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Clin. Cardiol. 23, 803–807 (2000)

Review

Bosentan and the Endothelin System in Congestive Heart Failure

Samer H. Ellahham, M.D., Vincent Charlon, Ph.D.,* Zaid Abassi, Ph.D.,† Karim A. Calis, Pharm.D.,† Wassim K. Choucair, M.D.

Division of Cardiology, Washington Hospital Center, Washington, D.C., USA; *Hoffmann-La Roche, Basel, Switzerland; †Warren G. Magnuson Clinical Center, National Institutes of Health, Bethesda, Maryland, USA

Summary: The endothelin system appears to play an important role in the pathophysiology of congestive heart failure (CHF). Endothelin receptor antagonists represent a novel class of agents that are being evaluated for their potential benefits in treating various cardiovascular disorders. Bosentan is an orally active endothelin receptor antagonist that has been studied for the treatment of CHF. Early clinical experience with bosentan has confirmed some benefits on hemodynamic parameters in patients with CHF. Its role in slowing the progression of the disease and improving survival remains to be elucidated.

Key words: endothelin, bosentan, chronic heart failure, receptor antagonist

Introduction

Congestive heart failure (CHF) is a progressive clinical syndrome resulting from cardiac dysfunction and characterized by signs and symptoms of intravascular and interstitial volume overload, including rales, edema, and shortness of breath, or manifestations of inadequate tissue perfusion, such as poor exercise tolerance or fatigue. The primary cause of CHF is impairment of the ability of the heart to fill or empty the left ventricle properly. In the United States, it is estimated that more than 2 million people have heart failure, and an additional 400,000 cases are diagnosed each year. The prevalence of CHF is increasing as the population continues to age. Mortality during the first 5 years from the time of diagnosis of CHF continues to be high, even among patients on the best available treatments. The last few years have brought significant advances in the understanding of the pathogenesis of CHF. Increasing evidence suggests a potential role of the endothelin system in the pathophysiology of CHF. With the recent discovery and development of endothelin receptor antagonists, the clinical potential of therapeutic agents that target the endothelin system is being actively evaluated. Bosentan, an orally active endothelin receptor antagonist, is to date the most well-studied of these agents for the treatment of CHF.

The Endothelin System

The endothelin family includes a group of three 21-amino acid peptides with very similar structures: endothelin-1 (ET-1), ET-2, and ET-3.1–4 Endothelin-1 is the most important endothelin synthesized in the blood vessels, mainly in endothelial cells.1 Almost 75% of ET-1 secreted by endothelial cells is directed toward the abluminal site (Fig. 1), where it can bind to specific receptors on the smooth muscle cells.5 Therefore, plasma ET-1 concentrations do not necessarily reflect endothelial cell production or the biological effect of ET-1 on smooth muscle cells.



Fig. 1 The endothelin system. NO = nitric oxide, PGI-2 = prostaglandin I-2, ANF = atrial natriuretic factor, ET = endothelin, LDL = low-density lipoprotein, ECE = endothelin-converting enzyme.


The development of specific ET receptor agonists and antagonists has led to the identification of two receptor subtypes in mammalian cells, ETA and ETB.6, 7 ETA receptors are present on smooth muscle cells and are responsible for the contractile response to ET-1. The vasoconstrictor effect persists even after ET-1 is removed from the receptor, probably because intracellular calcium concentrations remain elevated.8 Nitric oxide shortens the duration of this vasoconstriction by accelerating the decrease of intracellular calcium to its basal concentration.9, 10 ETB receptors are also present on vascular smooth muscle cells, where their activation produces vasoconstriction.12–14 Besides short-term regulation of vascular tone, ET-1 exerts a long-term modulation of cell function by affecting nuclear signal transduction mechanisms. Via these mechanisms, ET-1 may participate in the pathogenesis of proliferative disorders, such as atherosclerosis, and also in adaptive changes leading to vascular remodeling and cardiac hypertrophy as observed in CHF.

