In Vivo BCMA/GPRC5D Tandem Dual CAR-T Therapy for Relapsed/Refractory Plasma Cell Neoplasms

Inclusion Criteria: * Voluntary signing of informed consent by the subject or legally authorized representative, with willingness and ability to comply with scheduled visits, study treatment, laboratory tests, and other study procedures. * Diagnosis of relapsed or refractory plasma cell neoplasms meeting the following criteria: 1. Clonal plasma cells confirmed to be BCMA and/or GPRC5D positive by flow cytometry or immunohistochemistry; 2. Previously treated with at least 2 lines of anti-plasma cell neoplasms therapy, with at least 1 complete treatment cycle for each line, and evidence of disease progression within 12 months after the most recent anti-plasma cell neoplasms treatment, or being refractory to both immunomodulatory drugs and proteasome inhibitors, with disease progression within 2 months after the most recent anti-plasma cell neoplasms treatment (according to the IMWG diagnostic criteria) * Age 18 to 75 years (inclusive), male or female. * Eastern Cooperative Oncology Group (ECOG) performance status of 0 to 2. * Life expectancy \> 3 months from the date of informed consent. * Hemoglobin (HGB) ≥ 60 g/L (transfusion allowed). * Adequate organ function (hepatic, renal, cardiac, and pulmonary): 1. Creatinine ≤ 2 × ULN; 2. Left ventricular ejection fraction (LVEF) ≥ 50%; 3. Oxygen saturation \> 90%; 4. Total bilirubin ≤ 1.5 × ULN; ALT and AST ≤ 2.5 × ULN. * Willingness to use highly effective contraception from signing of informed consent until 1 year after SL4903 infusion. Exclusion Criteria: * Severe cardiac dysfunction with left ventricular ejection fraction (LVEF) \< 50%. * History of severe pulmonary impairment. * Concurrent diagnosis of another active malignancy. * Uncontrolled active infection. * History of severe autoimmune disease or primary immunodeficiency. * Active hepatitis (defined as HBV DNA or HCV RNA above the lower limit of detection). * Human immunodeficiency virus (HIV) infection or acquired immunodeficiency syndrome (AIDS), or active syphilis. * History of severe hypersensitivity to biological products (including antibiotics). * Allogeneic hematopoietic stem cell transplant recipients with ongoing acute graft-versus-host disease (GVHD) despite discontinuation of immunosuppressive therapy for at least one month prior to screening. * Any other severe comorbidities or laboratory abnormalities that, in the investigator's opinion, would increase the risk to the subject or interfere with study results, rendering the subject unsuitable for participation. * Pregnant or breastfeeding women (including women of childbearing potential who are pregnant or lactating).