Progress in anti- endothelin therapy in the treatment of pulmonary arterial hypertension and heart failure

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(Department of Cardiology，Zhongda Hospital，Southeast University，Nanjing 210009，China)

1 Introduction

1.1 Endothelin (ET) family

The 21- amino acid peptide ET- 1 is the predominant isoform of the endothelin peptide family， which also includes ET- 2， ET- 3， and ET- 4 (vasoactive intestinal contracting peptide). It exerts various biological effects， including vasoconstriction and the stimulation of cell proliferation in tissues both within and outside of the cardiovascular system[1]. ET- 1 is the predominant peptide in the cardiovascular system. ET- 1 is produced by vascular endothelial cells， smooth muscle cells， airway epithelial cells， macrophages， fibroblasts， cardiac myocytes， brain neurons， cells of cancer， mast cells， and pancreatic islets. ET- 2 is expressed in the ovary and intestinal epithelial cells. ET- 3 is found in endothelial cells and intestinal epithelial cells. ET- 3 mediates release of vasodilators， including nitric oxide (NO) and prostacyclin[2]. It is well established that the endothelin system contributes to the key physiological functions in normal tissue， acting as modulators of vasomotor tone， tissue differentiation， development， cell proliferation and hormone production. Perhaps the most important action is regulation of vascular physiology and participation in the pathophysiology of vascular disease. The discovery of several compounds acting as endothelin receptor antagonists (ERAs) has prompted research toward their use in clinical practice.

1.2 Endothelin receptors

ET- 1 binds two seven- transmembrane G protein coupled receptor subtypes， endothelin A (ETA)， and endothelin B (ETB). ETA receptors are located in vascular smooth muscle and mediate vasoconstriction. These receptors are also found in the endothelium where they can mediate vasodilatation. ETB receptors are involved in the control of resting tonic skin blood flow (SkBF) in both men and women. However， the ETB receptor antagonist BQ- 788 induced vasodilatation in men but vasoconstriction in women. The authors interpreted these findings to indicate that in cutaneous vessels the ETB receptors in men are located primarily in the endothelium and mediate vasodilatation[3].

The important discovery and identification of these receptors (ETA and ETB) indicated that the endothelin isoforms exert their physiological effects in a receptor- mediated fashion. The two endothelin receptors can be distinguished pharmacologically by the order of their affinity for the three endothelin isopeptides; ETA receptor is ET- 1- selective， with an affinity order of ET- 1 ≥ET- 2>ET- 3， whereas ETB receptor exhibits similar affinities for all three isopeptides[4]. These receptors are both distributed in various tissues and cells， but with different levels of expression， suggesting the presence of a multifunctional ET system. ETA receptors are located on vascular smooth muscle cells， and when activated， produce a sustained vasoconstriction that has a slow onset. In contrast， ETB receptors are located on both endothelial and vascular smooth muscle cells. Activation of ETB receptors on endothelial cells causes vasodilation through the release of vasodilators acting on smooth muscle cells[5]. Moreover， ETB receptor inhibits cell growth and vasoconstriction in the vascular system and functions as a clearance receptor， based on the evidence that selective ETB receptor blockade inhibits the accumulation of intravenously administered radiolabeled ET- 1 in tissue[6].

1.3 Production and clearance of ET

ET is formed by cleaving amino acids from pre- proendothelin (ProET) by endopeptidase to generate 38 to 39 aminoacids peptides called as big ETs which are biologically almost inactive precursors， by a membrane- bound Zn- dependent metalloprotease， endothelin- converting enzymes (ECEs). Big ETs are further converted into mature ETs by ECEs[7]. ETB has been regarded as a “clearance” receptor. Plasma ET- 1 has a half- life of less than 5 minutes in plasma， which is surprisingly short as a result of ETB receptors removing ET- 1 from the circulation. The main clearance is in the lungs， kidneys and liver[8]. The ETB receptor- mediated clearance mechanism is particularly important in the lung， which clears about 80% of circulating ET- 1. Clearance involves either catabolism via the neutral endopeptidase neprilysin， or lysosomal degradation in response to receptor- mediated uptake. It is likely that much higher concentrations of ET- 1 occur at the junctions between endothelial and vascular smooth muscle cells and that at least some of the plasma ET- 1 represents overspill from this site. Therefore， plasma levels of ET- 1 in pathological states may represent an unreliable index of vascular endothelin activity[9]. Conversely， urinary concentrations of ET- 1 may reflect local renal endothelin activity， but not systemic endothelin function．

