Effects of aldehyde dehydrogenase inhibitors on the ex vivo sensitivity of murine late spleen colony-forming cells (day-12 CFU-S) and hematopoietic repopulating cells to mafosfamide (ASTA Z 7557)

Oxazaphosphorine cytostatics: from serendipity to rational drug design

On the basis of the discovery that the proapoptotic aldehyde 3-hydroxypropanal is a cyclophosphamide metabolite, a novel mechanism of action of oxazaphosphorine cytostatics is presented and confirmed by animal experiments. Furthermore, it is shown that new oxazaphosphorine cytostatics, which are on orders of magnitude more effective than already existing, can be developed on the basis of the new model for the mechanism of action.

The role of human aldehyde dehydrogenase in normal and cancer stem cells

Normal stem cells and cancer stem cells (CSCs) share similar properties, in that both have the capacity to self-renew and differentiate into multiple cell types. In both the normal stem cell and cancer stem cell fields, there has been a great need for a universal marker that can effectively identify and isolate these rare populations of cells in order to characterize them and use this information for research and therapeutic purposes. Currently, it would appear that certain isoenzymes of the aldehyde dehydrogenase (ALDH) superfamily may be able to fulfill this role as a marker for both normal and cancer stem cells. ALDH has been identified as an important enzyme in the protection of normal hematopoietic stem cells, and is now also widely used as a marker to identify and isolate various types of normal stem cells and CSCs. In addition, emerging evidence suggests that ALDH1 is not only a marker for stem cells, but may also play important functional roles related to self-protection, differentiation, and expansion. This comprehensive review discusses the role that ALDH plays in normal stem cells and CSCs, with focus on ALDH1 and ALDH3A1. Discrepancies in the functional themes between cell types and future perspectives for therapeutic applications will also be discussed.

Metabolism and transport of oxazaphosphorines and the clinical implications

The oxazaphosphorines including cyclophosphamide (CPA), ifosfamide (IFO), and trofosfamide represent an important group of therapeutic agents due to their substantial antitumor and immuno-modulating activity. CPA is widely used as an anticancer drug, an immunosuppressant, and for the mobilization of hematopoetic progenitor cells from the bone marrow into peripheral blood prior to bone marrow transplantation for aplastic anemia, leukemia, and other malignancies. New oxazaphosphorines derivatives have been developed in an attempt to improve selectivity and response with reduced toxicity. These derivatives include mafosfamide (NSC 345842), glufosfamide (D19575, beta-D-glucosylisophosphoramide mustard), NSC 612567 (aldophosphamide perhydrothiazine), and NSC 613060 (aldophosphamide thiazolidine). This review highlights the metabolism and transport of these oxazaphosphorines (mainly CPA and IFO, as these two oxazaphosphorine drugs are the most widely used alkylating agents) and the clinical implications. Both CPA and IFO are prodrugs that require activation by hepatic cytochrome P450 (CYP)-catalyzed 4-hydroxylation, yielding cytotoxic nitrogen mustards capable of reacting with DNA molecules to form crosslinks and lead to cell apoptosis and/or necrosis. Such prodrug activation can be enhanced within tumor cells by the CYP-based gene directed-enzyme prodrug therapy (GDEPT) approach. However, those newly synthesized oxazaphosphorine derivatives such as glufosfamide, NSC 612567 and NSC 613060, do not need hepatic activation. They are activated through other enzymatic and/or non-enzymatic pathways. For example, both NSC 612567 and NSC 613060 can be activated by plain phosphodiesterase (PDEs) in plasma and other tissues or by the high-affinity nuclear 3'-5' exonucleases associated with DNA polymerases, such as DNA polymerases and epsilon. The alternative CYP-catalyzed inactivation pathway by N-dechloroethylation generates the neurotoxic and nephrotoxic byproduct chloroacetaldehyde (CAA). Various aldehyde dehydrogenases (ALDHs) and glutathione S-transferases (GSTs) are involved in the detoxification of oxazaphosphorine metabolites. The metabolism of oxazaphosphorines is auto-inducible, with the activation of the orphan nuclear receptor pregnane X receptor (PXR) being the major mechanism. Oxazaphosphorine metabolism is affected by a number of factors associated with the drugs (e.g., dosage, route of administration, chirality, and drug combination) and patients (e.g., age, gender, renal and hepatic function). Several drug transporters, such as breast cancer resistance protein (BCRP), multidrug resistance associated proteins (MRP1, MRP2, and MRP4) are involved in the active uptake and efflux of parental oxazaphosphorines, their cytotoxic mustards and conjugates in hepatocytes and tumor cells. Oxazaphosphorine metabolism and transport have a major impact on pharmacokinetic variability, pharmacokinetic-pharmacodynamic relationship, toxicity, resistance, and drug interactions since the drug-metabolizing enzymes and drug transporters involved are key determinants of the pharmacokinetics and pharmacodynamics of oxazaphosphorines. A better understanding of the factors that affect the metabolism and transport of oxazaphosphorines is important for their optional use in cancer chemotherapy.

