YTS109 in Pediatric Relapsed/Refractory Autoimmune Diseases

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1. Age 5 to \<18 years at screening; sex not restricted.
2. CD19 positivity: Presence of CD19-positive B cells in peripheral blood, confirmed by flow cytometry.
3. Adequate major organ function, meeting all of the following criteria:

1）Bone marrow function:

1. Absolute neutrophil count (ANC) ≥ 1.0 × 10\^9/L (no colony-stimulating factor use within 2 weeks prior to testing; neutropenia attributable to the underlying disease may be allowed);

2. Hemoglobin ≥ 60 g/L. 2）Hepatic function: ALT ≤ 3 × ULN (exceptions allowed for elevations attributable to the underlying disease); AST ≤ 3 × ULN (exceptions allowed for elevations attributable to the underlying disease); Total bilirubin (TBIL) ≤ 1.5 × ULN (exceptions allowed for elevations attributable to the underlying disease).

   3）Renal function: Estimated glomerular filtration rate (eGFR) ≥ 60 mL/min/1.73 m², calculated using the Schwartz formula (exceptions allowed for reduced renal function attributable to the underlying disease).

   4）Coagulation: International normalized ratio (INR) ≤ 1.5 × ULN and prothrombin time (PT) ≤ 1.5 × ULN.

   5）Cardiac status: Hemodynamically stable. 4. Females of childbearing potential must be not pregnant and not breastfeeding during screening and throughout the study period.

   5. The participant and the legal guardian are willing to participate, provide written informed consent, and can comply with study procedures and follow-up.

   Specific inclusion criteria:

   Recurrent refractory systemic lupus erythematosus

   1. Meets the 2019 EULAR/ACR classification criteria for systemic lupus erythematosus (SLE).

   2. Active disease, defined as either: SELENA-SLEDAI ≥ 6 and at least one BILAG-2004 organ domain score of A (severe activity) or two domains scored B (moderate activity), or a combination thereof; or SELENA-SLEDAI ≥ 8.

   3. Relapsed/refractory or intolerant to conventional therapy, defined as one of the following: Inadequate response after >3 months of conventional therapy; or intolerance to treatment-related adverse effects; or Disease flare/recurrence after achieving remission based on the LLDAS criteria. Conventional therapy is defined as treatment with glucocorticoids and cyclophosphamide, and one or more of the following immunomodulatory agents: antimalarials, azathioprine, mycophenolate mofetil, methotrexate, leflunomide, tacrolimus, cyclosporine, or any biologic agent, including rituximab, belimumab, and telitacicept.

   4. If renal involvement is present, a kidney biopsy must have been performed within 12 months prior to treatment, demonstrating active lesions or predominantly active lesions on pathology.

   Relapsing refractory/progressive diffuse systemic sclerosis

   1. Meets the 2013 ACR classification criteria for systemic sclerosis and is consistent with the diffuse cutaneous subtype (dcSSc).
   2. Positive for any antinuclear antibody (ANA) or systemic sclerosis-associated autoantibody.
   3. Evidence of diffuse cutaneous skin sclerosis and/or active interstitial lung disease (ILD), defined as ground-glass opacities on high-resolution computed tomography (HRCT).
   4. Inadequate response to conventional therapy for >3 months or disease relapse/recurrence after achieving remission. Conventional therapy is defined as treatment with glucocorticoids and cyclophosphamide, and one or more of the following immunomodulatory agents: antimalarials, azathioprine, mycophenolate mofetil, methotrexate, leflunomide, tacrolimus, cyclosporine, JAK inhibitors, or any biologic agent, including rituximab, tocilizumab, TNF-α inhibitors (etanercept, adalimumab, infliximab), and telitacicept.
   5. Progressive disease, defined as either:

   1） Rapid skin progression: mRSS increase >25%; or 2） Progressive lung disease: FVC decline ≥10%, or FVC decline ≥5% accompanied by DLCO decline ≥15%.

   Note: Criterion 4 or 5 must be met (either one is sufficient).

   Recurrent refractory/progressive inflammatory myopathy:

   1. Meets the 2017 EULAR/ACR classification criteria for idiopathic inflammatory myopathies (including dermatomyositis [DM], polymyositis [PM], antisynthetase syndrome [ASS], and necrotizing myopathy [NM]).

   2. Positive for myositis-specific and/or myositis-associated autoantibodies. 3. Evidence of active disease meeting either of the following:

   1) Muscle involvement: CMAS \< 30 and at least two abnormal findings among the following core measures: Physician Global Assessment (PhGA) ≥ 2, Patient/Parent Global Assessment (PtGA) ≥ 2, extra-muscular disease activity score ≥ 2, and/or serum muscle enzymes ≥ 1.5 × ULN; or 2) Interstitial lung disease (ILD): CMAS ≥ 30 but with active ILD, defined as ground-glass opacities on high-resolution computed tomography (HRCT).

