Acute endothelin A receptor blockade causes selective pulmonary vasodilation in patients with chronic heart failure

Sitaxentan

Sitaxentan is a highly selective endothelin (ET)(A) receptor antagonist, with an approximately 6500 higher affinity for ET(A) than ET(B) receptors. In pulmonary arterial hypertension (PAH), elevated ET-1 levels are strongly correlated with disease severity and prognosis. Sitaxentan 100 mg once daily was efficacious in the management of moderate to severe PAH in the pivotal, 12-18 week, large (n > or = 98), well designed, placebo-controlled STRIDE-1, -2 and -4 trials. In the STRIDE-1 and -2 trials (the majority of patients had New York Heart Association [NYHA]/WHO functional class III PAH), sitaxentan-treated patients experienced significantly greater improvements from baseline in distance walked over 6 minutes (6MWD; primary endpoint in STRIDE-2) and in NYHA/WHO functional class than placebo recipients. In STRIDE-4, although there was no between-group difference in terms of improvements in 6MWD in the primary analysis of patients across all WHO functional classes (61% were functional class II) [primary endpoint], improvements in 6MWD significantly favoured sitaxentan versus placebo-treated patients in a post hoc subgroup analysis of those with WHO functional class III or IV disease. The beneficial effects of sitaxentan therapy on exercise capacity and NYHA/WHO functional class were maintained after up to 2 years' treatment. Treatment with sitaxentan for up to 2 years was generally well tolerated in clinical trials.

Capillary pressure-induced lung injury: fact or fiction?

Lung capillary pressure in healthy humans at rest ranges between 6 and 10 mmHg. At maximal effort or in pathophysiological conditions such as left sided heart disease or massive pulmonary vasoconstriction, for example in high-altitude pulmonary disease, capillary pressure may be markedly elevated. Increased capillary pressure directly affects transendothelial fluid dynamics and thus results in the formation of hydrostatic lung edema. Excessive pressure increases may cause capillary stress failure. Recent studies, however, suggest that the microvascular response to lung capillary hypertension is more complex. Pressure, strain and shear stress cause dysfunction of the capillary endothelium characterized by an imbalanced release of vasoactive mediators. Endothelial dysfunction evokes a multicellular response with features of vasoconstriction, inflammation, and vascular leakage, thrombosis, and remodeling. These active cellular reactions contribute to the pathophysiological process and may be specifically targeted by new therapeutic strategies.

Future pharmacologic agents for treatment of heart failure in children

The addition of new agents to the armamentarium of treatment options for heart failure in pediatric patients is exciting and challenging. Administration of these therapies to pediatric patients will require careful scrutiny of the data and skilled application. Developmental changes in drug metabolism, excretion, and distribution are concerning in pediatric patients, and inappropriate evaluation of these parameters can have disastrous results. Manipulation of the neurohormonal pathways in heart failure has been the target of most recently developed pharmacologic agents. Angiotensin receptor blockers (ARBs), aldosterone antagonists, beta-blockers, and natriuretic peptides are seeing increased use in pediatrics. In particular, calcium sensitizing agents represent a new frontier in the treatment of acute decompensated heart failure and may replace traditional inotropic therapies. Endothelin receptor antagonists have shown benefit in the treatment of pulmonary hypertension, but their use in heart failure is still debatable. Vasopressin antagonists, tumor necrosis factor inhibitors, and neutral endopeptidase inhibitors are also targeting aspects of the neurohormonal cascade that are currently not completely understood. The future of pharmacologic therapies will include pharmacogenomic studies on new and preexisting therapies for pediatric heart failure. The education and skill of the practitioner when applying these agents in pediatric heart failure is of utmost importance.

Dual ET(A)/ET(B) vs. selective ET(A) endothelin receptor antagonism in patients with pulmonary hypertension

Since the identification of endothelin as a key mediator in the pathogenesis of several diseases, including pulmonary arterial hypertension (PAH), the pharmacologic control of the activated endothelin system with endothelin receptor antagonists (ETRA) has been a major therapeutic achievement for the treatment of patients with PAH. To date, dual ET(A)/ET(B) and selective ET(A) receptor antagonists have clinically been evaluated. To answer the question of whether selective or dual ETRA is preferable in patients with PAH, experimental and clinical data with relevance to the pulmonary circulation are reviewed in this article. Whereas experimental and clinical data provide unambiguous evidence that ET(A) receptors mediate the detrimental effects of ET-1, such as vasoconstriction and cell proliferation, the elucidation of the role of ET(B) receptors has been more complex. It has been shown that there is a subpopulation of ET(B) receptors on smooth muscle cells and fibroblasts mediating vasoconstriction and proliferation. On the contrary, there is clear evidence that endothelial ET(B) receptors continue to mediate vasodilation, vasoprotection and ET-1 clearance despite the pathology associated with pulmonary hypertension. More difficult to assess is the net effect of these mechanisms in patients to be treated with ETRA. When considering the available data from controlled clinical trials, nonselectivity does not appear to carry a relevant clinical benefit for the treatment of patients with PAH when compared with selective ET(A) receptor antagonism.

Sitaxsentan: a novel endothelin-A receptor antagonist for pulmonary arterial hypertension

Sitaxsentan is an orally active, selective endothelin-A receptor antagonist that may benefit patients with pulmonary arterial hypertension by blocking the vasoconstrictive effects of endothelin-A receptors, while maintaining the vasodilator and endothelin-1 clearance functions of the endothelin-B receptors. In its first randomized, placebo-controlled study, sitaxsentan improved exercise capacity assessed by the 6-min walk test, New York Heart Association functional class, cardiac index and pulmonary vascular resistance in New York Heart Association Class II, III and IV patients with idiopathic pulmonary arterial hypertension and pulmonary arterial hypertension related to connective tissue disease or congenital heart disease. Although doses of 100 and 300 mg once daily demonstrated equivalent efficacy, the lower dose had a better safety profile. Additional studies are ongoing to assess the relative safety and efficacy of 50 and 100 mg once-daily dosing. The most common side effects include rhinitis, headache, peripheral edema, chest pain, nausea, constipation, increased prothrombin time/international normalized ratio (in patients on warfarin), flushing and insomnia. As with other endothelin receptor antagonists, increases in hepatic transaminases have been observed with sitaxsentan. Initial studies using the selective oral endothelin-A receptor antagonist sitaxsentan in pulmonary arterial hypertension patients have revealed a favorable risk-benefit therapeutic profile with the 100 mg once-daily dose.

Bosentan, an endothelin receptor antagonist, reduces leucocyte adhesion and inflammation in a murine model of inflammatory bowel disease

BACKGROUND AND AIMS

Endothelins, a group of polyfunctional cytokines, induce the adhesion of circulating leucocytes to venous endothelium, an initial step in the pathogenesis of a cellular infiltrate in inflammatory bowel disease (IBD). The effect of bosentan, a non-selective endothelin receptor antagonist, on leucocyte adhesion and inflammation in a murine model of IBD was studied.

