Alternative models for carcinogenicity testing: weight of evidence evaluations across models

Carcinogenicity and testicular toxicity of 2-bromopropane in a 26-week inhalation study using the rasH2 mouse model

2-Bromopropane (2-BP) is a colorless liquid at room temperature and is used in closed systems in factories, mainly as an intermediate for medicines, pesticides, and other chemicals. However, the carcinogenicity of 2-BP is still unknown. The CByB6F1-Tg(HRAS)2Jic (rasH2) transgenic mouse model has been established as an alternative to long-term studies (1.5 years-lifetime) to detect carcinogenicity in as short a time as six months. We performed a 26-week inhalation exposure study of 2-BP using the rasH2 mouse model. Male and female rasH2 mice were exposed to 0, 67, 200, or 600 ppm of 2-BP for 6 h/day, 5 days/week for 26 weeks. All tissues and blood were collected and subjected to biological and histopathological analyses. The results showed a concentration-dependent increase in lung tumor development in male and female rasH2 mice exposed by inhalation to 2-BP, which was significant by Peto's and Poly-3 trend tests. Furthermore, in male rasH2 mice, 2-BP was found to be a testicular toxin. This study is the first to demonstrate that 2-BP is carcinogenic in male and female mice and a testicular toxin in male mice using the rasH2 mouse model.

No evidence for carcinogenicity of titanium dioxide nanoparticles in 26-week inhalation study in rasH2 mouse model

With the rapid development of alternative methods based on the spirit of animal welfare, the publications of animal studies evaluating endpoints such as cancer have been extremely reduced. We performed a 26-week inhalation exposure studies of titanium dioxide nanoparticles (TiO2 NPs) using CByB6F1-Tg(HRAS)2Jic (rasH2) mice model for detecting carcinogenicity. Male and female rasH2 mice were exposed to 2, 8 or 32 mg/m3 of TiO2 NPs for 6 h/day, 5 days/week for 26 weeks. All tissues and blood were collected and subjected to biological and histopathological analyses. TiO2 NPs exposure induced deposition of particles in lungs in a dose-dependent manner in each exposure group. Exposure to TiO2 NPs, as well as other organs, did not increase the incidence of lung tumors in any group, and pulmonary fibrosis and pre-neoplastic lesions were not observed in all groups. Finally, the cell proliferative activity of alveolar epithelial type 2 cells was examined, and it was not increased by exposure to TiO2 NPs. This is the first report showing the lack of pulmonary fibrogenicity and carcinogenicity (no evidence of carcinogenic activity) of TiO2 NPs in 26-week inhalation study in rasH2 mice exposed up to 32 mg/m3, which is considered to be a high concentration.

The 2-year rodent bioassay in drug and chemical carcinogenicity testing: Performance, utility, and configuration for cancer hazard identification

For several intended uses of chemicals, the 2-year rodent bioassay (RCB) has been the benchmark method to screen the carcinogenicity to humans of substances, according to the hazard identification sphere. Despite the ongoing controversy around this traditional testing, the RCB is in force and being used by stakeholders. After assembling the RCB's ability to forecast the carcinogenicity to humans of substances, the current review aimed to provide a discussion on the RCB's (1) sensitivity and specificity; (2) utility; (3) configuration, and (4) provisional role in the regulatory policy. In general, RCBs conducted at maximum tolerated doses (MTDs) exhibited a functional ability to (1) not missing the great majority of human carcinogens, and to (2) not responding to the large majority of human non-carcinogens. There is citable evidence supporting the use of MTDs to render RCBs as sensitive as possible, particularly provided the ethically-justified small samples used in RCBs. The literature shows that rodent-specific mechanisms of chemical carcinogenesis contribute significant unspecificity to RCBs. Nonetheless, the paradox between a functional sensitivity and a significant unspecificity can be predictively resolved through the application of Bayesian forecasting. In terms of performance to forecast the carcinogenicity to humans of either genotoxic or non-genotoxic substances, 2-species-RCBs added no value over the rat-RCB. Nevertheless, there is preliminary evidence cautioning that 15% of the rodent carcinogens probably carcinogenic to humans could be missed if mouse-RCBs are indiscriminately discontinued. More than thirteen RCB-related issues relevant to regulatory pharmacology and toxicology were discussed and summarized in this review.

