Inhibition of endothelin receptors in the treatment of pulmonary arterial hypertension: does selectivity matter?

Emerging drugs for pulmonary hypertension

IMPORTANCE OF THE FIELD

Pulmonary arterial hypertension (PAH) is a clinical syndrome characterized by structural narrowing of the small pulmonary arteries that often culminates in fatal right heart failure.

AREAS COVERED IN THIS REVIEW

PubMed was searched for PAH and treatment. Data from scientific meetings and pharmaceutical websites are also included. There are currently eight FDA approved drugs for PAH that fall into one of three classes: prostacyclins, endothelin-receptor antagonists and PDE-5 inhibitors. All have important limitations and morbidity and mortality remain high. Several new agents with similar mechanisms of action are in clinical development. Multiple novel therapeutic targets are being explored. New applications for PAH therapies, such as pulmonary hypertension due to left heart and lung disease, are also being investigated.

WHAT THE READER WILL GAIN

An understanding of currently available drugs and those in clinical development for pulmonary hypertension.

TAKE HOME MESSAGE

Drugs targeting the pulmonary vasculature have been an extremely active area of basic and clinical research for the past 20 years and will continue to be so for the foreseeable future. Considerable progress has been made, and yet there continues to be a great unmet medical need for developing more efficacious therapies.

Bosentan: a review of its use in the management of mildly symptomatic pulmonary arterial hypertension

Bosentan (Tracleer) is an orally administered dual endothelin-1 (ET-1) receptor antagonist approved for use in patients with WHO class II (mildly symptomatic) pulmonary arterial hypertension (PAH). Oral bosentan therapy was beneficial and generally well tolerated in patients with mildly symptomatic PAH. In a well designed, placebo-controlled trial in adolescents and adults with mildly symptomatic PAH, pulmonary vascular resistance was significantly reduced with bosentan relative to placebo, but the 6-minute walk distance did not increase significantly. Similarly, pediatric patients (most of whom had mildly symptomatic PAH) in a small uncontrolled trial experienced some improvement in hemodynamic variables with bosentan, but did not experience a significant increase in exercise capacity. Adverse events associated with bosentan were consistent with those seen in other indications, with major concerns being the potential for teratogenicity and hepatotoxicity, for which regular liver function monitoring is recommended. Overall, considering the progressive nature of PAH, bosentan extends the treatment options available to patients with mildly symptomatic PAH.

Selective endothelin-1 receptor type A inhibition in subjects undergoing cardiac surgery with preexisting left ventricular dysfunction: Influence on early postoperative hemodynamics

OBJECTIVE

A robust release of endothelin-1 with subsequent endothelin-A subtype receptor activation occurs in patients after cardiac surgery requiring cardiopulmonary bypass. Increased endothelin-A subtype receptor activation has been identified in patients with poor left ventricular function (reduced ejection fraction). Accordingly, this study tested the hypothesis that a selective endothelin-A subtype receptor antagonist administered perioperatively would favorably affect post-cardiopulmonary bypass hemodynamic profiles in patients with a preexisting poor left ventricular ejection fraction.

METHODS

Patients (n = 29; 66 +/- 2 years) with a reduced left ventricular ejection fraction (37% +/- 2%) were prospectively randomized in a blinded fashion, at the time of elective coronary revascularization or valve replacement requiring cardiopulmonary bypass, to infusion of the highly selective and potent endothelin-A subtype receptor antagonist sitaxsentan at 1 or 2 mg/kg (intravenous bolus; n = 9, 10 respectively) or vehicle (saline; n = 10). Infusion of the endothelin-A subtype receptor antagonist/vehicle was performed immediately before separation from cardiopulmonary bypass and again at 12 hours after cardiopulmonary bypass. Endothelin and hemodynamic measurements were performed at baseline, at separation from cardiopulmonary bypass (time 0), and at 0.5, 6, 12, and 24 hours after cardiopulmonary bypass.

RESULTS

Baseline plasma endothelin (4.0 +/- 0.3 fmol/mL) was identical across all 3 groups, but when compared with preoperative values, baseline values obtained from age-matched subjects with a normal left ventricular ejection fraction (n = 37; left ventricular ejection fraction > 50%) were significantly increased (2.9 +/- 0.2 fmol/mL, P < .05). Baseline systemic (1358 +/- 83 dynes/sec/cm(-5)) and pulmonary (180 +/- 23 dynes/sec/cm(-5)) vascular resistance were equivalent in all 3 groups. As a function of time 0, systemic vascular resistance changed in an equivalent fashion in the post-cardiopulmonary bypass period, but a significant endothelin-A subtype receptor antagonist effect was observed for pulmonary vascular resistance (analysis of variance; P < .05). For example, at 24 hours post-cardiopulmonary bypass, pulmonary vascular resistance increased by 40 dynes/sec/cm(-5) in the vehicle group but directionally decreased by more than 40 dynes/sec/cm(-5) in the 2 mg/kg endothelin-A subtype receptor antagonist group (P < .05). Total adverse events were equivalently distributed across the endothelin-A subtype receptor antagonist/placebo groups.

