The purpose of this study is to evaluate the effects of the addition of SG301 injection to pomalidomide and dexamethasone in subjects with relapsed or refractory multiple myeloma.

This is a randomized, placebo-controlled, double-blind, multicenter phase III clinical study to compare SG301 injection in combination with pomalidomide and dexamethasone versus placebo in combination with pomalidomide and dexamethasone in patients with relapsed or refractory multiple myeloma who have received at least 1 prior treatment regimen with both lenalidomide and a proteasome inhibitor and have demonstrated disease progression.

This study consists of two stages. Stage 1 is the dose exploration stage to confirm the recommended stage 2 dose of SG301 injection in combination with pomalidomide and dexamethasone in patients with relapsed/refractory multiple myeloma. Stage 2 is the randomized controlled stage of SG301 injection in combination with pomalidomide and dexamethasone versus placebo in combination with pomalidomide and dexamethasone in patients with relapsed/refractory multiple myeloma.

Inclusion Criteria:

1. Understand and voluntarily sign the informed consent form (ICF).
2. Males and females aged 18-75 years (inclusive)
3. Eastern Cooperative Oncology Group (ECOG) Performance Status score of 0, 1 or 2
4. Expected survival time of ≥3 months.
5. Subjects had a documented diagnosis of multiple myeloma with evidence of measurable disease.
6. Subjects had received at least 1 prior lines of anti-myeloma therapy, which must include lenalidomide and a proteasome inhibitor (bortezomib, carfilzomib or ixazomib) given alone or in combination.
7. Subjects must have documented evidence of PD on or after the last regimen.
8. Adequate function of vital organs
9. Women of childbearing potential (WOCBP) must agree to follow instructions for methods of contraception for 4 weeks before the start of study treatment, for the duration of study treatment, and for 6 months after cessation of SG301 or 4 weeks after cessation of pomalidomide, whichever is longer. WOCBP must have 2 negative serum or urine pregnancy tests, one 10-14 days prior to start of study treatment and one 24 hours prior to the start of study treatment.

Exclusion Criteria:

1. Primary refractory multiple myeloma defined as participants who had never achieved at least a minimal response (MR) with any treatment during the disease course.
2. Bone independent extramedullary disease at screening.
3. Subjects who are primary refractory to a prior CD38 monoclonal antibody therapy.
4. Previous exposure to pomalidomide.
5. Subject has received chemotherapy or small molecule antitumor therapy within 2 weeks or 5 pharmacokinetic half-lives of the treatment, whichever is shorter, before the first dose of study treatment;
6. Subject has received tumor biotherapy within 4 weeks or 5 pharmacokinetic half-lives of the treatment, whichever is shorter, before the first dose of study treatment;
7. Subject has received investigational agents within 4 weeks or 5 pharmacokinetic half-lives of the treatment, whichever is shorter (but not less than 14 days), before the first dose of study treatment;
8. Active hepatitis B, or C.
9. Known HIV infection.
10. Known active tuberculosis or positive treponema pallidum antibodies.
11. Subjects with clinical significant organ dysfunction that does not meet the study needs.
12. Previous allogenic stem cell transplant or autologous stem cell transplantation (ASCT) before the first dose of study treatment.
13. Known allergy to any component of the investigational medicinal product.
14. Any concurrent medical or psychiatric condition or disease (eg, active systemic infection, uncontrolled diabetes, acute diffuse infiltrative pulmonary disease) that is likely to interfere with the study procedures or results or that, in the opinion of the investigator, would constitute a hazard for participating in this study.

Stage 1#Open-label Stage 2#Double-blind