Endothelin receptor antagonists for pulmonary arterial hypertension: rationale and place in therapy

Molecular Mechanisms and Therapeutic Implications of Endothelial Dysfunction in Patients with Heart Failure

Heart failure is a complex medical syndrome that is attributed to a number of risk factors; nevertheless, its clinical presentation is quite similar among the different etiologies. Heart failure displays a rapidly increasing prevalence due to the aging of the population and the success of medical treatment and devices. The pathophysiology of heart failure comprises several mechanisms, such as activation of neurohormonal systems, oxidative stress, dysfunctional calcium handling, impaired energy utilization, mitochondrial dysfunction, and inflammation, which are also implicated in the development of endothelial dysfunction. Heart failure with reduced ejection fraction is usually the result of myocardial loss, which progressively ends in myocardial remodeling. On the other hand, heart failure with preserved ejection fraction is common in patients with comorbidities such as diabetes mellitus, obesity, and hypertension, which trigger the creation of a micro-environment of chronic, ongoing inflammation. Interestingly, endothelial dysfunction of both peripheral vessels and coronary epicardial vessels and microcirculation is a common characteristic of both categories of heart failure and has been associated with worse cardiovascular outcomes. Indeed, exercise training and several heart failure drug categories display favorable effects against endothelial dysfunction apart from their established direct myocardial benefit.

A meta-analysis of the safety and efficacy of bosentan therapy combined with prostacyclin analogues or phosphodiesterase type-5 inhibitors for pulmonary arterial hypertension

Bosentan is an effective drug for the treatment of pulmonary arterial hypertension (PAH). The aim of the present meta-analysis was to examine the evidence concerning the efficacy and safety of bosentan therapy combined with prostacyclin analogues or phosphodiesterase type 5 (PDE-5) inhibitors for treating PAH. Eligible published studies were collected from Embase, PubMed, The Cochrane Library and the www.clinicaltrials.gov website. Heterogeneity was assessed using the Cochran Q-statistic test. Results were presented as risk ratios or mean differences with 95% confidence intervals (CI). A total of five studies, comprising 310 patients were included for analysis. No significant improvements in six-minute walk distance (6MWD; mean difference, 16.43 m), clinical worsening (risk ratio, 0.54) and the World Health Organization functional classification (class I: risk ratio, 1.17; class II: risk ratio, 1.18) were observed in patients treated with bosentan in combination with prostacyclin analogues or PDE-5 inhibitors. However, a significant reduction in the mean pulmonary artery pressure (mPAP; 95% CI: -17.06, -6.83; P<0.0001) following bosentan combination therapy was observed. Comparisons of adverse event rates in the bosentan combination therapy (55.6%) and monotherapy (51.8%) suggested that there is no reduction in adverse events (risk ratio, 1.10). The results indicated that bosentan combined with prostacyclin analogues or PDE-5 inhibitors may not improve 6MWD, cardiac function, clinical worsening and adverse events. However, bosentan combined with prostacyclin analogues or PDE-5 inhibitor therapy was able to significantly reduce mPAP compared with the effect of bosentan monotherapy.

Inundation of asthma target research: Untangling asthma riddles

Asthma is an inveterate inflammatory disorder, delineated by the airway inflammation, bronchial hyperresponsiveness (BHR) and airway wall remodeling. Although, asthma is a vague term, and is recognized as heterogenous entity encompassing different phenotypes. Targeting single mediator or receptor did not prove much clinical significant, as asthma is complex disease involving myriad inflammatory mediators. Asthma may probably involve a large number of different types of molecular and cellular components interacting through complex pathophysiological pathways. This review covers the past, present, and future therapeutic approaches and pathophysiological mechanisms of asthma. Furthermore, review describe importance of targeting several mediators/modulators and receptor antagonists involved in the physiopathology of asthma. Novel targets for asthma research include Galectins, Immunological targets, K ⁺ Channels, Kinases and Transcription Factors, Toll-like receptors, Selectins and Transient receptor potential channels. But recent developments in asthma research are very promising, these include Bitter taste receptors (TAS2R) abated airway obstruction in mouse model of asthma and Calcium-sensing receptor obliterate inflammation and in bronchial hyperresponsiveness allergic asthma. All these progresses in asthma targets, and asthma phenotypes exploration are auspicious in untangling of asthma riddles.

EDN1 Gene Variant is Associated with Neonatal Persistent Pulmonary Hypertension

Recent studies have suggested associations between certain genetic variants and susceptibility to persistent pulmonary hypertension of the newborn (PPHN). The aim of the study was to evaluate the association of EDN1, NOS3, ACE and VEGFA genes with PPHN. Neonates with respiratory distress were enrolled in the study, whose gestational age ≥34 weeks, age ≤3 days. They were divided into PPHN and non-PPHN group. The EDN1, NOS3, ACE and VEGFA genes were detected by next-generation sequencing, and the results were validated by Sanger sequencing. Serum endothelin-1 (ET-1) levels were quantified by ELISA. A total of 112 neonates were enrolled (n = 55 in PPHN group; n = 57 in non-PPHN group). There is a significantly difference in the genotype distribution of EDN1 rs2070699 between the PPHN and non-PPHN group (P = 0). A higher frequency of the rs2070699 T allele was observed in the PPHN group (54.5% vs 27.2%; OR = 3.89; 95%CI 1.96-7.72). The rs2070699 T allele was associated with higher ET-1 levels (3.333 ± 2.517 pg/mL vs 1.223 ± 0.856 pg/mL; P = 0.002) and a longer ventilation period (5.8 ± 2.6 days vs 3.6 ± 3.3 days; P = 0). The results suggest there is an association between EDN1 and PPHN. The presence of the rs2070699 T allele increased the risk of PPHN in neonates with respiratory distress.

