Inclusion Criteria:
1. Able and willing to provide written informed consent and to comply with the study protocol;
2. Phase I: 18-70 years of age (inclusive) at the time of signing ICF; Phase II: ≥18 years of age;
3. Documented diagnosis of active or symptomatic multiple myeloma according to the 2014 IMWG diagnostic criteria; Phase I: at least 3 prior lines of systemic anti-myeloma therapy including at least one proteasome inhibitor (PI), one immunomodulatory drug (IMiD) and one anti-CD38 antibody, or no available standard treatment per investigator judgment, and relapsed or refractory to the last line of therapy or intolerant to prior treatment; Phase II: at least 2 prior lines including at least one PI, one IMiD and one anti-CD38 antibody, and relapsed or refractory to the last line of therapy or intolerant to prior treatment;
4. At least one measurable disease at screening： 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. Toxicity from prior anti-myeloma therapy recovered to Grade ≤1 per NCI CTCAE v6.0 (except alopecia, fatigue and Grade 2 peripheral neuropathy);
8. Left ventricular ejection fraction (LVEF) ≥50%;
9. Adequate hematologic, hepatic, renal and coagulation function: platelet count ≥75×10\^9/L (or ≥50×10\^9/L if ≥50% of bone marrow nucleated cells are plasma cells); absolute neutrophil count (NEUT) ≥1.5×10\^9/L; hemoglobin ≥75 g/L; AST and ALT ≤2.5×ULN; total bilirubin ≤1.5×ULN (≤3×ULN for participants with Gilbert syndrome); serum creatinine ≤1.5×ULN or creatinine clearance ≥40 mL/min; INR or PT ≤1.5×ULN; APTT ≤1.5×ULN; corrected serum calcium ≤14 mg/dL (≤3.5 mmol/L).
Exclusion Criteria:
1. Known active central nervous system (CNS) involvement or exhibits clinical signs of meningeal involvement of multiple myeloma.
2. Active infection within 14 days prior to the first MBS314 administration requiring anti-infective treatment or hospitalization (including infections requiring IV antibiotics and/or hospitalization within 4 weeks, or oral antibiotics within 2 weeks prior to first dose); participants with a positive SARS-CoV-2 test result, with or without symptoms or suspected infection, are excluded;
3. HBsAg and/or HBcAb positive with HBV DNA above the upper limit of normal; HCV antibody positive with HCV RNA above the upper limit of normal; HIV seropositive; Treponema pallidum antibody (TP-Ab) positive;
4. Received the following anti-myeloma treatments within the specified time frame prior to the first administration: targeted therapy, immunotherapy, antibody drugs, cytotoxic drugs, antibody-drug conjugates, radiotherapy, tumor embolization, epigenetic therapy or investigational drugs (or invasive investigational medical devices) within 21 days or 5 half-lives (whichever is shorter); PIs within 14 days; IMiDs within 14 days; anti-tumor Chinese herbal medicines within 14 days; cell therapy (including but not limited to CAR-T, NK cells, neoantigen vaccines, etc.) or autologous hematopoietic stem cell transplantation within 3 months; prior allogeneic hematopoietic stem cell transplantation;
5. Live, attenuated vaccines within 28 days prior to the first administration of MBS314, or expected to receive live, attenuated vaccines during the study period;
6. Major surgery within 28 days prior to the first administration of MBS314, or expected to undergo major surgery during the study treatment, or with serious unhealed wounds, trauma or ulcers (except diagnostic biopsy and central venous catheterization);
7. History of autoimmune diseases (except participants with autoimmune-related hypothyroidism on a stable dose of thyroid replacement therapy, and participants with well-controlled type 1 diabetes);
8. History of severe allergic reactions (≥Grade 3 per NCI CTCAE v6.0) to other antibody drugs, or known allergies or hypersensitivity to any components of MBS314;
9. Smoldering multiple myeloma, plasma cell leukemia (plasma cells \>2.0×10\^9/L), Waldenström macroglobulinemia, POEMS syndrome, or concurrent primary or secondary amyloidosis;
10. Prior treatment with agents targeting both BCMA and GPRC5D, including but not limited to BCMA/GPRC5D-targeted trispecific antibodies, chimeric antigen receptor T-cell (CAR-T) therapy, or simultaneous or sequential treatment with BCMA- and GPRC5D-targeted drugs; participants previously treated with only one of the BCMA or GPRC5D targets may enroll;
11. Other severe, poorly controlled comorbidities that may affect protocol compliance or interpretation of results, including but not limited to: clinically significant valvular heart disease; history of congestive heart failure; angina requiring medication or unstable angina; NYHA Class III or higher cardiac dysfunction; QTcF ≥470 ms (female) or ≥450 ms (male) or history of congenital long QT syndrome; myocardial infarction or acute coronary syndrome within 6 months prior to screening; thromboembolic or vascular events within 6 months prior to screening; clinically significant arrhythmias; poorly controlled hypertension (SBP \>160 mmHg or DBP \>100 mmHg); poorly controlled diabetes (fasting glucose ≥10 mmol/L); clinically significant pulmonary disease; uncontrolled pleural, pericardial or peritoneal effusions requiring repeated drainage or medical intervention;
12. Any other invasive malignancy within the past 3 years (except radically treated basal or squamous cell skin cancer, carcinoma in situ of the cervix, breast or bladder, or other malignancy with low metastasis or death risk \[5-year survival \>90%\]); progressive multifocal leukoencephalopathy; severe bleeding disorders such as hemophilia A, hemophilia B, von Willebrand disease, or spontaneous bleeding requiring transfusion or other medical intervention;
13. Pregnant or lactating women; women of childbearing potential who do not agree to use appropriate contraception (e.g., oral contraceptives, intrauterine device, barrier contraception or spermicide) during the treatment period and for at least 6 months after the last dose of study drug;
14. Men who do not agree to use barrier contraception during the treatment period and for at least 6 months after the last dose of study drug, and do not agree to have their partners use appropriate contraception;
15. Any other condition that, in the judgment of the investigator, makes the participant unsuitable for participation in this study.
