Last updated: September 8, 2026
WINREVAIR, Merck’s sotatercept-csrk, is the first approved therapy to target pulmonary arterial hypertension through the activin signaling pathway. FDA approval in March 2024 created a new treatment category within PAH and gave Merck a high-value growth asset following the loss of market exclusivity for key oncology products.
The commercial case rests on three factors: strong efficacy in the pivotal STELLAR trial, a large untreated or undertreated PAH population, and expansion into earlier-line and combination treatment. The principal risks are manufacturing scale-up, payer controls, treatment-center adoption, safety monitoring, and future competition from other pathway-modifying drugs.
Merck reported approximately $149 million in WINREVAIR revenue for 2024, its first partial commercial year.[1] The product is expected to move from launch-stage revenue into a multibillion-dollar franchise if Merck converts clinical differentiation into broad use across WHO functional class II and III PAH.
What is WINREVAIR and how does it work?
WINREVAIR is sotatercept-csrk, a recombinant fusion protein consisting of the extracellular domain of human activin receptor type IIA linked to the Fc portion of human IgG1. It is administered by subcutaneous injection once every three weeks.
Sotatercept binds selected transforming growth factor beta superfamily ligands, including activins and growth differentiation factors. The mechanism is intended to restore balance between proliferative and antiproliferative signaling in pulmonary vascular disease.
The FDA approved WINREVAIR for adults with pulmonary arterial hypertension, WHO Group 1, to increase exercise capacity and improve functional status.[2] Its principal commercial differentiation is mechanistic. Existing PAH therapies primarily affect nitric oxide, endothelin, or prostacyclin pathways. WINREVAIR addresses a separate disease pathway.
What did the STELLAR trial show?
The phase 3 STELLAR trial randomized 323 patients with WHO functional class II or III PAH to sotatercept or placebo, with background PAH therapy permitted. At week 24:
| Measure |
WINREVAIR result |
| Primary endpoint |
Significant improvement in 6-minute walk distance |
| Median treatment difference |
40.1 meters |
| Improvement in WHO functional class |
29.4% versus 13.8% with placebo |
| Reduction in clinical worsening or death |
Hazard ratio approximately 0.16 |
| Trial population |
163 sotatercept patients and 160 placebo patients |
The clinical-worsening result supports use as an add-on therapy rather than as a replacement for established PAH drugs.[3] Long-term data remain important because PAH treatment is chronic and the commercial opportunity depends on durable use.
How large is the WINREVAIR market?
The addressable market is a concentrated specialty market rather than a mass-market drug category. PAH is rare, but treatment costs are high and patients often receive multiple chronic therapies.
Commercial estimates commonly place the diagnosed U.S. PAH population in the tens of thousands, with a larger global population that is fragmented across specialist centers. The economically relevant population is smaller than the total prevalence estimate because WINREVAIR is initially indicated for adults with WHO Group 1 PAH and is generally used with background treatment.
What drives market expansion?
Market growth depends on four adoption stages:
- Use in high-risk patients with inadequate response to existing therapy.
- Earlier add-on use in WHO functional class II and III patients.
- Expansion into patients with less severe disease or treatment-naive patients.
- Additional indications, including possible use in other pulmonary hypertension subgroups.
Merck’s commercial opportunity increases if physicians use sotatercept before deterioration rather than reserving it for late-line rescue. The STELLAR population included patients already receiving background therapy, which supports combination use but may slow initial adoption because physicians must integrate WINREVAIR into established treatment pathways.
What is the estimated annual treatment cost?
Merck launched WINREVAIR with a U.S. wholesale acquisition cost of approximately $14,000 per vial, according to reporting at launch.[4] Because dosing depends on patient weight and may escalate from an initial dose, annual list-price exposure can exceed $200,000 per patient before discounts, rebates, administration costs, and patient assistance.
The product is supplied in 45 mg, 25 mg, and 10 mg vial configurations. Weight-based dosing creates a commercial advantage for revenue per treated patient but increases pharmacy, inventory, and administration complexity.
What are WINREVAIR’s sales and financial trajectory?
