Metabolism of 4-hydroxycyclophosphamide/aldophosphamide in vitro

Aldehyde oxidase and its role as a drug metabolizing enzyme

Aldehyde oxidase (AO) is a cytosolic enzyme that belongs to the family of structurally related molybdoflavoproteins like xanthine oxidase (XO). The enzyme is characterized by broad substrate specificity and marked species differences. It catalyzes the oxidation of aromatic and aliphatic aldehydes and various heteroaromatic rings as well as reduction of several functional groups. The references to AO and its role in metabolism date back to the 1950s, but the importance of this enzyme in the metabolism of drugs has emerged in the past fifteen years. Several reviews on the role of AO in drug metabolism have been published in the past decade indicative of the growing interest in the enzyme and its influence in drug metabolism. Here, we present a comprehensive monograph of AO as a drug metabolizing enzyme with emphasis on marketed drugs as well as other xenobiotics, as substrates and inhibitors. Although the number of drugs that are primarily metabolized by AO are few, the impact of AO on drug development has been extensive. We also discuss the effect of AO on the systemic exposure and clearance these clinical candidates. The review provides a comprehensive analysis of drug discovery compounds involving AO with the focus on developmental candidates that were reported in the past five years with regards to pharmacokinetics and toxicity. While there is only one known report of AO-mediated clinically relevant drug-drug interaction (DDI), a detailed description of inhibitors and inducers of AO known to date has been presented here and the potential risks associated with DDI. The increasing recognition of the importance of AO has led to significant progress in predicting the site of AO-mediated metabolism using computational methods. Additionally, marked species difference in expression of AO makes it is difficult to predict human clearance with high confidence. The progress made towards developing in vivo, in vitro and in silico approaches for predicting AO metabolism and estimating human clearance of compounds that are metabolized by AO have also been 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.

Effect of cyclophosphamide on serum cyclosporine levels at the conditioning of hematopoietic stem cell transplantation

We retrospectively analyzed the factors that affect serum cyclosporine (CsA) concentrations up to day 14 after allogeneic hematopoietic stem cell transplantation (HSCT). In all, 103 transplant recipients who received MTX and CsA for acute GVHD prophylaxis were analyzed. No significant relationships between serum CsA concentrations and gender, age, serum creatinine levels, AST/ALT levels, or antibiotic/fluconazole administration were found by comparing median CsA concentrations or by using longitudinal or regression multivariate analyses. However, the mean of the median serum CsA concentration in patients (n=54) receiving the regimen containing cyclophosphamide (CY) (149.7 ng/ml; 95% confidence interval (CI): 132.1-167.4) was significantly (P<0.0001) lower than that in patients (n=49) receiving the non-CY regimen (217.3 ng/ml; 95% CI: 198.9-235.6). Longitudinal analysis and regression multivariate analysis showed that only administration of CY had a significant effect on the serum CsA concentration. Our results suggest that administration of CY during conditioning can reduce the effects on serum CsA concentrations during the 2 weeks following HSCT. The mechanism of this effect is not clear, but it may be due to the autoinduction of CY.

Novel competitive irreversible inhibitors of aldehyde dehydrogenase (ALDH1): restoration of chemosensitivity of L1210 cells overexpressing ALDH1 and induction of apoptosis in BAF(3) cells overexpressing bcl(2)

