The purpose of this study is to evaluate the safety, tolerability, pharmacokinetics, pharmacodynamics, and efficacy of SCTC21C in participants with systemic lupus erythematosus

This study contains phase Ib and phase II studies. In phase Ib study, participants will be assigned to receive sequentially higher doses of SCTC21C to determine the recomended dose of SCTC21C for the randomized dose optimation-stage. In phase 2 study, 2 dose levels will be uased. A total of 120 participants will be randomized in a 1:1:1 ration to dose 1, dose 2 or placebo groups to better understand the exposure/efficacy/toxicity relationship.

Inclusion Criteria:

1. Aged 18-75 years;
2. Diagnosed with SLE for ≥12 weeks prior to screening, met the 2019 ACR/EULAR diagnostic criteria for SLE screening period;
3. SELENA-SLEDAI ≥8 at screening;
4. Positive immunological markers within 12 months prior to baseline;
5. Currently receiving ≥1 stable doses of standard treatment: oral corticosteroids, antimalarials, or conventional immunosuppressive drugs;
6. All male participants or females of reproductive potential must agree to use reliable contraception with their partner from signing the ICF through 5 months after the last dose of the study drug
7. Understanding of the study procedures and voluntary signing of the informed consent form

Exclusion Criteria:

1. Severe active or unstable lupus-related neuropsychiatric disorders;
2. Combined with active infections, malignant tumors, or other autoimmune diseases;
3. Catastrophic antiphospholipid syndrome;
4. severe lupus nephritis;
5. Study participants who received a live or attenuated vaccine within 28 days prior to baseline or during the screening period;
6. Received any investigational treatment within 30 days prior to baseline or within 5 half-lives of the investigational drug (whichever is longer);
7. Patients at high risk of severe clinical bleeding, or those requiring plasma transfusion, intravenous immunoglobulin, or acute blood products;;
8. Patients who experienced any of the following cardiovascular or cerebrovascular events within 24 weeks prior to baseline: myocardial infarction, unstable angina, ventricular arrhythmia, New York Heart Association (NYHA) Class II or higher heart failure, stroke, etc.;
9. In accordance with the protocol requirements, participants with abnormal laboratory test results;
10. Participants with positive results of viral serology, including Treponema pallidum, HIV, HCV, and HBV;
11. participants with a history of tuberculosis or latent tuberculosis infection;
12. Diagnosed with type 1 or type 2 diabetes with poor control;
13. Uncontrolled hypertension;
14. Intolerance to the study drug or contraindications, including a history of severe allergic reactions to monoclonal antibodies or any component of SCTC21C injection;
15. Required hospitalization for major surgery within 4 weeks prior to screening or within 12 weeks post-study drug administration;
16. History of solid organ or hematopoietic stem cell/bone marrow transplant, or expected to undergo transplant surgery during the study;
17. Pregnant or breastfeeding;
18. Any other condition that the investigator deems unsuitable for participation in the study