Spontaneous neoplasm incidences in Fischer 344 rats and B6C3F1 mice in two-year carcinogenicity studies: a National Toxicology Program update

A competing risks machine learning study of neutron dose, fractionation, age, and sex effects on mortality in 21,000 mice

This study explores the impact of densely-ionizing radiation on non-cancer and cancer diseases, focusing on dose, fractionation, age, and sex effects. Using historical mortality data from approximately 21,000 mice exposed to fission neutrons, we employed random survival forest (RSF), a powerful machine learning algorithm accommodating nonlinear dependencies and interactions, treating cancer and non-cancer outcomes as competing risks. Unlike traditional parametric models, RSF avoids strict assumptions and captures complex data relationships through decision tree ensembles. SHAP (SHapley Additive exPlanations) values and variable importance scores were employed for interpretation. The findings revealed clear dose-response trends, with cancer being the predominant cause of mortality. SHAP value dose-response shapes differed, showing saturation for cancer hazard at high doses (> 2 Gy) and a more linear pattern at lower doses. Non-cancer responses remained more linear throughout the entire dose range. There was a potential inverse dose rate effect for cancer, while the evidence for non-cancer was less conclusive. Sex and age effects were less pronounced. This investigation, utilizing machine learning, enhances our understanding of the patterns of non-cancer and cancer mortality induced by densely-ionizing radiations, emphasizing the importance of such approaches in radiation research, including space travel and radioprotection.

Highly malignant endometrial stromal sarcoma in a cat

An 11-year-old female Persian cat underwent ovariohysterectomy due to dilation of the uterine cavity with irregular thickening of the wall. Macroscopically, the middle and distal regions of the left uterine horn were swollen and the uterine wall was irregularly thickened due to the development of multiple coalescent, variably sized nodules. Microscopically, the nodules had originated in the endometrium and were composed of round to polygonal neoplastic cells arranged in dense sheets or ill-defined fascicles. The neoplastic cells had locally invaded the myometrium and reached the subserosa, with lymphovascular invasion. Immunohistochemically, the neoplastic cell population was partially positive for CD10, an established marker of endometrial stromal sarcoma (ESS) in humans, with focal and diffuse nuclear immunopositivity for oestrogen and progesterone receptors and immunonegativity for desmin and α-smooth muscle actin. Based on these findings, the uterine tumour was diagnosed as ESS and was considered to correspond morphologically to high-grade ESS in humans.

[The recombinant human parathyroid hormone, teriparatide as an alternative remedy for the medication-related osteonecrosis of the jaw]

In developed countries, osteoporosis is one of the most common debilitating conditions in the population over the age of 50. Unfortunately, the pathomechanism of the disease is still not fully understood. Nowadays, the administration of antiresorptive drugs blocking osteoclastic activity is the most commonly used medication to slow down the speed of the bone loss. One of the uncommon side effects of such drugs is the medication-related osteonecrosis of the jaw (MRONJ). Recently, a number of alternative therapeutic approaches has been tested and published, amongst them the recombinant human parathyroid hormone (rhPTH, teriparatide) use, which is turning into a promising treatment modality. According to certain meta-analyses, its pharmacological effect on increasing bone mineral density and controlling pathological vertebral fractures is superior to antiresorptive drugs; however, the so-called "off-label" application of teriparatide remains controversial. As intermittent administration of teriparatide stimulates bone formation, several animal and clinical studies indicated that systemic application of teriparatide shortened fracture healing time and improved quality of the callus and the newly formed bone. Furthermore, recently several clinical studies showed the beneficial effect of the intermittent rhPTH administration in the management of MRONJ. This article reviews the history of the anabolic effect of the low-dose rhPTH discovery, provides evidence-based data from animal and human studies, summarizes its biological mechanisms and the clinical benefits of the anabolic therapy and also their possible role in the management of MRONJ. The majority of the clinical data indicates that, in the case of therapy-resistant osteonecrosis, it may be worthwhile to apply short-term intermittent teriparatide therapy. Notwithstanding, more randomized clinical trials are necessary in order to confirm the efficacy and the safety of the use of teriparatide in the treatment of MRONJ. Orv Hetil. 2023; 164(36): 1406-1415.

Sex-specific respiratory and systemic endocrine effects of acute acrolein and trichloroethylene inhalation

Acrolein and trichloroethylene (TCE) are priority hazardous air pollutants due to environmental prevalence and adverse health effects; however, neuroendocrine stress-related systemic effects are not characterized. Comparing acrolein, an airway irritant, and TCE with low irritancy, we hypothesized that airway injury would be linked to neuroendocrine-mediated systemic alterations. Male and female Wistar-Kyoto rats were exposed nose-only to air, acrolein or TCE in incremental concentrations over 30 min, followed by 3.5-hr exposure to the highest concentration (acrolein - 0.0, 0.1, 0.316, 1, 3.16 ppm; TCE - 0.0, 3.16, 10, 31.6, 100 ppm). Real-time head-out plethysmography revealed acrolein decreased minute volume and increased inspiratory-time (males>females), while TCE reduced tidal-volume. Acrolein, but not TCE, inhalation increased nasal-lavage-fluid protein, lactate-dehydrogenase activity, and inflammatory cell influx (males>females). Neither acrolein nor TCE increased bronchoalveolar-lavage-fluid injury markers, although macrophages and neutrophils increased in acrolein-exposed males and females. Systemic neuroendocrine stress response assessment indicated acrolein, but not TCE, increased circulating adrenocorticotrophic hormone, and consequently corticosterone, and caused lymphopenia, but only in males. Acrolein also reduced circulating thyroid-stimulating hormone, prolactin, and testosterone in males. In conclusion, acute acrolein inhalation resulted in sex-specific upper respiratory irritation/inflammation and systemic neuroendocrine alterations linked to hypothalamic-pituitary-adrenal axes activation, which is critical in mediating extra-respiratory effects.

Experimental mouse models for translational human cancer research

The development and growth of tumors remains an important and ongoing threat to human life around the world. While advanced therapeutic strategies such as immune checkpoint therapy and CAR-T have achieved astonishing progress in the treatment of both solid and hematological malignancies, the malignant initiation and progression of cancer remains a controversial issue, and further research is urgently required. The experimental animal model not only has great advantages in simulating the occurrence, development, and malignant transformation mechanisms of tumors, but also can be used to evaluate the therapeutic effects of a diverse array of clinical interventions, gradually becoming an indispensable method for cancer research. In this paper, we have reviewed recent research progress in relation to mouse and rat models, focusing on spontaneous, induced, transgenic, and transplantable tumor models, to help guide the future study of malignant mechanisms and tumor prevention.

Gastrointestinal Mast Cell Tumor in an African Dormouse (Graphiurus sp.)

Mast cell tumors (MCTs) are well-known neoplasms derived from either mucosal or connective tissue mast cells. While well studied in several domestic species, MCTs are rarely documented in rodents. A three-year-old, male African dormouse (Graphiurus sp.) presented with a history of vomiting and anorexia for 3 months. Sonography revealed thickened gastric mucosa and hyperperistalsis. The patient died after receiving symptomatic treatment for 2 months. At necropsy, locally extensive, pale, thickened mucosal foci obscuring the first half of the stomach lumen was noted. Histological examination revealed moderately polymorphic, round, oval to spindle cells with amphophilic cytoplasmic granules infiltrating the mucosa to tunica muscularis, with moderate numbers of eosinophils. The mucosa was severely ulcerated with the proliferation of granulation tissue. The granules in most tumor cells exhibited metachromasia with the toluidine blue stain. Neoplastic cells revealed positive membranous immunoreactivity to KIT. Herein, we report the first case report of MCT in dormouse but also the first gastrointestinal MCT in a rodent species.

