Discovery of a series of pyrrolidine-based endothelin receptor antagonists with enhanced ETA receptor selectivity Original Research Article

Endothelins, ET-1, ET-2, and ET-3 are potent vasoconstricting and mitogenic 21-amino acid bicyclic peptides, which exert their effects upon binding to the ETA and ETB receptors. The ETA receptor mediates vasoconstriction and smooth muscle cell proliferation, and the ETB receptor mediates different effects in different tissues, including nitric oxide release from endothelial cells, and vasoconstriction in certain vascular cell types. Selective antagonists of endothelin receptor subtypes may prove useful in determining the role of endothelin in various tissue types and disease states, and hence as therapeutic agents for such diseases. The pyrrolidine carboxylic acid A-127722 has been disclosed as a potent and ETA-selective antagonist, and is currently undergoing clinical trials. In our efforts to find antagonists with altered selectivity (ETA-selective, ETB-selective, or nonselective), we investigated the SAR of the 2-substituent on the pyrrolidine. Compounds with alkyl groups at the 2-position possessed ETA selectivity improved over A-127722 (1400-fold selective), with the best of these compounds showing nearly 19,000-fold selectivity.