Study to Assess the Tolerability and the Safety of the Transition From Inhaled Treprostinil to Oral Selexipag in Patients With Pulmonary Arterial Hypertension

Inclusion Criteria: * Male and female patients aged from 18 to 75 years (inclusive) with pulmonary arterial hypertension (PAH). * Etiology of PAH belonging to one of the following subgroups: idiopathic PAH, Heritable PAH, drug or toxin induced, associated with connective tissue disease, associated with HIV infection, associated with congenital heart disease with simple systemic-to-pulmonary shunt at least 1 year after surgical repair. * Women of childbearing potential are eligible only if the following apply: Negative serum pregnancy test at Visit 1 and a negative urine pregnancy test at Visit on Day 1, agreement to undertake monthly urine pregnancy tests during the study and up to 30 days after study drug discontinuation, agreement to use efficient methods of birth control from Visit 1 up to at least 30 days after study treatment discontinuation. * Documented hemodynamic diagnosis of PAH by right heart catheterization (RHC). * Inhaled treprostinil treatment ongoing for at least 90 days and at stable dose for at least 30 days prior to Day 1. * WHO functional class (FC) II or III at Visit 1 and Visit 2. * 6-minute walk distance (6MWD) ≥ 300 m at Visit 1. * On background oral PAH therapy for at least 90 days and on a stable dose for 30 days prior to Visit 2. Acceptable concomitant PAH therapies are one or two of the following: a) Endothelin receptor antagonist (ERA), b) Phosphodiesterase type 5 (PDE-5) inhibitor or soluble guanylate cyclase (sGC) stimulator. Exclusion Criteria: * Treatment with any prostacyclin or prostacyclin analogs other than inhaled treprostinil within 90 days before Day 1, or patients scheduled to receive any of these treatments within the duration of the study. * Any hospitalization within 90 days before Day 1. * Worsening in WHO FC within 30 days prior to Day 1. * At any time prior to Day 1, documented moderate or severe obstructive or restrictive lung disease. * Known or suspicion of pulmonary veno-occlusive disease (PVOD). * Anemia: \< 80 g/L (5.0 mmol/L) hemoglobin. * Clinically relevant thyroid disease (hypo- or hyperthyroidism). * Known and documented severe hepatic impairment. * Uncontrolled hypertension. * Sitting systolic blood pressure \< 85 mmHg. * Acute myocardial infarction within the last 90 days prior to Visit 1. * History of left-sided heart disease. * Left ventricular disease/dysfunction risk factors. * Documented pericardial effusion within 90 days prior to Visit 1. * Documented severe renal insufficiency. * Receiving or having received any investigational drugs within 90 days before Day 1. * Having received selexipag at any time before Day 1. * Acute or chronic impairment (other than dyspnea), limiting the ability to comply with study requirements. * Recently conducted or planned cardio-pulmonary rehabilitation program based on exercise training during the study. * Psychotic, addictive or other disorder limiting the ability to provide informed consent or to comply with study requirements. * Known concomitant life-threatening disease with a life expectancy \< 12 months. * Females who are lactating or pregnant or plan to become pregnant during the study. * Known hypersensitivity to any of the excipients of the drug formulation.