A Study to Test the Safety, Tolerability, and Efficacy of an Antibody, REGN7999, Injected Under the Skin for the Treatment of Iron Overload in Adult Participants With Non-Transfusion Dependent β-thalassemia, Using MRI Scans to Measure Iron Levels in the Body

Key Inclusion Criteria:

1. Clinical diagnosis of NTDT as described in the protocol

2. IOL, defined as LIC ≥ 5 mg Fe/g DW as measured by R2* MRI at screening

3. Serum ferritin ≥ 300 ng/mL as described in the protocol

Key Exclusion Criteria:

1. Hemoglobin ≤ 8 g/dL

2. Any RBC transfusion within 12 weeks of visit 3

3. For Part A only: Any ICT use in approximately 12 weeks prior to screening asdescribed in the protocol

4. For Part B only: If on ICT, any change in Iron chelation therapy (ICT) dose inapproximately 12 weeks prior to screening as described in the protocol

5. Any use of luspatercept or mitapivat in 6 months prior to screening as described inthe protocol

6. Absolute contraindication to MRI

7. Diagnosis of cirrhosis of the liver

8. Diagnosis of Chronic kidney disease (CKD) stage 4 or higher

Note: Other protocol-defined Inclusion/ Exclusion Criteria apply