Chromosome aberrations in Chinese hamster and human cells: a comparison using compounds with various genotoxicity profiles

Application of the adverse outcome pathway framework to genotoxic modes of action

In May 2017, the Health and Environmental Sciences Institute's Genetic Toxicology Technical Committee hosted a workshop to discuss whether mode of action (MOA) investigation is enhanced through the application of the adverse outcome pathway (AOP) framework. As AOPs are a relatively new approach in genetic toxicology, this report describes how AOPs could be harnessed to advance MOA analysis of genotoxicity pathways using five example case studies. Each of these genetic toxicology AOPs proposed for further development includes the relevant molecular initiating events, key events, and adverse outcomes (AOs), identification and/or further development of the appropriate assays to link an agent to these events, and discussion regarding the biological plausibility of the proposed AOP. A key difference between these proposed genetic toxicology AOPs versus traditional AOPs is that the AO is a genetic toxicology endpoint of potential significance in risk characterization, in contrast to an adverse state of an organism or a population. The first two detailed case studies describe provisional AOPs for aurora kinase inhibition and tubulin binding, leading to the common AO of aneuploidy. The remaining three case studies highlight provisional AOPs that lead to chromosome breakage or mutation via indirect DNA interaction (inhibition of topoisomerase II, production of cellular reactive oxygen species, and inhibition of DNA synthesis). These case studies serve as starting points for genotoxicity AOPs that could ultimately be published and utilized by the broader toxicology community and illustrate the practical considerations and evidence required to formalize such AOPs so that they may be applied to genetic toxicity evaluation schemes. Environ. Mol. Mutagen. 61:114-134, 2020. © 2019 Wiley Periodicals, Inc.

Construction and application of (Q)SAR models to predict chemical-induced in vitro chromosome aberrations

In drug development, genetic toxicology studies are conducted using in vitro and in vivo assays to identify potential mutagenic and clastogenic effects, as outlined in the International Council for Harmonisation (ICH) S2 regulatory guideline. (Quantitative) structure-activity relationship ((Q)SAR) models that predict assay outcomes can be used as an early screen to prioritize pharmaceutical candidates, or later during product development to evaluate safety when experimental data are unavailable or inconclusive. In the current study, two commercial QSAR platforms were used to build models for in vitro chromosomal aberrations in Chinese hamster lung (CHL) and Chinese hamster ovary (CHO) cells. Cross-validated CHL model predictive performance showed sensitivity of 80 and 82%, and negative predictivity of 75 and 76% based on 875 training set compounds. For CHO, sensitivity of 61 and 67% and negative predictivity of 68 and 74% was achieved based on 817 training set compounds. The predictive performance of structural alerts in a commercial expert rule-based SAR software was also investigated and showed positive predictivity of 48-100% for selected alerts. Case studies examining incorrectly-predicted compounds, non-DNA-reactive clastogens, and recently-approved pharmaceuticals are presented, exploring how an investigational approach using similarity searching and expert knowledge can improve upon individual (Q)SAR predictions of the clastogenicity of drugs.

Cell Processing Engineering for Regenerative Medicine : Noninvasive Cell Quality Estimation and Automatic Cell Processing

The cell processing engineering including automatic cell processing and noninvasive cell quality estimation of adherent mammalian cells for regenerative medicine was reviewed. Automatic cell processing necessary for the industrialization of regenerative medicine was introduced. The cell quality such as cell heterogeneity should be noninvasively estimated before transplantation to patient, because cultured cells are usually not homogeneous but heterogeneous and most protocols of regenerative medicine are autologous system. The differentiation level could be estimated by two-dimensional cell morphology analysis using a conventional phase-contrast microscope. The phase-shifting laser microscope (PLM) could determine laser phase shift at all pixel in a view, which is caused by the transmitted laser through cell, and might be more noninvasive and more useful than the atomic force microscope and digital holographic microscope. The noninvasive determination of the laser phase shift of a cell using a PLM was carried out to determine the three-dimensional cell morphology and estimate the cell cycle phase of each adhesive cell and the mean proliferation activity of a cell population. The noninvasive discrimination of cancer cells from normal cells by measuring the phase shift was performed based on the difference in cytoskeleton density. Chemical analysis of the culture supernatant was also useful to estimate the differentiation level of a cell population. A probe beam, an infrared beam, and Raman spectroscopy are useful for diagnosing the viability, apoptosis, and differentiation of each adhesive cell.