MATERIALS AND METHODS

Thirty BALB/c mice were divided into three groups of 10 animals: untreated controls, chronic colitis [dextran sodium sulphate (DSS), 3% w/v for 30 days], and treatment with bosentan (30 mg/kg i.p. daily on days 26-30). On day 30, adherent and rolling leucocytes and the average rolling velocity were assessed by intravital microscopy. Clinical and histological activity of inflammation were assessed by the disease activity index and modified Dieleman score, respectively.

STATISTICS

Kruskal-Wallis test was used, followed by Dunn's method. A value of p<0.05 was considered significant.

RESULTS

Compared to healthy controls, mice treated with DSS showed pronounced clinical and histological inflammation, and a higher number of rolling and adhering leucocytes in colonic submucosal venules. Therapy with bosentan significantly reduced clinical and histological inflammation. Adherent leucocyte levels were markedly lower (1.2+/-0.3 vs 23.7+/-2.8 adherent cells per 0.01 mm2, p<0.05). The number of rolling leucocytes was lower but not significantly different. However, rolling velocity was significantly higher (91.5+/-14.0 vs 19.0+/-1.6 microm/s, p<0.05).

CONCLUSIONS

Bosentan reduces the adhesion of leucocytes in colonic submucosal venules and reduces inflammation in this mouse model of IBD. By inhibiting leucocyte adhesion, a crucial step in the recruitment of leucocytes to the inflamed tissue, bosentan is a potent therapeutic drug in this animal model. Further studies are necessary to investigate the role of bosentan as a novel drug in human IBD.

Endothelin receptor antagonists

Endothelin receptor antagonists (ERAs) have been developed to block the effects of endothelin-1 (ET-1) in a variety of cardiovascular conditions. ET-1 is a powerful vasoconstrictor with mitogenic or co-mitogenic properties, which acts through the stimulation of 2 subtypes of receptors [endothelin receptor subtype A (ETA) and endothelin receptor subtype B (ETB) receptors]. Endogenous ET-1 is involved in a variety of conditions including systemic and pulmonary hypertension (PH), congestive heart failure (CHF), vascular remodeling (restenosis, atherosclerosis), renal failure, cancer, and cerebrovascular disease. The first dual ETA/ETB receptor blocker, bosentan, has already been approved by the Food and Drug Administration for the treatment of pulmonary arterial hypertension (PAH). Trials of endothelin receptor antagonists in heart failure have been completed with mixed results so far. Studies are ongoing on the effects of selective ETA antagonists or dual ETA/ETB antagonists in lung fibrosis, cancer, and subarachnoid hemorrhage. While non-peptidic ET-1 receptor antagonists suitable for oral intake with excellent bioavailability have become available, proven efficacy is limited to pulmonary hypertension, but it is possible that these agents might find a place in the treatment of several cardiovascular and non-cardiovascular diseases in the coming future.

Sitaxsentan: an endothelin-A receptor antagonist for the treatment of pulmonary arterial hypertension

Endothelin (ET-1) is a potent vasoconstrictor and smooth muscle mitogen that mediates its effects through activation of ET-A and ET-B receptors. Pulmonary arterial hypertension (PAH) encompasses a heterogeneous group of disorders characterised by inappropriate overactivation of the ET system. There is clear evidence that strategies that block both ET receptors are associated with clinical improvement in PAH. However, there are theoretical physiological advantages to treatments that specifically inhibit only the ET-A receptor. Sitaxsentan is an orally active selective ET-A receptor antagonist that in recent clinical trials has demonstrated improvements in exercise capacity, functional class and haemodynamics in PAH patients with modified New York Heart Association class II, III and IV symptoms.

Endothelins: Pathophysiology and treatment implications in chronic heart failure

Less than 20 years after its discovery, endothelin is recognized as playing a central role in the pathogenesis of chronic heart failure. Endothelin is not only one of the most potent known vasoconstrictors; it also has multiple other actions. It mediates pathologic hypertrophy and fibrosis of both ventricular and vascular tissues, it potentiates the effects of other neurohormones, and it acts as a proarrhythmic. Endothelin receptor antagonists have been developed to investigate the hypothesis that these adverse effects could be prevented, and experimental studies showed promise in this regard. Clinical studies have confirmed the ability of these new agents to improve hemodynamics, but beneficial effects on clinical outcomes have been more difficult to demonstrate. Further analysis of the data from these trials, as well as other ongoing studies, may provide insight into these disparate findings and guidance for future investigations.

Type 5 phosphodiesterase inhibition in heart failure and pulmonary hypertension

The availability of selective inhibitors of the cyclic guanosine monophosphate (cGMP)-specific type 5 phosphodiesterase (PDE5) has created increasing interest in unlocking the therapeutic potential of PDE5 inhibition in cardiovascular diseases that are marked by dysfunction of nitric oxide (NO)-cGMP signaling. Pulmonary arterial hypertension (PAH) and heart failure (HF) are characterized by pulmonary arterial vasoconstriction that is thought to be caused by relative deficiencies of vasodilators such as NO and exaggerated production of vasoconstrictors such as endothelin. PDE5 is abundant in the pulmonary vasculature where it catabolizes cGMP, the second messenger of NO. Inhibition of PDE5 has been shown to lower pulmonary vascular resistance in PAH and HF by augmenting local cGMP. This review outlines the therapeutic potential of PDE5 inhibition for the treatment of PAH and HF.

Direct comparison of selective endothelin A and non-selective endothelin A/B receptor blockade in chronic heart failure

OBJECTIVE

To investigate the potential differential effects of selective endothelin (ET) A and dual ET-A/B receptor blockade in patients with chronic heart failure.

METHODS

Nine patients with chronic heart failure (New York Heart Association class II-III) each received intravenous infusions of BQ-123 alone (selective ET-A blockade) and combined BQ-123 and BQ-788 (dual ET-A/B blockade) in a randomised, placebo controlled, three way crossover study.

RESULTS

Selective ET-A blockade increased cardiac output (maximum mean (SEM) 33 (12)%, p < 0.001) and reduced mean arterial pressure (maximum -13 (4)%, p < 0.001) and systemic vascular resistance (maximum -26 (8)%, p < 0.001), without changing heart rate (p = 0.38). Dual ET-A/B blockade significantly reduced the changes in all these haemodynamic variables compared with selective ET-A blockade (p < 0.05). Selective ET-A blockade reduced pulmonary artery pressure (maximum 25 (7)%, p = 0.01) and pulmonary vascular resistance (maximum 72 (39)%, p < 0.001). However, there was no difference between these effects and those seen with dual ET-A/B blockade. Unlike selective ET-A blockade, dual ET-A/B blockade increased plasma ET-1 concentrations (by 47 (4)% with low dose and 61 (8)% with high dose, both p < 0.05).