4-Amino-4-methyl-pent-2-ynthioc acid S-methyl ester (ampal thiolester: ATE) was used as a lead compound to synthesise new amino-substituted derivatives of alpha, beta acetylenic thiolester compounds as inhibitors of aldehyde dehydrogenase 1, (ALDH1). Of these compounds, the dimethyl derivative (DIMATE) was a competitive irreversible inhibitor (K(i) approximately 280 microM) of baker's yeast ALDH1 in vitro showing 80% inhibition at 400 microM when preincubated with the enzyme for 30min, whereas the trimethyl ammonium and the morpholine derivatives showed only 15% inhibition at 600 microM even after 60min preincubation. ATE inhibited ALDH1 activity in ALDH1-transfected L1210 T cells resistant to hydroperoxycyclophosphamide (HCPA) and inhibited growth synergistically in the presence of HCPA. In non-transfected L1210 counterparts ATE did not potentiate growth inhibition by HCPA. DIMATE was a 30-100-fold more effective growth inhibitor than ATE. Endogenous ALDH1 activities of BAF(3) cells over-expressing different levels of bcl(2) (0-100%) were similar (16-20mU/mg protein) and were all inhibited by DIMATE, reaching 20-30% at 4 microM. Up to 4 microM no apoptosis, as measured by DNA-fragmentation was observed, but at 8 and 10 microM DIMATE, DNA-fragmentation increased concomitantly with ALDH1 inhibition. No DNA-fragmentation was observed with ALDH1 irreversible inhibitors devoid of a thiolester group or with thiolesters which were not inhibitors of ALDH1. It was seen only with competitive irreversible inhibitors having the methanethiol and enzyme-inhibitory moieties. The methanethiol putatively released from DIMATE by ALDH1 esterase activity plays a role, albeit undefined, in lowering intramitochondrial glutathione levels which decreased by 47% as DNA-fragmentation increased.

A mechanism-based pharmacokinetic-enzyme model for cyclophosphamide autoinduction in breast cancer patients

AIMS

This study investigated the pharmacokinetics of cyclophosphamide (CP) and its main metabolite 4-hydroxycyclophosphamide (4-OH-CP) in patients with breast cancer undergoing high dose chemotherapy prior to autologous stem cell transplantation. An enzyme turn-over model was also developed to study the time course of cyclophosphamide induction.

METHODS

Fourteen patients received a combination of CP (6 g m-2 ), thiotepum (500 mg m-2 ) and carboplatin (800 mg m-2 ) as a 96 h infusion. Plasma concentrations of CP and 4-OH-CP were determined with h.p.l.c. and a pharmacokinetic and enzyme turn-over model applied to data using NONMEM.

RESULTS

CP plasma concentrations were described by a two-compartment model with a noninducible and an inducible pathway, the latter forming 4-OH-CP. In the final enzyme model, CP affects the amount of enzymes by increasing the enzyme production rate. CP concentrations decreased during the infusion with no subsequent change in 4-OH-CP concentrations. CP inducible and noninducible clearance were estimated to 1.76 l h-1 (90% C.I. 0.92-2.58) and 1.14 l h-1 (0.31-1.85), respectively. The induction resulted in an approximately doubled CP clearance through the inducible pathway at the end of treatment. The model predicted the enzyme turn-over half-life to be 24 h.

CONCLUSIONS

The presented mechanism-based enzyme induction model where the pharmacokinetics of the inducer and the enzyme pool counterbalance each other successfully described CP autoinduction. It is reasonable to believe that CP affects its own elimination by increasing the enzyme production rate and thereby increasing the amount of enzyme by which CP is eliminated.

Direct gas chromatographic determination of dechloroethylcyclophosphamide following microsomal incubation of cyclophosphamide

A method for the sensitive determination of dechloroethylcylclophosphamide (3-DCl) in microsomal incubation mixtures was developed. 3-DCl, a side-chain oxidation product of cyclophosphamide (CP), was isolated by extraction with acetic acid ethyl ester following solid-phase extraction on C8 cartridges. Quantification of the metabolite was performed by direct capillary gas chromatography with a nitrogen-phosphorus detector without prior derivatization. The method showed good sensitivity and reproducibility with a detection limit of 1 ng/ml and a limit of quantification of 5 ng/ml. The suitability of the method is shown for the quantification of 3-DCl following incubation of CP with human liver microsomes.