CONCLUSION

These unique findings demonstrated that infusion of an endothelin-A subtype receptor antagonist in high-risk patients undergoing cardiac surgery was not associated with significant hemodynamic compromise. Moreover, the endothelin-A subtype receptor antagonist favorably affected pulmonary vascular resistance in the early postoperative period. Thus, the endothelin-A subtype receptor serves as a potential pharmacologic target for improving outcomes after cardiac surgery in patients with compromised left ventricular function.

[Pulmonary arterial hypertension in collagenoses]

Pulmonary arterial hypertension is a rare disease of small pulmonary arteries of unknown origin characterised by endothelial dysfunction and cellular proliferation throughout all vessel layers, resulting in progressively elevated pulmonary arterial resistance with increasing right heart strain and finally right heart failure. The condition may develop in connective tissue diseases with variable frequency leading to a substantial worsening of prognosis. However, the spectrum of therapeutic options has broadened significantly in recent years. Several compounds have gained approval that act mainly as pulmonary vasodilators. Further drugs are under investigation, some of which target pulmonary vascular remodeling. Echocardiography remains the primary examination for disease detection. To classify pulmonary hypertension definite hemodynamic evaluation by means of right heart catheterisation and a thorough differential diagnosis are essential to provide the basis for further treatment. For differential therapy and assessment of follow-up profound knowledge is required, pointing to the need for close cooperation with specialised centres.

Bosentan: a review of its use in the management of digital ulcers associated with systemic sclerosis

Bosentan (Tracleer) is an orally administered dual endothelin-1 (ET-1) receptor antagonist approved in the EU for reducing the number of new digital ulcers in patients with systemic sclerosis and ongoing digital ulcer disease. Oral bosentan therapy was beneficial and generally well tolerated in patients with digital ulcers associated with systemic sclerosis. In well designed, placebo-controlled trials, bosentan treatment significantly reduced the number of new ulcers, but had no effect on ulcer healing, in patients with digital ulcers. Adverse events associated with bosentan were consistent with those seen during treatment for other indications, with major concerns being the potential for teratogenicity and hepatotoxicity, for which regular liver function monitoring is recommended. Overall, considering the large unmet need for therapeutic options in patients with digital ulcers, bosentan extends the treatment options available to patients with systemic sclerosis-associated digital ulcers.

Bosentan in the treatment of pulmonary arterial hypertension with the focus on the mildly symptomatic patient

Pulmonary arterial hypertension (PAH) is a progressive disease with poor survival outcomes. Bosentan is an oral endothelin-1 receptor antagonist (ERA) that has been shown in a large randomized placebo-controlled trial (BREATHE-1) to be effective at improving exercise tolerance in patients with PAH in functional class III and IV. Further studies have been conducted showing: benefit in smaller subgroups of PAH, eg, congenital heart disease, efficacy in combination with other PAH therapies, eg, sildenafil, improved long-term survival compared with historical controls. More recently, controlled trials of new ERAs have included patients with milder symptoms; those in functional class II. Analysis of the functional class II data is often limited by small numbers. These trials have generally shown a similar treatment effect to bosentan, but there are no controlled trials directly comparing these new ERAs. The EARLY trial exclusively enrolled functional class II patients and assessed hemodynamics at 6 months. Though significant, the reduction in pulmonary vascular resistance is merely a surrogate marker for the intended aim of delaying disease progression. Significant adverse effects associated with bosentan include edema, anemia and transaminase elevation. These may preclude a long duration of treatment. Further studies are required to determine optimum treatment strategy in mild disease.

