Allogeneic UCB-derived, Dual-targeting BCMA/CD19 CAR-T for Relapsed/Refractory Multiple Myeloma

The purpose of this clinical trial is to learn if allogeneic cord blood-derived CAR-T cell drug works to treat Multiple Myeloma (MM) including Bone-related extramedullary (EMB) disease and extramedullary extraosseous disease(EME) in adults. It will also learn about the safety of the allogeneic cord blood-derived CAR-T cell drug. The main questions it aims to answer are: 1. What adverse events occur and the incidence rate of DLTs within 28 days and UCAR-T-related AEs within 28 days after the allogeneic cord blood-derived CAR-T cell injection for multiple myeloma (MM)? 2. Which dose level is the optimal biological dose (OBD)? 3. What is the overall responserate (ORR), including stringent complete response (sCR), completeresponse (CR),very good partial response (VGPR), partial response (PR), minimal Response (MR) and DOR, PFS, RFS, OS? Participants will: 1. be pretreated with FC regimen, fludarabine (30mg/m²/d, days -5, -4, and -3) and cyclophosphamide (300\~500 mg/m²/d, day -5,-4, and -3). 2. rest for 2 days on Day-2 and Day-1. Tumor burden should be re-evaluated and chemotherapy side effects assessment. 3. receive allogeneic cord blood-derived CAR-T cells infusion 4. Visit the clinic at D28, 1 month, 2 months, 3 months, 4 months, 6 months, 9 months and 1 year after CAR-T cells infusion.