NMR spectroscopic studies of intermediary metabolites of cyclophosphamide. A comprehensive kinetic analysis of the interconversion of cis- and trans-4-hydroxycyclophosphamide with aldophosphamide and the concomitant partitioning of aldophosphamide between irreversible fragmentation and reversible conjugation pathways

Intracellular activation of 4-hydroxycyclophosphamide into a DNA-alkylating agent in human leucocytes

1.The conversion of the cyclophosphamide intermediate metabolite 4-hydroxycyclophosphamide (4-OHCP) to the final cytotoxic metabolite phosphoramide mustard (PAM) is classically assumed to occur via chemical hydrolysis of the phospho-ester bond. Whilst it has been suggested previously that this reaction could be enzyme-catalysed, there was only indirect evidence for this (i.e. formation of the by-product acrolein).2. Using an assay to detect formation of DNA-alkylating adducts which block PCR amplification (QPCR-block assay), we have demonstrated that 4-OHCP can be activated by peripheral blood mononuclear cells (PBMC). The DNA-alkylating potency of 4-OHCP in PBMC increased >18-fold compared to the intrinsic reactivity of 4-OHCP for purified gDNA.3. We also found that immortalised T-cells (Jurkat) had a similar ability to activate 4-OHCP into a DNA alkylating agent, whereas there was no appreciable activation in epithelial derived (Caco-2) cells. This suggests the possibility of tissue-specific enzyme expression.4. Of the candidate enzymes tested only recombinant human cAMP-phosphodiesterase-PDE4B and snake-venom phosphodiesterase (PDE-I) could catalyse this activation into a DNA-alkylating agent.5. This enzymatic catalysis of the phospho-ester bond (P-O-C) is a hitherto unrecognised feature of this important immunomodulatory drug and should be investigated further.

Integrating cell-based and clinical genome-wide studies to identify genetic variants contributing to treatment failure in neuroblastoma patients

High-risk neuroblastoma is an aggressive malignancy with high rates of treatment failure. We evaluated genetic variants associated with in vitro sensitivity to two derivatives of cyclophosphamide for association with clinical response in a separate replication cohort of neuroblastoma patients (n=2,709). Lymphoblastoid cell lines (LCLs) were exposed to increasing concentrations of 4-hydroperoxycyclophosphamide [4HC n=422] and phosphoramide mustard [PM n=428] to determine sensitivity. Genome-wide association studies (GWAS) were performed to identify single nucleotide polymorphisms (SNPs) associated with 4HC and PM sensitivity. SNPs consistently associated with LCL sensitivity were analyzed for associations with event-free survival in patients. Two linked SNPs, rs9908694 and rs1453560, were found to be associated with PM sensitivity in LCLs across populations and were associated with event-free survival in all patients (P=0.01) and within the high-risk subset (P=0.05). Our study highlights the value of cell-based models to identify candidate variants that may predict response to treatment in patients with cancer.

Relationship of glutathione S-transferase genotypes with side-effects of pulsed cyclophosphamide therapy in patients with systemic lupus erythematosus

AIMS

Cyclophosphamide (CTX) is an established treatment of severe systemic lupus erythematosus (SLE). Cytotoxic CTX metabolites are mainly detoxified by multiple glutathione S-transferases (GSTs). However, data are lacking on the relationship between the short-term side-effects of CTX therapy and GST genotypes. In the present study, the effects of common GSTM1, GSTT1, and GSTP1 genetic mutations on the severity of myelosuppression, gastrointestinal (GI) toxicity, and infection incidences induced by pulsed CTX therapy were evaluated in patients SLE.

METHODS

DNA was extracted from peripheral leucocytes in patients with confirmed SLE diagnosis (n = 102). GSTM1 and GSTT1 null mutations were analyzed by a polymerase chain reaction (PCR)-multiplex procedure, whereas the GSTP1 codon 105 polymorphism (Ile-->Val) was analyzed by a PCR-restriction fragment length polymorphism (RFLP) assay.

RESULTS

Our study demonstrated that SLE patients carrying the genotypes with GSTP1 codon 105 mutation [GSTP1*-105I/V (heterozygote) and GSTP1*-105 V/V (homozygote)] had an increased risk of myelotoxicity when treated with pulsed high-dose CTX therapy (Odds ratio (OR) 5.00, 95% confidence interval (CI) 1.96, 12.76); especially in patients younger than 30 years (OR 7.50, 95% CI 2.14, 26.24), or in patients treated with a total CTX dose greater than 1.0 g (OR 12.88, 95% CI 3.16, 52.57). Similarly, patients with these genotypes (GSTP1*I/V and GSTP1*V/V) also had an increased risk of GI toxicity when treated with an initial pulsed high-dose CTX regimen (OR 3.33, 95% CI 1.03, 10.79). However, GSTM1 and GSTT1 null mutations did not significantly alter the risks of these short-term side-effects of pulsed high-dose CTX therapy in SLE patients.