ABCC2-Mediated Biliary Transport of 4-Glutathionylcyclophosphamide and Its Contribution to Elimination of 4-Hydroxycyclophosphamide in Rat

Hematopoietic stem cell transplantation patients conditioned with cyclophosphamide (CY) and total body irradiation have substantially greater risk of nonrelapse mortality when plasma area under the concentration-time curve (AUC) of O-carboxyethylcyclophosphoramide mustard (CEPM) is high. The discovery was paradoxical because CEPM is a nontoxic elimination route of the protoxic CY metabolite hydroxycyclophosphamide (HCY). CY was administered to Wistar and TR- rats (a Wistar strain lacking functional ABCC2) at doses of 100 and 200 mg/kg CY, respectively. After either dose, Wistar rats excreted 4-glutathionylcyclophosphamide (GSCY) abundantly in bile; GSCY was absent from bile of TR- rats. Liver AUC(GSCY) was 2- to 2.5-fold greater in TR- than Wistar rats after 100 and 200 mg/kg CY, respectively. Liver AUC(HCY) was 24-46% greater in TR- rats than in Wistar rats after the respective CY doses. Plasma AUC(CEPM) of TR- rats was approximately twice that of Wistar rats after 100 mg/kg, but did not differ between the two strains after 200 mg/kg. Conversely, plasma AUC(HCY) was not different after 100 mg/kg CY, but was 40% greater in TR- rats after 200 mg/kg. The dose dependence of plasma AUC(CEPM) and AUC(HCY) was explained by the concentrations of HCY attained and the in vitro K(m) of aldehyde dehydrogenase and inhibition of aldehyde dehydrogense in TR- rats. We conclude that GSCY is a substrate of ABCC2, and plasma AUC(CEPM) functions as a reporter of liver exposure to HCY and toxins formed from it when HCY concentration is below the K(m) of aldehyde dehydrogenase and the activity is not compromised.

Cyclophosphamide metabolism, liver toxicity, and mortality following hematopoietic stem cell transplantation

Liver toxicity caused by high-dose myeloablative therapy leads to significant morbidity after hematopoietic cell transplantation. We examined the hypothesis that liver toxicity after cyclophosphamide and total body irradiation is related to cyclophosphamide through its metabolism to toxins. Cyclophosphamide was infused at 60 mg/kg over 1 to 2 hours on each of 2 consecutive days, followed by total body irradiation. Plasma was analyzed for cyclophosphamide and its major metabolites. Liver toxicity was scored by the development of sinusoidal obstruction syndrome (veno-occlusive disease) and by total serum bilirubin levels. The hazards of liver toxicity, nonrelapse mortality, tumor relapse, and survival were calculated using regression analysis that included exposure to cyclophosphamide metabolites (as the area under the curve). Of 147 patients, 23 (16%) developed moderate or severe sinusoidal obstruction syndrome. The median peak serum bilirubin level through day 20 was 2.6 mg/dL (range, 0.5-41.1 mg/dL). Metabolism of cyclophosphamide was highly variable, particularly for the metabolite o-carboxyethyl-phosphoramide mustard, whose area under the curve varied 16-fold. Exposure to this metabolite was statistically significantly related to sinusoidal obstruction syndrome, bilirubin elevation, nonrelapse mortality, and survival, after adjusting for age and irradiation dose. Patients in the highest quartile of o-carboxyethyl-phosphoramide mustard exposure had a 5.9-fold higher risk for nonrelapse mortality than did patients in the lowest quartile. Engraftment and tumor relapse were not statistically significantly related to cyclophosphamide metabolite exposure. Increased exposure to toxic metabolites of cyclophosphamide leads to increased liver toxicity and nonrelapse mortality and lower overall survival after hematopoietic cell transplantation.

