Short-term exposure of human ovarian follicles to cyclophosphamide metabolites seems to promote follicular activation in vitro

How chemotherapy affects dormant ovarian primordial follicles is unclear. The 'burnout' theory, studied only in mice, suggests cyclophosphamide enhances primordial follicle activation. Using 4-hydroperoxycyclophosphamide (4hc) and phosphoramide mustard (PM), this study assessed how the active cyclophosphamide metabolites 4-hydroxycyclophosphamide (4-OHC) and PM, affect human primordial follicles. Frozen-thawed human ovarian samples were sliced and cultured with basic culture medium (cultured controls) or with 4hc/PM (3 µmol/l/10 µmol/l) (treated samples) for 24-48 h. Follicular counts and classification, Ki67 and anti-Müllerian hormone (AMH) immunohistochemistry and an apoptosis assay were used for evaluation, and 17β-oestradiol and AMH were measured in spent media samples. Generally, there was primordial follicle decrease and elevated developing follicle rates in treated samples compared with cultured (P = 0.04 to P < 0.0005) and uncultured controls (P < 0.05 to P < 0.0001). No traces of apoptosis were found. There were almost twicethe levels of AMH and 17β-oestradiol in treated compared with untreated samples (AMH with 4hc 3 µmol/l; P = 0.04). All follicles stained positively for AMHincluded treated samples. Ki67 positive staining was noted in all samples. Cyclophosphamide metabolites seem to enhance human primordial follicle activation to developing follicles, in vitro. Study findings support the 'burnout' theory as the mechanism of chemotherapy-induced ovarian toxicity.

Labeled oxazaphosphorines for applications in mass spectrometry studies. 2. Synthesis of deuterium-labeled 2-dechloroethylcyclophosphamides and 2- and 3-dechloroethylifosfamides

The prodrugs cyclophosphamide (CP) and ifosfamide (IF) each metabolize to an active alkylating agent through a cytochrome P450-mediated oxidation at the C-4 position. Competing with this activation pathway are enzymatic oxidations at the exocyclic α and α' carbons, which result in dechloroethylation of CP and IF. The incidence of oxidation at one position relative to another is believed to be at least one factor underlying the high degree of interpatient variability in both CP and IF pharmacokinetics. As standards for the mass spectrometry quantification of dechloroethylation, the following were synthesized: (1) [4,4,5,5-(2) H4 ]-2-dechloroethylcyclophosphamide (equivalent to [4,4,5,5-(2) H4 ]-3-dechloroethylifosfamide); (2) [α,α,4,4,5,5-(2) H6 ]-2-dechloroethylcyclophosphamide (equivalent to [α,α,4,4,5,5-(2) H6 ]-3-dechloroethylifosfamide); and (3) [α,α,4,4,5,5-(2) H6 ]-2-dechloroethylifosfamide. The common precursor to all of the target compounds was [2,2,3,3-(2) H4 ]-3-aminopropanol. A one-pot reaction of this compound with POCl3 and unlabeled or labeled 2-chloroethylamine hydrochloride gave the d4 and d6 labeled 2-dechloroethylcyclophosphamides. The construction of the 2-dechloroethylifosfamide from the aminopropanol required five discreet steps. Optimization of the synthetic pathways and stability studies are discussed.

Non-invasive monitoring of L-2-oxothiazolidine-4-carboxylate metabolism in the rat brain by in vivo 13C magnetic resonance spectroscopy

The cysteine precursor L-2-oxothiazolidine-4-carboxylate (OTZ, procysteine) can raise cysteine concentration, and thus glutathione levels, in some tissues. OTZ has therefore been proposed as a prodrug for combating oxidative stress. We have synthesized stable isotope labeled OTZ (i.e. L-2-oxo-[5-(13)C]-thiazolidine-4-carboxylate, (13)C-OTZ) and tracked its uptake and metabolism in vivo in rat brain by (13)C magnetic resonance spectroscopy. Although uptake and clearance of (13)C-OTZ was detectable in rat brain following a bolus dose by in vivo spectroscopy, no incorporation of isotope label into brain glutathione was detectable. Continuous infusion of (13)C-OTZ over 20 h, however, resulted in (13)C-label incorporation into glutathione, taurine, hypotaurine and lactate at levels sufficient for detection by in vivo magnetic resonance spectroscopy. Examination of brain tissue extracts by mass spectrometry confirmed only low levels of isotope incorporation into glutathione in rats treated with a bolus dose and much higher levels after 20 h of continuous infusion. In contrast to some previous studies, bolus administration of OTZ did not alter brain glutathione levels. Even a continuous infusion of OTZ over 20 h failed to raise brain glutathione levels. These studies demonstrate the utility of in vivo magnetic resonance for non-invasive monitoring of antioxidant uptake and metabolism in intact brain. These types of experiments can be used to evaluate the efficacy of various interventions for maintenance of brain glutathione.

Drug focus: Pharmacogenetic studies related to cyclophosphamide-based therapy

Cyclophosphamide is a cornerstone in the treatment of many pediatric and adult malignancies, as well as in the treatment of refractory autoimmune conditions. Genetic factors are thought to play a role in the interindividual variation in both response and toxicities associated with cyclophosphamide-based therapies. This drug focus reviews the most compelling studies conducted on the pharmacogenetics of cyclophosphamide-based therapies. Broader pharmacogenomic studies are needed and may reveal additional factors important in susceptibility to toxicity and/or response to therapy.

Pharmacodynamic and pharmacokinetic study of chronic low-dose metronomic cyclophosphamide therapy in mice

Prolonged, frequently administered low-dose metronomic chemotherapy (LDM) is being explored (pre)clinically as a promising antiangiogenic antitumor strategy. Although appealing because of a favorable side effect profile and mostly oral dosing, LDM involves new challenges different from conventional maximum tolerated dose chemotherapy. These include possible altered pharmacokinetic characteristics due to long-term drug exposure potentially resulting in acquired resistance and increased risk of unfavorable drug interactions. We therefore compared the antitumor and antivascular effects of LDM cyclophosphamide (CPA) given to mice that had been pretreated with either LDM CPA or normal saline, obtained blood 4-hydroxy-CPA (activated CPA) concentrations using either gas chromatography/mass spectrometry or liquid chromatography/tandem mass spectrometry in mice treated with LDM CPA, and measured hepatic and intratumoral activity of enzymes involved in the biotransformation of CPA and many other drugs [i.e., cytochrome P450 3A4 (CYP3A4) and aldehyde dehydrogenase]. Exposure of mice to LDM CPA for >or=8 weeks did not compromise subsequent activity of LDM CPA therapy, and biologically active 4-hydroxy-CPA levels were maintained during long-term LDM CPA administration. Whereas the effects on CYP3A4 were complex, aldehyde dehydrogenase activity was not affected. In summary, our findings suggest that acquired resistance to LDM CPA is unlikely accounted for by altered CPA biotransformation. In the absence of reliable pharmacodynamic surrogate markers, pharmacokinetic parameters might become helpful to individualize/optimize LDM CPA therapy. LDM CPA-associated changes of CYP3A4 activity point to a potential risk of unfavorable drug interactions when compounds that are metabolized by CYP3A4 are coadministered with LDM CPA.

