This study will assess the safety, tolerability, pharmacokinetics (PK) and the therapeutic potential of HDP-101 in patients with plasma cell disorders including multiple myeloma.

The study will consists of two parts: a Part 1 dose escalation phase and a Part 2a expansion phase for safety, tolerability, PK, PD, and clinical activity testing. The study will enroll subjects with relapsed/refractory MM or other plasma cell disorders expressing BCMA. An adaptive 2-parameter Bayesian logistic regression model (BLRM) for dose-escalation with overdose control will be used in the dose-escalation phase for determination of the MTD or the RP2D. Dose-expansion phase of the study aims to collect preliminary evidence of antitumor activity and to confirm the safety of the HDP-101 as a monotherapy.

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.