Abstract Introduction PAH impacts right ventricular (RV) structure and function but also leads to changes in the LV due to RV/LV interaction and underfilling. REPAIR, the first PAH study to use a primary endpoint assessed by cardiac MRI (cMRI), reported that RV stroke volume (RVSV) increased by 12 mL and pulmonary vascular resistance (PVR) decreased by 38% from baseline (BL) to Week 26 with macitentan. Purpose To assess the effect of macitentan on LV function in patients with PAH. Methods REPAIR (NCT02310672) was a 52-week, multicentre, open-label, single-arm, phase 4 study assessing the effect of macitentan primarily on RV structure and function, determined by cMRI and right heart catheterisation. Macitentan 10 mg was initiated in treatment-naïve patients, in patients receiving stable background phosphodiesterase type-5 inhibitor (PDE5i) at BL, or in initial combination with PDE5i. Exploratory LV endpoints were assessed by cMRI at Weeks 26 and 52. Safety was assessed up to end of study treatment +30 days in all patients who received ≥1 dose of macitentan (N=87). Patients with BL and Week 26 assessments for both PVR and RVSV were included in the modified Full Analysis Set (mFAS; N=71). Results In the mFAS, 57 (80%) patients were female. At BL, median age was 45 years; median (Q1, Q3) six-minute walk distance was 395 (323, 483) m; 48%/51% of patients were WHO functional class II/III; 59% had idiopathic PAH. Compared to BL, at Weeks 26 and 52 there were significant changes in LV cMRI parameters (table). The most common AEs were peripheral oedema (22%), headache (21%) and dizziness (14%). Conclusions Macitentan led to improvements in LV mass, volume and function, including clinically-relevant increases in LV stroke volume, at both 26 and 52 weeks in patients with PAH. Safety was consistent with other macitentan clinical trial data. Funding Acknowledgement Type of funding source: Other. Main funding source(s): Actelion Pharmaceuticals Ltd