First In Human (FIH) Study of REGN5459 in Adult Patients With Relapsed or Refractory Multiple Myeloma (MM)

Key Inclusion Criteria:

1. Eastern Cooperative Oncology Group (ECOG) performance status ≤1
2. Patients must have myeloma that is response-evaluable according to the 2016 International Myeloma Working Group (IMWG) response criteria

Measurable disease is defined as 1 or more of the following:

1. Serum M-protein ≥1 g/dL,
2. Urine M-protein ≥200 mg/24-hour, and/or
3. Free light chain (FLC) assay with involved FLC level ≥10 mg/dL with an abnormal serum FLC ratio
4. A patient with Immunoglobulin A (IgA) myeloma but without measurable M-protein may be enrolled if quantitative IgA levels are ≥400 mg/dL and can be followed longitudinally
5. A patient with non-secretory MM may be considered for enrollment after discussion with the sponsor that includes the feasibility of an individualized plan for response assessment

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 imide drug (IMiD), and an anti-CD38 antibody, OR
2. Progression on or after an anti-CD38 antibody and having 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 function as measured by:

1. Platelet count > 50 x 109/L. A patient may not have received a platelet transfusion within 7 days to meet this platelet eligibility requirement.
2. ANC > 1.0 x 109/L. A patient may not have received granulocyte colony stimulating factor (G-CSF) within 2 days to meet this absolute neutrophil count eligibility requirement.
3. Hemoglobin > 8.0 g/dL

Adequate hepatic function, defined as:

1. Total bilirubin ≤1.5 x ULN
2. Transaminase (ALT, AST) ≤2.5 x ULN
3. Alkaline phosphatase ≤2.5 x ULN

Patients with Gilbert syndrome do not need to meet this total bilirubin requirement provided that the total bilirubin is unchanged from the baseline value.

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

1. A patient with a creatinine clearance by Cockcroft-Gault who does not meet eligibility criteria may be considered for enrollment if a measured creatinine clearance (based on 24-hour urine collection or other reliable method) is >30 mL/min
2. Life expectancy of at least 6 months

Key Exclusion Criteria:

1. Patients with known MM brain lesions or meningeal involvement
2. History of neurodegenerative condition or central nervous system (CNS) movement disorder, or patients with a history of seizure within 12 months before study enrollment are excluded
3. Cardiac ejection fraction <40% by echocardiogram or multi-gated acquisition scan (MUGA)
4. 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

1. Patients with HIV who have controlled infection (undetectable viral load and CD4 count above 350 cells/microliter either spontaneously or on a stable antiviral regimen) are permitted.
2. Patients with hepatitis B (Hepatitis B Surface Antigen Test positive [HepBsAg+]) who have controlled infection (serum HBV DNA polymerase chain reaction [PCR] that is below the limit of detection AND receiving anti-viral therapy for hepatitis B) are permitted.
3. Patients who are HCV antibody-positive (HCV Ab+) who have controlled infection (undetectable HCV RNA by polymerase chain reaction (PCR) either spontaneously or in response to a successful prior course of anti-HCV therapy) are permitted.
4. History of allogeneic stem cell transplantation at any time, or autologous stem cell transplantation within 12 weeks of the start of study treatment

NOTE: Other protocol defined Inclusion/Exclusion Criteria apply.