Switching Sildenafil to Riociguat in Chronic Thromboembolic Pulmonary Hypertension After Balloon Pulmonary Angioplasty

Chronic thromboembolic pulmonary hypertension (CTEPH) results from the obstruction of the pulmonary arteries by organised fibrotic thrombi and the associated microvasculopathy, leading to increased pulmonary vascular resistance and progressive right-sided heart failure. CTEPH is associated with significant mortality and morbidity, so prompt initiation of treatments are necessary to improve the prognosis.

For those with accessible pulmonary arteries occlusions, surgical pulmonary endarterectomy (PEA) is the treatment of choice. Nevertheless, about 40% of CTEPH patients are not considered to be operable due to occlusion of distal pulmonary vessels. For patients with inoperative CTEPH, current treatment options include balloon pulmonary angioplasty (BPA) and medical therapies.

Several medical therapies that target microvascular components of CTEPH, such as phosphodiesterase type 5 inhibitor (PDE5i) and endothelin receptor antagonist (ERA), have been used off-label, as the efficacy of those medications in inoperable CTEPH has not been proven in randomised controlled trials or registry data. The CHEST-1 randomised controlled trial demonstrated that the soluble guanylate cyclase stimulator (sGCs), riociguat, significantly reduced pulmonary vascular resistance and improved exercise capacity in patients with inoperative CTEPH or persistent or recurrent pulmonary hypertension after PEA. Based on the finding of this study, riociguat has been approved for treatment for symptomatic inoperable patients with CTEPH.

Both PDE5i and sGCs act via the same nitric oxide (NO)-soluble guanylate cyclase (sGC)-cyclic guanosine monophosphate (cGMP) pathway, but these two classes of medications target different molecular targets in the same pathway. PDE5i inhibits the degradation of cGMP, so its efficacy is dependent on a functioning NO-sGC-cGMP axis and the presence of intracellular cGMP. In contrast, riociguat stimulates sGC directly, thus it increases intracellular cGMP level regardless the presence of NO. Therefore, based on this biological rationale, it is postulated that riociguat may be more effective in increasing intracellular cGMP compared to PDE5i. Currently there is no head-to-head trials comparing the efficacy of PDE5i and riociguat in treating pulmonary hypertension. Nevertheless, 2 clinical trials have demonstrated improvement in the clinical and biochemical parameters after switching from PDE5i to sGCs in selected patients with pulmonary arterial hypertension (PAH) with insufficient response to PDE5i. It is currently unknown whether this switching will also apply to patients with CTEPH as those 2 clinical trials do not include patients with CTEPH.

In addition to medical therapies, BPA, an endovascular procedure to dilate the occlusions and stricture in segmental or subsegmental pulmonary arteries, has emerged as a treatment for patients with inoperable CTEPH or persistent or recurrent pulmonary hypertension after PEA. Two randomised controlled trials comparing BPA and riociguat have demonstrated that BPA was associated with a greater improvement in mean pulmonary artery pressure and reduction in pulmonary vascular resistance in inoperable CTEPH patients.