This study is researching an experimental drug called REGN5837 in combination with another drug, odronextamab (called "study drug[s]"), in patients with relapsed or refractory aggressive B-cell Non-Hodgkin Lymphomas (B-NHLs).

The study has 2 parts. The aim of the first part (dose escalation) is to find a safe dose of REGN5837 when given in combination with odronextamab.

The goal of the second part (dose expansion) is to use the REGN5837 drug dose found in the first part to see how well REGN5837 in combination with odronextamab works.

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

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

Key Inclusion Criteria:

1. Have documented CD20+ aggressive B-NHL, with disease that has progressed after at least 2 lines of systemic therapy containing an anti-CD20 antibody and an alkylating agent, as described in the protocol.
2. Measurable disease on cross sectional imaging as defined in the protocol
3. Eastern Cooperative Oncology Group (ECOG) performance status 0 or 1
4. Adequate bone marrow, renal and hepatic function as defined in the protocol
5. Availability of tumor tissue for submission to central laboratory is required for study enrollment. Archival tumor tissue for histological assessment prior to enrollment is allowed
6. During dose expansion phase of the study, participant should be willing to undergo mandatory tumor biopsies, if in the opinion of the investigator, the participant has an accessible lesion that can be biopsied without significant risk to the participant.

Key Exclusion Criteria:

1. Prior treatments with allogeneic stem cell transplantation or solid organ transplantation, treatment with anti-CD20 x anti- CD3 bispecific antibody, such as odronextamab
2. Diagnosis of Mantle Cell Lymphoma (MCL)
3. Primary Central Nervous System (CNS) lymphoma or known involvement by non-primary CNS lymphoma, as described in the protocol
4. Treatment with any systemic anti-lymphoma therapy within 5 half-lives or within 14 days prior to first administration of study drug, whichever is shorter, as described in the protocol
5. Standard radiotherapy within 14 days of first administration of study drug, as described in the protocol
6. Continuous systemic corticosteroid treatment with more than 10 mg per day of prednisone or corticosteroid equivalent within 72 hours of start of odronextamab
7. Co-morbid conditions, as described in the protocol
8. Infections, as described in the protocol
9. Allergy/hypersensitivity: Known hypersensitivity to both allopurinol and rasburicase

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