This is a Phase I/Ⅱ, multicenter, open-label, dose-escalation study designed to evaluate the safety, tolerability, pharmacokinetics (PK), pharmacodynamics(PD) and efficacy of a novel asymmetric trivalent tri-specific humanized antibody, MBS314, administered by intravenous (IV) infusion in participants with relapsed or refractory multiple myeloma. This entry-to-human study is divided in 2 parts: a dose escalation part (Phase Ⅰa) and an expansion part (Phase Ⅰb/Ⅱ).

Inclusion Criteria:

1. Able and willing to provide written informed consent and to comply with the study protocol;
2. ≥18 years of age;
3. Documented diagnosis of multiple myeloma according to 2014 IMWG diagnostic criteria.
4. Phase Ⅰb/Ⅱ: At least one measurable disease: Serum monoclonal paraprotein (M-protein) ≥5 g/L or Urine M-protein ≥200 mg/24 hours or Serum immunoglobulin free-light chains (FLCs) ≥100 mg/L and abnormal kappa/lambda FLC ratio (<0.26 or >1.65)
5. Eastern Cooperative Oncology Group (ECOG) Performance Status score of 0 or 1.
6. Life expectancy ≥3 months.
7. Adequate hematologic, hepatic, and renal function.

Exclusion Criteria:

1. Known active central nervous system (CNS) involvement or exhibits clinical signs of meningeal involvement of multiple myeloma.
2. Participants with known active infection within 14 days prior to the first MBS314.
3. Infection with human immunodeficiency virus (HIV), hepatitis B or hepatitis C (including HBsAg, HBcAb positive with abnormal hepatitis B virus DNA or hepatitis C virus RNA).
4. Previously received anti-myeloma treatment within the specified time frame prior to the first administration.
5. Live, attenuated vaccines within 28 days prior to the first infusion of MBS314, or expected to receive live, attenuated vaccines during the study period.
6. Major surgery within 28 days prior to the first infusion of MBS314, or expected to undergo major surgery during the study treatment.
7. Participants with a history of autoimmune diseases.
8. Known severe allergic reactions to other antibodies, or known allergies or hypersensitivity to any components of MBS314.