Role of aldehyde dehydrogenase in the biological activity of spermine dialdehyde, a novel immunosuppressive/purging agent

The antitumour and immunosuppressive activities of spermine dialdehyde (SDA), a synthetic, oxidized form of spermine, were examined using L1210 cell lines and murine bone marrow cells. SDA acted as a high affinity substrate for aldehyde dehydrogenase (ADH) derived from different sources, with kinetic profiles similar to other aldehyde substrates. The murine leukaemic, cyclophosphamide-resistant L1210/CPA cells, having high levels of intracellular ADH activity, were less sensitive to SDA compared to ADH deficient L1210/O cells as measured by [3H]-thymidine incorporation in proliferation studies. Furthermore, pretreatment of L1210/CPA cells with the ADH inhibitor, diethyl aminobenzaldehyde (DEAB), resulted in potentiation of the SDA response. Murine bone marrow cells were more resistant to SDA than splenic T cells. However, addition of DEAB to bone marrow cultures potentiated the sensitivity of progenitor cells to SDA, as measured by colony formation. The results indicate that levels of ADH in the target tissues would determine the potency of SDA and subsequently offer selectivity and specificity to the therapeutic potentials of this putative purging agent.

Aldehyde dehydrogenases and their role in carcinogenesis

Aldehydes are highly reactive molecules that may have a variety of effects on biological systems. They can be generated from a virtually limitless number of endogenous and exogenous sources. Although some aldehyde-mediated effects such as vision are beneficial, many effects are deleterious, including cytotoxicity, mutagenicity, and carcinogenicity. A variety of enzymes have evolved to metabolize aldehydes to less reactive forms. Among the most effective pathways for aldehyde metabolism is their oxidation to carboxylic acids by aldehyde dehydrogenases (ALDHs). ALDHs are a family of NADP-dependent enzymes with common structural and functional features that catalyze the oxidation of a broad spectrum of aliphatic and aromatic aldehydes. Based on primary sequence analysis, three major classes of mammalian ALDHs--1, 2, and 3--have been identified. Classes 1 and 3 contain both constitutively expressed and inducible cytosolic forms. Class 2 consists of constitutive mitochondrial enzymes. Each class appears to oxidize a variety of substrates that may be derived either from endogenous sources such as amino acid, biogenic amine, or lipid metabolism or from exogenous sources, including aldehydes derived from xenobiotic metabolism. Changes in ALDH activity have been observed during experimental liver and urinary bladder carcinogenesis and in a number of human tumors, including some liver, colon, and mammary cancers. Changes in ALDH define at least one population of preneoplastic cells having a high probability of progressing to overt neoplasms. The most common change is the appearance of class 3 ALDH dehydrogenase activity in tumors arising in tissues that normally do not express this form. The changes in enzyme activity occur early in tumorigenesis and are the result of permanent changes in ALDH gene expression. This review discusses several aspects of ALDH expression during carcinogenesis. A brief introduction examines the variety of sources of aldehydes. This is followed by a discussion of the mammalian ALDHs. Because the ALDHs are a relatively understudied family of enzymes, this section presents what is currently known about the general structural and functional properties of the enzymes and the interrelationships of the various forms. The remainder of the review discusses various aspects of the ALDHs in relation to tumorigenesis. The expression of ALDH during experimental carcinogenesis and what is known about the molecular mechanisms underlying those changes are discussed. This is followed by an extended discussion of the potential roles for ALDH in tumorigenesis. The role of ALDH in the metabolism of cyclophosphamidelike chemotherapeutic agents is described. This work suggests that modulation of ALDH activity may an important determinant of the effectiveness of certain chemotherapeutic agents.(ABSTRACT TRUNCATED AT 400 WORDS)

Identification of mouse liver aldehyde dehydrogenases that catalyze the oxidation of retinaldehyde to retinoic acid

