BCMA/CD3 Bispecific Antibody as Bridging Therapy Before CAR-T Cell Infusion in RRMM

Inclusion Criteria: * 1.Voluntary Participation: Ability to understand and voluntarily sign the informed consent form (ICF).2.Age ≥18 years.3.Confirmed symptomatic MM diagnosis per the Chinese Guidelines for Diagnosis and Management of Multiple Myeloma (2022 Revision).4.Relapsed/Refractory MM (RRMM) meeting one of the following:Triple-class refractory RRMM: Resistant to ≥1 immunomodulatory drug (IMiD), ≥1 proteasome inhibitor (PI), and ≥1 anti-CD38 monoclonal antibody.Penta-drug refractory RRMM: Resistant to ≥2 IMiDs, ≥2 PIs, and ≥1 anti-CD38 antibody.Secondary plasma cell leukemia (sPCL):MM diagnosis per Chinese Guidelines (2022), plusPeripheral blood plasma cells ≥20% of leukocytes or absolute circulating plasma cells \>2×10⁹/L.5.Successful apheresis for CAR-T cell manufacturing.6.ECOG performance status ≤3.7.No active infections:HBV-DNA negative, HCV-RNA negative, HIV negative.8.Liver function:Total bilirubin \<1.5×ULN (\<3×ULN for Gilbert's syndrome).AST/ALT \<3×ULN.9.Renal function: Calculated CrCl ≥30 mL/min (Cockcroft-Gault formula).10.Baseline oxygen saturation \>92% (room air).11.Hematologic criteria (within 7 days of screening):WBC ≥1.0×10⁹/L, ANC ≥1.0×10⁹/L, hemoglobin ≥70 g/L, andPlatelets ≥75×10⁹/L (or ≥50×10⁹/L if bone marrow plasma cells ≥50%).Investigator discretion permitted for clinical justification.12.Growth factor restrictions:2-week washout required for erythropoietin, G-CSF, GM-CSF, or thrombopoietin agonists (e.g., eltrombopag).13.Reproductive require…