Endothelin-1 (ET-1) is the most potent endogenous vasoconstrictor peptide known, with sustained effects lasting hours after a single exposure. It is synthesized predominantly by vascular endothelial cells and exerts its effects through two structurally distinct G-protein-coupled receptors whose signaling consequences are largely opposite. Selective versus non-selective pharmacological antagonism of these receptors produces substantially different clinical profiles.
The endothelin biosynthetic pathway begins with transcription of the preproendothelin-1 gene, producing a 212-amino-acid preproendothelin-1 precursor. Furin, a ubiquitous proprotein convertase, cleaves the signal peptide and the C-terminus to generate big endothelin-1 (big ET-1), a 38-amino-acid biologically inactive intermediate. Big ET-1 is cleaved at the Trp21-Val22 bond by endothelin-converting enzyme-1 (ECE-1), a membrane-bound zinc metallopeptidase, to yield the mature 21-amino-acid ET-1. ECE-1 is expressed on the surface of vascular endothelial cells and is concentrated in the pulmonary vasculature, which serves as the primary site of ET-1 production and processing. The lungs extract a significant fraction of circulating big ET-1 on first pass and convert it to ET-1, making pulmonary endothelial ECE-1 the rate-limiting step in systemic ET-1 generation. ET-1 is secreted predominantly abluminally toward underlying vascular smooth muscle, giving it a paracrine rather than endocrine mode of action in most vascular beds.1
Two G-protein-coupled receptor subtypes mediate ET-1 effects: ETA and ETB. ETA receptors are expressed primarily on vascular smooth muscle cells and cardiac myocytes. They have high affinity for ET-1 and ET-2 but not ET-3. ETA receptor coupling to Gq activates phospholipase C (PLC), generating inositol trisphosphate (IP3, also written IP3) and diacylglycerol (DAG); IP3 releases calcium from the sarcoplasmic reticulum and DAG activates protein kinase C (PKC), producing sustained vasoconstriction, mitogenesis, and pro-fibrotic signaling. ETA also couples to Gi to inhibit adenylyl cyclase. The sustained vasoconstriction produced by ETA activation reflects both the IP3-dependent initial calcium release and the PKC-mediated sensitization of the contractile apparatus, making ETA-mediated contraction more prolonged than that produced by most other vasoconstrictors.2
ETB receptors are expressed on two distinct cell populations with opposing functional consequences. On vascular endothelial cells, ETB activation stimulates production of nitric oxide (NO) via endothelial nitric oxide synthase (eNOS) and prostacyclin (PGI2) via cyclooxygenase, producing vasodilation and opposing the vasoconstrictor actions of ETA. ETB receptors on endothelial cells also mediate clearance of circulating ET-1 through receptor-mediated internalization and lysosomal degradation, representing a major route of ET-1 elimination: approximately 50% of circulating ET-1 is cleared by lung ETB receptors on each pass. On vascular smooth muscle cells, however, ETB also mediates vasoconstriction through a mechanism similar to ETA. This ETB smooth muscle population is normally quantitatively minor in healthy vasculature but becomes upregulated in disease states, particularly pulmonary arterial hypertension (PAH), where it contributes to the pathological vasoconstrictive state and complicates the rationale for selective versus non-selective receptor antagonism.2
ECE-1 is an attractive pharmacological target because inhibiting it would prevent ET-1 formation from all upstream big ET-1, circumventing both receptor subtypes simultaneously. However, ECE-1 also cleaves other biologically important substrates including bradykinin and substance P. ECE-1 inhibitors developed to date have produced unacceptable off-target effects including bradykinin accumulation leading to angioedema and increased substance P-mediated neurotransmission. No ECE-1 inhibitor has entered clinical use. The current pharmacological approach targets downstream receptor antagonism, accepting the continued synthesis of ET-1 in exchange for cleaner selectivity.