NAD(P)-linked aldehyde dehydrogenases catalyze the oxidation of a wide variety of aldehydes. Thirteen of these enzymes have been identified in mouse tissues; eleven are found in the liver. Some are substrate-nonspecific; others are relatively substrate-specific. The present investigation sought to determine which of these enzymes are operative in catalyzing the oxidation of retinaldehyde to retinoic acid, a metabolite of vitamin A that promotes the differentiation of epithelial and other cells. Spectrophotometric and HPLC assays were used for this purpose. Enzyme-catalyzed oxidation of retinaldehyde (25 microM) was restricted to the cytosol (105,000 g supernatant fraction) and occurred at a rate of 211 nmol/min/g liver; oxidation of acetaldehyde (4 mM) by this fraction proceeds about ten times faster. At least 90% of this activity was NAD dependent. Of the approximately 10% that was apparently NAD independent, two-thirds was inhibited by 1 mM pyridoxal, a known inhibitor of aldehyde oxidase. Of the six cytosolic aldehyde dehydrogenases, only two, viz. AHD-2 and AHD-7, catalyzed the oxidation of retinaldehyde to retinoic acid. An additional NAD-dependent enzyme, viz. xanthine oxidase (dehydrogenase form), also catalyzed the reaction. Catalysis by AHD-2 accounted for more than 90% of the total NAD-dependent activity. Km values were 0.7, 0.6 and 0.9 microM, respectively, for the AHD-2-, AHD-7- and xanthine oxidase (dehydrogenase form)-catalyzed reaction. AHD-4, an aldehyde dehydrogenase found in the cytosol of mouse stomach epithelium and cornea, did not catalyze the reaction.

Cyclophosphamide metabolism in the primary immune organs of the chick: assays of drug activation, P450 expression, and aldehyde dehydrogenase

Several diagnostic catalytic assays were used to determine whether organ-specific metabolic activation or detoxification of cyclophosphamide (CP) contributes to the selective toxicity of CP directed towards differentiating B cells as compared to T cells in the developing chicken. An assay for the alkylation of 4-[p-nitrobenzyl] pyridine (NBP) was used to assess comparative levels of CP activation products generated from microsomal preparations from liver, bursa of Fabricius (B cells), and thymus (T cells) of day-old chicks. Three catalytic assays were used to characterize and compare cytochrome P450-associated enzyme activities in neonatal hepatic and lymphoid tissues. Aldrin epoxidase (AE) was used to detect phenobarbital (PB)-inducible P450 activity. Ethoxyresorufin-O-deethylase (EROD) and aryl hydrocarbon hydroxylase (AHH) were used for the evaluation of polycyclic aromatic hydrocarbon (PAH)-inducible P450 activities in control and PB- or 3,3',4,4'-tetrachlorobiphenyl (TCB)-induced animals. Using the NBP assay, basal and PB-induced CP activation were observed using chick liver microsomes. However, no evidence of CP activation from immune organ microsomes was observed in control, PB-, or TCB-induced chicks. Basal and PB-induced AE activities were observed in thymus, but not bursa, and represented less than 1% of basal liver activity. EROD activity was detected in TCB-induced samples from both thymus and bursa, the thymus having the greater activity. Activities of aldehyde dehydrogenase (ALDH), an enzyme involved in CP detoxification, were about equal in cytosolic fractions from the bursa and thymus. These studies suggest strongly that tissue-specific differences in metabolic capacities are not the major factors governing the selective toxicity of CP directed towards differentiating B lymphocytes in vivo.

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.

Detoxifying enzymes in human ovarian tissues: comparison of normal and tumor tissues and effects of chemotherapy

Many anticancer drugs exert their cytotoxic effects via formation of oxygen free radicals. Cellular thiols, glutathione (GSH)-dependent enzymes and other redox enzymes are involved in the metabolism of these anticancer drugs and of the oxygen free radicals that may be generated during their metabolism. We quantified these biochemical parameters in cytosol from human ovarian tissues. We compared non-protein thiol levels, GSH transferase, GSH peroxidase, superoxide dismutase, catalase, DT diaphorase and aldehyde dehydrogenase activity in serous ovarian tumors (n = 15), other malignant ovarian tumors (n = 12), benign ovarian tissue (n = 10) and histologically normal ovarian tissue (n = 12). Mean GSH transferase and DT diaphorase activities were similar in serous and other malignant ovarian tumors. GSH transferase activity was decreased in malignant tissues relative to normal and benign tissues. Mean DT diaphorase and superoxide dismutase activities were increased in the malignant tissues, although this was not statistically significant. The mean levels of all enzymes except superoxide dismutase and aldehyde dehydrogenase in benign tissues were fairly similar to the mean levels found in normal tissue samples. Tissues from patients with serous ovarian tumors, who had received cyclophosphamide and cisplatin prior to surgery, also were analyzed (n = 7). Except for aldehyde dehydrogenase, all the parameters measured were decreased in these samples relative to serous tissue from untreated patients. These biochemical analyses may be useful in understanding the mechanisms involved in the response to chemotherapy.

