An assessment of the utility of the yeast GreenScreen assay in pharmaceutical screening

Sensing chemical-induced DNA damage using CRISPR/Cas9-mediated gene-deletion yeast-reporter strains

Microorganism-based genotoxicity assessments are vital for evaluating potential chemical-induced DNA damage. In this study, we developed both chromosomally integrated and single-copy plasmid-based reporter assays in budding yeast using a RNR3 promoter-driven luciferase gene. These assays were designed to compare the response to genotoxic chemicals with a pre-established multicopy plasmid-based assay. Despite exhibiting the lowest luciferase activity, the chromosomally integrated reporter assay showed the highest fold induction (i.e., the ratio of luciferase activity in the presence and absence of the chemical) compared with the established plasmid-based assay. Using CRISPR/Cas9 technology, we generated mutants with single- or double-gene deletions, affecting major DNA repair pathways or cell permeability. This enabled us to evaluate reporter gene responses to genotoxicants in a single-copy plasmid-based assay. Elevated background activities were observed in several mutants, such as mag1Δ cells, even without exposure to chemicals. However, substantial luciferase induction was detected in single-deletion mutants following exposure to specific chemicals, including mag1Δ, mms2Δ, and rad59Δ cells treated with methyl methanesulfonate; rad59Δ cells exposed to camptothecin; and mms2Δ and rad10Δ cells treated with mitomycin C (MMC) and cisplatin (CDDP). Notably, mms2Δ/rad10Δ cells treated with MMC or CDDP exhibited significantly enhanced luciferase induction compared with the parent single-deletion mutants, suggesting that postreplication and for nucleotide excision repair processes predominantly contribute to repairing DNA crosslinks. Overall, our findings demonstrate the utility of yeast-based reporter assays employing strains with multiple-deletion mutations in DNA repair genes. These assays serve as valuable tools for investigating DNA repair mechanisms and assessing chemical-induced DNA damage. KEY POINTS: • Responses to genotoxic chemicals were investigated in three types of reporter yeast. • Yeast strains with single- and double-deletions of DNA repair genes were tested. • Two DNA repair pathways predominantly contributed to DNA crosslink repair in yeast.

Search for the optimal genotoxicity assay for routine testing of chemicals: Sensitivity and specificity of conventional and new test systems

Many conventional in vitro tests that are currently widely used for routine screening of chemicals have a sensitivity/specificity in the range between 60 % and 80 % for the detection of carcinogens. Most procedures were developed 30-40 years ago. In the last decades several assays became available which are based on the use of metabolically competent cell lines, improvement of the cultivation conditions and development of new endpoints. Validation studies indicate that some of these models may be more reliable for the detection of genotoxicants (i.e. many of them have sensitivity and specificity values between 80 % and 95 %). Therefore, they could replace conventional tests in the future. The bone marrow micronucleus (MN) assay with rodents is at present the most widely used in vivo test. The majority of studies indicate that it detects only 5-6 out of 10 carcinogens while experiments with transgenic rodents and comet assays seem to have a higher predictive value and detect genotoxic carcinogens that are negative in MN experiments. Alternatives to rodent experiments could be MN experiments with hen eggs or their replacement by combinations of new in vitro tests. Examples for promising candidates are ToxTracker, TGx-DDI, multiplex flow cytometry, γH2AX experiments, measurement of p53 activation and MN experiments with metabolically competent human derived liver cells. However, the realization of multicentric collaborative validation studies is mandatory to identify the most reliable tests.

