*Purpose: Studies examining the use of leflunomide in treating BK polyomavirus (BKPyV) viremia in children after kidney transplant are limited to case reports with small numbers of patients.

*Methods: In this retrospective study we report our experience using leflunomide in 22 children with BKPyV viremia over a 10-year period. Charts were abstracted for demographic and clinical data. The primary endpoint was the proportion of patients with viremia clearance (undetectable once or below dynamic range for two consecutive blood PCRs). Secondary endpoints included the proportion of patients with a clinically significant reduction (≥0.7 log copies/mL) in viremia, time to reduction in viremia, time to viremia clearance, incidence of thrombocytopenia, transaminitis, and nephropathy leading to graft loss.

*Results: One hundred sixty-one children received a kidney transplant at our center from 2011-2021, 22 received leflunomide for BKPyV viremia. The average age of the cohort was 9.1 ± 5.7 years, 14 (64%) were male, and 10 (46%) received a living donor transplant. The indications for kidney transplant were congenital anomalies (n=15, 68%), cystic disease (n=4, 18%), or other (n=3 [2 glomerular/1 unknown], 14%). The median time to first positive viremia after transplant was 96 days [IQR, 35-147 days]. Ten patients (46%) were treated with leflunomide as first-line therapy and 12 (54%) received reduction in immunosuppression prior to initiating leflunomide. Mycophenolate was discontinued in 21 (96%) patients when leflunomide was started. Additional reductions in immunosuppression, before or after leflunomide initiation, were made at the discretion of the treating team. Viral clearance was observed in 19 (86%) patients, a median of 209 days [IQR 100-432 days] after starting leflunomide. All patients had a clinically significant reduction in viral load a median of 63 days [IQR 37-102 days] after starting leflunomide. No adverse reactions requiring discontinuation of leflunomide occurred. No patients developed thrombocytopenia. Three (14%) developed mild transaminitis. Six patients with active viremia underwent biopsy, of which 2 (33%) revealed BKPvV nephropathy. There was no graft loss.

*Conclusions: To our knowledge, this is the largest reported cohort of children treated with leflunomide after kidney transplant. Leflunomide was well tolerated. While the response rates appeared high, the time to viral clearance was prolonged and the independent effect of leflunomide may have been confounded by concomitant changes in immunosuppression.

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