Clinical Pharmacology of Endothelin
Receptor Antagonists Used in the
Treatment of Pulmonary Arterial
Hypertension
Marie-Camille Chaumais, Christophe
Guignabert, Laurent Savale, Xavier Jaïs,
Athénaïs Boucly, David Montani, Gérald
Simonneau, et al.
American Journal of Cardiovascular
Drugs
ISSN 1175-3277
Am J Cardiovasc Drugs
DOI 10.1007/s40256-014-0095-y
1 23
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1 23
Author's personal copy
Am J Cardiovasc Drugs
DOI 10.1007/s40256-014-0095-y
REVIEW ARTICLE
Clinical Pharmacology of Endothelin Receptor Antagonists Used
in the Treatment of Pulmonary Arterial Hypertension
Marie-Camille Chaumais • Christophe Guignabert •
Laurent Savale • Xavier Jaı̈s • Athénaı̈s Boucly • David Montani
Gérald Simonneau • Marc Humbert • Olivier Sitbon
•
Ó Springer International Publishing Switzerland 2014
Abstract Pulmonary arterial hypertension (PAH) is a
devastating life-threatening disorder characterized by elevated pulmonary vascular resistance leading to elevated
pulmonary arterial pressures, right ventricular failure, and
ultimately death. Vascular endothelial cells mainly produce
and secrete endothelin (ET-1) in vessels that lead to a
potent and long-lasting vasoconstrictive effect in pulmonary arterial smooth muscle cells. Along with its strong
vasoconstrictive action, ET-1 can promote smooth muscle
cell proliferation. Thus, ET-1 blockers have attracted
attention as an antihypertensive drug, and the ET-1 signaling system has paved a new therapeutic avenue for the
treatment of PAH. We outline the current understanding of
not only the pathogenic role played by ET-1 signaling
systems in the pathogenesis of PH but also the clinical
pharmacology of endothelin receptor antagonists (ERA)
used in the treatment of PAH.
1 Introduction
M.-C. Chaumais
Faculté de Pharmacie, Univ. Paris-Sud, Châtenay Malabry,
France
D. Montani G. Simonneau M. Humbert O. Sitbon
Faculté de Médecine, Univ. Paris-Sud, Le Kremlin-Bicêtre,
France
M.-C. Chaumais C. Guignabert L. Savale X. Jaı̈s
D. Montani G. Simonneau M. Humbert O. Sitbon
INSERM UMR_S 999, LabEx LERMIT, Centre Chirurgical
Marie Lannelongue, Le Plessis-Robinson, France
O. Sitbon (&)
Service de Pneumologie et Soins Intensifs, Hôpital Universitaire
de Bicêtre, 78, rue du Général Leclerc,
94270 Le Kremlin-Bicêtre, France
e-mail: olivier.sitbon@bct.aphp.fr
M.-C. Chaumais
AP-HP, Service de Pharmacie, Département HospitaloUniversitaire (DHU) Thorax Innovation, Hôpital Antoine
Béclère, Clamart, France
L. Savale X. Jaı̈s A. Boucly D. Montani G. Simonneau
M. Humbert O. Sitbon
AP-HP, Centre de Référence de l’Hypertension Pumonaire
Sévère, Département Hospitalo-Universitaire (DHU) Thorax
Innovation, Service de Pneumologie et, Hôpital Bicêtre,
Le Kremlin-Bicêtre, France
Pulmonary arterial hypertension (PAH) is a rare condition
characterized by severe remodeling of the small pulmonary
arteries, leading to chronic pre-capillary pulmonary
hypertension (defined by a mean pulmonary artery pressure
C25 mmHg with a mean pulmonary artery wedge pressure
B15 mmHg), right heart failure, and ultimately death.
Development of therapeutic agents that modulate the three
main dysfunctional pathobiologic pathways (endothelin
[ET-1], prostacyclin [PGI2], and nitric oxide [NO]) have
revolutionized our approach to the treatment of PAH and
have changed the course of this devastating disease [1].
However, although the spectrum of therapeutic options for
PAH has expanded in the last decade, available therapies
remain essentially palliative. Since 2002, the dual endothelin receptor antagonist (ERA) bosentan has been avail-
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M.-C. Chaumais et al.
able for the management of PAH. Research on new ERAs
has resulted in the development of two other drugs, ambrisentan and, more recently, macitentan.
recognized as important modulators of endothelial
dysfunction.
3 The Endothelin (ET)-1 Signaling Pathway in PAH
2 Pulmonary Endothelial Dysfunction in PAH
The various conditions that make up PAH share a broadly
similar pathobiology, leading to the establishment of a
treatment-based classification of the disease that is
endorsed by the World Health Organization (WHO).
Although the exact mechanisms of pulmonary vascular
remodeling associated with PAH are still unclear, many
contributing factors have been identified, including pulmonary endothelial dysfunction and excessive proliferation
of pulmonary vascular cells. Indeed, pulmonary endothelial
cells (ECs) are recognized as major regulators of vascular
function through their production of vasoconstrictors (e.g.
ET-1, serotonin [5-HT], angiotensin II [A-II]), vasodilators
(NO and prostacyclin), activators and inhibitors of smooth
muscle cell (SMC) growth and migration, prothrombotic
and antithrombotic mediators, and proinflammatory signals
[2]. In healthy individuals, a balance between these molecules is thought to mediate the low basal pulmonary vascular tone and homeostasis [3]. However, the dysfunctional
endothelium displays, to varying degrees, an imbalanced
production of several mediators, leading towards an excess
of vasoconstriction, smooth muscle hyperplasia, and pulmonary vascular remodeling [4–7]. Furthermore, our group
[8] and others [9] have clearly shown that both proliferation and survival of pulmonary ECs in PAH are enhanced
not only in situ but also in vitro when removed from their
in vivo environment. An autocrine fibroblast growth factor
(FGF)-2 activation loop is among the mechanisms that
partly drive this abnormal hyper-proliferative and apoptosis-resistant endothelial phenotype [8]. In addition, we
obtained evidence that pulmonary ECs from patients with
idiopathic PAH (IPAH) release excessive amounts of soluble growth factors and cytokines able to act on different
types of pulmonary vascular cells in the vascular wall (i.e.
