In patients with pulmonary arterial hypertension (PAH), switching from phosphodiesterase-5 inhibitors (PDE5i) to riociguat has been associated with greater clinical improvement. Balloon pulmonary angioplasty (BPA) is a treatment option for patients with chronic thromboembolic pulmonary hypertension (CTEPH) and it has been associated with a greater reduction in mean pulmonary artery pressure (mPAP) and pulmonary vascular resistance (PVR) when compared to riociguat. In CTEPH patients who have completed BPA treatment, the potential benefits of switching from PDE5i to riociguat remain unknown. A prospective, single-arm, open-label study was conducted on CTEPH patients who remained symptomatic after BPA treatment. PDE5i was switched to riociguat after a 24-hour washout period at baseline and patients were followed up for 26 weeks. The primary endpoints were changes in haemodynamic parameters including mPAP and PVR at week 26. The secondary endpoints included changes in cardiac index, functional parameters including WHO functional class, 6-minute walking distance test (6MWD) and REVEAL Lite 2 score; biochemical parameters including N-terminal prohormone of brain natriuretic peptide (NT-proBNP); echocardiographic parameters including measurements of right heart functions and any treatment-related adverse events and clinical worsening. Between July 2024 and January 2025, 16 patients (mean age 62.3±14.6 years; 75% female) were recruited, and 14 (87.5%) completed the 26-week follow-up. At week 26, significant reductions were observed in mPAP (-4.79 mmHg; Confidence Interval [CI] -8.05 to -1.52; p=0.007) and PVR (-2.16 Wood units; CI -3.64 to -0.69; p=0.007). Significant improvements were also observed in cardiac index, WHO functional class, NT-proBNP levels, 6MWD and REVEAL Lite 2 score. No significant improvement was observed in echocardiographic parameters. Treatment-related adverse events occurred in 11 patients (68.75%). Four patients experienced clinical worsening, including two non-treatment-related deaths and two unplanned hospitalisations due to pulmonary hypertension. Conclusion: In patients with CTEPH who have completed BPA treatment, switching from PDE5i to riociguat significantly improved pulmonary haemodynamic and functional capacity but was associated with notable treatment-related adverse events.

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