Tg.rasH2 Mouse Model for Assessing Carcinogenic Potential of Pharmaceuticals: Industry Survey of Current Practices

The Tg.rasH2 mouse was developed as an alternative model to the traditional 2-year mouse bioassay for pharmaceutical carcinogenicity testing. This model has found extensive use in support of pharmaceutical drug development over the last few decades. It has the potential to improve quality and timeliness, reduce animal usage, and in some instances allow expedient decision-making regarding the human carcinogenicity potential of a drug candidate. Despite the increased use of the Tg.rasH2 model, there has been no systematic survey of current practices in the design, interpretation of results from the bioassay, and global health authority perspectives. Therefore, the aim of this work was to poll the pharmaceutical industry on study design practices used in the dose range finding and definitive 6-month studies and on results relative to the ongoing negotiations to revise The International Council for Harmonisation of Technical Requirements for Pharmaceuticals for Human Use S1 Guidance. Twenty-two member companies of International Consortium for Innovation and Quality in Pharmaceutical Development DruSafe Leadership Group participated in the survey, sharing experiences from studies conducted with 55 test compounds between 2010 and 2018. The survey results provide very useful insights into study design and interpretation. Importantly, the results identified several key opportunities for reducing animal use and increasing the value of testing for potential human carcinogenicity using this model. Recommended changes to study designs that would reduce animal usage include eliminating the requirement to include positive control groups in every study, use of nontransgenic wild-type littermates in the dose range finding study, and use of microsampling to reduce or eliminate satellite groups for toxicokinetics.

Alternative Multiorgan Initiation–Promotion Assay for Chemical Carcinogenesis in the Wistar Rat

The medium-term multiorgan initiation–promotion chemical bioassay (diethylnitrosamine, methyl-nitrosourea, butyl-hydroxybutylnitrosamine, dihydroxypropylnitrosamine, dimethylhydrazine [DMBDD]) with the Fischer 344 rat was proposed as an alternative to the conventional 2-year carcinogenesis bioassay for regulatory purposes. The acronym DMBDD stands for the names of five genotoxic agents used for initiation of multiorgan carcinogenesis. The Brazilian Agency for the Environment officially recognized a variation of this assay (DMBDDb) as a valid method to assess the carcinogenic potential of agrochemicals. Different from the original protocol, this DMBDDb is 30-week long, uses Wistar rats and two positive control groups exposed to carcinogenesis promoters sodium phenobarbital (PB) or 2-acetylaminofluorene (2-AAF). This report presents the experience of an academic laboratory with the DMBDDb assay and contributes to the establishment of this alternative DMBDD bioassay in a different rat strain. Frequent lesions observed in positive groups to evaluate the promoting potential of pesticides and the immunohistochemical expressions of liver cytochrome P450 (CYP) 2B1/2B2 and CYP1A2 enzymes were assessed. Commonly affected organs were liver, kidney, intestines, urinary bladder, and thyroid. PB promoting activity was less evident than that of 2-AAF, especially in males. This study provides a repository of characteristic lesions occurring in positive control animals submitted to a modified alternative 2-stage multiorgan protocol for carcinogenesis in Wistar rat.

Risk Assessment in the Genomic Era

The sequencing of the human and mouse genomes, and soon that of the rat, offers a foundation to evaluate biological phenomena, including toxicologic effects. Numerous tools are being developed to evaluate aspects of biology based on the DNA sequence. These tools can be utilized to evaluate absorption, distribution, metabolism and excretion, and effects of the toxicologically active product on the target organ. The genes involved can be broadly categorized as those affecting susceptibility to a toxicologic effect and those that are involved in the biologic response. For risk assessment to be performed in a rational manner, fundamental mechanisms of toxicologic processes must be ascertained. Based on successes already achieved, such as development of transgenic and knockout mouse strains, the application of aspects of the genomics revolution could be useful in developing a better understanding of mechanisms, and possibly in identifying specific markers of responses. In addition, genomics are likely to be useful in translating effects between species. However, genomics are being portrayed as the ultimate solution to all of toxicology. This is hardly the case. Basic chemistry, biochemistry, toxicokinetics, pharmacology, and pathology will continue to be needed in the overall weight of evidence approach to risk assessment. Genomics are likely to be of limited usefulness in predicting individual, in contrast to population susceptibility to various toxicological responses. Concordance of various diseases in identical twins, for example, different cancers, is rarely greater than 20% over the lifetime of these individuals. Similarly, genomics are likely to be of limited usefulness in screening for toxicologic end points. As with other tools of biology, those to be developed based on the genome are likely to provide greater usefulness in dissecting the mechanistic processes involved and defining the basis for susceptibility.