Role of MGMT in protecting against cyclophosphamide-induced toxicity in cells and animals

O(6)-Methylguanine-DNA methyltransferase (MGMT) is a DNA repair protein that protects cells from the biological consequences of alkylating agents by removing alkyl groups from the O(6)-position of guanine. Cyclophosphamide and ifosfamide are oxazaphosphorines used clinically to treat a wide variety of cancers; however, the role of MGMT in recognizing DNA damage induced by these agents is unclear. In vitro evidence suggests that MGMT may protect against the urotoxic oxazaphosphorine metabolite, acrolein. Here, we demonstrate that Chinese hamster ovary cells transfected with MGMT are protected against cytotoxicity following treatment with chloroacetaldehyde (CAA), a neuro- and nephrotoxic metabolite of cyclophosphamide and ifosfamide. The mechanism by which MGMT recognizes damage induced by acrolein and CAA is unknown. CHO cells expressing a mutant form of MGMT (MGMT(R128A)), known to have >1000-fold less repair activity towards alkylated DNA while maintaining full active site transferase activity towards low molecular weight substrates, exhibited equivalent CAA- and acrolein-induced cytotoxicity to that of CHO cells transfected with plasmid control. These results imply that direct reaction of acrolein or CAA with the active site cysteine residue of MGMT, i.e. scavenging, is unlikely a mechanism to explain MGMT protection from CAA and acrolein-induced toxicity. In vivo, no difference was detected between Mgmt-/- and Mgmt+/+ mice in the lethal effects of cyclophosphamide. While MGMT may be important at the cellular level, mice deficient in MGMT are not significantly more susceptible to cyclophosphamide, acrolein or CAA. Thus, our data does not support targeting MGMT to improve oxazaphosphorine therapy.

1,3- vs 1,5-Intramolecular Alkylation Reactions in Isophosphoramide and Phosphoramide Mustards

It is well-established that at pH 7.4, intramolecular 1,3-N-alkylation reactions in isophosphoramide mustard (IPM) and phosphoramide mustard (PM) produce electrophilic alkylating agents with aziridinyl moieties. To investigate the role of 1,5-intramolecular cyclizations in the chemistry of IPM and PM, the five-membered ring phospholidine products of these reactions were independently synthesized and characterized by (31)P NMR. In 0.33 M BisTris, pH 7.4, 37 degrees C, the intramolecular O-alkylation product of IPM [2-(2-chloroethylamino)-2-tetrahydro-2H-1,3,2-oxazaphospholidine-2-oxide (11)] had a chemical shift of delta 33.0 and a half-life of 3.3 h. The O-alkylation product of PM [2-amino-3-(2-chloroethyl)tetrahydro-2H-1,3,2-oxazaphospholidine-2-oxide (12)] displayed a chemical shift of delta 30.6 and a half-life of 26.9 h. For both IPM and PM, 1,5-N-alkylation provides the same product [1-(2-chloroethyl)-2-hydroxy-tetrahydro-2H-1,3,2-diazaphospholidine-2-oxide (13)]. Because of its instability, 13 was generated in situ and was not isolated; however, the chemical shift (delta 33.0) and reactivity (half-life 0.3 h at 25 degrees C) of the species attributed to 13 were consistent with the assigned structure. Resonances with (31)P NMR chemical shifts indicative of 11 or 12 did not appear in reaction solutions of IPM or PM. The compound assigned as 13 gave hydrolysis products that were not found in reaction solutions of IPM or PM. The collective data supported the conclusion that intramolecular 1,5-alkylations do not contribute to the chemistry of IPM or PM in aqueous solutions at pH 7.4, 37 degrees C. Conversely, 11 and 12 were found to be the major if not exclusive products formed in DMSO solutions of the respective cyclohexylammonium salts of IPM and PM. Both 11 and 12 were relatively noncytotoxic against a series of cell lines, but there were differences in mutagenicities. Chinese hamster ovary cells were exposed to 11 or 12 for one half-life of each compound; 11 was nonmutagenic up to 500 microM, while 12 (500 microM) was mutagenic with 246 mutant colonies/10(6) surviving cells.

Selective enhancement of ifosfamide-induced toxicity in Chinese hamster ovary cells

PURPOSE

O6-benzylguanine (BG) is a unique purine analog that has been shown to influence nitrogen mustard activity and increase cytotoxicity. Ifosfamide is a nitrogen mustard with growing clinical applications; effective modulation may lead to improved efficacy. We thus undertook a preliminary investigation of BG's effects on ifosfamide and ifosfamide derivatives in vitro.

EXPERIMENTAL DESIGN

BG's effect on ifosfamide toxicity was studied in CHO cells transfected with O6-alkylguanine-DNA alkyltransferase (AGT) (CHOwtAGT) or control plasmid pcDNA3 (CHOpcDNA) using five ifosfamide derivatives and two control compounds: 4-hydroperoxyifosfamide (4HI), isophosphoramide mustard (IPM), phenylketoifosfamide (PKIF), 4-hydroperoxydidechloroifosfamide (4HDI), chloroacetaldehyde (CAA), didechloroisophosphoramide mustard (d-IPM), didechlorophenylketoifosfamide (d-PKIF). To further explore the mechanism of interaction, BG's effect on apoptosis (annexin V-FITC) and cell cycle distribution in cells exposed to ifosfamide was also analyzed.

RESULTS

BG substantially enhanced cytotoxicity induced only by agents that produce IPM (4HI, IPM, PKIF) in both CHOwtAGT and CHOpcDNA cell lines. BG did not modulate 4HDI or CAA cytotoxicity. The addition of BG to IPM in CHO cells increased the percentage of apoptotic cells from 5.5% to 28.9% at 72 h after treatment. Cell cycle analysis showed that BG exposure was associated with G1 arrest. At 16 h following treatment with IPM, PKIF, or phosphoramide mustard (PM), BG increased the percentage of cells in G1 from 16-20% to 29-64%.

