Oxyphenbutazone ameliorates carfilzomib induced cardiotoxicity in rats via inhibition of oxidative free radical burst and NF-κB/IκB-α pathway

Carfilzomib (CFZ), a chemotherapeutic agent used for multiple myeloma treatments reported to cause high incidence of cardiac events either new onset and/or exacerbate formerly diagnosed heart failure with ventricular and myocardial dysfunction. Purpose: Current research designed to explore and examine the preventive effect of oxyphenbutazone in the CFZ -instigated cardiotoxicity. Methodology: Female Wistar Rats weighing 200-250 g selected randomly and grouped as follows: Group 1 designated as the Normal control and receive normal saline only. Group 2 served toxic control and exposed to CFZ (4 mg/kg, intraperitoneally [i.p.]). Group 3 & 4 served as treatment groups and administered with CFZ concomitantly orally fed with oxyphenbutazone at doses of 35 and 70 mg/kg/three times a week, respectively. The total duration of experimental protocol was of 21 days. After completion of the experiments animals subjected to blood collection using light ether anesthesia and serum was separated for biochemical analysis further. The serum levels of Mg+2, Ca+2 and cardiac enzymes (aspartate transaminase (AST), lactate dehydrogenase (LDH), creatine kinase (CK) and creatine kinase-MB (CK-MB) levels were estimated. Later animals sacrificed and heart tissue isolated for further examinations. Intracellular proteins NFkB and IkBα were estimated by western blot. Results: The serum analysis revealed that CFZ administration significantly elevated the levels of LDH, CK and CKMB in CFZ exposed animals when compared to normal animals while administration of oxyphenbutazone significantly reduced these biochemical changes, Intracellular antioxidant enzymes and NF-kB in treatment groups as compared to disease control animals. Conclusion: Findings of the research protocol suggests significant injuries to cardiac tissues when animals exposed to CFZ and Oxyphenbutazone protected the cardiac tissues.

Single-Fe-Atom Catalyst for Sensitive Electrochemical Detection of Caffeic Acid

A single-atom catalyst (Fe SAs/-N-C) with excellent stability and conductivity was strategically fabricated via high-temperature calcination using the NiFe layered double hydroxide (LDH)/ZIF-8 composite as precursors. With the help of Ni as a catalyst, a great number of carbon nanotubes were produced whereby the isolated carbon bulks were interconnected to form an "island-bridge"-like 3D network structure, which greatly enhanced the exposure of active sites and the electron transfer. Accordingly, caffeic acid (CA) with versatile biological and pharmacological activities was chosen as the model analyte. The Fe SAs/-N-C with Fe-N4 as the catalytic active site was employed to establish the electrochemical sensing of CA with satisfactory sensitivity, selectivity, and long-term stability. This work expands the application range of single-atom catalysts and contributes a significant reference for the synthesis of hybrid double-atom catalysts.

Scientific opinion on the renewal of the authorisation of Scansmoke SEF7525 (SF-004) as a smoke flavouring Primary Product

The EFSA Panel on Food Additives and Flavourings (FAF) was requested to evaluate the safety of the smoke flavouring Primary Product Scansmoke SEF7525 (SF-004), for which a renewal application was submitted in accordance with Article 12(1) of Regulation (EC) No 2065/2003. This opinion refers to the assessment of data submitted on chemical characterisation, dietary exposure and genotoxicity of the Primary Product. Scansmoke SEF7525 is obtained from a tar produced from a mixture of red oak, white oak, maple, beech and hickory. Based on the compositional data, the Panel noted that the identified and quantified proportion of the solvent-free fraction amounts to 32.6 weight (wt)%, thus the applied method does not meet the legal quality criterion that at least 50% of the solvent-free fraction shall be identified and quantified. At the maximum proposed use levels, dietary exposure estimates calculated with Food Additive Intake Model (FAIM) ranged from 0.6 to 3.8 mg/kg body weight (bw) per day at the mean and from 1.1 to 10.1 mg/kg bw per day at the 95th percentile. Based on the available information on genotoxicity on 44 identified components, the Panel concluded that two substances in the Primary Product, styrene and benzofuran, raise a potential concern for genotoxicity. In addition, a potential concern for genotoxicity was identified for the unidentified part of the mixture. Considering that the exposure estimates for styrene and benzofuran are above the threshold of toxicological concern (TTC) value of 0.0025 kg/kg bw per day for DNA-reactive mutagens and/or carcinogens and since further data are needed to clarify their potential genotoxicity, the Panel concluded that the potential safety concern for genotoxicity of the Primary Product cannot be ruled out.