Kinetic Modeling Reveals the Roles of Reactive Oxygen Species Scavenging and DNA Repair Processes in Shaping the Dose-Response Curve of KBrO₃-Induced DNA Damage

We have developed a kinetic model to investigate how DNA repair processes and scavengers of reactive oxygen species (ROS) can affect the dose-response shape of prooxidant induced DNA damage. We used as an example chemical KBrO₃ which is activated by glutathione and forms reactive intermediates that directly interact with DNA to form 8-hydroxy-2-deoxyguanosine DNA adducts (8-OH-dG). The single strand breaks (SSB) that can result from failed base excision repair of these adducts were considered as an effect downstream from 8-OH-dG. We previously demonstrated that, in the presence of effective base excision repair, 8-OH-dG can exhibit threshold-like dose-response dependence, while the downstream SSB can still exhibit a linear dose-response. Here we demonstrate that this result holds for a variety of conditions, including low levels of GSH, the presence of additional SSB repair mechanisms, or a scavenger. It has been shown that melatonin, a terminal scavenger, inhibits KBrO₃-caused oxidative damage. Our modeling revealed that sustained exposure to KBrO₃ can lead to fast scavenger exhaustion, in which case the dose-response shapes for both endpoints are not substantially affected. The results are important to consider when forming conclusions on a chemical's toxicity dose dependence based on the dose-response of early genotoxic events.

A critical appraisal of the process of regulatory implementation of novel in vivo and in vitro methods for chemical hazard and risk assessment

Regulatory toxicology urgently needs applicable alternative test systems that reduce animal use, testing time, and cost. European regulation on cosmetic ingredients has already banned animal experimentation for hazard identification, and public awareness drives toward additional restrictions in other regulatory frameworks as well. In addition, scientific progress stimulates a more mechanistic approach of hazard identification. Nevertheless, the implementation of alternative methods is lagging far behind their development. In search for general bottlenecks for the implementation of alternative methods, this manuscript reviews the state of the art as to the development and implementation of 10 diverse test systems in various areas of toxicological hazard assessment. They vary widely in complexity and regulatory acceptance status. The assays are reviewed as to parameters assessed, biological system involved, standardization, interpretation of results, extrapolation to human hazard, position in testing strategies, and current regulatory acceptance status. Given the diversity of alternative methods in many aspects, no common bottlenecks could be identified that hamper implementation of individual alternative assays in general. However, specific issues for the regulatory acceptance and application were identified for each assay. Acceptance of one-in-one replacement of complex in vivo tests by relatively simple in vitro assays is not feasible. Rather, innovative approaches using test batteries are required together with metabolic information and in vitro to in vivo dose extrapolation to convincingly provide the same level of information of current in vivo tests. A mechanistically based alternative approach using the Adverse Outcome Pathway concept could stimulate further (regulatory) acceptance of non-animal tests.

Reduction in the Number of Animals and the Evaluation Period for the Positive Control Group in Tg.rasH2 Short-Term Carcinogenicity Studies

The lack of a clear guidance on the adequate number of animals used for positive controls in the short-term (26-weeks) transgenic mouse carcinogenicity studies has resulted in the use of high number of animals. In our earlier Tg.rasH2 studies, 25 mice/sex were used in the urethane-positive control dose groups that were sacrificed by 18 weeks. Based on a robust response, several of our protocols for Tg.rasH2 studies with 15 mice/sex and terminal sacrifice at 17 ± 1 weeks were submitted and accepted by the Carcinogenicity Assessment Committee of the US Food and Drug Administration since we demonstrated close to 100% response for the development of lung and splenic tumors (target organs) in 500 mice/sex. These 500 mice/sex included 17 groups of 25 mice/sex and 5 groups of 15 mice/sex.  The objective of this investigation was to determine whether the number of animals can be further reduced along with the shortened duration of exposure to urethane. Accordingly, 10 Tg.rasH2 mice/sex/group were administered a total of 3 intraperitoneal (IP) injections of urethane (1000 mg/kg per day) on study days 1, 3, and 5, and the presence of tumors in the lungs and spleen was evaluated after 8, 10, 12, 14, or 16 weeks. Our results demonstrate that 100% of the mice at 8 weeks had developed lung tumors, whereas close to 100% of the mice at 14 weeks had developed splenic tumors. Based on the development of lung tumors alone in 100% of the mice, we recommend that 10 mice/sex are sufficient and that these mice can also be sacrificed as early as 10 ± 1 weeks following the administration of urethane.

