Nephrotic Syndrome Steroid-Dependent

The goal of this clinical trial is to learn if dupilumab works to treat severe nephrotic syndrome in children. It will also learn about the safety of dupilumab. The main questions it aims to answer are: * Does dupilumab reduce the time to relapse of nephrotic syndrome? * What medical problems do participants have when taking dupilumab? Researchers will compare dupilumab to a placebo (a look-alike substance that contains no drug) to see if dupilumab works to treat severe nephrotic syndrome. Participants will: * Receive an injection of dupilumab or placebo (just under the skin) every 2 weeks (if ≥30kg) or every 4 weeks (if \<30kg) for 24 weeks (6 months) * Wean down their prednisolone dose after starting the injections of dupilumab or placebo * Visit the clinic once every 2 weeks for checkups and tests * Keep a nephrotic diary to record down the urine dipstick result each day, together with the dose of prednisolone taken If protein returns in participant's urine, they will have completed the study at that point. However, if the participant is found to have received the placebo, they will be offered to receive dupilumab for up to 24 weeks.

Idiopathic Nephrotic Syndrome is a rare disease of the kidneys, which typically affects children. For most affected children there is the need of a prolonged treatment with drugs reducing the activity of the immune system, also resulting in many side effects. Those patients, who do not respond to treatment, are at risk of kidney damage and of dialysis or kidney transplantation. It is currently impossible to predict the response to treatment, leading to unnecessary therapies with side effects as well as unclear prognosis in the affected children. The response of the idiopathic nephrotic syndrome to medications acting on the immune system explains its important role in the occurrence of the disease. With this study we aim to obtain predictors of the response to treatment right at the beginning of the disease, to adapt the therapy avoiding needless side effects. This will be done evaluating the blood and urine of affected children using state of the art molecular characterisation. We will evaluate the genetic predisposition, the cell trait changes and the presence of molecules in blood and urine that may affect the interaction between the immune system and the kidneys. We expect that the findings will improve treatment of children with idiopathic nephrotic syndrome and reduce the number of children suffering from unnecessary drugs related side effects.