Pulmonary arterial hypertension (PAH) is a progressive vasculopathy characterized by sustained elevation of mean pulmonary arterial pressure beyond 20 mmHg at rest with pulmonary arterial wedge pressure at or below 15 mmHg, in the absence of secondary causes. The pathological hallmark is obliterative remodeling of the small pulmonary arterioles driven by vasoconstriction, smooth muscle cell proliferation, endothelial dysfunction, in situ thrombosis, and adventitial fibrosis, with ET-1 playing a central mechanistic role in all of these processes.
In PAH, circulating ET-1 levels are markedly elevated, often three to ten times above normal, and the degree of ET-1 elevation correlates with both hemodynamic severity (mean pulmonary arterial pressure and pulmonary vascular resistance) and prognosis. The pathological overproduction of ET-1 in the pulmonary vasculature arises from endothelial dysfunction: factors including hypoxia, shear stress, inflammatory cytokines, and oxidative stress upregulate preproendothelin-1 gene transcription and impair ETB-mediated ET-1 clearance simultaneously, producing a dramatic net increase in local and circulating ET-1. The increased ET-1 activates ETA receptors on pulmonary arterial smooth muscle cells to produce sustained vasoconstriction, raises pulmonary vascular resistance, and initiates the mitogenic signaling cascades that drive smooth muscle hyperplasia and hypertrophy in the arterial media. This structural thickening of the arterial wall reduces luminal area and further elevates resistance independently of the vasomotor component.3
Beyond vasoconstriction and smooth muscle proliferation, ET-1 signaling promotes adventitial fibrosis through activation of pulmonary adventitial fibroblasts and their transformation into myofibroblasts, which deposit extracellular matrix components including collagen and fibronectin. ET-1 also promotes in situ microvascular thrombosis by suppressing prostacyclin production and upregulating plasminogen activator inhibitor-1 (PAI-1). The net result is obliterative vascular remodeling that is largely irreversible once established, explaining why early intervention before fixed structural remodeling is critical and why response to vasodilator testing predicts some but not all therapeutic responses. The right ventricle, exposed to progressively increasing afterload from rising pulmonary vascular resistance, undergoes initially adaptive and then maladaptive remodeling, progressing from compensated right ventricular hypertrophy to right ventricular dilation, tricuspid regurgitation, reduced cardiac output, and ultimately right ventricular failure and death.4
Within the WHO classification of pulmonary hypertension, PAH constitutes Group 1, which includes idiopathic PAH, heritable PAH (most commonly associated with BMPR2 mutations), drug- and toxin-induced PAH (anorexigens, methamphetamine, dasatinib), and PAH associated with connective tissue disease (systemic sclerosis predominating), congenital heart disease, HIV infection, and portal hypertension. ERAs (endothelin receptor antagonists) are approved and guideline-recommended specifically for Group 1 PAH; they are generally not used in Groups 2 through 5, where the driving pathophysiology differs and pulmonary hypertension is secondary to left heart disease, lung disease, thromboembolic disease, or other mechanisms. The distinction matters clinically because ERA use in Group 2 PAH (pulmonary hypertension due to left heart disease) can worsen pulmonary edema by reducing afterload-dependent left ventricular filling pressure adaptation.3
Pulmonary hypertension (PH) is a hemodynamic descriptor; PAH (WHO Group 1) is a specific pathophysiological entity. ERAs are Group 1-specific; prescribing them in Group 2 (left heart disease-related PH), which is the most common cause of PH overall, is contraindicated because the underlying elevated left-sided pressures mean afterload reduction with ERAs can precipitate flash pulmonary edema rather than benefit. Before initiating ERA therapy, right heart catheterization is required to confirm the hemodynamic definition of PAH and exclude left heart disease as the driver (wedge pressure ≤15 mmHg is a required diagnostic criterion). ERAs require specialist management in a PAH center.