Science, politics, and health in the brave new world of pharmaceutical carcinogenic risk assessment: technical progress or cycle of regulatory capture?

The carcinogenicity (cancer-inducing potential) of pharmaceuticals is an important risk factor for health when considering whether thousands of patients on drug trials or millions/billions of consumers in the marketplace should be exposed to a new drug. Drawing on fieldwork involving over 50 interviews and documentary research spanning 2002–2010 in Europe and the US, and on regulatory capture theory, this article investigates how the techno-regulatory standards for carcinogenicity testing of pharmaceuticals have altered since 1998. It focuses on the replacement of long-term carcinogenicity tests in rodents (especially mice) with shorter-term tests involving genetically-engineered mice (GEM). Based on evidence regarding financial/organizational control, methodological design, and interpretation of the validation and application of these new GEM tests, it is argued that regulatory agencies permitted the drug industry to shape such validation and application in ways that prioritized commercial interests over the need to protect public health. Boundary-work enabling industry scientists to define some standards of public-health policy facilitated such capture. However, as the scientific credibility of GEM tests as tools to protect public health by screening out carcinogens became inescapably problematic, a regulatory resurgence, impelled by reputational concerns, exercised more control over industry’s construction and use of the tests, The extensive problems with GEM tests as public-health protective regulatory science raises the spectre that alterations to pharmaceutical carcinogenicity-testing standards since the 1990s may have been boundary-work in which the political project of decreasing the chance that companies’ products are defined as carcinogenic has masqueraded as techno-science.

Cell transformation assays for prediction of carcinogenic potential: state of the science and future research needs

Cell transformation assays (CTAs) have long been proposed as in vitro methods for the identification of potential chemical carcinogens. Despite showing good correlation with rodent bioassay data, concerns over the subjective nature of using morphological criteria for identifying transformed cells and a lack of understanding of the mechanistic basis of the assays has limited their acceptance for regulatory purposes. However, recent drivers to find alternative carcinogenicity assessment methodologies, such as the Seventh Amendment to the EU Cosmetics Directive, have fuelled renewed interest in CTAs. Research is currently ongoing to improve the objectivity of the assays, reveal the underlying molecular changes leading to transformation and explore the use of novel cell types. The UK NC3Rs held an international workshop in November 2010 to review the current state of the art in this field and provide directions for future research. This paper outlines the key points highlighted at this meeting.

Transgenic animal models in toxicology: historical perspectives and future outlook

Transgenic animal models are powerful tools for developing a more detailed understanding on the roles of specific genes in biological pathways and systems. Applications of these models have been made within the field of toxicology, most notably for the screening of mutagenic and carcinogenic potential and for the characterization of toxic mechanisms of action. It has long been a goal of research toxicologists to use the data from these models to refine hazard identification and characterization to better inform human health risk assessments. This review provides an overview on the applications of transgenic animal models in the assessment of mutagenicity and carcinogenicity, their use as reporter systems, and as tools for understanding the roles of xenobiotic-metabolizing enzymes and biological receptors in the etiology of chemical toxicity. Perspectives are also shared on the future outlook for these models in toxicology and risk assessment and how transgenic technologies are likely to be an integral tool for toxicity testing in the 21st century.

An Industry Perspective on the Utility of Short-Term Carcinogenicity Testing in Transgenic Mice in Pharmaceutical Development

International guidelines allow for use of a short-term cancer bioassay (twenty-six weeks) in transgenic mice as a substitute for one of the two required long-term rodent bioassays in the preclinical safety evaluation of pharmaceuticals. The two models that have gained the widest acceptance by sponsors and regulatory authorities are the CB6F1-RasH2 mouse hemizygous for a human H-ras transgene and the B6.129N5-Trp53 mouse heterozygous for a p53 null allele. The p53+/- model is of particular value for compounds with residual concern that genotoxic activity may contribute to tumorigenesis. The rasH2 model is an appropriate alternative without regard to evidence of genotoxic potential. Since results from a short-term bioassay can be obtained relatively early in drug development, there is the potential for more timely assessment of cancer risk for individuals in long-term clinical trials. Use of these models in preclinical safety evaluation also significantly reduces animal use, time, and manpower. Preliminary findings indicate that prediction of two-year rat bioassay outcomes based on data from chronic rat toxicity studies, together with early assessment of carcinogenic potential in short-term transgenic models, may have the potential to increase the timeliness and efficiency of strategies for the identification of human carcinogenic hazards.

