Selective endothelin A receptor antagonists .2. Discovery and structure-activity relationships of 5-ketopentanoic acid derivatives

The second in this series of papers describes the further progress made in the discovery of a potent and selective endothelin ETA receptor antagonist for the potential treatment of diseases in which endothelin has been shown to have a pathophysiological role including hypertension, ischaemic diseases and atherosclerosis. We describe herein the synthesis and structure-activity relationships of a novel series of 5-ketopentanoic acid derivatives exemplified by the lead compound 1 (IC50 0.72 mu M, rat aortic ETAR). Optimisation of the in vitro binding of 1 led to the identification of a more potent compound (37) which exhibited an IC50 < 0.1 mu M with > 300-fold selectivity for the ETA receptor over the ETB receptor. This compound demonstrated functional antagonism of endothelin-induced vasoconstriction in vitro.