A Study of Elritercept to Treat Anemia in Adults With Very Low, Low, or Intermediate Risk Myelodysplastic Syndromes (MDS)

Inclusion Criteria: 1. Ability to understand the purpose and risks of the study and provide signed and dated informed consent and authorization to use protected health information in accordance with national and local study participant privacy regulations. 2. Male or female ≥ 18 years of age, at the time of signing informed consent. 3. Eastern Cooperative Oncology Group (ECOG) performance status of 0, 1, or 2 (if related to anemia). 4. Females of childbearing potential and sexually active males must agree to use highly effective methods of contraception. 5. In the opinion of the Investigator, the participant is able and willing to comply with the requirements of the protocol (e.g., all study procedures, return for follow-up visits). Part 1 Inclusion Criteria Participants are eligible to be included in Part 1 of the study only if all the following criteria apply: 1. Diagnosis of MDS according to WHO classification that meets International Prognostic Scoring System-Revised (IPSS-R) classification of very low, low, or intermediate risk disease. 2. Less than (\<)5 percent (%) blasts in bone marrow during the Pretreatment Period. 3. Peripheral blood white blood cell (WBC) count \<13,000/microliter (μL) during the Pretreatment Period. 4. Anemia defined as: 1. In non-transfused participants, having received no RBC transfusions within 8 weeks, Hgb concentration ≤ 10.0 g/dL during the Pretreatment Period. OR 2. In LTB participants, having received 1 to 3 units of RBCs for Hgb ≤ 9.0 g/dL within 8 weeks of the Pretreatment Period. OR 3. In HTB participants, having received ≥ 4 units of RBCs for Hgb ≤ 9.0 g/dL within 8 weeks of the Pretreatment Period. Part 1 Extension - Abbreviated Inclusion Criteria Participants from Part 1 are eligible to be included in Part 1 Extension of the study only if all the following criteria apply: 1. Previously completed 4 cycles of elritercept in Part 1 with no dose-limiting toxicities (DLTs). 2. Participant has the potential to benefit from administration of elritercept, in the opinion of the Investigator. 3. \< 5% blasts in bone marrow. 4. Peripheral WBC count \< 13,000/μL during the 28 days prior to cycle 5 day 1 (C5D1). Part 2 Inclusion Criteria Participants are eligible to be included in Part 2 of the study only if all the following criteria apply: 1. Cohort A: * Diagnosis of MDS according to WHO 2016 classification that meets IPSS-R classification of very low, low, or intermediate risk disease. * ring sideroblast (RS)-positive as defined by WHO 2016 criteria. * Requiring at least 2 units of RBC transfusions in the preceding 8 weeks before cycle 1 day 1 (C1D1). 2. Cohort B: * Diagnosis of MDS according to WHO 2016 classification that meets IPSS-R classification of very low, low, or intermediate risk disease. * Non-RS as defined by WHO 2016 criteria. * Requiring at least 2 units of RBC transfusions in the 8 weeks before C1D1. 3. Cohort C: * Diagnosis of MDS according to WHO 2016 classification that meets IPSS-R classification of very low, low, or intermediate risk disease. * Has anemia, defined by Hgb ≤ 10 g/dL during the Pretreatment Period, and received no RBC transfusion in the 8 weeks before C1D1. 4. Cohort D: * Diagnosis of CMML according to WHO 2016 classification. * Has anemia, defined by Hgb ≤ 10 g/dL during the Pretreatment Period, and received no RBC transfusion in the 8 weeks before C1D1. OR * Received at least 2 units of RBC transfusions for anemia in the 8 weeks before C1D1. 5. Cohort E: * Diagnosis of MDS according to WHO 2016 classification that meets IPSS-R classification of very low, low, or intermediate risk disease. * Requiring ≥ 2 units of RBC transfusions in the preceding 8 weeks before C1D1. * Receipt of ≥ 20 units of RBC in transfusion over the participant's lifetime. * Serum ferritin \> 1000 nanograms per milliliter (ng/mL) on ≥ 2 assessments in the preceding 8 weeks before C1D1. * Treated with stable dose of iron chelation therapy for ≥ 8 weeks prior to C1D1. 6. Cohort F: * Diagnosis of MDS according to WHO 2016 classification that meets IPSS-R classification of very low, low, or intermediate risk disease. * Requiring ≥ 2 units of RBC transfusions in the preceding 8 weeks before C1D1. * Receipt of ≥ 20 units of RBC in transfusion over the participant's lifetime. * Serum ferritin \> 1000 ng/mL on ≥ 2 assessments in the preceding 8 weeks before C1D1. * Not treated with iron chelation therapy in the preceding 8 weeks before C1D1 and not eligible to initiate iron chelation therapy in the opinion of the Investigator and in accordance with local treatment guidelines for initiation of iron chelation therapy. 7. Cohort G: * Diagnosis of MDS according to WHO 2016 classification that meets IPSS-R classification of very low, low, or intermediate risk disease. * RS-positive as defined by WHO 2016 criteria OR non-RS as defined by WHO 2016 criteria. * Relapsed, refractory, or intolerant to luspatercept treatment * Relapsed is defined as documentation of response to luspatercept therapy followed by subsequent development of a need for transfusion(s), whether the transfusion need develops during luspatercept treatment or after luspatercept has been discontinued. * Refractory is defined as documentation of no response with luspatercept ≥ 1 mg/kg administered for ≥ 12 weeks duration. * Intolerant is defined as documentation of discontinuation of luspatercept therapy due to intolerance including, discontinuation because of any adverse event (serious or nonserious) at any time after treatment initiation. * Has anemia, defined as either of the following: * Has received at least 1 unit of RBC transfusions for anemia in the 8 weeks before C1D1; OR * Hgb ≤ 10 g/dL, and received no RBC transfusion in the 8 weeks before C1D1 * Erythropoietin (EPO) \< 500 international units per liter (U/L) at Baseline. * Last dose of luspatercep