The purpose of this study is to characterize the safety of QLS4131 injection and to determine the recommended Phase 2 dose (RP2D) and to further evaluate the efficacy and safety of QLS4131 injection in participants with relapsed or refractory multiple myeloma at the recommended Phase 2 dose.

Multiple myeloma (MM) is a malignant disease with abnormal proliferation of clonal plasma cell, which is often accompanied by multiple osteolytic damage, hypercalcemia, anemia, and kidney damage. QLS4131 is a novel tri-specific humanized antibody. The study consists 3 periods: screening phase, treatment phase and a post-treatment follow-up phase. The study will evaluate safety, tolerability, pharmacokinetics and preliminary antitumor activity of QLS4131 administered to participants with relapsed or refractory multiple myeloma. Total duration of study is up to 2 years.

Inclusion Criteria:

1. Age ≥18 years old, regardless of gender.
2. Subjects should be willing and able to comply with the study schedule and protocols.
3. Documented initial diagnosis of multiple myeloma according to International Myeloma Working Group (IMWG) diagnostic criteria.
4. Must have measurable disease as defined by the following: Serum M-protein greater than or equal to 1 g/dL; OR Urine M-protein greater than or equal to 200 mg/24 hours; OR Serum free light chain (FLC) assay; involved FLC level greater than or equal to 10 mg/dL provided the serum FLC ratio is abnormal.

Exclusion Criteria:

1. Known hypersensitivity to any of the ingredients of this product.
2. Known active central nervous system (CNS) involvement or exhibits clinical signs of meningeal involvement of multiple myeloma.
3. Has any active severe mental illness, medical illness, or other symptoms/conditions that may affect treatment, compliance, or the ability to provide informed consent, as determined by the investigator.