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

Evaluation and interpretation of cytogenetic test results based on biological relevance

The evaluation and interpretation of cytogenetic test data are discussed from the perspective of biological relevance. The reliability of tests must be considered, before evaluation and interpretation. Statistical procedures are important for the evaluation of test data, but for human health risk assessment, biological relevance is essential. Cell culture conditions must be carefully considered. Cells must be healthy in the physiologically controlled culture medium. Osmolality, pH, and temperature are critical factors in keeping the culture medium physiologically normal and avoiding artifactual responses. Careful attention must be paid to the exposure of test chemicals to target cells, in both in vitro and in vivo tests. For in vivo tests, absorption, distribution, metabolism, and excretion are critical issues that affect the exposure of the target cells to the test chemical. The dose-response relationship and reproducibility are also critical factors in biological reliability. I also discuss why so many chemicals show positive results in in vitro cytogenetic assays.

Multiple-endpoint in vitro carcinogenicity test in human cell line TK6 distinguishes carcinogens from non-carcinogens and highlights mechanisms of action

Current in vitro genotoxicity tests can produce misleading positive results, indicating an inability to effectively predict a compound's subsequent carcinogenic potential in vivo. Such oversensitivity can incur unnecessary in vivo tests to further investigate positive in vitro results, supporting the need to improve in vitro tests to better inform risk assessment. It is increasingly acknowledged that more informative in vitro tests using multiple endpoints may support the correct identification of carcinogenic potential. The present study, therefore, employed a holistic, multiple-endpoint approach using low doses of selected carcinogens and non-carcinogens (0.001-770 µM) to assess whether these chemicals caused perturbations in molecular and cellular endpoints relating to the Hallmarks of Cancer. Endpoints included micronucleus induction, alterations in gene expression, cell cycle dynamics, cell morphology and bioenergetics in the human lymphoblastoid cell line TK6. Carcinogens ochratoxin A and oestradiol produced greater Integrated Signature of Carcinogenicity scores for the combined endpoints than the "misleading" in vitro positive compounds, quercetin, 2,4-dichlorophenol and quinacrine dihydrochloride and toxic non-carcinogens, caffeine, cycloheximide and phenformin HCl. This study provides compelling evidence that carcinogens can successfully be distinguished from non-carcinogens using a holistic in vitro test system. Avoidance of misleading in vitro outcomes could lead to the reduction and replacement of animals in carcinogenicity testing.

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.

Value and limitation of in vitro bioassays to support the application of the threshold of toxicological concern to prioritise unidentified chemicals in food contact materials

Some of the chemicals in materials used for packaging food may leak into the food, resulting in human exposure. These include so-called Non-intentionally Added Substances (NIAS), many of them being unidentified and toxicologically uncharacterized. This raises the question of how to address their safety. An approach consisting of identification and toxicologically testing all of them appears neither feasible nor necessary. Instead, it has been proposed to use the threshold of toxicological concern (TTC) Cramer class III to prioritise unknown NIAS on which further safety investigations should focus. Use of the Cramer class III TTC for this purpose would be appropriate if amongst others sufficient evidence were available that the unknown chemicals were not acetylcholinesterase inhibitors or direct DNA-reactive mutagens. While knowledge of the material and analytical chemistry may efficiently address the first concern, the second could not be addressed in this way. An alternative would be use of a bioassay capable of detecting DNA-reactive mutagens at very low levels. No fully satisfactory bioassay was identified. The Ames test appeared the most suitable since it specifically detects DNA-reactive mutagens and the limit of biological detection of highly potent genotoxic carcinogens is low. It is proposed that for a specific migrate, the evidence for absence of mutagenicity based on the Ames test, together with analytical chemistry and information on packaging manufacture could allow application of the Cramer class III TTC to prioritise unknown NIAS. Recommendations, as well as research proposals, have been developed on sample preparation and bioassay improvement with the ultimate aim of improving limits of biological detection of mutagens. Although research is still necessary, the proposed approach should bring significant benefits over the current practices used for safety evaluation of food contact materials.

