*Purpose: Tacrolimus is an immunosuppressant used in post-liver transplantation, has wide pharmacokinetic variability, particularly in pediatric patients. Therefore, this study aimed to evaluate different weight-based dose regimens required to achieve tacrolimus therapeutic target early post-liver-transplant as well as identify the factors affecting time needed to achieve therapeutic target to determine dosing regimen that can promptly achieve the target

*Methods: Ongoing multi-center retrospective study IRB was granted from 2 transplant centers. Patients included in the study if age < 18 years, underwent liver transplantation, and were on tacrolimus therapy. Tacrolimus was started with a fixed dose of 0.5 mg every 12 hours adjusted based on tacrolimus level to achieve therapeutic target level (7-12 ng/ml). Doses adjusted for body weight and serum levels were analyzed daily for the first two weeks post-transplant[Aa1] . Demographic variables and other patient’s specific factors known to affect dose requirements such as donor status, concurrent immunosuppressants, surrogate metabolism marker; concentration to dose ratio (C0/D ratio), surgery, and feeding details were collected. The primary outcome was to describe tacrolimus daily dose requirement needed to achieve therapeutic target in the first 2 weeks post-transplant. The secondary outcome is to identify factors affecting time needed to reach therapeutic tacrolimus level within first 2 weeks post-transplant

*Results: So far, 45 patients included in study 20 (44%) were female, mean age 4.29±4.42 years. The most common indication for liver transplantation were metabolic diseases (70%). Majority of patients (60% ) started on tacrolimus with a dose of < 0.1mg/kg/day mean of 0.07 ± 0.05 mg/kg/day. By the day that those patients were on target level only 30% remain on this dose range while 70% maintained on doses ³ 0.1mg/kg/day and, by day 14 this percentage further increased to 83% with a mean dose of 0.24 ±0.18 mg/kg/day. Eighty-two percent reach therapeutic target during the first 2 weeks. The average time needed to reach therapeutic is 5.92 ±2.55 days. When evaluating factors affecting time needed to reach the therapeutic level, C0/D ratio was significantly higher in patients reaching target level within 7 days compared to those who reach later in > 7 days p = 0.0020. Additionally, patients on interacting medications like caspofungin require significantly longer time reach therapeutic range when compared to those who are not p = 0.046

*Conclusions: Dose requirements appear to be higher than the usual starting tacrolimus dose in pediatric liver transplants during early phase post-transplantation. Multiple factors could affect the time needed to reach therapeutic range including tacrolimus metabolism and interacting medications

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