CONCLUSIONS

While there appeared to be similar reductions in pulmonary pressures with selective ET-A and dual ET-A/B blockade, selective ET-A blockade caused greater systemic vasodilatation and did not affect ET-1 clearance. In conclusion, there are significant haemodynamic differences between selective ET-A and dual ET-A/B blockade, which may determine responses in individual patients.

Hemodynamic effects of sildenafil in patients with congestive heart failure and pulmonary hypertension: combined administration with inhaled nitric oxide

STUDY OBJECTIVES

In patients with pulmonary hypertension (PH) secondary to congestive heart failure, inhaled nitric oxide (NO) increases pulmonary vascular smooth-muscle intracellular cyclic guanosine monophosphate (cGMP) concentration, thereby decreasing pulmonary vascular resistance (PVR) and increasing cardiac index (CI). However, these beneficial effects of inhaled NO are limited in magnitude and duration, at least in part due to cGMP hydrolysis by the type 5 isoform of phosphodiesterase (PDE5). The goal of this study was to determine the acute pulmonary and systemic hemodynamic effects of the selective PDE5 inhibitor, sildenafil, administered alone or in combination with inhaled NO in patients with congestive heart failure and PH.

DESIGN

Single center, case series, pharmacohemodynamic study.

SETTING

Cardiac catheterization laboratory of a tertiary care academic teaching hospital.

PATIENTS

We studied 11 patients with left ventricular systolic dysfunction due to coronary artery disease or idiopathic dilated cardiomyopathy who had PH.

INTERVENTIONS

We administered oral sildenafil (50 mg), inhaled NO (80 ppm), and the combination of sildenafil and inhaled NO during right-heart and micromanometer left-heart catheterization.

MEASUREMENTS AND RESULTS

Sildenafil administered alone decreased mean pulmonary artery pressure by 12 +/- 5%, PVR by 12 +/- 5%, systemic vascular resistance (SVR) by 13 +/- 6%, and pulmonary capillary wedge pressure by 12 +/- 7%, and increased CI by 14 +/- 5% (all p < 0.05) [+/- SEM]. The combination of inhaled NO and sildenafil decreased PVR by 50 +/- 4%, decreased SVR by 24 +/- 3%, and increased CI by 30 +/- 4% (all p < 0.01). These effects were greater than those observed with either agent alone (p < 0.05). In addition, sildenafil prolonged the pulmonary vasodilator effect of inhaled NO. Administration of sildenafil alone or in combination with inhaled NO did not change systemic arterial pressure or indexes of myocardial systolic or diastolic function.

CONCLUSIONS

PDE5 inhibition with sildenafil improves cardiac output by balanced pulmonary and systemic vasodilation, and augments and prolongs the hemodynamic effects of inhaled NO in patients with chronic congestive heart failure and PH.

Endothelin-A receptor blockade prevents and partially reverses neonatal hypoxic pulmonary vascular remodeling

Hypoxia-induced pulmonary vascular remodeling (HPVR) may lead to persistent pulmonary hypertension of the newborn or cor pulmonale. Endothelin-1 (ET-1), via endothelin-A (ET(A)) receptor activation, mediates hypoxic pulmonary vasoconstriction. Our objectives were to develop a newborn mouse model of HPVR and to test the hypothesis that ET(A) blockade would prevent and reverse HPVR in this model. C57BL/6 mice (n = 64) were exposed to 12% oxygen (HYP group) or room air (RA group) from birth to 2 wk of age. The mice were injected intraperitoneally daily with either BQ-610 (ET(A) blocker) or vehicle (cottonseed oil) from birth (prevention study) or from 6 d of age (reversal study). HPVR was assessed histologically by pulmonary vascular morphometry by an examiner masked to study group, and by measurement of the right ventricle to left ventricle (RV/LV) thickness ratio. Hypoxia increased medial wall thickness (%WT) in pulmonary arteries <100 mum in diameter and RV/LV thickness ratio. BQ-610 prevented the hypoxia-induced increase in %WT and RV/LV thickness ratio when given from birth, and later therapy partially reversed the hypoxia-induced increase in %WT but not RV/LV thickness ratio. These data show that in the newborn mouse model, chronic hypoxia leads to HPVR that can be completely prevented and partially reversed by ET(A) blockade. These results indicate that ET-1, acting via ET(A) receptors, is a mechanism of pathophysiologic significance underlying neonatal HPVR. Development of this newborn mouse model of HPVR facilitates investigation of mechanisms underlying this important and severe disease entity in human infants.

Pulmonary arterial hypertension in systemic sclerosis: clinical manifestations, pathophysiology, evaluation, and management

It is increasingly recognized that significant pulmonary arterial hypertension (PAH) develops in more than 15% of patients with systemic sclerosis (SSc). As this complication of SSc may occur even in the absence of overt interstitial lung disease (isolated PAH), it has been likened to primary PAH and is attributable to intrinsic vascular pathology that is the hallmark of SSc. Deregulated activity of mediators controlling vasomotor tone has been implicated, and levels of endothelin-1 (ET-1) are elevated in the circulation and in the lungs. By causing enhanced vasoconstriction, vascular endothelial cell proliferation, smooth muscle hypertrophy, and irreversible vascular remodeling in the lungs, ET-1 appears to play a significant role in the pathogenesis of SSc-associated PAH. Although patients with the limited cutaneous form of SSc are more likely to develop PAH than those with the diffuse form, the true prevalence of PAH in SSc, and the risk factors for its development, are not yet known. Because the prognosis of patients with SSc-associated PAH is substantially worse than that of patients without this complication, intensive efforts are underway to develop sensitive screening strategies and effective treatments. Serial evaluation of SSc patients with Doppler echocardiography appears to be prudent. Antibodies against the centromere or fibrillarin proteins may be useful in identifying those patients with SSc at highest risk for developing PAH. The US FDA has approved a number of novel treatments, including long-acting oral ET-1 receptor antagonists such as bosentan and short-acting parenteral prostacyclin analogs, such as epoprostenol, for PAH. In particular, bosentan appear to be well tolerated, and short-term therapy results in improved exercise tolerance, improved hemodynamics, and possibly improved survival in patients with advanced PAH. These agents may be used alone, or possibly in combination with prostacyclin analogs. Therapeutic agents that modulate the synthesis of nitric oxide, and additional agents targeting the ET-1 signaling system are under preclinical development. Although the large-scale clinical trials that resulted in obtaining FDA approval for these agents were generally carried out in patients with primary PAH, it appears that patients with SSc-associated PAH respond similarly. Therefore, it is reasonable to conclude that ET-1 receptor antagonists and parenteral prostacyclin analogs should be used in SSc patients with moderate to severe PAH. The efficacy of these agents for treating patients with PAH who also experience significant interstitial lung disease, as occurs in many SSc patients, remains unknown. Additional important unresolved issues relate to the long-term efficacy of ET-1 receptor antagonists, and their effects on survival and progression of PAH. Additionally, it is not yet clear if early intervention for SSc patients with mild PAH is beneficial.