Three endothelin receptor antagonists (ERAs) are approved for PAH: bosentan (dual ETA/ETB), ambrisentan (selective ETA), and macitentan (dual ETA/ETB, tissue-targeting). The debate between selective and non-selective strategies centers on the ETB receptor paradox: ETB on endothelial cells mediates beneficial vasodilation and ET-1 (endothelin-1) clearance, suggesting selective ETA blockade might be superior; however, in PAH the disease-state upregulation of ETB on smooth muscle cells means dual blockade may be necessary to fully prevent pathological vasoconstriction. Clinical trial data ultimately showed equivalent outcomes for selective and dual ERAs, and the choice between agents is now driven primarily by tolerability profile, drug interaction potential, teratogenicity risk management, and adherence considerations.
Bosentan was the first approved ERA and remains the reference ERA for both dual ETA/ETB antagonism and for the generation of clinical trial experience. It is a sulfonamide derivative that blocks both ETA and ETB with a roughly 20-fold selectivity for ETA (Ki approximately 4.7 nM for ETA vs. 95 nM for ETB). Its binding is competitive and reversible. The dual ETB blockade of bosentan raises plasma ET-1 levels during treatment (because ETB-mediated clearance is impaired) by 100–200% above baseline. This is not indicative of worsening disease but rather a pharmacodynamic consequence of ETB blockade and serves as an indirect marker of drug exposure. Elevated ET-1 levels during bosentan therapy do not predict clinical worsening; the beneficial effects result from ETA blockade despite the elevated ET-1 levels.5
Ambrisentan is a propanoic acid derivative with high selectivity for ETA (approximately 4,000-fold selectivity for ETA over ETB). This selectivity preserves ETB-mediated endothelial vasodilation and ET-1 clearance by the pulmonary circulation, theoretically maintaining the counter-regulatory function of endothelial ETB. It raises plasma ET-1 levels less than bosentan because ETB clearance remains functional. In the pivotal ARIES-1 and ARIES-2 trials, ambrisentan significantly improved exercise capacity and delayed clinical worsening in PAH patients, with a hepatic safety profile superior to bosentan (elevated liver enzymes occurred at rates approaching placebo). This hepatic safety advantage led to the removal of mandatory liver function monitoring requirements for ambrisentan, distinguishing it from bosentan in terms of monitoring burden.6
Macitentan is a dual ETA/ETB antagonist structurally derived from bosentan but engineered for tissue targeting through enhanced lipophilicity and slower receptor dissociation kinetics. Its receptor binding is non-competitive with a very slow off-rate, meaning it has a longer duration of receptor occupancy relative to its plasma half-life, allowing once-daily dosing with sustained receptor coverage. Macitentan and its active metabolite ACT-132577 contribute additively to receptor occupancy. The SERAPHIN trial, which evaluated macitentan against placebo in 742 PAH patients over a median follow-up of approximately 115 weeks, was the first ERA trial to use a long-term morbidity-mortality endpoint: the composite of first event of clinical worsening or death. Macitentan 10 mg significantly reduced this composite endpoint by 45% versus placebo (hazard ratio 0.55, p less than 0.001), representing the most robust long-term ERA evidence base. Macitentan has a hepatic safety profile comparable to ambrisentan, with liver enzyme elevations at rates similar to placebo in SERAPHIN.7
The theoretical advantage of ETA selectivity (preserving beneficial ETB-mediated vasodilation and clearance) did not translate into demonstrable clinical superiority over dual blockade in randomized trials. Both selective and dual ERAs show similar improvements in exercise capacity, hemodynamics, and time to clinical worsening in head-to-head analyses and cross-trial comparisons. The choice between bosentan, ambrisentan, and macitentan in PAH practice is therefore driven by practical considerations: ambrisentan and macitentan carry lower hepatotoxicity risk and reduced monitoring burden; macitentan has the strongest morbidity-mortality endpoint data; bosentan has the longest experience and established interaction profile. Combination therapy with an ERA plus a phosphodiesterase-5 inhibitor (or soluble guanylate cyclase stimulator) is now the standard of care for most newly diagnosed PAH patients.