SMCs, myofibroblasts, pericytes). In this altered EC–mural
cell communication, many different endothelial factors
have been found to be critical: paracrine overproduction of
ET-1, FGF-2 [8], 5-HT [10], A-II [11, 12], and leptin [13].
Furthermore, recent evidence also suggests that pulmonary
vascular cells, including ECs, exhibit a chronic shift in
energy production from mitochondrial oxidative phosphorylation to glycolysis, a phenomenon that may participate in the pathogenesis [7, 14]. The initial trigger for
endothelial injury is not known, although hemodynamic
forces (as shear stress), reactive oxygen species, toxins,
inflammatory mediators, and/or genetic predisposition are
The endothelins (ET-1, -2, and -3) constitute a family of 21
amino acid peptides that are encoded by a 38-amino-acid
precursor known as big-endothelins. Endothelins are
expressed in many tissues, including lung, brain, kidney,
pituitary gland, and placenta. ET-1 is one of the most potent
vasoconstrictor proteins produced by vascular EC. ET-1 is a
small peptide (21 amino acids with two disulfide bridges),
with the free carboxy terminal function involved in the
activity of the peptide [15]. It results from the secretion of
the inactive pre-pro-endothelin transformed in big-endothelins through endopeptidase action and finally to ET-1 via
the action of endothelin-converting enzyme (ECE). ET-1
biosynthesis is stimulated by different stimuli, including
hypoxia, growth factors, cytokines, shear stress, thrombin,
and A-II (Fig. 1). Among endogenous vasoconstrictors, ET1 is one of the stronger constrictors associated with a longer
pharmacological effect [16, 17]. In addition to vasoconstriction, ET-1 induces cellular proliferation, collagen
deposition, and inflammation. Although ECs mainly produce ET-1, pulmonary arterial SMC and lung fibroblasts
[18] have been found to be a potential source of ET–1 [19].
ET-1 acts through two receptors, ETA and ETB. Both of
these receptors are coupled to a Gq-protein and the formation of inositol triphosphate (IP3). Increased IP3 causes
calcium release by the sarcoplasmic reticulum, which causes
SMC contraction. In general, binding of ET-1 to ETA and
ETB receptors on pulmonary arterial SMCs promotes
vasoconstriction, whereas activation of ETB receptors on
ECs causes vasodilation through an increase in PGI2 and NO
levels [20, 21] as well as through circulating ET-1 clearance
elevation [22] (Table 1). This duality is more complex
because expression of ETA receptors on the endothelium of
human peripheral pulmonary arteries in normal and IPAH
lungs and in cultured pulmonary artery ECs has recently
been shown [23]. Moreover, in physiological conditions,
ETB receptors have a predominantly vasodilatory effect;
while in PAH, ETB receptors are upregulated in pulmonary
arterial SMCs, leading to vasoconstriction and proliferation
[16, 24]. ET-1 and ETA (but not ETB) receptors are also
found in the normal right ventricle (RV). Expression of ET-1
and ETA receptors is increased in PAH patients with RV
hypertrophy (RVH), which could be explained as a compensatory mechanism to preserve RV contractility as the
after-load increases [25]. ET-1 affinity for ETA receptors is
100 times higher than that of ET-3, whereas all three isoforms have the same affinity for ETB receptors.
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Endothelin Receptor Antagonists in PAH
Fig. 1 The endothelin-1
signaling system. ECE
endothelin-converting enzyme,
ET-1 endothelin-1, ETA
endothelin-1 receptor A, ETB
endothelin-1 receptor B, Ca2?
calcium
Table 1 Localization of ETA and ETB receptors and biological
effects after binding with endothelin-1
ETA
Localization
Smooth muscle cells
Heart (cardiomyocytes)
Endothelial cells?
Biological effects
Vasoconstriction
Cellular proliferation
Tissue hypertrophy
Fibrosis
ETB
Localization
Smooth muscle cells
Endothelium
and lung ET-1 levels than control subjects. The high level
of ET-1 could be due to increased production by the pulmonary vasculature, reduced lung clearance, or a combination of these two processes [26–28]. Importantly, these
ET-1 levels are correlated with disease prognosis [29–31].
In addition, ET-1, ETA, and ETB expression has been
observed to increase in experimental pulmonary hypertension [32–34]. All these observations have led to the
development of effective oral treatments that are able to
modulate the activity of ET-1 and that are currently used in
the management of PAH.
Heart (fibroblasts)
Adrenal gland
Biological effects
Vasoconstriction
4 Endothelin Receptor Antagonists (ERAs)
Vasodilatation
Hypertrophy, fibrosis, apoptosis
Aldosterone production
ET-1 endothelin-1, ETA endothelin-1 receptor A, ETB endothelin-1
receptor B
The pathogenic role of ET-1 for PAH pathophysiology
has its roots in several crucial observations, including that
PAH patients have been reported as having higher plasma
Prospective clinical randomized controlled trials (RCTs)
have demonstrated the efficacy and safety of three available active ERAs (bosentan, ambrisentan, and macitentan),
leading to their approval for the treatment of PAH. The
chemical structures of these ERAs are shown in Fig. 2.
Before ambrisentan, sitaxsentan (Thelin) was the first
selective ETA receptor antagonist made available by the
European Regulatory Agency, in 2006 [35]. Multi-center,
randomized, placebo-controlled clinical trials have
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M.-C. Chaumais et al.
Fig. 2 Chemical structures of the three available endothelin receptor antagonists
demonstrated that sitaxsentan has beneficial effects on
exercise capacity (i.e., 6-min walk distance [6MWD]),
functional class (FC), and hemodynamic parameters in
PAH patients [36]. However, cases of fatal liver toxicity
led the European Medicines Agency (EMA) to withdraw
marketing authorization for sitaxsentan in 2010 [37–40].
