Assays to predict the genotoxicity of the chromosomal mutagen etoposide -- focussing on the best assay

OUP accepted manuscript

The regulatory 2D in vitro micronucleus (MN) assay is part of a battery of tests, used to test for genotoxicity of new and existing compounds before they are assessed in vivo (ICH S2). The 2D MN assay consists of a monolayer of cells, whereas the in vivo bone marrow (BM) setting comprises a multicellular environment within a three-dimensional extracellular matrix. Although the in vitro MN assay follows a robust protocol set out by the Organisation for Economic Co-operation and Development (OECD) to comply with regulatory bodies, some compounds have been identified as negative genotoxicants within the in vitro MN assay but marginally positive when assessed in vivo. The glucocorticoids, which are weakly positive in vivo, have generally been suggested to pose no long-term carcinogenic risk; however, for novel compounds of unknown activity, improved prediction of genotoxicity is imperative. To help address this observation, we describe a novel 3D in vitro assay which aims to replicate the results seen within the in vivo BM microenvironment. AlgiMatrix scaffolds were optimized for seeding with HS-5 human BM stromal cells as a BM microenvironment, to which the human lymphoblast cell line TK6 was added. An MN assay was performed aligning with the 2D regulatory assay protocol. Utilizing this novel 3D in vitro model of the BM, known genotoxicants (mitomycin C, etoposide, and paclitaxel), a negative control (caffeine), and in vivo positive glucocorticoids (dexamethasone and prednisolone) were investigated for the induction of MN. It was found, in agreement with historical in vivo data, that the model could accurately predict the in vivo outcome of the glucocorticoids, unlike the regulatory 2D in vitro MN assay. These preliminary results suggest our 3D MN assay may better predict the outcome of in vivo MN tests, compared with the standard 2D assay.

Utility of a next-generation framework for assessment of genomic damage: A case study using the pharmaceutical drug candidate etoposide

We present a hypothetical case study to examine the use of a next-generation framework developed by the Genetic Toxicology Technical Committee of the Health and Environmental Sciences Institute for assessing the potential risk of genetic damage from a pharmaceutical perspective. We used etoposide, a genotoxic carcinogen, as a representative pharmaceutical for the purposes of this case study. Using the framework as guidance, we formulated a hypothetical scenario for the use of etoposide to illustrate the application of the framework to pharmaceuticals. We collected available data on etoposide considered relevant for assessment of genetic toxicity risk. From the data collected, we conducted a quantitative analysis to estimate margins of exposure (MOEs) to characterize the risk of genetic damage that could be used for decision-making regarding the predefined hypothetical use. We found the framework useful for guiding the selection of appropriate tests and selecting relevant endpoints that reflected the potential for genetic damage in patients. The risk characterization, presented as MOEs, allows decision makers to discern how much benefit is critical to balance any adverse effect(s) that may be induced by the pharmaceutical. Interestingly, pharmaceutical development already incorporates several aspects of the framework per regulations and health authority expectations. Moreover, we observed that quality dose response data can be obtained with carefully planned but routinely conducted genetic toxicity testing. This case study demonstrates the utility of the next-generation framework to quantitatively model human risk based on genetic damage, as applicable to pharmaceuticals.

In Vitro Study of the Cytotoxic, Cytostatic, and Antigenotoxic Profile of Hemidesmus indicus (L.) R.Br. (Apocynaceae) Crude Drug Extract on T Lymphoblastic Cells