Assessment of the performance of the TGx-DDI biomarker to detect DNA damage-inducing agents using quantitative RT-PCR in TK6 cells

Gene expression biomarkers are now available for application in the identification of genotoxic hazards. The TGx-DDI transcriptomic biomarker can accurately distinguish DNA damage-inducing (DDI) from non-DDI exposures based on changes in the expression of 64 biomarker genes. The 64 genes were previously derived from whole transcriptome DNA microarray profiles of 28 reference agents (14 DDI and 14 non-DDI) after 4 h treatments of TK6 human lymphoblastoid cells. To broaden the applicability of TGx-DDI, we tested the biomarker using quantitative RT-PCR (qPCR), which is accessible to most molecular biology laboratories. First, we selectively profiled the expression of the 64 biomarker genes using TaqMan qPCR assays in 96-well arrays after exposing TK6 cells to the 28 reference agents for 4 h. To evaluate the classification capability of the qPCR profiles, we used the reference qPCR signature to classify 24 external validation chemicals using two different methods-a combination of three statistical analyses and an alternative, the Running Fisher test. The qPCR results for the reference set were comparable to the original microarray biomarker; 27 of the 28 reference agents (96%) were accurately classified. Moreover, the two classification approaches supported the conservation of TGx-DDI classification capability using qPCR; the combination of the two approaches accurately classified 21 of the 24 external validation chemicals, demonstrating 100% sensitivity, 81% specificity, and 91% balanced accuracy. This study demonstrates that qPCR can be used when applying the TGx-DDI biomarker and will improve the accessibility of TGx-DDI for genotoxicity screening. Environ. Mol. Mutagen. 60: 122-133, 2019. © 2018 Her Majesty the Queen in Right of Canada Environmental and Molecular Mutagenesis.

One science-driven approach for the regulatory implementation of alternative methods: A multi-sector perspective

EU regulations call for the use of alternative methods to animal testing. During the last decade, an increasing number of alternative approaches have been formally adopted. In parallel, new 3Rs-relevant technologies and mechanistic approaches have increasingly contributed to hazard identification and risk assessment evolution. In this changing landscape, an EPAA meeting reviewed the challenges that different industry sectors face in the implementation of alternative methods following a science-driven approach. Although clear progress was acknowledged in animal testing reduction and refinement thanks to an integration of scientifically robust approaches, the following challenges were identified: i) further characterization of toxicity pathways; ii) development of assays covering current scientific gaps, iii) better characterization of links between in vitro readouts and outcome in the target species; iv) better definition of alternative method applicability domains, and v) appropriate implementation of the available approaches. For areas having regulatory adopted alternative methods (e.g., vaccine batch testing), harmonised acceptance across geographical regions was considered critical for broader application. Overall, the main constraints to the application of non-animal alternatives are the still existing gaps in scientific knowledge and technological limitations. The science-driven identification of most appropriate methods is key for furthering a multi-sectorial decrease in animal testing.

A novel, integrated in vitro carcinogenicity test to identify genotoxic and non-genotoxic carcinogens using human lymphoblastoid cells

Human exposure to carcinogens occurs via a plethora of environmental sources, with 70-90% of cancers caused by extrinsic factors. Aberrant phenotypes induced by such carcinogenic agents may provide universal biomarkers for cancer causation. Both current in vitro genotoxicity tests and the animal-testing paradigm in human cancer risk assessment fail to accurately represent and predict whether a chemical causes human carcinogenesis. The study aimed to establish whether the integrated analysis of multiple cellular endpoints related to the Hallmarks of Cancer could advance in vitro carcinogenicity assessment. Human lymphoblastoid cells (TK6, MCL-5) were treated for either 4 or 23 h with 8 known in vivo carcinogens, with doses up to 50% Relative Population Doubling (maximum 66.6 mM). The adverse effects of carcinogens on wide-ranging aspects of cellular health were quantified using several approaches; these included chromosome damage, cell signalling, cell morphology, cell-cycle dynamics and bioenergetic perturbations. Cell morphology and gene expression alterations proved particularly sensitive for environmental carcinogen identification. Composite scores for the carcinogens' adverse effects revealed that this approach could identify both DNA-reactive and non-DNA reactive carcinogens in vitro. The richer datasets generated proved that the holistic evaluation of integrated phenotypic alterations is valuable for effective in vitro risk assessment, while also supporting animal test replacement. Crucially, the study offers valuable insights into the mechanisms of human carcinogenesis resulting from exposure to chemicals that humans are likely to encounter in their environment. Such an understanding of cancer induction via environmental agents is essential for cancer prevention.