CONCLUSIONS

The GSTP1 codon 105 polymorphism, but not GSTM1 or GSTT1 null mutations, significantly increased the risks of short-term side-effects of pulsed high-dose CTX therapy in SLE patients. Because of the lack of selective substrates for a GST enzyme phenotyping study, timely detection of this mutation on codon 105 may assist in optimizing pulsed high-dose CTX therapy in SLE patients.

New approach to the activation of anti-cancer pro-drugs by metalloporphyrin-based cytochrome P450 mimics in all-aqueous biologically relevant system

The low-molecular weight water-soluble Fe(III) and Mn(III) porphyrins--in biologically relevant phosphate-buffered saline medium with ascorbic acid as a source of electrons, under aerobic conditions but without co-oxidant - catalyze the hydroxylation of anti-cancer drug cyclophosphamide to active metabolite 4-hydroxycyclophosphamide in yields similar or higher than those typically obtained by the action of liver enzymes in vivo. The Fe(III) meso tetrakis(2,6-difluoro-3-sulfonatophenyl)porphyrin, highly electron-deficient at the metal site, was the most effective catalyst. If proven viable in vivo, this methodology could be expanded to localized or systemic activation of the entire family of oxazaphosphorine-based (and many other) anti-cancer drugs and become a powerful tool for an aggressive treatment of tumors with less toxic side effects to the patient.

Phosphoramidate and phosphate prodrugs of (−)-β-d-(2R,4R)-dioxolane-thymine: Synthesis, anti-HIV activity and stability studies

A series of phosphoramidate and phosphate prodrugs of DOT were synthesized via dichlorophosphate or H-phosphonate chemistry and evaluated for their anti-HIV activity against LAI M184V mutants in PBM cells as well as for their cytotoxicity. The antiviral and cytotoxic profiles of the prodrugs were compared with that of the parent compound (DOT), and it was found that four aryl phosphoramidates 5, 18, 20, and 26 showed a significant enhancement (8- to 12-fold) in anti-HIV activity without cytotoxicity. Chemical stability of these prodrugs was evaluated in phosphate buffer at pH values of biological relevance (i.e., pH 2.0 and 7.4). Enzymatic hydrolysis was also studied in esterase or lipase in buffer solution. Chemical stability studies indicate that the phosphoramidates have good chemical stability at pH 2.0 and at pH 7.4 phosphate buffer. Phosphoramidate prodrugs were hydrolyzed in vitro by esterase or lipase and found to be better substrates for lipases than for esterases. 1,3-Diol cyclic phosphates showed potent anti-HIV activity without increasing the cytotoxicity compared with that of DOT and have good chemical and enzymatic stability. Long-chain lipid phosphates, although showed potent anti-HIV activity, exhibited increased cytotoxicity.

Characterizing the ovotoxicity of cyclophosphamide metabolites on cultured mouse ovaries

Cyclophosphamide (CPA) is reported to target dormant primordial ovarian follicles in rodents and humans. However, mechanistic studies are complicated due to the complex ovarian structure. We present here the characterization of the sensitivity of ovaries to CPA metabolites and the timing of morphological alterations induced by phosphoramide mustard (PM) in an in vitro system. Intact mouse ovaries (postnatal-day-4) were cultured in vitro and exposed to multiple breakdown products of CPA on day 0 (d0). Tissues were cultured up to d8, and then follicle counts and immunohistochemistry were performed. 4-Hydroperoxy-CPA (4-HC), a precursor of an activated form of CPA, and PM depleted primordial and primary follicles (> or =1 microM and > or =3 microM, respectively, p < 0.05); acrolein had effects on follicle numbers only under continuous exposure (> =30 microM); carboxycyclophosphamide and 4-ketocyclophosphamide reduced primordial and small primary follicles only at high concentrations (100 microM). PM-induced follicle loss became significant (p < 0.05) by d1 or d2 following exposures to 10 microM or 3 microM PM, respectively, as determined by the numbers of pyknotic or TUNEL-positive follicles. Cellular targets were oocytes in the smallest follicles, but granulosa cells in large primary follicles. TUNEL staining was observed in both cell types, but caspase-3, a marker of apoptosis, was absent from primordial follicles. In addition, a pan-caspase inhibitor could not prevent follicle losses when administered prior to PM. Thus, brief exposures to 4-HC or PM are sufficient to induce permanent follicle loss in ovaries, and PM is likely the ultimate ovotoxicant. Furthermore, the cell death pathway is likely caspase-independent.

