A Trial to Study if REGN17372 in Combination With Linvoseltamab is Tolerable for Adult Participants With Relapsed/Refractory Multiple Myeloma

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