Triskellines A (3,6,8-Trisubstituted [1,2,4]triazolo[4,3- b ]pyridazines) as Potent Antagonists of Human Adenosine Receptor A3

Adenosine receptor A3 (A3AR) is coupled to Gi proteins, leading to adenylate cyclase inhibition, and to Go proteins, leading to phospholipase C activation and Ca++ release. We here describe the synthesis and biological characterization of a family of 3,6,8-trisubstituted [1,2,4]-triazolo-[4,3-b]-pyridazines (Triskellines A), initially inspired from the cyclin-dependent kinase inhibitory purine (R)-Roscovitine. Triskellines A act as selective A3AR antagonists. Among 234 synthesized compounds, some display extremely potent binding activity (2-digit picomolar IC50 values in a human A3AR agonist radioligand binding assay) and very high selectivity toward A3AR compared to adenosine receptors A1AR, A2AAR, and A2BAR (4-Log difference in IC50 values). Triskellines A do not inhibit protein kinases. A structure−activity relationship study points toward optimal potency structural features. Molecular docking with A3AR reveals interaction of Triskellines in the adenosine binding pocket at the agonist drug Piclidenoson (IB-MECA) binding site. Triskelline A-143 displays favorable pharmacological and safety properties.