1.4 ECEs

ECEs are expressed in the blood vessel epithelium and are members of the M13 family of zinc- metalloproteases; membrane bound endopeptidases that preferentially cleave at the amino side of hydrophobic residues. ECEs are characterized by their ability to hydrolyze a family of biologically inactive intermediates， big ETs (big ET- 1，- 2 and- 3) precisely at the Trp21- Val/Ile22 bond to form ET- 1， ET- 2 and ET- 3. So far， three members of the ECE family have been identified; Endothelin- converting enzyme- 1(ECE- 1)， endothelin- converting enzyme- 2(ECE- 2)， and endothelin- converting enzyme- 3(ECE- 3)， each encoded by a different gene. ECE gene knockout studies suggest that ECE- 1 is the major functional ECE for all three endothelin isoformsinvivo[10].

Inhibition of ECE blocks the conversion of big ET- 1 to ET- 1 and is associated with vasodilatation and hypotension. Plasma membrane ECE activity could be inhibited by phosphoramidon (potent inhibitor of ECE)， thiorphan (metalloprotease inhibitor) and phenanthroline (inhibitor of zinc- dependent proteases). The ECE activity may emerge as a possible target in preventing ET- 1- induced increase in cardiovascular diseases[11]. Most ECE inhibitors currently under development also inhibit neutral endopeptidase (NEP)， so that they have the combined effect of inhibiting ET- 1 production and potentiating endogenous vasodilator mediators metabolised by NEP， such as atrial natriuretic peptide and bradykinin. This inhibition might also be extended to include triple inhibition of ECE， NEP， and angiotensin converting enzyme (ACE)， with a potentially beneficial action in pathological conditions such as heart failure and renal dysfunction. In addition， by blocking ET- 1 generation， ECE inhibitors would act as mixed ETA/ETB antagonists and， perhaps importantly， would leave ET- 1 clearance unaffected. However， so far， less progress has been made clinically with the development of drugs based on ECE inhibition than endothelin receptor antagonism[12].

1.5 ET antagonism in cardiovascular disease

The ET system can be antagonized by the blockade of ET receptors and by the inhibition of the ECE. Endothelin receptor blockers or ERAs have emerged as a potential therapeutic option that operates on the basis of physiological and pathophysiological effects of endothelin. An ERA is a drug that blocks endothelin receptors. Three main kinds of ERAs exist:selective ETA receptor antagonists which affect ETA receptors， dual antagonists which affect both ETA and ETB receptors. and selective ETB receptor antagonists which affect ETB receptors are used in research but have not yet reached the clinical trial stage[13]．

The inhibition of the ECE reduces the production of ET- 1 but the effectiveness of these drugs is limited by independent pathways contributing to ET- 1 formation， such as chymase and metalloproteases. Therefore， the more efficient way to antagonize the ET system is the use of ET- 1 receptor antagonists that can block either ETA- or ETA- and ETB receptors. Currently， several peptides and nonpeptide compounds that block ET receptors are available， and some have been tested in both animal models and clinical trials in patients with pulmonary arterial hypertension (PAH)[14]．