Endothelin-1 is the most potent endogenous vasoconstrictor. It is 100 times more potent than norepinephrine and 10 times more potent than angiotensin II on a molar basis. Endothelin-1-induced contraction in isolated blood vessels develops slowly, but is maintained for a longer time and is more resistant to removal than that evoked by any other vasoconstrictor. It also potentiates the vasoconstriction caused by norepinephrine and angiotensin II. The vascular effect of ET-1 in healthy humans has been investigated by local infusion of the peptide into the brachial artery. Endothelin-1 administration causes a dose-dependent vasoconstriction that is slow in onset and may be prevented by verapamil or nifedipine through blockade of voltage-operated calcium channels.8 Because of its high vasoconstrictor potency and long-lasting actions, the continuous release of small amounts of ET could contribute to the maintenance of vascular tone in normal physiology.15, 16 However, ETB receptors are present on endothelial cells,11 where they bind ET-1 and ET-3 with similar affinity, and their stimulation leads to a transient vasodilation, probably caused by increased production of nitric oxide (NO) and prostacyclin.

One postulated mechanism for the maintenance of basal tone is the production of vasoactive substances by endothelial cells. Endothelin-induced vascular contraction is effectively antagonized by endothelium-derived vasorelaxant substances, such as prostacyclin (PGI2) and the potent endogenous vasodilator NO.9, 10, 17 An imbalance between the production of ET and NO could lead to a pathologically elevated vascular tone. Moreover, the vasoconstricting properties of ET-1 are greatly enhanced in atherosclerotic vessels in which the opposing biological effect of nitric oxide is lost. In patients with CHF, higher plasma ET-1 levels were measured.22

A role of the ET system has been postulated in various conditions of disturbed vascular homeostasis, such as hypertension, coronary artery disease, and CHF.18–24 The suspected role of ET-1 in the pathophysiology of CHF relies on several observations: (1) increased local production of the peptide by vascular tissues; (2) increased vasoconstrictor activity because of increased responsiveness of target cells and reduced bioavailability of vasodilator substances; (3) beneficial effects of ET receptor antagonists in animal models and in humans; (4) significant correlation between ET plasma levels and exercise capacity,25 vascular resistance,26 and clinical prognosis in CHF.27

Endothelins are synthesized primarily in the endothelium, but also produced by kidney, brain, and lung. Many factors affect the expression and release of ETs such as shear stress and angiotensin II. Endothelin of either local or circulatory origin significantly contributes to the increased vascular resistance in the renal vasculature.28 The kidney is considered a major site of ET production and an important target organ of this peptide.29 The highest immunoreactive levels of ET in mammalian cells exist in the renal medulla. However, ET has also been localized in the renal cortex.30 The renal vasculature is preferentially sensitive to the vasoconstrictive effects of ET compared with other arteries or veins. In vitro studies utilizing isolated perfused kidney of either rat or rabbit demonstrated that ET is the most potent vasoconstrictor of renal arteries known to date, and its effects exceed those of other well-known vasoconstricting agents such as angiotensin II and norepinephrine. Exogenous endothelin markedly decreases renal blood flow as a result of a severe and sustained increase in renal vascular resistance. In contrast to the consistent effects of ET on the renal hemodynamics, its effects on the excretion of sodium and water are variable. Systemic infusions of high doses of ET results in antinatriuretic and antidiuretic effects, probably as a result of the decrease in the renal blood flow and glomerular filtration rate. In contrast, administration of low doses of the peptide induces natriuresis and diuresis. Also, administration of big ET, the precursor of ET, has been shown to cause effects similar to those of low doses of ET. This finding supports the notion that local ET acts in an autocrine/paracrine manner on the tubular epithelial cells where it inhibits sodium reabsorption, thereby inducing increased salt and water excretion.31–39

Bosentan

Bosentan is the most studied endothelin receptor antagonist to date. Several other compounds with various affinities for endothelin receptors have been described and are currently under evaluation for various clinical indications, including CHF. Bosentan (Ro 47-0203) is a low-molecular weight, orally active, specific antagonist of the endothelin receptors, ETA and ETB.40 Its chemical structure is depicted in Figure 2. The affinity of bosentan for the ETA receptor is about 100 times greater than that for the ETB receptor in cultured cells.40



Fig. 2 Chemical structure of bosentan.


