Unique Positive Cooperativity Between the β-Arrestin–Biased β-Blocker Carvedilol and a Small Molecule Positive Allosteric Modulator of the β2-Adrenergic Receptor

Among b-blockers that are clinically prescribed for heart failure, carvedilol is a first-choice agent with unique pharmacological properties. Carvedilol is distinct from other b-blockers in its ability to elicit b-arrestin-biased agonism, which has been suggested to underlie its cardioprotective effects. Augmenting the pharmacologic properties of carvedilol thus holds the promise of developing more efficacious and/or biased b-blockers. We recently identified compound-6 (cmpd-6), the first small molecule positive allosteric modulator of the b2-adrenergic receptor (b2AR). Cmpd-6 is positively cooperative with orthosteric agonists at the b2AR and enhances agonistmediated transducer (G-protein and b-arrestin) signaling in an unbiased manner. Here, we report that cmpd-6, quite unexpectedly, displays strong positive cooperativity only with carvedilol among a panel of structurally diverse b-blockers. Cmpd-6 enhances the binding affinity of carvedilol for the b2AR and augments its ability to competitively antagonize agonist-induced cAMP generation. Cmpd-6 potentiates b-arrestin1but not Gs-protein-mediated high-affinity binding of carvedilol at the b2AR and b-arrestin-mediated cellular functions in response to carvedilol including extracellular signal-regulated kinase phos-phorylation, receptor endocytosis, and trafficking into lysosomes.