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 A3 adenosine receptor (hA3AR) 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 N2 position of the pyrazole ring, while acyl groups with different electronic properties were introduced at C6 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 hA3AR, as indicated by the low micromolar range of Ki values and among them, compound 63 with N2 neopentyl substituents showed most potent hA3AR affinity with Ki value of 0.9 lM and high selectivity (hA1AR/hA3AR = >111 & hA2AAR/hA3AR = >111) towards other adenosine receptor subtypes. Interestingly, small isopropyl groups at
N2 position displayed high affinity at another receptor subtype (hA2AAR, e.g., compound 55, with Ki hA2AAR
= 0.8 lM), while they were less favorable at the hA3AR. Molecular docking analysis was also performed to predict the possible binding mode of target compounds inside the hA3AR and hA2AAR. Overall, PP derivatives represent promising starting points for new AR antagonists.
