Weight-based tacrolimus trough concentrations post liver transplant

Background: Tacrolimus is one of the most widely used liver transplant medications. With the increasing number of obese patients requiring liver transplants, knowledge of the effect of body size affecting post-transplant outcomes, for example drug exposure is increasingly required. Aims: (i) To investigate whether patient body size (i.e. total bodyweight) affects trough plasma concentrations of tacrolimus when a standard mg/kg dosing regimen is used; and (ii) to investigate whether obese patients have different numbers of plasma concentrations outside the therapeutic range compared to non-obese patients in the first months after liver transplant. Methods: Using a transplant database, data tacrolimus concentrations were available for 69 patients. Tacrolimus was initially dosed at a standard 0.1 mg/kg/day after liver transplant, and adjusted to maintain a target trough concentration. Trough blood samples, phenotypic and outcome variables were analysed. Results: Trough concentrations were similar between obese and non-obese patients (P > 0.05) at each sampling day. At day 7 post-transplant, 85.7% and 79.5% of the observed plasma concentrations were outside the recommended therapeutic range for obese and non-obese patients respectively, at day 30, 52.9% and 57.4%, and at 6 months, 18.7% and 27.5%. Conclusion: In the first week post-transplant, tacrolimus trough concentrations after standard mg/kg dosing post liver transplant appear to be corrected by total bodyweight. Obese patients have a similar number of trough plasma concentrations outside the therapeutic range compared to non-obese patients.