Performance of the in vitro transgene mutation assay in MutaMouse FE1 cells: Evaluation of nine misleading ("False") positive chemicals

The screening of chemicals for the protection of human health and the environment requires the assessment of genetic toxicity. However, existing, internationally-accepted in vitro mammalian genotoxicity tests have been criticized for their low specificity (i.e. high frequency of "false" or "misleading" positive results for compounds that are negative in vivo). An in vitro transgene mutation assay has been established that uses a metabolically competent cell line derived from MutaMouse lung (i.e. FE1 cells). Mutation scoring employs the well-characterized lacZ positive selection system, and the assay is proposed as an alternative in vitro assessment tool. In this study, the performance of the FE1 cell assay was evaluated by examining responses to nine non-DNA-reactive chemicals that previously elicited misleading positive results in other mammalian cell genotoxicity assays. FE1 cells were exposed to concentrations up to approximately 10 mM and/or concentrations that yielded approximately 80-90% cytotoxicity (as measured by relative increase in cell count). The assay demonstrated excellent specificity; exposures to the chemicals examined did not yield any positive responses even when tested in the presence of an exogenous metabolic activation system (i.e. S9) or with an extended sampling time. These results indicate that the FE1 cell mutagenicity assay is an effective and practical alternative to traditional mammalian cell gene mutation assays. The development and validation of effective in vitro tools such as the MutaMouse FE1 cell assay will contribute to international efforts to reduce, refine, and replace experimental animals for toxicity assessment. Environ. Mol. Mutagen. 58:582-591, 2017. © 2017 Wiley Periodicals, Inc.

Integration of metabolic activation with a predictive toxicogenomics signature to classify genotoxic versus nongenotoxic chemicals in human TK6 cells

The use of integrated approaches in genetic toxicology, including the incorporation of gene expression data to determine the molecular pathways involved in the response, is becoming more common. In a companion article, a genomic biomarker was developed in human TK6 cells to classify chemicals as genotoxic or nongenotoxic. Because TK6 cells are not metabolically competent, we set out to broaden the utility of the biomarker for use with chemicals requiring metabolic activation. Specifically, chemical exposures were conducted in the presence of rat liver S9. The ability of the biomarker to classify genotoxic (benzo[a]pyrene, BaP; aflatoxin B1, AFB1) and nongenotoxic (dexamethasone, DEX; phenobarbital, PB) agents correctly was evaluated. Cells were exposed to increasing chemical concentrations for 4 hr and collected 0 hr, 4 hr, and 20 hr postexposure. Relative survival, apoptosis, and micronucleus frequency were measured at 24 hr. Transcriptome profiles were measured with Agilent microarrays. Statistical modeling and bioinformatics tools were applied to classify each chemical using the genomic biomarker. BaP and AFB1 were correctly classified as genotoxic at the mid- and high concentrations at all three time points, whereas DEX was correctly classified as nongenotoxic at all concentrations and time points. The high concentration of PB was misclassified at 24 hr, suggesting that cytotoxicity at later time points may cause misclassification. The data suggest that the use of S9 does not impair the ability of the biomarker to classify genotoxicity in TK6 cells. Finally, we demonstrate that the biomarker is also able to accurately classify genotoxicity using a publicly available dataset derived from human HepaRG cells.

