Endothelin Receptor Antagonists: Status Quo and Future Perspectives for Targeted Therapy

The endothelin axis, recognized for its vasoconstrictive action, plays a central role in the pathology of pulmonary arterial hypertension (PAH). Treatment with approved endothelin receptor antagonists (ERAs), such as bosentan, ambrisentan, or macitentan, slow down PAH progression and relieves symptoms. Several findings have indicated that endothelin is further involved in the pathogenesis of certain other diseases, making ERAs potentially beneficial in the treatment of various conditions. In addition to PAH, this review summarizes the use and perspectives of ERAs in cancer, renal disease, fibrotic disorders, systemic scleroderma, vasospasm, and pain management. Bosentan has proven to be effective in systemic sclerosis PAH and in decreasing the development of vasospasm-related digital ulcers. The selective ERA clazosentan has been shown to be effective in preventing cerebral vasospasm and delaying ischemic neurological deficits and new infarcts. Furthermore, in the SONAR (Study Of Diabetic Nephropathy With Atrasentan) trial, the selective ERA atrasentan reduced the risk of renal events in patients with diabetes and chronic kidney disease. These data suggest atrasentan as a new therapy in the treatment of diabetic nephropathy and possibly other renal diseases. Preclinical studies regarding heart failure, cancer, and fibrotic diseases have demonstrated promising effects, but clinical trials have not yet produced measurable results. Nevertheless, the potential benefits of ERAs may not be fully realized.

A patient-centered approach to the burden of symptoms in patients with scleroderma treated with Bosentan: A prospective single-center observational study

Systemic sclerosis (SSc) is a rare and complex autoimmune disease associated with poor vital and functional outcomes. The functional hindrance in patients derives from various disease-specific manifestations, including Raynaud's phenomenon and digital ulcers (DUs). Bosentan is an endothelin receptor antagonist capable of preventing the appearance of new DUs in patients with scleroderma. The present study aimed to evaluate the effects of Bosentan on the severity of Raynaud's phenomenon, DU-related symptoms and functional impairment during the first year of treatment. A prospective study that included adult patients with SSc admitted to the Rheumatology Department between January 2016 and January 2017 that were candidates for Bosentan therapy, was performed. All patients were asked to evaluate the burden of symptoms secondary to Raynaud's and DUs using a visual analogue scale (VAS), whereas functional hindrance was assessed via Health Assessment Questionnaire Disability Index (HAQ-DI). The outcomes were assessed at baseline and every 3 months during 1 year of therapy. Among the 41 patients included initially, 2 participants discontinued the treatment after 1 month due to adverse events (elevation of liver enzymes). The study cohort exhibited a significant improvement in HAQ-DI, VAS-R and VAS-DU scores in response to Bosentan therapy over the 1-year follow-up period. Higher scores at baseline predicted a weaker treatment-related improvement, with the risk of a poor outcome being increased by 220% for VAS-R, 116% for VAS-DU, whereas no increase was observed for HAQ-DI. The post-treatment improvement in VAS-DU levels was associated with a better outcome for HAQ-DI (R=0.44; P=0.005). This association was not identified for VAS-R (R=0.24; P=0.137). Throughout the follow-up period, patients with dyspnea presented with significantly higher HAQ-DI scores compared with non-dyspneic patients. Bosentan therapy may indirectly influence functionality and quality of life in patients with scleroderma by reducing the burden of Raynaud's and DU-related symptoms. Nonetheless, patients with SSc with a decreased symptom burden at baseline exhibited improved outcomes.

Bosentan monohydrate

In the title compound, C₂₇H₂₉N₅O₆S·H₂O {systematic name: 4-tert-butyl-N-[6-(2-hy­droxy­eth­oxy)-5-(2-meth­oxy­phen­oxy)-2-(pyrimidin-2-yl)pyrimidin-4-yl]benzene-1-sulfonamide monohydrate], the dihedral angle between the mean planes of the pyrimidine rings is 27.0 (1)°. The dihedral angle between the mean planes of the benzene rings is 47.7 (8)°, forming a U-shaped channel around the chain of twisted pyrimidine rings. The crystal packing is stabilized by O—H⋯O, O—H⋯N and N—H⋯O hydrogen bonds with a single water mol­ecule, and weak O—H⋯N inter­molecular inter­actions between the hy­droxy group and one of the pyrimidine rings producing an two-dimensional supra­molecular array in the bc plane. The title compound studied was a merohedral twin with the major component being approximately 57%.

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.

Role of endogenous endothelin in endothelial dysfunction in murine model of systemic sclerosis: tight skin mice 1

Systemic sclerosis (SSc) is a systemic inflammatory disorder, resulting in severe vascular dysfunction. The endothelin (ET) system has vasoconstrictor and profibrotic properties and has been shown to be activated in SSc. ET antagonists are currently used in SSc-related pulmonary arterial hypertension, but the endothelial impact of ET antagonists remains less known in SSc. We thus assessed the effects of the dual ET(A)-ET(B) antagonist, bosentan, on endothelial dysfunction in a murine model of SSc, the heterozygous tight-skin mice 1 (TSK1(+)). Six-week-old TSK1(+) were either untreated or treated for 6 weeks with bosentan (100 mg/kg/day), and compared with controls. Endothelial function was evaluated in isolated mesenteric resistance arteries, using a small vessel myograph. TSK1(+) displayed endothelial dysfunction, as shown by a decreased response of mesenteric arteries to acetylcholine, especially in the presence of L-nitro-arginine methyl ester (L-NAME), corresponding to NO-independent, prostaglandin-mediated relaxation. The NO-independent relaxation was partially restored in bosentan-treated TSK1(+), and this was abolished by a cyclo-oxygenase inhibitor. Therefore, the murine model of SSc, TSK1(+) exhibits severe endothelial dysfunction of peripheral resistance arteries. The ET antagonist bosentan prevents endothelial alterations, suggesting a major role of ET in the adverse vascular effects of SSc.