Clinical Pharmacology

Following oral administration of an aqueous solution of bosentan, peak plasma concentrations were reached within 2–3 h.41 Bosentan exhibits a strong binding to plasma proteins, especially albumin.42 This drug has a low systemic plasma clearance and a terminal half-life of approximately 4 h. The clearance and volume of distribution of bosentan were 10 l/h and 0.2-0.3 l/kg, respectively, after an intravenous dose of 250 mg (systemic exposure comparable with 500 mg of the oral solution). Bosentan is metabolized by the liver and undergoes some biliary excretion.

Animal Studies

Bosentan improves hemodynamics, left ventricular function, and cardiac remodeling in animal models of chronic heart failure. Several factors may account for the cardioprotective effects of bosentan, including reduced cardiac preload and afterload, improved coronary blood flow, inhibition of neurohormonal activation, and chronic structural effects (inhibition of cardiac remodeling, cardiac hypertrophy and cardiac fibrosis) by direct inhibition of the actions of ET-1 on myocardial cells.43–51 In animal experiments, treatment with bosentan has been associated with beneficial pharmacologic effects, including vasodilation, prevention of cardiac remodeling, and improvement of ventricular performance. In several models of hypertension in rats, bosentan reduced blood pressure, and in the DOCA-salt model, decreased cardiac hypertrophy and fibrosis.43–45 These effects may result from the blockade of the cardiac actions of endothelins (i.e., myocardial hypertrophy,46 smooth muscle cell,47 and fibroblast48 proliferation, and protein [glycoproteins, thrombospondin, fibronectin] synthesis and secretion).49 In a rat model of heart failure, following acute coronary ligation, bosentan increased survival.50 Bosentan treatment resulted in decreased preload and afterload; increased cardiac output; and decreased left ventricular hypertrophy, left ventricular dilatation, and cardiac fibrosis. Heart rate was slightly decreased, and neurohormonal activation was reduced.50 In another model, in which heart failure results from aortocaval fistula, renal blood flow increased following treatment with bosentan, suggesting that the vasodilatory properties of bosentan could be beneficial in the treatment of altered renal hemodynamics associated with heart failure. In dogs with heart failure due to repeated coronary embolization, acute injection of bosentan had no significant effect on mean aortic blood pressure but reduced left ventricular end-diastolic pressure and systemic and pulmonary vascular resistance. Furthermore, bosentan increased cardiac output.51 Given these pharmacologic effects, bosentan could potentially reduce the pulmonary hypertension which occurs in CHF.

By blocking both ETA and ETB receptors, bosentan may be of particular benefit in heart failure. Indeed, stimulation of ETA receptors contributes to renal and systemic vasoconstriction as well as cardiac hypertrophy. On the other hand, ETB receptors are upregulated in the media of coronary arteries from patients with ischemic heart failure.52 The ETB receptors contribute to vasoconstriction in dogs and in humans with heart failure.52, 53 In addition, ETB receptors are important mediators of cardiac fibrosis54 and of aldosterone secretion.55 Oral administration of bosentan is associated with an increase in the levels of circulating ET-1 in various animal species.40 The mechanism leading to this reactive increase in ET-1 remains unknown, although it has been suggested that it may result from blockade of ETB receptors involved in the clearance of ETs from the circulation. The apparent absence of functional consequences of increased ET-1 concentrations may be due to complete inhibition of the ET system as result of bosentan's blockade of both ETA and ETB.

Human Studies

Three clinical studies have been reported to date including patients with moderate to severe chronic heart failure. In addition, one large dose-ranging study has been completed in patients with mild to moderate essential hypertension. All three studies were placebo-controlled, double-blind trials that provide the first evidence of the potential clinical benefits of bosentan.

In the study by Krum et al.,56 293 patients with mild to moderate hypertension were randomized to receive treatment with placebo, enalapril 20 mg once daily, or bosentan (100, 500, or 1,000 mg once daily, or 1,000 mg twice daily). Patients receiving bosentan exhibited blood pressure reductions similar to those receiving the angiotensin-converting enzyme (ACE) inhibitor enalapril. Heart rate, angiotensin II, renin, norepinephrine, and epinephrine plasma concentrations remained unchanged during therapy with bosentan, whereas ET-1 plasma levels increased by approximately 50% from baseline. Bosentan was well tolerated overall, with only a few reports of adverse effects. These adverse effects were ones commonly seen with other vasodilators and included headache, leg edema, and dizziness. Fewer than 5% of the patients receiving bosentan developed increases in hepatic transaminases that were asymptomatic and reversible. Concomitant changes in bilirubin serum concentrations were not reported.