A review on genotoxicity in connection to infertility and cancer

Genotoxicity has been identified as the main cause of infertility and a variety of cancers. The mechanisms affect the structure, quality of the information or the segregation of DNA and are not inherently correlated with mutagenicity. The concept of genotoxicity, the chemical classes that cause genetic damage and the associated mechanisms of action are discussed here. Hazardous effects of pharmaceuticals, cosmetics, agrochemicals, industrial compounds, food additives, natural toxins and nanomaterials are, in large part, identified by genotoxicity and mutagenicity tests. These are critical and early steps in industrial and regulatory health assessment. Though several in vitro experiments are commonly used and approval by regulatory agencies for commercial licensing of drugs, their accuracy in human predictions for genotoxic and mutagenic effects is frequently questioned. Treatment of real and functional genetic toxicity problems depends in detail on the knowledge of mechanisms of DNA damage in the molecular, subcellular, cellular and tissue or organ system levels. Current strategies for risk assessment of human health need revisions to achieve robust and reliable results for optimizing their effectiveness. Additionally, computerized methods, neo-biomarkers leveraging '-omics' approaches, all of which can provide a convincing genotoxicity evaluation to reduce infertility and cancer risk.

Caffeic Acid Genotoxicity: Correlation of the Pig-a Assay with Regulatory Genetic Toxicology In Vivo Endpoints

Caffeic acid is found in variety of fruits and vegetables. It is considered as possible human carcinogen (Group 2B). It is negative in Ames and mouse micronucleus (MN), but positive in mouse lymphoma and chromosomal aberration assays. The objective of this study was to evaluate the in vivo genotoxicity of caffeic acid using three different endpoints: in vivo MN, Pig-a, and comet assay. Two sets of six rats per group were administered vehicle (0.5% hydroxypropyl methylcellulose), 500, 1,000, or 2,000 mg/kg/day of caffeic acid for three consecutive days via oral gavage. One set of animals was used for the Pig-a and MN assay and the other set was used for the comet assay. N-Ethyl N-Nitrosourea was used as positive control for the Pig-a and MN assay, and ethyl methanesulfonate for the comet assay. From one set of animals, peripheral blood was collected on Days -1, 14, and 30 for the Pig-a assay and on Day 4 for the MN assay. The other set of animals was euthanized 3 hr after the last dose; liver and blood were collected for the comet assay. A statistically significant increase in the MN frequency was observed at 2,000 mg/kg/day. No increase in the red blood cells (RBC^CD59- ) or reticulocytes (RET^CD59- ) Pig-a mutant frequencies was observed on Days 14 or 30. No increase in DNA strand breaks was observed in the peripheral blood or liver in the comet assay. Environ. Mol. Mutagen. 2019. © 2019 Wiley Periodicals, Inc.

Mode-of-action analysis of the effects induced by nicotine in the in vitro micronucleus assay

Nicotine's genotoxic potential has been extensively studied in vitro. While the results of mammalian cell-based studies have inferred that it can potentially damage chromosomes, in general and with few exceptions, adverse DNA effects have been observed primarily at supraphysiological concentrations in nonregulatory assays that provide little information on its mode-of-action (MoA). In this study, a modern-day regulatory genotoxicity assessment was conducted using a flow cytometry-based in vitro micronucleus (MN) assay, Good Laboratory Practice study conditions, Chinese hamster ovary cells of known provenance, and acceptance/evaluation criteria from the current OECD Test Guideline 487. Nicotine concentrations up to 3.95 mM had no effect on background levels of DNA damage; however, concentrations above the point-of-departure range of 3.94-4.54 mM induced increases in MN and hypodiploid nuclei, indicating a possible aneugenicity hazard. Follow-up experiments designed to elucidate nicotine's MoA revealed cellular vacuolization, accompanying distortions in microtubules, inhibition of tubulin polymerization, centromere-positive DNA, and multinucleate cells at MN-inducing concentrations. Vacuoles likely originated from acidic cellular compartments (e.g., lysosomes). Remarkably, genotoxicity was suppressed by chemicals that raised the luminal pH of these organelles. Other endpoints (e.g., changes in phosphorylated histones) measured in the study cast doubt on the biological relevance of this apparent genotoxicity. In addition, three major nicotine metabolites, including cotinine, had no MN effects but nornicotine induced a nicotine-like profile. It is possible that nicotine's lysosomotropic properties drive the genotoxicity observed in vitro; however, the potency and mechanistic insights revealed here indicate that it is likely of minimal physiological relevance for nicotine consumers. Environ. Mol. Mutagen. 2019. © 2019 The Authors. Environmental and Molecular Mutagenesis published by Wiley Periodicals, Inc. on behalf of Environmental Mutagen Society.