4.1 Structure and Specificity
4.1.1 Bosentan
The discovery of ET-1 in 1988 led to the development of
ERAs based on the pyrimidic sulfamide class, from which
the Ro 46-2005 molecule, a dual ETA and ETB receptor
antagonist, was selected [41]. In order to improve the
pharmacokinetic/pharmacodynamic properties, structural
optimization was performed, leading to the development of
a new molecule in 1991: Ro 47-0203 (bosentan) [16].
Bosentan is a non-peptide pyrimidine derivative that
competitively antagonizes the binding of ET-1 to both ETA
and ETB receptor subtypes and irreversibly blocks their
activities [16]. Bosentan has a specific inhibition of ET-1
receptors, with non-binding to other receptors, and was the
first ERA studied and approved in PAH.
4.1.2 Ambrisentan
Selective ETA receptor inhibition has theoretical benefits in
terms of preserving vasodilator and clearance functions
specific to ETB receptors, while preventing vasoconstriction
and cellular proliferation mediated by ETA receptors [42].
Based on these findings, ambrisentan, a highly selective
ETA receptor antagonist, was developed [43]. Approved by
the US FDA in 2007 and by the EMA in 2008 [44], it is the
only selective ERA available for the treatment of PAH.
Unlike bosentan (sulfonamide ERA), ambrisentan belongs
to the carboxylic ERA group.
4.1.3 Macitentan
Macitentan is a new potent non-peptide non-selective ERA
with a 50-fold higher affinity for ETA than for ETB
receptors. Development of macitentan led to a high level of
tissue targeting and sustained receptor binding compared
with other ERAs. The FDA (October 2013) and the EMA
(December 2013) approved OpsumitÒ (macitentan) for the
long-term treatment of PAH as monotherapy or in combination in adult patients of WHO FC II–III.
4.2 Pharmacokinetics
4.2.1 Bosentan
Bosentan, like the other ERAs, is an oral medication. The
usual dosage is 125 mg twice daily after a titration period
of 4 weeks (62.5 mg twice daily). Bosentan is also available for children in a dispersible tablet formulation (32 mg)
that has the same pharmacokinetic properties as the adult
formulation [45, 46]. This formulation can also be used in
adults with swallowing disorders. The pharmacokinetics of
bosentan have mainly been studied in healthy populations.
Data obtained from PAH patients indicate that exposure to
bosentan is about twofold higher than in healthy populations, whereas the pharmacokinetics of bosentan in pediatric PAH patients is comparable to that in healthy
subjects.
The pharmacokinetics of bosentan is dose dependent
and proportional until 500 mg daily. Higher dosages lead
to a less proportional increase for maximum concentration
(Cmax) and area under the concentration–time curve
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Endothelin Receptor Antagonists in PAH
(AUC). Following oral administration, bosentan reaches
peak plasma concentrations in healthy subjects after
approximately 3–5 h. The absolute bioavailability is about
50 % and is not significantly modified with food at the
recommended dosage of 125 mg. Bosentan is highly
bound to albumin (around 98 %) and does not enter
erythrocytes. No dosage adjustment in adults is required
based on sex, age, ethnic origin, or bodyweight. Steadystate concentrations are achieved within 3–5 days after
multiple-dose administration, with a volume of distribution of 30 L and a clearance of 17 L/h [17]. At steady
state, concentrations of bosentan are around 50–60 % of
the observed concentrations after single administration,
probably due to induction of its metabolizing enzymes
leading to a twofold increase in its clearance. The
metabolism of bosentan is mainly hepatic and involves
cytochrome P450 (CYP) 2C9 and CYP3A4, with three
identified metabolites eliminated by biliary excretion.
Among these metabolites, Ro 48-5033 is pharmacologically active and contributes to around 20 % of the total
response following administration of bosentan. Less than
3 % of the oral dose of bosentan is found in urine,
therefore severe renal impairment (creatinine clearance
15–30 mL/min) has no clinically relevant influence on the
pharmacokinetics of bosentan. No dose adjustment is
required in mild hepatic impairment (Child-Pugh class A);
however, moderate and severe hepatic impairment are
contraindications for bosentan therapy.
4.2.2 Ambrisentan
After oral administration (5 or 10 mg once daily), ambrisentan is rapidly absorbed into the systemic circulation
with a bioavailability of about 90 % [47]. Food has no
impact on this bioavailability. In a phase II trial in PAH
patients, plasma levels of ambrisentan reached Cmax
between 1.7 and 3.3 h after oral administration, and the
mean elimination half-life at steady state ranged from 9 to
15 h, allowing for once-daily dosing [48, 49]. Ambrisentan
is highly bound to albumin in the same range as bosentan
and is sparsely distributed in erythrocytes. Steady state is
obtained after 4 days of treatment. The main metabolic
pathways of ambrisentan are glucuronidation (13 %), oxidation by CYP3A4 (and to a lesser extent CYP3A5 and
CYP2C19), leading to 4-hydroxymethyl ambrisentan
(21 %). Affinity of this metabolite on ETA receptors is
65 % less than that of ambrisentan and is not part of the
pharmacologic activity of the drug. Due to metabolization,
treatment with ambrisentan should be avoided in patients
with severe hepatic impairment. Both biliary (around
80 %) and urinary (around 20 %) routes are involved in
ambrisentan excretion.
