U P D A T E A R T I C L E JIACM 2004; 5(1): 55-9

Endothelins and Endothelin Receptor Antagonists

Sidhartha Das*, TK Mishra**, C Satpathy***, SN Routray***

Abstract

Endothelin-1 is a 21 amino acid peptide and is the most potent vasoconstrictor and pressor substance known. Generated by vascularendothelial cells in response to a variety of chemical and mechanical signals, endothelin-1 contributes to the pathophysiology ofconditions associated with sustained vasoconstriction, such as hypertension and heart failure. The endothelin receptor antagonistswere until recently regarded as drugs of great potential in patients with heart failure. However, trials like REACH-1, ENABLE, andRITZ-4 have not found this group of drugs to be beneficial. In contrast, bosentan, a non-selective endothelin receptor antagonist,has been proved to improve functional class and increase the time to clinical worsening in patients with pulmonary arterialhypertension.

* Senior Consultant and Professor, PG Department of Medicine, ** Assistant Professor, *** Lecturer,Department of Cardiology, SCB Medical College, Cuttack-753 001, Orissa.

Introduction

The vascular endothelium has been shown to produce a

variety of vasoactive substances that are crucial for the

regulation of vascular tone, both in health and disease.

Yanagisawa et al, were the first to purify the sequence and

clone the 21 amino acid structure of endothelin (ET) and

its mRNA from the culture supernatant of porcine aortic

endothelial cells in 19881. The aim of the present article is

to outline the biology of endothelin system, endothelin

receptors, and review the role of the endothelin receptor

antagonists in various diseases.

Endothelin system

Endothelin is synthesised not only in the endothelium but

also in the brain, lung, kidney, and some circulating cells2.

The human genome holds three distinct endothelin

genes, encoding the closely related products ET-1, ET-2,

and ET-33. While there is evidence that endothelin-2 (ET-

2) may possibly function as a mediator in the kidney and

that endothelin-3 (ET-3) may act as a mediator in the gut

and nervous system, endothelin-1 (ET-1) is the major

isoform generated in blood vessels and appears to be of

greatest relevance4.

Currently, regulation of endothelin synthesis is thought

to be primarily at the level of gene transcription, with de

novo production and release occurring in response to

endothelial cell stimulation4. Factors acting at this level to

stimulate ET-1 synthesis include vasoactive hormones,

inflammatory mediators, vascular shear stress, and

hypoxia. In contrast, factors like nitric oxide, natriuretic

peptides, and dilator prostanoids serve to inhibit ET-1

generation5 (Figure-1).

The product of ET-1 gene transcription is

preproendothelin-1, which undergoes cleavage to

generate big endothelin-1. Subsequent conversion to the

mature, biologically active peptide, ET-1, occurs through

the action of endothelin converting enzyme (ECE). ET-1,

once formed, acts as a paracrine and autocrine mediator

rather than an endocrine hormone, and its secretion by

the endothelial cells is largely abluminal, i.e., toward the

adjacent vascular smooth muscle. The half life of ET-1 in

blood is short (4 - 7 minutes), with clearance via receptor

binding and metabolism in the lungs and kidneys6.

Endothelin receptors and its blockers

The endothelins act on two receptor subtypes ETA and

ETB. ET-1 has a similar binding affinity for ET

A and ET

B

receptor but has a much higher binding affinity for the

ETA receptor than ET-2. In contrast, ET-1 and ET-3 have

equal affinity for ETB receptor4. The ET

A receptor

predominates on vascular smooth muscle cells and is

responsible for causing vasoconstriction in both large and

small blood vessels. It is also the major subtype in the

heart. The ETB receptors are present in endothelial and

vascular smooth cells and is predominantly found in brain,

lung, kidney, and aorta. The ETB receptors on endothelial

cells modulate vasoconstriction in response to ET-1

through the production of nitric oxide and prostacyclin.

The ETB receptors, present on vascular smooth muscle cells,

56 Journal, Indian Academy of Clinical Medicine Vol. 5, No. 1 January-March, 2004

can mediate vasoconstriction.

The understanding of the function of endothelin receptor

has been aided by the advent of specific pharmacological

antagonists. These endothelin receptor antagonists can

be classified into non-selective and selective ones. The

non-selective antagonists act on both ETA and ET

B

receptors. These include bosentan, tezosentan, and Tak-

044. The selective antagonists act either on ETA receptor

or ETB receptor only. The selective ET

A receptor antagonists

are sitaxentan, darusentan, and BQ-123 whereas BQ-288

is a selective ETB receptor antagonist.