Acrylonitrile induction of rodent neoplasia: Potential mechanism of action and relevance to humans

Acrylonitrile, an industrial chemical, is a multisite carcinogen in rats and mice, producing tumors in four tissues with barrier function, that is, brain, forestomach, Zymbal’s gland, and Harderian gland. To assess mechanism(s) of action (MoA) for induction of neoplasia and to evaluate whether the findings in rodents are indicative of human hazard, data on the potential key effects produced by acrylonitrile in the four rodent target tissues of carcinogenicity were evaluated. A notable finding was depletion of glutathione in various organs, including two target tissues, the brain, and forestomach, suggesting that this effect could be a critical initiating event. An additional combination of oxidative DNA damage and cytotoxic effects of acrylonitrile and its metabolites, cyanide, and 2-cyanoethylene oxide, could initiate pro-inflammatory signaling and sustained cell and tissue injury, leading to compensatory cell proliferation and neoplastic development. The in vivo DNA-binding and genotoxicity of acrylonitrile has been studied in several target tissues with no compelling positive results. Thus, while some mutagenic effects were reported in acrylonitrile-exposed rodents, data to determine whether this mutagenicity stems from direct DNA reactivity of acrylonitrile are insufficient. Accordingly, the induction of tumors in rodents is consistent primarily with a non-genotoxic MoA, although a contribution from weak mutagenicity cannot be ruled out. Mechanistic data to support conclusions regarding human hazard from acrylonitrile exposure is weak. Comparison of metabolism of acrylonitrile between rodents and humans provide little support for human hazard. Three of the tissues affected in bioassays (forestomach, Zymbal’s gland, and Harderian gland) are present only in rodents, while the brain is anatomically different between rodents and humans, diminishing relevance of tumor induction in these tissues to human hazard. Extensive epidemiological data has not revealed causation of human cancer by acrylonitrile.

Research-Relevant Conditions and Pathology of Laboratory Mice, Rats, Gerbils, Guinea Pigs, Hamsters, Naked Mole Rats, and Rabbits

Animals are valuable resources in biomedical research in investigations of biological processes, disease pathogenesis, therapeutic interventions, safety, toxicity, and carcinogenicity. Interpretation of data from animals requires knowledge not only of the processes or diseases (pathophysiology) under study but also recognition of spontaneous conditions and background lesions (pathology) that can influence or confound the study results. Species, strain/stock, sex, age, anatomy, physiology, spontaneous diseases (noninfectious and infectious), and neoplasia impact experimental results and interpretation as well as animal welfare. This review and the references selected aim to provide a pathology resource for researchers, pathologists, and veterinary personnel who strive to achieve research rigor and validity and must understand the spectrum of "normal" and expected conditions to accurately identify research-relevant experimental phenotypes as well as unusual illness, pathology, or other conditions that can compromise studies involving laboratory mice, rats, gerbils, guinea pigs, hamsters, naked mole rats, and rabbits.

Spontaneous hemangiosarcoma in the spleen and liver of a young rat

Spontaneous hemangiosarcoma in young rats is rare. In this report, we describe a case of a spontaneous hemangiosarcoma in the spleen and liver of young rats. At necropsy, multiple pale red masses were observed in the spleen. Histopathologically, solid growth and haphazardly arranged neoplastic cells were observed, although no characteristic growth pattern was observed. In contrast, irregularly sized small slit-shaped spaces containing erythrocytes were found among the neoplastic cells. Reticular fibers incompletely surrounding the neoplastic cells were observed by silver staining. Immunohistochemistry revealed that the neoplastic cells were positive for vWF and CD34. Electron microscopic examination revealed that the neoplastic cells had erythrocytes in the lumen and Weibel-Palade bodies in the cytoplasm and were arranged along a discontinuous basal lamina. These features indicate that the tumor originated from vascular endothelial cells. Based on these results, the tumor was diagnosed as a hemangiosarcoma in the spleen and liver.

Spontaneous Primary Pleural Mesothelioma in Fischer 344 (F344) and Other Rat Strains: A Retrospective Review

Spontaneous primary pleural mesotheliomas in Fischer 344 (F344) or other rat strains have rarely been reported. The objectives of this retrospective study were to develop historical incidence data and better characterize the light-microscopic morphology of these naturally occurring neoplasms in a large cohort of rats of several strains. A retrospective review was performed of National Toxicology Program (NTP) studies in rats conducted between 1980 and 2019 and comprising a total of 104,029 rats (51,326 males, 52,703 females), predominantly (90%) of the F344 strain. Of the 94,062 F344 rats surveyed, there were 30 cases of primary pleural mesotheliomas (22 males, 8 females). Of the 2998 Wistar Han rats surveyed, primary pleural mesotheliomas were present in 2 male rats. No primary pleural mesotheliomas were noted in male and female rats of other strains (6669 Sprague Dawley; 300 Osborne-Mendel). All primary pleural mesotheliomas in control and treated F344 and Wistar Han rats were considered spontaneous and unrelated to treatment. Based on light-microscopic evaluation of paraffin-embedded hematoxylin and eosin stained sections, only epithelioid and biphasic histologic subtypes were observed: 18 and 12 in F344 rats, respectively, and one each in Wistar Han rats. No sarcomatoid subtype cases were noted in any strain of rat.

Spontaneous adenocarcinoma with giant cell formation in the accessory sex glands in a male Sprague-Dawley rat

In this study, we report the features of an adenocarcinoma with giant cell formation spontaneously occurring in the accessory sex glands of a male 10-month-old Sprague-Dawley rat. A milky white mass was found in the region corresponding to the left seminal vesicle and the left coagulating gland. Histologically, tumor cells exhibited diverse growth patterns, including glandular/trabecular, cystic, and sheet-like growth areas. The tumor cells were pleomorphic, with round- or oval-shaped nuclei and abundant eosinophilic cytoplasm. Mitotic figures were occasionally observed. Giant cells were also prominent in the sheet-like growth area, with intracytoplasmic vacuoles containing eosinophilic material. The stroma was rich in collagen fibers and fibroblasts. Numerous inflammatory cells were observed in the glandular and cystic lumina and stroma. Immunohistochemically, the tumor cells were positive for cytokeratin AE1/AE3 and proliferating cell nuclear antigen. In the sheet-like growth area, some of the tumor cells and giant cells were positive for vimentin in the cytoplasm adjacent to the nucleus. Electron microscopy revealed that the tumor cells contained a small number of mitochondria and rough endoplasmic reticulum, and had no basement membrane or desmosome. The giant cells occasionally contained variably sized intracytoplasmic lumina and globular filamentous bodies, probably corresponding to vimentin. Considering these morphological features, the tumor was diagnosed as an adenocarcinoma with the formation of giant tumor cells originating from the male accessory sex glands.

Application of an in Vitro Assay to Identify Chemicals That Increase Estradiol and Progesterone Synthesis and Are Potential Breast Cancer Risk Factors

BACKGROUND

Established breast cancer risk factors, such as hormone replacement therapy and reproductive history, are thought to act by increasing estrogen and progesterone (P4) activity.

OBJECTIVE

We aimed to use in vitro screening data to identify chemicals that increase the synthesis of estradiol (E2) or P4 and evaluate potential risks.

METHOD

Using data from a high-throughput (HT) in vitro steroidogenesis assay developed for the U.S. Environmental Protection Agency (EPA) ToxCast program, we identified chemicals that increased estradiol (E2-up) or progesterone (P4-up) in human H295R adrenocortical carcinoma cells. We prioritized chemicals by their activity. We compiled in vivo studies and assessments about carcinogenicity and reproductive/developmental (repro/dev) toxicity. We identified exposure sources and predicted intakes from the U.S. EPA's ExpoCast.