4.2.3 Macitentan
Selection of macitentan was based on inhibitory potency
on both ET receptors and optimization of physicochemical
properties to achieve a high affinity for the lipophilic
environment [50]. The pharmacokinetics of macitentan are
dose proportional and characterized by slow absorption
due to low aqueous solubility. At a dose of 300 mg,
macitentan has a median time to Cmax (tmax) of about 8 h
and a half-life of 17.5 h, compatible with a once-daily
dosing regimen [51, 52]. In vivo, macitentan is metabolized into a major and pharmacologically active metabolite, ACT-132577, which is formed by oxidative
depropylation through CYP3A4. While ACT-132577 is
fivefold less potent than macitentan, its long half-life
(about 48 h) leaves it prone to accumulate upon repeated
dosing and therefore significantly contributes to the
overall effect. Urinary excretion is the most important
route of elimination of drug-related material compared
with feces in humans. In urine, four entities were identified, with the hydrolysis product of ACT-373898 the most
abundant. In feces, five entities were identified, with the
hydrolysis product of macitentan and ACT-132577 the
most abundant [53]. Based on two prospective, singlecenter, open-label studies that evaluated the pharmacokinetics of macitentan and its metabolites in healthy subjects
and in subjects with mild, moderate, and severe hepatic
impairment or severe renal function impairment, Sidharta
et al. [54] reported no clinical relevance and no requirement for dose adjustment in these populations treated with
macitentan.
The main pharmacokinetic features of ERAs are summarized in Table 2.
4.3 Drug Interactions
4.3.1 Bosentan
Generally, multiple drug interactions with bosentan are
reported due to its property of enzymatic induction of
CYP2C9, CYP3A4, and probably also CYP2C19 and
P-glycoprotein: bosentan decreases exposure to cyclosporine, glibenclamide, simvastatin, and warfarin by up to 50 %
because of induction of CYP3A4 and/or CYP2C9. In terms
of warfarin, which is often prescribed in PAH patients, no
significant international normalized ratio (INR) modifications were reported; therefore, no systematic dosage
adjustments are required. However, close monitoring of the
INR is recommended. Pharmacokinetic induction of bosentan also renders hormone-based contraception ineffective. Women taking bosentan must be aware of the risk of
pregnancy and the need to use another kind of contraception. Pregnancy is contraindicated for women with PAH
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M.-C. Chaumais et al.
Table 2 Main pharmacokinetic features of endothelin receptor antagonists
Bosentan (dual ERA)
Ambrisentan (selective ERA)
Macitentan (dual ERA)
Dosage schedule
65.5 mg for 4 weeks then 125 mg bid
5–10 mg od
10 mg od
Bioavailability (%)
50
90
_
Time to peak, onset (hours)
3–5
1.5–3
9–10
Protein binding (%)
[98
[98
[99
Metabolism
Hepatic (active metabolite)
Hepatic and extra hepatic
Hepatic (active metabolite)
Half-life (hours)
5.4
13.6–16.5
17
Excretion
Biliary
Biliary [ urinary
Urinary biliary
Substrate for
Inductor of
CYP3A4/CYP2C9 (CYP2C19)
UGT
CYP3A4
OATP1B1/P1B3
CYP3A4/CYP2C19
CYP 2C8/CYP2C9 /CYP2C19
PgP
OATP1
CYP3A4, CYP2C9 (CYP2C19)
None
None
bid twice daily, CYP cytochrome P450, ERA endothelin receptor antagonist, OATP organic anion transporting polypeptides, od once daily, PgP
P-glycoprotein
because of a high risk of maternal mortality if the child is
brought to term. Furthermore, bosentan is considered a
teratogen, capable of causing fetal defects early in development. Metabolization of bosentan through CYP2C9 and
CYP3A4 leads to other drug interactions: ketoconazole
approximately doubles the exposure to bosentan because of
inhibition of CYP3A4. Co-administration of cyclosporine
and bosentan markedly increases initial bosentan trough
concentrations, probably due to the inhibition of bosentan
transport protein in hepatocytes. Concomitant treatment
with glibenclamide and bosentan leads to an increase in the
incidence of aminotransferase elevations. Therefore, combined use with cyclosporine is contraindicated, and combined use with glibenclamide is not recommended.
Bosentan does not affect lopinavir and ritonavir exposure to
a clinically relevant extent, but tolerability of bosentan
should be monitored in patients with HIV receiving antiretroviral therapy with lopinavir/ritonavir. All ritonavirboosted protease inhibitors are assumed to have a similar
effect on bosentan pharmacokinetics [55]. Finally, in coadministration of bosentan and sildenafil in PAH, a decrease
in sildenafil was associated with an increase in bosentan.
However, many PAH patients combined these therapies and
no clinical relevance was observed [56]. Finally, rifampicin
is a potent inducer of CYP3A4 and CYP2C9 but is also
known to inhibit several members of the organic anion
transport (OAT) protein family involved in elimination of
bosentan. Chronic treatment with rifampicin led to a more
than 50 % decrease in bosentan exposure. However, probably due to OAT inhibition, acute exposure to rifampicin
led to high levels of bosentan plasma concentration, which
could cause clinical concern with a higher risk of abnormal
liver function. Therefore, it is recommended that liver
function be assessed weekly for the first 4 weeks of concomitant administration [57].
4.3.2 Ambrisentan
Unlike bosentan, ambrisentan has a low potential for drug–
drug interactions, explained by the small effect on hepatic
CYP450 induction or inhibition [58]. It can be safely
administered with warfarin or sildenafil without dose
adjustment [59]. Similarly, no relevant pharmacokinetic
changes were detected with combined administration of
ethinyl estradiol/norethindrone and ambrisentan, leading to
no requirement for dose adjustment [60]. Significant
interaction was only reported with cyclosporine A, with a
twofold increase in ambrisentan concentration leading to
fixed dose adjustment at 5 mg daily [57].
4.3.3 Macitentan
Although macitentan metabolism is indeed affected by
inhibition of CYP3A4, the changes are not considered
clinically significant, and macitentan can be administered
concomitantly with CYP3A4 inhibitors without dose
adjustment [61]. Macitentan has a potency for induction
and inhibition of drug-metabolizing enzymes and transporters that is similar to or higher than that of bosentan, and
it seems to have the same interactions. However, its low
plasma concentration and minimal accumulation in the
liver suggest that it will be markedly less prone to drug–
drug interactions than bosentan [62].
