*Purpose: The incidence of BK virus infection is high among rapid metabolizers (RM) in kidney transplant (KT) patients. RM can be identified using validated and clinically available information with tacrolimus concentration to dose ratio (c/d) < 1. The RM patients have high peak levels and greater tacrolimus exposure to achieve a therapeutic trough. As compared to immediate-release tacrolimus (IR-Tac), Envarsus, though its high bioavailability, has lower peak levels and overall tacrolimus exposure. This study investigated whether conversion of IR-Tac to Envarsus would reduce the incidence of BK infection among RM.

*Methods: RM kidney transplant patients were identified using c/d <1, starting in March 2019, and prospectively followed for 12 months. All patients received standard of care immunosuppression with thymoglobulin induction, tacrolimus (trough level 8-12 ng/mL), mycophenolate mofetil (2000 mg daily), and prednisone at month 1. Patients met the criteria for the study if tacrolimus c/d remained < 1 at month 2 and serum BK PCR was negative at month 1. The study group was converted from IR-Tac to equivalent Envarsus at month 2. We retrospectively obtained the control group from January 2018 to February 2019 by chronologically screening every patient who underwent a KT prior to the study start date. We included everyone who met the above criteria of the standard immunosuppression and negative serum BK PCR at month 1. Control group data was collected 12 months post-transplant. Data are presented as counts and percentages or means. Significant differences between cases and controls were identified using t test and chi-square testing as indicated.

*Results: We compared 35 prospective study patients with 46 historical controls. The cohort consisted of deceased donor KT (88.89%), with a mean age of 50.93 ± 10.65 years and predominantly black race (87.65%). There was no difference in baseline characteristics (Table 1). At 6 months post-transplant, there was no difference in the incidence of BK viruria or BK viremia (28 study group vs. 46 controls). As expected in an RM cohort, there was a high incidence of BK viruria (49.38%) and BK viremia (37.04%). It appears to date, at a mean follow-up of 10.43 months, there is no difference in the incidence of BK viruria, BK viremia, or BK nephropathy (Table 2) in the two groups.

*Conclusions: We hypothesized that among rapid metabolizers, a lower peak level, and overall drug exposure with Envarsus would lead to a lower incidence of BK infection. There were no significant differences observed among the groups

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