The pharmacology of bosentan

At the heart of tissue: endothelin system and end-organ damage

ET (endothelin)-1 was first described as a potent vasoconstrictor. Since then, many other deleterious properties mediated via its two receptors, ETA and ETB, have been described, such as inflammation, fibrosis and hyperplasia. These effects, combined with a wide tissue distribution of the ET system, its up-regulation in pathological situations and a local autocrine/paracrine activity due to a high tissue receptor binding, make the tissue ET system a key local player in end-organ damage. Furthermore, ET-1 interacts in tissues with other systems such as the RAAS (renin-angiotensin-aldosterone system) to exert its effects. In numerous genetically modified animal models, non-specific or organ-targeted ET-1 overexpression causes intense organ damage, especially hypertrophy and fibrosis, in the absence of haemodynamic changes, confirming a local activity of the ET system. ET receptor antagonists have been shown to prevent and sometimes reverse these tissue alterations in an organ-specific manner, leading to long-term benefits and an improvement in survival in different animal models. Potential for such benefits going beyond a pure haemodynamic effect have also been suggested by clinical trial results in which ET receptor antagonism decreased the occurrence of new digital ulcers in patients with systemic sclerosis and delayed the time to clinical worsening in patients with PAH (pulmonary arterial hypertension). The tissue ET system allows therapeutic interventions to provide organ selectivity and beneficial effects in diseases associated with tissue inflammation, hypertrophy or fibrosis.

Bosentan and the endothelin system in congestive heart failure

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

Pharmacology of macitentan, an orally active tissue-targeting dual endothelin receptor antagonist

Macitentan, also called Actelion-1 or ACT-064992 [N-[5-(4-bromophenyl)-6-(2-(5-bromopyrimidin-2-yloxy)ethoxy)-pyrimidin-4-yl]-N'-propylaminosulfonamide], is a new dual ET(A)/ET(B) endothelin (ET) receptor antagonist designed for tissue targeting. Selection of macitentan was based on inhibitory potency on both ET receptors and optimization of physicochemical properties to achieve high affinity for lipophilic milieu. In vivo, macitentan is metabolized into a major and pharmacologically active metabolite, ACT-132577. Macitentan and its metabolite antagonized the specific binding of ET-1 on membranes of cells overexpressing ET(A) and ET(B) receptors and blunted ET-1-induced calcium mobilization in various natural cell lines, with inhibitory constants within the nanomolar range. In functional assays, macitentan and ACT-132577 inhibited ET-1-induced contractions in isolated endothelium-denuded rat aorta (ET(A) receptors) and sarafotoxin S6c-induced contractions in isolated rat trachea (ET(B) receptors). In rats with pulmonary hypertension, macitentan prevented both the increase of pulmonary pressure and the right ventricle hypertrophy, and it markedly improved survival. In diabetic rats, chronic administration of macitentan decreased blood pressure and proteinuria and prevented end-organ damage (renal vascular hypertrophy and structural injury). In conclusion, macitentan, by its tissue-targeting properties and dual antagonism of ET receptors, protects against end-organ damage in diabetes and improves survival in pulmonary hypertensive rats. This profile makes macitentan a new agent to treat cardiovascular disorders associated with chronic tissue ET system activation.

Bosentan® inhibits tumor vascularization and bone metastasis in an immunocompetent skin-fold chamber model of breast carcinoma cell metastasis

Angiogenic factors including endothelin-1 (ET-1) play a key role in the progression of breast metastases to bone. We investigated the impact of ET-1 on the development of bone metastases in an immunocompetent murine skin-fold chamber model. Murine mammary carcinoma 4T1 was injected in a skin-fold chamber implanted on CB6 mice along with bone explants. Furthermore, mice were treated with or without a dual selective antagonist of both ET-1 receptors. The progression of the vascularization within the chamber was monitored over time by intravital microscopy (IVM). The tumor growth and the development of bone metastases were assessed by cytokeratin-19 gene expression and histological studies. Results indicate that this new model associated with IVM allows for the continuous monitoring of the change in vascularization associated with the development of bone metastases. Additionally, treatment with an antagonist of both ET-1 receptors was associated with the presence of significantly less vessels near the tumor mass compared to control mice. These changes were correlated with smaller tumor masses and reduced bone invasion (P < 0.05). Thus, in an immunocompetent murine model of breast carcinoma metastases to bone, our data support the hypothesis that vascularization plays a role in tumor development and progression and that ET-1 specifically modulates the angiogenesis associated with breast metastases to the bone.

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.

