Background: Prostacyclin analogues (epoprostenol, treprostinil and iloprost) induce vasodilation in advanced pulmonary arterial hypertension (PAH) but also inhibit platelets, increasing patients' bleeding risk. The antiplatelet effects of different prostacyclin analogues have never been compared head-to-head. The goal of the PAPAYA (Platelet Reactivity and Treatment With Prostacyclin Analogues in Pulmonary Arterial Hypertension) trial is(i) to compare platelet function (platelet reactivity, extracellular vesicles concentration and thrombus formation) in patients with PAH treated with prostacyclin analogues on top of endothelin receptor antagonists (ERA) and/or phosphodiesterase type 5 inhibitors (PDE5i) and patients treated only with ERA and PDE5i, and (ii) to compare the antiplatelet effect of different prostacyclin analogues.

Venous blood will be collected from patients treated with prostacyclin analogues (study group; n=40) and patients treated with ERA or PDE5i (control group; n=40). Platelet reactivity will be analysed in whole blood by impedance aggregometry using arachidonic acid, adenosine diphosphate and thrombin receptor-activating peptide as agonists. Concentrations of extracellular vesicles from all platelets (CD61+), activated platelets (CD62P+), leukocytes (CD45+) and endothelial cells (CD146+) will be analysed in platelet-depleted plasma using flow cytometry (A-60 Micro). Platelet-rich thrombus formation will be measured using whole blood perfusion system. The study will determine the antiplatelet effect of prostacyclin analogues and compare different prostacyclin analogues head-to-head to identify the best drugs to use in case of thrombosis or bleeding.

Inclusion Criteria:

• Informed consent to participate in the study
• Pulmonary arterial hypertension confirmed with right heart catheterization
• Treatment with prostacyclin analogues (epoprostenol, treprostinil, iloprost) - study group
• Treatment with endothelin receptor antagonists and phosphodiesterase type 5 inhibitors - control group

Exclusion Criteria:

• Known coagulopathy
• Active pathological bleeding
• Known history of bleeding disorder
• Severe thrombocytopenia (platelet count < 50,000/μL )
• Need for antiplatelet therapy with acetylsalicylic acid or P2Y12 antagonists
• Severe chronic renal failure (estimated glomerular filtration rate < 30 mL/min)
• Severe liver insufficiency (Child-Pugh class C)
• Known pregnancy, breast-feeding, or intention to become pregnant during the study period
• Study drug intolerance
• Participation in any previous study with prostacyclin analogues

Patients were eligible for enrolment if they (i) had PAH diagnosed according to the criteria of European Society of Cardiology and European Respiratory Society, confirmed during right heart catheterization, (ii) were treated with prostacyclin analogues on top of other drugs (study group) or only other drugs (ERA and PDE-5i; control group) for at least 1 month.