Study to Assess Safety of HDP-101 in Patients With Relapsed Refractory Multiple Myeloma

Inclusion Criteria: * Male or female aged ≥18 years. * Life expectancy \>12 weeks. * Eastern Cooperative Oncology Group Performance Status (PS) of 0 to 2. * 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.