The following criteria is provided for health care professionals.

Key Inclusion Criteria:

• Eastern Cooperative Oncology Group (ECOG) performance status ≤ 1
• Confirmed diagnosis of active Multiple Myeloma (MM) by International Myeloma Working Group (IMWG) diagnostic criteria
• Patients must have myeloma that is response-evaluable according to the 2016 IMWG response criteria.
• Phase 1 Dose Escalation: Patients with MM who have exhausted all therapeutic options that are expected to provide meaningful clinical benefit, either through disease relapse, treatment refractory disease or intolerance of the therapy and including either:
  1. Progression on or after at least 3 lines of therapy, or intolerance of therapy, including a proteasome inhibitor, an Immunomodulatory agent (IMiD), and an anti-CD38 antibody, OR
  2. Progression on or after an anti-CD38 antibody and have disease that is "double refractory" to a proteasome inhibitor and an IMiD, or intolerance of therapy. The anti-CD38 antibody may have been administered alone or in combination with another agent such as a proteasome inhibitor. Refractory disease is defined as lack of response or relapse within 60 days of last treatment.
  3. Phase 2: Patients must be triple-refractory, defined as being refractory to prior treatment with at least 1 anti-CD38 antibody, a proteasome inhibitor, and an IMiD.
• Adequate hematologic and hepatic function as defined in protocol
• Serum creatinine clearance by Cockcroft-Gault >30 mL/min

Key Exclusion Criteria:

• Diagnosis of plasma cell leukemia, amyloid light-chain amyloidosis,Waldenström macroglobulinemia (lymphoplasmacytic lymphoma), or POEMS syndrome (polyneuropathy, organomegaly, endocrinopathy, monoclonal protein, and skin changes)
• Patients with known MM brain lesions or meningeal involvement
• History of neurodegenerative condition, central nervous system (CNS) movement disorder, or patients with a history of seizure within 12 months prior to study enrollment are excluded.
• Continuous systemic corticosteroid treatment with more than 10 mg of prednisone or anti-inflammatory equivalent within 72 hours of start of study drug
• Treatment with any systemic standard or investigational anti-myeloma therapy within 5 half-lives or within 28 days before first administration of study drug, whichever is shorter.
• Prior treatment with any anti-BCMA antibody (including antibody drug conjugate or bispecific antibody) or BCMA-directed CAR T therapy
• Uncontrolled infection with human immunodeficiency virus (HIV), hepatitis B virus (HBV) or hepatitis C virus (HCV) infection; or other uncontrolled infection
• Has known allergy or hypersensitivity to components of REGN5458
• History of allogeneic stem cell transplantation at any time, or autologous stem cell transplantation within 12 weeks of the start of study treatment
• Known hypersensitivity to both allopurinol and rasburicase
• Pregnant or breastfeeding women
• Women of childbearing potential and men who are unwilling to practice highly effective contraception prior to the initial dose/start of the first treatment, during the study, and for at least 6 months after the last dose.
• Another malignancy in the past 5 years, with the exception of non-melanoma skin cancer that has undergone potentially curative therapy or in situ cervical carcinoma, or any other tumor that has been deemed to be effectively treated with definitive local control and with curative intent.

Note: Other protocol defined inclusion / exclusion criteria apply