In the study by Kiowski et al.,57 24 patients with CHF (New York Heart Association [NYHA] functional class III) in whom therapy for heart failure had been discontinued were studied. Patients received placebo or bosentan intravenously (100 mg followed by 200 mg 1 h later). Cardiac, pulmonary, and systemic hemodynamic parameters were assessed repeatedly for 2 h. Infusion of bosentan resulted in pronounced systemic, pulmonary, and venous vasodilation accompanied by an improvement in cardiac performance without reflex tachycardia. These effects were submaximal at the first 100-mg dose. Plasma concentrations of norepinephrine, angiotensin II, and renin remained unchanged, suggesting an absence of neurohormonal stimulation

The second study in patients with CHF was conducted in two phases. In Phase I, seven patients with CHF NYHA functional class III received bosentan 500 mg twice daily for 14 days in an open-label fashion.58 Hemodynamic and neurohormonal parameters were measured repeatedly after the first dose and after 14 days of therapy. All patients continued to receive their prestudy CHF medications. These included digoxin, diuretics, and ACE inhibitors in all patients. Bosentan therapy was well tolerated and was associated with a marked improvement in cardiac performance and decreased pulmonary resistance and systemic vascular resistance. Heart rate increased slightly at the initiation of therapy but was difficult to evaluate because of the absence of a control group. In Phase II,59 the same protocol was followed as in Phase I, but 24 patients received bosentan 1,000 mg twice daily, and 12 patients received placebo twice daily for 14 days in a double-blind fashion. The administration of the ACE inhibitor was delayed by 3 h on the days of repeated hemodynamic assessments. Statistically significant hemodynamic improvements were observed with the 1,000 mg dose of bosentan compared with placebo. As with the 500 mg dose used in Phase I, a slight increase in heart rate was observed during the first hours following administration of bosentan. However, a similar increase in heart rate was observed in patients receiving placebo, suggesting that this effect was related to the study protocol rather than to a true effect of bosentan. Norepinephrine, epinephrine, renin, and angiotensin II remained unchanged, and ET-1 increased in patients treated with bosentan. The most recent trial using bosentan in patients with CHF was the Research on Endothelin Antagonism in Chronic Heart Failure (REACH-1) trial.60 It demonstrated some worsening of heart failure upon initiation of therapy, as with the initiation of beta blockers in CHF. However, long-term therapy may have resulted in improved symptoms and reduced progression of heart failure. This study which included 370 patients with severe CHF (NYHA class IIIB and IV) was stopped following the decision to conduct future trials at a lower dose (due to elevations in hepatic transaminases); only 50% had reached the intended endpoint of 6 months. In the proportion of patients who received bosentan for 6 months, there was a significant clinical improvement and a reduction in risk of clinical events. Longer-term studies with a lower dose of bosentan in CHF have been launched.

Conclusions

In recent years, significant progress has been made in our understanding of the ET receptors and the role of the ET system in the pathophysiology of CHF. This research has led to the development of several selective and highly specific ET receptor antagonists. Bosentan is the most studied orally active endothelin receptor antagonist currently in clinical trials for the treatment of CHF. Early clinical experience with this compound has confirmed some short-term benefits, especially in terms of hemodynamic improvement. Endothelin receptor antagonism may also prove to be useful in essential hypertension, pulmonary hypertension, atherosclerosis, and nephropathy. However, long-term trials to investigate the effects of chronic inhibition of the ET system are needed. It is hoped that ET receptor antagonists such as bosentan will slow the progression of CHF and improve survival in patients who remain symptomatic despite optimal therapy with currently available pharmacologic treatments.

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This work was partially supported by the Medlantic Research Institute.

Address for reprints:
Samer H. Ellahham, M.D.
Division of Cardiology
Washington Hospital Center, Room 1103
110 Irving Street, N.W.
Washington, D.C. 20010, USA

Received: August 4, 1999
Accepted with revision: December 1, 1999


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