Single-Arm, Open, Multi-Center Phase II Clinical Trial of the Efficacy, Safety, Pharmacokinetics, and Immunogenicity of GR1803 Injection in Patients With Relapsed/Refractory Multiple Myeloma Complicated by Extramedullary Plasmacytoma

All subjects will receive GR1803 injection until intolerable toxicity or investigator-assessed disease progression occurs (except in cases of disease progression due to discontinuation of the drug as a result of an adverse event) or until the subject has been administered the drug for 2 years or until the subject withdraws consent or until the investigator determines that the subject needs to be discontinued.