Cyclophosphamide and related anticancer drugs

This article presents an overview of the methods of bioanalysis of oxazaphosphorines, in particular, cyclophosphamide, ifosfamide, and trofosfamide as well as their metabolites. The metabolism of oxazaphosphorines is complex and leads to a large variety of metabolites and therefore the spectrum of methods used is relatively broad. The various methods used are shown in a table and the particularly important assays are described.

Liquid chromatography-tandem mass spectrometric quantitation of cyclophosphamide and its hydroxy metabolite in plasma and tissue for determination of tissue distribution

Cyclophosphamide (CP) and its metabolite, hydroxycyclophosphamide (OH-CP) have been quantitated in mouse plasma and tissue by derivatization combined with liquid chromatography-tandem mass spectrometry (LC-MS-MS). The derivatization was conducted immediately upon sample collection, to trap the OH-CP metabolite intermediate prior to further conversion to phosphoramide mustard or other reaction products. This simple and straightforward derivatization procedure, combined with sample extraction via protein precipitation, allowed quantitation of CP and the oxime derivative of OH-CP in plasma for concentrations ranging from approximately 12.5-3333 ng/ml, and in spleen tissue for concentrations of 1,250-50,000 ng/g. The short cycle time (2.5 min) of the LC-MS-MS method allowed high throughput analysis with minimal matrix interference. Mouse plasma levels were quantitated for doses of 40, 65 and 120 mg/kg; spleen concentrations were determined for mice dosed at 120 mg/kg. The CP and oxime plasma levels correlated well with dose amounts. The CP levels in the spleen and plasma were similar while the oxime levels in the spleen were significantly lower than the plasma.

Inactivation of aldophosphamide by human aldehyde dehydrogenase isozyme 3

Tumors resistant to chemotherapeutic oxazaphosphorines such as cyclophosphamide often overexpress aldehyde dehydrogenase (ALDH), some isozymes of which catalyze the oxidization of aldophosphamide, an intermediate of cyclophosphamide activation, with formation of inert carboxyphosphamide. Since resistance to oxazaphosphorines can be produced in mammalian cells by transfecting them with the gene for human ALDH isozyme 3 (hALDH3), it seems possible that patients receiving therapy for solid tumors with cyclophosphamide might be protected from myelosuppression by their prior transplantation with autologous bone marrow that has been transduced with a retroviral vector causing overexpression of hALDH3. We investigated whether retroviral introduction of hALDH3 into a human leukemia cell line confers resistance to oxazaphosphorines. This was examined in the polyclonal transduced population, that is, without selecting out high expression clones. hALDH3 activity was 0.016 IU/mg protein in the transduced cells (compared with 2x10(-5) IU/mg in untransduced cells), but there was no detectable resistance to aldophosphamide-generating compounds (mafosfamide or 4-hydroperoxycyclophosphamide). The lack of protection was due, in part, to low catalytic activity of hALDH3 towards aldophosphamide, since, with NAD as cofactor, the catalytic efficiency of homogeneous, recombinant hALDH3 for aldophosphamide oxidation was shown to be about seven times lower than that of recombinant hALDH1. The two polymorphic forms of hALDH3 had identical kinetics with either benzaldehyde or aldophosphamide as substrate. Results of initial velocity measurements were consistent with an ordered sequential mechanism for ALDH1 but not for hALDH3; a kinetic mechanism for the latter is proposed, and the corresponding rate equation is presented.

Cyclophosphamides as hypoxia-activated diffusible cytotoxins: a theoretical study

Cyclophosphamides have been in clinical use as anti-cancer drugs for a long time and much research has been directed towards reducing their side effects. Here we have performed a theoretical investigation into the possibility of designing bioreductive analogues of cyclophosphamides. Our calculations have employed semiempirical molecular orbital AM1-SM2 and PM3-SM3 calculations, as implemented in MOPAC 93, which include a modified Born method for the treatment of solvation. We have investigated the effect of bioreductive activation on the beta-elimination reaction that is central to the activation of cyclophosphamides. The approach was tested on two known bioreductive agents, including CB1954, and gave results in agreement with experiment. Non-local density functional calculations on CB1954 and its metabolites, including the radical anion, were in agreement with the semiempirical calculations. The calculations have identified a number of potentially novel bioreductive cyclophosphamides. In particular, our calculations identified compounds in which the initial one-electron reduction was not activating. Such compounds are likely to be more effective bioreductive agents, as the beta-elimination will not compete under oxic conditions with the important re-oxidation required for the protection of oxic tissue.

