First in Human (FIH) Study of REGN5458 in Patients With Relapsed or Refractory Multiple Myeloma

Key Inclusion Criteria:

- Eastern Cooperative Oncology Group (ECOG) performance status ≤ 1

- Patients must have symptomatic myeloma at the time of study entry with myeloma-related
organ damage or tissue dysfunction

- Patients must have myeloma that is measurable by either serum or urine evaluation of
the monoclonal component or by assay of serum free light chains (FLC)

- Disease progression based on IMWG criteria

- Patients with MM who have exhausted all therapeutic options known to provide clinical
benefit, either through disease relapse or intolerance or refusal 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.

- Adequate hematologic and hepatic function

- Serum creatinine clearance by Cockcroft-Gault >30 mL/min

Key Exclusion Criteria:

- Non-secretory myeloma

- Presence of plasma cell leukemia, Waldenström macroglobulinemia (lymphoplasmacytic
lymphoma), or POEMS syndrome (polyneuropathy, organomegaly, endocrinopathy, monoclonal
protein, and skin changes)

- Patients with known MM brain lesions or meningeal involvement with MM

- History of neurodegenerative condition or central nervous system movement disorder

- Continuous systemic corticosteroid treatment

- Treatment with any systemic standard or investigational anti-myeloma therapy within 5
half-lives or within 28 days prior to first administration of study drug, whichever is
shorter.

- History of allogeneic stem cell transplantation at any time, or autologous stem cell
transplantation within 12 weeks of the start of study treatment

- 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 (HBV) or
hepatitis C (HCV) infection; or other uncontrolled infection

- Known hypersensitivity to both allopurinol and rasburicase

- Pregnant or breastfeeding women

- Women of childbearing potential and men unwilling to practice highly effective
contraception during the study

Note: Other inclusion/ exclusion criteria may apply