Genetic toxicology in silico protocol

In silico toxicology (IST) approaches to rapidly assess chemical hazard, and usage of such methods is increasing in all applications but especially for regulatory submissions, such as for assessing chemicals under REACH as well as the ICH M7 guideline for drug impurities. There are a number of obstacles to performing an IST assessment, including uncertainty in how such an assessment and associated expert review should be performed or what is fit for purpose, as well as a lack of confidence that the results will be accepted by colleagues, collaborators and regulatory authorities. To address this, a project to develop a series of IST protocols for different hazard endpoints has been initiated and this paper describes the genetic toxicity in silico (GIST) protocol. The protocol outlines a hazard assessment framework including key effects/mechanisms and their relationships to endpoints such as gene mutation and clastogenicity. IST models and data are reviewed that support the assessment of these effects/mechanisms along with defined approaches for combining the information and evaluating the confidence in the assessment. This protocol has been developed through a consortium of toxicologists, computational scientists, and regulatory scientists across several industries to support the implementation and acceptance of in silico approaches.

The influence of exogenous metabolism on the specificity of in vitro mammalian genotoxicity tests

A two-part study was designed to determine whether the inclusion of the rodent liver 'S9' exogenous metabolic activating system contributes to the generation of misleading positive results by the regulator-required in vitro mammalian genotoxicity tests. The mono-oxygenase enzymes in S9 produce direct-acting DNA-reactive electrophiles, and are included in in vitro genotoxicity tests to enhance the detection of substances which only become genotoxic following metabolism. However, as the S9 system lacks 'detoxifying' phase 2 factors it was hypothesised that increased chemical metabolism per se may lead to an increase in irrelevant S9 test outcomes in safety assessment. To test this, 89 compounds with positive or negative carcinogenicity data were identified, which produced negative Ames test data (+/- S9), and only produced positive in vitro mammalian test data in the presence of S9. This allowed a determination of whether or not misleading predictions of carcinogenicity by the in vitro mammalian tests were more or less prevalent in the presence of S9. A subset of these compounds was then tested with and without S9 in the GADD45a-GFP genotoxicity test, in order to determine whether misleading in vitro mammalian positive results were generally more prevalent with S9, or reflected particular tests' liabilities. This study suggests that the use of S9 metabolic activation in in vitro genotoxicity tests does not increase the prevalence of misleading positive results in in vitro mammalian genotoxicity assays, at least amongst Ames negative compounds.

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.

International regulatory requirements for genotoxicity testing for pharmaceuticals used in human medicine, and their impurities and metabolites

The process of developing international (ICH) guidelines is described, and the main guidelines reviewed are the ICH S2(R1) guideline that includes the genotoxicity test battery for human pharmaceuticals, and the ICH M7 guideline for assessing and limiting potentially mutagenic impurities and degradation products in drugs. Key aspects of the guidelines are reviewed in the context of drug development, for example the incorporation of genotoxicity assessment into non-clinical toxicity studies, and ways to develop and assess weight of evidence. In both guidelines, the existence of "thresholds" or non-linear dose responses for genotoxicity plays a part in the strategies. Differences in ICH S2(R1) protocol recommendations from OECD guidelines are highlighted and rationales explained. The use of genotoxicity data during clinical development and in assessment of carcinogenic potential is also described. There are no international guidelines on assessment of potentially genotoxic metabolites, but some approaches to safety assessment are discussed for these. Environ. Mol. Mutagen. 58:296-324, 2017. © 2017 Wiley Periodicals, Inc.