Yeast-based genotoxicity tests for assessing DNA alterations and DNA stress responses: a 40-year overview

By damaging DNA molecules, genotoxicants cause genetic mutations and also increase human susceptibility to cancers and genetic diseases. Over the past four decades, several assays have been developed in the budding yeast Saccharomyces cerevisiae to screen potential genotoxic substances and provide alternatives to animal-based genotoxicity tests. These yeast-based genotoxicity tests are either DNA alteration-based or DNA stress-response reporter-based. The former, which came first, were developed from the genetic studies conducted on various types of DNA alterations in yeast cells. Despite their limited throughput capabilities, some of these tests have been used as short-term genotoxicity tests in addition to bacteria- or mammalian cell-based tests. In contrast, the latter tests are based on the emergent transcriptional induction of DNA repair-related genes via activation of the DNA damage checkpoint kinase cascade triggered by DNA damage. Some of these reporter assays have been linked to DNA damage-responsive promoters to assess chemical carcinogenicity and ecotoxicity in environmental samples. Yeast-mediated genotoxicity tests are being continuously improved by increasing the permeability of yeast cell walls, by the ectopic expression of mammalian cytochrome P450 systems, by the use of DNA repair-deficient host strains, and by integrating them into high-throughput formats or microfluidic devices. Notably, yeast-based reporter assays linked with the newer toxicogenomic approaches are becoming powerful short-term genotoxicity tests for large numbers of compounds. These tests can also be used to detect polluted environmental samples, and as effective screening tools during anticancer drug development.

Cell-based in vitro models in environmental toxicology: a review

An analysis of biological effects induced by environmental toxins and exposure-related evaluation of potential risks for health and environment represent central tasks in classical biomonitoring. While epidemiological data and population surveys are clearly the methodological frontline of this scientific field, cellbased in vitro assays provide information on toxin-affected cellular pathways and mechanisms, and are important sources for the identification of relevant biomarkers. This review provides an overview on currently available in vitro methods based on cultured cells, as well as some limitations and considerations that are of specific interest in the context of environmental toxicology. Today, a large number of different endpoints can be determined to pinpoint basal and specific toxicological cellular effects. Technological progress and increasingly refined protocols are extending the possibilities of cell-based in vitro assays in environmental toxicology and promoting their increasingly important role in biomonitoring.

State-of-the-Art Metabolic Toxicity Screening and Pathway Evaluation

Routine in vitro bioassays and animal toxicity studies of drug and environmental chemical candidates fail to reveal toxicity in ∼30% of cases. This Feature article addresses research on new approaches to in vitro toxicity testing as well as our own efforts to produce high-throughput genotoxicity arrays and LC-MS/MS approaches to reveal possible chemical pathways of toxicity.

The BlueScreen-384 assay as an indicator of genotoxic hazard potential in early-stage drug discovery

High-throughput cell-based techniques that permit early detection of compound-induced genotoxic damage have recently become available. Methods based on induction of the GADD45a promoter are attractive because multiple intracellular mechanisms that detect genetic damage intersect at this checkpoint gene. Consequently, assays such as GreenScreen HC, which uses p53-competant human TK6 lymphoblastoid cells and a GADD45a-GFP reporter, have been developed. GreenScreen HC allows weekly testing of dozens of compounds using 96-well microplates, with high interassay consistency. BlueScreen HC is a recent advancement, coupling GADD45a to Gaussia luciferase, with several advantages over GADD45a-GFP including the potential for miniaturization. Here we describe implementation of a 384-well BlueScreen assay. For drug discovery programs carrying out iterative analogue synthesis around a chemical lead series, these assays permit assessment of compound genotoxic potential in parallel to, rather than subsequent to, determination of activity at a therapeutic target. We demonstrate comparability of BlueScreen-384 to GreenScreen HC and illustrate the use of BlueScreen-384 to explore the structure-activity relationship around a genotoxic lead molecule to identify nongenotoxic analogues. BlueScreen-384 can reduce the need for costly and time-consuming analogue testing in more traditional genotoxicity tests, such as the Ames test.

