ARIES - Ambrisentan in Patients With Moderate to Severe Pulmonary Arterial Hypertension (PAH)

Inclusion Criteria: * Diagnosis of idiopathic PAH (formally known as PPH), or PAH associated with collagen vascular disease, anorexigen use, or HIV infection; * Historical cardiac catheterization with the following hemodynamic criteria: Mean pulmonary artery pressure greater than or equal to 25 mmHg; Pulmonary vascular resistance greater than 3 mmHg/L/min; Pulmonary capillary wedge pressure or left ventricular end diastolic pressure less than 15 mmHg; * 6-minute walk distance of at least 150 meters, but no more than 450 meters; * Total lung capacity greater than or equal to 70% and FEV1 greater than or equal to 65% of predicted normal; Exclusion Criteria: * Portopulmonary hypertension; * Subjects with PAH due to or associated with coronary artery disease, left heart disease, interstitial lung disease, chronic obstructive pulmonary disease, veno-occlusive disease, chronic thromboembolic disease, or sleep apnea; * Bosentan (Tracleer®), sildenafil (Viagra®), or chronic prostanoid therapy within 4 weeks of screening; * Serum ALT or AST lab value that is greater than 1.5 times the upper limit of normal; * Contraindication to treatment with an endothelin receptor antagonist; * Subject with cardiovascular, hepatic, renal, hematologic, gastrointestinal, immunologic, endocrine, metabolic, or central nervous system disease that may adversely affect the safety of the subject; * Participation in a clinical study involving another investigational drug within 4 weeks of screening.