Background: BK nephropathy is an important cause of early graft loss after renal-pancreas transplantation. We sought to determine if conversion from tacrolimus/mycophenolate mofetil (TAC-MMF) to sirolimus and low-dose tacrolimus (mTOR-TAC) immunosuppression would reduce the incidence of BK viremia.

Methods: A retrospective single center review of renal-pancreas transplant recipients performed between February 2008 and June 2012. The mTOR-TAC cohort was converted at one month post-transplant from tacrolimus/MMF to sirolimus (target trough level of 6-8 ng/ml) and reduced dose tacrolimus (target trough level of 2-4 ng/ml) maintenance immunosuppression. Outcomes were compared to a similar group of renal-pancreas recipients maintained on tacrolimus (target trough 8-10 ng/ml) and MMF (1000 mg/day). BK testing by PCR was performed on all recipients at one, 3,6,9, 12, and every 6 months thereafter.

Results: After a median follow up of 18 months (range 6-48 months), 70% of the mTOR-TAC cohort have one-year follow-up versus 80% of the TAC-MMF group (p=ns). Forty percent of the mTOR-TAC cohort and 44% of the TAC-MMF groups were prednisone free (p=ns). Beyond one month post-transplantation, none of the 27 recipients on mTOR-TAC immunosuppression have developed BK viremia compared to 4 of 25 on TAC-MMF immunosuppression (p<0.05). BK viremia occurred at 3,3,6, and 9 months post-transplantation. Three of 27 (11%) mTOR-TAC versus 6 of 25 (24%) TAC-MMF treated recipients (p=ns) suffered an acute rejection episode. All rejections occurred within the first-year post-transplantation. At one-year the mean serum creatinine of the mTOR-TAC and TAC-MMF treated groups was 1.2 ± 0.2 versus 1.3 ± 0.5 mg/dl (p=ns) respectively. Within the first year there were no graft losses in the mTOR-TAC cohort versus 2 graft losses in the TAC-MMF group. Two rejections and one graft loss in the TAC-MMF group were associated with immunosuppression reduction after diagnosis of BK viremia.

Conclusion: Conversion from full-dose tacrolimus/MMF to sirolimus and reduced dose tacrolimus resulted in a significant reduction in the incidence of BK viremia with a comparable risk of acute rejection and equivalent renal function.

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