CONCLUSIONS

BG's ability to increase 4HI-, IPM-, and PKIF-mediated cytotoxicity in cells devoid of AGT activity suggests a novel AGT-independent mode of action that is associated with increased apoptosis and may involve G1 arrest. BG selectively enhanced IPM toxicity without enhancement of acrolein and CAA toxicity. The data strongly support further investigation into combinations of BG and nitrogen mustards.

Evaluation of an elastin-like polypeptide-doxorubicin conjugate for cancer therapy

Thermally responsive elastin-like polypeptides (ELPs) were synthesized by recombinant DNA techniques and conjugated to doxorubicin through an acid-labile hydrazone bond to enable release of the drug in the acidic environment of lysosomes. The thermal properties, intracellular localization and cytotoxicity of the conjugate were investigated in this study. The conjugation procedure resulted in a mixed population of free ELP and ELP-doxorubicin (ELP-dox) conjugates that exhibit a broader transition than the parent ELP. A simple centrifugation procedure was developed to purify the ELP-dox conjugate from other reactants and resulted in a sharper thermal transition, similar to the parent ELP. The ELP was endocytosed by squamous cell carcinoma cells (FaDu) and trafficked into lysosomes, as observed by the colocalization of the ELP with a lysosome-specific dye through confocal fluorescence microscopy. Interestingly, both the ELP-dox conjugate and free drug exhibited near equivalent in vitro cytotoxicity, although their subcellular localization was significantly different. The free drug was largely concentrated in the nucleus, while the conjugate was dispersed throughout the cytoplasm with limited nuclear accumulation. These differences are significant because they suggest a different mechanism of cytotoxicity for the conjugate as compared with the free drug.

Exposure to a deuterated analogue of phenylbutyrate retards S-phase progression in HT-29 colon cancer cells

Differentiation agents that induce neoplastic cells to regain a normal phenotype and/or cause growth arrest without significantly affecting normal cells represent an attractive option for cancer treatment. Analogues of short chain fatty acids, such as phenylbutyrate (PB), have been studied as clinically relevant agents. In an attempt to improve its pharmacokinetic profile, structural modifications of PB and other fatty acids have been studied. We hypothesize that strategic isotopic modification of PB would result in a longer half-life and thus translate into a more potent differentiation agent for clinical use. Using a colon cancer model, we demonstrated that 2,2,3,3-tetradeuterated PB (D4PB) significantly increased induction of apoptosis and inhibition of cell proliferation as compared with PB and butyrate. Difference in potency could not be explained by the effect of D4PB on the expression of specific regulatory proteins of the apoptotic cascade or from the inhibitory effect of D4PB on histone deacetylase activity. Interestingly, exposure of HT-29 colon cancer cells to D4PB resulted in a slowing of S transit, in contrast to butyrate and PB, which induced a G2/M cell cycle block. This difference in cell cycle effect may explain the differences seen in the potency of the phenotypic changes seen with treatment with D4PB. Further studies are needed to elucidate the mechanisms underlying effects of D4PB on the cell cycle.

Modified guanines representing O(6)-alkylation by the cyclophosphamide metabolites acrolein and chloroacetaldehyde: synthesis, stability, and ab initio studies

Alkylation of DNA by acrolein and/or chloroacetaldehyde may result in the mutations that lead to the therapy-induced leukemia associated with cyclophosphamide (and ifosfamide) treatment. O(6)-(n-Propanalyl)guanine (O(6)-PAG) and O(6)-(ethanalyl)guanine (O(6)-EAG) were synthesized for use as authentic standards in investigations of DNA alkylation by acrolein and chloroacetaldehyde, respectively. Preparation of the O-methyl oximes of these aldehydes aided in confirming the structural assignments of O(6)-PAG and O(6)-EAG. HPLC was used to study the stability of O(6)-PAG under a variety of conditions. The decomposition of O(6)-PAG was attributed to an alpha,beta-elimination reaction resulting in the formation of guanine and acrolein. In 0.1 M phosphate-DMSO (9:1), O(6)-PAG (1-10 mM) had a half-life of approximately 1 h (pH 7.4, 37 degrees C). In 0.05 M Tris-DMSO (9:1), the apparent half-life of O(6)-PAG (1-10 mM) was approximately 16 h (pH 7.4, 37 degrees C). The increased lifetime under the latter conditions was attributed to a reversible reaction between Tris and the aldehydic functionality of O(6)-PAG to give a more stable oxazolidine. Under conditions similar to those that would be used for hydrolysis of DNA [0.1 M HCl-DMSO (98:2), pH 1.3, 70 degrees C, 30 min], there was an estimated 10-35% loss of O(6)-PAG. Under the same conditions, O(6)-EAG had apparent half-lives of 6.6 h (phosphate-DMSO) and 2.5 days (Tris-DMSO) and the estimated loss at pH 1.3 over 30 min (70 degrees C) was 15-20%. Ab initio quantum chemical calculations were used to understand the energy factors that underlie the occurrence of O- versus N-alkylations as well as possible, subsequent intramolecular cyclizations. Simulations of the free energies of reactions between acrolein and guanine indicated that N-alkylation was favored over O(6)()-alkylation and that cyclizations to tautomers were most favorable if they involved the N-1 or NH(2) positions.

Effect of O6-benzylguanine on nitrogen mustard-induced toxicity, apoptosis, and mutagenicity in Chinese hamster ovary cells

O6-Benzylguanine (BG) inactivates O6-alkylguanine-DNA alkyltransferase (AGT), resulting in an increase in the sensitivity of cells to the toxic effects of O6-alkylating agents. BG significantly enhances the cytotoxicity and decreases the mutagenicity of nitrogen mustards [i.e., phosphoramide mustard (PM), melphalan, and chlorambucil], a group of alkylating agents not known to produce O6-adducts in DNA. The enhancement is observed in cells irrespective of AGT activity. Exposure of Chinese hamster ovary cells to 100 microM BG results in enhancement in the cytotoxicity of PM (300 microM), chlorambucil (40 microM), and melphalan (10 microM) by 9-, 7-, and 18-fold, respectively. In contrast, mutation frequency after treatment with 300 microM PM is decreased from 259 mutants/10(6) cells to 22 mutants/10(6) cells when cells are pretreated with BG. The enhancement of toxicity of these bis-alkylating agents appears to involve cross-link formation, because neither cytotoxicity nor mutagenicity of a monoalkylating PM analogue is significantly altered when combined with BG. Enhanced cytotoxicity and decreased mutagenicity is concomitant with a dramatic increase in the number of cells undergoing apoptosis when BG is combined with PM, melphalan, or chlorambucil at 72-94 h after treatment. Cell cycle analysis demonstrates that BG alone or combined with nitrogen mustards arrests cells in G1 phase of the cell cycle. At 16 h after treatment, 11 and 57% of cells treated with PM alone or with BG plus PM are in G1 phase, respectively. Our data suggest that treatment with BG causes G1 arrest and drives noncycling cells treated with nitrogen mustards into apoptosis, thus protecting against mutagenic DNA damage introduced by nitrogen mustards.