The vascular endothelium is a source of vasoactive substances such as NO[1]as well as ET- 1 that modulate vascular smooth muscle tone and proliferation. NO is a potent vasodilator and is formed fromL- arginine by endothelial NO synthase. NO as well as prostacyclin inhibit ET- 1 production and furthermore， reduce ET- 1 induced vasoconstriction in vascular smooth muscle cells of small arteries[15]. This indicates that NO protects small arteries against contractions to ET- 1 under physiological conditions. In hypercholesterolemia， a major risk factor predisposing to atherosclerosis， endothelial dysfunction is present before structural vascular changes occurs[16]. The purpose of this review is to provide insight into the progress of anti- endothelin therapy in PAH and heart failure (HF) treatment therapies．

2 Anti- endothelin therapy in PAH

PAH is defined as a group of diseases characterized by a progressive increase of pulmonary vascular resistance leading to right ventricular failure and death[17]. The pathogenesis of PAH involves multiple and complex mechanisms including endothelial dysfunction in the pulmonary circulation， resulting in pulmonary vasoconstriction and vascular remodeling. Increased plasma levels of ET- 1 were detected in both experimental models of PAH and， in patients with some forms of pulmonary hypertension a few years after the discovery of the peptide. Elevated ET- 1 plasma levels are found in patients suffering from PAH and are correlated with poor prognosis. Endothelin receptor antagonism has been shown to improve hemodynamics and exercise tolerance in these patients[18]. Activation of ETA and ETB on pulmonary artery smooth muscle cells induce proliferation and vasoconstriction， whereas activation of ETB on pulmonary endothelial cells leads to release of NO and prostacyclin and participate to the clearance of circulating ET- 1. By ligating the ETA， ET- 1 intracellular calcium concentrations increase and activates the protein kinase C pathway. ET- 1 is a potent pulmonary vasoconstrictor and stimulates mitosis of arterial smooth muscle cells， thus contributing to pulmonary vascular remodeling. Pulmonary and plasma levels of ET- 1 are elevated in human PAH and in experimental animal models of PAH. The therapeutic efficacy of endothelin receptor antagonists has been demonstrated in clinical trials in the pathophysiology of PAH[19]．

2.1 Bosentan

Bosentan (a nonpeptide)， an oral active dual ETA and ETB antagonist is the most widely tested ET receptor antagonist in PAH. Bosentan has been evaluated in PAH in five RCT’s (Pilot， BREATHE- 1， BREATHE- 2， BREATHE- 5， and EARLY) that have shown improvement in exercise capacity， functional class， haemodynamic， echocardiographic and Doppler variables， and time to clinical worsening. The first placebo- controlled study included 32 patients affected by idiopathic PAH presenting with PAH showing significant exercise improvement with a gain of 76 meters after three months of treatment with bosentan as compared to placebo. The BREATHE studies have confirmed the efficacity of treatment with bosentan in more than 200 assessed patients in NYHA functional class Ⅲ or Ⅳ compared to placebo in a three month trial. After three months of treatment with bosentan， improvements in NYHA functional class were observed in 42% of patients receiving bosentan compared with 30% in the placebo arm. 6- MWD was improved overall to 44 meters in favor of bosentan. Furthermore， delayed time to clinical worsening was also noted as well as better results in dyspnea scores[20].

Subsequently published data of treatment with bosentan suggested persistent improvements in pulmonary hemodynamics， exercise capacity and modified NYHA functional class and possibly survival rate of patients. Results from the EARLY study (double- blind， randomised controlled 6 months trial) showed that the effect of bosentan in patients with mildly symptomatic PAH could be beneficial for PAH patients in NYHA functional class Ⅱ[21]B228#B228. In a rat model of advanced flow- associated PAH， dual endothelin receptor antagonism using bosentan improved both pulmonary vascular and cardiac remodeling and was associated with improved pulmonary vascular， cardiac remodeling and cardiovascular function[22]. A study to investigate the effect of bosentan on pulmonary hypertension secondary to systolic heart failure showed that bosentan significantly decreased systemic arterial pressures， pulmonary pressures， pulmonary vascular resistance (PVR) and the transpulmonary gradient thereby allowing them to be considered candidates for heart transplantation and left ventricular assist device implantation[23]. Treatment is started at a dose of 62.5 mg twice a day and increased to the dose of 125 mg twice daily after one month of treatment. Bosentan at a dose of 125 mg twice per day improved exercise tolerance and extended the time to clinical worsening in patients with PAH of NYHA functional class class Ⅲ and Ⅳ severity. One accepted common side effect of treatment is increase in liver enzymes; this is why monthly monitoring of liver transaminases is mandatory in all patients receiving bosentan.

