Sotatercept (Winrevair)

Sotatercept (Winrevair) is a real, FDA-approved activin/GDF-trap fusion protein — the first-in-class treatment for pulmonary arterial hypertension, approved in March 2024 on the strength of an 84% reduction in death or clinical worsening in its pivotal trial, with a second, high-risk-population trial expanding the label in October 2025.

An activin trap, not a small peptide

Sotatercept is structurally similar in spirit to romiplostim, etanercept, and aflibercept elsewhere on this site: the extracellular, ligand-binding domain of a natural human receptor (here, activin receptor type IIA, or ActRIIA) genetically fused to the Fc fragment of human IgG1, extending its half-life enough for once-every-three-weeks dosing. Rather than blocking a single growth factor, sotatercept acts as a decoy trap for a family of TGF-beta-superfamily ligands — chiefly activin A, alongside other activins and growth differentiation factors — that drive the excess proliferation of pulmonary-artery smooth-muscle cells underlying pulmonary arterial hypertension (PAH). That targets a genuinely different biological axis than the three existing PAH drug classes (endothelin receptor antagonists, PDE5 inhibitors, and prostacyclin-pathway agents), which is why the FDA classed it as the first "activin signaling inhibitor" approved for the disease.

The STELLAR trial and approval

Sotatercept was originally developed by Acceleron Pharma; Merck acquired the company for approximately $11.5 billion in a deal announced 30 September 2021 and completed that November, specifically to gain access to it. The pivotal STELLAR trial (Hoeper MM, Badesch DB, Ghofrani HA, et al., "Phase 3 Trial of Sotatercept for Treatment of Pulmonary Arterial Hypertension," N Engl J Med 2023;388(16):1478-1490, PMID 36877098, DOI: 10.1056/NEJMoa2213558) randomized 323 PAH patients already on background therapy to sotatercept or placebo. At 24 weeks, sotatercept improved six-minute walk distance by a mean of 40.8 meters over placebo (95% CI 27.5-54.1, p<0.001) — the trial's primary endpoint — and cut the risk of death or PAH clinical worsening by 84% (hazard ratio 0.16, 95% CI 0.08-0.35) over a median follow-up of roughly 32 weeks. The FDA approved sotatercept (as Winrevair) on 26 March 2024 for adults with PAH (WHO Group 1), added on top of standard background therapy — the first new PAH drug mechanism approved in years, on unusually strong data.

Safety, and a second trial that expanded the label

Winrevair's label carries warnings and precautions rather than a boxed warning, built around its most distinctive on-target effects: it can raise hemoglobin enough to cause erythrocytosis (moderate elevations of more than 2 g/dL above the upper limit of normal occurred in about 15% of STELLAR's treated patients, though no severe elevations were seen), and it carries risks of severe thrombocytopenia and serious bleeding, alongside telangiectasia (16.6% of treated patients versus 4.4% on placebo) — findings consistent enough with its mechanism that the label requires hemoglobin and platelet monitoring before each of the first several doses. On 27 October 2025, the FDA approved an updated indication based on the separate ZENITH trial, which enrolled 172 PAH patients specifically at high risk of mortality and found a statistically significant 76% reduction in the combined risk of major mortality and morbidity events when sotatercept was added to background therapy — extending the drug's evidence base beyond STELLAR's broader PAH population into the sicker patients most likely to need it.

Current status

Winrevair remains Merck-owned, FDA-approved, and the only drug of its class on the market. Merck reported $1.4 billion in Winrevair sales for full-year 2025, its first full year on the market, reflecting rapid uptake for a genuinely new mechanism in a serious, historically undertreated disease. As a complex, cold-chain-distributed biologic administered under specialist monitoring, it has no wellness, bodybuilding, or grey-market presence of any kind.