In vitro genotoxicity testing-Can the performance be enhanced?

The assessment of genotoxicity represents an essential component of the safety assessment of all types of substances. Several in vitro tests are available at different stages of development and acceptance, yet they are not considered at present sufficient to fully replace animal tests needed to evaluate the safety of substances. For an overall improvement of the traditional genotoxicity testing paradigm, several recent activities have taken place. These include the improvement of existing tests, the development of novel tests, as well as, the establishment and exploration of approaches to optimise in vitro testing accuracy. Furthermore, useful tools, such as databases or reference chemical lists have been developed to support advances in this field.

An integrative test strategy for cancer hazard identification

Assessment of genotoxic and carcinogenic potential is considered one of the basic requirements when evaluating possible human health risks associated with exposure to chemicals. Test strategies currently in place focus primarily on identifying genotoxic potential due to the strong association between the accumulation of genetic damage and cancer. Using genotoxicity assays to predict carcinogenic potential has the significant drawback that risks from non-genotoxic carcinogens remain largely undetected unless carcinogenicity studies are performed. Furthermore, test systems already developed to reduce animal use are not easily accepted and implemented by either industries or regulators. This manuscript reviews the test methods for cancer hazard identification that have been adopted by the regulatory authorities, and discusses the most promising alternative methods that have been developed to date. Based on these findings, a generally applicable tiered test strategy is proposed that can be considered capable of detecting both genotoxic as well as non-genotoxic carcinogens and will improve understanding of the underlying mode of action. Finally, strengths and weaknesses of this new integrative test strategy for cancer hazard identification are presented.

Carboxylated short single-walled carbon nanotubes but not plain and multi-walled short carbon nanotubes show in vitro genotoxicity

Long carbon nanotubes (CNTs) resemble asbestos fibers due to their high length to diameter ratio and they thus have genotoxic effects. Another parameter that might explain their genotoxic effects is contamination with heavy metal ions. On the other hand, short (1-2 µm) CNTs do not resemble asbestos fibers, and, once purified from contaminations, they might be suitable for medical applications. To identify the role of fiber thickness and surface properties on genotoxicity, well-characterized short pristine and carboxylated single-walled (SCNTs) and multi-walled (MCNTs) CNTs of different diameters were studied for cytotoxicity, the cell's response to oxidative stress (immunoreactivity against hemoxygenase 1 and glutathione levels), and in a hypoxanthine guanine phosphoribosyltransferase (HPRT) assay using V79 chinese hamster fibroblasts and human lung adenocarcinoma A549 cells. DNA repair was demonstrated by measuring immunoreactivity against activated histone H2AX protein. The number of micronuclei as well as the number of multinucleated cells was determined. CNTs acted more cytotoxic in V79 than in A549 cells. Plain and carboxylated thin (<8 nm) SCNTs and MCNTs showed greater cytotoxic potential and carboxylated CNTs showed indication for generating oxidative stress. Multi-walled CNTs did not cause HPRT mutation, micronucleus formation, DNA damage, interference with cell division, and oxidative stress. Carboxylated, but not plain, SCNTs showed indication for in vitro DNA damage according to increase of H2AX-immunoreactive cells and HPRT mutation. Although short CNTs presented a low in vitro genotoxicity, functionalization of short SCNTs can render these particles genotoxic.

Is Phenylbutazone a Genotoxic Carcinogen? A Weight-of-Evidence Assessment

Published in silico, in vitro, in vivo laboratory animal and human data, together with information on biotransformation and data from structure-activity analyses with two decision-tree systems (ACToR and Toxtree), have been used in a weight-of-evidence (WoE) assessment to determine whether phenylbutazone (PBZ) is a genotoxic or a non-genotoxic carcinogen. This was undertaken to facilitate the risk assessment of human exposure to this veterinary drug via the consumption of horsemeat from treated animals. Despite problems with data interpretation at all tiers of the database, it was concluded that PBZ behaves like a genotoxic carcinogen with a threshold dose. This conclusion is based mainly on the results of a definitive rodent bioassay, and on the following observations: a) that PBZ has weak in vitro activity only at high concentrations in some genotoxicity assays, accompanied by high levels of cytotoxicity; b) that it (and a major metabolite) is able to cause sister chromatid exchanges in vivo in rodents; and c) that it can induce cytogenetic effects in vivo in humans. It also takes into account the known and predicted activities of the parent drug, some of its metabolites and two structural analogues, and, importantly, several of the drug's other biochemical effects that are unrelated to toxicity. However, this conclusion is not fully supported by all the evidence, and much of the information is based on old papers. Therefore, more studies are required to establish whether the concentration thresholds seen in vitro would translate to dose thresholds for carcinogenicity, such that a safe dose-level could be defined for the purposes of assessing risk. It was disappointing that a WoE approach to evaluating all of the available hazard data, as is increasingly being advocated to improve the hazard identification paradigm, was unable to provide definitive answers in this case, particularly in view of the large numbers of animals that had been used to provide much of the information.