Effect of treatment with iloprost with or without bosentan on nailfold videocapillaroscopic alterations in patients with systemic sclerosis

INTRODUCTION

Vascular involvement plays a decisive role in systemic sclerosis (SSc) pathogenesis; it is responsible for some important clinical manifestations of the disease such as Raynaud's phenomenon and digital ulcers (DU). Bosentan, a dual receptor endothelin antagonist, and iloprost, often in combination therapy, seems to be able to interfere with the scleroderma microangiopathy.

OBJECTIVES

Aim of the study was to evaluate the effect of bosentan and iloprost on scleroderma microangiopathy, analyzed by means of capillaroscopic skin ulcer risk index (CSURI), in SSc patients treated for the prevention of DU.

METHODS

Nailfold videocapillaroscopy (NVC) was performed in 95 SSc patients, treated with iloprost alone (group 1) or combination therapy with iloprost and bosentan (group 2), at baseline and after one year. In all patients CSURI was calculated according to the formula "diameter × number of megacapillaries/(total number of capillaries)²": in addition, total number of capillaries, giant capillaries, micro-hemorrhages, disorganization of the vascular array, and ramified capillaries were evaluated by means of a semiquantitative score.

RESULTS

After 12 months, we observed a reduction of the number of giant capillaries in both groups, while an increase of ramified capillaries was recorded only in group 2. CSURI improved slightly in group 2 without statistical significance; on the contrary, in group 1 a significant worsening was recorded (p ≤ 0.001).

CONCLUSIONS

Our study confirms the effectiveness of bosentan, in combination with iloprost, in SSc microangiopathy observed to NVC. Moreover, the observed findings further support the role of CSURI in the evaluation and monitoring of SSc microangiopathy.

Endothelin-Receptor Antagonists beyond Pulmonary Arterial Hypertension: Cancer and Fibrosis

The endothelin axis and in particular the two endothelin receptors, ETA and ETB, are targets for therapeutic intervention in human diseases. Endothelin-receptor antagonists are in clinical use to treat pulmonary arterial hypertension and have been under clinical investigation for the treatment of several other diseases, such as systemic hypertension, cancer, vasospasm, and fibrogenic diseases. In this Perspective, we review the molecules that have been evaluated in human clinical trials for the treatment of pulmonary arterial hypertension, as well as other cardiovascular diseases, cancer, and fibrosis. We will also discuss the therapeutic consequences of receptor selectivity with regard to ETA-selective, ETB-selective, or dual ETA/ETB antagonists. We will also consider which chemical characteristics are relevant to clinical use and the properties of molecules necessary for efficacy in treating diseases against which known molecules displayed suboptimal efficacy.

Peroxisome Proliferator-Activated Receptor γ and microRNA 98 in Hypoxia-Induced Endothelin-1 Signaling

Endothelin-1 (ET-1) plays a critical role in endothelial dysfunction and contributes to the pathogenesis of pulmonary hypertension (PH). We hypothesized that peroxisome proliferator-activated receptor γ (PPARγ) stimulates microRNAs that inhibit ET-1 and pulmonary artery endothelial cell (PAEC) proliferation. The objective of this study was to clarify molecular mechanisms by which PPARγ regulates ET-1 expression in vitro and in vivo. In PAECs isolated from patients with pulmonary arterial hypertension, microRNA (miR)-98 expression was reduced, and ET-1 protein levels and proliferation were increased. Similarly, hypoxia reduced miR-98 and increased ET-1 levels and PAEC proliferation in vitro. In vivo, hypoxia reduced miR-98 expression and increased ET-1 and proliferating cell nuclear antigen (PCNA) levels in mouse lung, derangements that were aggravated by treatment with the vascular endothelial growth factor receptor antagonist Sugen5416. Reporter assays confirmed that miR-98 binds directly to the ET-1 3'-untranslated region. Compared with littermate control mice, miR-98 levels were reduced and ET-1 and PCNA expression were increased in lungs from endothelial-targeted PPARγ knockout mice, whereas miR-98 levels were increased and ET-1 and PCNA expression was reduced in lungs from endothelial-targeted PPARγ-overexpression mice. Gain or loss of PPARγ function in PAECs in vitro confirmed that alterations in PPARγ were sufficient to regulate miR-98, ET-1, and PCNA expression. Finally, PPARγ activation with rosiglitazone regimens that attenuated hypoxia-induced PH in vivo and human PAEC proliferation in vitro restored miR-98 levels. The results of this study show that PPARγ regulates miR-98 to modulate ET-1 expression and PAEC proliferation. These results further clarify molecular mechanisms by which PPARγ participates in PH pathogenesis and therapy.