RESULTS

We found 296 chemicals increased E2 (182) or P4 (185), with 71 chemicals increasing both. In vivo data often showed effects consistent with this mechanism. Of the E2- and P4-up chemicals, about 30% were likely repro/dev toxicants or carcinogens, whereas only 5-13% were classified as unlikely. However, most of the chemicals had insufficient in vivo data to evaluate their effects. Of 45 chemicals associated with mammary gland effects, and also tested in the H294R assay, 29 increased E2 or P4, including the well-known mammary carcinogen 7,12-dimethylbenz(a)anthracene. E2- and P4-up chemicals include pesticides, consumer product ingredients, food additives, and drinking water contaminants.

DISCUSSION

The U.S. EPA's in vitro screening data identified several hundred chemicals that should be considered as potential risk factors for breast cancer because they increased E2 or P4 synthesis. In vitro data is a helpful addition to current toxicity assessments, which are not sensitive to mammary gland effects. Relevant effects on the mammary gland are often not noticed or are dismissed, including for 2,4-dichlorophenol and cyfluthrin. Fifty-three active E2-up and 59 active P4-up chemicals that are in consumer products, food, pesticides, or drugs have not been evaluated for carcinogenic potential and are priorities for study and exposure reduction. https://doi.org/10.1289/EHP8608.

Ethyl acrylate (EA) exposure and thyroid carcinogenicity in rats and mice with relevance to human health

Ethyl acrylate (EA) was classified by IARC as a Group-2B Carcinogen based, in part, on data suggesting increased incidence of thyroid neoplasia in rats and mice exposed chronically to EA vapors. We examined chronic exposure of rats and mice to EA vapors, evaluated the data on the incidence of thyroid follicular neoplasia, and determined the relevance of thyroid tumors to human health risk. The data revealed a small statistically significant increase in thyroid tumors in EA-exposed male rats and mice. The tumor incidences were within the range of historical controls and were not consistently dose-dependent. Most thyroid tumors in exposed animals were benign. Chronic exposure of EA to rats and mice (drinking water or gavage) and dogs (capsules) had no evidence of thyroid neoplasia. Results from chronic studies, in vivo and in vitro data, and ToxCastTM/Tox 21 HTPS did not support genotoxic/mutagenic potential for EA. This suggests that the associations between EA exposure and thyroid neoplasia represent chance or random observations rather than a compound-mediated effect. Due to species-specific physiological differences, the hypothalamic-pituitary-thyroid axis of rodents is more sensitive to endocrine disruptive chemicals than that of humans which further suggests that findings in rodents have questionable relevance to human health.

Spontaneous granulocytic leukemia in a NOD/Shi-scid IL-2Rγnull mouse

Here, we report a case of spontaneous granulocytic leukemia in a 51-week-old male NOD/Shi-scid IL-2Rγ ^null (NOG) mouse. The mouse showed progressive anemia and rough respiratory movement. Macroscopically, the spleen was discolored and enlarged. Histologically, the bone marrow of the sternum and femur was highly cellular and almost exclusively filled with neoplastic cells. The nuclei of neoplastic cells were large, oval to slightly irregular in shape, and a small number of cells had kidney- or ring-shaped nuclei. Neoplastic cells extensively infiltrated the organs, and the spleen and liver were prominently involved. Immunohistochemically, a large population of neoplastic cells in the red pulp of the spleen and sinusoid of the liver was positive for myeloperoxidase. Based on the histological features, this case was diagnosed with granulocytic leukemia. This novel information on spontaneous tumors may be helpful for the appropriate use of this mouse strain in further research.

Hazard characterization of carcinogenicity, mutagenicity, and reproductive toxicity for short chain primary nitroalkanes

Nitroalkanes are organic aliphatic hydrocarbon compounds with a nitro moiety that are commonly used as solvents or intermediates to synthesize a variety of organic compounds due to their inherent reactivity. In June 2020, a harmonized classification and labeling (CLH) proposal was submitted to the European Chemicals Agency (ECHA) for the following harmonized carcinogenicity, mutagenicity, and reproductive toxicity ("CMR") classifications for nitromethane (NM), nitroethane (NE), and 1-nitropropane (1-NP): NM Carc. 1B and Repr. 1B; NE Repr. 1B; and 1-NP Repr. 2. In this assessment, a weight of evidence (WoE) evaluation of studies on animal carcinogenicity and reproductive and developmental toxicity, genotoxicity, and mode of action for these three nitroalkanes was performed to critically assess the relevance of the proposed CMR classifications. Overall, the WoE indicates that NM, NE, and 1-NP are not carcinogenic, genotoxic, nor selective reproductive or developmental toxicants. Based on our analysis, classifying NM, NE, and 1-NP as Category 2 reproductive toxicants is most appropriate. Furthermore, not classifying NE and 1-NP with respect to their carcinogenicity is appropriate based on the available studies for this endpoint coupled with negative results in genotoxicity studies, metabolism data, and in silico predictions. We determined that the classification for NM of Carc. 1B is not appropriate, based on the fact that rat mammary and harderian tumors are likely not relevant to humans and lung and liver tumors reported in mice were equivocal in their dose-response and statistical significance.

Comprehensive analysis of chronic rodent inhalation toxicity studies for methyl acrylate with attention to test conditions exceeding a maximum tolerated concentration

MA is a chemical intermediate, manufactured and processed within closed systems. While so far available subacute to chronic inhalation toxicity studies performed in compliance with OECD TG principles gave no indication of any carcinogenic potential for MA, a recent 2-year inhalation study with F344/DuCrlCrlj rats published in 2017 by the JBRC showed a statistically significant increase of squamous cell carcinoma in the nasal cavity of male rats at the highest tested concentration of 160 ppm. However, the results of the different studies in total indicate that this high concentration exceeded the MTC. As MA has a low potential for genotoxic and mutagenic activity, the increased tumour incidence can be attributed to a non-genotoxic mechanism, namely to a strong inflammatory response observed in this study. Together with mechanistic and epidemiologic data for other compounds related to nasal carcinogenesis via this mode of action, it can be concluded that the relevance of this increased tumour incidence in male rats for humans is questionable. Also, a long-term exposure to higher concentrations of MA is highly unlikely to be reached in the environment or at workplaces. Therefore, a risk for humans including cancer hazard is considered implausible.

Weight of evidence analysis of the tumorigenic potential of 1,3-dichloropropene supports a threshold-based risk assessment

1,3-Dichloropropene (1,3-D; CAS #542-75-6) is a fumigant used for preplant treatment of soil to control parasitic nematodes and manage soil borne diseases for numerous fruit, vegetable, field and tree and vine crops across diverse global agricultural areas. In the USA, 1,3-D has historically been classified by the U.S. EPA as likely to be carcinogenic to humans via both oral and inhalation routes. This classification for the oral route was primarily based upon increases in multiple tumor types observed in National Toxicology Program (NTP) cancer bioassays in rats and mice, while the classification for the inhalation route was based upon increased benign bronchioloalveolar adenomas in a mouse study conducted by The Dow Chemical Company. Based on U.S. EPA standard risk assessment methodologies, a low-dose linear extrapolation approach has been used to estimate risks to humans. Furthermore, genotoxicity associated with 1,3-D was historically considered a potential mode of action (MOA) for its tumorigenicity. New information is available and additional studies have been conducted that reveal a different picture of the tumorigenic potential of 1,3-D. These data and information include: (1) initial cancer studies by the NTP were conducted on an antiquated form of 1,3-D which contained a known mutagen/carcinogen, epichlorohydrin, as a stabilizer while current 1,3-D fumigants use epoxidized soybean oil (ESO) as the stabilizer; (2) results from two additional oral rodent cancer bioassays conducted on the modern form of 1,3-D became available and these two studies reveal a lack of carcinogenicity; (3) a newly conducted Big Blue study in F344 rats via the oral route further confirms that 1,3-D is not an in vivo genotoxicant; and (4) a newly conducted repeat dose inhalation toxicokinetic (TK) study shows that linear dose proportionality is observed below 30 ppm, which demonstrates the non-relevance of 60 ppm 1,3-D-induced benign lung tumors in mice for human health assessment. This weight of evidence review is organized as follows: (a) the TK of 1,3-D are presented because of relevant considerations when evaluating test doses/concentrations and reported findings of tumorigenicity; (b) the genotoxicity profile of 1,3-D is presented, including a contemporary study in order to put a possible genotoxicity MOA into perspective; (c) the six available bioassays are reviewed followed by (d) scientifically supported points of departure (PODs) and evaluation of human exposure for use in risk assessment. Through this assessment, all available data support the conclusion that 1,3-D is not a tumorigen at doses below 12.5 mg/kg bw/day via the oral route or at doses below 30 ppm via the inhalation route. These findings and clearly identified PODs show that a linear low dose extrapolation approach is not appropriate and a threshold-based risk assessment for 1,3-D is human health protective. Finally, in 2019, the Cancer Assessment Review Committee (CARC) reevaluated the carcinogenic potential of 1,3-D. In accordance with the EPA's Final Guidelines for Carcinogen Risk Assessment, the CARC classified 1,3-D (Telone) as "Suggestive Evidence of Carcinogenic Potential based on the presence of liver tumors by the oral route in male rats only." Given this finding, EPA stated that "quantification of human cancer risk is not required. The CARC recommends using a non-linear approach (i.e. reference dose (RfD)) that will adequately account for all chronic toxicity including carcinogenicity, that could result from exposure to 1,3-dichloropropene."

