Susan M. Ludeman

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.

Modulation of cyclophosphamide activity by O 6-alkylguanine-DNA alkyltransferase

Purpose: The human medulloblastoma cell line D283 Med (4-HCR), a line resistant to 4-hydroperoxycyclophosphamide (4-HC), displays enhanced␣repair of DNA interstrand crosslinks induced by phosphoramide mustard. D283 Med (4-HCR) cells are cross-resistant to 1,3-bis(2-chloroethyl)-1-nitrosourea, but partial sensitivity is restored after elevated levels of O6-alkylguanine-DNA alkyltransferase (AGT) are depleted by O6-benzylguanine (O6-BG). Studies were conducted to define the activity of 4-HC and 4-hydroperoxydidechlorocyclophosphamide against D283 Med (4-HCR) after AGT is depleted by O6-BG. Methods: Limiting dilution and xenograft studies were conducted to define the activity of 4-HC and 4-hydroperoxydidechlorocyclophosphamide with or without O6-BG. Results: The activity of 4-HC and 4-hydroperoxydidechlorocyclophosphamide against D283 Med (4-HCR) was increased after AGT depletion by O6-BG preincubation. Similar studies with Chinese hamster ovary cells, with or without stable transfection with a plasmid expressing the human AGT protein, revealed that the AGT-expressing cells were significantly less sensitive to 4-HC and 4-hydroperoxydidechlorocyclophosphamide. Reaction of DNA with 4-HC, phosphoramide mustard, or acrolein revealed that only 4-HC and acrolein caused a decrease in AGT levels. Conclusions: We propose that a small but potentially significant part of the cellular toxicity of cyclophosphamide in these cells is due to acrolein, and that this toxicity is abrogated by removal of the acrolein adduct from DNA by AGT.

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 ≥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. [Mol Cancer Ther 2007;6(8):2280–9]

Evidence for a role of chloroethylaziridine in the cytotoxicity of cyclophosphamide

Abstract 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. Conclusions: 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.

Mechanisms of resistance against cyclophosphamide and ifosfamide: can they be overcome without sacrificing selectivity?

Cyclophosphamide (CP, Cytoxan) is a member of the nitrogen mustard class of alkylating agents and the oxazaphosphorine class of chemical compounds. First synthesized as an anticancer drug four decades ago, it continues to be widely used because of its unique efficacy against a broad range of human cancers1. To date, the structural isomer ifosfamide (IF, Ifos) is the most clinically useful CP analog, although its current applications in chemotherapy are somewhat modest relative to those of CP itself. On the other hand, IF appears to have some advantages over CP in potentiating the activity of other drugs2. This has led to new investigations of the efficacy of IF in combination therapies, including clinical whole body hyperthermia trials3

Hydroxylation and N-Dechloroethylation of ifosfamide and deuterated ifosfamide by the human cytochrome P450s and their commonly occurring polymorphisms

The hydroxylation and N-dechloroethylation of deuterated ifosfamide (d4IFO) and ifosfamide (IFO) by several human P450s have been determined and compared. d4IFO was synthesized with deuterium at the alpha and alpha′ carbons to decrease the rate of N-dechloroethylation and thereby enhance hydroxylation of the drug at the 4′ position. The purpose was to decrease the toxic and increase the efficacious metabolites of IFO. For all of the P450s tested, hydroxylation of d4IFO was improved and dechloroethylation was reduced as compared with nondeuterated IFO. Although the differences were not statistically significant, the trend favoring the 4′-hydroxylation pathway was noteworthy. CYP3A5 and CYP2C19 were the most efficient enzymes for catalyzing IFO hydroxylation. The importance of these enzymes in IFO metabolism has not been reported previously and warrants further investigation. The catalytic ability of the common polymorphisms of CYP2B6 and CYP2C9 for both reactions were tested with IFO and d4IFO. It was determined that the commonly expressed polymorphisms CYP2B6*4 and CYP2B6*6 had reduced catalytic ability for IFO compared with CYP2B6*1, whereas CYP2B6*7 and CYP2B6*9 had enhanced catalytic ability. As with the wild-type enzymes, d4IFO was more readily hydroxylated by the polymorphic variants than IFO, and d4IFO was not dechloroethylated by any of the polymorphic forms. We also assessed the use of specific inhibitors of P450 to favor hydroxylation in human liver microsomes. We were unable to separate the pathways with these experiments, suggesting that multiple P450s are responsible for catalyzing both metabolic pathways for IFO, which is not observed with the closely related drug cyclophosphamide.

Abstract 4766: The use of deuterium isotope effects to shift the partitioning of cyclophosphamide and ifosfamide among competing, P450-dependent, oxidative pathways

Prodrugs cyclophosphamide and ifosfamide are each ‘activated’ through a P450 oxidation at the C-4 position of the oxazaphosphorine ring. This reaction triggers a series of spontaneous (non-enzymatic) steps resulting in the formation of the ultimate DNA crosslinking agents, phosphoramide mustard and isophosphoramide mustard. Competing with reaction at C-4 are P450-catalyzed oxidations at the alpha positions of the chloroethyl side-chains; reactions at the side chains result in the production of the neurotoxic chloroacetaldehyde. Predominance of one pathway over another is isozyme-dependent and of greatest impact in ifosfamide therapy where about 50% of this drug is lost to side-chain oxidation (relative to an average of 10% for cyclophosphamide) and dose-limiting neurotoxicity. In this project, deuterium isotope effects are being explored as a means to effect ‘metabolic switching’ in the oxidations of cyclophosphamide and ifosfamide; that is, favoring oxidation at the C-4 position by disfavoring oxidations at the side chains. Cyclophosphamide and ifosfamide were synthesized with deuterium at the alpha and alpha’ positions of the chloroethyl chains. After optimizing reaction times, drug and enzyme concentrations, cyclophosphamide and ifosfamide (unlabeled and labeled) were incubated (20 min) with three, relevant, cDNA-expressed supersomes (human CYP2B6, CYP3A4, CYP3A5). The concentration of each metabolite produced by C-4 and the two possible side-chain oxidations was measured quantitatively using LC-MS-MS techniques developed for this project. For unlabeled cyclophosphamide, the fraction of products derived from C-4 oxidation was 73% (3A4), 90% (3A5) and 100% (2B6). Use of deuterium labeled cyclophosphamide gave 89% (3A4), 94% (3A5) and 100% (2B6); the amount of product produced by 2B6 and labeled drug was more than twice that given by unlabeled drug (56 micromoles versus 23 micromoles, respectively). For unlabeled ifosfamide, the fraction of products given by C-4 oxidation was 63% (3A4), 55% (3A5) and 36% (2B6). Use of deuterium labeled ifosfamide gave 82% (3A4), 75% (3A5) and 88% (2B6). Aside from the initial oxidation, the metabolic steps most likely to be impacted by deuterium isotope effects are those of the alkylation sequence by the metabolites phosphoramide mustard and isophosphoramide mustard. To determine the effect, if any, phosphoramide mustard and isophosphoramide mustard are being synthesized with deuterium at the alpha and alpha’ positions of the chloroethyl chains and their alkylation kinetics, relative to unlabeled compounds are being determined using 31 P NMR spectroscopy. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 4766. doi:1538-7445.AM2012-4766