Inclusion Criteria:  Male or female aged ≥18 years. Life expectancy >12 weeks. Eastern Cooperative Oncology Group Performance Status (PS) of 0 to 1. A confirmed diagnosis of active MM according to the diagnostic criteria established by the International Myeloma Working Group (IMWG). Must have undergone SCT or is considered transplant ineligible. Must have undergone prior treatments with antimyeloma therapy which must have included an immunomodulatory drug, proteasome inhibitor, and anti-CD38 treatment, alone or in combination. In addition, the patient should either refractory or intolerant to any established standard of care therapy providing a meaningful clinical benefit for the patient assessed by the Investigator. Measurable disease as per IMWG criteria. Adequate organ system function as defined in protocol.  Exclusion Criteria:  For patient entering the Phase 2a part only: Prior treatment with any approved or experimental BCMA-targeting modalities are not allowed. Known central nervous system involvement. Plasma cell leukemia. History of congestive heart failure. Autologous or allogenic SCT within 12 weeks before the first infusion or is planning for autologous SCT. Symptomatic graft versus host disease post allogenic hemopoietic cell transplant within 12 months prior to the first study treatment infusion. Radiotherapy within 21 days prior to the first study treatment infusion. History of any other malignancy known to be active. Known human immunodeficiency virus infection. Patients with active infection requiring systemic anti-infective. Patients with positive test results for hepatitis B surface antigen or Hepatitis B core antigen. Patients with positive test results for hepatitis C virus (HCV) infection. Current active liver or biliary disease.