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

Abstract

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 A₃ 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 A₁ receptors with selectivity vs. the A₃ subtype. A molecular modeling study suggests that differences in affinity toward A₁, A_2A, and A₃ receptors could be ascribed to two residues: one in the EL2, E168 in human A_2A/E172 in human A₁, that is occupied by the hydrophobic residue V169 in the human A₃ receptor; and the other in TM6, occupied by H250/H251 in human A_2A and A₁ receptors and by a less bulky S247 in the A₃ receptor. In the end, these findings could help to design new subtype-selective adenosine receptor ligands.

Keywords: GPCR; adenosine receptor; antagonists; molecular modeling; pyrazolo[4,3-e][1,2,4]triazolo[1,5-c]pyrimidine.

Scheme 1

Synthesis of compounds 1–7. Reagents and Conditions: i: RCOCl, TEA, THF, reflux, 18 h.

Figure 1

Docking pose of compound 11 (grey) at hA₁ (pink, (A)), hA_2A (cyan, (B)), hA₃ (orange, (C)), and hA_2B (blue, (D)) AR structures. The following receptor 3D structures were employed: experimental X-ray for hA₁ (PDB IDs: 5UEN) and hA_2A ARs (PDB IDs: 4EIY), homology models for hA_2B and hA₃ ARs (built on hA_2A and hA₁, respectively). A ZM-241385-like docking pose is displayed here.

Figure 2

Docking pose of compound 4 (yellow) at hA₁ (pink, (A)), hA_2A (cyan, (B)), hA₃ (orange, (C)), and hA_2B (blue, (D)) AR structures. The following receptor 3D structures were employed: experimental X-ray for hA₁ (PDB IDs: 5UEN) and hA_2A ARs (PDB IDs: 4EIY), homology models for hA_2B and hA₃ ARs (built on hA_2A and hA₁, respectively). Structures were refined with induced-fit docking of compound 4 before re-docking it rigidly.