Analysis of ifosfamide, 4-hydroxyifosfamide, N2-dechloroethylifosfamide, N3-dechloroethylifosfamide and iphosphoramide mustard in plasma by gas chromatography-mass spectrometry

A sensitive and specific method for the simultaneous quantitation of ifosfamide (IF), 4-hydroxylifosfamide (4-OHIF), N2-dechloroethylifosfamide (N2D), N3-dechloroethylifosfamide (N3D) and iphosphoramide mustard (IPM) has been developed using gas chromatography-mass spectrometry (GC-MS) with an ion-trap mass spectrometer. Deuterium labeled analogues for each of these analytes were synthesized as the internal standards. The labile 4-OHIF in plasma was first converted to the more stable cyanohydrin adducts before dichloromethane extraction. IPM was extracted by C18 reversed-phase resin. All analytes were converted to their silyl derivatives before GC-MS analysis. The sensitivity limits ranged from 0.1 to 0.5 microgram/ml when 100 microliters of plasma was used. This method was validated with within-run coefficients of variation less than 5% (n = 8) and between-run coefficients of variation less than 12% (n = 6). The method was applied to the determination of plasma levels of IF and metabolites in the rat.

Effect of stereochemistry on the oxidative metabolism of the cyclophosphamide metabolite aldophosphamide

31P NMR and cell perfusion techniques were used to investigate the conversion of the individual enantiomers of aldophosphamide (AP) to carboxyphosphamide (CBP) as catalyzed by aldehyde dehydrogenase in human erythroleukemia K562 cells. R- and S-cyclophosphamides (CPs) were treated with ozone and hydrogen peroxide to yield Rp- and Sp-cis-4-hydroperoxycyclophosphamides (Rp- and Sp-cis-4-HO2-CP); reduction of each hydroperoxide gave the corresponding enantiomer of AP [along with its tautomer 4-hydroxycyclophosphamide (4-HO-CP)]. In separate experiments, K562 cells embedded in agarose gel threads were perfused at pH 7.4, 21 +/- 1 degrees, with solutions of 1.4 mM Rp- and Sp-4-HO-CP/AP, both with and without added mesna (an acrolein scavenger). A comparison of the 31P NMR spectral data derived from the experiments revealed little statistical difference (+/- 10-20% error limits) in the normalized intensities of the CBP peaks arising from the individual AP enantiomers [with added mesna, the ratio Rp-CBP:Sp-CBP was 1.00:1.24 +/- 0.13 (average deviation); without mesna, the same ratio was 1.00:1.35]. Using conventional methods for evaluating the in vitro drug toxicities, CP-resistant L1210 cells were treated in separate experiments with Rp- and Sp-cis-4-HO2-CP; there were no significant differences between the toxicities exhibited by the stereoisomers.

Quantitation of 4-hydroxycyclophosphamide/aldophosphamide in whole blood

There is considerable interest in determining 4-hydroxycylcophosphamide/aldophosphamide (4-HO-CP/AP) blood levels in patients receiving the prodrug, cyclophosphamide (CP). Phosphoramide mustard (PM), the alkylating metabolite of CP, is relatively impermeable to cell membranes and it is generally believed that circulating intermediary metabolites, including aldophosphamide, the immediate precursor of PM, is transported by circulating blood to tumor tissue. Therefore, circulating 4-HO-CP/AP blood levels should more closely reflect the oncostatic and cytotoxic effects of CP than the parent drug. We have developed a gas chromatographic electron-impact mass spectrometric (GC-EIMS) method suitable for routine monitoring of 4-HO-CP/AP levels in whole blood over the range 0.085 microM (25 ng/ml) to 34 microM (10 micrograms/ml). The unstable metabolites were derivatized with O-(2,3,4,5,6-pentafluorobenzyl)hydroxylamine-HCl to form a stable aldophosphamide oxime derivative (PBOX). [2H4]PBOX was used as an internal standard. For clinical samples, tubes were prepared prior to blood drawing, which contained the derivatizing reagent solution and the internal standard. These solutions were stable for up to 3 months when stored at room temperature. Following addition of blood to the reaction tubes, PBOX formation was rapid and the resulting derivative was stable under these conditions for up to 8 days at room temperature. Application of the method was demonstrated by quantitating 4-HO-CP/AP blood levels in patients receiving 4 g/m2 intravenous infusion of CP over a period of 90 min.