An enhanced 13-week bioassay: an alternative to the 2-year bioassay to screen for human carcinogenesis

The 2-year rodent bioassay has become the standard carcinogenicity screen for chemicals, despite concerns of costs, time, and excessive doses. More importantly, there are increasing concerns regarding its relevance to human carcinogenic risk, especially for non-DNA reactive chemicals. Cancer risk can be increased either by direct damage to DNA (DNA reactivity) or by increased cell proliferation. Utilizing the scientific basis for mode of action analysis in the framework that has been developed for extrapolating to human relevance, a short-term screen is proposed as a replacement for the 2-year bioassay. Chemicals are evaluated for DNA reactivity, immunosuppression, and estrogenic activity, known mechanisms of human carcinogenesis, by in vitro and/or in vivo tests. The chemical can then be evaluated for toxicity and/or increased cell proliferation in target tissues. This relies primarily on evaluation of organ weights and histopathology, and also utilizes data from blood and urine chemistries and DNA-labeling indices. Significant concern is raised regarding the relevance of evaluation of tissues that are present in rats or mice but not humans, and the relevance of proliferative responses in rodent endocrine tissues. In developing alternative procedures to evaluate chemicals for possible carcinogenic activity in humans, it is important not to rely on the 2-year rodent bioassay for validation of the new procedure. It is time to discontinue the performance of the 2-year rodent bioassay.

Aspartame: a safety evaluation based on current use levels, regulations, and toxicological and epidemiological studies

Aspartame is a methyl ester of a dipeptide used as a synthetic nonnutritive sweetener in over 90 countries worldwide in over 6000 products. The purpose of this investigation was to review the scientific literature on the absorption and metabolism, the current consumption levels worldwide, the toxicology, and recent epidemiological studies on aspartame. Current use levels of aspartame, even by high users in special subgroups, remains well below the U.S. Food and Drug Administration and European Food Safety Authority established acceptable daily intake levels of 50 and 40 mg/kg bw/day, respectively. Consumption of large doses of aspartame in a single bolus dose will have an effect on some biochemical parameters, including plasma amino acid levels and brain neurotransmitter levels. The rise in plasma levels of phenylalanine and aspartic acid following administration of aspartame at doses less than or equal to 50 mg/kg bw do not exceed those observed postprandially. Acute, subacute and chronic toxicity studies with aspartame, and its decomposition products, conducted in mice, rats, hamsters and dogs have consistently found no adverse effect of aspartame with doses up to at least 4000 mg/kg bw/day. Critical review of all carcinogenicity studies conducted on aspartame found no credible evidence that aspartame is carcinogenic. The data from the extensive investigations into the possibility of neurotoxic effects of aspartame, in general, do not support the hypothesis that aspartame in the human diet will affect nervous system function, learning or behavior. Epidemiological studies on aspartame include several case-control studies and one well-conducted prospective epidemiological study with a large cohort, in which the consumption of aspartame was measured. The studies provide no evidence to support an association between aspartame and cancer in any tissue. The weight of existing evidence is that aspartame is safe at current levels of consumption as a nonnutritive sweetener.

A tiered approach to systemic toxicity testing for agricultural chemical safety assessment

A proposal has been developed by the Agricultural Chemical Safety Assessment (ACSA) Technical Committee of the ILSI Health and Environmental Sciences Institute (HESI) for an improved approach to assessing the safety of crop protection chemicals. The goal is to ensure that studies are scientifically appropriate and necessary without being redundant, and that tests emphasize toxicological endpoints and exposure durations that are relevant for risk assessment. The ACSA Systemic Toxicity Task Force proposes an approach to systemic toxicity testing as one part of the overall assessment of a compound's potential to cause adverse effects on health. The approach is designed to provide more relevant data for deriving reference doses for shorter time periods of human exposure, and includes fewer studies for deriving longer term reference doses-that is, neither a 12-month dog study nor a mouse carcinogenicity study is recommended. All available data, including toxicokinetics and metabolism data and life stages information, are taken into account. The proposed tiered testing approach has the potential to provide new risk assessment information for shorter human exposure durations while reducing the number of animals used and without compromising the sensitivity of the determination of longer term reference doses.

