The anti-phospholipid syndrome (APS) is a systemic autoimmune disorder characterized by recurrent thrombosis and/or obstetrical morbidity along with persistent anti-phospholipid antibodies (APLA), including lupus anticoagulant (LA), anti-β2-glycoprotein I (anti-β2GPI) and/or anti-cardiolipin (aCL) antibodies. APS is one of the most common acquired thrombophilias, and unlike most of the genetic thrombophilias, ise associated with both venous and arterial thrombosis. The deep veins of the lower extremities and the cerebral arterial circulation are the most commonly affected venous and arterial sites, respectively. It has been posited that "two hits" are necessary to trigger a thrombotic event in antiphospholipid syndrome. This hypothesis argues that antiphospholipid antibodies provide the first hit by creating a generalized procoagulant state, which conspires with a second hit (often cryptic and potentially the result of subclinical vascular injury, stasis, or inflammation) that triggers thrombosis. The evidence that IgG antibodies targeting domain I of β2GPI contribute to the prothrombotic state of antiphospholipid syndrome is convincing. Meanwhile, data also support roles for other isotypes of aβ2GPI, antibodies that target heterotypic complexes of phospholipids and phospholipid binding proteins, and antibodies that target phospholipids directly. Given that most mechanistic studies have focused on either polyclonal IgG fractions isolated from patients with antiphospholipid syndrome or IgG anti-β2GPI, researchers unfortunately could not build a nuanced model of how diverse species of antiphospholipid antibodies conspire to cause various manifestations of antiphospholipid syndrome.

The Sapporo classification criteria for APS were first proposed in 1999, and updated at the Eleventh International Congress on Antiphospholipid Antibodies in Sydney in 2006.Patient must have both clinical and laboratory criteria to meet a diagnosis of APS. Clinical criteria include either objectively confirmed venous, arterial or small vessel thrombosis, or obstetric morbidity including the unexplained death of one or more morphologically normal fetuses at or beyond the 10th week of gestation, the premature birth of one or more morphologically normal neonates before the 34th week of gestation, and/or three or more unexplained, consecutive spontaneous abortions before the 10th week of gestation. Since these clinical manifestations are prevalent in the general population and may have a multifactorial etiology, laboratory investigations are central to the diagnosis of APS. The updated Sydney classification scheme also requires specific laboratory criteria: a lupus anticoagulant detected according to guidelines published by the International Society on Thrombosis and Hemostasis (ISTH), anticardiolipin (aCL) antibodies (IgG or IgM) exceeding 40 IgG or IgM antiphospholipid units, or anti-β2GPI antibodies (IgG or IgM) at levels exceeding the 99th percentile, measured by enzyme-linked immunosorbent assay (ELISA). To minimize the risk of making a diagnosis based on transient ntiphospholipid antibodies, the recommendations are to perform assays on two separate occasions, at least twelve weeks apart.The major changes made in the 2006 revision of the Sapporo criteria were that anti-β2GPI antibody was included for the first time and a recommendation was made to classify patients into those with only one positive APLA and those with two or three positive APLA, based on accumulating information suggesting that positivity in more than a single assay was associated with higher thrombotic risk.

Thrombocytopenia occurs in APS with a frequency ranging from 20% to 50%, and the estimated bleeding risk associated with it is much lower than the thrombotic risk associated with aPL. In some patients with a confirmed APS diagnosis, secondary immune thrombocytopenia (ITP) may lead to severe thrombocytopenia with consequent major bleeding. At the same time, the presence of antiphospholipid antibodies (aPL) in patients with a diagnosis of primary ITP has been reported in several studies, although with some specific characteristics especially related to the variety of antigenic targets. Even though it does not enter the APS defining criteria, thrombocytopenia should be regarded as a warning sign of a "high risk" APS and thus thoroughly evaluated. The presence of aPL in patients with ITP should be assessed as well to stratify the risk of paradoxical thrombosis. In detail, besides the high hemorrhagic risk in secondary thrombocytopenia, patients with a co-diagnosis of APS or only antibodies are also at risk of arterial and venous thrombosis. The pathogenesis of thrombocytopenia in antiphospholipid antibody-positive patients is not fully understood. However, based on currently available data, four different pathogenesis hypothesis were proposed, including secondary Immune thrombocytopenia, decreased platelet production, increased platelet pooling and increased platelet consumption. The APS associated thrombocytopenia treatment guidelines can be drawn lessons from the treatment strategies of ITP. Conventional treatment of adult APS associated thrombocytopenia includes first-line glucocorticoid and immunoglobulin therapy. B-cell targeted drugs are effective in the treatment of APS. Studies have found that rituximab can improve the clinical manifestations (including thrombocytopenia) and serological indicators in the late stage of APS treatment. Splenectomy is reserved for refractory patients who present with severe bleeding symptoms and are resistant to glucocorticoids and immunosuppressants. Plasmapheresis can be used in patients with CAPS when severe thrombocytopenia occurs. Thrombopoietin receptor agonists may increase the risk of thrombosis in APS patients and should be used with caution. When thrombosis coexists with bleeding events associated with thrombocytopenia, if platelet counts reach safe levels (\&gt; 50×109/L), glucocorticoid combined with anticoagulant therapy is recommended. It is difficult to treat APS patients with severe thrombocytopenia and severe bleeding. It has been reported in previous literature that platelets can be significantly increased in some patients after treatment with hormones or rituximab, immunosuppressants, etc., but it is easy to relapse and difficult to maintain. The decrease of aPL is not obvious and it is difficult to completely remove. It is a difficult point in the treatment of hematology.

Anti-CD38 antibody, which targets plasma cells, has been used in multiple myeloma in a number of clinical studies and has shown good therapeutic effects. In addition, the clinical trials of anti-CD38 antibody analogues such as fizumatumumab in the treatment of autoimmune diseases including membranous nephropathy and systemic lupus erythematosus (SLE) are also being carried out simultaneously. A case report of anti-CD38 antibody in the treatment of APS has been reported abroad, and it has been found that it can significantly reduce the titer of antiphospholipid antibodies. In the process of treating ITP patients with antiphospholipid antibodies, investigators found that anti-CD38 antibody could not only rapidly increase platelet count, but also significantly reduce the titer of antiphospholipid antibodies, which may have a significant therapeutic effect on APS.Therefore, the use of daratumumab to clear APS with secondary thrombocytopenia may be effective and represents a new therapeutic strategy.

Therefore, the investigators designed this clinical trial to evaluate the safety and efficacy of anti-CD38 antibody in the treatment of antiphospholipid syndrome with secondary thrombocytopenia.