Summary:Tacrolimus is an immunosuppressant commonly used in the prevention of graft-versus-host disease (GVHD) following allogeneic HCT. Unfortunately, the use of tacrolimus is associated with variable immunosuppression and toxicity. The purpose of this study was to describe tacrolimus population pharmacokinetic parameters, to identify relationships between clinical covariates and pharmacokinetic estimates, and to develop a model to predict tacrolimus clearance in HCT patients. Steadystate whole blood tacrolimus concentrations (n = 1625) obtained during intravenous and oral therapy were analyzed in 122 patients. Population clearance (CL) was 5.22 l/h and bioavailability (F) was 0.28. The influence of clinical covariates on population estimates of CL and F of tacrolimus were tested with nonlinear mixed effects models (NONMEM). CL was significantly reduced by elevations in total bilirubin 2.0-9.9 mg/dl (CL Ɇ 0.797), bilirubin у10 mg/dl (CL Ɇ 0.581), serum creatinine у2 mg/dl (CL Ɇ 0.587), grade III/IV graft-versus-host disease (CL Ɇ 0.814) and veno-occlusive disease (CL 0.814). No covariates were predictive of oral F. The interindividual variabilities in CL and F were 33% and 44%, respectively. Residual variability was 27.5% and 16.8% at tacrolimus concentrations of 10 g/l and 20 g/l, respectively. These models may be used to predict tacrolimus clearance and doses in adult patients following HCT. graft-versus-host disease (GVHD), are barriers to successful transplantation. Optimal pharmacologic immunosuppression is critical in reducing the risk of GVHD. Tacrolimus has been investigated as an alternative to cyclosporine for the prevention of GVHD following HCT. [1][2][3][4][5][6][7] Randomized clinical trials comparing tacrolimus with cyclosporine showed that a tacrolimus/methotrexate regimen is superior to cyclosporine/methotrexate in preventing GVHD. 6,8 The pharmacokinetics of tacrolimus are well described in patients undergoing solid organ transplantation, 9,10 however, they have been evaluated in only small numbers of HCT patients. 2,11,12 Unfortunately, pharmacokinetic data from the solid organ setting poorly represent the pharmacokinetic disposition of tacrolimus in patients undergoing HCT. HCT patients receive lower doses of tacrolimus, receive long courses of intravenous therapy and develop chemotherapy-related complications including mucositis, renal dysfunction, veno-occlusive disease (VOD) and GVHD of the gastrointestinal tract and/or liver. These factors may alter the bioavailability and clearance of tacrolimus.Nephrotoxicity and neurotoxicity secondary to tacrolimus therapy is prevalent following HCT 13 and is associated with tacrolimus blood concentrations Ͼ20 ng/ml. 14,15 Nephrotoxicity (SCr Ͼ2.0 mg/dl) occurs in 32-92.6% of patients undergoing related or unrelated donor HCT. [2][3][4][5]7,13,16 In one study, the risk of developing nephrotoxicity (SCr Ͼ2 mg/dl) was increased by 84.4% when the mean tacrolimus concentration was Ͼ20 ng/ml. 14 Neurotoxicity ranging from mild tremors and paresthesia to s...