The biological activity of ET-1

ET-1 has potent and long lasting vasopressor activity and

helps regulate the arterial and venous tone9. Local

selective ETA receptor blockade in the fore-arm

vasculature of healthy volunteers substantially increases

forearm blood flow, suggesting that endogenous

generation of ET contributes to maintenance of basal

vascular tone in humans10.

ET-1 has potent inotropic effects in vitro, although its effect

on overall cardiac function is more difficult to assess.

Systemic infusion of ET-1 in healthy volunteers and

patients with heart failure causes cardiac output to fall11.

This may be a secondary effect due to potent peripheral

and coronary vasoconstriction, rather than a negative

inotropic effect of ET-1 on cardiac myocytes12. BQ-123, an

ETA selective antagonist, has been shown to reduce left

ventricular contractility in humans, but has no effect on

patients with dilated cardiomyopthy13. This suggests that

ET-1 has a positive inotropic effect in normal subjects, but

this effect is lost in the failing human heart. Onishi et al

have shown that elevated levels of plasma ET-1 in chronic

heart failure may directly impair cardiac contractility and

hence contribute to the functional impairment seen in

chronic heart failure14.

In addition to its direct arterial and vasoconstrictor and

neuroendocrine action, ET-1 has been shown to enhance

conversion of angiotensin I to angiotensin II15 and adrenal

synthesis of adrenaline16. It can also augment plasma renin

activity17. ET-1 may amplify vasoconstrictor reflexes and

be of pathophysiological relevance even when plasma ET-

1 concentration is not clearly elevated12. ET-1 can also

stimulate vascular smooth muscle proliferation and

cardiac hypertrophy and is consequently thought to have

a role in myocardial and vascular remodelling18.

Given the diversity of actions of ET-1, it has clearly a role in

the pathophysiology of cardiovascular disease and has

emerged as a novel therapeutic target in this area. In

subsequent sections, we will focus on the role of endothelin

system in various cardiovascular diseases and the impact of

the endothelin receptor antagonists on them.

Congestive heart failure (CHF)

CHF is a disease process characterised by impaired left

ventricular function, increased peripheral and pulmonary

vascular resistance, and sodium and water retention19. In

patients with CHF, circulating ET-1 is elevated and

correlates with symptoms20 and haemodynamic severity21.

The plasma level of ET-1 precursor, big ET-1 is a strong

independent predictor of death22. Parallel to ET-1, the ETA

receptors are up-regulated in the failing human heart23.

In contrast, the ETB receptors are down-regulated24. The

vasoconstrictor response to exogenous ET-1 is blunted in

CHF as compared with healthy subjects, both in the arterial

and venous arm of the circulation.

The mechanisms leading to increased ET-1 expression in

CHF remain incompletely understood. The main source

of ET-1 seems to be the pulmonary vascular bed in CHF25.

ET-1 production is modulated by baroreflexes. Decreased

shear stress caused by low cardiac output contributes to

the elevated ET-1 release. Down-regulation of lung ETB

receptors, which are involved in clearance of ET-1, further

contributes to elevated circulating levels of ET-1 in CHF26.

In addition, other neurohumoral systems activated in CHF,

such as angiotensin II and catecholamines, also stimulate

ET-1 production27.

Endothelin receptor antagonists and CHF

Beneficial role of neurohumoral inhibitors like angiotensin

converting enzyme (ACE) inhibitors and -blockersprompted investigators to assess the utility of endothelin

receptor antagonists in patients with CHF.

Infusion of the mixed ETA/B

antagonist, bosentan, has been

shown to improve systemic and pulmonary

haemodynamics in patients with CHF28.

Journal, Indian Academy of Clinical Medicine Vol. 5, No. 1 January-March, 2004 57

A clinical trial, REACH-1 (Research on Endothelin

Antagonists in Chronic Heart Failure) investigating the

long term effects of bosentan on clinical events in CHF

showed an improvement in symptoms29. Unfortunately,

the trial had to be stopped prematurely because of

elevation of liver transaminases.

The endothelin antagonism with Bosentan and Lowering

of Events (ENABLE) investigators studied 1,613 patients

with CHF and could not demonstrate any improvement

in mortality or hospitalisations due to heart failure30.

Results of RITZ-4 (Randomised Intravenous Tezosentan

Study) have been published recently31. Tezosentan,

another non-selective ETA/B

receptor antagonist, was tried

in patients with acute decompensated heart failure

associated with acute coronary syndrome. There was no

significant difference in deaths, worsening heart failure,

and recurrent ischaemia between the tezosentan group

when compared with the placebo group.

