Luspatercept

A recombinant ActRIIB-IgG1 Fc fusion protein that traps TGF-beta superfamily ligands to promote late-stage red cell maturation, approved by the FDA as Reblozyl in 2019 for transfusion-dependent beta-thalassemia and later for lower-risk MDS anemia.

Also referenced as: Luspatercept-aamt, ACE-536, BMS-986346, Reblozyl

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What is Luspatercept?

Luspatercept is not a peptide. It is a recombinant fusion protein with a calculated molecular mass of about 76 kDa in which a modified extracellular domain of the human activin receptor type IIB (ActRIIB) is linked to the Fc domain of human IgG1, produced in Chinese hamster ovary cells; the US nonproprietary name carries the four-letter biologic suffix, luspatercept-aamt. The ActRIIB domain was engineered to reduce activin binding so that the molecule acts as a selective trap for other transforming growth factor-beta (TGF-beta) superfamily ligands, including GDF11, that hold back the final stages of red cell maturation (Suragani et al., Nature Medicine, 2014;20(4):408–414). Acceleron Pharma discovered it as ACE-536 and developed it with Celgene, now part of Bristol Myers Squibb, which lists it as BMS-986346 in newer trial registrations (Kubasch et al., Blood Advances, 2021;5(5):1565–1575).

The FDA approved it as Reblozyl on November 8, 2019 under BLA 761136 for anemia in adults with beta-thalassemia who require regular red blood cell transfusions, the first product of the class the label calls erythroid maturation agents. Two myelodysplastic syndrome (MDS) indications followed: on April 3, 2020 for anemia failing an erythropoiesis-stimulating agent (ESA) in lower-risk MDS with ring sideroblasts or MDS/myeloproliferative neoplasm with ring sideroblasts and thrombocytosis, and on August 28, 2023 for ESA-naive lower-risk MDS on the strength of the COMMANDS trial. It is a hematology-clinic biologic with no presence in the research-peptide market.

How it works

• Ligand trap for late-stage erythropoiesis. Erythropoietin drives proliferation of early erythroid precursors, but several EPO-resistant anemias are caused by a block in late-stage, EPO-independent maturation. ACE-536 bound GDF11, potently inhibited GDF11-mediated Smad2/3 signaling, and raised red cell counts rapidly in several species; unlike EPO, its murine version RAP-536 promoted maturation of late-stage erythroid precursors in vivo, and co-treatment with EPO was synergistic (Suragani et al., Nature Medicine, 2014;20(4):408–414). The FDA label describes the mechanism in the same terms: binding of several TGF-beta superfamily ligands diminishes Smad2/3 signaling and increases erythroid precursors in humans.
• Corrects ineffective erythropoiesis in disease models. In a mouse model of beta-thalassemia, the ligand trap mitigated ineffective erythropoiesis and the downstream complications of the disease, and in an MDS model RAP-536 reduced Smad2/3 activation, anemia, erythroid hyperplasia and ineffective erythropoiesis (Suragani et al., Blood, 2014;123(25):3864–3872; Suragani et al., Nature Medicine, 2014;20(4):408–414).
• Antibody-like pharmacokinetics. After subcutaneous injection the label reports linear kinetics, a median time to peak concentration of about 5 days in beta-thalassemia and 6 days in MDS, geometric mean half-lives of about 11 and 14 days respectively (Chen et al., Journal of Clinical Pharmacology, 2021;61(1):52–63), and steady state after three doses given every 3 weeks. The effect is a rise in hemoglobin or a fall in transfusion need over weeks, which is why the label states that the product is not a substitute for transfusion when anemia needs immediate correction.

Research status

Phase 2 dose-finding. PACE-MDS enrolled 58 patients with lower-risk MDS at nine German centers; 32 of 51 (63%) receiving the higher dose range (0.75 to 1.75 mg/kg) achieved hematologic improvement-erythroid, against 2 of 9 (22%) at lower doses (Platzbecker et al., Lancet Oncology, 2017;18(10):1338–1347). A parallel open-label study in 64 adults with beta-thalassemia found that 58% of non-transfusion-dependent patients at the higher dose levels had a mean hemoglobin rise of at least 1.5 g/dL, and 81% of transfusion-dependent patients cut transfusion burden by at least 20% (Piga et al., Blood, 2019;133(12):1279–1289).

