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.