As the non-selective ETA/B

receptor antagonists proved to

be ineffective in patients with CHF, therapy with selective

ETA agent was tried. It was thought that as ET

B receptor

causes beneficial nitric oxide mediated vasodilation a

selective ETA receptor antagonist may be beneficial by

leaving ETB receptor untouched. However, in a study of

157 patients, the selective agent darusentan caused

haemodynamic improvement, but there was no

improvement in symptoms32. Rather, there was a trend

towards increased mortality.

The explanations for these negative results remain

speculative. It is possible that the dose administered in

RITZ-4 may have been too high and that the study

was relatively small and underpowered. More

likely benefits cannot be accrued by further

antagonising the neurohormonal system after

administration of b-blocker and ACE-inhibitor33.

Role of endothelin receptor antagonistsin hypertension

Human data on the role of endothelin in

hypertension is conflicting2. Systemic administration

of TAK 044, a non-selective ETA/B

receptor

antagonist has been shown to reduce systemic

vascular resistance and lower blood pressure4. The

potential additional benefits of the ET receptor

antagonists on atherogenesis, cardiac and vascular

hypertrophy, and progression of renal impairment

makes this group of drugs an attractive option as

antihypertensive agents. Research is going on in

this regard.

Role of endothelin receptor antagonistsin pulmonary arterial hypertension

Bosentan, an orally active non-selective

endothelin receptor antagonist, has been proved

to be effective in the treatment of pulmonary

arterial hypertension (PAH). The drug is now

approved in USA, Canada, and the European Union

countries for treatment of PAH associated with

Fig. 1 : Factors that alter endothelin-1 (ET-1) synthesis and the pathway for ET-1 generation.IL-1, interleukin-1; TGF-, transforming growth factor ; LDL, low density lipoprotein; ANP,BNP, CNP, atrial, brain and c-type natriuretic peptides, respectively.

58 Journal, Indian Academy of Clinical Medicine Vol. 5, No. 1 January-March, 2004

collagen disease like scleroderma4.

BREATH-1 (Bosentan : Randomised Trial of Endothelin

Receptor antagonist Therapy for Pulmonary Arterial

Hypertension) trial was a first large scale (n = 213), double

blind, placebo-controlled study of an orally active

endothelin receptor antagonist in PAH34. In BREATH-1, 16

weeks of bosentan therapy resulted in significant

symptomatic improvement in the overall population.

There was also significant increase in time to clinical

worsening. Favourable results were noted also in

combined end-points that included lung transplantation,

hospitalisation, and death. Headache was the most

common adverse effect, seen in 19%. Abnormal hepatic

function was noted in 9%.

At present BREATH-2 trial is in progress. This trial is

evaluating the effect of bosentan in combination with

intravenous epoprostenol in patients with severe PAH35.

Preliminary results from BREATH-3, a study of the

pharmacokinetics and tolerability of bosentan in

paediatric patients with PAH, appear positive35.

Thus, bosentan is definitely useful in patients having

primary pulmonary arterial hypertension or PAH

associated with collagen vascular disease like scleroderma.

The drug is well tolerated at dose of 125mg twice daily.

Future directions

Research is being done to assess the role of endothelins

and ET receptor antagonists in various diseases like

atherosclerosis, subarachnoid haemorrhage, and

proteinuric nephropathies. Though the selective ETA

receptor antagonist holds exciting promise, it may worsen

renal vasoconstriction. Moreover, the safety side of

endothelin receptor antagonists is yet to be fully

addressed. Inhibition of endothelin during embryogenesis

can be devastating.

Summary and conclusions

Endothelin-1 is a potent vasoconstricting agent that acts

as a local autocrine and paracrine mediator. ET is the most

potent and sustained vasoconstrictor and pressor

substance yet identified. Abnormalities of the endothelin

system occur in a range of diseases associated with

vasoconstriction, vasospasm, and vascular hypertrophy. ET

receptor antagonists were only till recently regarded as a

drug of great promise in patients with CHF. However, the

result of trial like REACH-1 has been disappointing in this

regard. Still, an orally active selective ETA receptor

antagonist may tilt the balance in favour of its use in CHF

patients.

Role of ET receptor antagonists in PAH (Primary or

secondary to collagen vascular disease) is fairly well

established. Now, bosentan is an approved ET receptor

antagonist for use in patients with PAH associated with

collagen diseases.

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A N N O U N C E M E N T

IACM Credentials Committee 2003-2004

1. VN Kaushal Chairman2. SK Sharma Convener3. Nitya Nand Ex-Officio Member4. KK Dang Member5. Ajay Kumar Member6. HK Aggarwal Member7. DP Agarwal Member8. MS Gupta Member