Genotoxicity of pyrroloquinoline quinone (PQQ) disodium salt (BioPQQ™)

The genotoxic potential of pyrroloquinoline quinone (PQQ) disodium salt (BioPQQ™) was evaluated in a battery of genotoxicity tests. The results of the bacterial mutation assay (Ames test) were negative. Weak positive results were obtained in 2 separate in vitro chromosomal aberration test in Chinese hamster lung (CHL) fibroblasts. Upon testing in an in vitro chromosomal aberration test in human peripheral blood lymphocytes, no genotoxic activity of PQQ was noted. In the in vivo micronucleus assay in mice, PQQ at doses up to 2,000 mg/kg body weight demonstrated that no genotoxic effects are expressed in vivo in bone marrow erythrocytes. The weak responses in the in vitro test CHL cells were considered of little relevance under conditions of likely human exposure. PQQ disodium was concluded to have no genotoxic activity in vivo.

Genotoxicity guidelines recommended by International Conference of Harmonization (ICH)

Genotoxicity tests are designed to detect the genetic damage by various mechanisms. Several guidelines have provided various tests to be conducted for testing the genotoxicity and each of the regulatory agencies around the world have developed their own requirements for mutagenicity, without realizing that the products developed or registered in one country are also going to be registered and marketed around the world. The ICH guideline of genotoxicity helps to optimize the standard battery for genetic toxicology and to provide guidance on interpretation of results. These suggested standard set of tests does not imply that other genotoxicity tests are inadequate or inappropriate, but they help in improving risk characterization for carcinogenic effects that have their basis in changes in the genetic material.

Time-lapse analysis of laser phase shift for noninvasive discrimination of human normal cells and malignant tumor cells

Time-lapse analysis of phase shift using phase-shifting laser microscopy (PLM) revealed that the laser phase shifts of primary normal human prostate epithelial cells (PRECs) and human prostatic carcinoma epithelial cell line (PC-3) cells during the mitotic phase were markedly higher than those in the interphase. The phase shift of PC-3 cells during the interphase was markedly lower than that of PRECs throughout the cell cycle. In conclusion, it was proposed that adherent PC-3 cancer cells could be noninvasively discriminated from normal adherent PRECs by the periodical measurement of phase shift using PLM during culture.

Possible structural and functional determinants contributing to the clastogenicity of pharmaceuticals

The battery of regulatory genotoxicity studies required in support of new drug registration has remained largely static since its inception some thirty years ago. The Ames mutagenicity assay still forms the foundation for this testing being highly reproducible and, for the most part, transparent. The same is not necessarily true of in vitro mammalian chromosome aberration assays since there is a fairly large and growing number of molecules without clear structural genotoxicity alerts (DEREK, MCASE), which are negative in Ames testing but positive in aberration studies, often only at high concentrations and/or cytotoxicity. Interpretation and risk assessment of these positive results can be problematic since there is no clear understanding of the process that generates them. The present paper builds on our previous observations suggesting that non covalent drug/DNA interactions, which are not adequately modeled in computational programs, may help explain some of these unexpected positive results. In particular, it is suggested that N-dimethyl groups and certain pyridine/piperidine aryl ketones may play a contributory role in genotoxicity, perhaps via DNA intercalation and topoisomerase inhibition. Clastogenicity arising from topoisomerase inhibition would be expected to be a threshold phenomenon and, as such, may carry a distinctly reduced risk relative to clastogenicity associated with covalent drug/DNA interactions.

Noninvasive discrimination of human normal cells and malignant tumor cells by phase-shifting laser microscopy

The noninvasive discrimination of cancer cells from normal cells in adherent culture by the measuring of the phase shift using phase-shifting laser microscopy (PLM) was investigated with the aim of noninvasive quality control of cell processing for transplantation. A human prostatic carcinoma epithelial cell line (PC-3) and human hepatocellular carcinoma cell lines (Hep3B, PLC, HLF, and Huh7) showed markedly lower phase shifts as measured by PLM than those of human prostate epithelial cells (PREC) and cryopreserved human hepatocytes, respectively, although there was no apparent difference in cell morphology between these sets of cells. Mixed cultures of PC-3 and PREC, in which the percentages of PC-3 cells were determined to be 43.3%, 14.0%, and 10.6% by immunofluorescence staining, were prepared, and the percentages were calculated to be 50.0%, 8.0%, and 2.5% using the distribution of the phase shift data of the cells, respectively. In conclusion, these adherent cancer cells can be noninvasively discriminated from normal adherent cells by phase shift measurement using PLM, and the sensitivity of the method of detecting cancer cell contamination reached 10%.