Background data of 2-year-old male and female F344 gpt delta rats

Although gpt delta rats, as reporter gene-transgenic rats, were originally developed for in vivo mutation assays, they have also been used to evaluate chemical carcinogenesis and comprehensive toxicity. Therefore, it is necessary to accumulate background data on carcinogenicity and general toxicity in gpt delta rats. Here, we investigated the background data of 110-week-old male and female F344 gpt delta rats and wild-type rats. There was no effect of reporter gene transfection on animal survival rates and body weights during the experiment. The relative weight of male gpt delta rat adrenals was significantly higher than that of wild-type rats, possibly due to the higher incidence of pheochromocytoma. There were no intergenotype differences in the incidence of nonneoplastic lesions in both sexes, including chronic progressive nephropathy and focus of cellular alteration in the liver, which had a higher incidence in both genotypes. Additionally, the significantly higher incidence of adrenal pheochromocytoma in male gpt delta rats than that in wild-type rats was likely incidental because of the lack of differences in the incidences of preneoplastic (male and female) and neoplastic (female) adrenal lesions in both genotypes. Other neoplastic lesions in both sexes showed no intergenotype differences in incidence rates, although large granular lymphocytic leukemia in the spleen and Leydig cell tumors in the testes of males showed higher incidence rates. Overall, there were no effects of reporter gene transfection on the spectrum of spontaneous lesions in F344 gpt delta rats, thus supporting their applicability in evaluating chemical toxicity and carcinogenicity.

Bring Back the Rat!

As 2020 is "The Year of the Rat" in the Chinese astrological calendar, it seems an appropriate time to consider whether we should bring back the laboratory rat to front-and-center in research on the basic biology of mammalian aging. Beginning in the 1970s, aging research with rats became common, peaking in 1992 but then declined dramatically by 2018 as the mouse became preeminent. The purpose of this review is to highlight some of the historical contributions as well as current advantages of the rat as a mammalian model of human aging, because we suspect at least a generation of researchers is no longer aware of this history or these advantages. Herein, we compare and contrast the mouse and rat in the context of several biological domains relevant to their use as appropriate models of aging: phylogeny/domestication, longevity interventions, pathology/physiology, and behavior/cognition. It is not the goal of this review to give a complete characterization of the differences between mice and rats, but to provide important examples of why using rats as well as mice is important to advance our understanding of the biology of aging.

A comprehensive view on mechanistic approaches for cancer risk assessment of non-genotoxic agrochemicals

Currently the only methods for non-genotoxic carcinogenic hazard assessment accepted by most regulatory authorities are lifetime carcinogenicity studies. However, these involve the use of large numbers of animals and the relevance of their predictive power and results has been scientifically challenged. With increased availability of innovative test methods and enhanced understanding of carcinogenic processes, it is believed that tumour formation can now be better predicted using mechanistic information. A workshop organised by the European Partnership on Alternative Approaches to Animal Testing brought together experts to discuss an alternative, mechanism-based approach for cancer risk assessment of agrochemicals. Data from a toolbox of test methods for detecting modes of action (MOAs) underlying non-genotoxic carcinogenicity are combined with information from subchronic toxicity studies in a weight-of-evidence approach to identify carcinogenic potential of a test substance. The workshop included interactive sessions to discuss the approach using case studies. These showed that fine-tuning is needed, to build confidence in the proposed approach, to ensure scientific correctness, and to address different regulatory needs. This novel approach was considered realistic, and its regulatory acceptance and implementation can be facilitated in the coming years through continued dialogue between all stakeholders and building confidence in alternative approaches.

Quality assessment tests for tumorigenicity of human iPS cell-derived cartilage

Articular cartilage damage does not heal spontaneously and causes joint dysfunction. The implantation of induced pluripotent stem cell (iPSC)-derived cartilage (iPS-Cart) is one candidate treatment to regenerate the damaged cartilage. However, concerns of tumorigenicity are associated with iPS-Cart, because the iPSC reprogramming process and long culture time for cartilage induction could increase the chance of malignancy. We evaluated the tumorigenic risks of iPS-Cart using HeLa cells as the reference. Spike tests revealed that contamination with 100 HeLa cells in 150 mg of iPS-Cart accelerated the cell growth rate. On the other hand, 150 mg of iPS-Cart without HeLa cells reached growth arrest and senescence after culture, suggesting less than 100 tumorigenic cells, assuming they behave like HeLa cells, contaminated iPS-Cart. The implantation of 10,000 or fewer HeLa cells into joint surface defects in the knee joint of nude rat did not cause tumor formation. These in vitro and in vivo studies collectively suggest that the implantation of 15 g or less iPS-Cart in the knee joint does not risk tumor formation if assuming that the tumorigenic cells in iPS-Cart are equivalent to HeLa cells and that nude rat knee joints are comparable to human knee joints in terms of tumorigenicity. However, considering the limited immunodeficiency of nude rats, the clinical amount of iPS-Cart for implantation needs to be determined cautiously.

Angiomatous hyperplasia in the heart of a young rat

This case report describes angiomatous hyperplasia in the heart which is an unusual location in a young male Sprague-Dawley rat in a short-term toxicity study. Histologically, the lesion was characterized by blood-filled vascular channels of variable diameter lined by a thin wall and surrounded by a thin fibrous stroma and minimal lympho-plasmacytic and neutrophilic infiltrate in the apex of the heart. Immunohistopathology using CD31 confirmed the blood vessel origin, and using Ki67 confirmed low cell-proliferative activity in the vascular endothelial cells. To the authors’ knowledge, this is the first report of spontaneous angiomatous hyperplasia in the heart of a young rat.

Spontaneous abdominal hemangiosarcoma in a ferret

Herein, we describe the case of a 6-year-old female ferret that died within a few days of the onset of anorexia and reduced spontaneous locomotor activity. Necropsy revealed a dark red abdominal mass of unknown origin between the right lobes of the pancreas and the proximal jejunum, with massive blood retention in the peritoneal cavity. Histopathologically, spindle-shaped or sometimes polygonal tumor cells were proliferating with irregularly shaped vascular spaces containing blood components and surrounding-tissue infiltration. In some areas, tumor cells formed distinctly dilated blood vessel-like structures. Immunohistochemically, most of the tumor cells were strongly positive for CD31, but factor VIII-related antigen immunoreactivity was confined to the area with dilated blood vessel-like structures. Based on these findings, the tumor was diagnosed as an abdominal hemangiosarcoma. Abdominal hemangiosarcoma excluding cases of the liver and spleen are rare in ferrets.

