What Is an Endothelin Receptor Antagonist?

An endothelin receptor antagonist blocks access to endothelin receptor sites on smooth muscles and organs. Endothelin-1 is one of three endothelin amino acid peptides and has two subtypes that work together to regulate blood pressure. Researchers developed these medications that nullify the actions of these two subtypes, ensuring vascular relaxation. The possibility of developing liver damage while using the medications requires that patients taking them be monitored closely.

The endothelium of vascular tissues produces endothelin-1 and the subtypes, endothelin-A (ET-A) and endothelin-B (ET-B). ET-A binds to endothelin-A receptor sites, which are located on the ends of smooth muscles, causing vasoconstriction while encouraging cellular reproduction. The substance also combines with a protein known as Gq, which initiates chemical processes that trigger intracellular sarcoplasmic reticulum to release calcium. The increased calcium levels also cause vasoconstriction, vasospasm, and increased contractility and heart rate. Scientists also suggest that the peptide contributes to congestive heart failure and pulmonary artery hypertension (PAH).

Endothelin-B binds to ET-B receptor sites, causing site activation that triggers nitric oxide formation, providing subsequent vasodilatation and inhibition of cellular reproduction. Studies indicate that under normal conditions, the two subtypes maintain adequate circulation by performing alternating tasks as needed. Researchers believe that ET-A holds the key to the pathology behind high blood pressure, commonly referred to as hypertension. Scientists now believe that at some point the balance between ET-A and ET-B goes awry, causing the dominating hypertensive effects of ET-A.

Pharmaceutical researchers developed selective (ambrisentan) endothelin receptor antagonist and non-selective (bosentan) endothelin receptor antagonist medications. Physicians may prescribe either medication as one of the medical treatments for PAH. Ambrisentan mainly inhibits access to ET-A receptor sites, while bosentan inhibits access to both A and B sites. By blocking receptor sites, both medications prevent the chemical reactions that contribute to hypertension, ensuring vascular muscle relaxation and blood pressure reduction.

Studies indicate that individuals taking either endothelin receptor antagonist medication have an increased risk of developing liver damage or failure. Prior to receiving a prescription for the medications, patients undergo liver enzyme testing that determines baseline liver function. After the patient begins a prescription, his liver function is tested monthly. If testing reveals increased levels of liver transferase enzymes, cessation of the medication may occur. Patients who experience appetite loss, nausea, jaundice, and right-sided tenderness, accompanied by dark-colored urine and light to clay-colored stools should consult a physician.

The endothelin receptor antagonist medications are also not recommended for pregnant women because of the risk of birth defects. Frequently reported side effects associated with the medications include headache, skin flushing, and swelling of the feet and ankles, caused by fluid retention. Patients might also experience anemia, decreased sperm counts, and respiratory infections.