BELIEVE (NCT02604433). The pivotal thalassemia trial randomized 336 adults with transfusion-dependent beta-thalassemia 2:1 to luspatercept (224) or placebo (112) at 65 sites in 15 countries. The primary endpoint, a reduction in transfusion burden of at least 33% with at least 2 fewer units during weeks 13 to 24, was met by 21.4% versus 4.5% (p<0.001); over any 12-week interval, 70.5% versus 29.5% reached a 33% reduction, and serum ferritin at week 48 was 348 µg/L lower than placebo. Transient bone pain, arthralgia, dizziness, hypertension and hyperuricemia were more common with luspatercept (Cappellini et al., New England Journal of Medicine, 2020;382(13):1219–1231). In the final analysis, after a median of 153.6 weeks of treatment, 77% of patients had reached a 33% reduction over some 12-week interval, the median duration of response was 586 days, and there were no treatment-related deaths (Cappellini et al., Lancet Haematology, 2025;12(3):e180–e189).

MEDALIST (NCT02631070). This double-blind trial randomized 229 transfusion-dependent patients with very-low- to intermediate-risk MDS with ring sideroblasts who were refractory to, intolerant of, or unlikely to respond to ESAs, 153 to luspatercept and 76 to placebo. Transfusion independence for 8 weeks or longer during weeks 1 to 24 was reached by 38% versus 13% (p<0.001), and independence for 12 weeks or longer by 28% versus 8% in weeks 1 to 24 and 33% versus 12% in weeks 1 to 48 (Fenaux et al., New England Journal of Medicine, 2020;382(2):140–151). The label’s subgroup analysis shows how strongly baseline burden shaped the result: 80.4% of luspatercept patients transfused 2 to 3 units per 8 weeks became transfusion independent, against 9.1% of those needing 6 or more units.

COMMANDS (NCT03682536). The open-label phase 3 trial compared luspatercept with epoetin alfa in ESA-naive, transfusion-dependent lower-risk MDS at 142 sites in 26 countries, randomizing 363 patients. In the interim analysis that supported the August 28, 2023 approval, the primary endpoint of transfusion independence for at least 12 weeks with a concurrent mean hemoglobin rise of at least 1.5 g/dL during weeks 1 to 24 was met by 86 of 147 patients (59%) versus 48 of 154 (31%) (Platzbecker et al., Lancet, 2023;402(10399):373–385). In the primary analysis it was met by 110 patients (60%) versus 63 (35%), a 25.4-point difference (95% CI 15.8 to 35.0; p<0.0001); grade 3 to 4 hypertension occurred in 10% versus 4%, and one death from acute myeloid leukemia was considered luspatercept-related (Della Porta et al., Lancet Haematology, 2024;11(9):e646–e658). The label notes that more than 90% of participants were outside the US and received a non-US-licensed epoetin alfa. The study is active but not recruiting, with completion expected September 28, 2027.

BEYOND (NCT03342404). A phase 2, double-blind trial at 12 centers in six countries randomized 145 adults with non-transfusion-dependent beta-thalassemia 2:1 to luspatercept (96) or placebo (49) for 48 weeks. A mean hemoglobin rise of at least 1 g/dL over weeks 13 to 24 without transfusion was achieved by 74 patients (77%) and by none on placebo (p<0.0001); serious adverse events were less frequent with luspatercept (12% versus 25%), bone pain (37%), headache (30%) and arthralgia (29%) were the commonest adverse events, and no thromboembolic events or deaths occurred (Taher et al., Lancet Haematology, 2022;9(10):e733–e744). The European Commission’s authorization covers both transfusion-dependent and non-transfusion-dependent beta-thalassemia, whereas the US label states that the product is not indicated for non-transfusion-dependent disease and records extramedullary hematopoietic masses in 6.3% of luspatercept-treated patients versus 2% on placebo in that trial.