Oxime derivatives of the intermediary oncostatic metabolites of cyclophosphamide and ifosfamide: synthesis and deuterium labeling for applications to metabolite quantification

There is ongoing interest in the selective, quantitative analysis of the cyclophosphamide metabolites 4-hydroxycyclophosphamide (2a) and aldophosphamide (3a) because these tautomers are generally believed to play a key role in oncostatic selectivity and metabolite transport. O-(2,3,4,5,6-Pentafluorobenzyl)hydroxylamine (C6F5CH2ONH2, 1 equiv) provided for the complete conversion (by 31P NMR, 60% reaction within 15 min at 20 degrees C) of 2a/3a (17 mM in H2O/CH3OH) to E/Z-aldophosphamide O-(2,3,4,5,6-pentafluorobenzyl)oxime [C6F5CH2ON = CHCH2CH2OP-(O)(NH2)N(CH2CH2Cl)2; E:Z = 54:46 (+/- 3% average deviation)]. Under these conditions, the oxime exhibited little (6%) decomposition over 3 weeks. Parallel studies showed that 4-hydroxyifosfamide/aldoifosfamide reacted completely to give the analogous aldoifosfamide oxime [C6F5CH2ON = CHCH2CH2OP(O)(NHCH2CH2Cl)2; E:Z = 52:48 (+/- 1% average deviation)] with 50% reaction within 15 min at 20 degrees C with no product decomposition over 3 weeks. In aqueous methanol and with 2 equiv C6F5CH2ONH2, clinically useful 4-hydroperoxycyclophosphamide (10 mM; tau 1/2 = 10 min, 37 degrees C) and its isomer 4-hydroperoxyifosfamide (10 mM; tau 1/2 = 25 min, 20 degrees C) underwent complete conversion to the corresponding aldehyde oximes. Each oxime was synthesized with deuterium in the chloroethyl moieties for use as internal standards in GC/MS applications.

The interaction of glutathione with 4-hydroxycyclophosphamide and phosphoramide mustard, studied by 31P nuclear magnetic resonance spectroscopy

Development of resistance of cancer cells against cyclophosphamide (CP) is probably associated with an increased conjugation with glutathione. 31P NMR spectroscopy was used to monitor the time courses for the chemical conjugation with glutathione of the CP metabolites 4-hydroxycyclophosphamide (4-OHCP) and phosphoramide mustard (PM) at 24 degrees C. PM incubated with a 10-fold molar excess of glutathione showed a disappearance of the PM signal (t1/2 = 112 min), accompanied by an increase of two signals, attributed to the intermediate PM monoglutathione conjugate and the PM diglutathione conjugate. After 680 min, only a signal assigned to the PM diglutathione conjugate was found. This conjugate was relatively stable. The formation of the PM diglutathione conjugate was confirmed with fast atom bombardment mass spectrometry (FAB-MS). The rate constant for the disappearance of the PM signal in incubations with glutathione was 6.2 x 10(-3) min-1, and was 5.4 x 10(-3) min-1 in incubations without glutathione, indicating that the rate-limiting step in both reactions in the formation of aziridinium ions. When 4-OHCP was incubated with a 10-fold molar excess of glutathione, six signals was found which were not present in spectra of incubations without glutathione. In addition to the signals assigned to the mono- and diglutathionyl conjugates of PM, four signals were found of which the pattern of formation in time was identical. These four signals correspond to the four stereoisomers of 4-glutathionylcyclophosphamide (4-GSCP). The formation of 4-GSCP was confirmed with FAB-MS. Within 120 min after the start of the reaction no free 4-OHCP or aldophosphamide signals were found in the spectra. Free PM was detected in all spectra indicating that degradation of 4-GSCP gives rise to PM, the ultimate cytotoxic metabolite of CP, 4-GSCP therefore appears an important pool of phosphoramide mustard, which in turn can be deactivated by glutathione.

