7-substituted 5-amino-2-(2-furyl)pyrazolo[4,3-e]-1,2,4-triazolo[1,5-c]pyrimidines as A2A adenosine receptor antagonists:: A study on the importance of modifications at the side chain on the activity and solubility

It was demonstrated in the early 1990s that adenosine exerts many physiological functions through the interaction with four different receptors, named A(1), A(2A), A(2B), and A(3). In the past few years, our group has been involved in the development of A(2A) antagonists, which led to the synthesis of SCH 58261 (1), the first potent and selective adenosine A(2A) antagonist, which has been widely used as a reference compound. In this paper, we present an extended series of pyrazolotriazolopyrimidines synthesized with the aim to investigate the influence of the substitutions on the pyrazole ring. The choice of the substituents was based on their capability to improve water solubility while retaining high affinity and selectivity at the human A(2A) adenosine receptor subtype. In this series, some structural characteristics that are important for activity, i.e., tricyclic structure, free amino group at 5-position, furan ring, and substituent at 7-position on the pyrazole moiety, have been maintained. We focused our attention on the nature of the phenyl ring substituent to improve water solubility. Following this strategy, we developed new compounds with good affinity and selectivity for A(2A) adenosine receptors, such as 8d (K-i 0.12 nM; hA(1)/hA(2A) ratio = 1025; R-m = 2.8), 8h (K-i 0.22; hA(1)/hA(2A) ratio = 9818; R-m = 3.4), 8i (K-i 0.18 nM; hA(1)/hA(2A) ratio = 994; R-m = 2.8), 8k (K-i 0.13 nM; hA(1)/hA(2A) ratio = 4430; R-m = 3.6), and 14b (K-i 0.19 nM; hA(1)/hA(2A) ratio = 2273; R-m = 2.7). All the new synthesized compounds have no significant interaction with either-A(2B) or A(3) receptor subtypes. This new series of compounds deeply enlightens some structural requirements to display high affinity and selectivity for the A(2A) adenosine receptor subtype, although our goal of identifying new compounds with increased water solubility was not completely achieved. On this basis, other strategies will be devised to improve this class of compounds with a profile that appears to be promising for treatment of neurodegenerative disorders, such as Parkinson's disease.