Myelofibrosis. The phase 2 ACE-536-MF-001 study (NCT03194542) treated 95 patients with myeloproliferative neoplasm-associated myelofibrosis in four cohorts; anemia response in the first 24 weeks ranged from 10% to 26% by cohort, and hypertension (18%) was the most frequent treatment-related adverse event (Gerds et al., Blood Advances, 2024;8(17):4511–4522). The phase 3 INDEPENDENCE trial (NCT04717414) randomized 313 transfusion-dependent patients on a JAK inhibitor 2:1 to luspatercept or placebo from February 25, 2021, reaching primary completion on May 26, 2025. The sponsor reported in July 2025 that the primary endpoint was not met, and the registry posts transfusion independence for at least 12 weeks in 23.08% versus 13.33% (p=0.0674). As of November 2026, no regulator has approved luspatercept for myelofibrosis.

Ongoing and regional. A phase 2a study in 99 children with beta-thalassemia (NCT04143724) is recruiting, with primary completion estimated for January 31, 2028, and a phase 2 trial in 189 adults and adolescents with alpha-thalassemia (NCT05664737) is active but not recruiting, with primary completion estimated for July 16, 2027. The European Commission granted marketing authorization on June 25, 2020; Health Canada issued Notices of Compliance on September 25, 2020 for transfusion-dependent beta-thalassemia, in February 2021 for ESA-refractory MDS with ring sideroblasts, and on September 13, 2024 for ESA-naive MDS; and Japan approved luspatercept for myelodysplastic syndromes in 2024 (Matsumura and Mandai, International Journal of Clinical Oncology, 2024;29(5):493–494).

Common dosage forms

• Reblozyl for injection, 25 mg and 75 mg vials. A sterile, preservative-free lyophilized powder in single-dose vials for subcutaneous injection administered by a healthcare professional.
• Single US presentation. The two vial strengths are the only presentations listed under BLA 761136; no oral, intravenous or prefilled-syringe form exists.

Key considerations

• Regulatory status. The FDA Approved status reflects the November 8, 2019 approval of Reblozyl (luspatercept-aamt) for injection under BLA 761136 for anemia in adults with beta-thalassemia who require regular red blood cell transfusions. Supplemental approvals on April 3, 2020 (ESA-refractory lower-risk MDS with ring sideroblasts) and August 28, 2023 (ESA-naive lower-risk MDS) added the MDS indications, and the product is also authorized in the European Union, Canada and Japan.
• A protein therapeutic, not a peptide. The active molecule is a 76 kDa Fc fusion with an engineered ActRIIB domain, manufactured in mammalian cells and dosed every 3 weeks on the basis of an antibody-like half-life. It is distinct from sotatercept, an ActRIIA-Fc fusion from the same lineage approved as Winrevair on March 26, 2024, and from the erythropoiesis-stimulating agents it is compared with; none of those names are aliases.
• Thrombosis, hypertension and extramedullary masses are the labeled signals. Each occurred in a small share of treated patients across the trials, and the label ties monitoring to them. The higher rate of extramedullary masses in non-transfusion-dependent disease is the reason the US label excludes that population while the European Union includes it.
• Benefit is largest at low transfusion burden. Response falls steeply as baseline transfusion need rises, and only a minority of heavily transfused thalassemia patients meet the strict primary endpoint. The evidence base is strongest for lower-risk MDS and transfusion-dependent beta-thalassemia, with an open-label comparison in ESA-naive MDS and a negative phase 3 trial in myelofibrosis.
• No pediatric use yet. The label states that safety and effectiveness have not been established in children and, on the basis of findings in juvenile animals, does not recommend pediatric use. The beta-thalassemia and alpha-thalassemia pediatric studies, with primary completions estimated for 2028 and 2027, are the registered programs that could change that.