Stem cell toxicity of oxazaphosphorine metabolites in comparison to their antileukemic activity

Influence of Short-Term Use of Dexamethasone on the Pharmacokinetics of Ifosfamide in Patients

Dexamethasone induces the hepatic cytochrome P450 3A and, therefore, is predicted to change the pharmacokinetics, activities, and side effects of drugs metabolized by cytochrome P450 3A. The aim of this study was to determine whether the pharmacokinetics of the cytochrome P450 3A-dependent oxazaphosphorine cytostatic drug ifosfamide is influenced by short-term antiemetic use of dexamethasone in patients. The peak concentration and area under the curve (AUC) were determined for the parent compound and the metabolites 4-hydroxyifosfamide and chloracetaldehyde in eight patients who received two cycles of ICE chemotherapy (ifosfamide 5 g/m(2) day 1, carboplatin 300 mg/m(2) day 1, etoposide 100 mg/m(2) days 1-3). One cycle included concomitant administration of dexamethasone (40 mg over 30 min, 16 h and 1 h before chemotherapy), whereas the other did not. The half-lives of ifosfamide, 4-hydroxyifosfamide, and chloracetaldehyde were shorter with concomitant administration of dexamethasone, but the differences were not statistically significant. In addition, there were no significant differences in the ifosfamide and active 4-hydroxyifosfamide peak concentrations and AUCs when dexamethasone was included. After dexamethasone administration, the chloracetaldehyde peak concentration was slightly increased by 1.5-fold and the AUC by 1.3-fold; however, these increases were not statistically significant. In conclusion, dexamethasone comedication in ICE chemotherapy did not change the ifosfamide pharmacokinetics. Thus, dexamethasone can be used safely as an antiemetic drug in ifosfamide chemotherapy.

Necrotic pathway in human osteosarcoma Saos-2 cell death induced by chloroacetaldehyde

Chloroacetaldehyde, a metabolite of the anticancer drug ifosfamide, may be responsible for serious adverse effects like encephalopathy in ifosfamide chemotherapy. In this study, we demonstrate that chloroacetaldehyde, but not ifosfamide, induces cell death in human osteosarcoma Saos-2 cells and we investigated the mechanism by which this occurs. Chloroacetaldehyde above 30 micromol/l induced significant cell death in a time-dependent manner. Thiol compounds such as N-acetyl cysteine, glutathione and dithiothreitol protected the cells against chloroacetaldehyde-induced cell death, although other nonthiol compounds and the antioxidative enzymes superoxide dismutase and catalase did not, suggesting that reactive oxygen species might not mediate cell death. In cells exposed to chloroacetaldehyde, levels of both total thiols and glutathione were significantly reduced. Chloroacetaldehyde also collapsed the mitochondrial membrane potential of these cells, induced the release of cytochrome c from mitochondria to the cytosol and significantly reduced cellular ATP levels during the course of death. The mitochondrial potential collapse was also prevented by thiol compounds. Flow cytometric analyses by means of annexin-V and propidium iodide double staining and immunofluorescence staining of active caspase-3 revealed that cells subjected to a lethal dose of chloroacetaldehyde displayed features characteristic of necrosis and that caspase-3 was not activated in response to chloroacetaldehyde. Taken together, these findings suggest that Saos-2 cells exposed to chloroacetaldehyde die by necrosis resulting from a decrease in intracellular thiols, disruption of the mitochondrial membrane potential and the depletion of cellular ATP.

Chloroacetaldehyde: mode of antitumor action of the ifosfamide metabolite

BACKGROUND

The ifosfamide metabolite chloroacetaldehyde had been made responsible for side effects only. We found in previous studies a strong cytotoxicity on human MX-1 tumor cells and xenografts in nude mice. Chloroacetaldehyde is supposed to act via alkylation or by inhibition of mitochondrial oxidative phosphorylation with decrease of ATP. The aim of this study was to further elucidate chloroacetaldehyde's mode of action.

METHODS

MX-1 breast carcinoma cells were measured for ATP-content after exposure to chloroacetaldehyde. Further, the effect of chloroacetaldehyde on DNA-synthesis and its potency of causing strand-breaks or cross-links were investigated by bromodeoxyuridine-incorporation, comet-assay and a DNA interstrand cross-linking-assay.

RESULTS

Chloroacetaldehyde in high concentrations induces a reduction of ATP-levels when anaerobic glycolysis is blocked by oxamate and reduces the bromodeoxyuridine-incorporation to 46.3% after 4 h when used in IC(50) concentrations (7.49 mumol/l). In addition we observed DNA single strand-breaks in MX-1 cells treated with chloroacetaldehyde visible in the Comet assay, but no DNA-cross-linking by comet assay and cross-linking assay.

CONCLUSION

In summary, our results show that chloroacetaldehyde influences the oxidative phosphorylation in mitochondria, however, this is observed only in high concentrations and is not of clinical relevance because the tumor cells regenerate ATP by anaerobic glycolysis. Nevertheless, chloroacetaldehyde causes DNA-strand-breaks and strong inhibition of DNA-synthesis.

Enantioselective metabolism and cytotoxicity of R-ifosfamide and S-ifosfamide by tumor cell-expressed cytochromes P450

The anticancer prodrug ifosfamide (IFA) contains a chiral phosphorous atom and is administered in the clinic as a racemic mixture of R-IFA and S-IFA. Hepatic cytochrome P450 (P450) enzymes exhibit enantioselective preferences in the metabolism of R-IFA and S-IFA; however, the impact of this selectivity on P450-dependent anticancer activity is not known. Presently, the metabolism and cytotoxicity of R-IFA and S-IFA were determined in 9L gliosarcoma and Chinese hamster ovary tumor cells expressing an IFA-activating P450 enzyme and by in vitro steady-state kinetic analysis using cDNA-expressed P450 enzymes. Tumor cells expressing P450 enzyme CYP3A4 were the most sensitive to R-IFA cytotoxicity, whereas tumor cells expressing CYP2B1 or CYP2B6 were most sensitive to cyclophosphamide (CPA), an isomer of IFA. Correspondingly, CYP3A4-expressing cells and cDNA-expressed CYP3A4 metabolized R-IFA to yield the active, 4-hydroxylated metabolite at a 2- to 3-fold higher rate than they metabolized S-IFA or CPA. CYP2B cells and cDNA-expressed CYP2B enzymes metabolized CPA almost exclusively by 4-hydroxylation, whereas R-IFA and S-IFA were substantially converted to inactive, N-dechloroethylated metabolites. Further investigation revealed that CYP3A1, a rat enzyme, exhibited superior kinetic properties compared with the human enzyme CYP3A4, with R-IFA and S-IFA both metabolized with high catalytic efficiency by 4-hydroxylation and with a K(m) value of 200 microM, approximately 5-fold lower than CYP3A4. Based on these kinetic parameters and metabolic profiles, R-IFA is expected to exert greater anticancer activity than S-IFA or CPA against tumors that express CYP3A enzymes, whereas tumors expressing CYP2B enzymes may be more sensitive to CPA treatment.