Merck reported approximately $149 million in WINREVAIR sales in 2024.[1] That figure reflects a limited launch period beginning in the second quarter and does not represent a mature annual run rate.
| Period |
Commercial status |
Financial interpretation |
| 2024 |
U.S. launch and initial international rollout |
Early revenue; access and supply infrastructure established |
| 2025 |
Broader specialist adoption |
Primary ramp year; payer coverage and treatment-center capacity become material |
| 2026-2028 |
Potential expansion into earlier-line treatment |
Revenue growth depends on duration, diagnosis, and label expansion |
| Long term |
Mature specialty franchise |
Patent, regulatory exclusivity, competition, and biosimilar timing determine peak sales |
Merck identified WINREVAIR as one of the company’s principal growth products following the acquisition of Acceleron.[5] Industry forecasts have placed potential peak annual sales in the multibillion-dollar range, with estimates commonly clustering around $3 billion to $5 billion. These forecasts assume sustained superiority or differentiation, broad combination use, and expansion beyond the initial launch segment.
Why is WINREVAIR strategically important to Merck?
Merck paid approximately $11.5 billion to acquire Acceleron in 2021, including the acquisition of full rights to sotatercept.[6] The transaction gave Merck control of a late-stage cardiovascular asset with orphan-drug characteristics, high pricing potential, and limited direct competition at launch.
WINREVAIR also diversifies Merck’s revenue base beyond oncology and vaccines. Its growth profile is strategically important because Keytruda, Merck’s largest product, faces eventual loss-of-exclusivity pressure in the 2028-2029 period in major markets, subject to formulation, combination, and patent litigation outcomes.
What is the competitive landscape for WINREVAIR?
WINREVAIR competes against established PAH therapies rather than against a single direct substitute.
| Drug class |
Representative products |
Primary pathway |
Competitive effect |
| Endothelin receptor antagonists |
Opsumit, Tracleer, Letairis, generic ambrisentan |
Endothelin |
Common background therapy |
| PDE-5 inhibitors |
Adcirca, tadalafil, Revatio, sildenafil |
Nitric oxide |
Low-cost generic competition |
| Soluble guanylate cyclase stimulators |
Adempas |
Nitric oxide |
Alternative oral pathway therapy |
| Prostacyclin therapies |
Remodulin, Tyvaso, Flolan, Veletri |
Prostacyclin |
Strong efficacy but administration burden |
| Activin signaling inhibitor |
WINREVAIR |
TGF-beta superfamily |
First-in-class mechanistic entrant |
WINREVAIR’s main commercial advantage is incremental efficacy on top of combination therapy. Its main disadvantage is that it adds injection burden, laboratory monitoring, and cost to a population already receiving several medicines.
How does WINREVAIR compare with prostacyclin therapy?
Prostacyclin agents remain important for advanced or high-risk PAH, particularly when rapid hemodynamic improvement is required. WINREVAIR is more likely to compete as an earlier or less burdensome add-on therapy. Its subcutaneous injection every three weeks is less demanding than continuous intravenous or frequent inhaled prostacyclin administration.
Can generic drugs materially limit WINREVAIR sales?
Generic competition is unlikely to affect WINREVAIR materially during the initial commercial period. Generic tadalafil and sildenafil reduce the cost of background therapy, but they do not reproduce sotatercept’s mechanism. Future biosimilar competition is the more relevant long-term risk.
What is the FDA regulatory status of WINREVAIR?
The FDA approved WINREVAIR on March 26, 2024, under the standard approval pathway.[2] The approval covers adults with pulmonary arterial hypertension, WHO Group 1, to improve exercise capacity and reduce clinical worsening.
Key regulatory requirements include monitoring hemoglobin and platelets before treatment and during dose escalation. The label warns of erythrocytosis, thrombocytopenia, bleeding, telangiectasia, and embryo-fetal toxicity.[2]
WINREVAIR is not a traditional small-molecule drug. Its regulatory exclusivity is governed by biologic and orphan-drug provisions rather than only by small-molecule Hatch-Waxman rules.
