A Safety Study of SEA-BCMA in Patients With Multiple Myeloma

This trial will study SEA-BCMA to find out whether it is an effective treatment for multiple myeloma (MM) and what side effects (unwanted effects) may occur. The study will have several parts. In Parts A and B, participants get SEA-BCMA by itself. This part of the study will find out how much SEA-BCMA should be given for treatment and how often. It will also find out how safe the treatment is and how well it works. In Part C of the study, participants will get SEA-BCMA and dexamethasone. In Part D, participants will get SEA-BCMA, dexamethasone, and pomalidomide. Dexamethasone and pomalidomide are both drugs that can be used to treat multiple myeloma. These parts of the study will find out whether these drugs are safe when used together.