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

Fly casting with ligand sliding and orientational selection supporting complex formation of a GPCR and a middle sized flexible molecule

A GA-guided multidimensional virtual-system coupled molecular dynamics (GA-mD-VcMD) simulation was conducted to elucidate binding mechanisms of a middle-sized flexible molecule, bosentan, to a GPCR protein, human endothelin receptor type B (hETB). GA-mD-VcMD is a generalized ensemble method that produces a free-energy landscape of the ligand-receptor binding by searching large-scale motions accompanied with stable maintenance of the fragile cell-membrane structure. All molecular components (bosentan, hETB, membrane, and solvent) were represented with an all-atom model. Then sampling was conducted from conformations where bosentan was distant from the binding site in the hETB binding pocket. The deepest basin in the resultant free-energy landscape was assigned to native-like complex conformation. The following binding mechanism was inferred. First, bosentan fluctuating randomly in solution is captured using a tip region of the flexible N-terminal tail of hETB via nonspecific attractive interactions (fly casting). Bosentan then slides occasionally from the tip to the root of the N-terminal tail (ligand–sliding). During this sliding, bosentan passes the gate of the binding pocket from outside to inside of the pocket with an accompanying rapid reduction of the molecular orientational variety of bosentan (orientational selection). Last, in the pocket, ligand–receptor attractive native contacts are formed. Eventually, the native-like complex is completed. The bosentan-captured conformations by the tip-region and root-region of the N-terminal tail correspond to two basins in the free-energy landscape. The ligand-sliding corresponds to overcoming of a free-energy barrier between the basins.

The endothelin receptor antagonist macitentan for the treatment of pulmonary arterial hypertension: A cross-species comparison of its cytochrome P450 induction pattern

The dual endothelin receptor antagonist macitentan was approved in 2013 for the treatment of pulmonary arterial hypertension. Macitentan is an inducer of cytochrome P450 expression in vivo in animal species but not in man. In rat and dog, changes in P450 expression manifest as autoinduction upon repeat dosing. The induction pattern, however, significantly differed between both species, and between male and female rats. While macitentan exposure steadily declined with dose in the dog, P450 induction was saturable in the rat reaching levels of 40%-60% and 60%-80% at steady-state in male and female animals, respectively. The nature and number of P450 enzymes involved in macitentan clearance were identified as a major reason for the observed species differences. In the dog, macitentan was metabolized by a single P450 enzyme, that is, Cyp3a12, whereas several members of the Cyp2c and Cyp3a families were involved in the rat. Macitentan selectively upregulated Cyp3a expression in rat, whereas the expression of the Cyp2c enzymes involved in macitentan metabolism remained mostly unchanged, eventually leading to a higher contribution of Cyp3a upon induction. Macitentan also induced CYP3A4 expression in human hepatocytes via initial activation of the human pregnane X receptor. No such induction was evident in humans at the therapeutic macitentan dose of 10 mg as shown in a clinical drug-drug interaction study with the CYP3A4 substrate sildenafil.

Transfer and Vascular Effect of Endothelin Receptor Antagonists in the Human Placenta

Increasing evidence suggests a role for the ET (endothelin) system in preeclampsia. Hence, blocking this system with endothelin receptor antagonists (ERAs) could be a therapeutic strategy. Yet, clinical studies are lacking due to possible teratogenic effects of ERAs. In this study, we investigated the placental transfer of ERAs and their effect on ET-1-mediated vasoconstriction. Term placentas were dually perfused with the selective ET_AR (ET type A receptor) antagonists sitaxentan and ambrisentan or the nonselective ET_AR/ET_BR antagonist macitentan and subsequently exposed to ET-1 in the fetal circulation. ET-1 concentration-response curves after incubation with sitaxentan, ambrisentan, macitentan, or the selective ET_BR antagonist BQ-788 were also constructed in isolated chorionic plate arteries using wire-myography, and gene expression of the ET-system was quantified in healthy and early onset preeclamptic placentas. At steady state, the mean fetal-to-maternal transfer ratios were 0.32±0.05 for sitaxentan, 0.21±0.02 for ambrisentan, and 0.05±0.01 for macitentan. Except for BQ-788, all ERAs lowered the response to ET-1, both in the perfused cotyledon and isolated chorionic plate arteries. Placental gene expression of ECE-1, ET_AR, and ET_BR were comparable in healthy and preeclamptic placentas, while ET-1 expression was higher in preeclampsia. Our study is the first to show direct transfer of ERAs across the term human placenta. Furthermore, ET_AR exclusively mediates ET-1-induced constriction in the fetoplacental vasculature. Given its limited transfer, macitentan could be considered as potential preeclampsia therapy. Extending knowledge on placental transfer to placentas of preeclamptic pregnancies is required to determine whether ERAs might be applied safely in preeclampsia.