Incorporation of metabolic activation potentiates cyclophosphamide-induced DNA damage response in isogenic DT40 mutant cells

Elucidating the DNA repair pathways that are activated in the presence of genotoxic agents is critical to understand their modes of action. Although the DT40 cell-based DNA damage response (DDR) assay provides rapid and sensitive results, the assay cannot be used on genotoxic compounds that require metabolic activation to be reactive. Here, we applied the metabolic activation system to a DDR and micronucleus (MN) assays in DT40 cells. Cyclophosphamide (CP), a well-known cross-linking agent requiring metabolic activation, was preincubated with liver S9 fractions. When DT40 cells and mutant cells were exposed to the preactivated CP, CP caused increased cytotoxicity in FANC-, RAD9-, REV3- and RAD18-mutant cells compared to isogenic wild-type cells. We then performed a MN assay on DT40 cells treated with preactivated CP. An increase in the MN was observed in REV3- and FANC-mutant cells at lower concentrations of activated CP than in the parental DT40 cells. These results demonstrated that the incorporation of metabolic preactivation system using S9 fractions significantly potentiates DDR caused by CP in DT40 cells and their mutants. In addition, our data suggest that the metabolic preactivation system for DDR and MN assays has a potential to increase the relevance of this assay to screening various compounds for potential genotoxicity.

IWGT report on quantitative approaches to genotoxicity risk assessment II. Use of point-of-departure (PoD) metrics in defining acceptable exposure limits and assessing human risk

This is the second of two reports from the International Workshops on Genotoxicity Testing (IWGT) Working Group on Quantitative Approaches to Genetic Toxicology Risk Assessment (the QWG). The first report summarized the discussions and recommendations of the QWG related to the need for quantitative dose-response analysis of genetic toxicology data, the existence and appropriate evaluation of threshold responses, and methods to analyze exposure-response relationships and derive points of departure (PoDs) from which acceptable exposure levels could be determined. This report summarizes the QWG discussions and recommendations regarding appropriate approaches to evaluate exposure-related risks of genotoxic damage, including extrapolation below identified PoDs and across test systems and species. Recommendations include the selection of appropriate genetic endpoints and target tissues, uncertainty factors and extrapolation methods to be considered, the importance and use of information on mode of action, toxicokinetics, metabolism, and exposure biomarkers when using quantitative exposure-response data to determine acceptable exposure levels in human populations or to assess the risk associated with known or anticipated exposures. The empirical relationship between genetic damage (mutation and chromosomal aberration) and cancer in animal models was also examined. It was concluded that there is a general correlation between cancer induction and mutagenic and/or clastogenic damage for agents thought to act via a genotoxic mechanism, but that the correlation is limited due to an inadequate number of cases in which mutation and cancer can be compared at a sufficient number of doses in the same target tissues of the same species and strain exposed under directly comparable routes and experimental protocols.

Fluorescence-based recombination assay for sensitive and specific detection of genotoxic carcinogens in human cells

In vitro genotoxicity tests are known to suffer from several shortcomings, mammalian cell-based assays, in particular, from low specificities. Following a novel concept of genotoxicity detection, we developed a fluorescence-based method in living human cells. The assay quantifies DNA recombination events triggered by DNA double-strand breaks and damage-induced replication fork stalling predicted to detect a broad spectrum of genotoxic modes of action. To maximize sensitivities, we engineered a DNA substrate encompassing a chemoresponsive element from the human genome. Using this substrate, we screened various human tumor and non-transformed cell types differing in the DNA damage response, which revealed that detection of genotoxic carcinogens was independent of the p53 status but abrogated by apoptosis. Cell types enabling robust and sensitive genotoxicity detection were selected for the generation of reporter clones with chromosomally integrated DNA recombination substrate. Reporter cell lines were scrutinized with 21 compounds, stratified into five sets according to the established categories for identification of carcinogenic compounds: genotoxic carcinogens ("true positives"), non-genotoxic carcinogens, compounds without genotoxic or carcinogenic effect ("true negatives") and non-carcinogenic compounds, which have been reported to induce chromosomal aberrations or mutations in mammalian cell-based assays ("false positives"). Our results document detection of genotoxic carcinogens in independent cell clones and at levels of cellular toxicities <60 % with a sensitivity of >85 %, specificity of ≥90 % and detection of false-positive compounds <17 %. Importantly, through testing cyclophosphamide in combination with primary hepatocyte cultures, we additionally provide proof-of-concept for the identification of carcinogens requiring metabolic activation using this novel assay system.