In traditional Indian medicine, the crude drug Hemidesmus indicus root—commonly known as Indian sarsaparilla—is used alone or in poly-herbal preparations for the treatment of a wide range of diseases. The present study focuses on the cancer chemopreventive and therapeutic potential of H. indicus extracts on an acute lymphoblastic leukemia cell line (CCRF-CEM). With this aim in mind, we subjected H. indicus roots to two subsequent extractions (hydro-alcoholic extraction and soxhlet extraction). As DNA damage is an important prerequisite for the induction of mutations/cancer by genotoxic carcinogens, cancer chemoprevention may be achieved by preventing genotoxicity. Through an integrated experimental approach, we explored the genoprotective potential of the soxhlet H. indicus extract against different mutagenic compounds and its cytotoxic, proapoptotic, and cytostatic properties. In our experimental conditions, H. indicus induced a cytotoxic effect involving the activation of both intrinsic and extrinsic apoptotic pathways and blocked the cell cycle in the S phase. Moreover, the antigenotoxicity results showed that the extract was able to mitigate DNA damage, an essential mechanism for its applicability as a chemopreventive agent, via either the modulation of extracellular and intracellular events involved in DNA damage. These data add to the growing body of evidence that H. indicus can represent a noteworthy strategy to target early and late stages of cancer.

Deleterious effects of reactive metabolites

A number of drugs have been withdrawn from the market or severely restricted in their use because of unexpected toxicities that become apparent only after the launch of new drug entities. Circumstantial evidence suggests that, in most cases, reactive metabolites are responsible for these unexpected toxicities. In this review, a general overview of the types of reactive metabolites and the consequences of their formation are presented. The current approaches to evaluate bioactivation potential of new compounds with particular emphasis on the advantages and limitation of these procedures will be discussed. Reasonable reasons for the excellent safety record of certain drugs susceptible to bioactivation will also be explored and should provide valuable guidance in the use of reactive-metabolite assessments when nominating drug candidates for development. This will, in turn, help us to design and bring safer drugs to the market.

Wogonin attenuates etoposide-induced oxidative DNA damage and apoptosis via suppression of oxidative DNA stress and modulation of OGG1 expression

Damage to DNA can lead to many different acute and chronic pathophysiological conditions, ranging from cancer to endothelial damage. The current study has been initiated to determine whether the flavonoid wogonin can attenuate etoposide-induced oxidative DNA damage and apoptosis in mouse bone marrow cells. We found that oral administration of wogonin before etoposide injection significantly attenuates etoposide-induced oxidative DNA damage and apoptosis in a dose dependent manner. Etoposide induced a significant down-regulation of mRNA expression of the OGG1 repair gene and marked biochemical alterations characteristic of oxidative DNA stress, including increased 8-OHdG, enhanced lipid peroxidation and reduction in reduced glutathione. Prior administration of wogonin ahead of etoposide challenge restored these altered parameters. Importantly, wogonin had no antagonizing effect on etoposide-induce topoisomerase-II inhibition. Conclusively, our study indicates that wogonin has a protective role in the abatement of etoposide-induced oxidative DNA damage and apoptosis in the bone marrow cells of mice via suppression of oxidative DNA stress and enhancing DNA repair through modulation of OGG1 repair gene expression. Therefore, wogonin can be a promising chemoprotective agent and might be useful to avert secondary leukemia and other drug-related cancers in cured cancer patients and medical personnel exposing to the potent carcinogen etoposide.

Cell transformation assays: are we barking up the wrong tree?

There has been a current resurgence of interest in the use of cell transformation for predicting carcinogenicity, which is based mainly on rodent carcinogenicity data. In view of this renewed interest, this paper critically reviews the published literature concerning the ability of the available assays to detect IARC Group 1 agents (known human carcinogens) and Group 2A agents (probable human carcinogens). The predictivity of the available assays for human and rodent non-genotoxic carcinogens (NGCs), in comparison with standard and supplementary in vitro and in vivo genotoxicity tests, is also discussed. The principal finding is that a surprising number of human carcinogens have not been tested for cell transformation across the three main assays (SHE, Balb/c 3T3 and C3H10T1/2), confounding comparative assessment of these methods for detecting human carcinogens. This issue is not being addressed in the ongoing validation studies for the first two of these assays, despite the lack of any serious logistical issues associated with the use of most of these chemicals. In addition, there seem to be no plans for using exogenous bio-transformation systems for the metabolic activation of pro-carcinogens, as recommended in an ECVAM workshop held in 1999. To address these important issues, it is strongly recommended that consideration be given to the inclusion of more human carcinogens and an exogenous source of xenobiotic metabolism, such as an S9 fraction, in ongoing and future validation studies. While cell transformation systems detect a high level of NGCs, it is considered premature to rely only on this endpoint for screening for such chemicals, as recently suggested. This is particularly important, in view of the fact that there is still doubt as to the relevance of morphological transformation to tumorigenesis in vivo, and the wide diversity of potential mechanisms by which NGCs are known to act. Recent progress with regard to increasing the objectivity of scoring the transformed phenotype, and prospects for developing human cell-based transformation assays, are reviewed.

