This is a open-label to determine the efficacy and safety of IM21 CAR-T cells in adult with R/R multiple myeloma.

Inclusion Criteria:

• Diagnosis of MM with relapsed or refractory disease and have had at least 3 different prior lines of therapy including proteasome inhibitor .
• Evidence of cell membrane BCMA expression.
• Subjects must have measurable disease,including 1) Serum M-protein greater or equal to10 g/L. 2) Urine M-protein greater or equal to 200 mg/24 h. 3)Serum free light chain (FLC) assay: involved FLC level greater or equal to 100 mg/L provided serum FLC ratio is abnormal.
• ≥ 18 years of age at the time of signing informed consent.
• Estimated life expectancy >3 months.
• Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1.
• Women of childbearing age who had a negative blood pregnancy test before the start of the trial and agreed to take effective contraceptive measures during the trial period until the last follow-up; male subjects with fertility partners agreed to take effective contraceptive measures during the trial period until the last follow-up.
• Adequate organ function.
• Voluntarily sign informed consent form(s).

Exclusion Criteria:

• Subjects with graft versus host disease and need to use immunosuppressive agents.
• Subjects who had received chemotherapy or radiotherapy within 3 days prior to the blood collection period.
• Use of systemic steroids in combination within 5 days prior to the blood collection period (except for recent or current use of inhaled steroids)
• Subjects who had previously used any gene therapy product.
• Subjects with known central nervous system disease.
• Subjects with plasmacytic leukemia, Wallenian macroglobulinemia, POEMS syndrome, or primary light-chain amyloidosis.
• Subjects had the following cardiac conditions, including but not limited to unstable angina pectoris, myocardial infarction or coronary artery bypass graft in the 6 months prior to enrollment, severe arrhythmias with poor drug control;
• Subjects infected with active HBV or HCV, HIV, syphilis or other untreated active infections;
• Pregnant or lactating women.
• Subjects who have other uncontrolled diseases and are considered by the researchers to be unsuitable to participate in the study.
• Any situation that the researcher believes may increase the risk of subjects or interfere with the results of clinical trials.