Animal carcinogenicity studies: 3. Alternatives to the bioassay

Conventional animal carcinogenicity tests take around three years to design, conduct and interpret. Consequently, only a tiny fraction of the thousands of industrial chemicals currently in use have been tested for carcinogenicity. Despite the costs of hundreds of millions of dollars and millions of skilled personnel hours, as well as millions of animal lives, several investigations have revealed that animal carcinogenicity data lack human specificity (i.e. the ability to identify human non-carcinogens), which severely limits the human predictivity of the bioassay. This is due to the scientific inadequacies of many carcinogenicity bioassays, and numerous serious biological obstacles, which render profoundly difficult any attempts to accurately extrapolate animal data in order to predict carcinogenic hazards to humans. Proposed modifications to the conventional bioassays have included the elimination of mice as a second species, and the use of genetically-altered or neonatal mice, decreased study durations, initiation-promotion models, the greater incorporation of toxicokinetic and toxicodynamic assessments, structure-activity relationship (computerised) systems, in vitro assays, cDNA microarrays for detecting changes in gene expression, limited human clinical trials, and epidemiological research. The potential advantages of non-animal assays when compared to bioassays include the superior human specificity of the results, substantially reduced time-frames, and greatly reduced demands on financial, personnel and animal resources. Inexplicably, however, the regulatory agencies have been frustratingly slow to adopt alternative protocols. In order to decrease the enormous cost of cancer to society, a substantial redirection of resources away from excessively slow and resource-intensive rodent bioassays, into the further development and implementation of non-animal assays, is both strongly justified and urgently required.

Weight of evidence: a review of concept and methods

"Weight of evidence" (WOE) is a common term in the published scientific and policy-making literature, most often seen in the context of risk assessment (RA). Its definition, however, is unclear. A systematic review of the scientific literature was undertaken to characterize the concept. For the years 1994 through 2004, PubMed was searched for publications in which "weight of evidence" appeared in the abstract and/or title. Of the 276 papers that met these criteria, 92 were selected for review: 71 papers published in 2003 and 2004 (WOE appeared in abstract/title) and 21 from 1994 through 2002 (WOE appeared in title). WOE has three characteristic uses in this literature: (1) metaphorical, where WOE refers to a collection of studies or to an unspecified methodological approach; (2) methodological, where WOE points to established interpretative methodologies (e.g., systematic narrative review, meta-analysis, causal criteria, and/or quality criteria for toxicological studies) or where WOE means that "all" rather than some subset of the evidence is examined, or rarely, where WOE points to methods using quantitative weights for evidence; and (3) theoretical, where WOE serves as a label for a conceptual framework. Several problems are identified: the frequent lack of definition of the term "weight of evidence," multiple uses of the term and a lack of consensus about its meaning, and the many different kinds of weights, both qualitative and quantitative, which can be used in RA. A practical recommendation emerges: the WOE concept and its associated methods should be fully described when used. A research agenda should examine the advantages of quantitative versus qualitative weighting schemes, how best to improve existing methods, and how best to combine those methods (e.g., epidemiology's causal criteria with toxicology's quality criteria).

Probing p53 biological functions through the use of genetically engineered mouse models

The p53 tumor suppressor gene is rendered dysfunctional in the majority of human cancers. To model the effects of p53 dysfunction in an experimentally manipulable organismal context, genetically engineered inbred mice have been the models of choice. Transgenic and knock-out technologies have been utilized to generate an array of different p53 germ line alterations. As expected, many (though not all) of the mutant p53 mouse models are susceptible to enhanced spontaneous and carcinogen-induced tumors of a variety of types. A number of different variables affect the incidence and spectrum of tumors in p53 mutant mice. These include strain background, the nature of the p53 mutation, the presence of wild-type p53 (in addition to mutant p53), exposure to physical and chemical mutagens, or introduction of other cancer-associated genes into the mutant p53 background. In addition to their role in furthering our understanding of the mechanisms of cancer initiation and progression, these models have led to unexpected insights into p53 function in embryogenesis and aging. With the development of ever more sophisticated methods for manipulating the mouse genome, new p53 models are on the horizon, which should deliver advances that will provide not only important mechanistic insights but also discoveries of great clinical relevance.

