Pyrazolo[4,3-e]1,2,4-triazolo[1,5-c]pyrimidine derivatives as adenosine receptor ligands: A starting point for searching A(2B) adenosine receptor antagonists

A series of novel pyrazolo[4,3-e]1,2,4-triazolo[1,5-c]pyrimidine substitute d at N-8 pyrazole nitrogen and at the 5-amino group was synthesized and the ir affinities to all four cloned human adenosine receptor subtypes were eva luated by competition binding assays using [H-3]-DPCPX (A(1) and A(2B)), [H -3]-SCH 58261 (A(2A)), and [H-3]-MRE3008-F20 (A(3)) as radioligands. In par ticular, the structural requirements necessary for adenosine A2B receptor r ecognition were investigated. These preliminary results confirm that the fr ee amino group at the 5-position confers better A(2B) affinity, while the e ffect of the chain at nitrogen pyrazole nucleus, the 8-position, seems to b e preferred with respect to the corresponding N-7 regioisomers. The introdu ction of an aminoacyl chain at the 5-amino group produces better selectivit y for A(2B) receptors vs. A2A, but the compounds still remain more potent f or A, and a significant decrease (about 5-fold) of affinity at A2B receptor s was observed. Surprisingly, with these polar chains a good affinity at hu man A3 adenosine receptors was also detected. The most interesting compound s in binding studies proved to be potent antagonists (nM range) in function al assays, measuring the inhibition of cAMP production induced by 100 nM NE CA. (C) 2001 Wiley-Liss, Inc.