This study is a 2-part study with a dose-escalation part and a dose-expansion part. The aim of the dose-escalation part is to determine the maximum tolerated dose (MTD) and/or establish the recommended Phase 2 dose (RP2D) in the Chinese population, in order to select the treatment dose for the dose-expansion part. The dose-escalation part will be followed by the dose-expansion part once the MTD(s) and/or RP2D of HDP-101 monotherapy in the Chinese population have been determined. The dose-expansion part of the study is intended to collect preliminary evidence of antitumor activity and to confirm the safety of the HDP-101 as monotherapy in Chinese patients with r/r MM.

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 (Dose-escalation part only: patients with non-secretory or oligo-secretory myeloma (NSMM) not meeting the measurability criteria are eligible).
• Adequate organ system function as defined in protocol.

Exclusion Criteria:

• 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 therapy.
• Patients with positive hepatitis B virus (HBV) infection or positive hepatitis C virus (HCV) infection.
• Current active liver or biliary disease.
• Pregnancy or breast feeding.
• Pneumonia or symptomatic pneumonitis.