Animal models of human disease in drug safety assessment

Animal models of human disease have been widely used in drug discovery, but they are rarely utilized in toxicological research and screening (except for transgenic models in carcinogenicity testing). Although genetic and/or acquired pathophysiological alterations associated with a particular disease may greatly exacerbate toxic responses to drugs in certain patient subsets, these pre-existing pathological conditions are usually not considered in preclinical safety assessment. Examples of disease-related determinants of susceptibility include disruption of the cytokine network in pro-inflammatory conditions, mitochondrial alterations and oxidative stress in certain neurodegenerative diseases, altered antioxidant defense in certain viral infections, and altered gene expression and mitochondrial dysfunction in type 2 diabetes. Hence, if cellular stress caused by drugs or metabolites and the disease-related effects are superimposed, then an individual can become sensitized to potential drug toxicity. Animal models of modest inflammation indeed can potentiate the toxicity of certain drugs. Similarly, rodent models of type 2 diabetes predispose the animals to hepatotoxic effects of thiazolidinediones antidiabetics. In conclusion, it is suggested that tailor-made and simplified models be adopted and increasingly used, in spite of clear limitations, as optimal substrates for satellite toxicity studies to facilitate candidate selection, help predict rare and unexpected toxicity, and identify new biomarkers.

New therapeutics that antagonize endothelin: promises and frustrations

The discovery of endothelin--a highly potent endogenous vasoconstrictor - in 1988 has led to considerable efforts to develop antagonists of endothelin receptors that could have therapeutic potential in disorders including hypertension, heart failure and renal diseases. However, in general, the results of trials in humans have not mirrored the highly promising effects in animal disease models. Here, we discuss preclinical and clinical results with endothelin antagonists, and consider possible approaches to fully realizing the potential of endothelin antagonism.

Entry-into-humans study with tezosentan, an intravenous dual endothelin receptor antagonist

The purpose of this study was to investigate the tolerability, pharmacokinetics, and pharmacodynamics of ascending doses of tezosentan, an IV dual endothelin receptor antagonist, during first administration in humans. Tezosentan infused at doses of 5, 20, 50, 100, 200, 400, and 600 mg for 1 h was administered to sequential groups of six male subjects in a randomized, placebo-controlled, double-blind design. Recording of vital signs, electrocardiogram, adverse events, and clinical laboratory parameters monitored tolerability and safety. Blood samples were collected frequently for pharmacokinetic determinations and measurement of plasma endothelin-1 concentrations. Tezosentan was well tolerated at all dose levels. Headache was the most frequently reported adverse event and occurred at a higher incidence than with placebo at doses of > or = 100 mg. No clinically relevant changes in vital signs, electrocardiographic, or clinical laboratory parameters occurred. Plasma concentrations of tezosentan rapidly approached steady state and could be described by a two-compartment model. The volume of distribution at steady state (approximately 16 l) and the clearance (approximately 30 l/h) were considered independent of dose, in view of the wide dose range explored. A pronounced and rapid disposition phase (half-life 6 min), accounting for the major part of the elimination, was followed by a slower phase (half-life 3 h), probably caused by distribution from tissues. Endothelin-1 concentrations increased in a dose- and concentration-dependent fashion and returned slowly to baseline after termination of the infusion. Tezosentan warrants further clinical development in view of its tolerability and pharmacokinetic profile, which appears advantageous for application in emergency situations.

Coronary arterial lesions induced by endothelin antagonists

Endothelins are potent vasoconstrictors and pressor peptides and are important mediators of cardiac, renal and endocrine functions. Increased ET-1 levels in disease states such as congestive heart failure, pulmonary hypertension, acute myocardial infarction, and renal failure suggest the endothelin system as an attractive target for pharmacotherapy. A non-peptidic, selective, competitive endothelin receptor antagonist with an affinity for the ET(A) receptor in the subnanomolar range was administered by continuous intravenous infusion to beagle dogs, rats, and Goettingen minipigs. It caused mild arteriopathy characterised by segmental degeneration in the media of mid- to large-size coronary arteries in the heart of dog, but not rat or minipig. The lesions only occurred in the atrium and ventricle. Frequency and severity of the vascular lesions was not sex or dose related. No effects were noted in blood vessels in other organs or tissue. Plasma concentrations at steady state, and overall exposure in terms of AUC((0-24h)) were higher in minipig and rat than the dog but did not cause cardiac arteriopathy. These findings concur with those caused by other endothelin anatagonists, vasodilators and positive inotropic/vasodilating drugs such as potassium channel openers, phosphodiesterase inhibitors and peripheral vasodilators, and confirm that dogs appear to be uniquely sensitive to the development of cardiac vascular lesions.