Sirolimus Monotherapy in the Treatment of Antiphospholipid Antibody Related Thrombocytopenia

The goal of this clinical trial is to evaluate the efficacy and safety of sirolimus in patients with anti-phospholipid antibody-associated thrombocytopenia. In the 6-month randomized, double-blind, placebo-controlled phase, participants will receive either sirolimus 1 mg once daily or matching placebo. Participants will be followed at 2 weeks, 1 month, 3 months, and 6 months after enrollment. Participants who do not achieve the primary endpoint at 6 months may enter a 3-month open-label extension and receive sirolimus 1.5 mg once daily, with follow-up through month 9. The main question is whether sirolimus improves the overall response rate at 6 months compared with placebo. Primary outcome: \- Overall response rate at 6 months Secondary outcomes: * Overall response rate at 1 month and 3 months * Complete response rate at 6 months * Partial response rate at 6 months * Change in anti-phospholipid antibody titers at 6 months * Change in oral glucocorticoid dosage at 6 months Exploratory outcomes: * Proportion and reasons for early discontinuation during the double-blind phase * Among participants who do not achieve the primary endpoint at 6 months and enter the open-label extension, overall response rate, complete response rate, partial response rate, and safety outcomes after 3 months of open-label treatment

Inclusion Criteria: * persistent positive of antiphospholipid antibody (either lupus anticoagulant, anti-cardiolipin antibody, or anti-b2GP1 antibody, at least two times with 12 weeks apart) * persistent thrombocytopenia (30-100×10\^9/L, at least for 2 weeks) Eligible concomitant treatment: * prednisone or equivalent dose less than 10mg per day is allowed, and dose should be stable for more than 2 weeks * hydroxychloroquine less than 400mg per day is allowed, and dose should be stable for more than 1 month * anti-platelet and/or anti-coagulant therapy is allowed, and strength should be the stable for 1 week * these following therapies should be discontinued for more than 5 half-lives before the enrollment, including thrombopoietin or thrombopoietin receptor antagonist, intravenous immunoglobulin, immunosuppressants, B cell inhibitors (Belimumab or Talitacicept) and B cell depletion therapy (Rituximab or Obinutuzumab). Exclusion Criteria: * fulling the criteria of other connective tissue disease other than antiphospholipid syndrome * received oral/intravenous antibiotics within 2 weeks before the enrollment. * new onset of thrombosis within 4 weeks before the enrollment. * apparent bleeding tendency. * life or organ threatening manifestations, includes but not limit to catastrophic antiphospholipid syndrome and thrombotic microangiopathy. * liver and renal dysfunction: ALT or AST more than three times of upper limit of normal range; eGFR\