This clinical trial is focused on studying a condition known as Pulmonary Arterial Hypertension (PAH). PAH is a type of high blood pressure that affects the arteries in the lungs and the right side of the heart. The study is testing a medication called Macitentan, which is already used in a 10 mg dose, and comparing it to a higher dose of 75 mg. The purpose of the study is to see if the higher dose of Macitentan is more effective in delaying the time to the first serious health event related to PAH.

Participants in the study will be randomly assigned to receive either the 10 mg or 75 mg dose of Macitentan. Some participants may receive a placebo, which looks like the medication but does not contain the active ingredient. The study will be conducted in a way that neither the participants nor the researchers know who is receiving which treatment, to ensure unbiased results. After the initial phase, all participants will have the opportunity to receive the 75 mg dose of Macitentan in an open-label period, meaning everyone will know they are receiving the active medication.

The study will last for a period of up to 72 weeks, during which participants will be monitored for any changes in their condition and any side effects they may experience. The goal is to determine if the higher dose of Macitentan can better manage the symptoms of PAH and improve the quality of life for those affected by this condition. Participants will be required to visit the study site regularly for check-ups and assessments throughout the study period.