Imatinib inhibition of fludarabine uptake in T-lymphocytes.

PURPOSE: We investigated the potential drug-drug interaction between imatinib and fludarabine, which may be concomitantly administered in chronic myeloid leukemia (CML) patients receiving fludarabine-based conditioning for allogeneic hematopoietic cell transplantation (HCT). Imatinib is an inhibitor of human equilibrative transporters (hENTs), which are responsible for the intracellular uptake of fludarabine. METHODS: Intracellular accumulation of fludarabine triphosphate (F-ara-ATP), the active metabolite of fludarabine, was measured in CD4(+) and CD8(+) T-lymphocytes isolated from healthy volunteers, which were treated in vitro with fludarabine alone, and in the presence of either imatinib or NBMPR, a known hENT inhibitor. RESULTS: Imatinib significantly inhibited F-ara-ATP accumulation in CD4(+) and CD8(+) T-lymphocytes in a concentration-dependent manner. The observed imatinib inhibition was comparable to inhibition observed with NBMPR. The inhibition of F-ara-ATP by imatinib is likely due to inhibition of nucleoside transporters hENT1 and hENT2. CONCLUSIONS: There is significant in vitro drug interaction between imatinib and fludarabine. This effect may be of important consideration in patients receiving fludarabine-based conditioning prior to HCT.

Real-time dose adjustment of cyclophosphamide in a preparative regimen for hematopoietic cell transplant: a Bayesian pharmacokinetic approach.

PURPOSE: Dose-related toxicity of cyclophosphamide may be reduced and therapeutic efficacy may be improved by pharmacokinetic sampling and dose adjustment to achieve a target area under the curve (AUC) for two of its metabolites, hydroxycyclophosphamide (HCY) and carboxyethylphosphoramide mustard (CEPM). To facilitate real-time dose adjustment, we developed open-source code within the statistical software R that incorporates individual data into a population pharmacokinetic model. EXPERIMENTAL DESIGN: Dosage prediction performance was compared to that obtained with nonlinear mixed-effects modeling using NONMEM in 20 cancer patients receiving cyclophosphamide. Bayesian estimation of individual pharmacokinetic parameters was accomplished from limited (i.e., five samples over 0-16 hours) sampling of plasma HCY and CEPM after the initial cyclophosphamide dose. Conditional on individual pharmacokinetics, simulations of the AUC of both HCY and CEPM were provided for a range of second doses (i.e., 0-100 mg/kg cyclophosphamide). RESULTS: The results compared favorably with NONMEM and returned accurate predictions for AUCs of HCY and CEPM with comparable mean absolute prediction error and root mean square prediction error. With our method, the mean absolute prediction error and root mean square prediction error of AUC CEPM were 11.0% and 12.8% and AUC HCY were 31.7% and 44.8%, respectively. CONCLUSIONS: We developed dose adjustment software that potentially can be used to adjust cyclophosphamide dosing in a clinical setting, thus expanding the opportunity for pharmacokinetic individualization of cyclophosphamide. The software is simple to use (requiring no programming experience), reads individual patient data directly from an Excel spreadsheet, and runs in less than 5 minutes on a desktop PC.

Metabolism-based cyclophosphamide dosing for hematopoietic cell transplant.

When cyclophosphamide (120 mg/kg) is used for hematopoietic cell transplant, the increased area under the curve of carboxyethylphosphoramide mustard (AUC(CEPM)) is related to liver toxicity and death. We determined the feasibility of dose-adjusting cyclophosphamide to a preset metabolic endpoint (AUC(CEPM), 325 +/- 25 micromol/L.h). In 20 patients blood sampling was done over a 16-hour period after administration of 45 mg/kg cyclophosphamide; AUC(CEPM) from 0 to 16 hours was calculated by noncompartmental analysis. The expected AUC(CEPM) for 0 to 48 hours was estimated, and the second cyclophosphamide dose was determined. The mean second cyclophosphamide dose was 42 mg/kg, and the mean total cyclophosphamide dose was 86 mg/kg (range, 54-120 mg/kg). The mean AUC(CEPM) for the time from 0 to 48 hours was 296 micromol/L.h (95% confidence interval, 275-317 micromol/L.h). A retrospective analysis indicated that AUC(CEPM) could be more accurately predicted by use of a population pharmacokinetic model. We conclude that metabolism-based dosing of cyclophosphamide is feasible and that a lower cyclophosphamide dose does not affect engraftment.

A highly sensitive high-performance liquid chromatography-mass spectrometry method for quantification of fludarabine triphosphate in leukemic cells.

A high-performance liquid chromatography (HPLC)-mass spectrometry (MS) method has been developed for the analysis of 9-beta-D-arabinofuranosyl-2-fluoroadenine 5'-triphosphate (F-ara-ATP) from biological samples. Quantification is carried out by selected ion monitoring of the parent ion. Baseline separation of the monophosphate (F-ara-AMP) and diphosphate (F-ara-ADP) is achieved using the volatile ion-pairing reagent dimethylhexylamine. This method is selective and sensitive with an on-column detection limit of approximately 50 fmol. It also permits simultaneous monitoring of endogenous adenosine phosphates. The utility of the assay has been demonstrated by the analysis of F-ara-ATP in human leukemic cells after incubation with 9-beta-D-arabinosyl-2-fluoroadenine (F-ara-A) at clinically relevant concentrations.