Effect of Ambrisentan on Exercise Capacity in Adult Patients After the Fontan Procedure

The Fontan operation is a common end point for children born with a single functional ventricle. Fontan patients typically experience physiological deterioration leading to transplant or death in their third or fourth decades of life. This deterioration is partially attributable to progressive increases in pulmonary vascular resistance (PVR) and as such endothelin receptor antagonists, which are known to decrease pulmonary vascular resistance, have been proposed as potentially beneficial in this population. We conducted a single-center, randomized, double-blind, placebo-controlled, crossover study of 12 weeks of ambrisentan therapy (10 mg per day) versus placebo to test the hypothesis that endothelin receptor antagonism will improve cardiopulmonary exercise test parameters and 36-item short form (SF-36) assessed quality of life in adult Fontan patients. Twenty-eight patients entered the trial, 19 patients completed the protocol. Ambrisentan therapy improved peak oxygen consumption by 1.7 ml/kg/min in patients who achieved a respiratory exchange ratio of >0.95 (p = 0.05) and decreased the slope of the ventilatory equivalent ratio for oxygen (-2.8, p = 0.019) in all completers. It did not change SF-36 physical function score compared with placebo (p = 0.28). Ambrisentan therapy resulted in a decrease in (-1.4 g/dl, p <0.001) with no change in liver or renal function. Therapy was generally well tolerated, with no greater rate of side effects than placebo. In conclusion, ambrisentan is well tolerated and improves exercise capacity in adult Fontan patients.

Endothelin-1 induces VCAM-1 expression-mediated inflammation via receptor tyrosine kinases and Elk/p300 in human tracheal smooth muscle cells

The elevated level of endothelin-1 (ET-1) has been detected in the bronchoalveolar lavage of patients with severe asthma, acute lung injury, acute respiratory distress syndrome, and sepsis. ET-1 may affect vessel tone together with lung physiology and pathology. Vascular cell adhesion molecule-1 (VCAM-1) is one kind of adhesion molecules participating in the process of polymorphonuclear leukocyte transmigration and regulating the occurrence and amplification of tissue inflammation. However, the molecular mechanisms underlying ET-1-mediated expression of VCAM-1 on human tracheal smooth muscle cells (HTSMCs) were largely unknown. Here we reported that ET-1 stimulated expression of VCAM-1 gene on HTSMCs, which was blocked by pretreatment with the inhibitors of ET receptors, Src, matrix metalloproteinases (MMPs), epidermal growth factor receptor (EGFR), platelet-derived growth factor receptor (PDGFR), phosphatidylinositol 3-kinase (PI3K), AKT, MEK1/2, and p300, suggesting the participation of these signaling components in ET-1-regulated HTSMC responses. Furthermore, transfection with small-interfering RNA (siRNA) of Src, AKT, p42 mitogen-activated protein kinase (MAPK), or p300 downregulated the respective proteins and significantly attenuated ET-1-induced VCAM-1 expression. ET-1 also stimulated phosphorylation of Src, EGFR, PDGFR, AKT, p42/p44 MAPK, and Elk-1 and acetylation of histone H4 on HTSMCs. Immunoprecipitation assay showed the association between Elk-1 and p300 in the nucleus. Adhesion assay revealed that the adhesion of THP-1 to HTSMCs challenged with ET-1 was increased, which was attenuated by the inhibitors of ET receptors, Src, MMPs, EGFR, PDGFR, PI3K, AKT, p42/p44 MAPK, and p300. Taken together, these data suggested that ET-1 promotes occurrence and amplification of pathology-related airway inflammation via enhancing VCAM-1 expression in an ET receptor/Src/MMP/EGFR, PDGFR/PI3K/AKT/p42/p44 MAPK/Elk-1/p300 pathway in HTSMCs.

Macitentan: a review of its use in patients with pulmonary arterial hypertension

Macitentan (Opsumit(®)) is an orally active, dual endothelin receptor antagonist (ERA) with tissue targeting properties. Macitentan was approved recently in the EU (as monotherapy or combination therapy) for the long-term treatment of pulmonary arterial hypertension (PAH) in adults of WHO functional class II or III, and in the USA for the treatment of PAH (WHO group I) to delay disease progression and reduce hospitalization for PAH. This article reviews the pharmacological properties, efficacy and tolerability data relevant to the use of macitentan in this indication. Treatment with macitentan 10 mg once daily significantly reduced the risk for the primary composite endpoint of morbidity and mortality in patients with PAH (mostly WHO functional class II or III) in the large, randomized, placebo-controlled SERAPHIN study. Other efficacy outcomes, including exercise capacity, haemodynamic parameters and health-related quality of life also improved significantly with macitentan relative to placebo. Macitentan was generally well tolerated in this study. As with other ERAs, haemoglobin levels decreased with macitentan therapy; however, these were not progressive and stabilized following longer-term treatment. Although comparative studies are needed to definitively position macitentan with respect to other approved agents, current evidence suggests that macitentan is a useful treatment option for initial therapy in patients with WHO functional class II or III PAH, which has the potential advantage of once-daily administration.