Sitaxsentan, a selective endothelin-A receptor antagonist for the treatment of pulmonary arterial hypertension

Sitaxsentan, a highly selective endothelin-A (ET(A)) receptor antagonist (6500-fold more selective for ET(A) receptors than endothelin-B (ET(B)) receptors), may benefit patients with pulmonary artery hypertension (PAH) by blocking the vasoconstrictor effects of ET(A) receptors while maintaining the vasodilator/clearance functions of ET(B) receptors. In its first randomised, placebo-controlled study, Sitaxsentan to Relieve Impaired Exercise-1 (STRIDE-1), sitaxsentan improved exercise capacity assessed by 6 min walk, New York Heart Association functional class, cardiac index and pulmonary vascular resistance in New York Heart Association class II, III and IV patients with idiopathic PAH, PAH related to connective tissue disease or PAH related to congenital heart disease. In STRIDE-1, doses of 100 and 300 mg/day p.o. were evaluated. Although both doses showed equivalent efficacy, the lower dose had a more tolerable safety profile. Additional studies are ongoing to assess the relative safety and efficacy of 50 and 100 mg/day doses, both in de novo patients and in patients previously treated with the ET(A)/ET(B) receptor antagonist bosentan. Long-term comparative studies are necessary to determine whether there is a clinically meaningful difference between selective ET(A) receptor antagonism and ET(A)/ET(B) receptor antagonism.

The role of endothelin receptor antagonists in cardiovascular pharmacotherapy

Endothelin (ET) is a hormone produced predominantly by endothelial cells which has been recognised to play a significant role in the development of several cardiovascular disease states. In order to combat the deleterious effects of ET, several ET-receptor antagonists (ETRA) are currently in clinical development. The agents developed thus far inhibit the actions of ET through either selective inhibition of the ET(A) receptors or non-selective inhibition of both ET(A) and ET(B) receptors. However, due to the differing proportions of the two receptor subtypes in various tissues, animal models and pathologies, it remains a matter of debate whether receptor selective agents impart significant clinical benefits over non-selective agents. This paper seeks to briefly summarise the important preclinical and clinical effects that have been reported in the literature and will attempt to provide a rationale for the use of both types of ETRAs in the treatment of both systemic and pulmonary hypertension as well as chronic heart failure (CHF).

Comparison of selective ETAand ETBreceptor antagonists in patients with chronic heart failure

BACKGROUND

The vasoconstrictor action of endothelin-1 (ET-1) is mediated through ET(A) and ET(B) receptor subtypes on vascular smooth muscle. ET(B) receptors are also present on the vascular endothelium where they mediate vasodilation. Animal studies suggest that the ET(B) receptor also acts as a clearance receptor for endothelin.

AIMS

To investigate the effects of a selective ET(A) and a selective ET(B) receptor antagonist alone and in combination on haemodynamics and circulating concentrations of ET-1 in patients with chronic heart failure.

RESULTS

Infusion of BQ-123 (n=10), a selective ET(A) receptor antagonist, led to systemic vasodilation and did not change plasma ET-1 concentrations (1.38+/-0.82 to 1.38+/-0.91 fmol/ml, ns). Infusion of BQ-788 (n=8) led to systemic vasoconstriction with a rise in plasma ET-1 (1.84+/-1.06 to 2.73+/-0.99 fmol/ml, p<0.01). The addition of BQ-123 to BQ-788 led to systemic and pulmonary vasodilation with no further increase in plasma ET-1 concentrations (2.80+/-1.14 to 2.90+/-1.20 fmol/ml, ns).

CONCLUSION

The rise in plasma ET-1 concentrations in response to selective blockade of ET(B) receptors and the associated adverse haemodynamic effects suggest that ET(B) receptors have a role in the clearance of ET-1 in man and that their blockade may not be advantageous for patients with heart failure.

Endothelin receptor antagonists in heart failure: current status and future directions

Experimental evidence suggests that endothelin substantially contributes to left ventricular remodelling and progression of heart failure. Plasma endothelin (ET)-1 levels are increased in patients with heart failure, independent of the aetiology, and correlate with the severity of the disease. Furthermore, tissue endothelin levels and endothelin receptors are upregulated in myocardium from animals and humans with heart failure. In several experimental models 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 heart failure, 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. Furthermore, in the recently completed Endothelin-A Receptor Antagonist Trial in Heart Failure (EARTH) trial in patients with chronic heart failure, 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 heart failure include the following. 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 heart failure and remodelling processes has not been clearly defined. Finally, for the detection of improvement in left ventricular remodelling, a study needs to be conducted in patients with recent myocardial infarction and signs of heart failure.

Pathophysiological plasma ET-1 levels antagonize β-adrenergic dilation of coronary resistance vessels in conscious dogs

On the basis of in vitro experiments showing that endothelin (ET)-1 interferes with smooth muscle ATP-sensitive K+ (KATP) channel opening, which is pivotal in β-adrenergic coronary dilation, we hypothesized that pathophysiological plasma ET-1 levels impair β-adrenergic dilation of resistance coronary vessels. In conscious instrumented dogs, graded intravenous doses of dobutamine caused the expected inotropic responses. As myocardial O2 consumption (MV̇o2) increased, the disproportionate rise in coronary sinus (CS) Po2 indicates that increases in coronary blood flow (CBF) exceeded metabolic requirements, consistent with β-adrenergic dilation. ET-1 intravenous infusions, to reach pathophysiological plasma levels, reduced slopes of the Po2-MV̇o2 and CBF-MV̇o2 relations. In contrast, the first derivative of left ventricular pressure over time responses to dobutamine were not impaired during ET-1 delivery. Clazosentan, an ETA receptor blocker, prevented reduction of the slope of Po2-MV̇o2 and CBF-MV̇o2 relations. After ganglionic blockade to exclude reflex influences, ET-1 still reduced slopes of Po2-MV̇o2 and CBF-MV̇o2 relations. To assess effects of ET-1 on endothelium-dependent and -independent coronary vascular responses, intracoronary ACh and nitroglycerin were given to directly target coronary vessels. CBF responses to ACh and nitroglycerin were maintained during ET-1 delivery. In contrast, responses to intracoronary KATP channel-dependent dilators adenosine and lemakalim were impaired by ET-1. In conclusion, pathophysiological levels of ET-1 impaired β-adrenergic dilation of resistance coronary vessels through an ETA receptor-dependent process. In contrast, left ventricular inotropic responses to dobutamine were not impaired during ET-1 delivery. Our data suggest that ET-1 may interfere with smooth muscle KATP channels to impair β-adrenergic coronary dilation.