   4. Inadequate response to conventional therapy for >3 months, or intolerance to treatment-related adverse effects, or relapse/recurrence after achieving remission. Conventional therapy is defined as treatment with glucocorticoids and cyclophosphamide, and one or more of the following immunomodulatory agents: antimalarials, intravenous immunoglobulin (IVIG), azathioprine, mycophenolate mofetil, methotrexate, leflunomide, tacrolimus, cyclosporine, JAK inhibitors, or any biologic agent, including rituximab, tocilizumab, TNF-α inhibitors (etanercept, adalimumab, infliximab), and telitacicept.

   5. Progressive disease, defined as worsening myositis or rapidly progressive interstitial lung disease (RP-ILD).

   Note: Criterion 4 or 5 must be met (either one is sufficient). Recurrent refractory sjogren's syndrome

   1. Meets the 2002 American-European Consensus Group (AECG) classification criteria for primary Sjögren's syndrome or the 2018 Japanese classification criteria.
   2. Active disease, defined as ESSDAI ≥ 6.
   3. Positive for anti-SSA/Ro antibodies.
   4. Inadequate response to conventional therapy for >3 months or relapse/recurrence after achieving remission. Conventional therapy is defined as treatment with glucocorticoids and cyclophosphamide, and one or more of the following immunomodulatory agents: antimalarials, azathioprine, mycophenolate mofetil, methotrexate, leflunomide, tacrolimus, cyclosporine, or any biologic agent, including rituximab, belimumab, and telitacicept.

   Recurrent/refractory ANCA-associated vasculitis:

   1. Meets the 2022 ACR/EULAR classification criteria for ANCA-associated vasculitis, including microscopic polyangiitis (MPA), granulomatosis with polyangiitis (GPA), and eosinophilic granulomatosis with polyangiitis (EGPA).
   2. Positive for ANCA, defined as MPO-ANCA and/or PR3-ANCA positivity.
   3. Active vasculitis, defined as Paediatric Vasculitis Activity Score (PVAS) ≥ 15 (maximum score 65).

   4. Inadequate response to conventional therapy for >3 months or relapse/recurrence after achieving remission. Conventional therapy is defined as treatment with glucocorticoids and cyclophosphamide, and one or more of the following immunomodulatory agents: azathioprine, mycophenolate mofetil, methotrexate, leflunomide, tacrolimus, cyclosporine, or any biologic agent, including rituximab, belimumab, and telitacicept.

      Recurrent refractory/catastrophic antiphospholipid syndrome:

   1. Meets the 2006 revised Sydney classification criteria for primary antiphospholipid syndrome (APS).

   2. Positive for antiphospholipid antibodies (aPL) at medium to high titers, defined as lupus anticoagulant (LA), anti-β2-glycoprotein I (anti-β2GPI) IgG/IgM, and/or anticardiolipin (aCL) IgG/IgM, with ≥2 positive tests at least 12 weeks apart.

   3. Relapsed/refractory APS, defined as recurrent thrombosis despite standard-of-care therapy, including:

   1. Anticoagulation with warfarin or another vitamin K antagonist (VKA) with INR maintained within the target therapeutic range, or therapeutic-dose low-molecular-weight heparin (LMWH), and
   2. Prior treatment with glucocorticoids and cyclophosphamide, with subsequent recurrent thrombosis.

   4. Catastrophic antiphospholipid syndrome (CAPS), defined by fulfillment of all four criteria:1) Involvement of three or more organs, systems, and/or tissues; 2) Development of manifestations within one week; 3)Histopathologic confirmation of small-vessel thrombosis/occlusion in at least one organ or tissue; 4) aPL positivity.

   Note: Criterion 3 or 4 must be met (either one is sufficient).

Exclusion Criteria:

1. History of severe drug allergy or a known allergic predisposition.
2. Presence of, or suspected uncontrolled infection requiring treatment, including fungal, bacterial, viral, or other infections.
3. Central nervous system (CNS) disorders, except for prior seizures, psychosis, organic brain syndrome, cerebrovascular accident, encephalitis, or CNS vasculitis attributable to the underlying disease, as determined by the investigator.
4. Cardiac dysfunction deemed unable to tolerate study treatment (i.e., inadequate cardiac function at the investigator's discretion).
5. Known congenital immunoglobulin deficiency.
6. Presence of severe congenital structural malformations or syndromic birth defects (e.g., severe cardiovascular malformations, severe CNS malformations), or a confirmed diagnosis of a severe inherited metabolic disorder that, in the investigator's judgment, may significantly increase trial-related risk or interfere with compliance and interpretation of results.
7. History of malignancy within the past 5 years.
8. End-stage renal disease.
9. Evidence of certain chronic/active infections, including: HBsAg-positive, or HBcAb-positive with peripheral blood HBV DNA above the upper limit of detection; Anti-HCV antibody positive with detectable HCV RNA; HIV antibody positive; Positive syphilis test.
10. Psychiatric illness or severe cognitive impairment.
11. Participation in another clinical trial within 3 months prior to enrollment.
12. Use of immunosuppressive agents with therapeutic effects on the underlying disease within five half-lives prior to enrollment, or use of biologic agents within 4 weeks prior to enrollment.
13. Pregnant or planning pregnancy (females).
14. Any other condition that, in the investigator's opinion, makes the participant unsuitable for enrollment in this study.