Mizoribine pulse therapy for a pediatric patient with steroid-resistant nephrotic syndrome.

A 12-year-old Japanese boy was referred to our hospital with a 2-month history of persistent proteinuria. Despite urinary protein excretion in the nephrotic range, associated with hypoproteinemia, the patient did not complain of any disability. A percutaneous renal biopsy revealed minor glomerular abnormalities, without any evidence of immune complex deposition. Therapy with prednisolone (60 mg/day) was initiated, and while the proteinuria decreased after 4-week therapy, elevated urinary protein excretion persisted thereafter, at 1-2 g/day. Because of the steroid-resistant proteinuria, mizoribine (MZR), was started at 150 mg/day (3 mg/kg), administered as a single daily dose an immunosuppressive agent, in combination with prednisolone. Although there was some fluctuation in the urinary protein excretion, heavy proteinuria persisted for the next 4 weeks. The peak blood level of MZR was 0.9 microg/ml. Since we have previously reported the efficacy and safety of oral MZR pulse therapy, which is associated with higher peak serum MZR levels than conventional MZR therapy in selected patients with lupus nephritis, we adopted MZR pulse therapy for this patient, after obtaining informed consent. MZR was started at the daily dose of 300 mg (6 mg/kg), administered as a single dose before breakfast, twice a week (on Monday and Thursday). The peak blood level of MZR then increased to 1.29 microg/ml. Thereafter, despite a gradual reduction of the concomitantly administered prednisolone dose, the urinary protein excretion decreased rapidly to around 0.3 g/day and remained at this level thereafter. No adverse effects of MZR were observed. Based on these clinical observations, we suggest that oral MZR pulse therapy may be the treatment of choice in selected patients of steroid-resistant nephrotic syndrome, in addition to those of lupus nephritis.