Piplartine induces genotoxicity in eukaryotic but not in prokaryotic model systems

Piplartine {5,6-dihydro-1-[(2E)-1-oxo-3-(3,4,5-trimethoxyphenyl)-2-propen-1-yl]-2(1H)-pyridinone} is an alkamide present in Piper species that exhibits promising anticancer properties. It was previously shown that piplartine is mutagenic in yeast and cultured mammalian cells. This study was performed to increase the knowledge on the mutagenic potential of piplartine using the Salmonella/microsome assay, V79 cell micronucleus and chromosome aberration assays, and mouse bone-marrow micronucleus tests. Piplartine was isolated from the roots of Piper tuberculatum. This extracted compound was unable to induce a mutagenic response in any Salmonella typhimurium strain either in the presence or absence of metabolic activation. Piplartine showed mutagenic effects in V79 cells, as there was an increased frequency of aberrant cells and micronuclei formation. In addition, piplartine administered at 50mg/kg did not induce micronucleus formation in vivo, but a dose of 100mg/kg induced an increase in the levels of micronucleus polychromatic erythrocytes (MNPCEs). Overall, these results provide further support that piplartine induces in vivo and in vitro mutagenicity in eukaryotic models.

Protection of mouse bone marrow from etoposide-induced genomic damage by dexrazoxane

PURPOSE

The objective of the current investigation is to determine whether non-toxic doses of the catalytic topoisomerase-II inhibitor, dexrazoxane, have influence on the genomic damage induced by the anticancer topoisomerase-II poison, etoposide, on mice bone marrow cells.

METHOD

The scoring of micronuclei, chromosomal aberrations, and mitotic activity were undertaken as markers of cyto- and genotoxicity. Oxidative damage markers such as reduced glutathione and lipid peroxidation were assessed as a possible mechanism underlying this amelioration.

RESULTS

Dexrazoxane pre-treatment significantly reduced the etoposide-induced micronuclei formation, chromosomal aberrations, and also the suppression of erythroblast proliferation in bone marrow cells of mice. These effects were dose dependent. Etoposide induced marked biochemical alterations characteristic of oxidative stress including enhanced lipid peroxidation and reduction in the reduced glutathione level. Prior administration of dexrazoxane ahead of etoposide challenge ameliorated these biochemical markers.

CONCLUSION

Based on our data presented, strategies can be developed to decrease the etoposide-induced genomic damage in normal cells using dexrazoxane.

Early events in the mammalian response to DNA double-strand breaks

Physical and chemical agents that induce DNA double-strand breaks (DSBs) are among the most potent mutagens. The mammalian cell response to DSB comprises a highly co-ordinated, yet complex network of proteins that have been categorized as sensors, signal transducers, mediators and effectors of damage and repair. While this provides an accessible classification system, review of the literature indicates that many proteins satisfy the criteria of more than one category, pointing towards a series of highly co-operative pathways with overlapping function. In summary, the MRE11-NBS1-RAD50 complex is necessary for achieving optimal activation of ataxia-telangiectasia-mutated (ATM) kinase, which catalyses a phosphorylation-mediated signal transduction cascade. Among the subset of proteins phosphorylated by ATM are histone H2AX (H2AX), mediator of damage checkpoint protein 1, nibrin (NBS1), P53-binding protein 1 and breast cancer protein 1, all of which subsequently redistribute into DSB-containing sub-nuclear compartments. Post-translational modification of DSB responding proteins achieves a rapid and reversible change in protein behaviour and mediates damage-specific interactions, hence imparting a high degree of vigilance to the cell. This review highlights events fundamental in maintaining genetic integrity with emphasis on early stages of the DSB response.