Pharmacokinetic and pharmacodynamic evaluation of macitentan , a novel endothelin receptor antagonist for the treatment of pulmonary arterial hypertension

INTRODUCTION

Pulmonary arterial hypertension (PAH) is a chronic disorder of the pulmonary vasculature characterized by elevated mean pulmonary arterial pressure eventually leading to right-sided heart failure and premature death. Macitentan is an oral, once-daily, dual endothelin (ET)A and ETB receptor antagonist with high affinity and sustained receptor binding that was approved in the USA, Europe, Canada, and Switzerland for the treatment of PAH.

AREAS COVERED

This review discusses the pharmacokinetics (PK) and pharmacodynamics (PD) of macitentan and its drug interaction potential based on preclinical and clinical data.

EXPERT OPINION

Up to date, macitentan is the only registered treatment for PAH that significantly reduced morbidity and mortality as a combined endpoint in a long-term event-driven study. The safety profile of macitentan is favorable with respect to hepatic safety and edema/fluid retention and may be better than that of other ET receptor antagonists such as bosentan and ambrisentan. The PK profile supports a once-a-day dosing regimen. Macitentan has limited interactions with other drugs. Based on these characteristics macitentan is an important new addition to the treatment of PAH.

Pharmacokinetic-pharmacodynamic relationships of macitentan, a new endothelin receptor antagonist, after multiple dosing in healthy Korean subjects

BACKGROUND AND OBJECTIVES

Macitentan is a novel dual endothelin (ET)-1 receptor antagonist to be used in patients with pulmonary arterial hypertension. This study aimed to assess the pharmacokinetics (PK) and pharmacodynamics (PD) of macitentan after administration of multiple doses to healthy Korean male subjects.

METHODS

A randomized, double-blind, placebo-controlled, multiple-ascending dose study was performed in 30 healthy male subjects receiving oral macitentan (3, 10, or 30 mg) or placebo once daily for 10 days. Plasma concentrations of macitentan, its active metabolite ACT-13277, and ET-1 were evaluated. Safety and tolerability measurements were conducted throughout the study.

RESULTS

The concentration-time profile of macitentan was characterized by slow absorption (median time to maximum plasma concentration [t(max)] 9-10 h) and slow elimination (mean elimination half-life [t ½] 11-15 h). After repeated doses of 3, 10, and 30 mg of macitentan over the course of 10 days, the peak concentration (C(max)) increased as the dose increased and the area under the plasma concentration-time curve during the dosing interval (AUC(τ)) increased in a dose-proportional manner. Plasma concentrations showed approximately 1.5- to 1.9-fold accumulation on day 10 compared with day 1. ACT-132577 showed higher levels of exposure than macitentan, its mean half-life was 46-48 h, and it accumulated 7- to 12-fold. Macitentan increased plasma ET-1 concentrations at all doses tested and was well tolerated and elicited no serious adverse events.

CONCLUSION

Multiple oral doses of 3, 10, and 30 mg of macitentan were well tolerated in healthy Korean subjects, and its pharmacokinetics correlated positively with ET-1 concentrations.

Distinct ETA receptor binding mode of macitentan as determined by site directed mutagenesis

The competitive endothelin receptor antagonists (ERA) bosentan and ambrisentan, which have long been approved for the treatment of pulmonary arterial hypertension, are characterized by very short (1 min) occupancy half-lives at the ET_A receptor. The novel ERA macitentan, displays a 20-fold increased receptor occupancy half-life, causing insurmountable antagonism of ET-1-induced signaling in pulmonary arterial smooth muscle cells. We show here that the slow ET_A receptor dissociation rate of macitentan was shared with a set of structural analogs, whereas compounds structurally related to bosentan displayed fast dissociation kinetics. NMR analysis showed that macitentan adopts a compact structure in aqueous solution and molecular modeling suggests that this conformation tightly fits into a well-defined ET_A receptor binding pocket. In contrast the structurally different and negatively charged bosentan-type molecules only partially filled this pocket and expanded into an extended endothelin binding site. To further investigate these different ET_A receptor-antagonist interaction modes, we performed functional studies using ET_A receptor variants harboring amino acid point mutations in the presumed ERA interaction site. Three ET_A receptor residues significantly and differentially affected ERA activity: Mutation R326Q did not affect the antagonist activity of macitentan, however the potencies of bosentan and ambrisentan were significantly reduced; mutation L322A rendered macitentan less potent, whereas bosentan and ambrisentan were unaffected; mutation I355A significantly reduced bosentan potency, but not ambrisentan and macitentan potencies. This suggests that – in contrast to bosentan and ambrisentan - macitentan-ET_A receptor binding is not dependent on strong charge-charge interactions, but depends predominantly on hydrophobic interactions. This different binding mode could be the reason for macitentan's sustained target occupancy and insurmountable antagonism.

