This is a Phase I, open-label, multicenter study to evaluate the safety, tolerability, pharmacokinetics (PK), pharmacodynamics (PD), and preliminary efficacy of SHR-5421 in patients with multiple myeloma (MM).

Inclusion Criteria:

1. Age ≥ 18 years, male or female.
2. Expected survival ≥ 3 months.
3. ECOG performance status 0 or 1.
4. Diagnosed with active multiple myeloma per IMWG criteria.
5. Relapsed or refractory multiple myeloma and refractory to the most recent line of therapy.
6. Disease progression or non-response to the most recent therapy.
7. Measurable disease as defined by serum/urine M-protein or serum free light chain.
8. Adequate bone marrow, hepatic, renal, and coagulation function.
9. Female participants of childbearing potential and male participants with partners of childbearing potential must agree to use effective contraception.
10. Willing and able to provide written informed consent and comply with study procedures.

Exclusion Criteria:

1. Diagnosis of amyloidosis, plasma cell leukemia, Waldenström's macroglobulinemia, or POEMS syndrome.
2. Central nervous system involvement by tumor.
3. History of ≥ Grade 3 CRS with prior T-cell redirecting therapy.
4. Requirement for dialysis or plasmapheresis within 14 days prior to study drug.
5. Prior treatment-related AEs not recovered to ≤ Grade 1 (except alopecia, peripheral neuropathy, or controlled endocrinopathies).
6. Concurrent participation in another clinical study.
7. Prior anti-tumor therapy within specified washout periods.
8. Major surgery within 28 days prior to informed consent or planned during the study.
9. Live attenuated vaccine within 4 weeks prior to first dose or planned during the study.
10. Significant comorbidity that in the investigator's judgment would interfere with study participation or safety.
11. Active autoimmune disease (except controlled type I diabetes, hypothyroidism, or vitiligo).
12. Active infection.
13. Known hypersensitivity or contraindication to SHR-5421.