Effects of riociguat on right ventricular size and function in pulmonary arterial hypertension (RIVER II): a prospective, phase IV study

Abstract Background Pulmonary arterial hypertension (PAH) is characterized by right ventricular (RV) pressure overload, dilatation and dysfunction, which are key prognostic determinants. While riociguat improves exercise capacity and hemodynamics, prospective data on reverse remodeling remain limited. This prospective, phase IV study evaluated the effects of riociguat on right-heart size, function, hemodynamics, exercise capacity and safety in PAH patients. Methods Treatment-naïve or pretreated PAH patients were enrolled. Patients receiving phosphodiesterase-5 inhibitors (PDE5i) could be switched to riociguat upon clinical indication. The primary endpoint was the change in right atrial (RA) and RV area at 24 weeks. Secondary endpoints included additional echocardiographic, clinical, exercise, hemodynamic and laboratory parameters, as well as quality-of-life (QoL) and safety. Results Thirty patients (61.2 ± 14.5 years; 76.7% male; 24 PDE5i-pretreated) were enrolled. Marked right-heart dilatation and impaired RV function were prominent at baseline. At week 24, riociguat significantly reduced RA (Δ -3.17 ± 5.91 cm²; p = 0.006) and RV area (Δ -6.00 ± 3.80 cm²; p < 0.0001). Furthermore, RV-fractional area change, 6-minute walking distance and functional class improved significantly. In patients with invasive follow-up, cardiac index increased significantly, with favorable trends in further parameters. N-terminal pro-brain natriuretic peptide and QoL remained unchanged. The study terminated early following the decision by the supplier Merck/MSD; the termination was not safety related. Riociguat was generally well tolerated with no new safety concerns. Conclusion Riociguat therapy resulted in a statistically significant improvement in right ventricular and atrial size and function with further improvements in exercise capacity and functional class. This prospective trial confirms the findings of previous retrospective studies and supports riociguat as an effective treatment option to improve RV geometry and performance. Trial registration This trial was registered on clinicaltrials.gov with the ClinicalTrials.gov ID NCT04954742.