Concomitant treatment with cyclosporine A had no
clinically relevant effect on exposure to macitentan or its
metabolites at steady state. Concomitant treatment with
rifampin (a strong inducer of CYP3A4) significantly
reduced exposure to macitentan and its metabolite ACT373898 at steady state but did not affect exposure to the
active metabolite ACT-132577 to a clinically relevant
extent [63]. Regarding drug interactions with warfarin or
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Endothelin Receptor Antagonists in PAH
sildenafil, which are often used in the management of PAH,
concomitant administration of macitentan did not lead to a
clinically relevant modification for each drug. Therefore,
no dose adjustment is required between macitentan and
warfarin or sildenafil [64, 65]. Similarly, although specific
drug–drug interaction studies with hormonal contraceptives
have not been conducted, macitentan did not affect exposure to other CYP3A4 substrates such as sildenafil.
Therefore, no reduced efficacy of hormonal contraceptives
is expected.
4.4 Pharmacodynamics
Several studies in animal models document that both
selective and non-selective ERAs prevent or attenuate
experimental pulmonary hypertension by inducing pulmonary vasodilation and a decrease in pulmonary vascular
remodeling and RVH.
contractions in isolated endothelium-denuded rat aorta
(ETA receptors) and sarafotoxin S6c-induced contractions
in isolated rat trachea (ETB receptors). Macitentan
increased binding to receptors as compared with existing
ERAs, indicating a more potent pharmacological activity
in vivo: when administered to normotensive rats, macitentan increased plasma ET-1 concentrations, which
occurred at a tenfold lower dose than with bosentan [50]. In
pulmonary hypertension rats, macitentan prevented both
increases in pulmonary pressures and RVH and improved
survival without any effect on systemic arterial blood
pressure [50].
4.5 Efficacy of ERAs in PAH
The use of ERAs in PAH management is relatively new.
Bosentan (TracleerÒ), ambrisentan (VolibrisÒ), and,
recently, macitentan (OpsumitÒ) have been approved
within the last 12 years.
4.4.1 Bosentan
4.5.1 Bosentan
In pulmonary hypertension models induced by chronic
hypoxia or monocrotaline, bosentan attenuated the increase
in pulmonary artery pressures, prevented pulmonary vascular remodeling, and decreased RVH [66–69]. In terms of
cardiac tissue, bosentan inhibits RV collagen expression in
rats exposed to chronic hypoxia [68]. In addition, bosentan
has been reported to inhibit pulmonary arterial SMC proliferation of and inflammatory response in pulmonary tissue to injection of ET-1 in guinea pig and mouse lung [70–
72]. Moreover, in PAH patients, treatment with bosentan
led to a reduction of intercellular adhesion molecule
(ICAM)-1 and plasmatic interleukin (IL)-6 levels that
correlated with hemodynamic improvement [73].
4.4.2 Ambrisentan
Unlike other ERAs, no experimental data are available on
the effects of ambrisentan in pulmonary hypertension. In
terms of inflammation, treatment with ambrisentan
decreases expression of pro-inflammatory genes in ischemia/reperfusion models, leading to a cytoprotective effect
on vascular microcirculation [74].
4.4.3 Macitentan
Macitentan is a competitive ERA with significantly slower
receptor dissociation kinetics than the other approved
ERAs. Slow dissociation caused insurmountable antagonism in functional pulmonary arterial SMC-based assays;
this could contribute to an enhanced pharmacological
activity of macitentan in PAH [75]. In functional assays,
macitentan and ACT-132577 inhibited ET-1-induced
Bosentan was the first ERA and first oral medication
approved for use in PAH. Its availability represents major
progress in the management of the disease by improving
clinical status, exercise capacity, and hemodynamic
parameters, and delaying clinical worsening of PAH [26,
76, 77]. Moreover, bosentan significantly improves quality
of life in patients with IPAH or PAH associated with
connective tissue diseases [78]. Three pivotal studies led to
the approval of bosentan for IPAH and heritable PAH as
well as PAH associated with connective tissue disease [76,
77, 79]. More recently, it was also approved for PAH
associated with congenital heart disease [80]. Table 3
summarizes the RCTs performed with bosentan and other
ERAs.
In the first pilot study, performed in 32 patients with
IPAH and PAH associated with connective tissue diseases,
12 weeks of treatment with bosentan was shown to
improve exercise capacity (6MWD) and pulmonary
hemodynamics [76]. The pivotal study BREATHE-1 confirmed these findings, with bosentan leading to a significant
improvement in exercise capacity as well as delaying time
to clinical worsening in patients with FC III–IV PAH [77].
In patients with mildly symptomatic PAH (i.e., FC II), the
EARLY study showed that bosentan prevented clinical
deterioration (delayed time to clinical worsening) without
significant improvement in exercise capacity [79].
In the RCT BREATHE-5, bosentan was shown to
improve 6MWD without deterioration in oxygen saturation
in patients with PAH associated with non-repaired congenital pulmonary-to-systemic shunt (Eisenmenger’s syndrome) [80].
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M.-C. Chaumais et al.
