*Purpose: Major causes of de novo development of donor specific antibodies (DSA) after kidney transplant include medication noncompliance and immunosuppression minimization or elimination that may increase the risk for graft loss. Different formulations of tacrolimus have been compared in terms of pharmacokinetics, but not in the context of alloimmune-related injury and de novo DSA formation. Thus, we aimed to compare the incidence of de novo DSA formation in kidney transplant recipients maintained on twice-daily immediate-release tacrolimus (Prograf, IR-FK) versus those maintained on once-daily extended-release tacrolimus (Envarsus XR, LCPT) in the first 2 years post-transplant.

*Methods: This is a retrospective, single-center, comparative study of adult kidney transplant recipients between 2018 and 2019 who received either IR-FK or LCPT for maintenance immunosuppression. Exclusion criteria included multi-organ transplants, presence of current or historic pre-formed DSA, or desensitization therapy in the 90 days preceding transplant. The primary outcome was treatment failure defined as a composite of de novo DSA formation, biopsy-proven acute rejection (BPAR), graft failure, or death at 24 months. The presence of immune activation (IA) was also evaluated using peripheral blood molecular biomarkers, defined as a positive gene expression profile (Trugraf, Transplant Genomics, Inc., USA) or positive percentage dd-cfDNA (TRAC, Eurofins Viracor, Inc., USA) at 24 months.

*Results: A total of 240 patients were screened, and 150 patients met the pre-specified inclusion criteria. There were no statistical differences in baseline characteristics (Table 1). Preliminary results show that incidence of the composite primary outcome was 13.2% in the LCPT group versus 24.3% in the IR-FK group (Table 2, p=0.079) by 24 months post-transplant. The incidence of BK virus nephropathy was 8.1% and 6.8% (p=0.754), respectively. Further results to follow.

*Conclusions: At 24 months post-transplant, the composite primary outcome of treatment failure was numerically higher in the IR-FK group, though this did not reach statistical significance. Additional prospective studies are needed to determine the clinical significance of these findings and further explore means of improving adherence and providing consistent drug exposure.

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