Evaluation of in vitro assays for assessing the toxicity of cigarette smoke and smokeless tobacco

BACKGROUND

In vitro toxicology studies of tobacco and tobacco smoke have been used to understand why tobacco use causes cancer and to assess the toxicologic impact of tobacco product design changes. The need for toxicology studies has been heightened given the Food and Drug Administration's newly granted authority over tobacco products for mandating tobacco product performance standards and evaluate manufacturers' health claims about modified tobacco products. The goal of this review is to critically evaluate in vitro toxicology methods related to cancer for assessing tobacco products and to identify related research gaps.

METHODS

PubMed database searches were used to identify tobacco-related in vitro toxicology studies published since 1980. Articles published before 1980 with high relevance also were identified. The data were compiled to examine (a) the goals of the study, (b) the methods for collecting test substances, (c) experimental designs, (d) toxicologic end points, and (e) relevance to cancer risk.

RESULTS

A variety of in vitro assays are available to assess tobacco smoke that address different modes of action, mostly using non-human cell models. However, smokeless tobacco products perform poorly in these assays. Although reliable as a screening tool for qualitative assessments, the available in vitro assays have been poorly validated for quantitative comparisons of different tobacco products. Assay batteries have not been developed, although they exist for nontobacco assessments. Extrapolating data from in vitro studies to human risks remains hypothetical.

CONCLUSIONS

In vitro toxicology methods are useful for screening toxicity, but better methods are needed for today's context of regulation and evaluation of health claims.

Nonclinical safety assessment of the histone deacetylase inhibitor vorinostat

Vorinostat (SAHA, Zolinza), a histone deacetylase inhibitor, is assessed in nonclinical studies to support its approval for cutaneous T-cell lymphoma. Vorinostat is weakly mutagenic in the Ames assay; is clastogenic in rodent (ie, CHO) cells but not in normal human lymphocytes; and is weakly positive in an in vivo mouse micronucleus assay. No effects are observed on potassium ion currents in the hERG assay up to 300 microM (safety margin approximately 300-fold the approximately 1 microM serum concentration associated with the 400 mg/d maximum recommended human dose. No rat respiratory or central nervous system effects are found at 150 mg/kg (>2-fold maximum recommended human dose). No cardiovascular effects, including effects on QTc interval, are observed after a single oral dose (150 mg/kg) in dogs. Vorinostat is orally dosed daily in rats (controls, 20, 50, or 150 mg/kg/d) and dogs (controls, 60, 80, or 100/125/160 mg/kg/d) for 26 weeks with a 4-week recovery. Rat vorinostat-related adverse findings are decreased food consumption, weight loss, and hematologic changes; a no observed adverse effects level is not established. In dogs, adverse effects are primarily gastrointestinal; the no observed adverse effects level is 60 mg/kg/d (approximately 6-fold maximum recommended human dose). Toxicities are reversible and can be monitored in the clinic.

Improving dose selection and identification of aneugens in the in vitro chromosome aberration test by integration of flow cytometry-based methods

Previously, this laboratory reported on the development of a flow cytometry-based method that automates the assessment of the mitotic index (MI) and numerical chromosome changes in chemically treated cultures of human lymphocytes [Muehlbauer PA and Schuler MJ, 2003, 2005]. With this method, testing design can easily include numerous well-spaced doses to better define the shape of MI dose response curves. In addition, the hypodiploid, hyperdiploid, and polyploid mitotic populations are available simultaneously to determine the biological relevance of polyploidy effects during the conduct of the assay. The current work describes the integration of this flow cytometry-based method into the routine conduct of good laboratory practice structural chromosome aberration assays in vitro, and discusses improvements in evaluating cytotoxicity and polyploidy endpoints. Additional methods for simultaneous assessment of cell death (sub-G1 DNA) are shown in combination with the MI to provide a more complete evaluation of cytotoxic conditions. A total of 30 pharmaceutical compounds were assayed in compliance with Organization for Economic Cooperation and Development and International Conference on Harmonization guidelines. The inclusion of numerous well-spaced doses improved high dose selection and resulted in fewer high dose artifacts. Only 1 compound in 30 produced a positive response in structural aberrations. In comparison, polyploidy induction was observed in 22 of 30 (73%) compounds, with no apparent increases in numerical chromosomal aberrations. These studies show that flow cytometry-based methods can be used to better characterize cytotoxicity dose-response relationships and improve the detection of aneugens.