Stem cell recovery from cyclophosphamide-induced myelosuppression requires the presence of CD4+ cells

Recently, we have reviewed studies regarding the growth-stimulating effect of CD4+ cells on haematopoietic cells in culture (Pantel & Nakeff, 1989a). In the present study we have tested the physiologic relevance of this interaction using a drug-perturbed mouse model. The long-term application of cyclophosphamide (CY, 30 mg/kg/d, five i.p. injections per week over 7 weeks) in B6D2F1 mice resulted in initial CY-induced cytotoxicity to CFU-S followed by the reestablishment to pretreatment values of the femoral content of CFU-S within 2-3 weeks of CY-treatment. An examination of CY-metabolism in these treated mice excluded a pharmacological explanation for the compensation of CY-cytotoxicity. However, a three-fold increase in the cycling fraction of CFU-S (determined by in vivo hydroxyurea suicide) was observed concomitant with a two-fold increase in the femoral content of L3T4+ cells (the murine equivalent to human CD4+ cells), as compared to the corresponding values in untreated mice. Ablating these L3T4+ cells in vivo by means of a cytotoxic monoclonal antibody (MoAb) to the L3T4 determinant resulted in a decrease in the cycling fraction of CFU-S from 56 +/- 8% to essentially zero and a decrease in the femoral content of CFU-S when comparing mice receiving either CY alone or CY plus MoAb, respectively. It would appear that the CY-induced increase in the proliferative activity of CFU-S requires the presence of L3T4+ cells. These data constitute the first in situ evidence for the physiologic relevance of immunocompetent L3T4+ cells as regulators involved in the recovery of stem cells from drug-induced myelosuppression.

Kinetic characterization of the catalysis of "activated" cyclophosphamide (4-hydroxycyclophosphamide/aldophosphamide) oxidation to carboxyphosphamide by mouse hepatic aldehyde dehydrogenases

A spectrophotometric assay was developed and utilized to directly characterize aldehyde dehydrogenase-catalyzed oxidation of aldophosphamide to carboxyphosphamide by soluble and solubilized particulate fractions prepared from mouse liver homogenates. Vmax values of 3310 and 1170 nmol/min/g liver were obtained for the soluble and solubilized particulate fractions respectively. Km values were 22 and 84 microM respectively. Alkaline pH optimums were observed in each case. Aldehyde dehydrogenase-catalyzed oxidation of aldophosphamide by the soluble fraction was markedly more temperature responsive. Catalysis of aldophosphamide and acetaldehyde or benzaldehyde oxidation was apparently by the same isozyme(s) in the soluble fraction. Similarly, low Km (acetaldehyde/benzaldehyde) and high Km (acetaldehyde/benzaldehyde) isozymes each apparently catalyzed the oxidation of aldophosphamide in the solubilized particulate fraction. Our findings suggest that (1) oxidation of aldophosphamide to carboxyphosphamide by mouse liver is catalyzed largely by the predominant aldehyde dehydrogenase isozyme present in the soluble fraction (cytosol) of this tissue, and (2) isozymes that catalyze aldophosphamide oxidation are not different from those that catalyze the oxidation of acetaldehyde and benzaldehyde, though the relative contribution of each isozyme within the solubilized particulate fraction to the catalysis of aldophosphamide oxidation remains to be determined.

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.

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)

