Discovery of simplified N2-substituted pyrazolo[3,4-d]pyrimidine derivatives as novel adenosine receptor antagonists: Efficient synthetic approaches, biological evaluations and molecular docking studies

Abstract In the present study, a molecular simplification approach was employed to design novel bicyclic pyrazolo[3,4- d ]pyrimidine (PP) derivatives from tricyclic pyrazolo[4,3- e ]-1,2,4-triazolo-[1,5- c ]pyrimidines (PTP) as promising human A 3 adenosine receptor (hA 3 AR) antagonists. All the target compounds were synthesized using novel and efficient synthetic schemes and the structure–activity relationship studies of these PPs were explored through the synthesis of a series of PTP analogues with various substituents. Substituents with different lipophilicity and steric hindrance (e.g., alkyl and aryl–alkyl) functions were introduced at N 2 position of the pyrazole ring, while acyl groups with different electronic properties were introduced at C 6 position of the bicyclic nucleus to probe both electronic and positional effects. Most of the synthesized derivatives of the PP series presented good affinity at the hA 3 AR, as indicated by the low micromolar range of K i values and among them, compound 63 with N 2 neopentyl substituents showed most potent hA 3 AR affinity with K i value of 0.9 μM and high selectivity (hA 1 AR/hA 3 AR = >111 & hA 2A AR/hA 3 AR = >111) towards other adenosine receptor subtypes. Interestingly, small isopropyl groups at N 2 position displayed high affinity at another receptor subtype (hA 2A AR, e.g., compound 55 , with K i hA 2A AR = 0.8 μM), while they were less favorable at the hA 3 AR. Molecular docking analysis was also performed to predict the possible binding mode of target compounds inside the hA 3 AR and hA 2A AR. Overall, PP derivatives represent promising starting points for new AR antagonists.