Topical Workshop 7:  Endothelins in Hypertension and Cardiovascular Disease

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Endothelin-1 (ET-1) by acting on receptor A and B subtypes elicits a number of biologic responses, including vasoconstriction, cell proliferation, aldosterone secretion in some species including humans, activation of the nuclear factor kappa B (NF-κB) family, nitric oxide (NO) adrenomedullin release, blunting of the NF-κB family transcription factor, and is responsible for ET-1 release. The endothelial system in integrated with other major pressor systems, including the renin-angiotensin system (RAS). Chymase, which is mainly responsible for the conversion of Angiotensin I to Angiotensin II in humans is responsible for the cleavage of ET-1 to ET-131, which mainly acts at the receptor A level. Other known biologic functions include hypertrophy, inflammation, and fibrosis.

The endothelin system has been implicated in systemic arterial hypertension, pulmonary hypertension, and renal damage, among other conditions.

In pulmonary arterial hypertension (PAH) the endothelin system is activated at the tissue level. Endothelin expression is increased in the lungs of patients with PAH, and this has also been shown in the mouse model of PAH and congestive heart failure (CHF). Endothelin plasma concentrations are elevated particularly in PAH, and in PAH due to scleroderma and in PAHD linked to congenital heart disease. Only about 20% of secreted ET-1 is found in the lumen, therefore, finding elevated plasma levels indicates that the system is markedly activated at the tissue level and the leakage of just a small amount is sufficient to increase the plasma levels.

A new assay has been developed focusing on the C-Terminal preproET-1 peptide, which is more stable than ET-1 or the mature peptide in the circulation. The investigators were able to show a good correlation between the C-Terminal preproET-1 and ET-1, which had been the best marker of endothelin synthesis (in press). They also showed that the assay is not affected by food intake nor are there gender differences. Further, the assay provided an accurate estimation of the activation of the endothelin system in heart failure and sepsis.

Systemic arterial hypertension and the endothelin system

Two studies provide the best evidence for the involvement of the endothelin system in arterial hypertension. One study showed vasoconstriction via the ETA receptors in response to brachial artery infusion of ET-1 in healthy human volunteers, and co-infusion of ET-1 and an ETA antagonist reversed vasoconstriction and induced vasodilation.

Several receptor antagonists of the endothelin system are under investigation. Bosentan, a mixed ETA ETB receptor blocker, was shown in one multicenter study to be equally as effective as enalapril in reducing systolic blood pressure (SBP) and diastolic blood pressure (DBP) in patients with mild to moderate hypertension.  Darusentan, an ETA receptor antagonist, was shown in the HEAT study to have a dose-dependent reduction of SBP and DBP.

The endothelin system could play more of a role in salt- and volume-dependent models of hypertension, compared to the high-renin, vasoconstriction model. In a small study of 11 patients with primary or secondary aldosteronism, Rossi and colleagues showed that both an ETA antagonist and an ETAETB antagonist in small doses (selected to affect only endogenous system) produced small but significant reductions in blood pressure.

Endothelins and target organ damage

Endothelin plays an important role in target organ damage, for example vascular remodeling and hypertrophy. Bosentan in one animal study was shown to reverse pulmonary artery hypertrophy, and in another animal study to blunt fibrogenesis in the setting of scleroderma, a condition characterized by inflammation and tissue fibrosis. Bosentan was also shown to prevent perivascular fibrosis in the heart in an animal model.

PAH and bosentan

In PAH, bosentan was shown to blunt perivascular peribronchial inflammation and induce favorable hemodynamics in a small pilot study. Further, ET-1 plasma levels predicted survival. In a proof-of-concept, bosentan was shown to improve survival in patients with primary PAH.

The blockade of the endothelin system is important to prevent cell proliferation, vasoconstriction, cell proliferation, and hypertrophy in the vasculature, and for preventing vascular remodeling, endothelial dysfunction, fibrosis, and inflammation. It could, therefore, be of further interest for research and therapeutic strategies.

Endothelin and endothelin antagonists and the kidney

Three important findings from the RENAAL study were that proteinuria itself is a risk factor for renal disease, the reduction of proteinuria in the first 6 months is related to outcomes, and that although the angiotensin receptor blocker produced a significant reduction in end stage renal disease (ESRD) the 28% reduction was modest. Clearly, other treatments are needed to reduce proteinuria and disease progression.

Dr. G.C. Viberti from London, England, stated there are two important questions: Is normoalbuminuria required for maximal organ protection? Are there new targets for treatment and prevention strategies?

Edothelin-1 (ET-1) has been shown to:

• Control glomerular hemodynamics, sodium and water homeostasis, and acid-base balance.
• To be the major endothelial isoform and the only one expressed at the protein level in the human kidney.
• To be produced by glomerular, endothelial and mesangial cells and renal tubular and medullary collecting duct cells.
• Play a role in fibrosis, cell proliferation, matrix formation, and proteinuria in renal disease.
• ET-1 plasma levels are raised in diabetes and kidney disease.
• ET-1 acts by binding to ETA and ETB.
• In the kidney, ETB primarily expressed in the collection system, and ETA expressed n the glomeruli and vascular structure.
• In animal models of renal disease, endothelin receptor antagonists (ERAs) have renoprotective effects.

Evidence of renoprotective effect of ERAs

Bosentan , a dual ETA ETB antagonist prevented in increase in mean arterial pressure in a model of diabetic rats, and the addition of enalapril did not produce an additional decrease. In OLEFT type 2 diabetic rats, proteinuria was prevented by YM698, an ETA selective antagonist. This agent also reduced urinary collagen type IV and heparin sulfate to nearly the same level as enalapril.