FEMA GRAS assessment of natural flavor complexes: Mint, buchu, dill and caraway derived flavoring ingredients

In 2015, the Expert Panel of the Flavor and Extract Manufacturers Association (FEMA) initiated a re-evaluation of the safety of over 250 natural flavor complexes (NFCs) used as flavor ingredients. NFC flavor materials include a variety of essential oils and botanical extracts. The re-evaluation of NFCs is conducted based on a constituent-based procedure outlined in 2005 and updated in 2018 that evaluates the safety of NFCs for their intended use as flavor ingredients. This procedure is applied in the re-evaluation of the generally recognized as safe (GRAS) status of NFCs with constituent profiles that are dominated by alicyclic ketones such as menthone and carvone, secondary alcohols such as menthol and carveol, and related compounds. The FEMA Expert Panel affirmed the GRAS status of Peppermint Oil (FEMA 2848), Spearmint Oil (FEMA 3032), Spearmint Extract (FEMA 3031), Cornmint Oil (FEMA 4219), Erospicata Oil (FEMA 4777), Curly Mint Oil (FEMA 4778), Pennyroyal Oil (FEMA 2839), Buchu Leaves Oil (FEMA 2169), Caraway Oil (FEMA 2238) and Dill Oil (FEMA 2383) and determined FEMA GRAS status for Buchu Leaves Extract (FEMA 4923), Peppermint Oil, Terpeneless (FEMA 4924) and Spearmint Oil, Terpeneless (FEMA 4925).

Comparative Incidences and Biological Outcomes for Thymoma in Various Rat Strains in National Toxicology Program Studies

Thymomas from 277 Fischer 344/N (F344/N), 10 Sprague Dawley (HSD:Sprague Dawley SD) (SD), 129 Wistar Han [Crl:WI(Han)] (WH), and 4 Wistar Outbred (WO) rats were reviewed from long-term studies in the National Toxicology Program (NTP) database. The incidence of thymomas in F344/N rats was slightly higher in males than in females, while the incidences in SD and WH rats were higher in females than in males. Only male WO rats were used in NTP studies. Of the 277 thymomas in F344/N rats, 235 (84.8%) were benign and 42 (15.2%) malignant, 14 of which exhibited metastasis. Of the 10 thymomas in SD rats, 5 (50%) were benign and 5 (50%) were malignant, one of which exhibited metastasis. Of the 129 thymomas in WH rats, 126 (98%) were benign and 3 (2%) were malignant, 1 with metastasis. Of the 4 thymomas in WO rats, 3 (75%) were benign and 1 (25%) was malignant, with no metastases. Malignant thymomas in F344/N and WH rats showed a propensity to be the cause of death and to result in early mortality, whereas the benign thymomas were associated less often with decreased survival. No occurrences of this neoplasm were reported to be related to exposure to any test articles.

Classification or non-classification of substances with positive tumor findings in animal studies: Guidance by the German MAK commission

One of the important tasks of the German Senate Commission for the Investigation of Health Hazards of Chemical Compounds in the Work Area (known as the MAK Commission) is in the evaluation of a potential for carcinogenicity of hazardous substances at the workplace. Often, this evaluation is critically based on data on carcinogenic responses seen in animal studies and, if positive tumor responses have been observed, this will mostly lead to a classification of the substance under investigation into one of the classes for carcinogens. However, there are cases where it can be demonstrated with a very high degree of confidence that the tumor findings in the experimental animals are not relevant for humans at the workplace and, therefore, the MAK Commission will not classify the respective substance into one of the classes for carcinogens. This paper will summarize the general criteria used by the MAK Commission for the categorization into "carcinogen" and "non-carcinogen" and compare this procedure with those used by other national and international organizations.

Evaluation of potential health effects associated with occupational and environmental exposure to styrene - an update

The potential chronic health risks of occupational and environmental exposure to styrene were evaluated to update health hazard and exposure information developed since the Harvard Center for Risk Analysis risk assessment for styrene was performed in 2002. The updated hazard assessment of styrene's health effects indicates human cancers and ototoxicity remain potential concerns. However, mechanistic research on mouse lung tumors demonstrates these tumors are mouse-specific and of low relevance to human cancer risk. The updated toxicity database supports toxicity reference levels of 20 ppm (equates to 400 mg urinary metabolites mandelic acid + phenylglyoxylic acid/g creatinine) for worker inhalation exposure and 3.7 ppm and 2.5 mg/kg bw/day, respectively, for general population inhalation and oral exposure. No cancer risk value estimates are proposed given the established lack of relevance of mouse lung tumors and inconsistent epidemiology evidence. The updated exposure assessment supports inhalation and ingestion routes as important. The updated risk assessment found estimated risks within acceptable ranges for all age groups of the general population and workers with occupational exposures in non-fiber-reinforced polymer composites industries and fiber-reinforced polymer composites (FRP) workers using closed-mold operations or open-mold operations with respiratory protection. Only FRP workers using open-mold operations not using respiratory protection have risk exceedances for styrene and should be considered for risk management measures. In addition, given the reported interaction of styrene exposure with noise, noise reduction to sustain levels below 85 dB(A) needs be in place.

Occupational-like organophosphate exposure disrupts microglia and accelerates deficits in a rat model of Alzheimer's disease

Occupational exposure to organophosphate pesticides, such as chlorpyrifos (CPF), increases the risk of Alzheimer's disease (AD), though the mechanism is unclear. To investigate this, we subjected 4-month-old male and female wild-type (WT) and TgF344-AD rats, a transgenic AD model, to an occupational CPF exposure paradigm that recapitulates biomarkers and behavioral impairments experienced by agricultural workers. Subsequent cognition and neuropathology were analyzed over the next 20 months. CPF exposure caused chronic microglial dysregulation and accelerated neurodegeneration in both males and females. The effect on neurodegeneration was more severe in males, and was also associated with accelerated cognitive impairment. Females did not exhibit accelerated cognitive impairment after CPF exposure, and amyloid deposition and tauopathy were unchanged in both males and females. Microglial dysregulation may mediate the increased risk of AD associated with occupational organophosphate exposure, and future therapies to preserve or restore normal microglia might help prevent AD in genetically vulnerable individuals exposed to CPF or other disease-accelerating environmental agents.

A model for RAS mutation patterns in cancers: finding the sweet spot

The three RAS genes - HRAS, NRAS and KRAS - are collectively mutated in one-third of human cancers, where they act as prototypic oncogenes. Interestingly, there are rather distinct patterns to RAS mutations; the isoform mutated as well as the position and type of substitution vary between different cancers. As RAS genes are among the earliest, if not the first, genes mutated in a variety of cancers, understanding how these mutation patterns arise could inform on not only how cancer begins but also the factors influencing this event, which has implications for cancer prevention. To this end, we suggest that there is a narrow window or 'sweet spot' by which oncogenic RAS signalling can promote tumour initiation in normal cells. As a consequence, RAS mutation patterns in each normal cell are a product of the specific RAS isoform mutated, as well as the position of the mutation and type of substitution to achieve an ideal level of signalling.

