A Study of Talquetamab for People With Multiple Myeloma Who Have Received BCMA CAR T-Cell Therapy

Inclusion Criteria: * Patient with multiple myeloma who has received prior treatment with an IMID, PI, and a CD38 monoclonal antibody * Received treatment with an FDA approved BCMA CART cell therapy ide-cel within 1-3 months prior to enrollment * Serum monoclonal protein \< 0.5 gm/dL; 24-hour urine monoclonal protein \< 200 mg; and serum involved free light chains \< 10 mg/dL * No evidence of disease progression based on IMWG criteria * ≥18 years of age at the time of signing informed consent. * ECOG performance status of 0 or 1 * Recovered to Grade 1 or baseline of any non-hematologic toxicities due to prior treatments, excluding Grade 2 neuropathy and Grade 2 alopecia. * No evidence of ongoing, any grade cytokine release syndrome or immune effector cell mediated neurotoxicity * No additional myeloma therapies after the CART cell therapy * Absolute neutrophil count (ANC) ≥ 1,000/mm\^3 without growth factor support for 7 days for G-CSF or GM-CSF and for 14 days for pegylated GCSF prior to the laboratory test. * Platelet count ≥ 75,000/mm\^3 (without platelet transfusion or thrombopoietin receptor agonist use in the previous 7 before the laboratory test). Platelets ≥ 50,000/mm\^3 is acceptable if the counts prior lymphodepleting chemotherapy for the preceding CAR T cell therapy was \< 75,000. * Hemoglobin ≥ 8 g/dL (without red blood cell transfusion support or erythropoietin use within 7 days of the laboratory test). * Creatinine Clearance (CrCl)/estimated glomerular filtration rate (eGFR) ≥ 30 mL/min, measured by 24 hour urine assessment or estimated by CKD-EPI2021. * Oxygen saturation ≥ 92% on room air * Hepatic Function: * Serum aspartate aminotransferase (AST) and alanine aminotransferase (ALT) ≤ 2.5 x upper limit of normal (ULN) * Serum total bilirubin ≤ 2 x ULN; except in participants with congenital bilirubinemia, such as Gilbert syndrome (in which case direct bilirubin ≤1.5 x ULN is required). * International ratio (INR) or partial thromboplastin time (PTT) \< 1.5 x ULN * Cardiac Function: left ventricular ejection fraction ≥ 45% by echocardiogram (ECHO) or multigated acquisition scan (MUGA). * Willing and able to adhere to the study visit schedule and other protocol requirements * Female patients of childbearing potential (FCBP) must: * Have a negative highly-sensitive serum β-human chorionic gonadotropin (β-hCG) pregnancy test (\<5 IU/mL) at screening and a negative urine or serum pregnancy test within 24 hours before the first dose of study drug. * Practicing a highly effective, preferably user-independent method of contraception (failure rate of \<1% per year when used consistently and correctly) and agrees to remain on a highly effective method while receiving study drug and until 100 days after last dose. * Agree to pregnancy testing (serum or urine) within 100 days after the last study drug administration. * Agree to use an additional contraceptive method in addition to the requirements listed above * Agree not to donate eggs (ova, oocytes) for the purposes of assisted reproduction during the study and for at least 100 days after the last dose of study drug. * Agree to abstain from breastfeeding from screening through at least 6 months after the last dose of talquetamab * Male patients must: * Agree to wear a condom when engaging in any activity that allows for passage of ejaculate to another person, during the study and for 100 days after the last dose of study drug. Male subjects should also be advised of the benefit for a female partner to use a highly effective method of contraception as a condom may break or leak. * Men must agree not to donate sperm for the purpose of reproduction during the study and for at least 100 days after receiving the last dose of study drug. * Must sign an ICF (or their legally acceptable representative must sign) indicating that the participant understands the purpose of, and procedures required for, the study and is willing to participate in the study. Exclusion Criteria: * Prior or concurrent exposure to any of the following in the specified time frame prior to enrollment: * Received any prior GRPRC5D-directed therapy * T-cell redirection therapy (for example, antibody therapy or BiTE's) within 3 months * Investigational vaccine other than SARS CoV-2 vaccine approved/ in use under emergency approval within 4 weeks * Live, attenuated vaccine within 4 weeks. * Monoclonal antibody therapy within 21 days (with the exception of COVID monoclonal antibodies) * Cytotoxic therapy within 21 days * PI therapy within 14 days * IMiD agent therapy within 14 days * Radiotherapy within 14 days or focal radiation within 7 days * Major surgery (as defined by the investigator) within 14 days * Plasmapheresis within 14 days * Received either of the following: * An allogeneic SCT within 6 months before the first dose of study treatment. Participants who received an allogeneic transplant must be off all immunosuppressive medications during the 6 weeks before the start of study treatment administration without signs of graft-versus-host disease. * An autologous SCT within 12 weeks before the start of study treatment administration * A maximum cumulative dose of corticosteroids of ≥ 140 mg of prednisone or equivalent within 14-day period before the first dose of study drug (does not include pretreatment medications). * Prior organ transplant requiring systemic immunosuppressive therapy * History of ≥ Grade 2 hemorrhage within 30 days of enrollment * Patient requiring ongoing treatment with chronic, therapeutic dosing of anticoagulants (e.g., Warfarin, low molecular weight heparin, Factor Xa inhibitors) can be enrolled with approval of the PI. * History or presence of clinically relevant CNS pathology such as epilepsy, seizure, paresis, aphasia, stroke, subarachnoid hemorrhage or other CNS bleed, severe brain injuries, dementia, Parkinson's disease, cerebellar disease, organic brain syndrome, or psychosis. 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