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 52 weeks or until the subject withdraws consent or until the investigator determines that the subject needs to be discontinued.

Inclusion Criteria:

• ECOG score 0-2.
• ≥18 years of age.
• Multiple myeloma must be measurable by central laboratory assessment:

Serum monoclonal paraprotein (M-protein) level ≥0.5 g/dL or urine M-protein level ≥200 mg/24 hours; or

Light chain multiple myeloma without measurable disease in the serum or the urine:

Serum immunoglobulin free light chain (FLC) ≥10 mg/dL and abnormal serum immunoglobulin kappa lambda FLC ratio.

Exclusion Criteria:

• Prior treatment with any BCMA-targeted therapy.
• Known active CNS involvement or exhibits clinical signs of meningeal involvement of multiple myeloma.
• Plasma cell leukemia , Waldenström's macroglobulinemia, POEMS syndrome (polyneuropathy, organomegaly, endocrinopathy, monoclonal protein, and skin changes), or primary amyloid light-chain amyloidosis.

Study Protocol