This study is researching a drug called REGN17372 used with another drug called linvoseltamab (each individually called "study drug" or "study drugs" when combined) in participants with relapsed (when a tumor comes back) or refractory (when a tumor does not respond to treatment) multiple myeloma. This study is the first time REGN17372 will be given to humans.

The aim of the study is to understand if REGN17372 can be given safely with linvoseltamab, and if so, what dosing regimen should be used for this treatment combination, in comparison with linvoseltamab alone.

The study is looking at:

• What side effects may happen from taking REGN17372 with linvoseltamab
• How well REGN17372 and linvoseltamab, or linvoseltamab alone, work in treating multiple myeloma
• What is the best dose of REGN17372 when given with linvoseltamab
• How much study drug(s) are in the blood at different times
• Whether the body makes antibodies against the study drugs (which could make the study drugs less effective or could lead to side effects)
• If and how REGN17372 and linvoseltamab affect the overall quality of life, daily activities, symptoms and treatment side effects based on participant own feedback (Phase 2)

Key Inclusion Criteria:

1. Participants with RRMM who have exhausted (or are not a candidate for) all therapeutic options that are expected to provide meaningful clinical benefit and have received at least 3 lines of therapy as defined in the protocol
2. ECOG performance status score ≤1
3. Participants must have measurable disease for response assessment as described in the protocol
4. Adequate hematologic, cardiac, hepatic, and renal function, as described in the protocol

Key Exclusion Criteria:

1. Participants with non-secretory MM, active plasma cell leukemia, known amyloidosis, Waldenström macroglobulinemia, or known POEMS syndrome as defined in the protocol
2. Participants who have known MM brain lesions or CNS involvement
3. Participants with a history of PML, a neurocognitive condition or CNS movement disorder, or a history of seizure within 12 months prior to entering screening
4. Prior treatment with GPRC5D-directed immunotherapies (phase 1 and phase 2) and/or prior treatment with a BCMAxCD3 bispecific antibody (phase 2)

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