Effect of Hematopoietic Stem Cell Transplantation Regimen on Tacrolimus Pharmacokinetics.

OBJECTIVES: Treatment with tacrolimus requires strict control of the whole-blood concentration in patients undergoing allogeneic hematopoietic stem cell transplantation (HSCT). In patients undergoing cord blood transplantation (CBT), there is a negative correlation between volume of distribution of tacrolimus and hemoglobin levels, which reflect the red blood cell (RBC) count. In this study, we evaluated the influence of the conditioning regimen (myeloablative and reduced-intensity conditioning) or donor source (cord blood, bone marrow, and peripheral blood stem cells) on the pharmacokinetics of tacrolimus in patients undergoing HSCT, including those undergoing CBT. We also examined applicability of dosing strategy of tacrolimus considering the RBC count. METHODS: We retrospectively analyzed clinical data-including whole-blood tacrolimus concentrations-from patients with HSCT. The observation period spanned from first continuous intravenous infusions until switch to oral medication, transfer to another hospital, relapse, or death. Population pharmacokinetic analysis was performed on whole-blood tacrolimus concentrations obtained from therapeutic drug monitoring during the observation period. Patient characteristics and laboratory data were evaluated as covariates. RESULTS: We enrolled 91 patients undergoing HSCT (CBT: n = 56; bone marrow transplantation: n = 22; and peripheral blood stem cell transplantation: n = 13); 58 and 33 patients received myeloablative conditioning and reduced-intensity conditioning, respectively. Whole-blood tacrolimus concentrations were accurately captured (n = 1,658 measurements) using a one-compartment and additive error model. The conditioning regimen and donor source did not have an impact on the pharmacokinetics of tacrolimus. Therefore, these factors were not considered when forming the dosing strategy. Nevertheless, a negative correlation between volume of distribution and hemoglobin level was confirmed, indicating that monitoring the RBC count is useful in assessing the dosing strategy. CONCLUSIONS: A tacrolimus dosing strategy that considers the variability in hemoglobin levels applies to all patients undergoing HSCT.