Pyrazolo-triazolo-pyrimidine Scaffold as a Molecular Passepartout for the Pan-Recognition of Human Adenosine Receptors

Adenosine receptors are largely distributed in our organism and are promising therapeutic targets for the treatment of many pathologies. In this perspective, investigating the structural features of the ligands leading to affinity and/or selectivity is of great interest. In this work, we have focused on a small series of pyrazolo-triazolo-pyrimidine antagonists substituted in positions 2, 5, and N8, where bulky acyl moieties at the N5 position and small alkyl groups at the N8 position are associated with affinity and selectivity at the A(3) adenosine receptor even if a good affinity toward the A(2B) adenosine receptor has also been observed. Conversely, a free amino function at the 5 position induces high affinity at the A(2A) and A(1) receptors with selectivity vs. the A(3) subtype. A molecular modeling study suggests that differences in affinity toward A(1), A(2A), and A(3) receptors could be ascribed to two residues: one in the EL2, E168 in human A(2A)/E172 in human A(1), that is occupied by the hydrophobic residue V169 in the human A(3) receptor; and the other in TM6, occupied by H250/H251 in human A(2A) and A(1) receptors and by a less bulky S247 in the A(3) receptor. In the end, these findings could help to design new subtype-selective adenosine receptor ligands.