Table 3 Pivotal randomized clinical trials with endothelin receptor antagonists
NYHA FC at
inclusion
Study duration
(weeks)
Primary
endpoint
References
32
III–IV
12
6MWD
[65]
213
III–IV
16
6MWD
[66]
54
III
18
SpO2 and
PVR
[68]
iPAH, hPAH, PAH-CTD, PAHHIV, PAH-CHD (repaired)
185
II
24
6MWD
and PVR
[69]
ARIES-1
and -2
iPAH, hPAH, PAH-anorexigens,
PAH-CTD, PAH-HIV
394 (202
and 192)
I–IV
12
6MWD
[36]
SERAPHIN
iPAH, hPAH, PAH-drugs and
toxins, PAH-CHD (repaired)
742
II–IV
Event-driven: 85
(PL) to 104
(10 mg)
Morbiditymortalitya
[76]
Drug
Study
acronym
PAH etiologies
Bosentan
Study 351
iPAH, hPAH, PAH-CTD
BREATHE1
iPAH, hPAH, PAH-CTD
BREATHE5
PAH-CHD (Eisenmenger’s)
EARLY
Ambrisentan
Macitentan
Pts (n)
FC functional class, hPAH heritable PAH, iPAH idiopathic PAH, NYHA New York Heart Association, PAH pulmonary arterial hypertension,
PAH-CHD PAH associated with congenital heart diseases, PAH-CTD PAH associated with connective tissue diseases, PAH-HIV PAH associated
with HIV infection, PL placebo, pts patients, PVR pulmonary vascular resistance, SpO2 systemic pulse oximetry, 6MWD 6-min walk distance
a
In the SERAPHIN study, the primary endpoint was the time from the initiation of treatment to the first occurrence of a composite endpoint of
death, atrial septostomy, lung transplantation, initiation of treatment with intravenous or subcutaneous prostanoids, or worsening of PAH
Bosentan was also shown to be notably effective in other
forms of PAH in open-label trials. In the BREATHE-4
study, performed in 16 patients with PAH associated with
HIV infection, 16 weeks of treatment with bosentan led to
a major improvement in hemodynamics and exercise
capacity without any change in the control of HIV infection
[81]. Patients with portopulmonary hypertension are usually excluded from RCTs with ERAs, and there is no
approval for bosentan in this population. However, several
non-controlled studies have shown that patients with portopulmonary hypertension with mild to moderate cirrhosis
(i.e., Child-Pugh score A or B) or extra-hepatic portal
hypertension (e.g., portal thrombosis) may benefit from
bosentan, with an acceptable safety profile [82].
In long-term retrospective observational studies of bosentan in children with IPAH or PAH associated with
congenital heart diseases or connective tissue diseases,
bosentan was reported to be safe and effective in slowing
disease progression [83, 84].
4.5.2 Ambrisentan
Ambrisentan was studied in two phase III placebo-controlled trials, ARIES-1 (n = 202, doses of 5 mg daily or
10 mg daily for 12 weeks) and ARIES-2 (n = 192, doses
of 2.5 mg daily and 5 mg daily for 12 weeks), in patients
with IPAH or heritable PAH or with PAH associated with
connective tissue disease, anorexigen use, or HIV infection
[44]. The primary endpoint was change from baseline in
6MWD at week 12. The 6MWD increased in all ambrisentan groups with mean placebo-corrected treatment
effects of 31–59 m. Improvements in time to clinical
worsening, FC, quality of life (SF-36 score), Borg dyspnea
score, and B-type natriuretic peptide were also observed.
No patient treated with ambrisentan developed aminotransferase concentrations greater than three times the
upper limit of normal. In the extension phase of these
studies (ARIES-E) [85], 2-year treatment with ambrisentan
was associated with sustained improvements in exercise
capacity and a low risk of clinical worsening and death.
Ambrisentan was generally well tolerated and had a low
risk of aminotransferase serum level elevation over the
study period. Another long-term study of ambrisentan in
PAH reported similar sustained benefit in exercise capacity
and pulmonary hemodynamics [86].
4.5.3 Macitentan
Macitentan was studied in a multi-center, double-blind,
placebo-controlled, long-term, event-driven randomized
study (SERAPHIN [Study with an Endothelin Receptor
Antagonist in Pulmonary arterial Hypertension to Improve
cliNical outcome]) [87]. This study, the first using a morbidity–mortality composite endpoint, was designed to
evaluate the efficacy and safety of macitentan through the
primary endpoint of time to first morbidity and all-cause
mortality event in 742 patients with symptomatic PAH and
treated for up to 3.5 years. Macitentan 3 mg daily and
10 mg daily has met the primary endpoint, decreasing the
risk of a morbidity/mortality event over the treatment
period versus placebo. This risk was reduced by 45 % in
the 10-mg group (P \ 0.0001). At 3 mg, the observed risk
reduction was 30 % (P = 0.0108). Worsening of PAH was
the main event contributing to primary endpoint. Interestingly, macitentan 10 mg significantly reduced the risk of
primary endpoint event versus placebo in both treatment-
Author's personal copy
Endothelin Receptor Antagonists in PAH
naı̈ve patients and patients receiving background therapy at
study entry (sildenafil in the vast majority of cases). Macitentan also significantly improved clinically important
secondary endpoints, including 6MWD, New York Heart
Association (NYHA) FC, and PAH-related death or hospitalization. Treatment with macitentan in the SERAPHIN
study was well tolerated; the more frequently reported
adverse events were headache, nasopharyngitis, and anemia. Elevations of liver aminotransferases greater than
three times the upper limit of normal were observed in
4.5 % of patients receiving placebo, in 3.6 % of patients
receiving macitentan 3 mg, and in 3.4 % of patients
receiving macitentan 10 mg. In addition, no difference was
observed between macitentan and placebo in occurrence of
peripheral edema. A decrease in hemoglobin—reported as
an adverse event—was observed more frequently in macitentan-treated groups than in those receiving placebo,
with no difference in treatment discontinuation between
groups [87]. Macitentan 10 mg has been recently approved
by the FDA and the EMA as OpsumitÒ, based on the
results of the SERAPHIN study.
4.6 Safety
4.6.1 General Side Effects
General side effects of ERAs are related to vasodilator
properties such as headache, peripheral edema, nasal congestion, flushing, or nausea and are dose dependent.
Hypotension and palpitations have also been reported in
treatment with ERAs. Of the ERAs available in PAH,
macitentan seems to be the most tolerated in terms of
vasodilation [51]. However, this observation could be
explained by the limited use of macitentan, which has just
obtained approval in the USA and the EU.
4.6.2 Elevation in Hepatic Aminotransferases
Elevation of hepatic aminotransferase is the main side
effect observed with ERAs. True hepatotoxicity was
observed with sitaxsentan, another selective ETA receptor
antagonist previously available in Europe, Australia, and
Canada [35], leading to its withdrawal from the market
after cases of fatal liver toxicity [37–40].