The three ERAs differ substantially in their metabolic profiles, drug interaction potential, and dosing requirements. Bosentan has the most complex pharmacokinetic profile, involving induction of its own metabolism and multiple drug interaction pathways. Ambrisentan has a simpler profile with once-daily dosing and a favorable hepatic safety record. Macitentan combines once-daily dosing with the most durable long-term trial evidence and a tissue-targeting pharmacological mechanism that sustains receptor blockade beyond plasma drug levels.
Bosentan is absorbed orally with a bioavailability of approximately 50%. It undergoes extensive hepatic metabolism via cytochrome P450 3A4 (CYP3A4) and CYP2C9, generating three metabolites; one metabolite (Ro 48-5033) is pharmacologically active and accounts for approximately 20% of bosentan's receptor-blocking activity. Bosentan is a potent inducer of both CYP3A4 and CYP2C9, substantially reducing plasma concentrations of co-administered CYP3A4 substrates including cyclosporine, sildenafil, simvastatin, and warfarin. It also induces its own metabolism, producing autoinduction: plasma bosentan concentrations fall by approximately 50% after 4–8 weeks of steady-state dosing as CYP induction develops. This autoinduction is clinically important because it means blood levels at steady state are substantially lower than during the first weeks of treatment, and dose escalation from 62.5 mg to 125 mg twice daily after 4 weeks is standard to compensate for this reduction. Bosentan is contraindicated with cyclosporine (which markedly raises bosentan levels) and with glyburide (which produces additive hepatotoxicity). Biliary excretion is the primary elimination route after hepatic glucuronidation.5
Ambrisentan is orally bioavailable at approximately 60–70%, with once-daily dosing (5 mg or 10 mg). It is a substrate for P-glycoprotein (P-gp) and organic anion-transporting polypeptides (OATPs), and is metabolized predominantly by CYP3A4 and UGT1A9S through glucuronidation; unlike bosentan, it does not induce CYP enzymes and does not cause autoinduction. Its half-life is approximately 15 hours. The ARIES-1 and ARIES-2 trials randomized 394 patients with PAH to ambrisentan 5 mg, 10 mg, or placebo, with the primary endpoint of change in 6-minute walk distance (6MWD) at 12 weeks. Both trials showed statistically significant improvements in 6MWD and delay in time to clinical worsening. A key secondary finding was that ambrisentan significantly reduced the rate of elevated liver aminotransferases compared to bosentan-treated historical controls, leading to the removal of monthly liver function testing requirements that had previously applied to all ERAs. Ambrisentan is also approved for use in combination with tadalafil based on the AMBITION trial, which showed that first-line combination therapy was superior to monotherapy for reducing the risk of clinical failure in treatment-naive PAH patients.68
Macitentan has a bioavailability of approximately 50%, a plasma half-life of approximately 16 hours, and generates the active metabolite ACT-132577 with a half-life of approximately 48 hours. Macitentan is predominantly metabolized by CYP3A4 but does not significantly induce CYP enzymes. It is a weak inducer of P-gp. The half-life of ACT-132577 contributes to sustained receptor coverage beyond the dosing interval, supporting once-daily administration. The pivotal SERAPHIN trial was a landmark in PAH trial design: unlike earlier 12-week 6MWD endpoint trials, SERAPHIN was an event-driven trial following 742 patients for a median of approximately 115 weeks. The primary endpoint was the composite of time to first event of worsening PAH or death; macitentan 10 mg reduced this composite by 45% versus placebo (hazard ratio 0.55, 97.5% confidence interval 0.39–0.76). Mortality alone did not reach statistical significance in this trial, consistent with the underpowering for mortality as an isolated endpoint, but the morbidity-mortality composite data provide the strongest long-term efficacy evidence of any ERA. Current PAH guidelines recommend ERAs as part of a combination treatment approach including a phosphodiesterase-5 inhibitor or soluble guanylate cyclase stimulator from treatment initiation in most intermediate- and high-risk patients.7810
The AMBITION trial randomized 500 treatment-naive PAH patients to first-line ambrisentan (10 mg/day) plus tadalafil (40 mg/day) versus either drug alone. The combination therapy group showed a 50% lower risk of clinical failure (composite of first event of clinical worsening, unsatisfactory clinical response, or death) compared to the pooled monotherapy arms (hazard ratio 0.50, 95% CI 0.35–0.72). This established that upfront combination ERA plus PDE5 (phosphodiesterase type 5) inhibitor therapy is superior to sequential add-on strategy in patients who can tolerate both drugs. Current guidelines incorporate this principle, recommending risk-stratified combination initiation over monotherapy for most newly diagnosed PAH patients.
