Follow-up actions from positive results of in vitro genetic toxicity testing (original) (raw)
Related papers
Mutagenesis, 1989
A series of multivariate statistical methods have been used to explore the results of a set of four in vitro short-term tests (STT) on 73 chemicals reported by the US National Toxicology Program (NTP). Cluster analysis showed that the mouse lymphoma mutation (MLY) and sister-chromatid exchange (SCE) were similar in performance, as were the Salmonella (STY) and chromosomal aberration test (CHA). The lack of association between tests using the same genetic end-point or at the same phylogenetic level found in previous analyses was confirmed in this study. Factor analysis was used to derive a scale of genetic damage. This measure was contrasted with rodent carcinogenicity; only a limited association was found (rank correlation coefficient, rs = 0.32). Linear discriminant analysis was used to study whether the STTs could be used to complement one another. The combination of STY with the other STTs did not improve significantly the prediction of rodent carcinogenicity of STY alone. In the entire set of chemicals, 33% were negative in STY and positive in at least two other STT, and 11% was negative in STY and positive in the three other tests. SCE and MLY were complementary to STY for identifying the most genotoxic chemicals, but CHA was not a useful complement. The presence of potential electrophilic sites in the chemicals was highly correlated with the STY results, but did not improve the ability of STY to identify genotoxic chemicals or predict rodent carcinogens. In conclusion, the other in vitro STTs did not complement STY for predicting carcinogenicity, but were an important complement for describing the potential genotoxicity of chemicals.
Mutation Research/Genetic Toxicology and Environmental Mutagenesis, 2011
A working group convened at the 2009 5th IWGT to discuss possibilities for improving in vivo genotoxicity assessment by investigating possible links to standard toxicity testing. The working group considered: (1) combination of acute micronucleus (MN) and Comet assays into a single study, (2) integration of MN assays into repeated-dose toxicity (RDT) studies, (3) integration of Comet assays into RDT studies, and (4) requirements for the top dose when integrating genotoxicity measurements into RDT studies. The working group reviewed current requirements for in vivo genotoxicity testing of different chemical product classes and identified opportunities for combination and integration of genotoxicity endpoints for each class. The combination of the acute in vivo MN and Comet assays was considered by the working group to represent a technically feasible and scientifically acceptable alternative to conducting independent assays. Two combination protocols, consisting of either a 3- or a 4-treament protocol, were considered equally acceptable. As the integration of MN assays into RDT studies had already been discussed in detail in previous IWGT meetings, the working group focussed on factors that could affect the results of the integrated MN assay, such as the possible effects of repeated bleeding and the need for early harvests. The working group reached the consensus that repeated bleeding at reasonable volumes is not a critical confounding factor for the MN assay in rats older than 9 weeks of age and that rats bled for toxicokinetic investigations or for other routine toxicological purposes can be used for MN analysis. The working group considered the available data as insufficient to conclude that there is a need for an early sampling point for MN analysis in RDT studies, in addition to the routine determination at terminal sacrifice. Specific scenarios were identified where an additional early sampling can have advantages, e.g., for compounds that exert toxic effects on hematopoiesis, including some aneugens. For the integration of Comet assays into RDT studies, the working group reached the consensus that, based upon the limited amount of data available, integration is scientifically acceptable and that the liver Comet assay can complement the MN assay in blood or bone marrow in detecting in vivo genotoxins. Practical issues need to be considered when conducting an integrated Comet assay study. Freezing of tissue samples for later Comet assay analysis could alleviate logistical problems. However, the working group concluded that freezing of tissue samples can presently not be recommended for routine use, although it was noted that results from some laboratories look promising. Another discussion topic centred around the question as to whether tissue toxicity, which is more likely observed in RDT than in acute toxicity studies, would affect the results of the Comet assay. Based on the available data from in vivo studies, the working group concluded that there are no clear examples where cytotoxicity, by itself, generates increases or decreases in DNA migration. The working group identified the need for a refined guidance on the use and interpretation of cytotoxicity methods used in the Comet assay, as the different methods used generally lead to inconsistent conclusions. Since top doses in RDT studies often are limited by toxicity that occurs only after several doses, the working group discussed whether the sensitivity of integrated genotoxicity studies is reduced under these circumstances. For compounds for which in vitro genotoxicity studies yielded negative results, the working group reached the consensus that integration of in vivo genotoxicity endpoints (typically the MN assay) into RDT studies is generally acceptable. If in vitro genotoxicity results are unavailable or positive, consensus was reached that the maximum tolerated dose (MTD) is acceptable as the top dose in RDT studies in many cases, such as when the RDT study MTD or exposure is close (50% or greater) to an acute study MTD or exposure. Finally, the group agreed that exceptions to this general rule might be acceptable, for example when human exposure is lower than the preclinical exposure by a large margin.
