Identification of a Novel Subtype-Selective α_1B-Adrenoceptor Antagonist

α_1A-, α_1B-, and α_1D-adrenoceptors (α₁-ARs) are members of the adrenoceptor G protein-coupled receptor family that are activated by adrenaline (epinephrine) and noradrenaline. α₁-ARs are clinically targeted using antagonists that have minimal subtype selectivity, such as prazosin and tamsulosin, to treat hypertension and benign prostatic hyperplasia, respectively. Abundant expression of α₁-ARs in the heart and central nervous system (CNS) makes these receptors potential targets for the treatment of cardiovascular and CNS disorders, such as heart failure, epilepsy, and Alzheimer's disease. Our understanding of the precise physiological roles of α₁-ARs, however, and their involvement in disease has been hindered by the lack of sufficiently subtype-selective tool compounds, especially for α_1B-AR. Here, we report the discovery of 4-[(2-hydroxyethyl)amino]-6-methyl-2H-chromen-2-one (Cpd1), as an α_1B-AR antagonist that has 10-15-fold selectivity over α_1A-AR and α_1D-AR. Through computational and site-directed mutagenesis studies, we have identified the binding site of Cpd1 in α_1B-AR and propose the molecular basis of α_1B-AR selectivity, where the nonconserved V197^45.52 residue plays a major role, with contributions from L314^6.55 within the α_1B-AR pocket. By exploring the structure-activity relationships of Cpd1 at α_1B-AR, we have also identified 3-[(cyclohexylamino)methyl]-6-methylquinolin-2(1H)-one (Cpd24), which has a stronger binding affinity than Cpd1, albeit with reduced selectivity for α_1B-AR. Cpd1 and Cpd24 represent potential leads for α_1B-AR-selective drug discovery and novel tool molecules to further study the physiology of α₁-ARs.