Evidence for a role of chloroethylaziridine in the cytotoxicity of cyclophosphamide

A number of investigators have observed that the use of 4-hydroperoxycyclophosphamide (4-HC) in multiwell plate cytotoxicity assays can be associated with toxicity to cells in wells that contain no drug. Previous reports have implicated diffusion of 4-HC decomposition products, and acrolein in particular, as the active species.

PURPOSE

The purpose of this study was to elucidate the species responsible for the airborne cytotoxicity of 4-HC, and to devise ways to minimize such effects in chemosensitivity assays.

METHODS

To this end, analogues of 4-HC were synthesized to identify the contributions of individual cyclophosphamide metabolites to cytotoxicity. The analogues were then tested for activity against three human breast tumor cell lines (including a line resistant to 4-HC), and one non-small-cell lung carcinoma line. Cytotoxicity was evaluated by assays that quantitate cellular metabolism and nucleic acid content.

RESULTS

Didechloro-4-hydroperoxycyclophosphamide, a compound that generates acrolein and a nontoxic analogue of phosphoramide mustard, gave no cross-well toxicity. In contrast, a significant neighboring well effect was observed with phenylketophosphamide, a compound that generates phosphoramide mustard but not acrolein. Addition of authentic chloroethylaziridine reproduced the airborne toxicity patterns generated by 4-HC and phenylketophosphamide. Increasing the buffering capacity of the growth medium and sealing the microtiter plates prevented airborne cytotoxicity.

CONCLUSION

Since it is unlikely that phosphoramide mustard is volatile, these findings implicate chloroethylaziridine rather than acrolein as the volatile metabolite of 4-HC that is responsible for airborne cytotoxicity. The fact that chloroethylaziridine is generated in amounts sufficient to volatilize, diffuse across wells and cause cytotoxicity indicates that it is an important component in the overall cytotoxicity of 4-HC in vitro. Furthermore, these findings suggest that chloroethylaziridine may also contribute to the toxicity of cyclophosphamide in vivo.

Intraneoplastic polymer-based delivery of cyclophosphamide for intratumoral bioconversion by a replicating oncolytic viral vector

rRp450 is an oncolytic herpesvirus that expresses the CYP2B1 cDNA, responsible for bioconverting cyclophosphamide (CPA) into the active metabolites 4-hydroxyCPA/aldophosphamide (AP). However, formal proof of prodrug activation is lacking. We report that activation of CPA in cells infected with rRp450 generates a time-dependent increase of diffusible 4-hydroxyCPA/AP. For in vivo applications, a CPA-impregnated polymer was implanted into human tumor xenografts inoculated with rRp450. The area under the curve for 4-hydroxyCPA/AP was 806 microg/g of tumor tissue/h when CPA was administered via intraneoplastic polymer and 3 microg/g of tumor tissue/h when CPA was administered i.p. Therefore, (a) a lytic virus expressing a "suicide" gene can activate a prodrug; and (b) within rRp450-infected tumor, more prolonged and higher concentrations of activated metabolites are generated by intraneoplastic compared with systemic administration of prodrug.

Effect of O6-benzylguanine on alkylating agent-induced toxicity and mutagenicity. In Chinese hamster ovary cells expressing wild-type and mutant O6-alkylguanine-DNA alkyltransferases

The DNA repair protein O6-alkylguanine-DNA alkyltransferase (AGT) has been shown to protect cells from the toxic and mutagenic effect of alkylating agents by removing lesions from the O6 position of guanine. O6-Benzylguanine (BG) is a potent inactivator of AGT, resulting in an increase in the sensitivity of cells to the toxic effects of chemotherapeutic alkylating agents. Chinese hamster ovary (CHO) cells and CHO cells transfected with wild-type AGT (CHOWTAGT) and a mutant AGT [P138 M/V139I/P140K (CHOMIK)] known to be resistant to BG were treated with BG and various alkylating agents. BG treatment alone dramatically decreased AGT activity in CHOWTAGT cells but resulted in no depletion in AGT activity in CHOMIK cells. In the absence of AGT, these cells are highly sensitive to the toxic and mutagenic effects of temozolomide and 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU), and no further sensitization occurs in the presence of BG. In contrast, CHOWTAGT cells are resistant to temozolomide and BCNU, and treatment with BG resulted in a significantly higher cell killing and mutation frequency. CHOMIK cells were completely resistant to temozolomide or BCNU in the presence and absence of BG. Both cell killing and mutation frequency of 4-hydroperoxycyclophosphamide (4-HC) in CHO, CHOWTAGT, and CHOMIK cells were increased in the presence of BG. 4-HC generates two active metabolites, phosphoramide mustard (PM) and acrolein. BG had no effect on 4hydroperoxydidechlorocyclophosphamide (which generates acrolein and a nonalkylating form of PM) in CHO cells and CHOMIK cells, but enhancement of toxicity was observed with PM in both these cell lines. Therefore, we attribute the enhancement to the PM metabolite of 4-HC. Our results demonstrate that wild-type AGT plays an important role in protecting against the toxic and mutagenic effect of O6 alkylating agents and that a mutant AGT resistant to inactivation by BG effectively prevents BG-enhanced toxicity and mutagenicity induced by these agents. Expression of the AGT protein contributes to resistance of 4-HC. BG also enhances the toxicity of 4-HC and PM by a mechanism that may not involve the AGT repair protein.

Isolation of primitive human hematopoietic progenitors on the basis of aldehyde dehydrogenase activity

Because hematopoietic stem cells are rich in aldehyde dehydrogenase (ALDH) activity, we developed a fluorescent substrate for ALDH, termed BODIPY aminoacetaldehyde (BAAA), and tested its potential for isolating primitive human hematopoietic cells. A population of cells with low orthogonal light scattering and bright fluorescence intensity (SSC(lo)ALDH(br) cells) could be readily fractionated from human umbilical cord blood cells costained with BAAA and the multidrug-resistance inhibitor verapamil. The SSC(lo)ALDH(br) population was depleted of lineage-committed cells, 40-90% pure for CD34(+)CD38(lo/-) cells, and enriched 50- to 100-fold for primitive hematopoietic progenitors detected in short- and long-term culture analyses. Together, these observations indicate that fractionating human hematopoietic stem cells on the basis of ALDH activity using BAAA is an effective method for isolating primitive human hematopoietic progenitors. This technique may be useful for isolating stem cells from other tissues as well.