Quantitative approaches for assessing dose-response relationships in genetic toxicology studies
Environmental and Molecular Mutagenesis, 2013
Genetic toxicology studies are required for the safety assessment of chemicals. Data from these studies have historically been interpreted in a qualitative, dichotomous ''yes'' or ''no'' manner without analysis of doseresponse relationships. This article is based upon the work of an international multi-sector group that examined how quantitative dose-response relationships for in vitro and in vivo genetic toxicology data might be used to improve human risk assessment. The group examined three quantitative approaches for analyzing dose-response curves and deriving point-of-departure (POD) metrics (i.e., the no-observed-genotoxic-effectlevel (NOGEL), the threshold effect level (Td), and the benchmark dose (BMD)), using data for the induction of micronuclei and gene mutations by methyl methanesulfonate or ethyl methanesulfonate in vitro and in vivo. These results suggest that the POD descriptors obtained using the different approaches are within the same order of magnitude, with more variability observed for the in vivo assays. The different approaches were found to be complementary as each has advantages and limitations. The results further indicate that the lower confidence limit of a benchmark response rate of 10% (BMDL 10 ) could be considered a satisfactory POD when analyzing genotoxicity data using the BMD approach. The models described permit the identification of POD values that could be combined with mode of action analysis to determine whether exposure(s) below a particular level constitutes a significant human risk. Subsequent analyses will expand the number of substances and endpoints investigated, and continue to evaluate the utility of quantitative approaches for analysis of genetic toxicity dose-response data. Environ. Mol. Mutagen. 54:8-18, 2013. V V C 2012 Wiley Periodicals, Inc.
Toxicological Sciences, 2007
Early screening of drug candidates for genotoxicity typically includes an analysis for mutagenicity in bacteria and for clastogenicity in cultured mammalian cells. In addition, in recent years, an early assessment of photogenotoxicity potential has become increasingly important. Also, for screening purposes, expert computer systems can be used to identify structural alerts. In cases where structural alerts are identified, mutagenicity testing limited to bacteria can be conducted. The sequence of computeraided analysis and limited testing using bacteria allows for screening a comparatively large number of drug candidates. In contrast, considerably more resources, in terms of supplies, technical time, and the amount of a test substance needed, are required when screening for clastogenic activity in mammalian cells. In addition, the relatively large percentage of false positive results for rodent carcinogenicity associated with clastogenicity assays is of considerable concern. As a consequence, mammalian cell-based alternatives to clastogenicity assays are needed for early screening of mammalian genotoxicity. The comet assay is a relatively fast, simple, and sensitive technique for the analysis of DNA damage in mammalian cells. This assay seems especially useful for screening purposes because false positives associated with excessive toxicity appear to occur less frequently, only relatively small amounts of a test compound are needed, and certain steps of the test procedure can be automated. Therefore, the in vitro comet assay is proposed as an alternative to cytogenetic assays in early genotoxicity/photogenotoxicity screening of drug candidates.