Summary of in vitro genetic toxicology assay results: expected and unexpected effects of recent study design modifications

Key modifications to in vitro genetic toxicology testing have been made in the last 5 years including the use of optimization approaches such as structure-activity relationships and screening assays to identify and eliminate potentially genotoxic chemicals from further consideration, better guidance on cytotoxicity assessment and dose selection, and greater use of p53-competent human cells. To determine the effect of these changes on testing outcomes, the pattern of positive results across assays conducted by BioReliance from 2005 to 2010 was examined. Data were tabulated for good laboratory practice (GLP)-compliant Ames, mouse lymphoma (MLA), chromosome aberration in Chinese hamster ovary (CHO) cells, and in human peripheral blood lymphocytes (HPBL) assays along with non-GLP screening Ames assays. A decrease in percentage of positive results in MLA and CHO chromosome aberration assays was observed, whereas the percentage of positive Ames assays remained consistent. This was not unexpected because MLA and CHO cytogenetic assays have undergone the most substantive changes (e.g., the establishment of the Global Evaluation Factor for the MLA and the use of the relative increase in cell counts in CHO chromosome aberration assays). Over the last 5 years, there has been an increase in the percentage of positive results observed in the chromosome aberration assay in HPBL. It is speculated that this may have led to an increase in HPBL-positive results if the chemicals routed to HPBL had previous positive genotoxicity results. Another factor may be the lack of a reliable cytotoxicity measurement in the HPBL assay.

Development of a High-Throughput Gaussia Luciferase Reporter Assay for the Activation of the GADD45a Gene by Mutagens, Promutagens, Clastogens, and Aneugens

Exposure to genotoxic carcinogens leads to increased expression of the GADD45a gene in mammalian cells. This signature of genotoxic hazard has previously been exploited in the GreenScreen HC assay, in which GADD45a expression is linked to green fluorescent protein (GFP) expression in the human TK6 lymphoblastoid cell line. This article describes the development and validation of an alternative assay (“BlueScreen HC”), in which expression is linked to Gaussia luciferase (GLuc) expression, yielding a luminescent reporter, the preferred optical output in high-throughput screening. The coelentrazine substrate of GLuc is relatively unstable, and a new buffer is reported that improves its stability. A more sensitive method is demonstrated for the measurement of cell densities in the assay, using the fluorescent cyanine dye thiazole orange. A protocol amendment also allows the assessment of pro-genotoxicity using S9 liver extracts. Compounds from the European Centre for the Validation of Alternative Methods (ECVAM) recommended list for the assessment of new or improved genotoxicity assays were evaluated with and without S9 in the new assay. The new GLuc assay was as effective as the GFP assay in producing positive results for all classes of genotoxic carcinogen and negative results for all nongenotoxins tested.

Need and potential value of the Pig-ain vivo mutation assay-a HESI perspective

The Health and Environmental Sciences Institute (HESI), a global branch of the International Life Sciences Institute (ILSI), initiated a project committee entitled "Relevance and Follow-up of Positive Results from In Vitro Genetic Toxicity Testing (IVGT)" with the overall objective of improving the scientific basis for the interpretation of results from genetic toxicology testing. The IVGT committee has also recognized the need to develop follow-up strategies for determining the relevance of in vitro test results to human health, and moving genetic toxicology testing from the sole purpose of hazard identification toward a more quantitative risk assessment approach. In this context, a group of experts evaluated the potential utility of the emerging in vivo mutational assessment model commonly known as the Pig-a gene mutation assay to follow-up positive in vitro genetic toxicology findings and to generate robust dose-response data for quantitative assessment of the in vivo mutagenicity. The IVGT experts participating in this effort represented academia, industry, and government agencies from across the globe and addressed such issues as the optimal sample size and experimental design for generating robust dose-response data. This expert group concluded that the emerging Pig-a gene mutation assay holds great promise as an in vivo mutagenicity assay, either as a stand-alone study or integrated into repeat-dose toxicology studies, and therefore supports further validation of the model.