Comparative study for carcinogenicity of 7 different multi-wall carbon nanotubes with different physicochemical characteristics by a single intraperitoneal injection in male Fischer 344 rats

The present study comparatively examined carcinogenicity of 7 different multi-wall carbon nanotubes (MWCNTs) with different physicochemical characteristics. Physicochemical characteristics of MWCNTs (referred to as M-, N-, WL-, SD1-, WS-, SD2- and T-CNTs in the present study) were determined using scanning electron and light microscopes and a collision type inductively coupled plasma mass spectrometer. Male Fischer 344 rats (10 weeks old, 15 animals per group) were administered MWCNTs at a single intraperitoneal dose of 1 mg/kg body weight, and sacrificed up to 52 weeks after the commencement. Fibers of M-, N-, WL- and SD1-CNTs were straight and acicular in shape, and contained few agglomerates. They were relatively long (38-59% of fibers were longer than 5 μm) and thick (33% to more than 70% of fibers were thicker than 60 nm). All of these 4 MWCNTs induced mesotheliomas at absolute incidences of 100%. Fibers of WS-, SD2- and T-CNTs were curled and tightly tangled to form frequent agglomerates. They were relatively short and thin (more than 90% of measured fibers were thinner than 50 nm). WS- CNT did not induce mesothelioma, and only one of 15 rat given SD2- or T-CNT developed tumor. Any correlations existed between the metal content and neither the size or form of fibers, nor the carcinogenicity. It is thus indicated that the physicochemical characteristics of MWCNTs are critical for their carcinogenicity. The straight and acicular shape without frequent agglomerates, and the relatively long and thick size, but not the iron content, may be critical factors. The present data can contribute to the risk management, practical use and social acceptance of MWCNTs.

Verification of a false positive in a two-year rat carcinogenicity study using dual control groups

There is sometimes controversy over whether or not statistically significant responses produced in carcinogenicity studies have biologically significance. Ambiguous results from our previous two-year oral carcinogenicity study on acotiamide hydrochloride hydrate (acotiamide-HH), a prokinetic drug for functional dyspepsia, in rats made it unclear whether the drug may exhibit uterine carcinogenicity. To check this finding, we performed a second long-term carcinogenicity study using two identical control groups to more accurately evaluate uterine carcinogenesis by considering the incidence of spontaneous neoplasms. Female Fischer 344 rats were divided into three groups: the two control groups (control 1 and 2) were administered vehicle (0.5% w/v methylcellulose) and the acotiamide-HH-treated group was administered 2,000 mg/kg/day of acotiamide-HH by oral gavage for two years. Among all groups, the incidence of endometrial adenocarcinoma (EmA) was highest in the control 2 group, followed by the acotiamide-HH-treated group and the control 1 group. Moreover, acotiamide-HH did not affect the incidence of precursor lesions of EmA. In cases where an ambiguous difference is observed, the use of two control groups allows for a more informed interpretation of the findings in the drug-treated groups. The outcomes in this study strongly support the hypothesis that the increase in EmA in rats treated with acotiamide-HH in our previous study is unrelated to administration of the drug.

Differences in Types and Incidence of Neoplasms in Wistar Han and Sprague-Dawley Rats

A substantial quantity of data on Sprague-Dawley (SD) and Hannover Wistar rats strains have been published concerning their source, diet, and housing conditions, as well as the incidences of nonneoplastic lesions and neoplasms observed in different laboratories. Differences between the commonly used rat strains provided by different breeders (i.e., CD (SD) vs. Harlan Sprague-Dawley strain or Crl: WI(Han) vs. Wistar Hannover (Han)-derived strain, continued breeding by RCC Ltd., Switzerland, thereafter continued breeding by Harlan) may include, but are not limited to, body weight, incidence, and onset of major nonneoplastic lesions and neoplasms, and these can impact the development of a nonclinical safety program. Fisher 344 (F344) and SD rat strains generally have the highest tumor incidences, exceeding that in Wistar rats. Certain tumors are more commonly observed in one strain, and for some, the difference in incidence may be so significant that the tumor may even be considered characteristic for a specific strain (e.g., thymoma in Wistar and amphophilic renal adenoma in SD).

Safety of betaine as a novel food pursuant to Regulation (EC) No 258/97

Following a request from the European Commission, the EFSA Panel on Dietetic Products, Nutrition and Allergies (NDA) was asked to deliver an opinion on betaine as a novel food (NF) pursuant to Regulation (EC) No 258/97. The information provided on the composition, the specifications, the batch-to-batch variability, stability and production process of the NF is sufficient and does not raise concerns about the safety of the NF. The NF is proposed to be used in foods intended to meet additional requirements for intense muscular effort with a maximum intake of 2.5 g/day of betaine for sports people above 10 years of age. Based on the lowest BMDL₀₅, which was derived from a chronic toxicity study in rats in which a dose-related increase in platelet counts was observed, and the anticipated daily intake of the NF in the target population, the Margins of Exposure are 3.6 and 5, which are generally regarded as not sufficient. However, the total exposure to betaine from the diet (about 830 mg/day) is not known to be associated with adverse effects. Moreover, no adverse effects on platelet counts were noted in human intervention studies with exposure levels of 4 g/day of betaine for up to 6 months. A significant increase in total and low-density lipoprotein (LDL)-cholesterol concentrations was noted at intakes of 4 g/day of betaine in overweight subjects with metabolic syndrome but not in healthy subjects, nor at intakes of 3 g/day. Thus, considering 4 g/day of betaine as a reference point and applying an uncertainty factor of 10 to account for interindividual variability, an amount of 400 mg/day of betaine (i.e. 6 mg/kg body weight (bw) per day for adults) in addition to the background exposure is considered as safe. The Panel considers that the NF is safe to be used at maximum intake of 6 mg/kg bw per day in the target population.

Editor's Highlight: Complete Attenuation of Mouse Lung Cell Proliferation and Tumorigenicity in CYP2F2 Knockout and CYP2F1 Humanized Mice Exposed to Inhaled Styrene for up to 2 Years Supports a Lack of Human Relevance

Styrene is a mouse-specific lung carcinogen, and short-term mode of action studies have demonstrated that cytotoxicity and/or cell proliferation, and genomic changes are dependent on CYP2F2 metabolism. The current study examined histopathology, cell proliferation, and genomic changes in CD-1, C57BL/6 (WT), CYP2F2(-/-) (KO), and CYP2F2(-/-) (CYP2F1, 2B6, 2A13-transgene) (TG; humanized) mice following exposure for up to 104 weeks to 0- or 120-ppm styrene vapor. Five mice per treatment group were sacrificed at 1, 26, 52, and 78 weeks. Additional 50 mice per treatment group were followed until death or 104 weeks of exposure. Cytotoxicity was present in the terminal bronchioles of some CD-1 and WT mice exposed to styrene, but not in KO or TG mice. Hyperplasia in the terminal bronchioles was present in CD-1 and WT mice exposed to styrene, but not in KO or TG mice. Increased cell proliferation, measured by KI-67 staining, occurred in CD-1 and WT mice exposed to styrene for 1 week, but not after 26, 52, or 78 weeks, nor in KO or TG mice. Styrene increased the incidence of bronchioloalveolar adenomas and carcinomas in CD-1 mice. No increase in lung tumors was found in WT despite clear evidence of lung toxicity, or, KO or TG mice. The absence of preneoplastic lesions and tumorigenicity in KO and TG mice indicates that mouse-specific CYP2F2 metabolism is responsible for both the short-term and chronic toxicity and tumorigenicity of styrene, and activation of styrene by CYP2F2 is a rodent MOA that is neither quantitatively or qualitatively relevant to humans.

Evidence-Based Advances in Rodent Medicine

The number of exotic companion pet rodents seen in veterinary practices is growing very rapidly. According to the American Veterinary Medical Association's surveys, more than 2,093,000 pet rodents were kept in US households in 2007 and in 2012 it was more than 2,349,000 animals. This article summarizes the most important evidence-based knowledge in exotic pet rodents (diagnostics of the hyperadrenocorticism in guinea pigs, pituitary tumors in rats, urolithiasis in guinea pigs, use of itopride as prokinetics, use of deslorelin acetate in rodents, cause of dental disease, and prevention of mammary gland tumors in rats).

