This study is researching an experimental drug called odronextamab, referred to as study drug, when used in combination with chemotherapy. The study is focused on patients with Diffuse Large B-cell Lymphoma (DLBCL) that have not been treated before (called "previously untreated"). Patients with DLBCL that have come back after treatment (called "relapsed"), or have not responded to treatment (called "refractory"), can also participate in this study.

This study will be made up of Part 1A, Part 1B, and Part 2.The aim of Part 1A and Part 1B of the study is to see how safe and tolerable the study drug in combination with chemotherapy is and to determine the dose and schedule of the study drug to be combined with chemotherapy in Part 2 of the study.

The aim of Part 2 of the study is to see how effective the combination of the study drug with chemotherapy is in comparison with the combination of rituximab (the comparator drug), and chemotherapy, the current standard of care treatment approved for DLBCL. Standard of care means the usual medication expected and used when receiving treatment for a condition.

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

• What side effects may happen from taking the study drug when combined with chemotherapy
• How much study drug is in the blood at different times
• Whether the body makes antibodies against the study drug (which could make the study drug less effective or could lead to side effects)
• The impact from the study drug on quality of life and ability to complete routine daily activities

Key Inclusion Criteria:

1. Previously untreated participants for lymphoma with documented Cluster of Differentiation 20+ (CD20+) DLBCL, as described in the protocol OR relapsed or refractory DLBCL, for whom next available standard of care therapy is not available or deemed ineligible according to the investigator (Part 1A only)
2. Measurable disease with at least one nodal lesion or at least one extranodal lesion, as described in the protocol
3. Eastern Cooperative Oncology Group (ECOG) performance status ≤2
4. Life expectancy ≥ 12 months
5. International Prognostic Index (IPI) of 3 to 5 (part 1 only) and ≥2 (part 2) for untreated DLBCL only
6. Adequate hematologic and organ function, as defined in the protocol.

Key Exclusion Criteria:

1. Primary Central Nervous System (CNS) lymphoma or known involvement by non-primary CNS NHL and history or current relevant CNS pathology
2. Another active malignancy, significant active disease or medical condition, as described in the protocol
3. Peripheral neuropathy Grade ≥3
4. Treatment with any systemic anti-lymphoma therapy, except for participants with Relapsed/Refractory (R/R) DLBCL and participants with DLBCL transformed from an indolent lymphoma after treatment with systemic anti-lymphoma therapy.
5. Any other therapy or investigational treatment within 28 days or 5 half-lives of the drug, whichever is shorter, prior to the start of study treatment
6. Recent major surgery, prior organ transplantation, or standard radiotherapy, as described in the protocol
7. Allergy/hypersensitivity to study drugs, as described in the protocol
8. Infections such as any active infection (bacterial, viral, fungal, mycobacterial, parasitic or other), active Coronavirus Disease (COVID-19) infection, uncontrolled infection with Human Immunodeficiency Virus (HIV), Hepatitis B Virus (HBV), or Hepatitis C Virus (HCV), Cytomegalovirus (CMV) infection, as described in the protocol.

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