The chemistry of the metabolites of cyclophosphamide

This is primarily an overview of the spontaneous (non-enzymatic) chemistry of the metabolites of cyclophosphamide, viz., cis- and trans-4-hydroxycyclophosphamide, aldophosphamide (and its hydrate), iminophosphamide, phosphoramide mustard, acrolein, and chloroethylaziridine. A brief description of detoxification products obtained through enzyme catalyzed reactions appears. Included as the historical basis for the development of cyclophosphamide is the chemistry of nitrogen mustards. Among the topics covered are: perturbations to metabolite distributions and half-lives effected by buffer, structure, pH and nucleophiles; effects of pH on mechanism; alkylation versus P-N bond hydrolysis; the influence of nucleophiles on alkylation product distributions; the influence of substituents on alkylation rates; and preactivated forms of cyclophosphamide as metabolite precursors (4-hydroperoxycyclophosphamide and mafosfamide). A review with 66 references.

Role of O6-alkylguanine-DNA alkyltransferase in protecting against cyclophosphamide-induced toxicity and mutagenicity

Cyclophosphamide is used to treat a wide range of human malignancies. However, it is also a known carcinogen associated with induction of therapy-related leukemia and bladder cancer. The DNA repair protein, O6-alkylguanine-DNA alkyltransferase (AGT), protects cells from the toxic and mutagenic effects of O6-alkylating agents. We report here the contribution of AGT in protecting against the toxic and mutagenic effects of cyclophosphamide. CHO cells transduced with wild-type human AGT (CHO(AGT)) and pcDNA3 (CHOpcDNA3) were treated with activated cyclophosphamide derivatives, 4-hydroperoxycyclophosphamide (4-HC), 4-hydroperoxydidechlorocyclophosphamide (4-HDC), a progenitor of acrolein, and phosphoramide mustard (PM). The results show that CHO(AGT) is 7- or 20-fold less sensitive to the toxic effects of 30 microM 4-HC or 300 microM 4-HDC, respectively, than CHOpcDNA3 cells as measured by cell survival using a colony-forming assay. CHO(AGT) cells treated with 20 microM 4-HC or 200 microM 4-HDC produced 4- or 7-fold lower mutation frequency as measured at the HPRT locus than CHOpcDNA3 cells treated with the same dose of drugs. At 30 microM acrolein, the cell survival for CHO(AGT) was 30% compared with 18.7% for CHOpcDNA3. The mutation frequency of acrolein at the same dose was 57 mutants/10(6) cells in CHOpcDNA3 compared with no mutants in CHO(AGT). In contrast, CHO(AGT) and CHOpcDNA3 cells treated with PM had similar survival curves and exhibited no difference in mutation frequency. The present study demonstrates that AGT plays an important role in protecting against the toxic and mutagenic effect of cyclophosphamide and suggests that acrolein, not PM, is responsible for generating the toxic and mutagenic lesion(s) protected by the AGT protein.

Isophosphoramide Mustard and Its Mechanism of Bisalkylation

To investigate the mechanism(s) of bisalkylation by isophosphoramide mustard (IPM), IPM-beta,beta,beta',beta'-d(4) was synthesized and the products of its reaction with thiosulfate (at pD 7.0) were analyzed by NMR. By both (1)H and (13)C NMR, the distribution of deuterium in the products was consistent with bisalkylation through sequential aziridinyl intermediates [(NCH(2)CD(2)S):(NCD(2)CH(2)S) = 53:47]. Under the given reaction conditions, label scrambling as a result of thiosulfate acting as a leaving group was ruled out through control experiments. The data gave a calculated kinetic isotope effect of 0.97 per deuterium. For the initial aziridine species formed from IPM, ab initio quantum chemical calculations gave a hybridization value of sp(2.4)(-)(2.5) for each of the C-H bonds of the reaction centers, and this correlated with the observed inverse isotope effect. Other structure and bond order data were also determined for this aziridine intermediate and related compounds.

The partitioning of phosphoramide mustard and its aziridinium ions among alkylation and P-N bond hydrolysis reactions

NMR (1H and 31P) and HPLC techniques were used to study the partitioning of phosphoramide mustard (PM) and its aziridinium ions among alkylation and P-N bond hydrolysis reactions as a function of the concentration and strength of added nucleophiles at 37 degrees C and pH 7.4. With water as the nucleophile, bisalkylation accounted for only 10-13% of the product distribution given by PM. The remainder of the products resulted from P-N bond hydrolysis reactions. With 50 mM thiosulfate or 55-110 mM glutathione (GSH), bisalkylation by a strong nucleophile increased to 55-76%. The rest of the PM was lost to either HOH alkylation or P-N bond hydrolysis reactions. Strong experimental and theoretical evidence was obtained to support the hypothesis that the P-N bond scission observed at neutral pH does not occur in the parent PM to produce nornitrogen mustard; rather it is an aziridinium ion derived from PM which undergoes P-N bond hydrolysis to give chloroethylaziridine. In every buffer studied (bis-Tris, lutidine, triethanolamine, and Tris), the decomposition of PM (with and without GSH) gave rise to 31P NMR signals which could not be attributed to products of HOH or GSH alkylation or P-N bond hydrolysis. The intensities of these unidentified signals were dependent on the concentration of buffer.

A structural basis for a phosphoramide mustard-induced DNA interstrand cross-link at 5'-d(GAC)

Phosphoramide mustard-induced DNA interstrand cross-links were studied both in vitro and by computer simulation. The local determinants for the formation of phosphoramide mustard-induced DNA interstrand cross-links were defined by using different pairs of synthetic oligonucleotide duplexes, each of which contained a single potentially cross-linkable site. Phosphoramide mustard was found to cross-link dG to dG at a 5'-d(GAC)-3'. The structural basis for the formation of this 1,3 cross-link was studied by molecular dynamics and quantum chemistry. Molecular dynamics indicated that the geometrical proximity of the binding sites also favored a 1,3 dG-to-dG linkage over a 1,2 dG-to-dG linkage in a 5'-d(GCC)-3' sequence. While the enthalpies of 1,2 and 1,3 mustard cross-linked DNA were found to be very close, a 1,3 structure was more flexible and may therefore be in a considerably higher entropic state.

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.

