Sotatercept

Details of the Concept

Sotatercept (brand name: Winrevair) is a first-in-class activin signaling inhibitor — an ACTRIIA-Fc fusion protein — that binds pro-proliferative members of the TGF-β superfamily (primarily activins and GDFs), restoring the balance between growth-promoting (activin/GDF) and growth-inhibiting (BMP) signals in the pulmonary vasculature. It is FDA-approved for the treatment of pulmonary arterial hypertension (PAH) and represents a mechanistic departure from all prior PAH therapies, which are vasodilators. Sotatercept addresses the underlying pulmonary vascular remodeling rather than its haemodynamic consequences.

Key Facts

Mechanism of Action

• Class: Activin signaling inhibitor; ACTRIIA-Fc fusion protein (activin receptor type IIA extracellular domain fused to IgG1-Fc)
• Target: Binds and sequesters pro-proliferative TGF-β superfamily ligands (activins, GDF8, GDF11); reduces excess activin/GDF signaling that drives smooth muscle cell proliferation and pulmonary vascular remodeling
• Pathway rebalancing: Corrects the activin/GDF (proliferative) vs. BMP (anti-proliferative/pro-apoptotic) imbalance that is central to PAH pathobiology; acts upstream and complementary to vasodilators
• Differentiation from vasodilators: ERAs, PDE5i, sGC stimulators, and prostacyclins address increased vascular tone/vasoconstriction; sotatercept addresses the structural vascular remodeling component — explaining why it provides additive benefit on top of maximally tolerated background therapy
• Route: subcutaneous injection every 21 days (Q21d)
• Starting dose: 0.3 mg/kg → target dose: 0.7 mg/kg (dose escalated based on tolerability)
• Off-target effects on erythropoiesis (↑ hemoglobin), hemostasis (bleeding, especially epistaxis), and microvascular structure (telangiectasia) (sources/sotatercept-zenith-nejm-2025, rating: very high)

Clinical Trial Program

PULSAR (Phase 2)

• Demonstrated favorable hemodynamic changes and RV function improvements in PAH patients receiving background therapy (sources/rv-failure-aha-2026, rating: very high)
• Long-term follow-up (PULSAR extension): decreased RV mass and improved RV function sustained

SPECTRA (Phase 2b)

• Demonstrated improved exercise tolerance and reduced RV mass with sotatercept
• Key finding: decreased RV mass + increased hemoglobin + increased peak VO₂ without change in resting cardiac output — suggesting direct cardioprotective or remodeling reversal effect on RV cardiomyocytes beyond pure afterload reduction (sources/rv-failure-aha-2026, rating: very high)

STELLAR (Phase 3) — WHO FC II/III

• Population: group 1 PAH, WHO FC II or III; on stable background therapy; n=323
• Results: improved 6MWD (+34.4 m), PVR, PA compliance, NT-proBNP, PA–RV coupling, RV function
• 84% reduction in clinical worsening events (composite including deterioration in exercise capacity, WHO-FC worsening, clinical deterioration, and transplant/death)
• Established sotatercept as a practice-changing add-on therapy for less advanced PAH (sources/rv-failure-aha-2026, rating: very high)

ZENITH (Phase 3) — WHO FC III/IV, High-Risk (REVEAL Lite 2 ≥9)

• Population: group 1 PAH, WHO FC III or IV, REVEAL Lite 2 risk score ≥9; on maximum tolerated double or triple background PAH therapy; n=172 (86 each); stopped early for overwhelming efficacy
• Primary composite (all-cause death / lung transplantation / PAH hospitalization ≥24h): HR 0.24 (95% CI 0.13–0.43, P<0.001); 17.4% vs 54.7%
• PAH hospitalization (dominant component): 9.3% vs 50.0%
• All-cause death: 8.1% vs 15.1% (HR 0.42; NS due to early-stopping alpha boundary constraint)
• Transplantation-free survival: HR 0.34 (95% CI 0.15–0.78)
• NT-proBNP, PVR, mPAP, WHO-FC, 6MWD, EQ-5D-5L: all improved
• No drug discontinuations due to serious AEs in sotatercept group; serious AEs lower despite longer follow-up (sources/sotatercept-zenith-nejm-2025, rating: very high)