Calcitonin gene-related peptide selectively relaxes contractile responses to endothelin-1 in rat mesenteric resistance arteries

We tested the hypothesis that endothelin-1 (ET-1) modulates sensory-motor nervous arterial relaxation by prejunctional and postjunctional mechanisms. Isolated rat mesenteric resistance arteries were investigated with immunohistochemistry, wire-myography, and pharmacological tools. ET(A)- and ET(B)-receptors could be visualized on the endothelium and smooth muscle and on periarterial fibers containing calcitonin gene-related peptide (CGRP). Arterial contractile responses to ET-1 (0.25-16 nM) were not modified by blockade of ET(B)-receptors, NO-synthase, and cyclooxygenase or desensitization of transient receptor potential cation channel, subfamily V, member 1 (TRPV1) with capsaicin. ET-1 reversed relaxing responses to CGRP in depolarized arteries. This effect was inhibited by ET(A)-antagonists. It was not selective because ET-1 also reversed relaxing responses to Na-nitroprusside (SNP) and because phenylephrine (PHE; 0.25-16 microM) similarly reversed relaxing responses to CGRP or SNP. Conversely, contractile responses to ET-1 were, compared with PHE, hypersensitive to the relaxing effects of the TRPV1-agonist capsaicin and to exogenous CGRP, but not to acetylcholine, forskolin, pinacidil, or SNP. In conclusion, ET-1 does not stimulate sensory-motor nervous arterial relaxation, but ET(A)-mediated arterial contractions are selectively sensitive to relaxation by the sensory neurotransmitter CGRP. This does not involve NO, cAMP, or ATP-sensitive K(+) channels.

Ambrisentan for the treatment of pulmonary arterial hypertension

Ambrisentan is an endothelin receptor antagonist (ERA) that was recently approved for treatment of pulmonary arterial hypertension (PAH). Endothelin (ET) is a potent vasoconstrictor with mitogenic, hypertrophic and pro-inflammatory properties that is upregulated in pulmonary hypertensive diseases. The biologic effects of ET are mediated by 2 cell surface receptors termed ET_A and ET_B. ET_A mediates the vasoconstrictor effect of ET on vascular smooth muscle, whereas ET_B is expressed primarily on vascular endothelial cells where it induces nitric oxide synthesis and acts to clear ET from the circulation. Ambrisentan is the first ET_A selective ERA approved for use in the US. Recently published clinical trials in patients with PAH demonstrate improvement in functional capacity and pulmonary hemodynamics similar to other ET_A selective and non-selective ERAs. Its once daily dosing and lower incidence of serum aminotransferase elevation offer potential advantages over other ERAs, but further experience with this agent is needed to fully understand its long-term efficacy and safety. This review discusses the endothelin family of proteins and receptors and their role in the pathophysiology of pulmonary hypertensive diseases. It also examines the development process, safety profile and clinical trials that have resulted in ambrisentan being approved for treatment of PAH.

Ambrisentan

Elevated endothelin (ET)-1 levels are strongly correlated with the pathogenesis and prognosis of pulmonary arterial hypertension (PAH). Ambrisentan is an orally active, highly selective ETA receptor antagonist with >4000-fold higher selectivity over the ETB receptor. In two large, well designed, 12-week, placebo-controlled, phase III trials (ARIES-1, n = 202 and ARIES-2, n = 192) in patients with PAH (WHO group I), ambrisentan 2.5-10 mg once daily significantly increased 6-minute walk distance by 31-59 m from baseline (primary outcome measure) versus placebo. The incidence of clinical worsening (secondary outcome measure) was significantly delayed for the combined ambrisentan 5 mg once daily groups versus the combined placebo groups from ARIES-1 and -2. At week 12, WHO functional class distribution was significantly improved with once-daily ambrisentan 5 mg, and Borg dyspnoea scores were significantly improved with ambrisentan 2.5-10 mg versus placebo in combined data from the ARIES-1 and -2 trials. The beneficial effects of ambrisentan on exercise capacity, WHO functional class and Borg dyspnoea scores seen at 12 weeks were maintained at 48 weeks in the ARIES-E phase III extension trial (n = 361). One-year survival rates with ambrisentan were 95-97%. Treatment with ambrisentan for up to 2.8 years was generally well tolerated in clinical trials.

Ethanol alters the physiology of neuron-glia communication

In the central nervous system (CNS), both neurones and astrocytes play crucial roles. On a cellular level, brain activity involves continuous interactions within complex cellular circuits established between neural cells and glia. Although it was initially considered that neurones were the major cell type in cerebral function, nowadays astrocytes are considered to contribute to cerebral function too. Astrocytes support normal neuronal activity, including synaptic function, by regulating the extracellular environment with respect to ions and neurotransmitters. There is a plethora of noxious agents which can lead to the development of alterations in organs and functional systems, and that will end in a chronic prognosis. Among the potentially harmful external agents we can find ethanol consumption, whose consequences have been recognized as a major public health concern. Deregulation of cell cycle has devastating effects on the integrity of cells, and has been closely associated with the development of pathologies which can lead to dysfunction and cell death. An alteration of normal neuronal-glial physiology could represent the basis of neurodegenerative processes. In this review we will pay attention on to the recent findings in astrocyte function and their role toward neurons under ethanol consumption.