Comparison of the protonation of isophosphoramide mustard and phosphoramide mustard

The alkylating agent isophosphoramide mustard (IPM) spontaneously forms a relatively stable aziridine derivative which can be directly observed using NMR spectroscopy. The protonations of IMP and its aziridine were probed using 1H, 31P, 15N, and 17O NMR spectroscopy. The positions of the 31P, 15N, and 17O resonances of IPM between pH 2 and 10 each exhibit a single monobasic titration curve with the same pKa of 4.31 +/- 0.02. On the basis of a comparison with other compounds and our earlier work with phosphoramide mustard, the NMR results for IPM indicate that protonation occurs at nitrogen and not oxygen. Over this same pH range, each of the 1H, 31P, and 15N resonances of IPM-aziridine also show a single monobasic titration with a pKa of 5.30 +/- 0.09. The magnitude of the change in chemical shifts suggests that the protonation of the IPM-aziridine occurs at the ring nitrogen. Theoretical gas-phase calculations of PM, IPM, and IPM-aziridine suggest O-protonation to be more likely; however, aqueous phase calculations predict the N-protonated forms to be most stable. Furthermore, for PM and IPM-aziridine, which contain nonequivalent nitrogens, the theoretical calculations and experimental data both agree as to which nitrogen undergoes protonation. These results suggest that the IMP-aziridine remains unprotonated under physiological conditions and may, in part, explain the lower alkylating activity of IPM as compared to PM.

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.

Assessment of aldehyde dehydrogenase in viable cells

Cytosolic aldehyde dehydrogenase (ALDH), an enzyme responsible for oxidizing intracellular aldehydes, has an important role in ethanol, vitamin A, and cyclophosphamide metabolism. High expression of this enzyme in primitive stem cells from multiple tissues, including bone marrow and intestine, appears to be an important mechanism by which these cells are resistant to cyclophosphamide. However, although hematopoietic stem cells (HSC) express high levels of cytosolic ALDH, isolating viable HSC by their ALDH expression has not been possible because ALDH is an intracellular protein. We found that a fluorescent aldehyde, dansyl aminoacetaldehyde (DAAA), could be used in flow cytometry experiments to isolate viable mouse and human cells based on their ALDH content. The level of dansyl fluorescence exhibited by cells after incubation with DAAA paralleled cytosolic ALDH levels determined by Western blotting and the sensitivity of the cells to cyclophosphamide. Moreover, DAAA appeared to be a more sensitive means of assessing cytosolic ALDH levels than Western blotting. Bone marrow progenitors treated with DAAA proliferated normally. Furthermore, marrow cells expressing high levels of dansyl fluorescence after incubation with DAAA were enriched for hematopoietic progenitors. The ability to isolate viable cells that express high levels of cytosolic ALDH could be an important component of methodology for identifying and purifying HSC and for studying cyclophosphamide-resistant tumor cell populations.

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.

Direct detection of the intracellular formation of carboxyphosphamides using nuclear magnetic resonance spectroscopy

31P nuclear magnetic resonance (NMR) spectroscopy was used in conjunction with cell perfusion techniques to monitor the intracellular chemistry of the cyclophosphamide (CP, CAS 6055-19-2) metabolites 4-hydroxy-cyclophosphamide (4-HO-CP) and aldophosphamide (AP) in U937 human histiocytic (CP-sensitive) and K562 human erythroleukemia (CP-resistant) cells. Similar experiments were carried out using the ifosfamide (IF, CAS3778-73-2) metabolites 4-hydroxyifosfamide (4-HO-IF) and aldoifosfamide (AIF). The hydroxy and aldehydic metabolites were generated by the triphenylphosphine reduction of 4-hydroperoxycyclophosphamide (4-HO2-CP) or 4-hydroperoxyifosfamide (4-HO2-IF) or by a spontaneous elimination/addition reaction involving water and 4-thiocyclophosphamide analogs 4-(2-hydroxyethyl) thiocyclophosphamide (4-ESCP) or mafosfamide. Cell death resulting from 4-HO-CP/AP perfusions was mimicked by perfusion with acrolein or an acrolein producing but non-alkylating, dechloro-CP analog. Acrolein toxicity was minimized by the presence of 2-mercaptoethanol or mesna (sodium 2-mercaptoethanesulfonate) in perfusion solutions as well as by fractional dose drug perfusions (sequential 2.5-3.0 h perfusions separated by cell washes with drug-free medium). The intracellular half-life for phosphoramide mustard (PM) at an intracellular pH value of 7.1 +/- 0.1 and an ambient probe temperature of 23 +/- 1 degree C in U937 cells was 2.1 h [k = (5.4 +/- 0.3) x 10(-3) min-1] and in K562 cells was 3.1 h [k = (3.7 +/- 0.4) x 10(-3) min-1]. Similar half-lives (2-4 h) were determined for intracellular isophosphoramide mustard (IPM). Fractional dose perfusion of U937 or K562 cells with 1.5 mmol/l 4-HO-CP/AP (generated from 4-HO2-CP) and 0.3 mmol/l mesna allowed for the observation of intracellular carboxyphosphamide (CBP); CBP was formed in higher concentrations in the CP-resistant K562 cells. Similar results were obtained using 4-ESCP and mafosfamide as sources of 4-HO-CP/AP. Identification of CBP was based on chemical shift, chemical stability, and membrane permeability studies of synthetic CBP. Concentrations of carboxyifosfamide (CBIF) formed in K562 cells were also greater than that in U937 cells.

Protonation of phosphoramide mustard and other phosphoramides

The chemistry of the bifunctional alkylating agent phosphoramide mustard and model phosphoramides was probed by multinuclear NMR spectroscopy as a function of pH. Between pH 1 and 11, both the 31P and 15N resonances for phosphoramide mustard displayed a single monobasic titration curve with a pKa of 4.9. The protonation below pH 4.9 correlates with the loss in reactivity of the mustard. The 17O NMR spectrum of 17O-enriched phosphoramide mustard shows little change with pH. The data on the mustard was compared to 15N and 31P NMR data on 15N-enriched phosphoramidic acid, phosphorodiamidic acid, and phosphoric triamide. Contrary to the conclusions of previous studies, our combined 31P, 15N, and 17O NMR results are more consistent with N-protonation of phosphoramide mustard rather than an O-protonation. Theoretical calculations on the phosphoramidic acid, phosphorodiamidic acid, and phosphoric triamide show O-protonation to be more stable in the gas phase. For the latter two compounds, the calculations suggest that N-protonation may be the most stable protonated form in the aqueous phase. These findings influence our understanding of the structure-activity relationships of phosphoramide mustards.

