Bosentan is a dual endothelin receptor antagonist important in the treatment of pulmonary artery hypertension (PAH). It is licensed in the United States, the European Union and other countries by Actelion Pharmaceuticals for the management of PAH under the trade name Tracleer®. Bosentan is used to treat pulmonary hypertension by blocking the action of endothelin molecules that would otherwise promote narrowing of the blood vessels and lead to high blood pressure. Bosentan competitively antagonizes the binding of 125I-labeled ET-1 to human vascular smooth muscle cells (predominantly ETA receptors) with an inhibition constant (Ki ) of 4.7 nM and to human placenta membranes (predominantly ETB receptors) with a Ki of 95 nM. Furthermore, bosentan is specific for endothelin receptors and does not interfere with the binding of a variety of peptides, neurotransmitters, growth factors, or eicosanoids to their receptors.