The effects of inhibitors of aldehyde dehydrogenase activity on the sensitivity of murine pluripotent hematopoietic stem cells to oxazaphosphorine anticancer agents, e.g. mafosfamide, were examined using two different assay procedures. In the first part of the investigation, the ex vivo sensitivity of murine day-12 spleen colony-forming cells (CFU-S) to mafosfamide was determined in the absence and presence of known inhibitors of aldehyde dehydrogenase activity, viz. diethyldithiocarbamate and cyanamide. These results were compared to those generated for day-8 CFU-S. Day-12 CFU-S were less sensitive to mafosfamide, and to phosphoramide mustard, although the difference in sensitivity to the latter was less marked. Diethyldithiocarbamate and cyanamide each potentiated the cytotoxic action of mafosfamide toward both day-12 and day-8 CFU-S; they did not potentiate the cytotoxic action of phosphoramide mustard toward these cells. Since cellular aldehyde dehydrogenases are known to catalyze the oxidation of 4-hydroxycyclophosphamide/aldophosphamide, the major transport form of mafosfamide, to the relatively nontoxic acid, carboxyphosphamide, the results suggest that intracellular aldehyde dehydrogenase activity is a determinant of the sensitivity of day-12 CFU-S, as well as of day-8 CFU-S, to mafosfamide and other oxazaphosphorines, e.g. cyclophosphamide. In the second part of this investigation, a murine syngeneic bone marrow transplantation model was used to determine the ex vivo sensitivity of murine hematopoietic repopulating cells to mafosfamide in the absence and presence of diethyldithiocarbamate. Specifically, the ability of treated marrow grafts to repopulate the hematopoietic system, and thereby save recipients from the otherwise lethal effect of total body irradiation, was determined. Diethyldithiocarbamate potentiated the cytotoxic action of mafosfamide, but not that of phosphoramide mustard, toward hematopoietic repopulating cells. These observations support our previous contention that aldehyde dehydrogenase activity is an operative determinant with regard to the sensitivity of murine pluripotent hematopoietic stem cells to oxazaphosphorines.

Effect of the aldehyde dehydrogenase inhibitor, cyanamide, on the ex vivo sensitivity of murine multipotent and committed hematopoietic progenitor cells to mafosfamide (ASTA Z 7557)

The ex vivo sensitivity of murine multipotent (CFU-GEMM) and committed (CFU-Mk, CFU-GM, BFU-E and CFU-E) hematopoietic progenitor cells to mafosfamide was quantified with and without concurrent exposure to cyanamide, an inhibitor of aldehyde dehydrogenase activity. In the absence of cyanamide, CFU-GEMM, CFU-Mk and CFU-GM were approximately equisensitive to mafosfamide while the erythroid progenitors were more sensitive to the drug. Cyanamide potentiated the cytotoxicity of mafosfamide toward CFU-GEMM and CFU-Mk, but not toward CFU-GM, BFU-E and CFU-E. Cellular aldehyde dehydrogenases are known to catalyze the oxidation of 4-hydroxycyclophosphamide/aldophosphamide, the major intermediate in cyclophosphamide and mafosfamide activation, to the relatively nontoxic acid, carboxyphosphamide. Thus, our findings indicate that 1) murine CFU-GEMM contain the relevant aldehyde dehydrogenase activity, and 2) the relevant aldehyde dehydrogenase activity is retained upon differentiation to progenitors committed to the megakaryocytoid lineage, but lost upon differentiation to progenitors committed to the granulocytoid/monocytoid and erythroid lineages. The relative insensitivity of CFU-GM to mafosfamide is apparently due to a cellular determinant that influences their sensitivity to all cross-linking agents since CFU-GM were found to be relatively insensitive to non-oxazaphosphorine cross-linking agents as well.

Aldehyde dehydrogenase activity as the basis for the relative insensitivity of murine pluripotent hematopoietic stem cells to oxazaphosphorines

