Di-aryl guanidinium derivatives: Towards improved α2-Adrenergic affinity and antagonist activity

Compounds with excellent receptor engagement displaying α₂-AR antagonist activity are useful not only for therapeutic purposes (e.g. antidepressants), but also to help in the crystallization of this particular GPCR. Therefore, based on our broad experience in the topic, we have prepared eighteen di-aryl (phenyl and/or pyridin-2-yl) mono- or di-substituted guanidines and 2-aminoimidazolines. The in vitro α₂-AR binding affinity experiments in human brain tissue showed the advantage of a 2-aminoimidazolinium cation, a di-arylmethylene core, a conformationally locked pyridin-2-yl-guanidine and a di-substituted guanidinium to achieve good α₂-AR engagement. After different in vitro [³⁵S]GTPγS binding experiments in human prefrontal cortex tissue, it was possible to identify that compounds 7a, 7b and 7c were α₂-AR partial agonist, whereas 8h was a potent α₂-AR antagonist. Docking and MD studies with a model of α_2A-AR and two crystal structures suggest that antagonism is achieved by compounds carrying a di-substituted guanidine which substituent occupy a pocket adjacent to TM5 without engaging S200^5.42 or S204^5.46, and a mono-substituted cationic group, which favorably interacts with E94^2.65.