Allosteric modulator potentiates β(2)AR agonist–promoted bronchoprotection in asthma models

Asthma is a chronic inflammatory disease associated with episodic airway narrowing. Inhaled β(2)-adrenergic receptor (β(2)AR) agonists (β(2)-agonists) promote — with limited efficacy — bronchodilation in asthma. All β(2)-agonists are canonical orthosteric ligands that bind the same site as endogenous epinephrine. We recently isolated a β(2)AR-selective positive allosteric modulator (PAM), compound-6 (Cmpd-6), which binds outside of the orthosteric site and modulates orthosteric ligand functions. With the emerging therapeutic potential of G-protein coupled receptor allosteric ligands, we investigated the impact of Cmpd-6 on β(2)AR-mediated bronchoprotection. Consistent with our findings using human β(2)ARs, Cmpd-6 allosterically potentiated β(2)-agonist binding to guinea pig β(2)ARs and downstream signaling of β(2)ARs. In contrast, Cmpd-6 had no such effect on murine β(2)ARs, which lack a crucial amino acid in the Cmpd-6 allosteric binding site. Importantly, Cmpd-6 enhanced β(2) agonist–mediated bronchoprotection against methacholine-induced bronchoconstriction in guinea pig lung slices, but — in line with the binding studies — not in mice. Moreover, Cmpd-6 robustly potentiated β(2) agonist–mediated bronchoprotection against allergen-induced airway constriction in lung slices obtained from a guinea pig model of allergic asthma. Cmpd-6 similarly enhanced β(2) agonist–mediated bronchoprotection against methacholine-induced bronchoconstriction in human lung slices. Our results highlight the potential of β(2)AR-selective PAMs in the treatment of airway narrowing in asthma and other obstructive respiratory diseases.