Cyclophosphamide resistance in medulloblastoma

Mechanisms of tumor resistance to 4-hydroperoxycyclophosphamide (4-HC) were studied by using a panel of human medulloblastoma cell lines either passaged in the laboratory for resistance to 4-HC or established from tumors showing clinical resistance to cyclophosphamide. Multiple distinct mechanisms of resistance were demonstrated. Daoy (4-HCR), a line that was 6-fold more resistant than Daoy, contained elevated levels of aldehyde dehydrogenase (ALDH). Most of the difference in sensitivity between the Daoy (4-HCR) and Daoy cell lines was abolished when 4-HC was replaced with phenylketocyclophosphamide, a 4-HC analogue that cannot be detoxified by ALDH. Thus, elevated levels of ALDH appear to play a role in the resistance of Daoy (4-HCR). Several of the cell lines [D283 Med (4-HCR), D341 Med (4-HCR), Daoy (4-HCR), D458 Med] contained elevated levels of glutathione (GSH). No changes in glutathione-S-transferase activity or isozyme pattern were observed, but in two of these three lines, the elevation in GSH was accompanied by elevated levels of gamma-glutamyl transpeptidase. To confirm the role of elevated GSH content in 4-HC resistance, the sensitivity of the cell lines to 4-HC was repeated after depletion of GSH by treatment with L-buthionine-S,R-sulfoximine. In medulloblastoma cell lines without other mechanisms of resistance, a linear relationship was seen between GSH content and resistance to 4-HC. Moreover, cells with GSH content greater than 5 nmol/mg protein and no other overriding mechanism of resistance could be sensitized to 4-HC treatment with L-buthionine-S,R-sulfoximine. Finally, D283 Med (4-HCR) cells had mild elevations in both ALDH and GSH content, but were resistant to phenylketocyclophosphamide and were not significantly sensitized by L-buthionine-S,R-sulfoximine. This cell line appears to demonstrate a third mechanism of resistance to 4-HC. These results suggest that 4-HC resistance in medulloblastoma can be multifactorial.

31P NMR studies of the kinetics of bisalkylation by isophosphoramide mustard: comparisons with phosphoramide mustard

31P nuclear magnetic resonance spectroscopy was used to measure the pKa (4.28 +/- 0.2) of isophosphoramide mustard (IPM) at 20 degrees C and to study the kinetics and products of the decomposition of IPM at a solution pH value of ca. 7.4 and at temperatures between 20 and 47 degrees C in the presence of nucleophilic trapping agents. At 37 degrees C, the half-life for the first alkylation was ca. 77 min and ca. 171 min for the second alkylation; these data may be compared with those for phosphoramide mustard (Engle, T.W.; Zon, G.; Egan, W.J. Med. Chem. 1982, 25, 1347), wherein the half-lives for the first and second alkylations are approximately the same (18 min). The rate of fragmentation of aldoifosfamide to IPM and acrolein was also studied by NMR spectroscopy (pH 7.0; 37 degrees C; 0.07 M phosphate); under the noted conditions, the half-life of aldoifosfamide was found to be ca. 60 min.

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.

Experimental chemotherapy of human medulloblastoma cell lines and transplantable xenografts with bifunctional alkylating agents

A series of bifunctional alkylators were tested against the genotypically and phenotypically heterogeneous continuous human medulloblastoma cell lines, TE-671, Daoy, and D283 Med in vitro and against TE-671 and Daoy growing as s.c. and intracranial xenografts in athymic mice. Drugs tested included melphalan, cyclophosphamide, iphosphamide, phenylketocyclophosphamide, thiotepa, 1,3-bis(2-chloroethyl)-1-nitrosourea (in vivo), and busulfan (in vivo). Melphalan and phenylketocyclophosphamide were the most active agents in vitro with drug doses at which there is a 90% reduction in the number of colonies in comparison to controls of 2.13, 5.29, and 4.72 microM for melphalan and 4.60, 5.01, and 4.34 microM for phenylketocyclophosphamide against TE-671, D283 Med, and Daoy, respectively. Melphalan, cyclophosphamide, iphosphamide, phenylketocyclophosphamide, and thiotepa produced significant growth delays against s.c. TE-671 and Daoy xenografts, while no activity could be demonstrated for 1,3-bis(2-chloroethyl)-1-nitrosourea or busulfan. Melphalan, cyclophosphamide, iphosphamide, and thiotepa also produced significant increases in median survival in mice bearing intracranial TE-671 and Daoy xenografts. These results extend our previous studies demonstrating the antitumor activity of nitrogen and phosphoramide mustard-based bifunctional alkylating agents in the treatment of human medulloblastoma continuous cell lines and transplantable xenografts, and support the continued use of these agents in clinical trials.

NMR spectroscopic studies of intermediary metabolites of cyclophosphamide. 2. Direct observation, characterization, and reactivity studies of iminocyclophosphamide and related species

4-Hydroxy-5,5-dimethylcyclophosphamide (6) was synthesized as a stable (to fragmentation) analogue of 4-hydroxycyclophosphamide (1). In anhydrous Me2SO-d6 (less than or equal to 0.03 mol % water), cis- and trans-6 were observed by multinuclear NMR spectroscopy to equilibrate with alpha, alpha-dimethylaldophosphamide (7) and 5,5-dimethyliminocyclophosphamide (8). Identification of 8 was based on 1H, 13C, and 31P chemical shifts, selective INEPT and two-dimensional NMR correlation experiments, and temperature-dependent equilibria data. The interconversion of cis-/trans-6 and -7 was also observed in lutidine buffer; 8 was not detected under the aqueous conditions. In Me2SO-d6, hydroxy metabolite 1 underwent dehydration to give iminocyclophosphamide (5), as evidenced by chemical shift data and a selective INEPT experiment. Concentrations of cis-/trans-1, aldophosphamide (2), and 5 were found to be temperature-dependent with higher temperatures favoring 2 and 5 in a reversible manner, thus indicating that 1/2/5 were intercoverting. The addition of small amounts of water to Me2SO-d6 solutions of imine 5 resulted in the immediate disappearance of its NMR signals. The role of imine 5 in the conversion of 1 to C-4 substituted analogues of 1 was elucidated for the formation of 4-cyanocyclophosphamide (3a) from 1 and sodium cyanide in lutidine buffer.