Metabolic toxicity screening using electrochemiluminescence arrays coupled with enzyme-DNA biocolloid reactors and liquid chromatography-mass spectrometry

New chemicals or drugs must be guaranteed safe before they can be marketed. Despite widespread use of bioassay panels for toxicity prediction, products that are toxic to a subset of the population often are not identified until clinical trials. This article reviews new array methodologies based on enzyme/DNA films that form and identify DNA-reactive metabolites that are indicators of potentially genotoxic species. This molecularly based methodology is designed in a rapid screening array that utilizes electrochemiluminescence (ECL) to detect metabolite-DNA reactions, as well as biocolloid reactors that provide the DNA adducts and metabolites for liquid chromatography-mass spectrometry (LC-MS) analysis. ECL arrays provide rapid toxicity screening, and the biocolloid reactor LC-MS approach provides a valuable follow-up on structure, identification, and formation rates of DNA adducts for toxicity hits from the ECL array screening. Specific examples using this strategy are discussed. Integration of high-throughput versions of these toxicity-screening methods with existing drug toxicity bioassays should allow for better human toxicity prediction as well as more informed decision making regarding new chemical and drug candidates.

No time to lose--high throughput screening to assess nanomaterial safety

Nanomaterials hold great promise for medical, technological and economical benefits. Knowledge concerning the toxicological properties of these novel materials is typically lacking. At the same time, it is becoming evident that some nanomaterials could have a toxic potential in humans and the environment. Animal based systems lack the needed capacity to cope with the abundance of novel nanomaterials being produced, and thus we have to employ in vitro methods with high throughput to manage the rush logistically and use high content readouts wherever needed in order to gain more depth of information. Towards this end, high throughput screening (HTS) and high content screening (HCS) approaches can be used to speed up the safety analysis on a scale that commensurate with the rate of expansion of new materials and new properties. The insights gained from HTS/HCS should aid in our understanding of the tenets of nanomaterial hazard at biological level as well as assist the development of safe-by-design approaches. This review aims to provide a comprehensive introduction to the HTS/HCS methodology employed for safety assessment of engineered nanomaterials (ENMs), including data analysis and prediction of potentially hazardous material properties. Given the current pace of nanomaterial development, HTS/HCS is a potentially effective means of keeping up with the rapid progress in this field--we have literally no time to lose.

Microscreening toxicity system based on living magnetic yeast and gradient chips

There is an increasing demand for easy and cost-effective methods to screen the toxicological impact of the growing number of chemical mixtures being generated by industry. Such a screening method has been developed using viable, genetically modified green fluorescent protein (GFP) reporter yeast that was magnetically functionalised and held within a microfluidic device. The GFP reporter yeast was used to detect genotoxicity by monitoring the exposure of the cells to a well-known genotoxic chemical (methyl methane sulfonate, MMS). The cells were magnetised using biocompatible positively charged PAH-stabilised magnetic nanoparticles with diameters around 15 nm. Gradient mixing was utilised to simultaneously expose yeast to a range of concentrations of toxins, and the effective fluorescence emitted from the produced GFP was measured. The magnetically enhanced retention of the yeast cells, with their facile subsequent removal and reloading, allowed for very convenient and rapid toxicity screening of a wide range of chemicals. This is the first report showing magnetic yeast within microfluidic devices in a simple bioassay, with potential applications to other types of fluorescent reporter yeast in toxicological and biomedical research. The microfluidic chip offers a simple and low-cost screening test that can be automated to allow multiple uses (adapted to different cell types) of the device on a wide range of chemicals and concentrations.

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.

Evaluation of the Vitotox and RadarScreen assays for the rapid assessment of genotoxicity in the early research phase of drug development

