Pyrazolo[4,3‐e]1,2,4‐triazolo[1,5‐c]pyrimidine derivatives as adenosine receptor ligands: A starting point for searching A2B adenosine receptor antagonists

A series of novel pyrazolo[4,3‐e]1,2,4‐triazolo[1,5‐c]pyrimidine substituted at N8 pyrazole nitrogen and at the 5‐amino group was synthesized and their affinities to all four cloned human adenosine receptor subtypes were evaluated by competition binding assays using [3H]‐DPCPX (A1 and A2B), [3H]‐SCH 58261 (A2A), and [3H]‐MRE3008‐F20 (A3) as radioligands. In particular, the structural requirements necessary for adenosine A2B receptor recognition were investigated. These preliminary results confirm that the free amino group at the 5‐position confers better A2B affinity, while the effect of the chain at nitrogen pyrazole nucleus, the 8‐position, seems to be preferred with respect to the corresponding N7 regioisomers. The introduction of an aminoacyl chain at the 5‐amino group produces better selectivity for A2B receptors vs. A2A, but the compounds still remain more potent for A1 and a significant decrease (about 5‐fold) of affinity at A2B receptors was observed. Surprisingly, with these polar chains a good affinity at human A3 adenosine receptors was also detected. The most interesting compounds in binding studies proved to be potent antagonists (nM range) in functional assays, measuring the inhibition of cAMP production induced by 100 nM NECA. Drug Dev. Res. 53:225–235, 2001.