2.2 Ambrisentan

Ambrisentan is a selective ETA receptor antagonist administrated once daily at a dose of 5 mg or 10 mg. Two large RCTs (ARIES Ⅰ and Ⅱ， i.e. ambrisentan in pulmonary hypertension， randomized， double blind， placebo- controlled multicenter， efficacy studies Ⅰ and Ⅱ) have demonstrated efficacy on symptoms， haemodynamics and time to clinical worsening of patients with idiopathic PAH and associated to connective tissue disease (CTD) and HIV infection. An extension study of the ARIES study is the recently published ARIES- E study by Klinger and colleagues. They followed patients for a mean period of 60 weeks in where patients underwent hemodynamic evaluation. The authors concluded that treatment with ambrisentan leads to hemodynamic stability in PAH patients[24].

2.3 Sitaxsentan

Sitaxsentan is a selective ETA receptor antagonist， has shown to improve 6- minute walk distance (6MWD)， functional class， pulmonary vascular resistance， and cardiac index after 12 weeks of treatment in patients with PAH. The incidence of liver function abnormalities was much lower for the 100- mg dose compared with the 300- mg dose， suggesting a distinct dose response for safety and tolerability[19]. In an open- label study to investigated the effects on hemodynamics and 6- minute walk test distance in 6 children and 14 adults who were given sitaxsentan at doses of 100 to 500 mg twice per day for 12 weeks showed exercise distance walked in 6 minutes improved significantly， from 466.132 m at baseline to 507.153 m at week 12. Right heart catheterization at the end of the trial showed a 17% reduction in mean pulmonary artery pressure and a 22% increase in cardiac index. But serious liver function abnormalities occurred in 2 of the 20 patients in this study (including 1 death related to fulminant hepatic failure). Selective antagonism of ETA receptors may be more beneficial than antagonism of both ETA and ETB receptors for the treatment of PAH by blocking the vasoconstrictor effects of ETA while maintaining the vasodilator and clearance functions of ETB receptors[25].

2.4 Macitentan

Macitentan， is a novel， highly potent， tissue- targeting dual ERA characterized by a high lipophilicity. Macitentan has been tested in the largest， long- term， event- driven randomized， controlled study (Seraphin， study with an ERA in PAH to improve clinical outcome)[26]. The Seraphin study was designed to evaluate the efficacy and safety of macitentan through the primary endpoint of time to first morbidity and all- cause mortality event in 742 patients with symptomatic PAH. Macitentan has met its primary endpoint， decreasing the risk of a morbidity/mortality event over the treatment period versus placebo. Secondary efficacy endpoints， including change from baseline to month six in 6- MWD， change from baseline to month six in NYHA FC and time- over the whole treatment period- to either death due to PAH or hospitalization due to PAH， also showed a dose- dependent effect. Treatment with macitentan in this study was well tolerated; headache， nausea and vomiting were reported as minor adverse events[27]. The safety set comprised 741 patients， who received at least one dose of study treatment (placebo， 3 mg or 10 mg). The number of adverse events reported and patients discontinuing treatment due to adverse events was similar across all groups. Similar elevations of liver aminotransferases greater than three times the upper limit of normal were observed in all groups. In addition， no difference was observed between macitentan and placebo in terms of fluid retention (edema). A decrease in hemoglobin (adverse event) was observed more frequently on macitentan than placebo， with no difference in treatment discontinuation between groups. In the Seraphin study， 10 mg of macitentan significantly reduced the risk of morbidity and mortality by 45% (P<0.001). Macitentan was well tolerated by the patients in this trial and， notably， adverse events commonly associated with the ERA drug class (elevated liver aminotransferases and peripheral edema) occurred at a similar rate to patients who received placebo across all groups[28].