The Vitotox and RadarScreen assays were evaluated as early screens for mutagenicity and clastogenicity, respectively. The Vitotox assay is a bacterial reporter assay in Salmonella typhimurium based on the SOS-response, and it contains a luciferase gene under control of the recN promoter. The RadarScreen assay is a RAD54 promoter-linked beta-galactosidase reporter assay in yeast. The expression of this beta-galactosidase can easily be quantified by use of the substrate d-luciferin-o-beta-galactopyranoside, which is converted into galactose and luciferin that can be measured luminometrically. Recently, an ECVAM workgroup defined a list of 20 genotoxic and 42 non-genotoxic compounds [D. Kirkland, P. Kasper, L. Muller, R. Corvi, G. Speit, Recommended lists of genotoxic and non-genotoxic chemicals for assessment of the performance of new or improved genotoxicity tests: a follow-up to an ECVAM workshop, Mutat. Res. 653 (2008) 99-108.] that can be used for the validation and/or optimization of in vitro genotoxicity assays. In the present study, this compound set was used for the validation of the assays. Moreover, an additional set of 192 compounds was used to broaden this validation study. The compounds of this additional set can be classified as non-genotoxins and genotoxins and consists of both in-house and reference compounds. In case of the ECVAM compound list, the results from the Vitotox and RadarScreen assays were compared to the genotoxic/non-genotoxic classification of the compounds in this list. In case of the additionally tested compounds, the results of the Vitotox and RadarScreen assays were compared, respectively, with bacterial mutagenicity (Ames) results or in vitro clastogenicity data obtained in-house or from the literature. The validation with respect to the ECVAM compound list resulted in a sensitivity for both the Vitotox and RadarScreen assay of 70% (14/20). If both assays were combined the sensitivity increased to 85% (17/20). Both tests also gave a low number of false positive results. The specificity of the Vitotox and RadarScreen assays was 93% (39/42) and 83% (35/42), respectively. This resulted in a predictivity of the Vitotox and RadarScreen assay of 85% (53/62) and 79% (49/62), respectively. In case both tests were combined the specificity and the predictivity of the Vitotox and RadarScreen assay turned out to be 81% (34/42) and 82% (51/62), respectively. The results from the additional list of 192 compounds confirmed the results found with the ECVAM compound list. The results from the Vitotox assay showed a high correlation with Ames test of 91% (132/145). Subsequently, the RadarScreen assay had a correlation with in vitro clastogenicity of 76% (93/123). The specificity of the Vitotox assay was 94% (90/96) for Ames test results and that of the RadarScreen assay was 74% (34/46) for clastogenicity. Moreover, the sensitivities of the Vitotox and RadarScreen assays were 86% (42/49) and 77% (59/77), respectively. Implementation of the Vitotox and RadarScreen assays in the early research phase of drug development can lead to fast de-selection for genotoxicity. It is expected that this application will reduce the number of compounds that have a positive score in the regulatory Ames and clastogenicity tests. Moreover, problems with a complete compound class can be foreseen at an early time point in the research phase, which gives more time for issue resolution than late detection of these problems with the regulatory tests.

Predictive models for carcinogenicity and mutagenicity: frameworks, state-of-the-art, and perspectives

Mutagenicity and carcinogenicity are endpoints of major environmental and regulatory concern. These endpoints are also important targets for development of alternative methods for screening and prediction due to the large number of chemicals of potential concern and the tremendous cost (in time, money, animals) of rodent carcinogenicity bioassays. Both mutagenicity and carcinogenicity involve complex, cellular processes that are only partially understood. Advances in technologies and generation of new data will permit a much deeper understanding. In silico methods for predicting mutagenicity and rodent carcinogenicity based on chemical structural features, along with current mutagenicity and carcinogenicity data sets, have performed well for local prediction (i.e., within specific chemical classes), but are less successful for global prediction (i.e., for a broad range of chemicals). The predictivity of in silico methods can be improved by improving the quality of the data base and endpoints used for modelling. In particular, in vitro assays for clastogenicity need to be improved to reduce false positives (relative to rodent carcinogenicity) and to detect compounds that do not interact directly with DNA or have epigenetic activities. New assays emerging to complement or replace some of the standard assays include Vitotox, GreenScreenGC, and RadarScreen. The needs of industry and regulators to assess thousands of compounds necessitate the development of high-throughput assays combined with innovative data-mining and in silico methods. Various initiatives in this regard have begun, including CAESAR, OSIRIS, CHEMOMENTUM, CHEMPREDICT, OpenTox, EPAA, and ToxCast. In silico methods can be used for priority setting, mechanistic studies, and to estimate potency. Ultimately, such efforts should lead to improvements in application of in silico methods for predicting carcinogenicity to assist industry and regulators and to enhance protection of public health.

