in rat brain membranes, most of the compounds showed affinity fo r A(2A) receptors in the low nanomolar range with a different degree o f 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 substitu tion decreases the A(1) affinity with the concomitant increase of A(2A ) selectivity. Specifically, the introduction of a 3-phenylpropyl grou p 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) r eceptors, also the compound 10n, bearing in the 7-position a beta-morp holin-4-ylethyl group, deserves attention (K-i = 5.6 nM) even though t he A(2A) selectivity (84-fold) was not as high as that of 101. Convers ely, the compound 10m (N-7-4-phenylbutyl derivative) showed a remarkab le selectivity (A(1)/A(2A) ratio = 129) associated with lower A(2A) af finity (K-i = 21 nM). In functional studies, most of the compounds exa mined reversed 5'-(N-ethylcarbamoyl)adenosine-induced inhibition of ra bbit platelet aggregation inhibition which is a biological response me diated by the A(2A) receptor subtype. The compounds are potent and sel ective A(2A) antagonists which can be useful to elucidate the pathophy siological role of this adenosine receptor subtype. These compounds de serve to be further developed to assess their potential for treatment of neurodegenerative disorders such as Parkinson's disease.