Segmentation features and structural organization of the intrapulmonary artery of the yak

This study aims to systematically investigate intrapulmonary artery segmentation, blood vessel wall characteristics and structure organization, and the interrelation between intrapulmonary artery structure and plateau hypoxia adaptation in yak. The normal intrapulmonary artery structure of the yak had been studied using histological methods and transmission electron microscopy. The intrapulmonary artery of the yak was also examined using morphometric analysis and angiography. Results showed that the elastic intrapulmonary artery is divided into two types, namely, classical and transitional elastic segments. The muscular intrapulmonary artery is divided into three types, namely, transitional, classical muscular, and muscular arteriole segments. In the transitional elastic artery, elastic fibers and smooth muscles are linked through three models of ends, lateral branches, and branch tops. Two phenomena are possible for the transition from the elastic intrapulmonary artery to the muscular artery. One phenomenon postulates that a less elastic membrane is first increased and then suddenly decreased, and another supposes that the elastic membrane is gradually reduced and assembled in one to two layers before entering the transitional muscular artery. The smooth muscle of the intrapulmonary artery tunica media had more apophysis; it was physically connected with elastic membrane or fiber and composed of functionally resilient unit of the intrapulmonary arterial wall. Glycogenosomes increased in the muscular intrapulmonary artery smooth muscle cells. It exist one to two layers intact smooth muscle in intrapulmonary arteriole, the presence of intact smooth muscle in the intrapulmonary arteriole of the yak is a kind of structure adaptation to low-oxygen environment.

Safety, tolerability, pharmacokinetics, and pharmacodynamics of macitentan, an endothelin receptor antagonist, in an ascending multiple-dose study in healthy subjects

This multiple-ascending-dose study investigated safety, tolerability, pharmacokinetics, and pharmacodynamics, of macitentan, a new endothelin receptor antagonist (ERA) with sustained receptor binding and enhanced tissue penetration properties compared to other ERAs. Healthy male subjects (n = 32) received once daily oral doses of macitentan (1 - 30 mg) or placebo for 10 days. Administration of macitentan was safe and well tolerated. Macitentan had no effect on bile salts, suggesting an improved liver safety profile. The multiple-dose pharmacokinetics of macitentan were dose-proportional and were characterized by a median tmax and apparent elimination half-life varying from 6.0 to 8.5 and 14.3 to 18.5 hours, respectively, for the different doses and minimal accumulation. ACT-132577, a metabolite with lower potency than macitentan, had a half-life of about 48 hours and accumulated approximately 8.5-fold. Compared to placebo, administration of macitentan caused a dose-dependent increase in plasma ET-1 with maximum effects attained at 10 mg. A small dose-dependent increase in the 6β-hydroxycortisol/cortisol urinary excretion ratio was observed, although there were no statistically significant differences between treatments including placebo. Effects of macitentan on cytochrome P450 enzyme 3A4 should be further evaluated in dedicated studies. The present results support investigation of macitentan in the management of pulmonary arterial hypertension and ET-1-dependent pathologies.

Combination Pharmacotherapy in the Treatment of Pulmonary Arterial Hypertension

Pulmonary arterial hypertension (PAH) is a disorder of the small pulmonary arteries characterized by progressive fibrotic and proliferative changes that result in an increased pulmonary vascular resistance. It is a progressive and debilitating disease that leads to right ventricular dysfunction, impairment in activity tolerance and eventually right-sided heart failure, and premature death. The treatment goals for PAH include improvement in symptoms, improvement in functional class and exercise class, decreased morbidity, and preventing mortality. Combination therapy in the treatment of PAH is an emerging therapeutic option. Combining therapies with differing mechanisms of action will maximize therapeutic benefits such as symptom control and increased rate of survival. The updated 2007 American College of Chest Physicians evidence-based clinical practice guidelines recommend combination therapy in functional classes III and IV if there is no improvement with current therapy or if there is deterioration in class. PAH monotherapy has been shown to improve symptoms, but the patients’ hemodynamic parameters may not be normalized, leading to further pulmonary vascular remodeling. Combination therapy offers an additional option for those patients who are unable to stabilize on monotherapy.

Slow receptor dissociation kinetics differentiate macitentan from other endothelin receptor antagonists in pulmonary arterial smooth muscle cells