Integrated decision-tree testing strategies for mutagenicity and carcinogenicity with respect to the requirements of the EU REACH legislation

Liverpool John Moores University and FRAME recently conducted a research project sponsored by Defra, on the status of alternatives to animal testing with regard to the European Union REACH (Registration, Evaluation and Authorisation of Chemicals) system for the safety testing and risk assessment of chemicals. The project covered all the main toxicity endpoints associated with the REACH system. This paper focuses on the prospects for using alternative methods (both in vitro and in silico) for mutagenicity (genotoxicity) and carcinogenicity testing--two toxicity endpoints, which, together with reproductive toxicity, are of pivotal importance for the REACH system. The manuscript critically discusses well-established testing approaches, and in particular, the requirement for short-term in vivo tests for confirming positive mutagenicity, and the need for the rodent bioassay for detecting non-genotoxic carcinogens. Recently-proposed testing strategies focusing on non-animal approaches are also considered, and our own testing scheme is presented and supported with background information. This scheme makes maximum use of pre-existing data, computer (in silico) and in vitro methods, with weight-of-evidence assessments at each major stage. The need for the improvement of in vitro methods, to reduce the generation of false-positive results, is also discussed. Lastly, ways in which reduction and refinement measures can be used are also considered, and some recommendations are made for future research to facilitate the implementation of the proposed testing scheme.

High-throughput screening for analysis of in vitro toxicity

The influence of combinatorial chemistry and high-throughput screening (HTS) technologies in the pharmaceutical industry during the last 10 years has been enormous. However, the attrition rate of drugs in the clinic due to toxicity during this period still remained 40-50%. The need for reduced toxicity failure led to the development of early toxicity screening assays. This chapter describes the state of the art for assays in the area of genotoxicity, cytotoxicity, carcinogenicity, induction of specific enzymes from phase I and II metabolism, competition assays for enzymes of phase I and II metabolism, embryotoxicity as well as endocrine disruption and reprotoxicity. With respect to genotoxicity, the full Ames, Ames II, Vitotox, GreenScreen GC, RadarScreen, and non-genotoxic carcinogenicity assays are discussed. For cytotoxicity, cellular proliferation, calcein uptake, oxygen consumption, mitochondrial activity, radical formation, glutathione depletion as well as apoptosis are described. For high-content screening (HCS), the possibilities for analysis of cytotoxicity, micronuclei, centrosome formation and phospholipidosis are examined. For embryotoxicity, endocrine disruption and reprotoxicity alternative assays are reviewed for fast track analysis by means of nuclear receptors and membrane receptors. Moreover, solutions for analyzing enzyme induction by activation of nuclear receptors, like AhR, CAR, PXR, PPAR, FXR, LXR, TR and RAR are given.

Development and validation of a higher throughput screening approach to genotoxicity testing using the GADD45a-GFP GreenScreen HC assay

There is a pressing need to develop rapid yet accurate screening assays for the identification of genotoxic liability and for early hazard assessment in drug discovery. The GADD45a-GFP human cell-based genotoxicity assay (GreenScreen HC) has been reformatted to test 12 compounds per 96-well microplate in a higher throughput, automated screening mode and the protocol applied to the analysis of 1266 diverse, pharmacologically active compounds. Testing from a fixed starting concentration of 100 AmicroM and over 3 serial dilutions, the hit rates for genotoxicity (7.3%) and cytotoxicity (33%) endpoints of the assay have been determined in a much wider chemical space than previously reported. The degree of interference from color, autofluorescence, and low solubility has also been assessed. The assay results have been compared to an in silico approach to genotoxicity assessment using Derek for Windows software. Where carcinogenicity data were available, GreenScreen HC demonstrated a higher specificity than in silico methods while identifying genotoxic species that were not highlighted for genotoxic liability in structure-activity relationship software. Higher throughput screening from a fixed, low concentration reduces sensitivity to less potent genotoxins, but the maintenance of the previously reported high specificity is essential in early hazard assessment where misclassification can lead to the needless rejection of potentially useful compounds in drug development.

