Identification of a Novel Subtype-Selective α 1B -Adrenoceptor Antagonist.

α 1A -, α 1B -, and α 1D -adrenoceptors (α 1 -ARs) are members of the adrenoceptor G protein-coupled receptor family that are activated by adrenaline (epinephrine) and noradrenaline. α 1 -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 α 1 -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 α 1 -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 α 1 -ARs.