Toxicity Testing in the 21st Century: Implications for Human Health Risk Assessment
Risk Analysis, 2009
The risk analysis perspective by Daniel Krewski and colleagues lays out the long-term vision and strategic plan developed by a National Research Council committee, (1) sponsored by the U.S. Environmental Protection Agency (EPA) with support from the U.S. National Toxicology Program (NTP), to "advance the practices of toxicity testing and human health assessment of environmental agents." Components of the vision include chemical characterization; the use of human-cell-based, high-throughput assays that cover the diversity of toxicity pathways; targeted testing using animals to fill in data gaps; dose-response and extrapolation modeling; and the generation and use of population-based and human exposure data for interpreting the results of toxicity tests. The strategic plan recognizes that meeting this vision will require a major research effort conducted over a period of a decade or more to identify all of the important toxicity pathways, and that a clear distinction must be made between which pathway perturbations are truly adverse (i.e., would likely lead to adverse health outcomes in humans) and those that are not. Krewski et al. note that achieving this vision in a reasonable timeframe (i.e., decades) would require the involvement of an interdisciplinary research
The Future of Toxicity Testing
Journal of Toxicology and Environmental Health, Part B, 2010
In 2007, the U.S. National Research Council (NRC) released a report, "Toxicity Testing in the 21st Century: A Vision and a Strategy," that proposes a paradigm shift for toxicity testing of environmental agents. The vision is based on the notion that exposure to environmental agents leads to adverse health outcomes through the perturbation of toxicity pathways that are operative in humans. Implementation of the NRC vision will involve a fundamental change in the assessment of toxicity of environmental agents, moving away from adverse health outcomes observed in experimental animals to the identification of critical perturbations of toxicity pathways. Pathway perturbations will be identified using in vitro assays and quantified for dose response using methods in computational toxicology and other recent scientific advances in basic biology. Implementation of the NRC vision will require a major research effort, not unlike that required to successfully map the human genome, extending over 10 to 20 years, involving the broad scientific community to map important toxicity pathways operative in humans. This article provides an overview of the scientific tools and technologies that will form the core of the NRC vision for toxicity testing. Of particular importance will be the development of rapidly performed in vitro screening assays using human cells and cell lines or human tissue surrogates to efficiently identify environmental agents producing critical pathway perturbations. In addition to the overview of the NRC vision, this study documents the reaction by a number of stakeholder groups since 2007, including the scientific, risk assessment, regulatory, and animal welfare communities.
Commentary on ''Toxicity Testing in the 21st Century: A vision and a Strategy
Human & Experimental Toxicology, 2010
Toxicity Testing in the 21st Century: A Vision and a Strategy, from the National Research Council Committee on Toxicity Testing and Assessment of Environmental Agents, presents a vision wherein toxicology testing moves from feeding test substances to animals for their lifetimes, and assessing clinical laboratory and histopathological changes, to human tissue studies made suitable by recent technological advances in computational biology, toxicogenomics, and the like. This is to be accomplished by elucidating toxicity pathways complemented by targeted testing. The report focuses on the array of available new concepts and attendant technology that the committee considers relevant to its proffer, but, in the final analysis, it describes little in the way of robust strategy for achieving the stated goals. From that perspective, the vision, as described, is no more innovative or far-reaching than goals directed at the utility of cellular metabolism measurements put forth fifty years ago. The report generally lacks the coherence and organization that could have given greater credibility to the committee's deliberative effort.
Statistical Studies in Genetic Toxicology: A Perspective from the U.S. National Toxicology Program
Environmental Health Perspectives, 1985
This paper surveys recent, as yet unpublished, statistical studies arising from research in genetic toxicology within the U.S. National Toxicology Program (NTP). These studies all involve analyses of data from Ames Salmonella/microsome mutagenicity tests, but the statistical methodologies are broadly applicable. Three issues are addressed: First, what is a tenable sampling model for Ames test data, and how does one best test the adequacy of the Poisson sampling assumption? Second, given that nonmonotone dose-response curves are fairly common in the Salmonella assay, what new statistical techniques or modifications of existing ones seem appropriate to accommodate to this reality? Finally, an intriguing question: How can the extensive NTP Ames test data base be used to assess the characteristics of any mutagen-nonmutagen decision rule? The last issue is illustrated with the commonly used "two-times background" rule.