A prototype microfluidic chip using fluorescent yeast for detection of toxic compounds

A microfluidic chip has been developed to enable the screening of chemicals for environmental toxicity. The microfluidic approach offers several advantages over macro-scale systems for toxicity screening, including low cost and flexibility in design. This design flexibility means the chips can be produced with multiple channels or chambers which can be used to screen for different toxic compounds, or the same toxicant at different concentrations. Saccharomyces cerevisiae containing fluorescent markers are ideal candidates for the microfluidic screening system as fluorescence is emitted without the need of additional reagents. Microfluidic chips containing eight multi-parallel channels have been developed to retain yeast within the chip and allow exposure of them to toxic compounds. The recombinant yeast used was GreenScreentrade mark which expresses green fluorescent proteins when is exposed to genotoxins. After exposure of the yeast to target compounds, the fluorescence emission was detected using an inverted microscope. Qualitative and quantitative comparisons of the fluorescent emission were performed. Results indicated that fluorescent intensity per area significantly increases upon exposure to methyl-methanesulfonate, a well known genotoxic compound. The microfluidic approach reported here is an excellent tool for cell-based screening and detection of different toxicities. The device has the potential for use by industrial manufacturers to detect and reduce the production and discharge of toxic compounds, as well as to characterise already polluted environments.

Synergistic metabolic toxicity screening using microsome/DNA electrochemiluminescent arrays and nanoreactors

Platforms based on thin enzyme/DNA films were used in two-tier screening of chemicals for reactive metabolites capable of producing toxicity. Microsomes were used for the first time as sources of cytochrome (cyt) P450 enzymes in these devices. Initial rapid screening involved electrochemiluminescent (ECL) arrays featuring spots containing ruthenium poly(vinylpyridine), DNA, and rat liver microsomes or bicistronically expressed human cyt P450 2E1 (h2E1). Cyt P450 enzymes were activated via the NADPH/reductase cycle. When bioactivation of substrates in the films gives reactive metabolites, they are trapped by covalent attachment to DNA bases. The rate of increase in ECL with enzyme reaction time reflects relative DNA damage rates. "Toxic hits" uncovered by the array were studied in structural detail by using enzyme/DNA films on silica nanospheres as "nanoreactors" to provide nucleobase adducts from reactive metabolites. The utility of this synergistic approach was demonstrated by estimating relative DNA damage rates of three mutagenic N-nitroso compounds and styrene. Relative enzyme turnover rates for these compounds using ECL arrays and LC-UV-MS correlated well with TD 50 values for liver tumor formation in rats. Combining ECL array and nanoreactor/LC-MS technologies has the potential for rapid, high-throughput, cost-effective screening for reactive metabolites and provides chemical structure information that is complementary to conventional toxicity bioassays.

An analysis of results from 305 compounds tested with the yeast RAD54-GFP genotoxicity assay (GreenScreen GC)—including relative predictivity of regulatory tests and rodent carcinogenesis and performance with autofluorescent and coloured compounds

Data from 305 non-proprietary compounds tested using the yeast RAD54-GFP (Green Fluorescent Protein) assay, GreenScreen GC, are presented, together with a detailed comparison with results from in vitro and in vivo genotoxicity tests and rodent carcinogenesis. In addition, observations on reproducibility and the performance of the test with autofluorescent and coloured compounds are described. Like the Ames test, the GreenScreen assay is shown to exhibit high specificity (82%), meaning that compounds with positive results are very likely to be genotoxic carcinogens. This is in contrast to mammalian cell tests established for use in regulatory testing that provide disappointingly low specificity and the inevitable generation of confounding false positive data. The analysis confirmed the observations of earlier studies, showing that a combination of an Ames test (or surrogate) with the yeast test provides high specificity as well as high sensitivity in the identification of rodent carcinogens.