Pharmacokinetics, metabolism and clinical effect of ifosfamide in breast cancer patients

Ifosfamide (IFO) at a dose of 5 g/m2, was administered as a 24-h infusion to 15 patients with metastatic (12) or locally advanced (3) breast cancer (age range 33-59 years, median 46). Concurrent chemotherapy was doxorubicin (40 mg/m2) or epirubicin (60 mg/m2). Ifosfamide and its metabolites were measured in plasma and urine during and for 24 h after the infusion using a high performance thin layer chromatography (HPTLC) technique. Patients' haematological toxicity and biochemistry were monitored during treatment and patients were followed for up to 2 years after therapy. At the time of evaluation, 5 of the patients were alive, 2 of whom had not relapsed. A marked variation was observed in the pharmacokinetics and metabolism of ifosfamide in the evaluable patients. Clearance, volume of distribution and half-life of the drug were 3.48 +/- 0.88 1/h/m2, 0.56 +/- 0.22 l/kg and 4.68 +/- 2.01 h, respectively. There was no apparent correlation between these pharmacokinetic variables and patient age, weight or renal function. AUCs of the ultimate alkylating species isophosphoramide mustard (IPM) varied over 6-fold, as did those of the inactivated metabolite carboxyifosfamide (CX). AUCs of dechloroethylated metabolites varied 4-fold (3-dechloroethylifosfamide, 3-DCI) or 8-fold (2-DCI), while that of the parent compound varied only 2.5-fold. Variation in recovery of the metabolites in urine varied over an even wider range, total recovery varying from 17.5 to 81.8% of the dose administered. There was little apparent correlation between pharmacokinetic and metabolite parameters of IFO and haematological toxicity. However, there was a marked negative correlation between both progression-free interval and survival and the AUCs of the products of IFO activation (IPM and CX). In addition, the recovery of IPM in urine was higher in patients experiencing a partial response compared to those with progressive or stable disease. Recovery of dechloroethylated metabolites correlated positively with survival, if 1 poor prognosis patient was excluded. Although far from conclusive, these results give some insight into a possible mechanism of action of ifosfamide and indicate that some species other than IPM, as measured systemically, is responsible for the pharmacological effects of this drug.

Sensitivity of aldehyde dehydrogenases in murine tumor and hematopoietic progenitor cells to inhibition by chloral hydrate as determined by the ability of chloral hydrate to potentiate the cytotoxic action of mafosfamide

Several murine aldehyde dehydrogenases, most notably AHD-2, are known to catalyze the detoxification of cyclophosphamide, mafosfamide, and other oxazaphosphorines. Thus, cellular sensitivity to these agents decreases as the relevant aldehyde dehydrogenase activity increases, and vice versa. Chloral hydrate is a sedative/hypnotic agent that is sometimes administered to patients being treated with cyclophosphamide. It is known to inhibit some, but not all, aldehyde dehydrogenases. Murine (CFU-S, CFU-GEMM and CFU-Mk) and human (CFU-Mix, CFU-GM, BFU-E and CFU-Mk) hematopoietic progenitor cells, as well as murine oxazaphosphorine-resistant (L1210/OAP and P388/CLA) tumor cells, are known to contain the relevant aldehyde dehydrogenase activity but the identity of the specific enzyme present in the normal cells is unknown and may be different than that, namely AHD-2, present in neoplastic cells. In that event, the potential exists to inhibit the detoxification of the oxazaphosphorines in tumor cells without inhibiting this event in normal cells; the net effect of such a selective inhibition would be to increase the margin of safety of the oxazaphosphorines. In ex vivo experiments, chloral hydrate markedly potentiated the antitumor activity of mafosfamide against oxazaphosphorine-resistant L1210/OAP and P388/CLA cells. It did not potentiate the cytotoxic action of mafosfamide against any of the murine or human hematopoietic cells tested, even at concentrations which fully restored the sensitivity of the resistant tumor cell lines to this agent. One explanation for these observations is that hematopoietic progenitor, and the resistant tumor, cells express different relevant aldehyde dehydrogenases and that these aldehyde dehydrogenases differ in their sensitivity to inhibition by chloral hydrate. Consistent with this notion were the observations that AHD-2 was exquisitely sensitive to inhibition by chloral hydrate, whereas two other aldehyde dehydrogenases that also catalyze the detoxification of aldophosphamide, namely AHD-12a, b and AHD-13, were relatively unaffected.

Identification of human liver aldehyde dehydrogenases that catalyze the oxidation of aldophosphamide and retinaldehyde