Reduced pulmonary clearance of endothelin in congestive heart failure: a marker of secondary pulmonary hypertension

BACKGROUND

Endothelin-1 (ET-1) levels are elevated in congestive heart failure (CHF) in relation with the severity of pulmonary hypertension. We evaluated whether a reduced pulmonary ET-1 clearance could contribute to this elevation.

METHODS AND RESULTS

We determined pulmonary ET-1 clearance in 24 patients with CHF in relation with hemodynamics, plasma ET-1, and N-terminal pro-brain natriuretic peptide (NT-proBNP) levels. Pulmonary ET-1 extraction, measured by the single bolus indicator-dilution technique, was reduced to 32 +/- 14% in comparison to historic controls (47 +/- 7%). Plasma ET-1 clearance by the lungs (924 +/- 588 mL/min) was also much lower than in controls (1424 +/- 79 mL/min). Clearance correlated inversely with mean pulmonary artery pressure (PAP, r = -.47, P = .017) and pulmonary capillary wedge pressure (r = -.47, P = .017) and positively with the rate of left ventricular (LV) relaxation LV -dP/dt (r = .593, P = .004). After multivariate analysis, only mean PAP and LV -dP/dt were independently correlated with ET-1 clearance (r = -.40, P = .03, and r = .55, P = .005, respectively). Plasma ET-1 levels did not correlate with clearance (r = .038, P = .86), and there was no significant arteriovenous ET-1 gradient. There was a mild nonsignificant correlation between plasma ET-1 and pulmonary artery systolic pressure (r = .38, P = .06), but a strong correlation with right atrial pressure (r = .696, P < .0001) and NT-proBNP levels (r = .51, P = .001), which were maintained after multivariate linear regression (r = .60, P = .001, and r = .32, P = .04, respectively).

CONCLUSION

Pulmonary ET-1 clearance is reduced in CHF in relation with the severity of pulmonary hypertension. This reduced clearance does not significantly modulate plasma ET-1 levels. Whether this is only a marker of secondary pulmonary hypertension or could modulate pulmonary vascular tone will require further studies.

Discovery, Modeling, and Human Pharmacokinetics ofN-(2-Acetyl-4,6-dimethylphenyl)-3-(3,4-dimethylisoxazol-5-ylsulfamoyl)thiophene-2-carboxamide (TBC3711), a Second Generation, ETASelective, and Orally Bioavailable Endothelin Antagonist1

Sitaxsentan (1) (Wu et al. J. Med. Chem. 1997, 40, 1690) is our first endothelin antagonist being evaluated in clinical trials. It has demonstrated biological effects in an acute hemodynamic study in CHF (Givertz et al. Circulation 2000, 101, 2922), an open-label 20-patient pulmonary hypertension trial (Barst et al. Chest 2002, 121, 1860-1868), and a 31-patient trial in essential hypertension (Calhoun et al. AHA Scientific Sessions 2000). In a phase 2b/3 pulmonary arterial hypertension trial, once a day treatment of 100 mg of sitaxsentan statistically significantly improved 6-min walk distance and NYHA class at 12 weeks (Barst et al. Am. J. Respir. Crit. Care Med. 2004, 169, 441). We have since reported on our efforts in generating follow-up compounds (Wu et al. J. Med. Chem. 1999, 42, 4485) and recently communicated that an ortho acyl group on the anilino ring enhanced oral absorption in this category of compounds (Wu et al. J. Med. Chem. 2001, 44, 1211). Here we report an expansion of this work by substituting a variety of electron-withdrawing groups at the ortho position and evaluating their effects on oral bioavailability as well as structure-activity relationships. As a result, TBC3711 (7z) was identified as our second endothelin antagonist to enter the clinic due to its good oral bioavailability (approximately 100%) in rats, high potency (ET(A) IC(50) = 0.08 nM), and optimal ET(A)/ET(B) selectivity (441 000-fold). Compound 7z has completed phase-I clinical development and was well tolerated with desirable pharmacokinetics in humans (t(1/2) = 6-7 h, oral availability > 80%).

Drug therapy in chronic heart failure

Chronic heart failure is widely recognised as a common and escalating problem that causes major disability and often shortens life. Diuretics and digoxin have formed the mainstay of treatment for many years. Clinical trials have demonstrated that angiotensin converting enzymes and beta-blockers, in selected patients, improve symptoms and reduce mortality. Angiotensin-II antagonists and spironolactone may also have a role in certain individuals. Newer pharmacological approaches to the management of this complex disease are being developed, but await full evaluation.

Acute endothelin A receptor antagonism improves pulmonary and systemic haemodynamics in patients with pulmonary arterial hypertension that is primary or autoimmune and related to congenital heart disease

OBJECTIVE

To evaluate the acute haemodynamic effect of BQ-123, a selective endothelin A receptor antagonist, in severe chronic pulmonary arterial hypertension (PAH) of primary or autoimmune origin or related to congenital heart disease.

DESIGN

Prospective open clinical study.

SETTING

Cardiology tertiary referral centre.

PATIENTS

26 patients with chronic PAH were studied, with mean (SEM) age 29 (3) years (range 4-71 years), mean pulmonary artery pressure 68 (4) mm Hg, and pulmonary vascular resistance index 1694 (170) dyne x s x cm(-5). Patients were divided in three groups according to PAH aetiology: primary or autoimmune PAH (n = 12), and PAH associated with congenital heart defects with (n = 6) or without (n = 8) complete mixing.

INTERVENTION

BQ-123 200 nmol/min was infused for 60 minutes in the right atrium with sequential haemodynamic measurements at 30 minute intervals.

RESULTS

BQ-123 improved mean pulmonary artery pressure from 68 (4) to 64 (4) mm Hg (p < 0.05), pulmonary vascular resistance index from 1694 (170) to 1378 (145) dyne x s x cm(-5) (p < 0.001), pulmonary cardiac index from 3.0 (0.2) to 3.4 (0.3) l/min/m2 (p < 0.001), and effective cardiac index from 2.5 (0.2) to 2.7 (0.2) l/min/m2 (p < 0.01). Haemodynamic response was similar in all groups except for systemic cardiac index where a different (p = 0.0001, F = 5.53) response was observed; systemic cardiac index increased from 2.7 (0.2) to 2.9 (0.2) l/min/m2 (p < 0.001) when patients with complete mixing were excluded, in whom systemic cardiac index tended to decrease from 3.4 (1.0) to 3.0 (0.6) l/min/m2 (p = 0.06).

CONCLUSIONS

Acute endothelin A receptor antagonism induces substantial haemodynamic improvement in severe chronic PAH of primary or autoimmune origin or related to congenital heart disease.

