Effect of Belimumab on Antibody Titers in Primary APS Patients

Inclusion Criteria:

• Age ≥ 18 years old.

• Positive for high-titer antiphospholipid antibodies according to the criteria of the "EULAR Guidelines for the Treatment of Antiphospholipid Syndrome in Adults"published in 2019.

• Stable APS regimen according to EULAR recommendations for antiphospholipid syndrome.

• Female patients who are not pregnant, breastfeeding, of childbearing potential, ornot using contraception.

Exclusion Criteria:

• Patients with a history of malignant tumor in the past 5 years, except for basalcell carcinoma or squamous cell carcinoma of skin or cervical carcinoma in situtreated locally, and there is no evidence of metastasis within 3 years;

• Patients with a history of primary immunodeficiency;

• Serious lack of IgG (IgG level < 400 mg/dL);

• IgA deficiency (IgA level < 10 mg/dL);

• Patients with a current history of infection;

• Patients with a current history of drug or alcohol abuse or dependence, or have ahistory of drug or alcohol abuse or dependence within 365 days before day 0;

• HIV test is historically positive or HIV screening is positive;

• Hepatitis status;

• Patients with a history of allergic reaction caused by injection of contrast agent,human or mouse protein or monoclonal antibody;

• Patients with other abnormal laboratory values with clinical significance;

• If women with reproductive potential (WCBP) are included, please refer to thefollowing special instructions;

• Patients with concurrent major medical or mental illnesss;

• Patients with diseases of liver, kidney, heart and other important organs,blood andEndocrine system;

• Patients who have been vaccinated with live vaccine in the last month;

• Patients who have participated in any clinical trial within 28 days before theinitial screening and/or within 5 times of the half-life of the study compound (whichever is longer);

• Patients who use B-cell targeted therapy drugs within one year, such as rituximab orepratuzumab;

• Patients who use tumor necrosis factor inhibitor and interleukin receptor blockerwithin one year;

• Patients who use Intravenous gamma globulin (IVIG) and prednisone ≥100mg/d for morethan 14 days within one year or plasma exchange;

• Patients with active infection (such as herpes zoster, HIV infection, activetuberculosis, etc.) during the screening period;

• Patients with depression or suicidal thoughts;

• Other conditions that the investigator considers would make the candidate unsuitablefor the study.