A Study to Compare Elritercept to Placebo in Adults With Myelofibrosis and Anemia Who Are Taking Ruxolitinib

The main aim of this study is to find out how well elritercept works to improve anemia in participants with myelofibrosis (MF) who are taking ruxolitinib when compared to placebo. Other aims are to learn how elritercept improves anemia compared to placebo; to learn if elritercept reduces tiredness, improves symptoms related to MF, and helps participants do physical activities more easily. The study also aims to find out how elritercept affects the bone marrow, the spleen, and whether participants develop antibodies to the study drug. The study will also check how safe elritercept is compared to placebo, and if elritercept stays safe over a long period of time. Participants will receive study treatment for at least 9 months (36 weeks). After this period, participants who received placebo will have the option to switch to elritercept.

Inclusion Criteria: 1. Aged ≥18 years at the time of signing the informed consent form (ICF). 2. Able to understand the purpose and risks of the trial and voluntarily sign an ICF. 3. Diagnosed with primary myelofibrosis (PMF), post-essential thrombocythemia (post-ET MF) or post-polycythemia vera (post-PV MF) according to the 2022 WHO criteria (WHO Classification of Tumours Editorial Board 2024), confirmed by local pathology report. 4. Transfusion status as assessed in the 12 weeks immediately preceding randomization classified as Transfusion Dependent: 3 to 8 RBC units over 12 weeks. 5. Receiving ruxolitinib (as approved in the country of the trial site) as the standard of care treatment for MF for at least 12 consecutive weeks, and on a stable daily dose for at least the 8 weeks immediately preceding the date of randomization. 6. Eastern Cooperative Oncology Group score less than or equal to (≤) 2. Exclusion Criteria: 1. Prior treatment with luspatercept, sotatercept, or other transforming growth factor beta inhibitors or activin receptor ligand traps. 2. Systemic treatment within 28 days before randomization with any of the following: 1. Androgens (including danazol). Participants on stable androgen dosing for hypogonadism for ≥8 weeks are allowed. 2. erythropoiesis-stimulating agents. 3. granulocyte colony stimulating factor or granulocyte-macrophage colony stimulating factor. 4. High dose corticosteroids. Participants on stable chronic steroid doses of prednisone ≤10 mg/day or corticosteroid equivalent for ≥4 weeks are allowed. Other treatments for autoimmune diseases may be allowed upon medical monitor review. 5. Hydroxyurea. 6. Immunomodulatory drugs (for example, thalidomide, pomalidomide, or lenalidomide). 7. Interferon. 8. Thrombopoietin receptor agonists. 9. Any investigational drug, including antihemojuvelin antibody. If the half-life of the investigational product is known, the exclusionary period prior to randomization is equal to 5 half-lives of the investigational product or 28 days, whichever is longer. 3. Initiation of new iron chelation therapy or dose adjustments to existing iron chelation therapy ≤8 weeks prior to randomization. Participants on stable doses of iron chelation therapy for ≥8 weeks are allowed. 4. Clinically significant anemia that is due to causes other than MF or Janus kinase (JAK) inhibitor therapy (for example, thalassemia, iron deficiency, vitamin B12 and/or folate deficiencies, autoimmune or hemolytic anemia, infections, or any active clinically significant bleeding or sequestration). 5. Receipt of RBC transfusion for any reason(s) other than underlying MF within 12 weeks before randomization. 6. Life expectancy \