ET-1 is a very potent vasoconstrictor; its use results in increased pulmonary vascular resistance [Raja, 2010a]. It also has proliferative effects on vascular smooth muscle cells [Raja and Dreyfus, 2008]. Blockade of the endothelin receptor has been used in the treatment of PAH. Several endothelin receptor antagonists (ERAs) have been identified and differ in their selectivity toward the ET-A and ET-B receptors. Currently, both selective and nonselective ERAs are approved and available for treating PAH.