An Exploratory Study of Fully Human Anti-B-Cell Maturation Antigen (BCMA)/G Protein-coupled Receptor, Class C Group 5 Member (GPRC5D) Chimeric Antigen Receptor T Cells (RD140) in Patients With Relapsed/Refractory Multiple Myeloma or Plasma Cell Leukemia

This study is divided into two stages: dose escalation and dose extension. The dose escalation stage sampled the "3+3" dose-escalation design, and set up three dose-increasing dose groups of 1.0×10\^5 CART cells/kg, 3.0×10\^5 CART cells/kg and 6.0×10\^5 CART cells/kg, and subjects will receive a single infusion of RD140. Each dose group level will include 3-6 subjects. In the dose expansion stage, 1\~2 dose groups were selected for expansion and 3\~6 subjects were included in each extended dose group, and the target dose was administered once.