This study is researching an experimental drug called REGN7999 (called "study drug"). The study is focused on patients with non-transfusion dependent beta-thalassemia. The aim of the study is to see how safe and effective the study drug is.

The study is looking at several other research questions, including:

• Whether the study drug lowers extra iron levels in the body
• What side effects may happen from taking the study drug
• How much study drug is in the blood at different times
• Whether the body makes antibodies against the study drug (which could make the drug less effective or could lead to side effects)

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 at screening
2. Any RBC transfusion within 12 weeks of visit 3
3. For Part A only: Any Iron Chelation Therapy (ICT) use in approximately 12 weeks prior to screening as described in the protocol
4. For Part B only: If on ICT, any change in ICT dose in approximately 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 in the 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

All Individual Patient Data (IPD) that underlie publicly available results will be considered for sharing.

When Regeneron has:

• received marketing authorization from major health authorities (e.g., FDA, European Medicines Agency (EMA), Pharmaceuticals and Medical Devices Agency (PMDA), etc.) for the product and indication or has globally discontinued development of the product for all indications on or after April 2020 and has no plans for future development
• made the study results publicly available (e.g., scientific publication, scientific conference, clinical trial registry)
• the legal authority to share the data, and
• ensured the ability to protect participant privacy

Qualified researchers can submit a proposal for access to individual patient or aggregate level data from a Regeneron-sponsored clinical trial through Vivli. Regeneron's Independent Research Request Evaluation Criteria can be found at: https://www.regeneron.com/sites/default/files/Regeneron-External-Data-Sharing-Policy-and-Independent-Research-Request-Evaluation-Criteria.pdf