HYPERION (Phase 3) — WHO FC II/III, Early Disease (<1 Year Since Diagnosis), Intermediate/High Risk

• Population: group 1 PAH, WHO FC II or III; diagnosis <1 year earlier; REVEAL Lite 2 ≥6 OR COMPERA 2.0 ≥2 (intermediate or high risk); stable double or triple background therapy ≥90 days; n=320 (160 each); stopped early for loss of clinical equipoise (ZENITH results), before planned interim analysis
• Median follow-up 13.2 months; majority WHO FC II (67%); COMPERA 2.0 intermediate-low (64%); only 17% on IV prostacyclin; older and more comorbid than STELLAR/ZENITH — more representative of real-world PAH registries
• Primary composite (clinical worsening: death/PAH hospitalization/atrial septostomy/lung transplant/exercise testing deterioration): HR 0.24 (95% CI 0.14–0.41, P<0.001); 10.6% vs 36.9%; NNT=5 at 12 months; curve separation after 3 doses
• Exercise testing deterioration (dominant driver): 5.0% vs 28.8%
• PAH hospitalization: 1.9% vs 8.8%
• All-cause death: 4.4% vs 3.8% ← balanced; no mortality benefit observed
• Atrial septostomy/lung transplantation: 0% in both groups
• Secondary endpoints: Multicomponent improvement at week 24 (P=0.003); low REVEAL Lite 2 ≤5 at week 24 (P=0.04); simplified French risk score did NOT reach significance → hierarchy stopped
• Benefit consistent across CTD-associated, double therapy, and intermediate REVEAL Lite 2 or intermediate-low COMPERA 2.0 subgroups (sources/sotatercept-hyperion-nejm-2025, rating: very high)

Safety Profile (Consolidated Across Trials)

• Epistaxis: Most common AE; 44% in ZENITH; driven by microvascular changes
• Telangiectasia: 26% in ZENITH (vs 3.5% placebo); all cutaneous; dose-related
• Elevated hemoglobin: 13% in ZENITH; off-target erythropoietic effect; dose adjustments required
• Thrombocytopenia: 14% in ZENITH (vs 8.1% placebo); does not explain bleeding imbalance
• Bleeding overall: 63% vs 35% — mostly nonserious epistaxis/gingival; serious bleeding similar (5.8% vs 4.7%)
• Serious adverse events (ZENITH): Despite longer follow-up, serious AEs and deaths lower in sotatercept group
• Serious bleeding (HYPERION): 3.8% vs 1.9% — higher with sotatercept (differs from ZENITH where serious bleeding was balanced); overall bleeding 41.2% vs 16.2%
• Dose modification criteria: hemoglobin levels, platelet counts, telangiectasia severity, serious bleeding

Position in PAH Treatment Algorithm

• Add-on to background therapy: Not a replacement for ERAs/PDE5i/prostacyclins; all ZENITH and STELLAR patients received double or triple background therapy
• High-risk PAH (REVEAL Lite 2 ≥9, WHO FC III/IV): ZENITH establishes sotatercept as the highest-evidence escalation therapy for advanced PAH on maximum conventional treatment — HR 0.24 for major outcomes
• Intermediate-risk PAH, early disease (WHO FC II/III, diagnosed <1 year, REVEAL Lite 2 ≥6): HYPERION establishes benefit in early-diagnosis, intermediate-risk PAH; benefit is predominantly prevention of exercise testing deterioration and functional progression rather than mortality at this disease stage
• First-line therapy: Whether sotatercept can be used as first-line monotherapy without prior background therapy remains unanswered — all trials required pre-established background therapy
• FDA approved for PAH; 2022 ESC/ERS guidelines predate ZENITH and HYPERION; updated guidelines expected to incorporate this evidence