The utilization of a Saccharomyces cerevisiae HUG1P-GFP promoter-reporter construct for the selective detection of DNA damage

In this study, we report the creation and characterization of a yeast-based promoter-reporter construct for the detection of genotoxic compounds within a cell's local environment. We have synthesized a fusion containing the HUG1 promoter and GFP and incorporated this cassette into the yeast genome creating a stable, sensitive genotoxicity indicator. To quantify biosensor performance, HUG1P-GFP cells were exposed to multiple doses of a wide variety of genotoxins, including alkylating agents, an oxidative agent, a ribonucleotide reductase inhibitor, a UV mimetic agent, an agent that causes double strand breaks, a topoisomerase I inhibitor, and ionizing radiation, all of which triggered a detectable and reproducible level of GFP production by the HUG1P-GFP strain. Furthermore, GFP was not induced by general cell stresses including starvation, heat shock, and acidic pH. These results suggest this system will be a valuable supplement to traditional genotoxicity assays.

Proposed integrated decision-tree testing strategies for mutagenicity and carcinogenicity in relation to the EU REACH legislation

Liverpool John Moores University and FRAME recently conducted a research project sponsored by Defra, on the status of alternatives to animal testing with regard to the European Union REACH (Registration, Evaluation and Authorisation of Chemicals) system for the safety testing and risk assessment of chemicals. The project covered all the main toxicity endpoints associated with the REACH system. This paper focuses on the prospects for using alternative methods (both in vitro and in silico) for mutagenicity (genotoxicity) and carcinogenicity testing - two toxicity endpoints, which, together with reproductive toxicity, are of pivotal importance for the REACH system. The manuscript critically discusses well-established testing approaches, and in particular, the requirement for short-term in vivo tests for confirming positive mutagenicity, and the need for the rodent bioassay for detecting non-genotoxic carcinogens. Recently-proposed testing strategies focusing on non-animal approaches are also considered, and our own testing scheme is presented and supported with background information. This scheme makes maximum use of pre-existing data, computer (in silico) and in vitro methods, with weight-of-evidence assessments at each major stage. The need for the improvement of in vitro methods, to reduce the generation of false-positive results, is also discussed. Lastly, ways in which reduction and refinement measures can be used are also considered, and some recommendations are made for future research to facilitate the implementation of the proposed testing scheme.

The application of discovery toxicology and pathology towards the design of safer pharmaceutical lead candidates

Toxicity is a leading cause of attrition at all stages of the drug development process. The majority of safety-related attrition occurs preclinically, suggesting that approaches to identify 'predictable' preclinical safety liabilities earlier in the drug development process could lead to the design and/or selection of better drug candidates that have increased probabilities of becoming marketed drugs. In this Review, we discuss how the early application of preclinical safety assessment--both new molecular technologies as well as more established approaches such as standard repeat-dose rodent toxicology studies--can identify predictable safety issues earlier in the testing paradigm. The earlier identification of dose-limiting toxicities will provide chemists and toxicologists the opportunity to characterize the dose-limiting toxicities, determine structure-toxicity relationships and minimize or circumvent adverse safety liabilities.

High-specificity and high-sensitivity genotoxicity assessment in a human cell line: Validation of the GreenScreen HC GADD45a-GFP genotoxicity assay

The battery of genetic toxicity tests required by most regulatory authorities includes both bacterial and mammalian cell assays and identifies practically all genotoxic carcinogens. However, the relatively high specificity of the Salmonella mutagenicity assay (Ames test) is offset by the low specificity of the established mammalian cell assays, which leads to difficulties in the interpretation of the biological relevance of results. This paper describes a new high-throughput assay that links the regulation of the human GADD45a gene to the production of Green Fluorescent Protein (GFP). A study of 75 well-characterised genotoxic and non-genotoxic compounds with diverse mechanisms of DNA-damage induction (including aneugens) reveals that the assay responds positively to all classes of genotoxic damage with both high specificity and high sensitivity. The current micro-well assay format does not include metabolic activation, but a separate low-throughput protocol demonstrates a successful proof-of-principle for an S9 metabolic activation assay with the model pro-mutagen cyclophosphamide. The test should be of value both as a tool in the selection of candidate compounds for further development, where additional data may be required because of conflicting information from the in vitro test battery, or in product development areas where the use of animals is to be discontinued. As a microplate assay however, it has the qualities of high throughput and low compound use that will facilitate its application in early screening for genotoxic liability.