John T. Slattery

Pharmacokinetics of cyclophosphamide and its metabolites in bone marrow transplantation patients

To characterize the pharmacokinetics of cyclophosphamide and 5 of its metabolites in bone marrow transplant patients and to identify the mechanism of the increase in 4‐hydroxycyclophosphamide area under the plasma concentration‐time curve (AUC) from day 1 to day 2 of cyclophosphamide administration.

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 (AUCCEPM) is related to liver toxicity and death. We determined the feasibility of dose‐adjusting cyclophosphamide to a preset metabolic endpoint (AUCCEPM, 325 ± 25 μmol/L · h). In 20 patients blood sampling was done over a 16‐hour period after administration of 45 mg/kg cyclophosphamide; AUCCEPM from 0 to 16 hours was calculated by noncompartmental analysis. The expected AUCCEPM 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 AUCCEPM for the time from 0 to 48 hours was 296 μmol/L · h (95% confidence interval, 275–317 μmol/L · h). A retrospective analysis indicated that AUCCEPM 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.

Phase I topotecan preparative regimen for high-risk neuroblastoma, high-grade glioma, and refractory/recurrent pediatric solid tumors.

We evaluated the toxicity and maximum tolerated dose of topotecan in a novel myeloablative regimen as treatment for high-risk pediatric tumors. Patients received an assigned topotecan dosage in combination with fixed doses of carboplatin and thiotepa, followed by autologous hematopoietic stem cells infusion. Topotecan dose was escalated in cohorts of four patients until the maximum tolerated dose of topotecan was defined or until accrual of 30 patients. Pharmacokinetics of topotecan were examined, and event-free survival was estimated. We describe preliminary results following treatment of 25 pediatric patients with high-risk solid tumors.

Overexpression of glutathione-S-transferase, MGSTII, confers resistance to busulfan and melphalan

A major obstacle to hematopoietic gene therapy is the lack of appropriate in vivo selection protocols that can raise the presently low numbers of gene-altered stem cells to therapeutically useful levels. Overexpression of glutathione-S-transferases (GST), in combination with busulfan treatment, may provide an exploitable selection mechanism for hematopoietic gene therapy strategies. GST provides a major route of detoxification of a variety of xenobiotics, including alkylating agents used for myeloablative chemotherapy. The only known route of clearance of busulfan is by GST-mediated conjugation. Using a fibroblast cell line as a model, we have tested the effects of overexpression of three human GST (GSTA1, GSTP1, and MGSTII) on cell survival under a busulfan or melphalan challenge. In two separate assay formats using chronic exposure to busulfan, MGSTII conferred a reproducible twofold selective advantage. GSTA1 and GSTP1 had no effect on busulfan resistance, and melphalan resistance was not affected by expression of any of the GSTs in these assays. In an acute (24-hour) melphalan exposure assay, MGSTII conferred about a twofold selective advantage. Busulfan was not toxic in this assay. RTPCR analysis of human bone marrow CD34+ cells showed that MGSTII is not highly expressed in this stem/early progenitor population. These data indicate that MGSTII may be a useful selective agent in hematopoietic gene therapy.

Inhibition of carboxyethylphosphoramide mustard formation from 4-hydroxycyclophosphamide by carmustine

It has been reported that the toxicity of carmustine (BCNU) cyclophosphamide (CY)/etoposide regimen (when BCNU is split into 4 doses) is less than that of BCNU/CY/cisplatin regimen (when the same amount of BCNU is administered as a single dose). We hypothesized that this might in part be due to the inhibition of aldehyde dehydrogenase 1 (ALDH1) by BCNU or its degradation product, 2-chloroethyl isocyanate, which is likely to be more pronounced at the higher BCNU dose. The effects of BCNU and 2-chloroethyl isocyanate on the formation of carboxyethylphosphoramide mustard (CEPM) from 4-hydroxycyclophosphamide (HCY) was evaluated in human liver cytosol incubations. We found that CEPM formation from HCY was inhibited strongly by BCNU and weakly by 2-chloroethyl isocyanate. The mechanism of inhibition of ALDH1 activity by BCNU was elucidated using indole-3-acetaldehyde (IAL) as the probe substrate in ALDH1 prepared from human erythrocytes. BCNU was a competitive inhibitor of ALDH1 activity with a Ki of 1.95 μM. The inhibition was independent of preincubation time and reversible by dialysis. The calculated %inhibition of ALDH1 activity by acrolein and BCNU in patients receiving BCNU in 4 split doses with CY was 81%, and it increased to 92% in single dose BCNU regimen. Thus, the calculation indicates that residual operating ALDH1 activity is halved in the presence of single-dose BCNU compared to split-dose BCNU. The inhibition of ALDH1 may contribute to the observed lower incidence of toxicity when BCNU was split into 4 doses compared with single dose and coadministered with CY although dose-dependent effects of BCNU on glutathione and glutathione reductase are also likely to contribute.