Intravenous vasodilator therapy in congestive heart failure

The prevalence of congestive heart failure (CHF) is increasing in the US and worldwide, partly because patients are living longer. Treatment of CHF is mostly on an outpatient basis, but inpatient care is required for decompensated CHF, acute CHF or poor response to outpatient treatment. Control of symptoms is usually achieved by diuresis. Intravenous (IV) vasodilators are an important adjunct to the inpatient treatment of CHF. They work mainly by reducing the afterload on the myocardium although preload reduction also occurs. After clinical stabilisation, the goal is to switch to a maintenance oral regimen to be continued as outpatient therapy. The range of IV vasodilators available for inpatient treatment of CHF includes nitrates, phosphodiesterase inhibitors, dobutamine, morphine, ACE inhibitors, B-type natriuretic peptides and endothelin receptor antagonists. As each agent may have a different mechanism or site of action, each agent may affect preload, contractility or afterload to a different extent and it may be desirable to choose one over the other in a particular clinical setting. Examples of standard therapy include dobutamine, milrinone and nitroglycerin. Nesiritide, a B-type natriuretic peptide, is a newer vasodilator and US FDA approved for use in acute CHF. However, most studies with this agent have been in small numbers of patients with anecdotal findings. Larger studies are warranted to pinpoint the efficacy and adverse effects of this agent. It is primarily used to reduce the acuity of decompensated CHF on admission to hospital.Endothelin receptor antagonists show promise in the management of acute CHF, but continue to be investigational. Long-term data on their efficacy and safety are limited. None of the endothelin receptor antagonists are FDA approved for use in patients with CHF.

Hemodynamic and clinical effects of tezosentan, an intravenous dual endothelin receptor antagonist, in patients hospitalized for acute decompensated heart failure

OBJECTIVES

We sought to investigate the efficacy and safety of tezosentan, a dual endothelin receptor antagonist, in patients hospitalized for acute heart failure (HF).

BACKGROUND

Tezosentan has been previously shown to improve hemodynamics in patients with stable chronic HF.

METHODS

In a double-blind fashion, 292 patients (cardiac index < or =2.5 l/min per m(2) and pulmonary capillary wedge pressure (PCWP) > or =15 mm Hg) who were admitted to the hospital and in need of intravenous treatment for acute HF and central hemodynamic monitoring were randomized to 24-h intravenous treatment with tezosentan (50 or 100 mg/h) or placebo. Central hemodynamic variables, the dyspnea score, and safety variables were measured.

RESULTS

After 6 h of treatment, significantly greater increases in the cardiac index and decreases in PCWP were observed with both tezosentan dosages than with placebo (mean treatment effects at 0.38 and 0.37 l/min per m(2) with 50 and 100 mg/h and -3.9 mm Hg for each dose, respectively; p < 0.0001). This effect was maintained during the remaining infusion and for > or =6 h after treatment cessation. A tendency for an improved dyspnea score and a decreased risk of clinical worsening was observed after 24 h of treatment with each tezosentan dose. Adverse events, more frequent with tezosentan than with placebo (headache, asymptomatic hypotension, early worsening of renal function, nausea, vomiting), were dose-related.

CONCLUSIONS

Intravenous tezosentan rapidly and effectively improved hemodynamics in these patients. The similar beneficial effects of the two dosages and the increased dose-related adverse events with the higher dosage suggest that the optimal dosing regimen is <50 mg/h.

Bosentan for the treatment of pulmonary arterial hypertension

OBJECTIVE

To describe the pharmacology, pharmacokinetics, efficacy, and safety of bosentan in the treatment of pulmonary arterial hypertension (PAH).

DATA SOURCES

A MEDLINE and Current Contents search (1966-June 2002) of the English-language literature was conducted to identify published dose-ranging, pharmacokinetic, pivotal efficacy trials and review articles of bosentan and endothelin antagonists. Additional references were identified from the bibliographies of the retrieved articles.

DATA SYNTHESIS

Bosentan is the first orally active, nonpeptide endothelin receptor antagonist approved by the Food and Drug Administration for use in patients with World Health Organization (WHO) functional class III and IV PAH. Titrated to a dose of 125 mg twice daily, bosentan produces pulmonary vasodilation, improving cardiopulmonary hemodynamics leading to better outcomes for patients. It is metabolized primarily by the hepatic system via the cytochrome P450 enzyme pathway and eliminated by biliary excretion. Bosentan is an inducer of the isoenzymes CYP3A4 and 2C9. It possesses a unique pharmacokinetic profile with a terminal elimination half-life of approximately 5 hours, yet steady-state plasma concentrations are not achieved for 3-5 days as a result of enhanced drug clearance and autoinduction following multiple daily dosing. The major adverse effects of bosentan are the potential for birth defects and hepatotoxicity.

CONCLUSIONS

The use of bosentan in patients with WHO functional class III and IV PAH is associated with improved exercise tolerance, cardiopulmonary hemodynamics, and increased time to clinical worsening when compared with placebo. It offers significant advantage in ease of administration and quality of life compared with epoprostenol therapy, with similar efficacy.

The endothelin system and its role in acute myocardial infarction

Immediately after an acute myocardial infarction (AMI) or in models of ischemia-reperfusion injury, cardiac endothelin (ET) system is markedly activated, and plasma levels of ET are increased. In the heart, expression of the main components of the ET system (ET-1 peptide, both receptor subtypes ETA and ETB, though not endothelin converting enzyme) are increased both at the gene level and protein level, in the viable myocardium, and--even more substantially--in the necrotic area. Despite these conspicuous abnormalities, the role of ET in this setting remains unclear. In the absence of human data, most short-term studies in animals (in terms of hours to up to 8 days post-AMI) and in the reperfused ischemic heart, have found beneficial effects of ET receptor blockade on survival rate, incidence of arrhythmias, cardiac function, and morphology. In contrast, many studies in which a long-term ET inhibition was started immediately post-infarction and the late effects were examined in animals with ensuing chronic heart failure (14-100 days postinfarction), adverse effects were also observed, such as scar thinning, further ventricular dilation, or even a worse survival rate. It appears that the ET system plays a dual role during the early post-AMI period. At present, it is not clear whether the short-term beneficial effects or long-term adverse effects of ET receptor blockade would prevail. Acute use of short-acting ET receptor antagonists in patients with AMI complicated by an acute heart failure is an attractive possibility that also remains to be investigated.