Two endothelin receptor antagonists (ERAs), bosentan and ambrisentan, are currently approved for the treatment of pulmonary arterial hypertension (PAH), a devastating disease involving an activated endothelin system and aberrant contraction and proliferation of pulmonary arterial smooth muscle cells (PASMC). The novel ERA macitentan has recently concluded testing in a Phase III morbidity/mortality clinical trial in PAH patients. Since the association and dissociation rates of G protein-coupled receptor antagonists can influence their pharmacological activity in vivo, we used human PASMC to characterize inhibitory potency and receptor inhibition kinetics of macitentan, ambrisentan and bosentan using calcium release and inositol-1-phosphate (IP₁) assays. In calcium release assays macitentan, ambrisentan and bosentan were highly potent ERAs with K_b values of 0.14 nM, 0.12 nM and 1.1 nM, respectively. Macitentan, but not ambrisentan and bosentan, displayed slow apparent receptor association kinetics as evidenced by increased antagonistic potency upon prolongation of antagonist pre-incubation times. In compound washout experiments, macitentan displayed a significantly lower receptor dissociation rate and longer receptor occupancy half-life (ROt_1/2) compared to bosentan and ambrisentan (ROt_1/2∶17 minutes versus 70 seconds and 40 seconds, respectively). Because of its lower dissociation rate macitentan behaved as an insurmountable antagonist in calcium release and IP₁ assays, and unlike bosentan and ambrisentan it blocked endothelin receptor activation across a wide range of endothelin-1 (ET-1) concentrations. However, prolongation of the ET-1 stimulation time beyond ROt_1/2 rendered macitentan a surmountable antagonist, revealing its competitive binding mode. Bosentan and ambrisentan behaved as surmountable antagonists irrespective of the assay duration and they lacked inhibitory activity at high ET-1 concentrations. Thus, macitentan is a competitive ERA with significantly slower receptor dissociation kinetics than the currently approved ERAs. Slow dissociation caused insurmountable antagonism in functional PASMC-based assays and this could contribute to an enhanced pharmacological activity of macitentan in ET-1-dependent pathologies.

Pulmonary arterial hypertension: advances in pathophysiology and management

Pulmonary arterial hypertension (PAH) is a heterogeneous, hemodynamic, and pathophysiological state which is commonly found throughout the world, but the disease burden is greater in India and in other developing countries. It is a disease characterized by vascular obstruction and vasoconstriction leading to progressive increase in pulmonary vascular resistance and right ventricular failure. PAH is a progressive disorder carrying a poor prognosis; however, dramatic progress has occurred in our knowledge of its pathogenesis and consequently, its treatment over the last two decades. In this article, we attempt to provide an overview of the etiology, pathophysiology, and current therapeutic modalities in the treatment of PAH. Patients suspected to have PAH should be submitted to a battery of investigations which help in establishing the diagnosis, identifying the etiology, guiding in treatment and informing the prognosis. All patients should be considered for standard therapy with oxygen, anticoagulation, and diuretics for right heart failure. Oral calcium channel blockers should be used in patients with a favorable response to acute vasodilator challenge. Disease targeted therapies include prostacyclines, endothelin receptor blockers, and phosphodiesterase-5 inhibitors. A brief mention of new and potential therapeutic strategies is also included.

Macitentan: entry-into-humans study with a new endothelin receptor antagonist

PURPOSE

To study the pharmacokinetics, pharmacodynamics, and tolerability of rising single doses of macitentan, an endothelin receptor antagonist, in healthy male subjects.

METHODS

This double-blind, placebo-controlled study was performed in seven groups of eight healthy male subjects. Doses of 0.2, 1, 5, 25, 100, 300 and 600 mg or placebo (two subjects per group) were administered. Plasma macitentan and endothelin-1 and serum total bile salt concentrations were measured and analysed non-compartmentally. Plasma and urine were analysed qualitatively for the presence of metabolites and one of these, ACT-132577, was also measured quantitatively in plasma. Standard tolerability measurements were performed throughout the study.

RESULTS

Macitentan was slowly absorbed and, at a dose of 300 mg, the t(1/2) (95% confidence interval, CI) was 17.5 h (14.1, 21.8). The dose-proportionality coefficient β for C(max) (95% CI) was 0.83 (0.79, 0.87) indicating less than dose-proportional pharmacokinetics of macitentan. In plasma, a pharmacologically active oxidative depropyl metabolite, ACT-132577, was found whereas in urine two minor metabolites were detected. The t(1/2) of ACT-132577 (95% CI) was 65.6 h (53.1, 80.9). Macitentan dose-dependently increased endothelin-1 concentrations up to 2.2-fold (95% CI 1.4, 2.4) at a dose of 600 mg, but had no consistent effect on total bile salts. Macitentan was well tolerated up to and including a dose of 300 mg, the maximum tolerated dose. Headache, nausea and vomiting were dose-limiting adverse events.

CONCLUSION

The pharmacokinetic and tolerability profile of macitentan is consistent with a once-a-day dosing regimen and warrants further investigation in clinical studies.

The PPARγ ligand rosiglitazone attenuates hypoxia-induced endothelin signaling in vitro and in vivo