ERAs share three class-wide safety concerns that govern their prescribing: absolute teratogenicity contraindication with mandatory REMS enrollment, class-wide fluid retention and peripheral edema through ET-mediated renal effects, and class-wide anemia. Bosentan adds hepatotoxicity as an agent-specific concern that drove the original REMS requirements; ambrisentan and macitentan have substantially lower hepatotoxicity risk. Drug interactions are most extensive with bosentan, which broadly induces cytochrome P450 and UDP-glucuronosyltransferase enzymes.
All ERAs carry an absolute contraindication in pregnancy due to demonstrated teratogenicity in animal models and the mechanistic implausibility of safe use in humans: ET-1 (endothelin-1) signaling is essential for normal cardiovascular development, and its blockade produces severe cardiac septal defects, craniofacial malformations, and major vessel abnormalities. In the United States, all three ERAs require enrollment in Risk Evaluation and Mitigation Strategy (REMS) programs: the Tracleer REMS (bosentan), the Letairis REMS (ambrisentan), and the Opsumit REMS (macitentan). REMS requirements mandate monthly pregnancy testing in all women of childbearing potential and require use of at least two reliable forms of contraception. Bosentan reduces oral contraceptive efficacy by inducing CYP3A4-mediated metabolism of estrogen and progestin components; clinicians must account for this interaction by ensuring barrier contraception is used in addition to hormonal methods. Ambrisentan and macitentan do not significantly induce oral contraceptive metabolism but REMS pregnancy testing is still required for all ERAs.8
Bosentan-associated hepatotoxicity affects approximately 10% of patients and is characterized by dose-dependent elevation of serum aminotransferases (alanine aminotransferase, ALT, and aspartate aminotransferase, AST). The mechanism involves bile salt export pump (BSEP) inhibition, reducing biliary bile salt secretion and causing intrahepatic bile salt accumulation that damages hepatocytes. This is distinct from direct cytotoxicity and is a form of drug-induced cholestatic hepatitis. Elevations are generally asymptomatic and reversible upon dose reduction or drug discontinuation, but severe hepatotoxicity with marked elevations (more than eight times the upper limit of normal) or associated clinical hepatitis has been reported. The monitoring protocol requires baseline liver function tests and monthly aminotransferase monitoring throughout bosentan therapy. Dose-response management thresholds are defined: if aminotransferases rise between 3 and 5 times the upper limit of normal, bosentan should be continued with more frequent monitoring; 5 to 8 times: reduce or interrupt; more than 8 times: discontinue permanently. Ambrisentan and macitentan do not inhibit BSEP and carry hepatotoxicity rates comparable to placebo; they do not require routine monthly liver function monitoring, though baseline and periodic testing remains appropriate.9
Peripheral edema affects 5–17% of ERA-treated patients and reflects ET receptor antagonism in the renal vasculature, impairing sodium excretion and promoting fluid retention. This effect is not dose-dependent and does not universally predict reduced clinical efficacy; in PAH (pulmonary arterial hypertension) patients it must be distinguished from right heart failure-related edema, which carries different prognostic implications. Anemia (hemoglobin reduction of approximately 1 g/dL from baseline) is a class effect occurring in approximately 13% of bosentan-treated and 8% of ambrisentan-treated patients, attributed to hemodilution from fluid retention and potential inhibition of erythropoietin signaling. Serious hepatic failure is a rare but potentially fatal complication of bosentan. Bosentan significantly reduces cyclosporine levels (by approximately 50%) through CYP3A4 induction, while cyclosporine increases bosentan concentrations several-fold through OATP inhibition and CYP3A4 competition; this bidirectional interaction makes concomitant use contraindicated. Bosentan also reduces sildenafil plasma concentrations by approximately 50%, requiring dose adjustment or alternative ERA selection in patients receiving combination phosphodiesterase-5 inhibitor therapy.9
Bosentan: CYP3A4/2C9 inducer (autoinduction at 4–8 weeks); reduces oral contraceptives, sildenafil, warfarin, simvastatin, and cyclosporine levels. Contraindicated with cyclosporine, glyburide. Monthly LFTs required throughout therapy. REMS: Tracleer.
