Cell-based and in-silico studies on the high intrinsic activity of two boron-containing salbutamol derivatives at the human β ₂- adrenoceptor

Salbutamol is a well-known β ₂ adrenoceptor (β ₂AR) partial agonist. We synthesized two boron-containing salbutamol derivatives (BCSDs) with greater potency and efficacy, compared to salbutamol, for inducing β ₂AR-mediated smooth-muscle relaxation in guinea-pig tracheal rings. However, the mechanism involved in this pharmacological effect remains unclear. In order to gain insight, we carried out binding and functional assays for BCSDs in HEK-293T cells transfected with the human β ₂AR (hβ ₂AR). The transfected hβ ₂AR showed similar affinity for BCSDs and salbutamol, but adenosine 3′,5′-cyclic phosphate (cAMP) accumulation induced by both BCSDs was similar to that elicited by isoproterenol and greater than that induced by salbutamol. The boron-containing precursors (boric and phenylboronic acids, 100 μM) had no significant effect on salbutamol binding or salbutamol-induced cAMP accumulation. These experimental results are in agreement with theoretical docking simulations on lipid bilayer membrane-embedded hβ ₂AR structures. These receptors showed slightly higher affinity for BCSDs than for salbutamol. An essential change between putative active and inactive conformational states depended on the interaction of the tested ligands with the fifth, sixth and seventh transmembrane domains. Overall, these data suggest that BCSDs induce and stabilize conformational states of the hβ ₂AR that are highly capable of stimulating cAMP production.