Bosentan is known to be associated with reversible,
dose-dependent, and, in most cases asymptomatic, elevation of aminotransferases [88]. This increase in liver
enzymes usually appears during the first 6 months of
treatment with bosentan but could also occur later. In order
to prevent this adverse event, a gradual dosage increase is
recommended (62 mg twice daily the first month, and
125 mg twice daily thereafter). In the same way, aminotransferase elevation could normalize after decrease of
bosentan dosage. The mechanism of this toxicity is actually
not fully understood. It has been hypothesized that it could
be a consequence of the cellular accumulation of bile salts
due to impaired canalicular excretion as a result of bile salt
export pump inhibition [89]. Another hypothesis resides in
the demonstration that bosentan, but not ambrisentan,
inhibits four human hepatic transporters, providing a
potential mechanism for the increased hepatotoxicity
observed with bosentan [90]. Genetic variability of
enzymes involved in drug metabolism is a preponderant
susceptibility factor for drug-induced liver injury and was
hypothesized in bosentan liver toxicity: it can influence the
metabolism of bosentan in a variety of ways. Recently,
Markova et al. [91] identified CYP2C9*2 as a potential
genetic marker for bosentan-induced liver injury, despite a
modest effect on bosentan metabolism. However, results of
this study were not marked and additional studies are
needed to validate this hypothesis. Another study published
in June 2014 did not support CYP2C9*2 as a genetic
marker of bosentan-induced liver injury. Moreover, functional polymorphisms of genes involved in bosentan
pharmacokinetics (SLCO1B1, SLCO1B3, and CYP2C9*3)
or in hepato-biliary transporters affected by bosentan
(ABCB11) were not found to be associated with bosentaninduced hepatotoxicity [92]. Other mechanisms could also
be involved in this toxicity as accumulation of bosentan in
hepatocytes leading to cytolysis or an immune-allergic
pathway.
In order to obtain further safety data for bosentan,
European authorities required the introduction of a postmarketing surveillance system. Within 30 months, this
system has assembled data from 4,994 patients, representing 79 % of those exposed to bosentan in Europe
during that time period [93]. The reported annual rate of
aminotransferase level elevation was 10.1, and 3.2 % of
patients had to discontinue the drug for this reason. Elevation of aminotransferase levels was reversible in all
cases, and no permanent liver injury occurred. In order to
afford the best efficacy/safety balance for PAH patients,
monthly monitoring of hepatic aminotransferase is mandatory in patients treated with bosentan. Similarly, in the
case of dosage modification and/or potential drug–drug
interaction, monitoring must be performed (after 2 weeks).
Ambrisentan has not been shown to increase the risk of
liver enzyme elevation over placebo [94]. Based on the
data obtained from the risk minimization action plan, in
March 2011 the FDA removed the requirement for mandatory monthly monitoring of liver function tests with
ambrisentan therapy [95]. However, monitoring is still
required by the EMA. Ambrisentan belongs to the group of
carboxylic ERAs which, unlike sulfonamide ERAs, are
devoid of hepatotoxicity. Ambrisentan is a safe alternative
when bosentan has to be discontinued because of increased
Author's personal copy
M.-C. Chaumais et al.
liver aminotransferase levels [94]. None of the 31 patients
who discontinued bosentan had a recurrence of liver
enzyme elevation necessitating ambrisentan discontinuation, and only one patient presented transient elevated
aminotransferases, resulting in a dose reduction with no
further elevations. However, elevations in liver aminotransferases induced by ERAs are typically seen after
repeated dosing and sometimes only after months of
treatment [93]. The results of the ARIES-E study, the longterm open-label extension of the ARIES-1 and ARIES-2
studies with ambrisentan, showed that the annual incidence
rate of elevation of aspartate transaminase/alanine transaminase levels of more than three times the upper limit of
normal was about 2 % [85].
Macitentan does not inhibit canalicular bile acid
transport in rats, which could lead to a better liver safety
profile than bosentan [96]. In the SERAPHIN study, the
incidence of aminotransferase levels more than three
times the upper limit of normal were similar to that of the
placebo group [87]. However, due to the limited use of
macitentan, these safety results need to be confirmed in
coming years.
need for prospective clinical trials to further characterize
these findings [100].
4.6.4 Anemia
Decreased hemoglobin levels and anemia are other biologic side effects that could appear during ERA treatment.
However, this adverse event is rare and usually manageable. The mechanism of the decrease in hemoglobin level
is not fully understood. This decrease could be related to
the hemodilution induced by vasodilation and intravascular
fluid retention. A direct effect of ERAs on hematopoiesis
has never been demonstrated. In clinical placebo-controlled
studies, bosentan-induced decreases in hemoglobin levels
have been stabilized after 4–12 weeks of treatment. Monitoring of hemoglobin is therefore recommended before
initiation of the treatment: monthly monitoring for the first
4 months and then quarterly. Monitoring is also recommended with ambrisentan and macitentan therapies before
and after its introduction and thereafter periodically
depending on clinical practice.
4.6.5 Teratogenicity
4.6.3 Edema
Peripheral edema is another side effects observed with
ERA use. One recent explanation is the up-regulation of
the myocardial ET axis in right heart failure during pulmonary hypertension, which could be a compensatory
mechanism to preserve RV contractility, as the afterload
increases. ERAs might therefore potentially worsen RV
function, explaining some of the peripheral edema noted
clinically with these agents [25]. Moreover, there is a
higher incidence of peripheral edema observed in PAH
patients treated with ambrisentan than in those treated with
dual ERAs [97]. In studies comparing bosentan or macitentan with placebo, the occurrence of peripheral edema
was similar in active-treatment and placebo groups [77,
87]. One possible explanation for different rates of edema
with selective versus dual ERAs may reflect differences in
affinities to the ETA receptor [97]. Another potential
explanation may involve the renin/angiotensin system
(RAS): selective ETA receptor blockade during early congestive heart failure causing sustained sodium retention by
activating the RAS, resulting in edema [98]. Finally, a
study in rats comparing bosentan and sitaxsentan suggested
that ERA-induced fluid retention was occurring via activation of the vasopressin system via secondary stimulation
by ET of the uninhibited ETB receptors [99]. Recently,
Maron et al. [100] analyzed the association of spironolactone and ETA receptor antagonism in order to avoid edema
as a side effect. This study reported that use of spironolactone may be clinically beneficial in PAH despite the
Due to teratogenic effects reported in animals treated with
bosentan, pregnancy is officially contraindicated [101]. Like
bosentan, ambrisentan and macitentan are considered teratogens, capable of causing fetal defects early in development.