31P nuclear magnetic resonance spectroscopic observation of the intracellular transformations of oncostatic cyclophosphamide metabolites

31P NMR spectroscopy was used to directly monitor, for the first time, the intracellular chemistry of the ultimate active metabolite of cyclophosphamide, namely, phosphoramide mustard. These NMR studies utilized a human histiocytic lymphoma cell line (U937), embedded in agarose gel threads, and perfused with medium containing synthetically derived metabolites (4-hydroxycyclophosphamide, aldophosphamide, and phosphoramide mustard). Metabolites 2 or 3 or both readily crossed the cell membrane; in contrast, the membrane was relatively impermeable to 4. Intracellular concentrations of 4 could, therefore, be attributed primarily to the intracellular fragmentation of 3. Signals suggestive of either carboxyphosphamide or 4-ketophosphamide were not detected. Spectral data were used to calculate a rate constant of (5.4 +/- 0.3) X 10(-3) min-1 for the intracellular disappearance of 4 at 23 degrees C. The intracellular pH was determined to be 7.1 from the chemical shift of the internal inorganic phosphate signal.

Experimental chemotherapy of human medulloblastoma with classical alkylators

Seven classical alkylators were tested for activity against the continuous human medulloblastoma cell line TE-671 grown in vitro and as s.c. and intracranial xenografts in athymic mice. Drugs tested included melphalan, cyclophosphamide (4-hydroperoxycyclophosphamide in vitro), iphosphamide (4-hydroperoxyiphosphamide in vitro), phenylketocyclo-phosphamide, phenylketoiphosphamide, Asta Z 7557, and thiotriethyl-enephosphoramide. All agents were active, with melphalan demonstrating the most activity in vitro and in vivo. Comparative studies of cyclophosphamide and phenylketocyclophosphamide revealed partition coefficients (log P) of 0.73 and greater than 1.69, respectively, and cyclophosphamide exhibited greater cytotoxic activity in post- (equitoxic) drug administration murine plasma. Hematological toxicity was limited to leukopenia/neutropenia for both of these agents. These studies suggest that the classical alkylators may have a role in the treatment of medulloblastoma and provide a means to further analyze their therapeutic potential.

Synthesis and antitumor activity of cyclophosphamide analogues. 4. Preparation, kinetic studies, and anticancer screening of "phenylketophosphamide" and similar compounds related to the cyclophosphamide metabolite aldophosphamide

Phenyl ketone phosphorodiamidates [C6H5C(O)CH2CH2OP(O)NHR1NR2R3] were synthesized in conjunction with an ongoing investigation into the effects of substituents on the dynamical solution chemistry of the metabolites of cyclophosphamide (1a). In contrast to aldophosphamide (3a), which readily interconverts with its cyclic isomer 4-hydroxycyclophosphamide (2a), phenylketophosphamide (14a: R1 = H, R2 = R3 = CH2CH2Cl) exhibited an apparent "resistance" toward an intramolecular addition reaction such that 4-hydroxy-4-phenylcyclophosphamide (13a) could not be detected either spectroscopically (31P or 13C NMR) or chemically (NaCN trapping experiment). Control studies that compared the relative reactivities of 14a and methylketophosphamide [20: CH3C(O)CH2CH2OP(O)NH2N-(CH2CH2Cl)2] revealed that the factors that modulate the ring closure/opening reactions were not peculiar to the phenyl group; however, differences between phenyl and methyl profoundly influenced the rates of fragmentation of 14a and 20. 31P NMR spectroscopy was used to determine the rates at which each compound generated a cytotoxic alkylating agent. Under a standard set of reaction conditions [1 M lutidine buffer with added Me2SO (8:2), pH 7.4, 37 degrees C], the half-lives of 2a/3a, 14a, phenylketoifosfamide (14b: R1 = R2 = CH2CH2Cl, R3 = H), phenylketotrofosfamide (14c: R1 = R2 = R3 = CH2CH2Cl), and 20 were 72, 66, 63, 56, and 173 min, respectively. Analogues 14a and 14b exhibited good anticancer activity against a variety of test systems.

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

Multinuclear (31P, 13C, 2H, and 1H) Fourier-transform NMR spectroscopy, with and without isotopically enriched materials, was used to identify and quantify, as a function of time, the following intermediary (short-lived) metabolites of the anticancer prodrug cyclophosphamide (1, Scheme I): cis-4-hydroxycyclophosphamide (cis-2), its trans isomer (trans-2), aldophosphamide (3), and its aldehyde-hydrate (5). Under a standard set of reaction conditions (1 M 2,6-dimethylpyridine buffer, pH 7.4, 37 degrees C), the stereospecific deoxygenation of synthetic cis-4-hydroperoxycyclophosphamide (cis-12, 20 mM) with 4 equiv of sodium thiosulfate (Na2S2O3) afforded, after approximately 20 min, a "pseudoequilibrium" distribution of cis-2, 3, 5, and trans-2, i.e., the relative proportions of these reactants (57:4:9:30, respectively) remained constant during their continual disappearance. NMR absorption signals indicative of "iminophosphamide" (8) and enol 6 were not detected (less than 0.5-1% of the synthetic metabolite mixture). A computerized least-squares fitting procedure was applied to the individual 31P NMR derived time courses for conversion of cis-2, 3 plus 5 (i.e., "3"), and trans-2 into acrolein and phosphoramide mustard (4), the latter of which gave an expected array of thiosulfate S-alkylation products (e.g., 16) and other phosphorus-containing materials derived from secondary decomposition reactions. This kinetic analysis gave the individual forward and reverse rate constants for the apparent tautomerization processes, viz., cis-2 in equilibrium "3" in equilibrium trans-2, as well as the rate constant (k3) for the irreversible fragmentation of 3. The values of k3 at pH 6.3, 7.4, and 7.8 were equal to 0.030 +/- 0.004, 0.090 +/- 0.008, and 0.169 +/- 0.006 min-1, respectively. Replacement of the HC(O)CH2 moiety n 3 with HC(O)CD2 led to a primary kinetic isotope effect (kH/kD = 5.6 +/- 0.4) for k3. The apparent half-lives (tau 1/2) for cis-2, "3", and trans-2 under the standard reaction conditions, at "pseudoequilibrium" (constant ratio of cis-2/"3"/trans-2), were each equal to approximately 38 min, which is considerably shorter than the widely cited colorimetrically derived half-lives reported by earlier investigators. The values of tau 1/2 for cis-2, "3", and trans-2 were affected by pH in the same manner as that found for k3 but were relatively insensitive to the presence of either K+, Na+, Ca2+, or Mg2+. The presence of certain primary amines led to marked decreases in tau 1/2 and, in some cases, the formation of acyclic adducts of aldehyde 3.(ABSTRACT TRUNCATED AT 400 WORDS)