Ambrisentan: CYP3A4/UGT1A9 substrate; does not induce CYP. P-gp substrate. Minimal clinically significant interactions. No routine monthly LFT requirement. REMS: Letairis. Use with caution in patients with pre-existing hepatic impairment.
Macitentan: CYP3A4 substrate (active metabolite ACT-132577 by CYP3A4); weak P-gp inducer. Strong CYP3A4 inhibitors (ketoconazole, ritonavir) increase macitentan exposure markedly; strong inducers (rifampin) reduce it. No routine monthly LFT requirement. REMS: Opsumit.
All ERAs: Absolute contraindication in pregnancy. Monthly pregnancy testing in women of childbearing potential. Two reliable contraceptive methods required. Peripheral edema and anemia are class-wide effects requiring monitoring.
Yanagisawa M, Kurihara H, Kimura S, et al. A novel potent vasoconstrictor peptide produced by vascular endothelial cells. Nature. 1988;332(6163):411–415.
doi:10.1038/332411a0Davenport AP, Hyndman KA, Dhaun N, et al. Endothelin. Pharmacol Rev. 2016;68(2):357–418.
doi:10.1124/pr.115.011833Galiè N, Humbert M, Vachiery JL, et al. 2015 ESC/ERS guidelines for the diagnosis and treatment of pulmonary hypertension. Eur Heart J. 2016;37(1):67–119.
doi:10.1093/eurheartj/ehv317Humbert M, Guignabert C, Bonnet S, et al. Pathology and pathobiology of pulmonary hypertension: state of the art and research perspectives. Eur Respir J. 2019;53(1):1801887.
doi:10.1183/13993003.01887-2018Channick RN, Simonneau G, Sitbon O, et al. Effects of the dual endothelin-receptor antagonist bosentan in patients with pulmonary hypertension: a randomised placebo-controlled study. Lancet. 2001;358(9288):1119–1123.
doi:10.1016/S0140-6736(01)06250-XGaliè N, Olschewski H, Oudiz RJ, et al. Ambrisentan for the treatment of pulmonary arterial hypertension: results of the ambrisentan in pulmonary arterial hypertension, randomized, double-blind, placebo-controlled, multicenter, efficacy (ARIES) study 1 and 2. Circulation. 2008;117(23):3010–3019.
doi:10.1161/CIRCULATIONAHA.107.742510Pulido T, Adzerikho I, Channick RN, et al. Macitentan and morbidity and mortality in pulmonary arterial hypertension. N Engl J Med. 2013;369(9):809–818.
doi:10.1056/NEJMoa1213917Humbert M, Kovacs G, Hoeper MM, et al. 2022 ESC/ERS guidelines for the diagnosis and treatment of pulmonary hypertension. Eur Heart J. 2022;43(38):3618–3731.
doi:10.1093/eurheartj/ehac237Rubin LJ, Badesch DB, Barst RJ, et al. Bosentan therapy for pulmonary arterial hypertension. N Engl J Med. 2002;346(12):896–903.
doi:10.1056/NEJMoa012212Sitbon O, Channick R, Chin KM, et al. Selexipag for the treatment of pulmonary arterial hypertension. N Engl J Med. 2015;373(26):2522–2533.
doi:10.1056/NEJMoa1503184