Peroxisome proliferator-activated receptor (PPAR) γ activation attenuates hypoxia-induced pulmonary hypertension (PH) in mice. The current study examined the hypothesis that PPARγ attenuates hypoxia-induced endothelin-1 (ET-1) signaling to mediate these therapeutic effects. To test this hypothesis, human pulmonary artery endothelial cells (HPAECs) were exposed to normoxia or hypoxia (1% O(2)) for 72 h and treated with or without the PPARγ ligand rosiglitazone (RSG, 10 μM) during the final 24 h of exposure. HPAEC proliferation was measured with MTT assays or cell counting, and mRNA and protein levels of ET-1 signaling components were determined. To explore the role of hypoxia-activated transcription factors, selected HPAECs were treated with inhibitors of hypoxia-inducible factor (HIF)-1α (chetomin) or nuclear factor (NF)-κB (caffeic acid phenethyl ester, CAPE). In parallel studies, male C57BL/6 mice were exposed to normoxia (21% O(2)) or hypoxia (10% O(2)) for 3 wk with or without gavage with RSG (10 mg·kg(-1)·day(-1)) for the final 10 days of exposure. Hypoxia increased ET-1, endothelin-converting enzyme-1, and endothelin receptor A and B levels in mouse lung and in HPAECs and increased HPAEC proliferation. Treatment with RSG attenuated hypoxia-induced activation of HIF-1α, NF-κB activation, and ET-1 signaling pathway components. Similarly, treatment with chetomin or CAPE prevented hypoxia-induced increases in HPAEC ET-1 mRNA and protein levels. These findings indicate that PPARγ activation attenuates a program of hypoxia-induced ET-1 signaling by inhibiting activation of hypoxia-responsive transcription factors. Targeting PPARγ represents a novel therapeutic strategy to inhibit enhanced ET-1 signaling in PH pathogenesis.

Efficacy of aminaftone in a rat model of monocrotaline-induced pulmonary hypertension

Pulmonary hypertension is characterized by increased vascular resistances, that could lead to right heart failure and death. Endothelin-1 (ET-1) is a peptide with strong vasoconstrictive and pro-fibrotic properties and is one of the main mediators of pulmonary hypertension. Aminaftone, a synthetic molecule derivative of 4-amynobenzoic acid, down-regulates ET-1 production in vitro by interfering with the transcription of the pre-pro-ET-1 gene. The aim of this study was to test whether the inhibition of ET-1 production by aminaftone attenuates the effects of pulmonary hypertension. Pulmonary hypertension was induced through s.c. injection of 60 mg/kg monocrotaline. The rats were randomly assigned to the following experimental groups: Control; Monocrotaline; Aminaftone 30 mg/kg/day; Aminaftone 150 mg/kg/day. After 5 weeks, mortality was significantly lower in the animals treated with aminaftone at both doses compared to monocrotaline alone. Aminaftone reduced plasma concentration of ET-1 and seemed to reduce right heart hypertrophy and the wall thickness of the pulmonary arteries at the highest dose. Aminaftone may represent a novel treatment strategy of pulmonary hypertension.

Regulation of blood pressure and salt homeostasis by endothelin

Endothelin (ET) peptides and their receptors are intimately involved in the physiological control of systemic blood pressure and body Na homeostasis, exerting these effects through alterations in a host of circulating and local factors. Hormonal systems affected by ET include natriuretic peptides, aldosterone, catecholamines, and angiotensin. ET also directly regulates cardiac output, central and peripheral nervous system activity, renal Na and water excretion, systemic vascular resistance, and venous capacitance. ET regulation of these systems is often complex, sometimes involving opposing actions depending on which receptor isoform is activated, which cells are affected, and what other prevailing factors exist. A detailed understanding of this system is important; disordered regulation of the ET system is strongly associated with hypertension and dysregulated extracellular fluid volume homeostasis. In addition, ET receptor antagonists are being increasingly used for the treatment of a variety of diseases; while demonstrating benefit, these agents also have adverse effects on fluid retention that may substantially limit their clinical utility. This review provides a detailed analysis of how the ET system is involved in the control of blood pressure and Na homeostasis, focusing primarily on physiological regulation with some discussion of the role of the ET system in hypertension.

Pharmacokinetic evaluation of ambrisentan

INTRODUCTION

Pulmonary arterial hypertension (PAH) is a life-threatening and progressive disease characterized by increasing pulmonary vascular resistance leading to right ventricular failure and premature death. Current therapies target three major pathways involving endothelin, prostacyclin and NO. Ambrisentan is an oral, once daily, selective endothelin receptor antagonist.

AREAS COVERED

This review focuses on, and critically appraises, the clinical efficacy and safety of ambrisentan as well as its pharmacokinetic and pharmacodynamic properties. The article also gives an expert perspective on the role of ambrisentan in the management of PAH.

EXPERT OPINION

Ambrisentan is an effective and safe treatment which is, in the authors' opinion, a valuable addition to the armamentarium against PAH. Ambrisentan offers a relative lack of drug interactions, once daily dosing and reassuring liver safety, offering safety and convenience advantages over bosentan. Presently, there is a lack of comparative studies between PDE5 inhibitors and endothelin receptor antagonists and a lack of data comparing bosentan with ambrisentan. This is hindering data-based conclusions regarding relative efficacy and further studies are needed to define the role of ambrisentan in the management of PAH.

