Merck has received a positive opinion from the Committee for Medicinal Products for Human Use (CHMP) of the European Medicines Agency recommending an expanded indication for WINREVAIR (sotatercept) in combination with other therapies for the treatment of pulmonary arterial hypertension (PAH) in adults across WHO Functional Classes II, III, and IV. The update follows pivotal results from the Phase 3 ZENITH trial and, if adopted by the European Commission, would mark a significant shift in the treatment landscape for severely ill PAH patients in Europe. This recommendation comes on top of an earlier EU approval that limited use of WINREVAIR to patients in Functional Classes II and III, with the primary goal of improving exercise capacity.

Expanded eligibility changes the baseline for PAH intervention

The inclusion of WHO Functional Class IV patients in the potential updated label represents a meaningful expansion into a population that has historically been excluded from many PAH drug trials. These patients, who often exhibit symptoms even at rest and carry the highest risk of hospitalization and death, have traditionally had fewer disease-modifying therapeutic options. If authorized, this change would enable Merck to market sotatercept as a treatment that not only improves exercise capacity but also modifies disease trajectory across the full spectrum of PAH severity.

For clinicians, this could reframe how and when sotatercept is deployed in the treatment algorithm. Many have reserved the most aggressive therapies, including prostacyclin infusions and transplant evaluations, for WHO Functional Class IV patients. The availability of WINREVAIR in this subgroup could position it earlier in decision trees, particularly for patients failing on standard triple therapy. This broader eligibility may also alter guidelines from European societies on first-line and combination treatment strategies.

ZENITH’s mortality reduction raises the bar for PAH outcomes

The ZENITH trial produced data that went beyond symptomatic relief, showing a 76 percent reduction in the risk of morbidity and mortality events in patients treated with sotatercept compared to placebo. This included endpoints such as all-cause death, lung transplantation, and PAH-related hospitalization. The trial was stopped early at interim analysis due to overwhelming efficacy. Industry analysts believe that this magnitude of benefit will be difficult for competing therapies to match, especially in a disease area where most approvals rely on surrogate markers.

The decision to halt ZENITH early based on efficacy signals strong therapeutic potential, but it also raises certain flags around long-term safety data. Regulators appear satisfied with the data package, but some clinicians may withhold full confidence until longer-term outcomes from the ongoing open-label extension become available. Real-world data from these follow-up patients may also influence future reimbursement decisions and clinical confidence in high-risk settings.

Functional Class IV inclusion reflects a shift in regulatory attitude

The CHMP recommendation marks one of the few cases in recent years where a therapy has advanced to approval consideration with solid evidence in WHO Functional Class IV PAH patients. This subgroup is often excluded from trials due to clinical instability, elevated transplant risk, and complex background therapy use. By including this population, ZENITH not only expands the eligible patient base but also signals a potential shift in regulator expectations around trial inclusivity and real-world relevance.

Approximately 26 percent of the ZENITH trial cohort were WHO Functional Class IV patients, a figure well above average for PAH trials. Regulators reviewing this data will have noted that clinical benefit extended across both FC III and FC IV categories. This adds weight to the hypothesis that sotatercept could serve as a bridge between intermediate and advanced stages of PAH, with the potential to defer more invasive interventions such as lung transplantation.

The competitive value of targeting the activin signaling pathway

WINREVAIR is the first and only approved therapy in Europe that targets the activin signaling pathway. Its mechanism involves modulating pro- and anti-proliferative vascular signaling, an upstream target compared to nitric oxide, endothelin, or prostacyclin pathways. The biological logic underpinning sotatercept’s effect is to reduce remodeling in the pulmonary arteries, thereby improving hemodynamics and right ventricular function.

Compared to traditional vasodilators, WINREVAIR presents a new model of disease modification rather than symptomatic management. Clinicians following the field believe this upstream approach could eventually lead to earlier initiation, particularly for newly diagnosed patients at intermediate risk. The ability to combine WINREVAIR with existing triple therapy regimens also enhances its value as an add-on rather than a replacement, which is likely to encourage uptake rather than resistance from prescribers accustomed to layering treatments.

Safety profile may shape how widely it is adopted

Despite the compelling efficacy, the safety data from ZENITH and STELLAR trials introduces important considerations. Adverse events such as thrombocytopenia, epistaxis, diarrhea, and rash occurred at higher frequencies among sotatercept-treated patients. Notably, the incidence of serious bleeding events, including gastrointestinal and intracranial hemorrhages, was elevated in those also receiving prostacyclin therapy or antithrombotic agents.

Clinical protocol will likely need to include routine monitoring of hemoglobin and platelet counts. The need for individualized dose modifications based on lab values may slow adoption in settings where frequent laboratory testing is impractical. Furthermore, sotatercept’s impact on fertility and its contraindication in pregnancy will necessitate counseling and contraception in women of reproductive potential, adding another layer of complexity to prescribing in younger adult populations.

Regulatory alignment between Europe and the United States strengthens Merck’s strategy

Merck’s commercial and regulatory strategy appears to be well-synchronized across geographies. The United States Food and Drug Administration approved an expanded WINREVAIR indication in October 2025 based on ZENITH data, and the CHMP’s recent opinion brings Europe close to matching that scope. Once the European Commission finalizes its review, Merck will be able to deliver a harmonized message to global clinicians and payers: that WINREVAIR is a foundational therapy across the full spectrum of PAH severity.

This level of alignment enhances operational efficiency and could drive formulary adoption in countries that look to both the European Union and the United States for reference pricing and guideline development. The decision is also likely to trigger adjustments in national reimbursement schemes, with health technology assessment bodies now required to weigh the therapy’s survival benefit against cost and delivery complexities.

What industry observers will watch in the next six months

The European Commission is expected to make a final decision on the expanded indication by the first quarter of 2026. If approved, the focus will quickly shift to national implementation, pricing negotiations, and real-world uptake. Pulmonology societies and cardiovascular guidelines committees will likely assess how best to integrate WINREVAIR into treatment pathways, particularly for WHO Functional Class IV patients who have previously been underserved by existing therapies.

Manufacturing scale and supply logistics also represent key operational questions. As a subcutaneously administered biologic requiring dose titration and ongoing lab monitoring, WINREVAIR’s delivery model will test Merck’s ability to maintain continuity across diverse healthcare systems. Countries with centralized hospital pharmacy models may be better positioned to manage these requirements compared to more fragmented outpatient networks.

There is also likely to be increased interest in head-to-head studies or post-marketing surveillance comparing sotatercept with emerging competitors or existing backbone therapies. Stakeholders will be particularly interested in whether WINREVAIR can reduce long-term dependence on intravenous prostacyclins or delay listing for lung transplantation.

Reimbursement and access decisions could define sotatercept’s real impact

While regulatory support is clearly building, the full commercial impact of WINREVAIR in Europe will depend heavily on payer decisions. Countries with stringent cost-effectiveness thresholds may scrutinize the therapy’s durability of benefit beyond the median follow-up period in ZENITH. Moreover, the healthcare resource use required for ongoing monitoring may impact its value assessment compared to oral agents that require less infrastructure.

In parallel, clinicians may demand further clarity on how to integrate sotatercept into existing risk stratification models like REVEAL Lite 2, particularly for patients transitioning from WHO Functional Class III to IV. This is a group where progression risk is high, and therapeutic inertia is common due to the lack of validated options beyond prostacyclin infusion.

  activin signaling inhibitors, CHMP, EMA, EU drug approvals, Merck, PAH, pulmonary arterial hypertension, sotatercept, STELLAR trial, WHO FC IV, WINREVAIR, ZENITH trial