Biotransformation of the biologically and pharmacologically important aldehydes, retinaldehyde and aldophosphamide, is mediated, in part, by NAD(P)-dependent aldehyde dehydrogenases catalyze the oxidation of the aldehydes to their respective acids, retinoic acid and carboxyphosphamide. Not known at the onset of this investigation was which of the several known human aldehyde dehydrogenases (ALDHs) catalyze these reactions. Thus, human liver aldehyde dehydrogenases were chromatographically resolved and the ability of each to catalyze the oxidation of retinaldehyde and aldophosphamide was assessed. Only one, namely ALDH-1, catalyzed the oxidation of retinaldehyde; the Km value was 0.3 microM. Three, namely ALDH-1, ALDH-2 and succinic semialdehyde dehydrogenase, catalyzed the oxidation of aldophosphamide; Km values were 52, 1193, and 560 microM, respectively. ALDH-4, ALDH-5 and betaine aldehyde dehydrogenase did not catalyze the oxidation of either aldophosphamide or retinaldehyde. ALDH-1 and succinic semialdehyde dehydrogenase accounted for 64 and 30%, respectively, of the total hepatic aldehyde dehydrogenase-catalyzed aldophosphamide (160 microM) oxidation. ALDH-1-catalyzed oxidation of aldophosphamide was noncompetitively inhibited by chloral hydrate; the Ki value was 13 microM. ALDH-2- and succinic semialdehyde dehydrogenase-catalyzed oxidation of aldophosphamide was relatively insensitive to inhibition by chloral hydrate. These observations strongly suggest an important in vivo role for ALDH-1 in the catalysis of retinaldehyde and aldophosphamide biotransformation. Succinic semialdehyde dehydrogenase-catalyzed biotransformation of aldophosphamide may also be of some in vivo importance.

Development and characterisation of a cyclophosphamide resistant variant of the BNML rat model for acute myelocytic leukaemia

A cyclophosphamide resistant subline (BNML/CPR) was developed in vivo in the BN rat acute myelocytic leukaemia (BNML) model. Full resistance was achieved after in vivo exposure of leukaemic animals to cyclophosphamide with, in total, 15 intraperitoneal injections of 100 mg/kg. The CPR line was cross-resistant to ifosfamide, but less so to mafosfamide. Continuous transplantation of the BNML/CPR line without a cyclophosphamide selection pressure resulted in the emergence of a subline (BNML/CPR greater than S) whose sensitivity to cyclophosphamide was similar to that of the parent BNML/S line. Both in the BNML parent line and in the BNML/CPR greater than S line, a 2p+ marker chromosome was present, whereas a 2p+q+ marker chromosome was characteristic for the BNML/CPR line. The mechanism of cyclophosphamide resistance can now be investigated in the BNML model at the DNA, at the mRNA and at the protein level.

Biotransformation of mafosfamide in P388 mice leukemia cells: intracellular 31P-NMR studies

The intracellular transformation of cis-mafosfamide has been studied in P388 mice leukemia cells using 31P-NMR spectroscopy. For this purpose the cells were entrapped in low-gelling-temperature agarose threads. Internal pH of the cells, determined from the position of the intracellular inorganic phosphate, was 7.2. The cell membrane was permeable to 4-hydroxycyclophosphamide and aldophosphamide and less permeable to phosphoramide mustard. 4-Ketocyclophosphamide and carboxyphosphamide signals were not detectable in cells either sensitive or resistant to oxazaphosphorine treatment.

Bone marrow purging with mafosfamide--a critical survey

Autologous bone marrow transplantation (ABMT) is increasingly used to consolidate remissions, primarily in hematological disease. Various purging strategies have been developed to minimize the risk of reimplantation of tumor cells with the bone marrow autotransplant. Pharmacological purging with the oxazaphosphorine derivative mafosfamide has been studied extensively, and recent clinical data suggest that purging with mafosfamide may translate into superior remission duration if compared to nonpurged ABMT in acute leukemia. Chemical and experimental data relevant to mafosfamide-purging and clinical results are reviewed, with special emphasis on safety aspects.

Ex vivo treatment of murine splenocyte-supplemented bone marrow inocula with mafosfamide prior to allogeneic transplantation in an attempt to prevent lethal graft-versus-host disease without compromising engraftment

Murine splenocyte-supplemented bone marrow cell suspensions were incubated with mafosfamide, an analog of "activated" cyclophosphamide, prior to transplantation across major histocompatibility barriers into lethally-irradiated recipient mice in an attempt to reduce the incidence of graft-versus-host disease (GvHD)-related mortality without compromising engraftment. Irradiated mice that received vehicle-treated splenocyte-supplemented bone marrow inocula developed symptoms of severe GvHD; the majority of such animals did not survive. Treatment of donor cells with 160 microM mafosfamide for 30 min resulted in a marked increase in animal survival without evidence of GvHD. Survival of bone marrow allografts was demonstrated by the persistence of donor-type mononuclear cells in the peripheral blood of surviving animals. Treatment of donor cells with a four-fold higher concentration of mafosfamide also resulted in a significant increase in survival without evidence of GvHD; however, host resistance to engraftment was indicated by a low percentage of donor mononuclear cells in the peripheral blood of the survivors. Treatment of donor cells with a four-fold lower concentration of mafosfamide resulted in a slight increase in survival; however, all animals developed symptoms of GvHD. These results indicate that, at appropriate concentrations, mafosfamide can effect the elimination of GvHD-causing T lymphocytes from donor bone marrow inocula without compromising its engraftment potential.