Prolonged quercetin administration diminishes the etoposide-induced DNA damage in bone marrow cells of rats

The DNA damage in bone marrow cells induced by etoposide (E) injected intraperitoneally to rats (100 mg/kg b.w.) decreased to the control level when quercetin (Q) was administered subcutaneously for 10 consecutive days (40 mg/kg b.w.per day) before E was injected. The antioxidant power (FRAP assay) increased significantly after Q or E compared with control rats but did not change when Q preceded the E injection. The superoxide dismutase activity significantly increased in Q+E-treated rats compared with quercetin given alone. The study provides evidence that Q protects bone marrow cells against long-lived E-induced DNA damage and alters the redox balance in lung tissue.

Micronucleus induction and FISH analysis in buccal cells and lymphocytes of nurses administering antineoplastic drugs

A genotoxic effect for antineoplastic drugs, in particular micronucleus induction, has been shown in several studies. The aim of our study was to assess genotoxic effects in nurses administering different mixtures of antineoplastic drugs in an oncology hospital by evaluating the frequency of micronuclei in exfoliated buccal cells and blood lymphocytes by use of the standard micronucleus (MN) test and by identifying, by means of FISH analysis with centromeric probes, the mechanism of micronucleus induction (clastogenic or aneugenic). The study group comprised 23 nurses, 10 of whom worked in the day-care hospital and 13 in the ward. Twenty healthy subjects were selected as controls. Pan-centromeric FISH analysis was performed on lymphocytes from a selected group of nurses (12/23 subjects) characterized by higher MN frequencies as observed by standard Giemsa staining. A significant increase of micronucleus frequency compared with controls was found in exfoliated buccal cells of both groups of nurses: day-care hospital nurses 0.92 versus 0.45 (p=0.034) and ward nurses 0.94 versus 0.45 (p=0.051). An increase, although not statistically significant, of mean MN frequency was also found by the MN standard test on lymphocytes of the day-care hospital nurses (10.9 versus 7.5; p=0.056), while no differences were found in ward nurses (8.15 versus 7.5; p=0.56). We found that the administration of antineoplastic drugs by nurses in ward units induced a higher frequency of FISH MN+ (43% of subjects) than in the day-care hospital (20%). This was associated with the micronucleus size percentage. This finding could be correlated with the different compositions of administered mixtures of antineoplastic drugs: in ward units the mixtures contained drugs, such as vinorelbine, that were absent in the mixtures administered in the day-care hospital. Our results show genetic damage induced by administration of antineoplastic drugs, particularly in exfoliated buccal cells. This result suggests the useful application of this non-invasive sampling to evaluate genotoxic effects of occupational exposure to mixtures of inhalable chemicals at low doses.

Genotoxic profile of inhibitors of topoisomerases I (camptothecin) and II (etoposide) in a mitotic recombination and sex-chromosome loss somatic eye assay of Drosophila melanogaster