Re-evaluation of tara gum (E 417) as a food additive

The Panel on Food Additives and Nutrient Sources added to Food (ANS) provides a scientific opinion re-evaluating the safety of tara gum (E 417) as a food additive. Tara gum (E 417) has been evaluated by the EU Scientific Committee for Food (SCF, 1992) and by the Joint FAO/WHO Expert Committee on Food Additives (JECFA, 1987), who both allocated an acceptable daily intake (ADI) 'not specified' for this gum. Following the conceptual framework for the risk assessment of certain food additives, re-evaluated under Commission Regulation (EU) No 257/2010, the Panel considered that adequate exposure and toxicity data were available for tara gum (E 417). Tara gum (E 417) is unlikely to be absorbed intact and is expected to be fermented by intestinal microbiota. No adverse effects were reported at the highest doses tested in subchronic, chronic and carcinogenicity studies and there is no concern with respect to the genotoxicity. The Panel concluded that there is no need for a numerical ADI for tara gum (E 417) and that there is no safety concern for the general population at the refined exposure assessment of tara gum (E 417) as a food additive at the reported uses and use levels.

Biological relevance of effects following chronic administration of octamethylcyclotetrasiloxane (D4) in Fischer 344 rats

Octamethylcyclotetrasiloxane (D4) is a cyclic siloxane primarily used as a monomer or intermediate in the production of silicone polymers resulting in potential exposure of workers, and potential low level inhalation or dermal exposure for consumers and the general public. Following a two-year inhalation toxicity study with D4 in rats, increases in uterine endometrial cystic hyperplasia and adenomas were observed at the highest concentration of D4 administered (700ppm). No other neoplasms were increased with D4 treatment. In addition, chronic inhalation exposure of rats to D4 induced changes in relative liver and kidney weights, and produced a chronic nephropathy. This manuscript examines the biological relevance and possible modes of action for the effects observed in the F344 rat following chronic inhalation exposure to D4. D4 is not genotoxic and appears to exert its effects through a nongenotoxic mode of action. An alteration in the estrous cycle in the aging F344 rat was the most likely mode of action for the observed uterine effects following chronic inhalation exposure. Data support the conclusion that D4 acts indirectly via a dopamine-like mechanism leading to alteration of the pituitary control of the estrous cycle in aging F344 rats with a decrease in progesterone and an increase in the estrogen/progesterone ratio most likely induced by a decrease in prolactin concentration. D4 also inhibited the pre-ovulatory LH surge causing a delay in ovulation, persistent follicles and thus a prolonged exposure to elevated estrogen in the adult Sprague Dawely rat. A lengthening of the estrous cycle in the F344 rat with an increase in endogenous estrogen was also induced by D4 inhalation. Although the mode of action responsible for induction of uterine adenomas in the female F344 rat has not been clearly confirmed, the subtlety of effects on the effects of D4 on cyclicity may prevent further assessment and definition of the mode of action. The occurrence of uterine endometrial adenoma in the rat is not relevant for human risk characterization because (1) there are differences in ovulatory cycle regulation in rats compared to humans, (2) cystic hyperplasia without atypia in women is not a cancer precursor, and (3) there is no endometrial lesion in women that is directly analogous to endometrial adenoma in the rat. The effects of D4 on liver are due to a phenobarbital-like mechanism that results in induction of cytochrome P450 and other enzymes of xenobiotic biotransformation. The liver effects are adaptive and not adverse. Kidney findings included chonic progressive nephropathy, a rat lesion that has no counterpart in the human and that should not be used in human risk assessment.

Prediction of the Carcinogenic Potential of Human Pharmaceuticals Using Repeated Dose Toxicity Data and Their Pharmacological Properties

In an exercise designed to reduce animal use, we analyzed the results of rat subchronic toxicity studies from 289 pharmaceutical compounds with the aim to predict the tumor outcome of carcinogenicity studies in this species. The results were obtained from the assessment reports available at the Medicines Evaluation Board of the Netherlands for 289 pharmaceutical compounds that had been shown to be non-genotoxic. One hundred forty-three of the 239 compounds not inducing putative preneoplastic lesions in the subchronic study did not induce tumors in the carcinogenicity study [true negatives (TNs)], whereas 96 compounds were categorized as false negatives (FNs) because tumors were observed in the carcinogenicity study. Of the remaining 50 compounds, 31 showed preneoplastic lesions in the subchronic study and tumors in the carcinogenicity study [true positives (TPs)], and 19 only showed preneoplastic lesions in subchronic studies but no tumors in the carcinogenicity study [false positives (FPs)]. In addition, we then re-assessed the prediction of the tumor outcome by integrating the pharmacological properties of these compounds. These pharmacological properties were evaluated with respect to the presence or absence of a direct or indirect proliferative action. We found support for the absence of cellular proliferation for 204 compounds (TN). For 67 compounds, the presence of cellular hyperplasia as evidence for proliferative action could be found (TP). Therefore, this approach resulted in an ability to predict non-carcinogens at a success rate of 92% and the ability to detect carcinogens at 98%. The combined evaluation of pharmacological and histopathological endpoints eventually led to only 18 unknown outcomes (17 categorized as FN and 1 as FP), thereby enhancing both the negative and positive predictivity of an evaluation based upon histopathological evaluation only. The data show the added value of a consideration of the pharmacological properties of compounds in relation to potential class effects, both in the negative and positive direction. A high negative and a high positive predictivity will both result in waiving the need for conducting 2-year rat carcinogenicity studies, if this is accepted by Regulatory Authorities, which will save large numbers of animals and reduce drug development costs and time.

Malignant mast cell tumor of the thymus in an Royal College of Surgeons (RCS) rat

A 152-week-old male Royal College of Surgeons (RCS) rat kept as a non-treated animal in a long-term animal study presented with a soft mass in the anterior mediastinum, which adhered to the pleura of the lung. Histopathologically, the mass mainly consisted of round to short spindle-shaped tumor cells that had infiltrated through the hyperplastic thymic tissue. The tumor cells were arranged in loose to dense sheets. Nuclei were moderate in size and round to spindle-shaped, with small nucleoli. Almost all tumor cells exhibited abundant eosinophilic cytoplasm, including eosinophilic granules of a range of sizes. The granules of tumor cells exhibited metachromasia with toluidine blue stain and were positive for c-kit and mast cell protease II. These findings indicate that the tumor described here represents a rare case of spontaneous malignant mast cell tumor with thymic epithelial hyperplasia.

Fischer-344 Tp53-knockout rats exhibit a high rate of bone and brain neoplasia with frequent metastasis

Somatic mutations in the Tp53 tumor suppressor gene are the most commonly seen genetic alterations in cancer, and germline mutations in Tp53 predispose individuals to a variety of early-onset cancers. Development of appropriate translational animal models that carry mutations in Tp53 and recapitulate human disease are important for drug discovery, biomarker development and disease modeling. Current Tp53 mouse and rat models have significant phenotypic and genetic limitations, and often do not recapitulate certain aspects of human disease. We used a marker-assisted speed congenic approach to transfer a well-characterized Tp53-mutant allele from an outbred rat to the genetically inbred Fischer-344 (F344) rat to create the F344-Tp53^{tm1(EGFP-Pac)Qly}/Rrrc (F344-Tp53) strain. On the F344 genetic background, the tumor spectrum shifted, with the primary tumor types being osteosarcomas and meningeal sarcomas, compared to the hepatic hemangiosarcoma and lymphoma identified in the original outbred stock model. The Fischer model is more consistent with the early onset of bone and central nervous system sarcomas found in humans with germline Tp53 mutations. The frequency of osteosarcomas in F344-Tp53 homozygous and heterozygous animals was 57% and 36%, respectively. Tumors were highly representative of human disease radiographically and histologically, with tumors found primarily on long bones with frequent pulmonary metastases. Importantly, the rapid onset of osteosarcomas in this promising new model fills a current void in animal models that recapitulate human pediatric osteosarcomas and could facilitate studies to identify therapeutic targets.

Editors' choice: Transferring a Tp53-knockout allele from an outbred rat stock to the F344 inbred rat genetic background alters the spectrum of tumors, providing a model of early-onset brain and bone sarcomas.