Drug interaction effects on antitumour drugs (XV): Disulfiram as protective agent against cyclophosphamide-induced urotoxicity without compromising antitumour activity in mice

The prevention of cyclophosphamide-induced urotoxicity by disulfiram was studied in mice. A single dose of cyclophosphamide (100-400 mg/kg, intraperitoneally) produced a significant dose-dependent increase in urinary bladder weight within 48 hr of treatment. Disulfiram prevented cyclophosphamide-induced bladder damage in a dose-dependent manner in mice when orally administered simultaneously with antitumour agents, but failed to diminish the acute toxicity, leukocytotoxicity and immunotoxicity of cyclophosphamide. The protective effect of disulfiram on the bladder was critically dependent on administration timing. Oral administration of disulfiram between 60 min. before and 60 min. after the injection of cyclophosphamide was found to be effective. The optimum time was simultaneous administration of both drugs. Diethyldithiocarbamate and carbon disulfide, metabolites of disulfiram, prevented cyclophosphamide-induced bladder damage when administered simultaneously with cyclophosphamide 1 to, 3 or 5 hr afterwards. Disulfiram slightly potentiated the antitumour activity of cyclophosphamide against Sarcoma 180 or EL-4 leukaemia in vivo when administered simultaneously with cyclophosphamide. In contrast, diethyldithiocarbamate or carbon disulfide did not interfere with cyclophosphamide antitumour activity when administered 3 hr after cyclophosphamide. From these preliminary studies, disulfiram appears to be a likely candidate for protection against cyclophosphamide-induced urotoxicity without compromising the therapeutic utility of the alkylating agent.

Synthesis and anticancer activity of 2-substituted 2,3-dihydro-1,3,2- benzoxazaphosphorin-4-one and its 2-oxide derivatives

As a result of the reaction of salicylic amide with PCl3 and POCl3 2-chloro-2,3-dihydro-1,3,2-benzoxazaphosphorin-4-one (1) and its 2-oxide 2 are obtained. Compounds 1 and 2 form amides with amines in 2-position. Antineoplastic action of the derivatives containing the bis(2-chloroethyl)amide group in 2-position was found.

Cardiotoxicity of commercial 5-fluorouracil vials stems from the alkaline hydrolysis of this drug

The cardiotoxicity of 5-fluorouracil (FU) was attributed to impurities present in the injected vials. One of these impurities was identified as fluoroacetaldehyde which is metabolised by isolated perfused rabbit hearts into fluoroacetate (FAC), a highly cardiotoxic compound. FAC was also detected in the urine of patients treated with FU. These impurities were found to be degradation products of FU that are formed in the basic medium employed to dissolve this compound. To avoid chemical degradation of this antineoplastic drug, the solution of FU that will be injected should be prepared immediately before use.

[Synthesis and cytostatic effect of 2-phenyl-3-alkyl(aryl)-2,3-dihydro-1,3,2(lambda-5)- benzoxazaphosphorine-4-one derivatives]

Preparation of 2,3-dihydro-1,3,2-benzoxazaphosphorin-4-one derivatives and their 2-oxo derivatives is described. Compounds 1 and 12 have anticancer activity.

Glutathione diminishes the anti-tumour activity of 4-hydroperoxycyclophosphamide by stabilising its spontaneous breakdown to alkylating metabolites

Evidence was obtained showing that GSH protects against the cytotoxicity of 4-hydroperoxycyclophosphamide (4-OOH-CP) by minimizing the spontaneous fission of 4-hydroxycyclophosphamide (4-OH-CP), its breakdown product, to the ultimate toxic species, phosphoramide mustard (PM). This conclusion was borne out in two series of experiments. The first demonstrated that 4-OH-CP was progressively more stable in aqueous solutions containing increasing concentrations of GSH. The second series of experiments were carried out with tumour cell lines with high (SKOV-3) and low (KHT) GSH contents. The cytotoxicity of 4-OOH-CP, a stable precursor that rapidly gives rise to 4-OH-CP spontaneously under physiological conditions, was enhanced in GSH-depleted SKOV-3 cells, but was unchanged in GSH-depleted KHT cells. It is concluded that the high GSH content of SKOV-3 cells provides a significant protection against 4-OH-CP by limiting the breakdown/activation of 4-OH-CP. Deschloro-4-hydroperoxycyclophosphamide (deschloro-4-OOH-CP), an analogue of 4-OOH-CP that generates acrolein (AC) but not PM in the spontaneous fission reaction, is essentially non-toxic when compared with 4-OOH-CP but is equally potent in depleting GSH. It is postulated that AC may promote the cytotoxicity of the parent 4-OH-CP by depleting cellular GSH. Consequently, the stabilising influence of GSH on 4-OH-CP is removed, leading to increased formation of PM, the ultimate cytotoxic agent.

