Eisenmenger syndrome and pulmonary arterial hypertension (PAH) due to unrepaired congenital shunts, including atrial septal defect (ASD), ventricular septal defect (VSD) and patent ductus arteriosus (PDA), remain life-threatening conditions despite advances in congenital heart disease (CHD) care. In this population, vasodilator-based therapies effective in other forms of PAH have shown limited benefit, and no disease-modifying treatment has been established. Sotatercept, an activin-signalling inhibitor, improved exercise capacity and haemodynamics in phase 2/3 PAH trials; however, patients with unrepaired CHD, including Eisenmenger syndrome, were excluded. The efficacy and safety of sotatercept in this population remain unknown.

The SuMILE trial is a prospective, exploratory, multicentre, open-label, randomised, controlled trial conducted at 11 Japanese tertiary centres. 36 adults with vasodilator-resistant PAH due to unrepaired ASD, VSD or PDA, including Eisenmenger syndrome, will be randomised 2:1 to sotatercept add-on therapy plus vasodilator-based PAH therapy versus vasodilator-based PAH therapy alone. Sotatercept will be administered subcutaneously every 3 weeks in accordance with label-approved dose-modification rules for haemoglobin and platelet changes. The primary endpoint is the change in 6-min walk distance from baseline to week 24. Key clinical events will be independently adjudicated. Secondary endpoints include all-cause mortality or lung transplantation; pulmonary hypertension-related hospitalisation or initiation of parenteral prostacyclin and changes in WHO functional class, N-terminal pro-brain natriuretic peptide and emPHasis-10. Exploratory endpoints include genotype, right heart catheterisation and cardiac MRI parameters. The primary analysis will use ANCOVA, adjusting for baseline 6-min walk distance and randomisation stratum in the intention-to-treat population.

The protocol has been reviewed and approved by the certified central review board (Kyushu University Hospital Clinical Ethics Review Board) and participating institutions. Written informed consent will be obtained from all participants. Findings will be disseminated through peer-reviewed journals, scientific conferences and trial registries.

Japan Registry of Clinical Trials no. 1071250069; ClinicalTrials.gov NCT07356778. Protocol version and date: V.1.3; 23 October 2025