Role of endothelins in congestive heart failure

Despite major advances in conventional medical therapy, patients with heart failure continue to experience significant morbidity and mortality. Endothelin-1 (ET-1) is a potent vasocontrictor and mitogenic peptide that is activated in heart failure. There is increasing experimental and clinical evidence in support of an important role of ET-1 in the pathophysiology of heart failure. Manipulation of the activity of ET-1, especially using endothelin receptor blockers, has allowed for the further elucidation of the role of this neurohormonal system and development of novel therapeutic strategies in heart failure. Published clinical studies of these agents to date have involved relatively small numbers of patients with severe heart failure, followed for a relatively short period of time, and have mainly examined surrogate endpoints. Large-scale trials that address to hard clinical outcomes are ongoing and their results forthcoming. A key question that remains concerns whether selective ETA or dual ETA-ETB receptor blockade will be more effective.

Will endothelin receptor antagonists have a role in heart failure?

Mixed ET(A/B) and selective ET(A) receptor antagonists showed promising hemodynamic and symptomatic improvements in patients with heart failure. Randomized, clinical trials to investigate the effects of ET receptor antagonists on survival in patients with heart failure still need to be conducted. Also, the effects of selective ET(A) and mixed ET(A/B) receptor antagonists on the clinical outcome of patients with CHF will have to be assessed.

Evaluation of pulmonary hypertension in heart transplant candidates

Assessment of the pulmonary circulation and right ventricular function is a cornerstone in the evaluation of the patient as a potential heart transplant recipient. The importance of pulmonary hypertension is linked to outcomes in the posttransplant period. Preoperative pulmonary vascular resistance is an independent risk factor for early death after heart transplantation. Pulmonary hypertension can be classified as reversible, or irreversible if not rapidly responsive to pharmacologic maneuvers. However, in most patients, the major component is likely to reverse with vasodilators, because of the central role played by the endothelium in the control of pulmonary vascular tone. To discriminate between patients with reversible and irreversible pulmonary hypertension, provocative therapies are used, and baseline and the postprovocation hemodynamic parameters are measured. To date, there is no reliable hemodynamic threshold beyond which right ventricular failure is certain to occur, nor are there values below which right ventricular failure is always avoidable. Because of this uncertainty, it becomes clear that only through careful preoperative assessment can this life-threatening condition be recognized preoperatively and, hence, managed in the posttransplant recovery period.

[New expansion of endothelin research: perspectives for clinical application of endothelin-receptor antagonists]

Three isopeptides of endothelin (ET-1, -2, and -3) exert various actions through stimulation of two sub-types of receptor (ETA and ETB). Vascular endothelial cells produce only ET-1. In addition to its powerful vasoconstrictor action, ET-1 has direct mitogenic actions on cardiovascular tissues, as well as comitogennic actions with a wide variety of growth factors and vasoactive substances. ET-1 also promotes the synthesis and secretion of growth factors and various substances, including extracellular constituents. These effects of endogenous ET-1 would naturally be thought to be concerned with the development and/or aggravation of chronic cardiovascular diseases; e.g., hypertension, pulmonary hypertension, vascular remodeling (stenosis, atherosclerosis), renal failure, and heart failure. A large number of peptide and orally active non-peptide endothelin receptor antagonists have been developed, and utilized to analyze physiological and pathophysiological roles of endogenous ET-1. These antagonists have been shown to exert excellent therapeutic effects in animal models of various kinds of diseases by either acute or chronic treatment. Therapeutic treatment of patients suffering from the above-mentioned cardiovascular diseases with ET-receptor antagonists have also been taking place, and bosentan (ETA/ETB antagonist) was recently approved by the FDA as a formal therapeutic drug for pulmonary hypertension. In this review, perspectives for therapeutic applicability of ET-receptor antagonists will be explored.

The nitric oxide-endothelin-1 connection

Nitric oxide (NO) and endothelin-1 (ET-1) are endothelium-derived mediators that play important roles in vascular homeostasis. This review is focused on the role and reciprocal interactions between NO and ET-1 in health and diseases associated with endothelium dysfunction. We will also discuss the clinical significance of NO donors and drugs that antagonize ET receptors.

The therapeutic potential of endothelin-1 receptor antagonists and endothelin-converting enzyme inhibitors on the cardiovascular system

Clinical trials have established bosentan, an orally active non-selective endothelin (ET) receptor antagonist, as a beneficial treatment in pulmonary hypertension. Trials have also shown short-term benefits of bosentan in systemic hypertension and congestive heart failure. However, bosentan also increased plasma levels of ET-1, probably by inhibiting the clearance of ET-1 by endothelin type B (ET(B)) receptors, and this may mean its effectiveness is reduced with long-term clinical use. Preliminary data suggests that selective endothelin type A (ET(A)) receptor antagonists (BQ-123, sitaxsentan) may be more beneficial than the non-selective ET receptor antagonists in heart failure, especially when the failure is associated with pulmonary hypertension. Experimental evidence in animal disease models suggests that non-selective ET or selective ET(A) receptor antagonism may have a role in the treatment of atherosclerosis, restenosis, myocarditis, shock and portal hypertension. In animal models of myocardial infarction and/or reperfusion injury, non-selective ET or selective ET(A) receptor antagonists have beneficial or detrimental effects depending on the conditions and agents used. Thus clinical trials of the non-selective ET or selective ET(A) receptor antagonists in these conditions are not presently warranted. Several selective endothelin-converting enzyme inhibitors have been synthesised recently, and these are only beginning to be tested in animal models of cardiovascular disease, and thus the clinical potential of these inhibitors is still to be defined.

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.

A combination of oral endothelin-A receptor antagonist and oral prostacyclin analogue is superior to each drug alone in ameliorating pulmonary hypertension in rats

OBJECTIVES

We sought to investigate whether the combination of an oral endothelin (ET)A receptor antagonist and an oral prostacyclin (PGI(2)) analogue is superior to the single use of each drug alone for treating pulmonary hypertension (PH).

BACKGROUND

Treatment with intravenous PGI(2) or an ET(A) receptor antagonist was effective for PH; however, the effect of both agents is unclear.

METHODS

We administered the oral ET(A) receptor antagonist TA-0201 and/or the oral PGI(2) analogue beraprost sodium (BPS) to rats with monocrotaline-induced PH for 19 days. The groups were: normal rats with vehicle treatment (Control group), PH rats with vehicle treatment (PH group), PH rats with TA-0201 treatment (PH + TA group), PH rats with BPS treatment (PH + BPS group) and PH rats with TA-0201 and BPS treatment (PH + TA + BPS group).

RESULTS

Right ventricular (RV) systolic pressure and the ratio of RV systolic pressure to systemic systolic blood pressure (Pp/Ps) were markedly higher in the PH group than in the Control group. The increased RV systolic pressure and Pp/Ps were significantly and comparably depressed in the PH + TA and PH + BPS groups; it was more greatly depressed in the PH + TA + BPS group than in the groups with each drug alone. The indexes of RV hypertrophy showed the same tendency as the increase in RV systolic pressure among the five groups. The expression of beta-myosin heavy chain messenger ribonucleic acid in the RV was markedly augmented in the PH group; the enhancement was inhibited in the PH + TA + BPS group to the greatest degree. Medial wall thickness of the pulmonary artery was markedly increased in the PH group; the increase was depressed in the PH + TA + BPS group. Combined treatment also ameliorated PH, even if it started after the onset of PH.

