GR1803 Injection in Patients With RRMM

This study is testing an experimental drug called GR1803 (a bispecific antibody targeting BCMA and CD3) in patients with relapsed or refractory multiple myeloma (a type of bone marrow cancer) that has also developed tumors outside the bone marrow (extramedullary plasmacytoma). The goal is to evaluate safety and effectiveness of this targeted therapy. **You may be eligible if...** - You are 18 years or older - You have been diagnosed with multiple myeloma that has also formed tumors outside the bone marrow (extramedullary plasmacytoma) - Your disease has relapsed or is no longer responding to treatment - You are in reasonable overall health (ECOG score 0-2) **You may NOT be eligible if...** - You have previously received any therapy specifically targeting BCMA - Your multiple myeloma has spread to the brain or spinal fluid (active CNS involvement) - You have a known allergy or intolerance to teclistamab or its ingredients - You have plasma cell leukemia, Waldenström's macroglobulinemia, POEMS syndrome, or primary amyloidosis Talk to your doctor to see if this trial is right for you.