Macitentan has sustained receptor binding properties， and optimised physicochemical properties leading to enhanced tissue penetration. Oral macitentan 10 mg once daily for the treatment of PAH. Studies have shown that macitentan has a limited drug- drug interaction profile. It also has no significant inhibitory effects on hepatic bile salt transport and， therefore， has the potential for a favourable liver safety profile[29- 31].

2.5 Combination therapy

Another therapeutic option in PAH is to combine drugs with different mechanisms of action， in order to optimize clinical benefit while minimizing side effects. The BREATHE- 2 trial compared the association of epoprostenol and oral bosentan with epoprostenol and placebo among 33 patients with severe PAH over a 12 weeks period[32]. Reduction in pulmonary vascular resistance was greater with combination therapy， although it did not reach statistical significance. Furthermore， no benefit could be shown on the 6- MWD with combination therapy compared to epoprostenol alone (+68 m and+74 m， respectively). These results however may be related to the small number of patients and the short term follow up in the context of the addition of a treatment to a drug already known to bring an important benefit in severe PAH patients. The combination of iloprost to bosentan in patients with idiopathic PAH or associated PAH in NYHA functional class Ⅲ was shown to be significantly better than placebo and bosentan in terms of 6- MWD (+26 m)， NYHA functional class and on post inhalation hemodynamic parameters (PVR- 26.4%)[33]. Several uncontrolled studies evaluated the efficacy of other associations with encouraging results. The open- label sequential addition of bosentan or sildenafil to epoprostenol， treprostinil or iloprost was shown to be of interest. Also， the association of ERA and PDE5， both available in oral form， offers an interesting option[34].

3 Anti- endothelin therapy in HF

Both ETA and ETB receptors are expressed throughout the cardiac muscle， including the coronary vasculature. Cardiomyocytes express predominantly ETA receptors while both receptors are equally represented on cardiofibroblasts. Activation of the ETA receptor results in increased contractility[35]. In consequence， overexpression of ET- 1 in mice reduces time of diastolic relaxation. The L- type Ca2+channels， which are responsible for the main depolarizing current， provide the response to ET- 1 after activation of ETA receptors， protein kinase C (PKC) and the Ca2+/calmodulin kinase[36]. ET- 1 induces an alkalization of the myocytes through an activation of the Na+/H+exchanger by a mechanism involving the DAG- stimulated PKC and tyrosine kinases. This alkalization increases the sensitivity of the myofilament to Ca2+. Endogenous ET- 1 is also responsible for the positive inotropic effect of angiotensin Ⅱ via the production of reactive oxygen species. In the mouse heart， endogenous ET- 1 may increase and decrease contractility via ETA and ETB receptors， respectively[37]. Moreover， the actual knowledge suggests that the effects of endogenous ET- 1 may differ depending on the pathological situations. ET- 1 has been showed to induce a negative inotropic effect reportedly after activation of ETB receptors and consecutive activation of Na+/Ca2+exchangers. This may be the case in the failing heart， where Na+/Ca2+exchanger are overexpressed[38].

3.1 Endothelin antagonists in congestive HF (CHF)

ET- 1 is increased in patients with congestive HF and appears to be a prognostic marker that strongly is correlated with the severity of disease. Experimental evidence suggests that endothelin substantially contributes to left ventricular remodelling and progression of HF. Plasma ET- 1 levels are increased in patients with HF， independent of the aetiology， and correlate with the severity of the disease. Intracoronary injection of the ETA receptor antagonist BQ- 123 to investigate the local effect of ET- 1 on the cardiac muscle independently from systemic effects on the vasculature revealed that， in healthy humans， the positive inotropic effect of ET- 1 is rather modest and that in patients with HF， blocking endogenous ET- 1 actions on ETA receptors tends to increase contractility[39]. Local intra- arterial administration of BQ- 123 in healthy subjects， induced a forearm blood flow increase， suggesting that ET- 1 contributes to the maintenance of basal vascular tone. A study on tezosentan， an intravenous dual ERA， in patients with class Ⅲ to Ⅳ congestive HF demonstrated that tezosentan can be safely administered to patients with moderate to severe HF and that by virtue of its ability to antagonize the effects of endothelin- 1， it induced vasodilatory responses leading to a significant improvement in cardiac index[40].

