Sitaxsentan sodium is primarily used in the treatment of pulmonary arterial hypertension (PAH), a condition characterized by high blood pressure in the arteries of the lungs. It works by selectively blocking endothelin receptors, specifically the ET-A receptor, which helps to relax blood vessels and reduce pressure in the pulmonary arteries. This improves blood flow and eases the strain on the heart, enhancing exercise capacity and slowing disease progression. It is often prescribed when other treatments are not suitable or effective, offering a targeted approach to managing PAH symptoms.