Predicting compounds that interact with the 2 known agonist-induced conformations of the human β1-adrenoceptor

The β1-adrenoceptor exists in at least 2 agonist-stabilized conformational ensembles: a "catecholamine" ensemble induced via the intrahelical binding site through which catecholamines and most agonists act and a "secondary" ensemble of conformations through which CGP12177 stimulates agonist responses. Several β-ligands stimulate agonist responses through both conformations, resulting in biphasic concentration responses, but little is known about the structure-activity relationship of such ligands. Using a structure-activity hypothesis built on the predicted poses CGP12177 and 3 biphasic agonists (alprenolol, oxprenolol, and bucindolol), predictions based on ligand similarity and structural compatibility reasoning were made about 11 other β1-ligands not yet tested for secondary conformation interaction and examined in radioligand binding and functional assays using human β1- and β2-adrenoceptors. Although the predictions matched with pharmacology in only 6/11 of cases, 3 novel compounds were found to induce an active-state secondary conformation. A CGP12177 derivative (methyl-pyrrole replacing the cyclic urea motif) retained catecholamine site antagonism with secondary site activation. Carteolol (related to CGP12177) and bunitrolol (similar to alprenolol) activated both conformations with biphasic concentration responses. Bunolol (CGP12177 derivative lacking nitrogen in the bicyclic system), as predicted, was a neutral antagonist with no secondary site activation. Moprolol and some bucindolol analogs appeared as conventional agonists, whereas other alprenolol and bucindolol analogs lost all receptor interaction. In a β1-adrenoceptor mutant (β1-V189T-L195Q-W199Y) where secondary site CGP12177 and pindolol interaction is lost, the 3 novel secondary-site compounds were also no longer able to stimulate secondary conformation responses, suggesting that there is a common TM4 secondary conformation-inducing interaction site. SIGNIFICANCE STATEMENT: The β1-adrenoceptor exists in 2 agonist-stabilized, pharmacologically distinguishable conformations. This study pinpointed the interaction site through which the alternative conformation is stabilized and suggested and evaluated additional ligands, thus providing possible molecular determinants.