Treatment of pulmonary arterial hypertension with the dual endothelin receptor antagonist macitentan: clinical evidence and experience

Macitentan (10 mg once daily orally), a dual endothelin receptor antagonist (ERA) developed by modifying the structure of bosentan to increase the efficacity and safety, is approved for the treatment of pulmonary arterial hypertension (PAH). The pivotal SERAPHIN trial, (a landmark trial in the history of PAH trials because of the large number of included patients, the long-term follow up and the first trial with morbidity/mortality as the primary endpoint) showed a reduction of the risk of a morbidity or mortality event by 45% over the treatment time compared with placebo. The positive effect on the primary endpoint was observed whether or not the patient was already on PAH therapy. There has been no direct comparison between macitentan and other ERAs, which were approved based on improved exercise capacity, but preclinical and clinical data suggest better pharmacological and safety profiles. Further analyses of the SERAPHIN trial investigated the predictive value of different indices and events on long-term outcome and mortality. The efficacy in children, the long-term effects and safety of macitentan and its place in combination therapy compared with other ERAs are still under investigation. This review presents the preclinical evidence of superiority of macitentan compared with other ERAs, and the available clinical trial data. The place of macitentan in the therapeutic algorithm for PAH treatment, post-marketing experience and future perspectives are discussed.

X-ray structures of endothelin ETB receptor bound to clinical antagonist bosentan and its analog

Endothelin receptors (ETRs) have crucial roles in vascular control and are targets for drugs designed to treat circulatory-system diseases and cancer progression. The nonpeptide dual-ETR antagonist bosentan is the first oral drug approved to treat pulmonary arterial hypertension. Here we report crystal structures of human endothelin ET_B receptor bound to bosentan and to the ET_B-selective analog K-8794, at 3.6-Å and 2.2-Å resolution, respectively. The K-8794-bound structure reveals the detailed water-mediated hydrogen-bonding network at the transmembrane core, which could account for the weak negative allosteric modulation of ET_B by Na⁺ ions. The bosentan-bound structure reveals detailed interactions with ET_B, which are probably conserved in the ET_A receptor. A comparison of the two structures shows unexpected similarity between antagonist and agonist binding. Despite this similarity, bosentan sterically prevents the inward movement of transmembrane helix 6 (TM6), and thus exerts its antagonistic activity. These structural insights will facilitate the rational design of new ETR-targeting drugs.

Endothelin research and the discovery of macitentan for the treatment of pulmonary arterial hypertension

Endothelin receptor antagonists (ERAs) are used for the treatment of pulmonary arterial hypertension (PAH). Macitentan, a dual (ETA+ETB) ERA approved for the long-term treatment of PAH, was discovered through a tailored research program aimed at improving efficacy and safety over the existing ERAs. The goal of improved efficacy was based on the understanding that not only the ETA receptor but also the ETB receptor contributed to the hemodynamic and structural changes induced by endothelin-1 (ET-1) in pathological conditions and on the predefined requirements for optimal tissue penetration and binding kinetics of the antagonist. The goal of improved safety was based on the discovery of the role of ETB receptors in vascular permeability and vasopressin release and on the elucidation of the mechanism by which bosentan (the first approved oral dual ETA/ETB ERA) caused liver enzyme changes. Our intention was to design a molecule that would block ETA and ETB receptors optimally and would not interfere with bile salt elimination. This review takes us through the drug discovery journey that led to the discovery, development, and registration of macitentan.

Comparison of Macitentan and Bosentan on Right Ventricular Remodeling in a Rat Model of Non-vasoreactive Pulmonary Hypertension

Supplemental Digital Content Is Available in the Text.

We compared the efficacy of macitentan, a novel dual endothelin A/endothelin B receptor antagonist, with that of another dual endothelin receptor antagonist, bosentan, in a rat model of non-vasoreactive pulmonary hypertension (PH) with particular emphasis on right ventricular (RV) remodeling.