Pharmacokinetics, pharmacodynamics, metabolism, toxicology and residues of phenylbutazone in humans and horses

The presence of horse meat in food products destined for human consumption and labelled as beef has raised several concerns of public interest. This review deals solely with one aspect of these concerns; samples of equine tissue from horses destined for the human food chain have tested positive for the non-steroidal anti-inflammatory drug, phenylbutazone. The safety of some or all such foods for human consumers is a major concern, because it was shown many years ago that phenylbutazone therapy in humans can be associated with life threatening blood dyscrasias. As an initial basis for assessing the potential toxicity of foods containing phenylbutazone and its metabolites, this article reviews (1) the pharmacokinetic, pharmacodynamic, metabolic and toxicological profiles of phenylbutazone, with particular reference to horses and humans; (2) toxicity data in laboratory animals; (3) phenylbutazone residues in food producing species, and (4) as a preliminary assessment, the potential hazard associated with the consumption of horse meat containing phenylbutazone and its metabolites. Since phenylbutazone cannot be classified as a carcinogenic substance in humans, and noting that blood dyscrasias in humans are likely to be dose and treatment duration-dependent, the illegal and erratic presence of trace amount residues of phenylbutazone in horse meat is not a public health issue.

Evaluation of the highest concentrations used in the in vitro chromosome aberrations assay

There is controversy over the highest concentration to which an article should be tested in in vitro mammalian cell assays of genetic toxicity. Until recently, most guidelines specified the use of concentrations of up to 10 mM or 5,000 μg/ml (whichever is lower) when not limited by the toxicity of the test article to the cells used for the test. Several recent publications have called for lowering those limits. We examined concentration/response curves for in vitro chromosome aberrations assays. Data was extracted from two published databases to evaluate the lowest dose at which a positive response was reported. Concentration/response curves were simulated using Monte Carlo procedures on log normal distributions of the data. These curves were then used to predict the loss in assay sensitivity that would be incurred by arbitrarily lowering the highest concentration to which the assay is conducted. The simulations suggest that lowering the current high concentration limit from 10 mM would dramatically impact the sensitivity of the assay. In contrast, lowering the high concentration limit using the μg/ml scale, the most commonly applied scale in regulatory submissions, would not have a similar impact on assay sensitivity until the limit concentration was lowered to more than half of the current 5,000 μg/ml limit. This analysis suggests that the current limits of 10 mM and 5,000 μg/ml are not equivalent to one another and challenges the assumption that lowering the 10 mM limit will not decrease assay sensitivity.

In vitro genotoxicity testing strategy for nanomaterials and the adaptation of current OECD guidelines

There is a pressing requirement to define a hazard identification and risk management strategy for nanomaterials due to the rapid growth in the nanotechnology industry and their promise of life-style revolutions through the development of wide-ranging nano-containing consumer products. Consequently, a battery of well defined and appropriate in vitro assays to assess a number of genotoxicity endpoints is required to minimise extensive and costly in vivo testing. However, the validity of the established protocols in current OECD recognised genotoxicity assays for nanomaterials is currently being questioned. In this report, we therefore consider the in vitro OECD genotoxicity test battery including the Ames, micronucleus and HPRT forward mutation assays, and their potential role in the safety assessment of nanomaterial induced DNA damage in vitro.

How accurate is in vitro prediction of carcinogenicity?