In a study to examine the effect of bosentan in patients with pulmonary hypertension secondary to HF， comparable dosages of bosentan were used in post- infarction rat model with pulmonary hypertension secondary to HF. This demonstrated no beneficial long- term effects on cardiac and pulmonary remodelling despite the use of sensitive measurements[41]. The strengths of this research are its integral approach， assessing both cardiac and pulmonary remodelling by functional and morphological parameters. Their findings， therefore， challenge the pathophysiological rationale for the use ERA in HF with secondary pulmonary hypertension. These results do not contradict the possibility that the activation of the ET system may be deleterious and contribute to the pathophysiology of PH associated with HF. The demonstration that pharmacological blockade of the ET receptors has no effect could suggest a minor role， or that alternate pathways play a more dominant role， such as the renin- angiotensin system[42].

3.2 Endothelin antagonists in CHF

Circulating levels of ET- 1 are elevated in patients with CHF caused by a combination of increased production and reduced clearance of ET- 1. Furthermore， activation of the ET system contributes to peripheral vasoconstriction and impaired endothelial function in patients with CHF， and plasma ET- 1 levels were shown to predict survival. In the majority of animal models， the expression of ET- 1 in the left ventricle was found to be upregulated. Some studies found upregulation of either ETA or ETB receptors， and sometimes both receptors. In humans， left ventricular expression of ET- 1 and ETA receptors were found to be increased， with no change in ETB receptors[43]. Despite the upregulation of ETA receptors， the inotropic response to ET- 1 is reduced in failing hearts， indicating reduced post- receptor signaling efficiency.

Most studies in animal models of CHF demonstrated beneficial effects of selective ETA receptor blockade on cardiac function and overall mortality， and the same is true for most studies with nonselective receptor blockade. Only two of these animal studies compared selective versus nonselective blockade directly. Both studies found that the effects on hemodynamics and cardiac function were similar. Early studies showed that endothelin blockade has beneficial hemodynamic effects in humans with CHF. Two recent clinical studies compared the hemodynamic effects of selective versus nonselective endothelin receptor blockade directly. In the first study， similar reductions of pulmonary and peripheral vascular resistance and increases in cardiac output were produced by the two types of antagonists， whereas in the second study[44]， these effects were found to be more pronounced with selective ETA receptor blockade. In both studies， circulating ET- 1 levels were only raised by nonselective receptor blockade. The clinical significance of a further increase in plasma ET- 1 levels with nonselective blockade is unknown， but may be of minor importance when ETA receptors are fully blocked.

The ENCOR trial was the first trial to examine the effects of endothelin receptor blockade in patients with CHF. Treatment with the nonselective antagonist， enrasentan， unfortunately led to a deterioration in clinical status and a tendency to increase mortality， as compared with the placebo treated group. A more recent study also showed that enrasentan has adverse effects on left ventricular structure in patients with asymptomatic left ventricular systolic dysfunction[45]. In the REACH- 1 trial， patients with CHF were treated with bosentan， but the study was interrupted early because of a high incidence of elevated liver enzymes. In the subsequent ENABLE 1 and 2 trials， a lower dose of 125 mg bid reduced hepatotoxicity did not improve outcome as compared to placebo. Especially in the first 2 weeks of treatment， bosentan led to a high incidence of fluid retention and edema. So far， two trials have examined the long- term effects of selective ETA receptor blockade in patients with CHF.

