*Purpose: Cidofovir is a potentially nephrotoxic antiviral drug used primarily as a treatment for CMV retinitis however due to its potent antiviral effects has been suggested also for polyoma BK virus nephropathy (BKVN) treatment, however clinical experience remains anecdotal.

*Methods: In this retrospective cohort study we analysed the efficacy and safety of administration of cidofovir in the treatment of BKVN after kidney transplantation. The diagnosis was confirmed by histology and/or quantitative nucleic acid test (BKV PCR). 24 patients received cidofovir for BKVN treatment in 2003-2017 when other measures (18x switch from tacrolimus to CsA, 6x reduction of the current immunosuppression) had failed in terms of deterioration of graft function, progression of morphological changes and BKV replication. Cidofovir (0.22-0.33 mg/kg) was given biweekly five times. Patients were followed for a minimum of 12 months.

*Results: Diagnosis of BKVN was made at median of 3 months (2.5-43) after kidney transplantation, with median donor age 53 years, median age of recipients 51.5 years, 58.3 % were males, median peak PRA 16 %, current PRA 8%, mean HLA mismatch in HLA-A 1± 0.7, HLA-B 1.5 ± 0.6, HLA-C 1.2 ± 0.6, 16 % had CMV mismatch, CIT was 13.5 ± 6 hours, with median of 2.4 years on dialysis. All patients had received triple immunosuppression based on tacrolimus, mycophenolate mofetil and corticosteroids, rATG was given as induction regimen to 12 patients. 7 out of 24 patients experienced simultaneous BKVN and acute rejection findings in histology. Graft function before and 12 months after cidofovir treatment was similar (creatinine 197.2 µmol/l vs. 212 µmol/l, p= n.s.). We did not observe a single case of acute nephrotoxicity. BKV virus replication decreased significantly at 3 and 12 months after the treatment (median BKV replication at the time of dg 75 530 copies/mL vs median 0 copies/mL at the 3 and 12 months after therapy, p< 0.0001). We did not confirm any severe adverse events after the therapy with cidofovir. 1-year graft survival in 78.8 %, 3-year graft survival since the diagnosis of BKVN was 74.1 %. Patient survival after the therapy with cidofovir was 95.8 % after 1 year and 91.7 % after 3 years.

*Conclusions: Treatment with cidofovir resulted in the significant decrease of BKV quantity while stabilizing renal function. We did not observe any graft failure due to nephrotoxic effect of cidofovir.

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