Pulmonary arterial hypertension (PAH) is a rare, progressive, and potentially fatal disease characterized by increased pulmonary vascular resistance and right ventricular dysfunction. Among the four major molecular pathways involved in PAH pathophysiology-nitric oxide, prostacyclin, activin, and endothelin-1 (ET-1)-the endothelin pathway plays a central role. Endothelin-1 acts on ETA and ETB receptors, inducing vasoconstriction and vascular remodeling. Endothelin receptor antagonists (ERAs) are cornerstone therapies in PAH. Ambrisentan is selective for ETA and associated with a lower risk of hepatotoxicity. Bosentan, a dual ERA (ETA/ETB), has well-established efficacy but a higher incidence of liver enzyme elevation, with approximately 9% of patients experiencing hepatic side effects and about 2% discontinuing therapy due to hepatotoxicity. While transitions between ERAs occur in routine clinical practice, data on their clinical impact are scarce. This prospective, observational, single-center cohort study aims to evaluate the effect of switching from ambrisentan to bosentan on risk stratification using the COMPERA 2.0 and REVEAL Lite 2.0 scores at 3-6 months post-switch. Secondary outcomes include variations in functional class (WHO/NYHA), 6-minute walk distance (6MWD), NT-proBNP levels, incidence of adverse events (with a focus on hepatotoxicity), and hematologic parameters such as anemia. The study will enroll adult patients (≥18 years) with confirmed PAH by right heart catheterization who have undergone a documented switch from ambrisentan 10 mg to bosentan 125 mg within the last 6 months. The primary endpoint is the proportion of patients whose risk category changes post-transition according to COMPERA 2.0 and REVEAL Lite 2.0. The results are expected to provide clinically relevant insights into therapeutic decisions involving ERA transitions in PAH management.