Another ETA selective antagonist, LU135252, in a diabetic rat model decreased glomerular and hyaline deposits. It also decreased TGF-β expression in early experimental diabetic nephropathy, but did not prevent podocyte loss, which has been implicated in renal disease and diabetes. An ACE inhibitor did inhibit podocyte loss, thus this does not seem to be the mechanism by which the ETA selective antagonist works to prevent proteinuria and disease progression.

Clinical evidence with ERAs

Avosentan reduced the urinary albumin excretion rate (UAER) by about 50% (from 1.-1.5 gm/day to 0.5 gm/day) in a 3-month proof-of-concept study. The patients had diabetic nephropathy, were 60 years old on average, and residual proteinuria despite treatment with an ACE inhibitor and/or angiotensin receptor blocker (ARB) and preserved renal function (creatinine clearance 75 ml/min). Avosentan had not effect on systemic arterial blood pressure.  A side effect was a reduction in hematocrit and hemoglobin levels, which must be considered in regard to clinical use of this drug.

This proof-of-concept study showed that all of the doses of avosentan studied significantly reduced UAER compared to placebo and also significant reduction in total cholesterol. Creatinine clearance was the same at 3 months. The drug was safe and well tolerated and the most common side effect was edema and headache. There was no significant effect on heart rate, blood pressure, glucose metabolism, triglyceride, body weight, or liver enzyme activity.

The ASCEND study is now underway, based on these findings. ASCEND is a large, randomized, double-blind, placebo-controlled study to evaluate avosentan (25 mg and 50 mg) in addition to an ACE inhibitor and/or ARB in type 2 diabetics with nephropathy, The primary endpoints are time to doubling of serum creatinine, ESRD, or death. The study began in July 2005 and is planned to close in 2009.

Viberti concluded that the ASCEND study should determine whether an ERA is effective in the management of renal disease, the incidence of which is increasing and is not sufficiently managed with current treatment approaches.

Do we need ET antagonists for the treatment of systemic arterial hypertension?

ET-1 can be involved in the pathogenesis of hypertension and target organ damage, and ET-1 antagonists have been shown to lower blood pressure in patients with essential hypertension and special patient populations. Also, these agents are potentially effective in target organ protection, such as the kidney. Whether selective or nonselective ETA antagonists are preferable must be established. It may be that nonselective ETA antagonists will be preferable for hypertension and selective ETA antagonists for target organ protection. Dr. S. Taddei from Pisa, Italy, presented data to support these conclusions.

Role of endothelin in human hypertension

Taddei and colleagues demonstrated that plasma levels of ET-1 are similar in normotensive and hypertensive persons, because only about 20% of endothelin is secreted in the lumen, and thus it is not a sensitive marker of tissue production of endothelin. However, in patients with impaired renal function and hypertension, increased plasma levels are detected, likely due to reduced clearance.

Persons with essential hypertension have a greater sensitivity to endothelin, and it has been shown that ET-1 causes vasoconstriction in essential hypertension compared to normotension. Work from this group also showed that the vasoconstrictive effect of endogenous endothelin is much greater in hypertensives compared to normotensives and that ET-1 receptor antagonists induce vasodilation.

ET-1 causes vasoconstriction but also stimulates the ADP receptor on the endothelial cell to produce nitric oxide (NO), and the balance of this is important for the activity of endothelin. Work from Taddei and colleagues confirmed there is increased endogenous vascular tone related to ET-1 in patients with essential hypertension. They also showed an inverse relation between NO levels and endogenous ET-1 vasoconstriction, with a decrease in NO resulting in an increase in ET-1 activity.

In another study, mean arterial pressure and peripheral resistance were increased with intravenous Tak-044, and treatment with bosentan produced a dose-dependent reduction in systolic blood pressure (SBP) and diastolic blood pressure (DBP), similar to the reduction found with enalapril. The effect of bosentan was related to blockade of the ADP receptor, as shown by the increase ET-1 during bosentan administration. In the HEAT study, darusentan, compared to placebo, produced a dose-dependent reduction in SBP and DBP.

Role of endothelin in special hypertensive populations

In Black persons with hypertension, ET-1 plays a greater role in hypertension, and the ETA receptor blocker BQ123 produces a greater vasodilatory effect compared to Caucasians with hypertension.  In both Black and Caucasian normotensive persons, ET-1 was similar.

In severe human hypertension, compared to mild hypertension and normotension, the endothelial expression of ET-1 mRNA is greatly increased.  In cyclosporin A-mediated hypertension in the rat model, bosentan reduced systolic hypertension.  In erythropoietin (EPO)-induced hypertension, which occurs in about 16-21% of patients with end stage renal disease, blood pressure is increased by the treatment with human recombinant EPO because of the potentiation of vascular ET production. LU135252, a selective ETA antagonist produced a clear reduction of blood pressure in EPO-induced hypertension, whereas bosentan, a nonselective ET antagonist, had no effect on blood pressure.

In obstructive sleep apnea, ET levels were elevated in some studies but not in others. In this condition, chronic hypoxia, vasoconstriction, inflammation activation, cytokine production, and endothelial damage could lead to hypertension. This may be a target for endothelial antagonists.

Endothelin and target organ damage

In renal disease, a small study has shown that bosentan reduced blood pressure, increases renal blood flow, and reduces protein excretion. This confirms the role of ET in renal disease.