This is an open - label study to evaluate the safety, pharmacokinetics, pharmacodynamics, and preliminary efficacy of SG301 Subcutaneous injection in children with frequently relapsing or steroid - dependent nephrotic syndrome.

This study will recruit children aged 6 - 18 with frequently relapsing or steroid - dependent nephrotic syndrome, who have morning urine protein < 1 + or urine protein / creatinine < 0.2 g/g (<20 mg/mmol) for at least three consecutive days after steroid treatment.

The study will stratify participants into two age groups:≥6 - <12 years and≥12 - <18 years. Recruitment will start with the 12 - 18 - year - old group. After collecting sufficient safety and clinical pharmacology data to support moving to lower age groups, the 6 - 12 - year - old group will be recruited. For every 1 to 3 participants enrolled, there will be a phased data review to guide further recruitment.

The trial has three phases: screening (D - 28~D - 1), treatment (12 - week), and follow - up (40 - week). The enrolled participants will be treated with 10 mg/kg SG301 Subcutaneous injection subcutaneously. They will receive weekly injections for the first six weeks, then every two weeks for three times, totaling nine injections.

Inclusion Criteria:

1. Aged 6 - 18 years.
2. Steroid-sensitive nephrotic syndrome with frequent relapses (FRNS) or steroid dependence (SDNS).
3. Normal renal function: eGFR≥90 ml/min/1.73m².
4. After steroid treatment, morning urine protein<1 + or urine protein/creatinine<0.2 g/g (<20 mg/mmol) for ≥3 consecutive days.
5. Within 7 days before enrollment, blood tests (without growth factors or transfusions) must meet: hemoglobin>80 g/L; platelets>75×10⁹/L; neutrophils>1.5×10⁹/L.
6. Prothrombin time/INR≤1.5×ULN, unless due to anticoagulation.
7. No Tacrolimus, cyclosporine A, mycophenolate mofetil, belimumab, levamisole, azathioprine, or cyclophosphamide in prior 2 months; no rituximab or obinutuzumab in prior 6 months.

Exclusion Criteria:

1. Family history of nephrotic syndrome, chronic glomerulonephritis, or uremia.
2. Alanine aminotransferase >2×ULN or total bilirubin>2×ULN with a sustained increase for 2 weeks.
3. HBsAg or HBcAb positive with Hepatitis B virus DNA above the lower limit of normal; Hepatitis C virus antibody - positive with Hepatitis C virus RNA positive; HIV antibody - positive.
4. Chronic active infections (e.g., Epstein-Barr Virus, Cytomegalovirus, tuberculosis) that may worsen with steroids/ immunosuppressants.
5. Acute active infection (excluding onychomycosis) requiring systemic antibiotics/antivirals.
6. Secondary nephrotic syndrome (e.g.,immunoglobulin A - associated, lupus nephritis); steroid - resistant nephrotic syndrome (SRNS).
7. Other autoimmune diseases, primary immunodeficiency, or malignancy.
8. Prior anti - cluster of differentiation 38 (CD38) treatment.
9. Live/attenuated vaccine receipt or major surgery (non - diagnostic) within 4 weeks before first dose.