The STELLAR Travel to BMPR2-based therapies for pulmonary arterial hypertension

Pulmonary Vascular & Cardiovascular Network

Pulmonary Vascular Disease Section

The recently published STELLAR trial was a phase 3, multicenter, double-blind, randomized, placebo-controlled study designed to evaluate patients with PAH receiving stable vasodilator therapy after treatment with sotatercept, a first-in-class recombinant fusion protein with parts of the activin receptor type IIA, a member of the BMPR2/TGF-beta superfamily of receptors and ligands (Hoeper. N Engl J Med. 2023;388:1478).

Sotatercept improved 6-minute walk distance, the primary endpoint of the trial at 24-weeks, as well as eight of the trial’s nine secondary endpoints including changes in PVR, NT-ProBNP levels, functional class, French risk score, and time-to-clinical worsening when compared with placebo. However, many questions remain about the mechanisms whereby sotatercept achieved its clinical endpoints, the answers to which may lie within its basic molecular biology.

The focus on BMPR2/TGF-beta cell signaling pathways originated from the identification of loss-of-function mutations in the BMPR2 gene in patients with heritable and idiopathic PAH (Morrell, NW. Eur Respir J. 2019;53[3]: 1900078). An imbalance in BMPR2/TGF-beta signaling (low BMPR2/high TGF-beta function) has been proposed as a central mechanism in the development of PAH. Specifically, researchers have shown increased levels of Activin A, one of 33 ligands that can bind either BMPR2 or TGF-beta receptors, within vascular lesions in the lungs of patients with PAH. It has been thus hypothesized that reducing the amount of circulating Activin A could treat PAH by rebalancing BMPR2/TGF-beta signaling in lung vascular cells. In preclinical experimental models of PAH with elevated Activin A levels, sotatercept has been shown to reduce distal small vessel medial thickness/muscularization and increase the number of patent small vessels (Yung, LM. Sci Transl Med. 2020;12).

The exact mechanism by which sotatercept improves hemodynamics and outcomes remains unclear. Indeed, whether de-remodeling of the lung vasculature or new vessel formation occurs in humans is unknown. The results from STELLAR mark a new era in the development of potential “disease-modifying agents” for PAH; however, the question is: what exactly are we modifying?

Jose Gomez-Arroyo, MD, PhD – Section Fellow-in-Training
Dana Kay, DO – Section Member-at-Large