Cellular glutathione as a protective agent against 4-hydroperoxycyclophosphamide cytotoxicity in K-562 cells

Exposure of cells of the K-562 erythroleukemia cell line to 4-hydroperoxycyclophosphamide (4-HC), an analog of activated cyclophosphamide, causes a concentration-dependent inhibition of in vitro colony formation by these cells. For investigation of the role of glutathione (GSH) in the metabolism of 4-HC, GSH levels of K-562 cells were modulated by exposing the cells to buthionine sulfoximine (BSO), a specific inhibitor of GSH synthesis, and/or to GSH ethyl esters. Both the mono- and diethyl esters of GSH were synthesized in our laboratories and their identities were determined by chromatographic methods and fast-atom-bombardment mass spectrometry. An HPLC method including electrochemical detection used for thiol determination was applied for the measurement of GSH esters. Incubation of the cells with BSO depleted GSH levels to approximately 11% of control values and potentiated the cytotoxicity of 4-HC. By contrast, exposure to GSH esters approximately doubled GSH levels and protected the cells against the toxicity of 4-HC. Moreover, when cellular GSH levels were first depleted by BSO exposure and then replenished by incubation with GSH esters, the BSO-associated potentiation of 4-HC cytotoxicity was abolished. The work described herein extends the application of an HPLC method used for thiol determination to the measurement of GSH ethyl esters. In addition, it established that GSH acts as a competitive protecting agent against the in vitro toxicity of 4-HC in the K-562 cell line.

Analysis of 4-hydroxycyclophosphamide by gas chromatography-mass spectrometry in plasma

A sensitive and specific method for the quantitative analysis of 4-hydroxycyclophosphamide/aldophosphamide has been developed using gas chromatography-mass spectrometry and a deuterium-labeled analogue as the internal standard. The labile 4-hydroxycyclophosphamide/aldophosphamide and the internal standard in plasma were first converted into the more stable cyanohydrin adducts before extraction. The isolated adducts were silylated and the products analyzed by gas chromatography-mass spectrometry. The assay was found to be linear from 50 to 5000 ng/ml in plasma with a routine detection limit of 50 ng/ml. The within- and between-run standard deviations at 100 ng/ml on eight replicate determinations were found to be 6.2 and 11.9%, respectively. The extraction recovery was ca. 80%. This analytical method was used to evaluate the stability of 4-hydroxycyclophosphamide/aldophosphamide in fresh rat and pooled human plasma and to measure plasma 4-hydroxycyclophosphamide/aldophosphamide concentrations in the rat.

Embryotoxicity of phenyl ketone analogs of cyclophosphamide

Phenylketophosphamide and phenylketoisophosphamide are preactivated acyclic ketone analogs of cyclophosphamide and isophosphamide with antitumor activity. These compounds undergo an elimination reaction to yield phosphoramide or isophosphoramide mustard and phenyl vinyl ketone. In this study, the embryotoxicity of phenylketophosphamide, phenylketoisophosphamide, and phenyl vinyl ketone were determined. Embryotoxicity was assessed in vitro in whole rat embryos cultured on day 10.5 of gestation in the absence and presence of an activating system derived from maternal liver. Both phenylketophosphamide and phenylketoisophosphamide were embryotoxic in the absence of metabolic activation. Moreover, there was no enhancement of this embryotoxicity in the presence of an activating system. A 10-microM concentration of phenylketophosphamide produced 100% malformed embryos, while this concentration of phenylketoisophosphamide was not teratogenic. At 25 microM phenylketoisophosphamide, all the surviving exposed embryos were malformed. Phenylketophosphamide was embryolethal to more than 50% of the exposed embryos at a concentration of 50 microM. In contrast, a concentration of phenylketoisophosphamide of 100 microM was required to produce significant embryolethality. Phenyl vinyl ketone was not embryotoxic at any of the concentrations tested. The major malformation observed, a hypoplastic prosencephalon, and the growth retardation effects were not only similar for phenylketophosphamide and phenylketoisophosphamide, but also similar to those previously reported for "activated" cyclophosphamide. Unlike the results with cyclophosphamide, where both phosphoramide mustard and the aldehydic metabolite of cyclophosphamide, acrolein, are toxic, the embryotoxic effects of phenylketophosphamide and phenylketoisophosphamide are mediated only by the mustard metabolite.

