Pharmacological characterization of PABSA, an orally active and highly potent endothelin-receptor antagonist

Function of the endothelinB receptor in cardiovascular physiology and pathophysiology

One of the two receptors by which the potent vasoactive effects of endothelin (ET)-1 are mediated is the ET(B) receptor (ET(BR)), which is found in several tissues, but, more importantly from a cardiovascular point of view, on the endothelial cell. The endothelial cell also has the unique capability of releasing ET-1, as well as other factors, such as the endothelial-derived relaxing factors and prostacyclin, which counteract the myotropic effects of the peptide. The secretory and contractile responses to ET-1 rely on G-protein-coupled ET(BR)s, as well as ET(A)-G-protein-coupled receptor-like proteins. The mitogenic properties of ET-1 via ET(A) receptors (ET(AR)s) coupled to mitogen-activated protein kinases and tyrosine kinases on the vascular smooth muscle may occur in conjunction with the anti-apoptotic characteristics of the endothelial ET(BR)s. Interestingly, most of the relevant antagonists and agonists for both ET(AR)s and ET(BR)s have been developed by the pharmaceutical industry. This highlights the therapeutical potential of compounds that act on ET receptors. In normal as well as in physiopathological conditions, the ET(BR) plays an important role in the control of vascular tone, and must be taken into account when using ET receptor antagonists for the treatment of cardiovascular diseases. For the management of congestive heart failure, renal failure and primary pulmonary hypertension, the most recent literature supports the use of selective ET(AR) antagonists rather than mixed antagonists of ET(AR)s and ET(BR)s. Nonetheless, validation of this view will have to await the first clinical trials comparing the actions of ET(A) to mixed ET(A)/ET(B) receptor antagonists.

Role of endothelin in cardiovascular disease

Endothelins are a family of peptides, which comprises endothelin-1 (ET-1), endothelin-2 (ET-2) and endothelin-3 (ET-3), each containing 21 amino-acids. ET-1 is a peptide secreted mostly by vascular endothelial cells, the predominant isoform expressed in vasculature and the most potent vasoconstrictor currently known. ET-1 also has inotropic, chemotactic and mitogenic properties. In addition, it influences salt and water homeostasis through its effects on the renin-angiotensin-aldosterone system (RAAS), vasopressin and atrial natriuretic peptide and stimulates the sympathetic nervous system. The overall action of endothelin is to increase blood pressure and vascular tone. Therefore, endothelin antagonists may play an important role in the treatment of cardiac, vascular and renal diseases associated with regional or systemic vasoconstriction and cell proliferation, such as essential hypertension, pulmonary hypertension, chronic heart failure and chronic renal failure. Long-term anti-endothelin therapy may improve symptoms and favourably alter the progression of heart failure. Endothelin appears to participate in induction and progression of sclerotic renal changes, leading to progression to end-stage renal disease. Anti-endothelin therapy might offer additional benefits in the prevention of progression of chronic renal failure in addition to the known benefits of RAAS inhibition. Clinical trials have demonstrated potentially important benefits of endothelin antagonists for patients with essential hypertension, pulmonary hypertension and heart failure. Further studies are necessary to determine the role of anti-endothelin therapy in the treatment of cardiovascular diseases and determine the different roles of selective receptor antagonism vs. mixed ET(A/B)-receptor antagonism in human diseases.

Pharmacologic characterization of S-1255, a highly potent and orally active endothelin A receptor antagonist

The pharmacologic properties of a novel nonpeptide endothelin (ET) receptor antagonist, S-1255 ([R]-[+]-2-[benzo(1,3)dioxol-5-yl]-6-isopropyl-4-[4-methoxyphenyl]-2H-chromene-3-carboxylic acid), was studied. [3H]S-1255 specifically bound to porcine aortic smooth muscle membranes expressing only ET(A) receptors with a Kd value of 0.39 nM. [3H]S-1255 binding was potently inhibited by ET-1 and selective ET(A) or ET(A)/ET(B) receptor antagonists, such as L-749329, SB209670, bosentan, and BQ-123, but the inhibitory effect of ET-3 and the selective ET(B) receptor antagonist, BQ-788, on the binding was weak. These inhibitory effects on [3H]S-1255 binding correlated well with those on [125I]ET-1 binding. S-1255 inhibited ET(A) receptor- and ET(B) receptor-mediated contractions in isolated rabbit femoral and pulmonary arteries with pA2 values of 8.8 and 6.3, respectively. The pA2 value of S-1255 for ET(B) receptor-mediated relaxation in isolated rabbit mesenteric artery was 7.4. Oral administration of S-1255 (0.3-10 mg/kg) caused dose-dependent inhibition of the pressor response to exogenous ET-1 (0.1 nmol/kg) in conscious normotensive rats, which was similar to that produced by intravenous administration (1 and 3 mg/kg). S-1255 (10 and 30 mg/kg, p.o.) significantly reduced blood pressure in deoxycorticosterone acetate-salt hypertensive rats from 6 h after administration, and the hypotensive effects were sustained up to 24-48 h. These results suggest that S-1255 is a highly potent and orally active ET(A) receptor antagonist.