Genotoxic carcinogens which interact with DNA may produce double-strand breaks as normal intermediates of homologous mitotic recombination, and may give rise to structural chromosome aberrations and inter-chromosomal deletion-recombination. The genotoxic profile of two inhibitors of DNA topoisomerases were evaluated using an in vivo somatic w/w+ eye assay of Drosophila melanogaster for the detection of loss of heterozygosity (LOH) by homologous mitotic recombination, intra-chromosomal recombination and structural chromosomal aberrations. We studied camptothecin (CPT) as a topoisomerase-I-interactive agent and etoposide (ETOP) as a topoisomerase II inhibitor. These drugs act by stabilizing a ternary complex consisting of topoisomerases covalently linked to DNA at single-strand or at double-strand breaks, thereby preventing the relegation step of the breakage/rejoining reaction mediated by the enzyme. The genotoxic profiles were determined from the appearance of eye tissue in adult flies, in which LOH and expression of the reporter gene white produced light clones. The results demonstrated that both compounds were significantly genotoxic, with CPT being more effective than ETOP. Inter-chromosomal mitotic recombination was the major mechanism responsible for the induction of light spots by both compounds in XX females. Loss of the ring X chromosome (rX), was significantly enhanced by CPT, and this topoisomerase blocker also produced intra-chromosomal recombination (XY males).

Inhibition of Hsp90 acts synergistically with topoisomerase II poisons to increase the apoptotic killing of cells due to an increase in topoisomerase II mediated DNA damage

Topoisomerase II plays a crucial role during chromosome condensation and segregation in mitosis and meiosis and is a highly attractive target for chemotherapeutic agents. We have identified previously topoisomerase II and heat shock protein 90 (Hsp90) as part of a complex. In this paper we demonstrate that drug combinations targeting these two enzymes cause a synergistic increase in apoptosis. The objective of our study was to identify the mode of cell killing and the mechanism behind the increase in topoisomerase II mediated DNA damage. Importantly we demonstrate that Hsp90 inhibition results in an increased topoiosmerase II activity but not degradation of topoisomerase II and it is this, in the presence of a topoisomerase II poison that causes the increase in cell death. Our results suggest a novel mechanism of action where the inhibition of Hsp90 disrupts the Hsp90-topoisomerase II interaction leading to an increase in and activation of unbound topoisomerase II, which, in the presence of a topoisomerase II poison leads to the formation of an increased number of cleavable complexes ultimately resulting in rise in DNA damage and a subsequent increase cell death.

Analysis of spontaneous, gamma ray- and ethylnitrosourea-induced hprt mutants in HL-60 cells with multiplex PCR

AIM: To explore the molecular spectra and mechanism of human hypoxanthine guanine phosphoribosyl transferase (hprt) gene mutation induced by ethyluitrosourea (ENU) and ⁶⁰Co γ-rays.

METHODS: Independent human promyelocytic leukemia cells (HL-60) mutants at the hprt locus were isolated from untreated, ethyluitrosourea (ENU) and ⁶⁰Co γ-ray-exposed cells, respectively, and verified by two-way screening. The genetic changes underlying the mutation were determined by multiplex polymerase chain reaction (PCR) amplification and electrophoresis technique.

RESULTS: With dosage increased, survival rate of plated cell reduced (in the group with dosage of ENU with 100-200 μg/mL, P < 0.01; in the group with dosage of ⁶⁰Co γ-ray with 2-4 Gy, P < 0.05) and mutational frequency increased (in the group of ENU 12.5-200.0 μg/mL, P < 0.05; in the group of ⁶⁰Co γ-ray with 1-4 Gy, P < 0.05) significantly. In the 13 spontaneous mutants analyzed, 92.3% of mutant clones did not show any change in number or size of exon, a single exon was lost in 7.7%, and no evidence indicated total gene deletion occurred in nine hprt exons. However, deletions were found in 79.7% of ENU-induced mutations (62.5%-89.4%, P < 0.01) and in 61.7% of gamma-ray-induced mutations (28.6%-76.5%, P < 0.01). There were deletion mutations in all 9 exons of hprt gene and the most of induced mutations were chain deletion with multiplex exons (97.9% in gamma-ray-induced mutants, 88.1% in ENU-induced mutants).

CONCLUSION: The spectra of spontaneous mutations differs completely from that induced by EUN or ⁶⁰Co γ-ray. Although both ENU and γ-ray can cause destruction of genetic structure, mechanism of mutagenesis between them may be different.