Contradictions / Open Questions

• Overall survival benefit technically unproven due to early stopping: HR 0.42 for all-cause death (8.1% vs 15.1%) strongly favors sotatercept, but the trial was stopped at interim analysis and the hierarchical testing strategy was not satisfied for survival (alpha boundary not reached). Clinical interpretation: highly likely a true mortality benefit, but statistically not formally established at this dataset (sources/sotatercept-zenith-nejm-2025, rating: very high)
• Hospitalization dominates the composite; longer follow-up needed for mortality clarity: 50% of placebo patients were hospitalized for PAH vs 9.3% with sotatercept; deaths were 15.1% vs 8.1%. With longer follow-up (currently median ~10 months), the mortality gap may become statistically significant. SOTERIA open-label extension study will provide longer-term safety and efficacy data
• Optimal positioning in treatment algorithm — early add-on vs escalation rescue: Should sotatercept be initiated early (at diagnosis alongside ERA+PDE5i, given STELLAR data) or reserved for high-risk/failing patients (ZENITH data)? No head-to-head data comparing strategies; cost, access, and patient selection criteria are unresolved
• Cardiac output unchanged despite PVR/mPAP improvements: Consistent across PULSAR, SPECTRA, STELLAR, ZENITH — the mechanism underlying this dissociation (vasodilation vs. structural remodeling reversal) requires further study; long-term consequences of direct RV myocardial effects are unknown (sources/rv-failure-aha-2026, rating: very high)
• Generalizability beyond high-risk enriched populations — partly resolved by HYPERION: ZENITH enrolled WHO FC III/IV with REVEAL Lite 2 ≥9; HYPERION extends evidence to FC II/III, intermediate-risk patients (REVEAL Lite 2 ≥6 / COMPERA 2.0 ≥2), diagnosed <1 year; benefit now demonstrated across a broad PAH risk spectrum; applicability to elderly with severe comorbidities, SSc-PAH specifically, and lower-resource settings remains uncertain (sources/sotatercept-hyperion-nejm-2025, rating: very high)
• Death not prevented in early-disease PAH (HYPERION): HR 0.24 composite benefit was driven entirely by exercise testing deterioration; deaths were balanced (4.4% vs 3.8%); the early-disease benefit of sotatercept is primarily functional/progression-prevention rather than survival — mortality divergence may emerge with longer follow-up (sources/sotatercept-hyperion-nejm-2025, rating: very high)
• Composite endpoint HR 0.24 represents qualitatively different events in HYPERION vs ZENITH: The identical numerical HR masks a critical difference — ZENITH's composite was death/transplant/PAH hospitalization (hard outcomes); HYPERION's composite was dominated by exercise testing deterioration (softer, more subjective, includes clinical judgement components). Prevention of different event types should not be conflated in cross-trial comparisons (sources/sotatercept-hyperion-nejm-2025, rating: very high)
• Simplified French risk score failed (HYPERION secondary endpoint): Despite significant multicomponent improvement and REVEAL Lite 2 ≤5 being achieved, the simplified French risk score (requiring 6MWD >440m) was not significantly different — likely because HYPERION's older, more comorbid population had lower walking capacity at baseline, creating a ceiling effect on achieving the 440m threshold. This suggests risk tools with fixed 6MWD thresholds may underperform in older PAH cohorts (sources/sotatercept-hyperion-nejm-2025, rating: very high)

Connections

• Related to entities/Pulmonary-Hypertension — PAH treatment; mechanism complementary to vasodilators; ZENITH and STELLAR within PAH treatment algorithm
• Related to concepts/PAH-Risk-Stratification — REVEAL Lite 2 ≥9 as high-risk threshold; sotatercept as the evidence-based treatment for this tier
• Related to concepts/RV-PA-Coupling — RV function improvement with sotatercept; cardiac output dissociation

Sources

• sources/sotatercept-zenith-nejm-2025
• sources/sotatercept-hyperion-nejm-2025
• sources/rv-failure-aha-2026