In childbearing women, bosentan and ambrisentan could be
prescribed if contraception is proved, along with a negative
pregnancy test performed before initiation of the treatment
and, thereafter, monthly. Type of contraception is particularly essential, notably with bosentan: estroprogestative
contraception, regardless of administration route, is not
reliable due to powerful enzymatic induction on CYP2C9
and CYP3A4. Consequently, double contraception is
required. Similar to other members of its drug class, macitentan carries a boxed warning alerting patients and
healthcare professionals that the drug should not be used in
pregnant women because it can harm the developing fetus.
In addition to the teratogenic properties of ERAs, pregnancy
is a formal contraindication in PAH as it could be an
aggravating factor in the prognosis of the disease.
For each ERA available in PAH treatment, a risk evaluation and mitigation strategy (REMS) has been implemented. REMS enables clinicians to go beyond product
labeling to manage risks and thereby ensure that the benefits outweigh the risks. REMS goals for bosentan, ambrisentan, and macitentan are to inform the population
about the serious risk of teratogenicity and to minimize the
risk of fetal exposure. Another REMS objective for bosentan is to minimize the risk of hepatotoxicity in patients
who are exposed to Tracleer.
Author's personal copy
Endothelin Receptor Antagonists in PAH
4.6.6 Male Fertility
In general, development of testicular tubular atrophy in
male animals has been linked to the chronic administration
of ERAs. Whereas fertility studies in rats showed no effects
on sperm parameters (sperm count, motility, and viability)
or fertility with bosentan, hypospermatogenesis was
observed in the life-long carcinogenicity study in rats and
in repeat-dose toxicity studies in dogs for macitentan;
decreases in the percentage of morphologically normal
sperm were noted at 300 mg/kg/day for ambrisentan. The
effect of ambrisentan and macitentan on male human fertility is not known.
5 Place of ERAs in the Treatment Algorithm for PAH
In the past 15 years, the number of available specific
therapies for PAH management has markedly increased.
Today nine drugs are approved for the treatment of patients
with PAH, including prostacyclin analogs (epoprostenol,
treprostinil, iloprost), ERAs (bosentan, ambrisentan, macitentan), and drugs targeting the NO/cyclic guanosine
monophosphate (cGMP) pathway (two PDE5 inhibitors,
sildenafil and tadalafil, and the guanylate cyclase activator
riociguat).
ERAs have proven their efficacy with relatively few side
effects, becoming an attractive option, either in monotherapy or in combination therapy with drugs targeting the
other pathways. The three ERAs are currently recommended
as first-line therapy in FC II and III PAH patients. The
advantage of ambrisentan and macitentan over bosentan is
the once daily oral dose, leading to improved quality of life
and adherence of PAH patients to treatment regimens.
Moreover, the reduction of hepatic adverse events compared
with bosentan seems to be a clinical argument for the choice
of therapy. However, more data are needed in the long term
to confirm this observation, especially with macitentan.
Regarding the general safety profile, macitentan appears to
be safer and has a low propensity for drug–drug interactions.
In addition, no dose adjustments are required in patients with
renal or hepatic impairment.
6 Conclusions and Future Directions
ERAs were the first oral therapy for PAH and remain a
critical component of the therapeutic algorithm in the
management of the disease. ERAs demonstrated improvements in pulmonary hemodynamics, exercise capacity,
functional status, and clinical outcome in several randomized placebo-controlled trials, therefore representing a
major therapy in PAH. Current clinical data suggest that
both dual and specific ERAs have a similar efficacy in
improving clinical outcomes in PAH patients, with differences in safety profiles. Recently, the randomized, doubleblind, multi-center, AMBITION study showed that firstline treatment with ambrisentan/tadalafil combination
therapy in naı̈ve PAH patients is superior in terms of the
primary endpoint (time to first clinical failure event)
compared with monotherapy (ambrisentan or tadalafil)
[102]. This result promotes arguments to treat de novo
PAH patients with a combination therapy in order to
improve clinical outcomes in PAH patients where ERAs
could take a primary role.
Conflict of interest M.C. Chaumais, C. Guignabert, and A. Boucly
have no conflicts of interest to declare. L. Savale has relationships
with pharmaceutical companies, including Actelion, Pfizer, GSK, and
Lilly. Relationships include consulting services and fees for speaking.
X. Jaı̈s has received honorarium for consulting services from Actelion, Pfizer, and GSK. D. Montani has received honorarium for
consultancy services from Actelion, Pfizer, Bayer, and GSK. G. Simonneau has relationships with pharmaceutical companies including
Actelin, Bayer, GSK, Novartis, and Pfizer. Relationships include
consultancy services and membership of scientific advisory boards.
M. Humbert has relationships with pharmaceutical companies
including Actelion, Bayer, GSK, Novartis, and Pfizer. In addition to
being investigator in trials involving these companies, relationships
include consultancy services, membership of scientific advisory
boards, fees for speaking, funds for research, and reimbursement for
attending symposia. O. Sitbon has relationships with pharmaceutical
companies including Actelion, Bayer, GSK, Pfizer, and United
Therapeutics. In addition to being investigator in trials involving these
companies, relationships include consultancy services, membership of
scientific advisory boards, fees for speaking, and reimbursement for
attending symposia.
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