This study is researching an experimental drug called odronextamab (referred to as study drug), in combination with lenalidomide. The study is focused on participants who have one of two types of cancer: follicular lymphoma (FL) or marginal zone lymphoma (MZL) that has come back after treatment (called "relapsed"), or did not respond to treatment (called "refractory"). FL and MZL are subtypes of Non-Hodgkin 's lymphoma (NHL).

This study will be made up of two parts (Part 1 not randomized, Part 2 randomized - controlled).

The aim of Part 1 of the study is to see how safe and tolerable the study drug is when used in combination with lenalidomide, in participants with FL or MZL, and to determine the dose of the study drug to be used in Part 2 of this study. This combination is considered "first-in-human" as it has not been tested as a combination treatment in humans before.

The aim of Part 2, of the study is to assess how well the combination of the study drug and lenalidomide works compared to the combination of rituximab (called "the comparator drug") and lenalidomide. The combination of comparator drug and lenalidomide is the current standard-of-care treatment for relapsed/refractory FL and/or MZL. 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 in combination with lenalidomide
• 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. Local histologic confirmation of FL grade 1-3a or MZL (nodal, splenic, or extra nodal MZL) as assessed by the investigator, as described in the protocol.

2. Must have refractory disease or relapsed after at least 1 prior line (with a duration of at least 2 cycles) of systemic chemo-immunotherapy or immunotherapy. Prior systemic therapy should have included at least one anti-Cluster of Differentiation 20 (CD20) monoclonal antibody and participant should meet indication for treatment, as described in the protocol.

3. Have measurable disease on cross sectional imaging documented by diagnostic Computed Tomography [CT], or Magnetic Resonance Imaging [MRI] imaging, as described in the protocol.

4. Eastern Cooperative Oncology Group (ECOG) performance status of 0 to 2.

5. Adequate hematologic and organ function, as described in the protocol.

6. All study participants must:

   1. Have an understanding that lenalidomide could have a potential teratogenic risk.
   2. Agree to abstain from donating blood while taking study drug therapy and for 28 days after discontinuation of lenalidomide.
   3. Agree not to share study medication with another person.
   4. Agree to be counseled about pregnancy precautions and risk of fetal exposure associated with lenalidomide.

Key Exclusion Criteria:

1. Primary Central Nervous System (CNS) lymphoma or known involvement (either current or prior history of CNS involvement) by non-primary CNS NHL, as described in the protocol.
2. Participants with current or past histological evidence of high-grade or diffuse large B-cell lymphoma, or any histology other than FL grade 1-3a or MZL.
3. History of or current relevant CNS pathology, as described in the protocol.
4. A malignancy other than NHL (inclusion diagnosis) unless the participant is adequately and definitively treated and is cancer free for at least 3 years, with the exception of localized prostate cancer treated with hormone therapy or local radiotherapy (ie, pellets), cervical carcinoma in situ, breast cancer in situ, or nonmelanoma skin cancer that was definitively treated.
5. Any other significant active disease or medical condition that could interfere with the conduct of the study or put the participant at significant risk, as described in the protocol.
6. Allergy/hypersensitivity to study drugs or excipients. as described in the protocol.
7. Active infection as defined 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