This is a Phase I, single-center, single-arm, open-label clinical study to evaluate the safety, tolerability, and preliminary efficacy of SL4903 autologous T-cell injection (CAR-T cell therapy) in adult patients with relapsed or refractory multiple myeloma (r/r MM) who have failed prior standard therapies.

The study employs a "3+3" dose-escalation design with three planned dose levels (1×10⁶, 2×10⁶, and 3×10⁶ CAR+ cells/kg). Approximately 9 to 18 evaluable subjects will be enrolled to determine the maximum tolerated dose (MTD) and recommended Phase II dose (RP2D), and to characterize the safety profile and potential anti-myeloma activity of SL4903.

Inclusion Criteria

Subjects must meet ALL of the following conditions:

1. Age 18-75 years; either sex.

2. Able to understand the study and provide voluntary written informed consent.

3. Histologically and/or cytologically confirmed multiple myeloma according to IMWG 2016 criteria, meeting the following conditions:

   1. Patients must have received at least one prior line of anti-myeloma therapy, including treatment with proteasome inhibitors (PIs), immunomodulatory drugs (IMiDs), and anti-CD38 monoclonal antibodies, or patients with multiple myeloma refractory to proteasome inhibitors, immunomodulatory drugs, and anti-CD38 monoclonal antibodies;
   2. Documented evidence of disease progression or failure to achieve complete response (CR) following the last line of therapy, as assessed by the investigator according to IMWG criteria.

4. Measurable disease at screening, defined as meeting one or more of the following criteria:

   • Serum M-protein ≥0.5 g/dL
   • Urinary M-protein ≥200 mg/24 h
   • Abnormal serum FLC ratio (<0.26 or >1.65) with involved FLC ≥10 mg/dL
   • Soft tissue extramedullary disease (EMD) identified by radiographic imaging with a longest diameter ≥ 2 cm

5. ECOG performance status 0-2.

6. Adequate organ function:

   1. Serum creatinine ≤150 µmol/L or calculated creatinine clearance (Cockcroft-Gault) ≥40 mL/min (may be relaxed for acute MM-related renal impairment at investigator discretion).
   2. Total bilirubin ≤2×ULN; ALT ≤3×ULN; AST ≤3×ULN.
   3. Normal diastolic function on echocardiography, LVEF ≥50 %, no significant arrhythmia.
   4. No active pulmonary infection; oxygen saturation on room air >90 %.

7. No contraindication to leukapheresis: Hemoglobin ≥ 60 g/L, platelets ≥ 50 × 10⁹/L, and lymphocytes ≥ 0.3 × 10⁹/L.

8. Life expectancy >12 weeks.

9. Women of child-bearing potential must have a negative urine pregnancy test and must not be breastfeeding; all men and women with reproductive potential must use effective contraception throughout the study.

Exclusion Criteria

A subject will be excluded if ANY of the following apply:

1. History of severe immediate hypersensitivity to any study drug.

2. Central nervous system (CNS) disorders such as epilepsy, paralysis, aphasia, stroke, severe brain injury, dementia, Parkinson's disease, or neuropathy (resolved cases without residual symptoms may be exempted at the investigator's discretion). Active CNS involvement, prior CNS myelomatous disease, or meningeal/spinal cord involvement signs must be excluded.

3. Prior traumatic brain injury, cerebrovascular accident, significant cerebral ischemia, or intracranial hemorrhage.

4. Concurrent uncontrolled malignancies other than adequately treated cervical carcinoma in situ, basal-cell or squamous-cell skin carcinoma, localized prostate cancer after radical surgery, ductal carcinoma in situ after radical surgery, or thyroid cancer after curative surgery.

5. Clinically significant cardiovascular disease, e.g., uncontrolled or symptomatic arrhythmia, congestive heart failure, or NYHA class III/IV cardiac disease; myocardial infarction, coronary angioplasty/stenting, unstable angina, or other clinically relevant cardiac disorders within 12 months before enrollment.

6. Any severe comorbidity or condition judged by the investigator to increase subject risk or interfere with the study, including but not limited to liver cirrhosis or recent major trauma.

7. Prior BCMA- and/or GPRC5D-directed CAR-T therapy.

8. Allogeneic hematopoietic stem-cell transplantation within 6 months before screening, or any immunosuppressive therapy for graft-versus-host disease during screening.

9. Autoimmune disease, immunodeficiency, or need for immunosuppressants (except low-dose corticosteroids).

10. Uncontrolled active infection, including but not limited to active tuberculosis; suspected or proven uncontrolled fungal, bacterial, viral, or other infections.

11. Live attenuated vaccine within 4 weeks before leukapheresis.

12. Active hepatitis (HBV-DNA or HCV-RNA above the lower limit of detection), syphilis infection, congenital or acquired immunodeficiency including HIV, EBV or CMV viremia (DNA above the lower limit of detection).

13. History of alcohol abuse, drug abuse, or psychiatric illness.

14. Inability to meet the following washout requirements prior to PBMC collection:

    1. Corticosteroids: no more than 5 mg prednisone (or equivalent) within 72 h.
    2. Anti-tumor therapies: targeted therapy, epigenetic therapy, investigational drugs/devices-discontinued ≥14 days or ≥5 half-lives (whichever is longer); anti-myeloma monoclonal antibodies-≥21 days; cytotoxic therapy-≥14 days; proteasome inhibitors-≥14 days; immunomodulatory drugs-≥7 days.
    3. Radiotherapy: completed ≥4 weeks before leukapheresis, except if the radiation field covers ≤5 % of marrow reserves.
    4. Anti-T-cell antibodies (e.g., alemtuzumab): discontinued ≥8 weeks.

15. Any condition judged by the investigator to render the subject unsuitable for enrollment.