Phosphodiesterase-5 inhibitors in management of pulmonary hypertension: safety, tolerability, and efficacy

Pulmonary arterial hypertension (PAH) is a progressive disease that causes severe disability and has no cure. Over the past 20 years, a variety of treatment options have evolved for the management of PAH. With an expanded therapeutic armamentarium come more complex decisions regarding treatment options. Agent selection depends upon several factors including efficacy, side effect profile, and cost, as well as convenience of administration. We have undertaken a review of phosphodiesterase-5 (PDE-5) inhibitors in PAH with a focus on efficacy and safety. A literature search was conducted using the Medline and Cochrane Central Register of Controlled Trials databases (1966-February 2010) for relevant randomized clinical studies. Overall, 10 studies met our inclusion criteria. Sildenafil was the most commonly studied agent, followed by tadalafil and vardenafil. Most trials found that the PDE-5 inhibitors significantly improved exercise capacity and lowered pulmonary pressures. However, there were conflicting results regarding these agents' impact on improving cardiac function and functional class. Overall, these medications were effective and well tolerated with a relatively benign side effect profile. The PDE-5 inhibitors are an important option in treating PAH. While most of the published clinical data involved sildenafil, the other PDE-5 inhibitors show promise as well. Further studies are needed to determine the optimal doses of this therapeutic drug class, as well as its effects as adjunctive therapy with other agents in PAH.

Pulmonary hypertension in chronic neonatal lung disease

Pulmonary hypertension is an uncommon but significantly challenging complication of chronic neonatal lung disease [CNLD] as it occurs in the "new bronchopulmonary dysplasia [BPD]". The presence of pulmonary hypertension may be sub-clinical and is often overlooked as it is not considered in all but the more severe cases of children with CNLD. Whilst the mainstays of therapy are supplemental oxygen and time and the majority of children will have resolution of their pulmonary hypertension with lung growth, the advent of newer pharmacological treatments has offered stability and perhaps a better prognosis for more severe cases of pulmonary hypertension.

Bosentan: in pediatric patients with pulmonary arterial hypertension

Bosentan is a dual endothelin-1 (ET-1) receptor antagonist that has affinity for ET-1 receptors A and B. In the EU, oral bosentan (Tracleer) is indicated to improve exercise capacity and symptoms in patients with pulmonary arterial hypertension (PAH) of WHO functional class III; benefits have also been seen in patients with WHO functional class II PAH. Bosentan is available as film-coated tablets, and a new dispersible formulation of bosentan has also recently been approved in the EU for the treatment of PAH in children aged > or =2 years. A noncomparative, multicenter, phase III trial (FUTURE-1), which was primarily designed to investigate the pharmacokinetics of dispersible bosentan in pediatric patients, demonstrated that increasing the dosage of bosentan from 2 to 4 mg/kg twice daily was unlikely to result in increased exposure to bosentan. Exploratory measures of efficacy in FUTURE-1 demonstrated that target dosages of twice-daily dispersible bosentan 4 mg/kg (in patients weighing <30 kg) or 120 mg (in patients weighing > or =30 kg) for 12 weeks were beneficial in pediatric patients (aged > or =2 to <12 years) with WHO functional class II or III PAH. Film-coated bosentan 31.25, 62.5, or 125 mg (in pediatric patients weighing 10-20, >20-40, or >40 kg, respectively) twice daily for 12 weeks significantly (p < 0.05) improved the majority of hemodynamic measures evaluated as an exploratory measure in pediatric patients (aged 3-15 years) with WHO functional class II or III PAH in another noncomparative, multicenter, pharmacokinetic trial (BREATHE-3). However, there was no significant change in peak oxygen consumption or mean walk distance in those patients capable (i.e. children aged > or =8 years) of performing the 6-minute walk test. Bosentan was generally well tolerated in clinical trials of pediatric patients with PAH, with most adverse events being mild or moderate in severity and resolving with continued treatment.

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.

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 affects 15-F2t-isoprostane adverse effects on postischemic rat hearts

BACKGROUND

15-F(2t)-isoprostane (IsoP), a marker of reactive oxygen species-induced oxidative stress, is increased after myocardial ischemia and reperfusion. It exerts deleterious effects on postischemic myocardium accompanied with increased release of endothelin-1 (ET-1), a potent vasoconstrictor. We hypothesized that IsoP exacerbates myocardial ischemia-reperfusion injury by stimulating ET-1 production, and that ET-1 blockade can attenuate or prevent these deleterious effects of IsoP.

METHODS

Adult rat hearts were perfused by the Langendorff technique with Krebs-Henseleit solution (KH) at a constant flow rate of 10 mL/min. Global myocardial ischemia was induced by stopping KH perfusion for 40 min followed by 60 min of reperfusion. Hearts were randomized to one of the five groups (n = 8 each): untreated control, treated with IsoP (100 nM), or the ET-1 receptor A/B antagonist bosentan (1 μM) alone or in combination 10 min prior to, during 40 min global ischemia and 15 min of reperfusion, or treated with IsoP as above plus delayed administration of bosentan after 15 min of reperfusion.

RESULTS

Coronary effluent ET-1 concentrations in the IsoP group were higher than those in the control group during ischemia and reperfusion (P < 0.05), which was associated with increased release of cardiac-specific creatine kinase, reduced cardiac contractility during reperfusion, and increased myocardial infarct size (all P < 0.05 versus control). Bosentan administration during early reperfusion exacerbated the IsoP deleterious effects, while delayed administration attenuated it.

CONCLUSION

15-F(2t)-isoprostane-induced ET-1 production during later reperfusion is detrimental to functional recovery of damaged myocardium, while ET-1 increase during early reperfusion seems to improve it.

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.