The ex vivo sensitivity of murine pluripotent hematopoietic stem cells (CFU-S) and myeloid progenitor cells (CFU-GM) to 4-hydroperoxycyclophosphamide, ASTA Z 7557, phosphoramide mustard, acrolein, melphalan, and cis-platinum was determined in the absence and presence of known (disulfiram, diethyldithiocarbamate, cyanamide) or suspected [ethylphenyl(2-formylethyl)phosphinate] inhibitors of aldehyde dehydrogenase activity. As compared to CFU-GM, CFU-S were less sensitive to the oxazaphosphorine agents, 4-hydroperoxycyclophosphamide and ASTA Z 7557. The two cell populations were approximately equisensitive to acrolein as well as to the non-oxazaphosphorine cross-linking agents, phosphoramide mustard, melphalan and cis-platinum. All four inhibitors of aldehyde dehydrogenase activity potentiated the cytotoxic action of the oxazaphosphorines toward CFU-S; they did not potentiate the cytotoxic action of acrolein or the non-oxazaphosphorines toward these cells. The inhibitors did not potentiate the cytotoxic action of the oxazaphosphorines, non-oxazaphosphorines, or acrolein toward CFU-GM. Pyridoxal, a substrate for aldehyde oxidase, did not potentiate the cytotoxic action of oxazaphosphorines toward CFU-S. Cellular NAD-linked aldehyde dehydrogenases are known to catalyze the oxidation of the major transport form of cyclophosphamide, 4-hydroxycyclophosphamide/aldophosphamide, to an inactive metabolite, carboxyphosphamide. Our observations suggest that (1) aldehyde dehydrogenase activity is an important determinant of the sensitivity of a cell population to the oxazaphosphorines, (2) CFU-GM lack the relevant aldehyde dehydrogenase activity, and (3) the phenotypic basis for the relative insensitivity of CFU-S to oxazaphosphorines is the aldehyde dehydrogenase activity contained by these cells.

A critical review of the literature on acrolein toxicity

A detailed literature review of human and animal toxicity studies of acrolein is presented, and information gaps identified that call for further investigation. Specific recommendations are suggested for additional short-/long-term studies, including chemical disposition and cytogenetic investigations. Two bibliographies are provided indicating the scope of the review: (1) literature actually cited and (2) literature examined but not included.

Mutagenic activities of cyclophosphamide (NSC-26271) and its main metabolites in Salmonella typhimurium, human peripheral lymphocytes and Chinese hamster ovary cells

Cyclophosphamide (CPA) and its main metabolites were analyzed with respect to their mutagenic activities in Salmonella, human peripheral lymphocytes (PL), and Chinese hamster ovary (CHO) cells. In Salmonella, the compounds were activated with S9 mix from rat livers, which were unstimulated or stimulated with Aroclor 1254 or phenobarbital. For the enzyme inducers the following order of efficiency was found for all test compounds except carboxyphosphamide: phenobarbital greater than Aroclor 1254 greater than non-induced. The most potent mutagens in all 3 test systems were 4-OH-CPA, PAM and nor-HN2. S9 mix transforms 4-OH-CPA to strong mutagenic compounds in the Salmonella assay. All metabolites tested in the Salmonella assay were activated by S9 mix to higher mutagenic potential.

Carbon tetrachloride-induced increase in the antitumor activity of cyclophosphamide in mice: a pharmacokinetic study

Carbon tetrachloride is an hepatotoxin that depresses hepatic microsomal cytochrome P-450 and other enzyme activities. Cyclophosphamide is an anticancer drug that is activated by hepatic microsomal cytochrome P-450, while the products of cyclophosphamide metabolism by cytochrome P-450 can be metabolized by other hepatic enzymes. Carbon tetrachloride pretreatment has been found to increase the in vivo antitumor activity of cyclophosphamide against murine leukemia P-388. Carbon tetrachloride did not, however, affect the direct cytotoxicity of cyclophosphamide or 4-hydroxycyclophosphamide to cells in culture. Pharmacokinetic studies in mice revealed a delayed plasma disappearance of cyclophosphamide after carbon-tetrachloride pretreatment with an apparent initial half-time of 20.4 min compared to 9.0 min in non carbon-tetrachloride-pretreated mice. Plasma levels of total alkylating activity and plasma 4-hydroxycyclophosphamide increased more slowly and reached a lower peak, but were maintained for a longer time period in mice pretreated with carbon-tetrachloride than in untreated mice. The half-life for plasma elimination of 4-hydroxycyclophosphamide in untreated mice was 12 min and in carbon-tetrachloride-pretreated mice 27 min. There was, however, no difference in the area under the curve for either plasma total alkylating activity or plasma 4-hydroxycyclophosphamide between the two groups. It is suggested that prolonged exposure of tumor cells to 4-hydroxycyclophosphamide might be responsible for the increased antitumor activity of cyclophosphamide following carbon-tetrachloride pretreatment.