In the HEAT trial， the hemodynamic effects of selective ETA receptor blockade with darusentan were determined before and after 3 weeks of treatment. Although there was a significant increase in cardiac output with active treatment， major safety concerns were raised following 4 deaths in the darusentan treated group. In addition， this study showed an increase in plasma ET- 1 levels at the higher doses of darusentan， indicating that blockade of the ETA receptor can increase circulating ET- 1 levels in CHF. The EARTH trial failed to detect a beneficial effect of 6 months treatment with darusentan on left ventricular remodeling in patients with CHF[46]. At this point， it is not clear why， but the beneficial effects in the acute hemodynamic studies of either selective or non- selective ET antagonists do not appear to translate into clinical benefits during long- term treatment. In addition， tissue endothelin levels and endothelin receptors are upregulated in myocardium from animals and humans with heart failure. In several model experiments of left ventricular remodelling and/or heart failure， treatment with nonselective ET- A and- B as well as selective ET- A antagonists exerted beneficial cardiovascular effects. In patients with HF， short- term studies of treatment with endothelin antagonists demonstrated an improvement of haemodynamic parameters; however， long- term treatment with these drugs did not significantly improve combined morbidity/mortality endpoints. The Endothelin- A Receptor Antagonist Trial in HF (EARTH trial) in patients with chronic heart failure， showed that the selective ET- A receptor antagonist darusentan did not significantly affect left ventricular remodelling as assessed by cardiac magnetic resonance imaging. Potential reasons for the lack of beneficial effects of long- term treatment with ET antagonists in patients with HF include:firstly adverse effects on left ventricular healing have been observed when endothelin antagonist therapy was introduced early after myocardial infarction in rats， secondly the role of the ET- B receptor in the pathophysiology of HF and remodelling processes has not been clearly defined. Selective ETB blockade have been indicated to cause potentially deleterious vasoconstriction in conditions of chronic HF.

A previous study suggested that the effects of endothelin receptor blockade， beneficial or harmful， may crucially depend on the stage of CHF when therapy is initiated. These investigators also reported that selective ETA receptor blockade during an early stage of CHF caused sustained sodium retention by activating the renin- angiotensin system[47]. Another recent study describes a new genetic model of CHF in mice produced by cardiac- specific overexpression of ET- 1. The mice suffered cardiac hypertrophy， inflammation， dilation and subsequently death. It has to be noted， however， that the levels of ET- 1 in the hearts of the transgenic mice were 10 times greater than in the control mice compared to 3- fold increases reported in humans with CHF[48]. Although highly problematic given the negative findings in human CHF， this study raises the question of whether there could be a role for endothelin blockade in inflammatory cardiac disease， such as in the early stages of virally induced cardiomyopathies．

3.3 Endothelin antagonists in acute HF

Several small studies have shown beneficial effects of the non- selective ERA， tezosentan， on hemodynamic parameters in patients with acute HF. The larger Value of Endothelin Receptor Inhibition with Tezosentan in Acute Heart Failure Studies (VERITAS) trial then examined the effect of tezosentan on the mortality of patients hospitalized with acute HF. Similar to the situation in chronic HF， the trial had to be discontinued prematurely because of lack of a beneficial effect on mortality[40].

4 Conclusion

This review indicates clinical efficacy of ET antagonism in PAH and HF but up to date， no disease entity has been attributed solely to an abnormality in ET alone. Consequently， it would be unrealistic to expect the use of ET antagonists to result completely in disease reversal. ET antagonists， however， have been studied in clinical trials involving a wide spectrum of cardiovascular diseases though some results are still argumentative due to discontinuation of therapy. ERAs predispose to hepatic toxicity as manifest by an increase in transaminases. These are generally reversible on discontinuation of therapy. Given the redundant pathways that control vascular tone and growth， it could be that ET antagonism will ultimately be used as part of a combined regimen. Future clinical studies to explore this potential are warranted.

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