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.

Lack of Pharmacokinetic Interactions Between Macitentan and a Combined Oral Contraceptive in Healthy Female Subjects

Macitentan, a dual endothelin receptor antagonist used in pulmonary arterial hypertension, induces cytochrome P450 (CYP) 3A at supratherapeutic concentrations in vitro. Most combined oral hormonal contraceptives (OCs) are CYP3A substrates and their efficacy can be affected by CYP3A inducers. This randomized crossover study assessed possible pharmacokinetic (PK) interactions between macitentan and an OC containing ethinyl estradiol and norethindrone (or norethisterone). Twenty-six healthy women received a single oral dose of OC alone (reference) and concomitantly with 10 mg macitentan at steady state (test). No PK interaction was concluded if the 90% confidence intervals (CIs) of geometric mean ratios (GMRs; test/reference) of the peak plasma concentration (Cmax ) and the exposure from 0 to infinity (AUC0 - ∞ ) to the OC components were within the equivalence limits of 0.8 to 1.25. Cmax and AUC0-∞ of the OC were within the equivalence limits. For ethinyl estradiol, GMRs (90%CIs) of Cmax and AUC0-∞ were 0.92 (0.85-0.99) and 0.95 (0.90-0.99). For norethindrone, these values were 1.02 (0.95-1.09) and 1.04 (0.98-1.09), respectively. Overall, study treatments were well tolerated. No major changes from baseline in safety parameters were reported in either treatment. Macitentan does not affect the PK of OCs.

Incident and prevalent cohorts with pulmonary arterial hypertension: insight from SERAPHIN

In SERAPHIN, a long-term, randomised, controlled trial (NCT00660179) in pulmonary arterial hypertension (PAH), macitentan significantly reduced the risk of morbidity/mortality and PAH-related death/hospitalisation. We evaluated disease progression and the effect of macitentan in treatment-naïve incident and prevalent cohorts.

Patients allocated to placebo, or macitentan 3 mg or 10 mg were classified by time from diagnosis to enrolment as incident (≤6 months; n=110) or prevalent (>6 months; n=157). The risk of morbidity/mortality and PAH-related death/hospitalisation was determined using Cox regression.

The risk of morbidity/mortality (Kaplan–Meier estimates at month 12: 54.4% versus 26.7%; p=0.006) and PAH-related death/hospitalisation (Kaplan–Meier estimates at month 12: 47.3% versus 19.9%; p=0.006) were significantly higher for incident versus prevalent patients receiving placebo, respectively. There was no significant difference in the risk of all-cause death between incident and prevalent cohorts (p=0.587). Macitentan 10 mg significantly reduced the risk of morbidity/mortality and PAH-related death/hospitalisation versus placebo in incident and prevalent cohorts.

Incident patients had a higher risk for PAH progression compared with prevalent patients but not a higher risk of death. Macitentan delayed disease progression in both incident and prevalent PAH patients.

In the SERAPHIN trial, incident PAH patients had a higher risk of disease progression than prevalent patientshttp://ow.ly/RvCei

Development of macitentan for the treatment of pulmonary arterial hypertension

Pulmonary arterial hypertension (PAH) is a serious, chronic condition that, without early recognition and treatment, leads to progressive right heart failure and death. The dual endothelin receptor antagonist macitentan was designed through a deliberate discovery process to maximize endothelin-axis blockade while improving adverse-effect profiles compared with previous compounds. Macitentan's efficacy was demonstrated in an event-driven morbidity and mortality study of treatment-naive and background PAH therapy-treated symptomatic patients. Compared to placebo, 10 mg of macitentan significantly reduced the relative risk of morbidity and mortality by 45%, primarily by delaying PAH worsening, most prominently in World Health Organization (WHO) functional class II and III PAH patients. Macitentan reduced the incidence of the composite end point of PAH-related hospitalizations and mortality and improved WHO FC and exercise capacity (6-min walk distance). Furthermore, it significantly improved cardiopulmonary hemodynamics and quality of life, and had a favorable safety and tolerability profile. To date, this was the largest and longest prospective trial for PAH. Macitentan, currently the only approved oral PAH treatment shown to be safe and effective in delaying long-term progression and reducing PAH-related hospitalizations, has changed treatment paradigms from goal-directed to long-term outcome-oriented therapy.