The MEMORINEPH study: Memory B cell level-based tailored treatment with Rixathon in children with complicated forms of steroid-sensitive Nephrotic syndrome

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• At screening, patients with any of the following characteristics will not be considered eligible for enrollment: 1. Corticosteroid resistance, defined as lack of response to 4 weeks of oral prednisone at standard dosage. 2. Onset of nephrotic syndrome after the age of 18. 3. Secondary nephrotic syndrome. 4. Past or present malignant tumors of any organ or system, even in the absence of active or previous metastatic signs or symptoms. 5. Pregnancy (defined as the condition of a female individual from conception until the end of gestation, confirmed by a positive laboratory test for beta human chorionic gonadotropin (hCG)) or breastfeeding. Women of childbearing age (from menarche onward) must use an effective contraceptive method (complete abstinence, barrier contraception, or hormonal contraception) throughout the duration of the study and for 12 months following the last study RTX infusion. 6. Lack of response to rituximab or mycophenolate mofetil, defined as persistence of nephrotic proteinuria >4 weeks despite such therapy. 7. Active severe infection, for example, hepatitis B (positive HBsAg, HBcAb), sepsis, or active tuberculosis (positive Quantiferon). 8. Severe immunosuppression, defined as congenital immunodeficiency and/or as circulating immunoglobulin levels 2x lower than the minimum normal range for age and/or as circulating neutrophil granulocytes <1.5x10^3/μl. 9. Renal insufficiency with eGFR < 15 ml/min/1.73 m² calculated using the modified Schwartz formula (for children) or the Cockcroft-Gault formula (for adults). 10. Severe heart failure (New York Heart Association class IV) or severe uncontrolled heart disease. 11. Known hypersensitivity to RTX treatment. 12. Known hypersensitivity to concomitant medications (prednisone, mycophenolate mofetil/mycophenolic acid, calcineurin inhibitors). 13. Use of live attenuated vaccines within 28 days prior to enrollment and throughout the duration of the study. 14. Weight < 10 kg. 15. Thrombocytopenia, defined as platelet level <100,000/mm². 16. Alanine aminotransferase (ALT) or aspartate aminotransferase (AST) levels >2.5x the upper limit of the normal range for age and sex.