CONCLUSIONS

The combination of an oral ETA receptor antagonist and an oral PGI(2) analogue is superior to the single use of each drug alone in inhibiting the progression of PH.

Present and future pharmacotherapy for heart failure

The pharmacotherapy currently recommended by the American College of Cardiology and the American Heart Association for heart failure (HF) is a diuretic, an angiotensin-converting enzyme inhibitor (ACEI), a beta-adrenoceptor antagonist and (usually) digitalis. This current treatment of HF may be improved by optimising the dose of ACEI used, as increasing the dose of lisinopril increases its benefits in HF. Selective angiotensin receptor-1 (AT(1)) antagonists are effective alternatives for those who cannot tolerate ACEIs. AT(1) antagonists may also be used in combination with ACEIs, as some studies have shown cumulative benefits for the combination. In addition to being used in Stage IV HF patients, in whom it has a marked benefit, spironolactone should be studied in less severe HF and in the presence of beta-blockers. The use of carvedilol, extended-release metoprolol and bisoprolol should be extended to severe HF patients as these agents have been shown to decrease mortality in this group. The ancillary properties of carvedilol, particularly antagonism at prejunctional beta -adrenoceptors, may give it additional benefits to selective beta(1)-adrenoceptor antagonists. Celiprolol and bucindolol are not the beta-blockers of choice in HF, as they do not decrease mortality. Although digitalis does not reduce mortality, it remains the only option for a long-term positive inotropic effect, as the long-term use of the phosphodiesterase inhibitors is associated with increased mortality. The calcium sensitising drug levosimendan may be useful in the hospital treatment of decompensated HF to increase cardiac output and improve dyspnoea and fatigue. The antiarrhythmic drug amiodarone should probably be used in patients at high risk of arrhythmic or sudden death, although this treatment may soon be superseded by the more expensive implanted cardioverter defibrillators, which are probably more effective and have fewer side effects. The natriuretic peptide nesiritide has recently been introduced for the hospital treatment of decompensated HF. Novel drugs that may be beneficial in the treatment of HF include the vasopeptidase inhibitors and the selective endothelin-A receptor antagonists but these require much more investigation. However, disappointing results have been obtained in a large clinical trial of the tumour necrosis factor alpha antagonist etanercept, where no likelihood of a difference between placebo and etanercept was observed. Small clinical trials with recombinant growth hormone to thicken ventricles in dilated cardiomyopathy have given variable results.

Clinical efficacy of sitaxsentan, an endothelin-A receptor antagonist, in patients with pulmonary arterial hypertension: open-label pilot study

STUDY OBJECTIVES

To evaluate the safety and efficacy of sitaxsentan, an endothelin-A receptor antagonist, in a 12-week, open-label trial of patients with pulmonary arterial hypertension (PAH).

PATIENTS

Six children and 14 adults with New York Heart Association (NYHA) functional class II, III, or IV primary pulmonary hypertension or PAH associated with either congenital systemic-to-pulmonary shunts or collagen vascular disease were enrolled.

MEASUREMENTS

Sitaxsentan was administered orally at 100 to 500 mg bid for 12 weeks. Cardiopulmonary hemodynamics via cardiac catheterization were obtained at baseline and week 12. Six-minute walk test distance was measured at baseline, week 6, and week 12.

RESULTS

Sitaxsentan treatment resulted in significant improvement in exercise capacity as assessed by the 6-min walk distance (baseline [mean +/- SD], 466 +/- 132 m; week 12, 515 +/- 141 m, n = 20, p = 0.006). Mean pulmonary artery pressure and pulmonary vascular resistance index also improved (63 +/- 20 to 52 +/- 22 mm Hg, n = 17, p = 0.0002; and 20 +/- 11 to 14 +/- 13 U x m(2), n = 17, p = 0.008, respectively). Serious adverse events included two cases of acute hepatitis (fatal in one patient).

CONCLUSIONS

Patients with NYHA functional class II, III, or IV PAH showed a significant improvement in exercise capacity and cardiopulmonary hemodynamics over a 12-week period of treatment with sitaxsentan, an endothelin-A receptor antagonist. Further investigation is warranted to evaluate the safety and efficacy of sitaxsentan in patients with PAH.

Current management of primary pulmonary hypertension

Primary pulmonary hypertension (PPH) is a rare disorder of the lung vasculature characterised by an increase in pulmonary artery pressure. Although the aetiology of this disease remains unknown, knowledge of the pathophysiology of the disease has advanced considerably. Diagnosis of PPH is largely by exclusion. The clinical symptoms associated with PPH are aspecific and similar to those seen in other cardiovascular and pulmonary diseases. Electrocardiography, echocardiography, pulmonary function tests, and a lung perfusion scan are necessary to exclude secondary forms of pulmonary hypertension and also help to confirm the diagnosis of PPH. A definite diagnosis of PPH is established by right-heart catheterisation which gives a precise measure of the blood pressure in the right side of the heart and the pulmonary artery, right ventricular function and cardiac output. Once a diagnosis of PPH is established, treatment involving drug therapy or surgery is commenced on the basis of the New York Heart Association functional class. Conventional treatment consists of lifetime administration of anticoagulants, oxygen, diuretics, and digoxin. Vasodilator therapy with calcium channel antagonists is indicated in patients who are 'vasoreactive' to acute vasodilator challenge as assessed by right-heart catheterisation. Promising results are obtained by continuous intravenous administration of epoprostenol (prostacyclin). Newer therapies for PPH include prostacyclin analogues, endothelin receptor antagonists, nitric oxide, phosphodiesterase-5 inhibitors, elastase inhibitors, and gene therapy. Surgical treatment consists of atrial septostomy, thromboendarterectomy, and lung or heart-lung transplantation.

Endothelin antagonists and heart failure

Chronic heart failure (CHF) is characterized by impaired left ventricular function, increased peripheral and pulmonary vascular resistance, reduced exercise tolerance, and dyspnea. Despite considerable progress in the treatment of CHF, especially in targeting activated neurohumoral systems, mortality in these patients remains high. Therefore, new treatment approaches are warranted. Endothelin-1 (ET-1) plasma levels are elevated in CHF and correlate with both hemodynamic severity and symptoms. Plasma levels of ET-1 are strong independent predictors of mortality in CHF. Combined ET(A/B) selective ET(A) receptor antagonists have been evaluated in patients with CHF showing impressive hemodynamic improvements. These results indicate that ET receptor antagonists indeed have a potential to improve hemodynamics, symptoms, and potentially prognosis in patients with CHF, which still carries a high mortality.