Endothelin Receptor Blockers (Antagonists)
Clinically Approved Agents
The clinically approved endothelin receptor antagonists include bosentan, ambrisentan, macitentan, and the recently FDA-approved aprocitentan. 1
Classification by Receptor Selectivity
Dual ETA and ETB Receptor Antagonists
- Bosentan: The first synthesized oral dual endothelin receptor antagonist, blocking both ETA and ETB receptors 2
- Macitentan: A dual receptor antagonist approved for pulmonary arterial hypertension 1
- Aprocitentan: The newest dual receptor antagonist, FDA-approved in March 2024 specifically for resistant hypertension 3, 4
Selective ETA Receptor Antagonists
- Ambrisentan: A non-sulfonamide, propanoic acid-class ERA that preferentially binds to the ETA receptor 2, 5
- Sitaxentan: A selective ETA receptor antagonist that was previously available but has been withdrawn from the market due to fatal hepatotoxicity at higher doses 2, 6
Primary Clinical Indications
Pulmonary Arterial Hypertension (PAH)
All three currently available ERAs for PAH (bosentan, ambrisentan, macitentan) have demonstrated efficacy in:
- Improving exercise capacity (6-minute walk distance) 1, 2
- Enhancing WHO functional class 2
- Improving hemodynamics (reducing pulmonary arterial pressure and pulmonary vascular resistance) 1
- Delaying time to clinical worsening 2, 5
Bosentan is approved for WHO functional class II and III PAH patients, including those with systemic sclerosis-associated PAH and Eisenmenger syndrome 2, 5
Ambrisentan is approved for WHO functional class II and III PAH, with demonstrated efficacy in idiopathic PAH and PAH associated with connective tissue disease and HIV 2, 7, 5
Macitentan is used for PAH treatment at a dose of 10 mg daily orally 1
Resistant Hypertension
Aprocitentan represents the first endothelin receptor antagonist approved specifically for resistant hypertension (defined as uncontrolled blood pressure despite three or more antihypertensive agents) 3, 4, 8
The PRECISION trial demonstrated significant blood pressure-lowering effects, with particular efficacy in:
- Patients over 75 years of age 3
- Those with higher cardiovascular risk profiles 3
- Patients with diabetes 3
- Individuals with advanced chronic kidney disease 3, 8
Dosing Regimens
For Pulmonary Arterial Hypertension
- Bosentan: 62.5 mg twice daily for 4 weeks, then uptitrated to 125 mg twice daily 1, 2, 1
- Ambrisentan: 5 mg once daily, can be increased to 10 mg once daily if tolerated 1, 2, 1
- Macitentan: 10 mg once daily 1
For Resistant Hypertension
- Aprocitentan: 12.5 mg or 25 mg once daily, with maximum plasma concentrations reached within 4-5 hours and an effective half-life of 41 hours 4
Important Safety Considerations
Hepatotoxicity
All ERAs carry risk of liver injury requiring monitoring:
- Bosentan: 10% incidence of elevated hepatic aminotransferases (dose-dependent and reversible); requires monthly liver function testing 2, 5
- Ambrisentan: 0.8-3% incidence of abnormal liver function tests; monthly monitoring required in some jurisdictions but not mandated in the USA 2, 7, 5
- Macitentan: Approximately 10% incidence of aminotransferase elevations 5
- Sitaxentan (withdrawn): Associated with fatal hepatitis at higher doses 6
Teratogenicity
ERAs are highly teratogenic and require strict pregnancy prevention:
- Animal studies demonstrate severe teratogenic effects, particularly craniofacial malformations 1
- Monthly pregnancy testing is required in women of childbearing potential 7
- Bosentan may decrease efficacy of hormonal contraceptives; double-barrier methods with spermicide are recommended 6, 9
- Despite animal data, a 2019 systematic review of 39 human pregnancy exposures found no fetal congenital abnormalities, though pregnancy remains dangerous for mothers with PAH due to increased cardiac output 1
Anemia
ERAs can cause reductions in hemoglobin levels:
Fluid Retention and Edema
- Ambrisentan: Increased incidence of peripheral edema (29% in patients over 65 years) 2, 7, 5
- Bosentan: Fluid retention and lower limb edema reported 9
- Aprocitentan: As a dual receptor antagonist, demonstrates lower risk of fluid retention and vascular leakage compared to other ERAs 3
Male Fertility
Concern exists that ERAs may cause testicular atrophy and male infertility; younger men considering conception should be counseled before starting therapy 6, 9
Drug Interactions
Sitaxentan (when it was available) inhibited CYP2C9 P450 enzyme, requiring warfarin dose reductions to maintain appropriate INR 2, 6
Bosentan may decrease efficacy of hormonal contraceptives through enzyme induction 6, 9
Emerging Applications
Chronic Kidney Disease
Selective ETA receptor antagonists show promise in CKD:
- Reduce proteinuria and slow kidney disease progression 10, 11
- Atrasentan reduced risk of renal events in the SONAR trial for diabetic nephropathy 12
- Sparsentan is now available for IgA nephropathy 11
- Combination with SGLT2 inhibitors may mitigate fluid retention side effects 10, 11
Other Conditions Under Investigation
ERAs show potential benefit in:
- Systemic sclerosis (bosentan effective for digital ulcers) 12
- Cerebral vasospasm prevention (clazosentan) 12
- Cancer, fibrotic disorders, and pain management (preclinical evidence) 12
Common Pitfalls to Avoid
- Do not use ERAs in pregnancy or without adequate contraception in women of childbearing potential 1, 6, 1
- Do not skip monthly liver function monitoring for bosentan, macitentan, and in some jurisdictions for ambrisentan 2, 7, 2
- Do not rely on hormonal contraception alone with bosentan; use additional barrier methods 6, 9
- Monitor for fluid retention carefully, especially in patients with diabetes, advanced CKD, or those over 65 years on ambrisentan 7, 3, 11
- Do not use high-dose calcium channel blockers instead of ERAs in PAH patients who are non-responders to acute vasoreactivity testing 5