Endothelin Receptor Antagonists (ERA)

Endothelin is a chemical messenger released by cells that line the inside of the blood vessels. There are many types of edothelin but all of them are hormones that cause blood vessels to tighten and constrict. Of the various types of Edothelin, Endothelin-1 (ET-1) is the most potent vasoconstrictor. There is evidence that patients with primary pulmonary hypertension have higher levels of ET-1 in their system and that this increased level of ET-1 may be the cause of primary pulmonary hypertension.

ET-1 causes blood vessels to narrow by joining with cell receptors on the smooth muscle layer that surround the blood vessel. When the ET-1 comes in contact with the cell receptors on the muscle cells, the cells are activated and contract, causing the blood vessel to narrow.

Recently drugs containing a substance known as ERAs have been developed to reverse this narrowing. ERAs work by joining with the ET-1 receptors much in the same manner as ET-1. The result is that ET-1 molecules no longer have access to the blocked receptor and vessels constriction is reduced.

Whether or not the higher ET-1 level found in victims of primary pulmonary hypertension is a consequence or cause of primary pulmonary hypertension is still unknown, but recent research in which ERAs have been used to treat primary pulmonary hypertension in animals suggests that it might be a cause. In other words, drugs that use ERAs might actually attack the root cause of primary pulmonary hypertension.

Clinical trials of an ERA drug called Tracleer, also known as Bosentan, have been carried out in humans with pulmonary hypertension. Based on two randomized, placebo-controlled clinical trials, Tracleer was found to be an effective treatment for pulmonary hypertension. However, because of serious side effects associated with Tracleer, the primary pulmonary hypertension treatment version of this drug is available only through a direct distribution program from Tracleer's manufacturer Actelion Pharmaceuticals. According to the FDA, the use of Tracleer by persons with pulmonary hypertension may present two significant risks; liver toxicity and potential damage to a fetus.

Reports are mixed on the exact effectiveness of Tracleer. One four month trial discovered that Tracleer demonstrated only "small but measurable beneficial effects" and concluded that though the duration of the test was not sufficient to determine a difference in mortality, its results suggested that the drug had a therapeutic role in some patients with primary pulmonary hypertension.

One of the main advantages of Tracleer is that it comes in tablet form. This makes it a lot easier to use than other PPH treatments such as intravenous or inhaled forms of Prostacyclin. Whether or not Tracleer is more effective than these other treatments is something that still has not been determined.


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