What patents and exclusivity protect WINREVAIR?
WINREVAIR is protected by a combination of biologic regulatory exclusivity, orphan-drug exclusivity, formulation and dosing claims, manufacturing know-how, and patents covering sotatercept-related compositions and therapeutic uses.
Does WINREVAIR have Orange Book listings?
No traditional Orange Book listing is expected to function as the principal exclusivity record because WINREVAIR is a biologic licensed under a biologics license application, or BLA. Biologic patent and exclusivity analysis is generally conducted through the Purple Book, BLA records, patent databases, and the Biologics Price Competition and Innovation Act, or BPCIA, framework.
The absence of an Orange Book-style listing does not mean that WINREVAIR lacks patent protection. It changes the pathway for biosimilar applicants and patent litigation.
When does WINREVAIR lose exclusivity?
The BPCIA provides 12 years of reference-product exclusivity from the date of first licensure, subject to statutory interpretation and possible pediatric-extension issues. Based on the March 2024 approval date, the principal U.S. biologic exclusivity period would extend into 2036.
Patent expiry may occur later than regulatory exclusivity and may differ by jurisdiction. Relevant protection can include:
- Composition-of-matter claims for sotatercept or related fusion proteins.
- Pharmaceutical-composition claims.
- Weight-based dosing and administration claims.
- Method-of-use claims for PAH.
- Manufacturing and formulation claims.
- Patent-term-adjusted expiration dates.
The commercial risk is therefore unlikely to be defined by a single expiration date. Biosimilar applicants can challenge patents under the BPCIA patent-exchange process, while Merck may pursue infringement actions involving formulation, dosing, manufacturing, or use patents.
What manufacturing barriers protect the product?
Sotatercept is a complex recombinant protein requiring cell-line development, controlled bioprocessing, purification, viral clearance, analytical characterization, sterile filling, and cold-chain distribution. These requirements create a higher entry barrier than generic tablets.
Manufacturing complexity does not eliminate biosimilar competition. It can delay approval, increase development costs, and narrow the number of credible competitors. The most valuable manufacturing protection may be process know-how and commercial-scale consistency rather than any single patent.
What biosimilar and generic entry risks exist?
The near-term entry risk is low because:
- WINREVAIR is a newly approved biologic.
- No interchangeable biosimilar is currently established in the U.S. market.
- Clinical development requires analytical, pharmacokinetic, immunogenicity, and potentially comparative clinical evidence.
- PAH treatment is specialist-driven, which favors incumbent supply reliability.
- Manufacturing scale and long-term safety data create barriers for new entrants.
The medium- to long-term risk is higher. A biosimilar could compete on price with payers and specialty pharmacies, particularly if Merck’s patents do not extend meaningfully beyond biologic exclusivity.
What litigation and settlement issues affect WINREVAIR?
No major public Paragraph IV litigation framework applies in the same way as it does to a conventional small-molecule product because WINREVAIR is a biologic. The relevant future dispute would likely involve a biosimilar applicant’s patent notice and BPCIA litigation.
As of the initial launch period, no major public biosimilar settlement involving WINREVAIR had shaped the market. Future settlements could establish an agreed launch date, royalty structure, or limited licensed entry. The commercial impact would depend on whether the settlement permits entry before the principal composition and use patents expire.
What licensing deals and ownership arrangements affect WINREVAIR?
Merck acquired Acceleron in 2021 and obtained full control of sotatercept through that transaction.[6] The acquisition reduced licensing complexity and allowed Merck to control clinical development, regulatory strategy, manufacturing investment, and global commercialization.
The transaction also transferred execution risk to Merck. The company must finance global launch infrastructure, maintain supply, support specialist education, and fund additional clinical trials. The return on the acquisition depends heavily on converting WINREVAIR into a durable chronic-use franchise.
How strong is the WINREVAIR patent estate?
The estate is commercially strong because patent protection is reinforced by biologic exclusivity, orphan-drug status, manufacturing complexity, and first-mover clinical differentiation.
Its principal vulnerabilities are:
- Potential invalidity or narrow claim scope in composition patents.