N-(2-hydroxyethyl)-N-[2-(7-guaninyl)ethyl]amine, the putative major DNA adduct of cyclophosphamide in vitro and in vivo in the rat

The anti-cancer agent, cyclophosphamide, metabolises to the cytotoxic alkylating agent phosphoramide mustard, which can be dephosphoramidated to give nornitrogen mustard. A rat liver mitochondrial supernatant system was used to study the binding of [chloroethyl 3H]cyclophosphamide to DNA. The reacted DNA was acid-hydrolysed and one major adduct was identified using Sephadex G-10 chromatography, followed by HPLC, using reversed-phase or ion-exchange systems. Further studies, using [14C]guanine as reaction substrate for [chloroethyl 3H]cyclophosphamide, phosphoramide mustard or nornitrogen mustard, demonstrated the main adduct from each reaction had identical chromatographic properties in these systems. The radiolabelled ratio in the [3H]cyclophosphamide-[14C]guanine reaction demonstrated a monoadducted product. From this evidence and from 1H NMR data, the common adduct was putatively identified as a hydroxylated nornitrogen mustard adduct (N-(2-hydroxyethyl)-N-[2-(7-guaninyl)ethyl]amine). In in vivo studies, rats were injected intraperitoneally with 2.775 MBq [3H]cyclophosphamide. Total organ [3H] content and DNA binding levels were ascertained. Maximal levels of [3H] binding to DNA were seen between 1-4 hr with the highest binding levels observed in the bladder. The in vivo adduct was shown, using various HPLC systems, to co-chromatograph with the in vitro adduct and thus the main in vivo adduct was putatively identified as N-(2-hydroxyethyl)-N-[2-(7-guaninyl)ethyl]amine.

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.

Combined thin-layer chromatography-photography-densitometry for the quantitation of cyclophosphamide and its four principal urinary metabolites

A novel method for the quantitative determination of the anti-cancer drug cyclophosphamide and its principal urinary metabolites 4-oxocyclophosphamide, carboxyphosphamide, phosphoramide mustard and bis(2-chloroethyl)amine has been devised. The assay combines adsorption of drug-related material onto Amberlite XAD-2 and thin-layer chromatography with spot visualization using 4-(4-nitrobenzyl)pyridine, rapid photography and densitometry. The intra-assay coefficient of variation for each compound was less than 6%. The limit of detection of the assay was 1 microgram ml-1 for cyclophosphamide, phosphoramide mustard and bis(2-chloroethyl)amine and 0.5 microgram ml-1 for 4-oxocyclophosphamide and carboxyphosphamide. The method was validated for cyclophosphamide and 4-oxocyclophosphamide using gas chromatography. It is concluded that the method provides the first means of determining the full metabolic spectrum for cyclophosphamide in patients without recourse to the administration of radioisotopically labelled drug.

Stability of solutions of antineoplastic agents during preparation and storage for in vitro assays. General considerations, the nitrosoureas and alkylating agents

In vitro drug sensitivity of tumour biopsies is currently being determined using a variety of methods. For these chemosensitivity assays many drugs are required at short notice, and this in turn means that the drugs must generally be stored in solution. There are, however, a number of potential problems associated with dissolving and storing drugs for in vitro use, which include (a) drug adsorption; (b) effects of freezing; (c) drug stability under the normal conditions of dilution and setting up of an in vitro assay; and (d) insolubility of drugs in normal saline (NS) or phosphate-buffered saline (PBS). These problems are considered in general, and some recommendations for use of solutions of drugs in in vitro assays are suggested. The nitrosoureas and alkylating agents are also investigated in greater detail in this respect. The nitrosoureas are found to be very labile in PBS at pH 7, with 5% degradation (t0.95) occurring in 10-50 min at room temperature. These values are increased about 10-fold on refrigeration and about 5- to 10-fold on reduction of the pH of the medium to pH 4-5. At pH 7 and room temperature, t0.95 is observed in under 1 h with the alkylating agents nitrogen mustard, chlorambucil, melphalan, 2,5-diaziridinyl-3,6-bis(2-hydroxyethylamino)-1,4-benzoquinone (BZQ), dibromodulcitol, dibromomannitol, treosulphan, and procarbazine. Of the other alkylating agents, 4-hydroperoxycylophosphamide (sometimes used in vitro in place of cyclophosphamide), busulphan, dianhydrogalactitol, aziridinylbenzoquinone (AZQ), and dacarbazine have a t0.95 of between 2 and 24 h, while ifosfamide and pentamethylmelamine are both stable in aqueous solution for greater than 7 days. About half the drugs studied in detail have been stored frozen in solution for in vitro use, although very little is known about their stability under these conditions.