Study to Evaluate the Safety and Efficacy of Luspatercept (ACE-536) in Participants With Beta-thalassemia (A536-04/MK-6143-002)

Key Inclusion Criteria: * Men or women ≥18 years of age * For the dose escalation phase of the study: documented diagnosis of β-thalassemia intermedia (transfusion dependent participants must not have begun regular transfusions at age \<4.0 years). For the expansion cohort: documented diagnosis of β-thalassemia (including β-thalassemia major or β-thalassemia intermedia) * Prior splenectomy or spleen size \<18 cm in the longest diameter by abdominal ultrasound (dose escalation cohorts only) * Anemia, defined as: (1) mean hemoglobin concentration \<10.0 g/dL of 2 measurements (one performed within one day prior to Cycle 1 Day 1 and the other performed during the screening period \[Day -28 to Day -1\]) in non-transfusion dependent participants, defined as having received \< 4 units of RBCs within 8 weeks prior to Cycle 1 Day 1, or (2) transfusion dependent participants, defined as requiring ≥ 4 units of RBCs every 8 weeks (confirmed over 6 months prior to Cycle 1 Day 1) * Aspartate aminotransferase (AST) or alanine aminotransferase (ALT) \<3 x upper limit of normal (ULN) * Serum creatinine ≤1.5 x ULN * Adequate pregnancy avoidance measures * Participants are able to adhere to the study visit schedule, understand, and comply with all protocol requirements * Understand and able to provide written informed consent Key Exclusion Criteria: * Any clinically significant pulmonary (including pulmonary hypertension), cardiovascular, endocrine, neurologic, hepatic, gastrointestinal, in…