Effects of bosentan on the skin lesions: an observational study from a single center in Japan

Effects of a dual endothelin receptor antagonist, bosentan on peripheral circulatioin and skin lesions as well as pulmonary arterial hypertension (PAH) were investigated in Japanese patients with connective tissue diseases (CTD). Fifteen patients with PAH associated with CTD [systemic sclerosis (SSc) 13, mixed connective tissue disease (MCTD) 2] were treated with bosentan for 40-96 weeks, and changes of exercise capacity (6-min walk distance and Borg's dyspnea scale), cardio-pulmonary hemodynamics (right ventricular pressure, specific activity scale and cardiac index), Raynaud's phenomenon, digital ulcers and dermal sclerosis were observed. Bosentan improved exercise capacity, had a positive effect on hemodynamic parameters, and was well tolerated as previously reported. After a median 8 weeks of treatment, 13 out of 15 patients had improved Raynaud's phenomenon. Digital ulcers also improved after a median 12 weeks' treatment in all of 8 patients. Modified Rodnan total skin score decreased from 21.0 +/- 5.9 to 11.5 +/- 3.9 in diffuse cutaneous SSc and from 17.0 +/- 6.5 to 9.5 +/- 4.5 in limited cutaneous SSc after 24 months' treatment, reaching significance after 6 months in both groups. These data suggest that bosentan is effective for both PAH and peripheral vascular diseases in Japanese patients with CTD. The pathological background to the improvement in dermal sclerosis observed in this study should be further investigated.

[Treatment of systemic sclerosis]

Treatment of systemic sclerosis is based upon 3 types of medications modifiers of disease: drugs which prevent vascular damage, antifibrotric agents, and immunomodulators and immunosupressors. Drugs that prevent vascular damage such as: calcium antagonists, prostaglandins analogues, receptors of endothelin blockers (bosentan), inhibitors of angiotensin converting enzyme, receptors of angiotensin antagonists and inhibitors of 5'-phosphodiesterase have been successful in treating the Raynaud's phenomenon, renal crisis and pulmonary arterial hypertension. In contrast, the results of treatment of fibrosis are discouraging and the D-penicilamine continues being a matter of controversy. The immunosupressor therapy with cyclophosphamide and the transplant of hematopoietic cells, may be beneficial. The knowledge of the pathogenesis of systemic sclerosis to molecular level will lead to new treatment strategies.

Pulmonary Involvement in Systemic Sclerosis

Scleroderma, also known as progressive systemic sclerosis (SSc), is a multisystem autoimmune disorder characterized by inflammation and fibrosis involving the skin as well as internal organs such as the vasculature, esophagus, and the respiratory tract. Pulmonary involvement consists most often of interstitial fibrosis and pulmonary vascular disease leading to pulmonary arterial hypertension (PAH). Bronchiectasis is an uncommon pulmonary manifestation of systemic sclerosis. Pulmonary hemorrhage with acute renal failure and diffuse alveolar hemorrhage in the absence of a history of renal involvement or penicillamine intake have rarely been reported in patients with systemic sclerosis.On high resolution CT, evidence of interstitial disease is seen in approximately 90% of patients, the main findings being a fine reticular pattern involving the subpleural regions of the lower lobe. Other common findings include ground-glass opacities, honeycombing, and parenchymal micronodules. The most distinctive pulmonary histologic findings in patients with scleroderma are the vascular changes found in PAH in the absence of significant interstitial fibrosis.There is no strong evidence that any drug alters the course of the two main types of lung disease in systemic sclerosis. This apparent failure of therapy may reflect the fact that pulmonary involvement is usually identified at an established or late stage. It has been suggested that, for fibrosing alveolitis, corticosteroids are most effective if given in combination with cyclophosphamide. In some patients with SSc, PAH has been considered as a major cause of morbidity and mortality. Centrally infused prostacyclin (epoprostenol) and its subcutaneously infused analog treprostinil improve hemodynamics, as well as the quality of life and survival in these patients. Iloprost has also shown a positive effect on PAH in SSc patients. More recently, bosentan, an endothelin receptor antagonist, has proved effective in controlling PAH after 6 months' treatment. Sildenafil has been used as a selective pulmonary vasodilator in SSc patients with isolated PAH. This drug decreased mean pulmonary artery pressure and pulmonary vascular resistance, and increased cardiac output, with much improvement of the physical condition of the patients. Lung transplant can be considered as a last option.Clinicians must be aware of the possibility of lung disease in patients with SSc so that it can be treated as early as possible.