Major Inclusion Criteria:

          1. Received at least 3 prior therapies (including proteasome inhibitor, immune modulatory
             drug, and an anti-CD38 antibody; patients should not be a candidate for or be
             intolerant of all established therapies known to provide clinical benefit in multiple
             myeloma).

          2. Measurable disease defined as at least one of the following:

               1. Serum M-protein ≥0.5 g/dL

               2. Urine M-protein ≥200 mg/24 hours

               3. Serum free light chain (FLC) assay: Involved FLC level ≥10 mg/dL (≥100 mg/L)

          3. Eastern Cooperative Oncology Group (ECOG) performance status ≤2.

          4. Adequate hematologic status, including:

               1. Absolute neutrophil count (ANC) ≥1000 cells/μL

               2. Platelet count ≥50,000/μL (without transfusions)

               3. Hemoglobin ≥8 g/dL

          5. Adequate renal function, including:

             a. Calculated creatinine clearance ≥30 mL/min using the formula of Cockcroft and Gault

          6. Adequate hepatic function, including

               1. Total bilirubin ≤1.5 × upper limit of normal (ULN), regardless of direct
                  bilirubin

               2. AST and ALT ≤3.0 × ULN (≤5.0× ULN if due to myeloma involvement)

               3. Alkaline phosphatase ≤3× ULN (≤5.0× ULN if due to myeloma involvement)

        Major Exclusion Criteria:

          1. Prior exposure to BCMA-targeting agents (Part 2 only)

          2. Concurrent treatment with anti-tumor necrosis factor alpha therapies, systemic
             corticosteroids (prednisone dose >10 mg per day or equivalent), or other immune
             suppressive drugs within the 2 weeks prior to Screening