Positive genetic toxicity data suggest carcinogenic hazard, and this can stop a candidate pharmaceutical reaching the clinic. However, during the last decade, it has become clear that many non-carcinogens produce misleading positive results in one or other of the regulatory genotoxicity assays. These doubtful conclusions cost a lot of time and money, as they trigger additional testing of apparently genotoxic candidates, both in vitro and in animals, to discover whether the suggested hazard is genuine. This in turn means that clinical trials can be put on hold. This review describes the current approaches to the 'misleading positive' problem as well as efforts to reduce the use of animals in genotoxicity assessment. The following issues are then addressed: the application of genotoxicity testing screens earlier in development; the search for new or improved in vitro genotoxicity tests; proposed changes to the International Committee on Harmonisation guidance on genotoxicity testing [S2(R1)]. Together, developments in all these areas offer good prospects of a more rapid and cost-effective way to understand genetic toxicity concerns.

Alternative (non-animal) methods for cosmetics testing: current status and future prospects-2010

The 7th amendment to the EU Cosmetics Directive prohibits to put animal-tested cosmetics on the market in Europe after 2013. In that context, the European Commission invited stakeholder bodies (industry, non-governmental organisations, EU Member States, and the Commission's Scientific Committee on Consumer Safety) to identify scientific experts in five toxicological areas, i.e. toxicokinetics, repeated dose toxicity, carcinogenicity, skin sensitisation, and reproductive toxicity for which the Directive foresees that the 2013 deadline could be further extended in case alternative and validated methods would not be available in time. The selected experts were asked to analyse the status and prospects of alternative methods and to provide a scientifically sound estimate of the time necessary to achieve full replacement of animal testing. In summary, the experts confirmed that it will take at least another 7-9 years for the replacement of the current in vivo animal tests used for the safety assessment of cosmetic ingredients for skin sensitisation. However, the experts were also of the opinion that alternative methods may be able to give hazard information, i.e. to differentiate between sensitisers and non-sensitisers, ahead of 2017. This would, however, not provide the complete picture of what is a safe exposure because the relative potency of a sensitiser would not be known. For toxicokinetics, the timeframe was 5-7 years to develop the models still lacking to predict lung absorption and renal/biliary excretion, and even longer to integrate the methods to fully replace the animal toxicokinetic models. For the systemic toxicological endpoints of repeated dose toxicity, carcinogenicity and reproductive toxicity, the time horizon for full replacement could not be estimated.

A core in vitro genotoxicity battery comprising the Ames test plus the in vitro micronucleus test is sufficient to detect rodent carcinogens and in vivo genotoxins

In vitro genotoxicity testing needs to include tests in both bacterial and mammalian cells, and be able to detect gene mutations, chromosomal damage and aneuploidy. This may be achieved by a combination of the Ames test (detects gene mutations) and the in vitro micronucleus test (MNvit), since the latter detects both chromosomal aberrations and aneuploidy. In this paper we therefore present an analysis of an existing database of rodent carcinogens and a new database of in vivo genotoxins in terms of the in vitro genotoxicity tests needed to detect their in vivo activity. Published in vitro data from at least one test system (most were from the Ames test) were available for 557 carcinogens and 405 in vivo genotoxins. Because there are fewer publications on the MNvit than for other mammalian cell tests, and because the concordance between the MNvit and the in vitro chromosomal aberration (CAvit) test is so high for clastogenic activity, positive results in the CAvit test were taken as indicative of a positive result in the MNvit where there were no, or only inadequate data for the latter. Also, because Hprt and Tk loci both detect gene-mutation activity, a positive Hprt test was taken as indicative of a mouse-lymphoma Tk assay (MLA)-positive, where there were no data for the latter. Almost all of the 962 rodent carcinogens and in vivo genotoxins were detected by an in vitro battery comprising Ames+MNvit. An additional 11 carcinogens and six in vivo genotoxins would apparently be detected by the MLA, but many of these had not been tested in the MNvit or CAvit tests. Only four chemicals emerge as potentially being more readily detected in MLA than in Ames+MNvit--benzyl acetate, toluene, morphine and thiabendazole--and none of these are convincing cases to argue for the inclusion of the MLA in addition to Ames+MNvit. Thus, there is no convincing evidence that any genotoxic rodent carcinogens or in vivo genotoxins would remain undetected in an in vitro test battery consisting of Ames+MNvit.