Non-proliferative and Proliferative Lesions of the Cardiovascular System of the Rat and Mouse

The INHAND Project (International Harmonization of Nomenclature and Diagnostic Criteria for Lesions in Rats and Mice) is a joint initiative of the Societies of Toxicologic Pathology from Japan (JSTP), Europe (ESTP), Great Britain (BSTP) and North America (STP) to develop an internationally-accepted nomenclature for proliferative and non-proliferative lesions in laboratory animals. The primary purpose of this publication is to provide a standardized nomenclature for characterizing lesions observed in the cardiovascular (CV) system of rats and mice commonly used in drug or chemical safety assessment. The standardized nomenclature presented in this document is also available electronically for society members on the internet (http://goreni.org). Accurate and precise morphologic descriptions of changes in the CV system are important for understanding the mechanisms and pathogenesis of those changes, differentiation of natural and induced injuries and their ultimate functional consequence. Challenges in nomenclature are associated with lesions or pathologic processes that may present as a temporal or pathogenic spectrum or when natural and induced injuries share indistinguishable features. Specific nomenclature recommendations are offered to provide a consistent approach.

Human health risk assessment of 2-mercaptobenzothiazole in drinking water

2-Mercaptobenzothiazole (MBT) is used as a vulcanization accelerator in rubber products that come into contact with potable drinking water. When such products are evaluated for contact with potable water and submitted for ANSI/NSF Standard 61 certification, any chemical extracting from these products must be below an appropriate action level of exposure. As defined by Standard 61, a total allowable concentration (TAC) is the maximum concentration of a nonregulated contaminant allowed in a public drinking water supply, and the single product allowable concentration (SPAC) is 10% of the TAC. Currently, MBT has a TAC of 40 mg/L and a SPAC of 4 mg/L. A comprehensive health effects evaluation of MBT was performed to determine whether these action levels should be revised. Epidemiological investigations indicate that workers occupationally exposed to MBT have an increased risk of death from bladder cancer. Genotoxicity investigations in bacterial and mammalian test systems provide some evidence indicating that MBT has the potential to induce mutations and chromosomal aberrations. Toxicity studies in rats and mice chronically exposed to MBT identified increases in various tumours, such as adrenal gland tumours, pituitary gland tumours, liver tumours and renal pelvis tumours. The biological significance of most of these tumours is questionable due to a variety of factors, such as a lack of dose-response between tumour incidence and dose, and the effect of test article vehicle (corn oil) upon tumour rates. Potential human health effects of exposure to MBT can be predicted from an NTP 2-year cancer study in rats, as well as epidemiological investigations in occupationally exposed workers. A comprehensive review of the epidemiological and toxicological dataset for MBT indicates that the induction of renal pelvis transitional cell tumours is the most sensitive and relevant health effects endpoint upon which to base a revised TAC and SPAC. A multistage model was used to extrapolate to low-dose MBT exposures due to the genotoxicity and clastogenicity of MBT. A TAC of 600 mg/L was derived for MBT, and was based upon an LED10 of 157.681 mg/kg per day. A SPAC of 60 mg/L was derived by multiplying the revised TAC by 0.10, and rounding to one significant figure. These revised action levels are based upon the most sensitive health effects endpoint, as well as current cancer risk assessment methodology.

Carcinogenicity Evaluation: Comparison of Tumor Data from Dual Control Groups in the Sprague–Dawley Rat

Following recent clarification in Europe that a single control group is now acceptable for rodent carcinogenicity studies, the use of dual controls may be reduced or disappear. To date, virtually nothing has been published on whether this latter situation has improved the identification of tumorigenic risk potential in these studies. In this paper, the results of 13 rat carcinogenicity studies, performed between 1991 and 2002, with 2 control groups, are presented. Although no major differences in tumor incidences between these dual control groups were found, some interstudy variation occurred. In cases where a notable difference was seen, the use of 2 control groups, as well as robust, contemporary background data, allowed an easier interpretation of findings in drug-treated groups. Thus, the continued use of dual control groups has a vital role in the assessment of tumoriogenic risk. The paper also presents an update on survival, on the range and extent of background spontaneous neoplasms, and comments on genetic drift in this commonly used rat strain.

Carcinogenicity Evaluation: Comparison of Tumor Data from Dual Control Groups in the CD–1 Mouse

Current regulatory thinking allows for the use of single control groups for rodent carcinogenicity testing although there has been a trend until recently to use dual control groups. To date, virtually nothing has been published on whether a shift from dual to single control groups will affect the identification of tumorigenic risk potential in these studies. A recent evaluation of dual control carcinogenicity data in the rat (Baldrick, Toxicol Pathol 2005, 33: 283–291) showed that although no major differences in tumor incidences between the control groups were found, some interstudy variation occurred and in cases were a notable difference was seen, the use of 2 control groups, as well as robust, contemporary background data, allowed an easier interpretation of findings in drug-treated groups. In this paper, the results of 10 mouse carcinogenicity studies, performed between 1991 and 2004, with 2 control groups, are presented. As in the rat, interstudy variation was seen and in some cases, the use of dual control groups assisted in the tumor risk assessment. Thus, the continued use of 2 control groups can have a vital role in mouse carcinogenicity studies. The paper also presents an update on survival, on the range and extent of background spontaneous neoplasms and comments on genetic drift in this commonly used mouse strain.

The legacy of the F344 rat as a cancer bioassay model (a retrospective summary of three common F344 rat neoplasms)

The Fischer 344 (F344) rat was used by the National Toxicology Program (NTP) for over 5 decades for toxicity and carcinogenicity studies. However, in 2006, the NTP decided to switch to a different rat stock due largely to high background control incidences of Leydig cell tumors (LCTs) and mononuclear cell leukemia (MNCL), also known as large granular lymphocytic (LGL) leukemia. In the current review, we aim (1) to provide a summary of NTP bioassays with treatment-associated effects involving MNCL and LCTs in addition to male F344-specific tunica vaginalis mesothelioma (TVM); (2) to describe important pathobiological differences between these F344 rat tumor responses and similar target tissue-tumor response in humans; and (3) to present the NTP reasons for switching away from the F344 rat. We show that due to the highly variable background incidence of F344 MNCL, more reliance on historical control data than is usual for most tumor responses is warranted to evaluate potential effect of any chemical treatment in this rat strain. The high spontaneous incidence of LCTs in the testes of male F344 rats has made this tumor endpoint of little practical use in identifying potential testicular carcinogenic responses. TVM responses in F344 rats have a biological plausible relationship to LCTs unlike TVM in humans. Given their high spontaneous background incidence and species-specific biology, we contend that MNCL and LCT, along with TVM responses, in F344 rat carcinogenicity studies are inappropriate tumor types for human health risk assessment and lack relevance in predicting human carcinogenicity.

Nasal Tumorigenesis in B6C3F1 Mice Following Intraperitoneal Diethylnitrosamine

Diethylnitrosamine (DEN) is a chemical broadly used in animal models as a hepatocarcinogen, reported to also cause pulmonary neoplasms in mice. The original objective was to evaluate the impact of a Western diet with or without 10% broccoli on DEN-induced on liver cancer. We administered DEN (45 mg/kg) intraperitoneally to young adult male B6C3F1 mice by 6 weekly injections and evaluated liver cancer 6 months after the DEN treatments. Here, we report unexpected primary tumorigenesis in nasal epithelium, independent of dietary treatment. More than 50% of DEN-treated B6C3F1 mice developed nasal neoplasm-related lesions, not reported previously in the literature. Only one of these neoplasms was visible externally prior to postmortem examination. Intraperitoneal DEN treatment used as a model for liver cancer can have a carcinogenic effect on the nasal epithelium in B6C3F1 mice, which should be carefully monitored in future liver cancer studies.