This study is for adults with **pulmonary arterial hypertension (PAH)**, a type of high blood pressure affecting the lungs and heart. It checks if a new medicine, **Ralinepag**, helps patients when added to their usual treatments. Participants will take either Ralinepag or a placebo (a pill with no active medicine) along with their regular PAH treatments. The goal is to see how long it takes for a participant's condition to worsen.

Participants will be in the study for about 28 weeks. If their condition gets worse, they might switch to an extension study to keep getting the real medicine. People who don't want to continue can stop the study drug but will still be checked for their health status.

• **Duration**: The study lasts for about 28 weeks.
• **Eligibility**: Participants must be 18 or older and have symptoms of PAH.
• **Safety**: Regular check-ups and tests will be done to ensure safety.

If interested, participants need to sign a consent form and agree to follow the study guidelines, including using birth control if needed. Some health conditions or prior treatments may prevent participation.