Biological profiles of highly potent novel endothelin antagonists selective for the ET A receptor

Abstract

We describe novel potent endothelin (ET) antagonists that are highly potent and selective for the ET A receptor (selective to ET-1). Of the synthetic analogs based on ET A antagonist BE-18257A isolated from Streptomyces misakiensis (IC 50 value for ET A receptor on porcine aortic smooth muscle cells (VSMCs); 1.4μM), the compounds BQ-123 and BQ-153 greatly improved the binding affinity of [ 125I]ET-1 for ET A receptors on VSMCs (IC 50; 7.3 and 8.6 nM, respectively), whereas they barely inhibited [ 125I]ET-1 binding to ET B receptors (nonselective with respect to isopeptides of ET family) in the cerebellar membranes (IC 50; 18 and 54 μM, respectively). Associated with the increased affinity for ET A receptors, these peptides antagonized ET-1-induced constriction of isolated porcine coronary artery. However, there was a small amount of ET-1-induced vasoconstriction resistant to these antagonists, which paralleled the incomplete inhibition of [ 125I]ET-1 binding in the membrane of the aortic smooth muscle layer. These data suggest that the artery has both ET A and ET B receptors responsible for ET-1-induced vasoconstriction. The antagonists shifted the concentration-response curve to the right for ET-1 in the coronary artery, and increased the apparent dissociation constant in the Scatchard analysis of [ 125I]ET-1 binding on the VSMCs without affecting the binding capacity, indicative of the competitive antagonism for ET A receptor. In conscious rats, pretreatment with the antagonists markedly antagonized ET-1-induced sustained pressor responses in dose-dependent fashion without affecting ET-1-induced transient depressor action, suggesting that the pressor action is mediated by ET A receptors, while the depressor action is mediated by ET B receptors. In addition, pretreatment with the potent antagonists prevented ET-1-induced sudden death in mice. Thus, these potent ET A antagonists should provide a powerful tool for exploring the therapeutic uses of ET A antagonists in putative ET-1-related disorders.