New and emerging technologies for genetic toxicity testing

The International Life Sciences Institute (ILSI) Health and Environmental Sciences Institute (HESI) Project Committee on the Relevance and Follow-up of Positive Results in In Vitro Genetic Toxicity (IVGT) Testing established an Emerging Technologies and New Strategies Workgroup to review the current State of the Art in genetic toxicology testing. The aim of the workgroup was to identify promising technologies that will improve genotoxicity testing and assessment of in vivo hazard and risk, and that have the potential to help meet the objectives of the IVGT. As part of this initiative, HESI convened a workshop in Washington, DC in May 2008 to discuss mature, maturing, and emerging technologies in genetic toxicology. This article collates the abstracts of the New and Emerging Technologies Workshop together with some additional technologies subsequently considered by the workgroup. Each abstract (available in the online version of the article) includes a section addressed specifically to the strengths, weaknesses, opportunities, and threats associated with the respective technology. Importantly, an overview of the technologies and an indication of how their use might be aligned with the objectives of IVGT are presented. In particular, consideration was given with regard to follow-up testing of positive results in the standard IVGT tests (i.e., Salmonella Ames test, chromosome aberration assay, and mouse lymphoma assay) to add weight of evidence and/or provide mechanism of action for improved genetic toxicity risk assessments in humans.

Genotoxicity testing: moving beyond qualitative "screen and bin" approach towards characterization of dose-response and thresholds

For more than 40+ years, genotoxicity data have been interpreted in a qualitative, binary mode; a chemical is considered either positive or negative for a response in the test system. Although dose-response information is sometimes used in this decision, it is not routine to obtain the amount of information needed to inform risk assessment, for example to determine no-observed-genotoxic-effect-levels, primarily due to the historical view of genotoxic responses as "linear, no-threshold." Only recently have researchers begun to address this issue through robust experimental designs and application of statistical models. A growing body-of-evidence supports the existence of response thresholds for a number of mutagenic agents, in vitro and in vivo. Clearly, simple observation of a "hockey-stick" dose-response curve is not sufficient to establish a threshold. Collection of robust empirical data must be supported with an analysis of biological plausibility for the observed threshold. In this context, a chemical-specific mode-of-action (MOA) approach, which identifies key events responsible for the observed mutagenic effect, is extremely valuable. Biomarkers of key events, providing qualitative and quantitative information, can be integrated in a weight-of-evidence-based assessment of genotoxicity data from multiple test systems and used to identify data gaps to resolve/reduce uncertainties during the risk assessment process. To this end, specific recommendations on study design and data analysis are proposed. As the Environmental Mutagen Society celebrates its 40th anniversary, the field of genetic toxicology is marking a milestone on the path to a new paradigm, using a MOA, data-driven approach to answer questions about thresholds for genotoxic agents.

Genetic toxicology of folpet and captan

Folpet and captan are fungicides whose genotoxicity depends on their chemical reaction with thiols. Multiple mutagenicity tests have been conducted on these compounds due to their positive activity in vitro and their association with gastrointestinal tumors in mice. A review of the collective data shows that these compounds have in vitro mutagenic activity but are not genotoxic in vivo. This dichotomy is primarily due to the rapid degradation of folpet and captan in the presence of thiol-rich matrices typically found in vivo. Genotoxicity has not been found in the duodenum, the mouse tumor target tissue. It is concluded that folpet like captan presents an unlikely risk of genotoxic effects in humans.

GADD45a-GFP GreenScreen HC genotoxicity screening assay

BACKGROUND

Genetic toxicology is getting very interesting. The International Conference on Harmonisation has drafted new guidance that allows for the registration of pharmaceuticals without the submission of data from in vitro mammalian genotoxicity tests (in vitro micronucleus test, chromosomal aberrations, mouse lymphoma assay). These tests often produce falsely positive predictions of genotoxic carcinogenicity.

OBJECTIVES

This article reviews the properties of the Gadd45a-GFP (green fluorescent protein) assay, for which positive results appear to provide more reliable predictions of genotoxic carcinogenicity. The criteria for assessment of genotoxicity assays are reviewed. Consideration is given to the value of genotoxicity hazard assessment early in pharmaceutical discovery.

METHODS

Peer-reviewed data have been reviewed, as well as information contributed to the public domain through conference presentations.

RESULTS/CONCLUSION

The Gadd45a assay is increasingly used as a screening tool, and has utility in the prioritisation of Ames-negative compounds prior to in vivo genotoxicity assessment.