Design, synthesis, and biological evaluation of a second generation of pyrazolo[4,3-e]-1,2,4-triazolo[1,5-c]pyrimidines as potent and selective A2A adenosine receptor antagonists

New A(2A) adenosine receptor antagonists in the series of pyrazolo[4,3-e]-1,2,4-triazolo[1,5-c]-pyrimidines, bearing oxygenated substituents on the phenylalkyl chains on the 7-position, have been synthesized. The compounds were tested in binding and functional assays to evaluate affinity, potency, and selectivity for rat A(2A) compared to rat A(1) and human A(3) receptor subtypes. The most interesting compounds (5d,e,h) were tested also in binding to human A(1) and A(2A) adenosine receptors. They showed very good affinity (K-i = 0.94 nM for compound 5h) and interesting selectivity with respect to both rA(1) and hA(3) (compound 5h: rA(1)/rA(2A) = 787, hA(3)/rA(2A) > 10000) These important findings make this new series of compounds the first really selective for A(2A) adenosine receptors. Thermodynamic parameters were evaluated; all the tested compounds displayed an enthalpy-driven binding as expected for antagonists. Moreover, compound 5h showed a negative entropy value. The highly negative enthalpic and entropic contributions could mean that 5h fits very well in the binding site where, probably, an electrostatic interaction is present associated to a scarce solvent reorganization around the receptor binding site. These compounds deserve to be further developed to assess their potential for treatment of neurodegenerative disorders such as Parkinson's disease.