Study of Safety & PK of Luspatercept (ACE-536) in Pediatric Participants With Beta (β)-Thalassemia

This is a Phase 2a study to evaluate the safety and pharmacokinetics (PK) of luspatercept in pediatric participants with β-thalassemia. The study will be conducted in 2 parts for both transfusion-dependent (TD) and non-transfusion-dependent (NTD) β-thalassemia participants: TD Part A will be in adolescent participants aged 12 to \<18 years with two dose escalation cohorts, followed by a dose expansion cohorts. NTD Part A will be conducted in the same age group participants as TD Part A with dose confirmation and expansion cohorts. After Part A TD participants have completed at least one year of treatment, all available safety data from Part A adolescent participants will be evaluated before initiating TD and NTD Part B in the age group from 6 to \<12 years old. Part B will consist of two dose escalation cohorts for TD and two dose escalation cohorts for NTD. Upon completion of the Treatment Period, participants of any cohort who are benefiting from the study treatment, will be offered the opportunity to continue luspatercept treatment in the Long-term Treatment Period for up to 5 years from their first dose. Participants who discontinue study treatment at any time will continue in the Posttreatment Follow-up Period for at least 5 years from their first dose of luspatercept, or 3 years from their last dose, whichever occurs later, or until they withdraw consent/assent, are lost to follow-up, or the End of Trial, whichever occurs first. If neither commercial treatment nor an LTFU (long-term follow-up) protocol is available at that time, continued treatment will be provided within this study or via an alternative mechanism, at the Sponsor's discretion.