- Design-around opportunities involving dosing or formulations.
- Biosimilar applicants challenging non-core use patents.
- Regulatory reliance on the reference product after exclusivity expires.
- Revenue concentration in one indication during the early launch period.
The estate is stronger than a conventional small-molecule estate if Merck secures overlapping composition, dosing, formulation, and manufacturing claims. It is weaker if protection depends mainly on method-of-use patents that can be avoided by a biosimilar label.
What generic launch scenarios should investors monitor?
| Scenario |
Timing |
Commercial effect |
| No early biosimilar challenge |
Before 2036 |
Merck retains high pricing and broad access control |
| Patent challenge with settlement |
Potentially before or around 2036 |
Entry date becomes the key valuation variable |
| At-risk biosimilar launch |
Before patent expiry |
Immediate litigation, payer disruption, and price pressure |
| Label-limited biosimilar entry |
After regulatory exclusivity |
Lower initial substitution, but payer-driven share loss |
| New mechanism competitor |
Before patent expiry |
Slower patient additions and lower long-term pricing power |
The largest near-term financial variable is not generic erosion. It is the speed at which physicians use WINREVAIR across eligible patients and the degree to which payers restrict access.
Key Takeaways
- WINREVAIR is Merck’s first approved activin-signaling therapy for PAH.
- FDA approval occurred on March 26, 2024.
- The STELLAR trial showed a 40.1-meter median improvement in 6-minute walk distance and a substantial reduction in clinical worsening.
- Merck reported approximately $149 million in 2024 WINREVAIR sales.
- Long-term peak-sales estimates commonly range from $3 billion to $5 billion.
- The product’s commercial case depends on add-on use, earlier-line treatment, and potential label expansion.
- WINREVAIR is a biologic, so Purple Book and BPCIA analysis is more relevant than conventional Orange Book Paragraph IV analysis.
- U.S. biologic exclusivity is expected to extend into 2036, subject to statutory and pediatric-extension considerations.
- Biosimilar entry is a longer-term risk, while payer access, specialist adoption, and supply execution are the main launch-stage risks.
- Merck’s 2021 Acceleron acquisition gave it full strategic control of the asset and exposed the company to both the product’s upside and commercialization costs.
FAQs
Is WINREVAIR a biologic or a small-molecule drug?
WINREVAIR is a biologic fusion protein. It is not a conventional small-molecule tablet or capsule.
Is WINREVAIR used alone?
No. WINREVAIR is generally used with background PAH therapy, although the FDA label does not require a specific combination regimen.
How often is WINREVAIR administered?
WINREVAIR is administered by subcutaneous injection once every three weeks after weight-based dose escalation.
Will WINREVAIR be subject to Medicare price negotiation?
WINREVAIR was newly launched in 2024 and is not an early candidate for the first Medicare negotiation cycles. Its future exposure will depend on statutory eligibility, product age, spending, and CMS selection decisions.
What is the largest threat to WINREVAIR’s revenue forecast?
The largest near-term threat is slower-than-expected adoption caused by payer restrictions, monitoring requirements, injection burden, or limited treatment-center capacity. Biosimilar erosion is a later-stage risk.
References
- Merck & Co., Inc. (2025). 2024 annual report.
- U.S. Food and Drug Administration. (2024, March 26). FDA approves new drug to treat pulmonary arterial hypertension.
- Hoeper, M. M., Badesch, D. B., Ghofrani, H. A., Gibbs, J. S. R., Gomberg-Maitland, M., McLaughlin, V. V., Preston, I. R., Souza, R., Waxman, A. B., Grünig, E., & STELLAR investigators. (2023). Phase 3 trial of sotatercept for treatment of pulmonary arterial hypertension. New England Journal of Medicine, 388(16), 1478-1490.
- Reuters. (2024, March 26). Merck prices Winrevair at about $14,000 per vial in the United States.
- Merck & Co., Inc. (2024). First-quarter 2024 financial results and business update.
- Merck & Co., Inc. (2021, September 30). Merck to acquire Acceleron Pharma Inc. for approximately $11.5 billion.