Relevance and follow-up of positive results in in vitro genetic toxicity assays: an ILSI-HESI initiative

In vitro genotoxicity assays are often used to screen and predict whether chemicals might represent mutagenic and carcinogenic risks for humans. Recent discussions have focused on the high rate of positive results in in vitro tests, especially in those assays performed in mammalian cells that are not confirmed in vivo. Currently, there is no general consensus in the scientific community on the interpretation of the significance of positive results from the in vitro genotoxicity assays. To address this issue, the Health and Environmental Sciences Institute (HESI), held an international workshop in June 2006 to discuss the relevance and follow-up of positive results in in vitro genetic toxicity assays. The goals of the meeting were to examine ways to advance the scientific basis for the interpretation of positive findings in in vitro assays, to facilitate the development of follow-up testing strategies and to define criteria for determining the relevance to human health. The workshop identified specific needs in two general categories, i.e., improved testing and improved data interpretation and risk assessment. Recommendations to improve testing included: (1) re-examine the maximum level of cytotoxicity currently required for in vitro tests; (2) re-examine the upper limit concentration for in vitro mammalian studies; (3) develop improved testing strategies using current in vitro assays; (4) define criteria to guide selection of the appropriate follow-up in vivo studies; (5) develop new and more predictive in vitro and in vivo tests. Recommendations for improving interpretation and assessment included: (1) examine the suitability of applying the threshold of toxicological concern concepts to genotoxicity data; (2) develop a structured weight of evidence approach for assessing genotoxic/carcinogenic hazard; and (3) re-examine in vitro and in vivo correlations qualitatively and quantitatively. Conclusions from the workshop highlighted a willingness of scientists from various sectors to change and improve the current paradigm and move from a hazard identification approach to a "realistic" risk-based approach that incorporates information on mechanism of action, kinetics, and human exposure..

In vitro approaches to develop weight of evidence (WoE) and mode of action (MoA) discussions with positive in vitro genotoxicity results

A recent analysis by Kirkland et al. [Kirkland, D., Aardema, M., Henderson, L. and Müller, L. (2005) Evaluation of the ability of a battery of 3 in vitro genotoxicity tests to discriminate rodent carcinogens and non-carcinogens. I. Sensitivity, specificity and relative predictivity. Mutat. Res. 584, 1-256] demonstrated an extremely high false positive rate for in vitro genotoxicity tests when compared with carcinogenicity in rodents. In many industries, decisions have to be made on the safety of new substances, and health risk to humans, without rodent carcinogenicity data being available. In such cases, the usual way to determine whether a positive in vitro genotoxicity result is relevant (i.e. indicates a hazard) for humans is to develop weight of evidence (WoE) or mode of action (MoA) arguments. These are based partly on further in vitro investigations, but usually rely heavily on tests for genotoxicity in one or more in vivo assays. However, for certain product types in the European Union, the use of animals for genotoxicity testing (as well as for other endpoints) will be prohibited within the next few years. Many different examples have been described that indicate DNA damage and genotoxic responses in vitro can arise through non-relevant in vitro events that are a result of the test systems and conditions used. The majority of these non-relevant in vitro events can be grouped under a category of 'overload of normal physiology' that would not be expected to occur in exposed humans. However, obtaining evidence in support of such MoAs is not easy, particularly for those industries prohibited from carrying out in vivo testing. It will become necessary to focus on in vitro studies to provide evidence of non-DNA, threshold or in vitro-specific processes and to discuss the potential for such genotoxic effects to occur in exposed humans. Toward this end, we surveyed the published literature for in vitro approaches that may be followed to determine whether a genotoxic effect observed in vitro will occur in humans. Unfortunately, many of the approaches we found are based on only a few published examples and validated approaches with consensus recommendations often do not exist. This analysis highlights the urgent need for developing consensus approaches that do not rely on animal studies for dealing with in vitro genotoxins.