p27(Kip1) (Cdkn1b)-deficient mice are susceptible to chemical carcinogenesis and may be a useful model for carcinogen screening

The two-year rodent bioassay is one of several tests that are widely used by governmental regulatory agencies as well as pharmaceutical and chemical companies to determine the carcinogenic potential of chemicals or environmental agents where human exposure is anticipated. That this assay has remained relatively unchanged for the last 25 years is a testament to the power of this approach to identify carcinogens and thus to minimize human exposure. However, there has long been controversy over the specificity and relevance of the rodent bioassay as well as its high cost in terms of time, expense, and numbers of mice. Much discussion has been generated in recent years over how to improve the 2-year rodent bioassay for more accurate and faster detection of potential human carcinogens. Here, we argue that the use of p27(Kip1) (Cdkn1b) knockout mouse for carcinogen screening may solve several shortcomings inherent in the conventional bioassay while preserving its best quality, that is, protecting public health by providing reliable in vivo information on the potential of chemicals to cause cancer.

A medium-term rat liver bioassay for rapid in vivo detection of carcinogenic potential of chemicals

A reliable medium-term bioassay system for rapid detection of carcinogenic potential of chemicals in the human environment has been developed. The 8-week-protocol consists of 2 stages; male F344 rats are given a single intraperitoneal injection of diethylnitrosamine (200 mg/kg) for initiation of liver carcinogenesis, followed by a 6-week test chemical treatment starting 2 weeks thereafter. Test chemicals are usually given in the diet or the drinking water and in the 2nd week of test chemical treatment, all rats are subjected to two-thirds partial hepatectomy in order to induce regenerative cell replication. The end-point marker is the glutathione S-transferase placental form (GST-P)-positive hepatic focus, the numbers and sizes of which are analyzed using an image-analyzer and expressed as values per unit liver section (1 cm2). When the yield of GST-P-positive foci is significantly enhanced (P<0.05) over the control value, a chemical is judged to possess carcinogenic or promotion potential for the liver. Among 313 chemicals already tested in this system in our laboratory, 30/31 (97%) mutagenic hepatocarcinogens and 29/33 (88%) non-mutagenic hepatocarcinogens gave positive results. Ten out of 43 (23%) agents known to be carcinogenic in organs other than the liver were also positive. It is particularly important that only one of 48 non-carcinogens gave a very weak positive result, so that the system has a very low false-positivity rate. It is now well documented that the assay system is highly effective for detecting hepatocarcinogens, bridging the gap between traditional long-term carcinogenicity tests and short-term screening assays. At the Fourth International Conference on Harmonization, our medium-term liver bioassay based on an initiation and promotion protocol was recommended in the guidelines as an acceptable alternative to the long-term rodent carcinogenicity test.

Cloning of the 5' upstream region of the rat p16 gene and its role in silencing

Hypermethylation of the 5' upstream region (5' region) of the human p16(CDKN2A) (p16) gene is known to cause silencing, which is involved in a wide range of human cancers. For the rat p16 gene, its 5' region has not been cloned, and it is uncertain whether surrogate use of exon 1 alpha is adequate for analysis of p16 silencing. In this study, we observed that methylation analysis of exon 1 alpha gave false positive results in three samples of normal rat mammary epithelia and in two of six primary mammary carcinomas. Therefore, we determined the nucleotide sequence of the 5' region of the rat p16 gene. To confirm that methylation status of the 5' region is correlated with p16 expression, the methylation status was analyzed by bisulfite sequencing and methylation-specific PCR in three samples of normal mammary glands, six samples of mammary carcinomas and four cell lines. The 5' region was demethylated in all of the three normal and six carcinoma samples that fully expressed p16. On the other hand, the 5' region was highly methylated in the 3Y1 cell line, which lacked p16 expression, but without deletion. These results showed that the methylation status of the 5' region was more closely correlated with p16 expression than that of the exon 1 alpha and analysis of the methylation status is useful in examining p16 silencing in various rat tumors.