2-year Bioassay Research Articles

An assessment of the genetic toxicology of novel boron‐containing therapeutic agents

Boron-containing compounds are being studied as potential therapeutic agents. As part of the safety assessment of these therapeutic agents, a battery of genetic toxicology studies was conducted. The battery included a bacterial reverse mutation (Ames) assay, an in vitro chromosome aberration assay in peripheral human lymphocytes, and an in vivo rat micronucleus study. The following compounds represent some of the boron-containing compounds that have been advanced to human clinical trials in various therapeutic areas. The borinic picolinate, AN0128, is an antibacterial compound with anti-inflammatory activity that has been studied in clinical trials for acne and the treatment of mild to moderate atopic dermatitis. AN2690 (tavaborole) is a benzoxaborole in Phase 3 clinical trials for the topical treatment of onychomycosis, a fungal infection of the toenails and fingernails. Another benzoxaborole derivative, AN2728, a phosphodiesterase-4 (PDE4) inhibitor, is in Phase 2 clinical trials for the treatment of atopic dermatitis. AN2898, also a PDE4 inhibitor, has been studied in clinical trials for atopic dermatitis and psoriasis. AN3365 is a leucyl-tRNA synthetase inhibitor that has been in clinical development for the treatment of various Gram-negative bacterial infections. These five representative compounds were negative in the three genotoxicity assays. Furthermore, AN2690 has been studied in mouse and rat 2-year bioassays and was not found to have any carcinogenic potential. These results demonstrate that it is possible to design boron-based therapeutic agents with no genetic toxicology liabilities.

Abstract 4792: Analysis of the relationship between experimentally induced rodent forestomach hyperplasia and tumorigenesis.

Abstract Hyperplasia is an increase in cell number resulting from physiological processes such as irritation, or pathological processes like carcinogenesis. The difficulty in differentiating reactive versus preneoplastic (or dedicated) hyperplasia has been the subject of debate in the literature, especially in regard to prediction of human health risks following chronic chemical exposure. Squamous hyperplasia and neoplasia are frequently detected in the rodent forestomach (FS) following chronic oral administration of chemicals evaluated in rodent cancer bioassays. Molecular level mechanistic details related to FS hyperplasia progression are not available for a wide range of chemicals, despite the increasing need for this level of mechanistic understanding to support evaluations for human health risk. In order to facilitate further discussion on the use of rodent FS lesions in the estimation of cancer and noncancer reference values, a comparison of chemical hyperplasia versus tumor induction was performed. FS lesion classification and incidence data were obtained from rat and mouse gavage bioassays on over 50 chemicals from National Toxicology Program (NTP) 2-year bioassays that reported dose-related increases in FS hyperplastic or neoplastic lesions in one or more rodent sex/strain combinations tested. Chemical effects were tabulated and sorted by strain, sex, genotoxicity assay results and irritation effects, and the relationship between FS hyperplasia and FS tumor (e.g., squamous cell papilloma or carcinoma) induction was analyzed using grouped animal data. When considering chemicals with positive results in genotoxicity assays, FS tumors positively correlated with FS hyperplasia incidence in both rats and mice. Increased incidence of FS hyperplasia was not correlated with FS tumors in animals exposed to chemicals that were negative for genotoxicity and irritation. The potential results of this analysis may be useful in the evaluation of chemicals with limited chronic bioassay data by informing predictions of long-term tumor formation based on chemical properties observable during short-term animal or in vitro studies (e.g., genotoxicity, irritation). The views expressed in this abstract are those of the authors and do not necessarily represent the views or policies of the U.S. Environmental Protection Agency. Citation Format: Jason M. Fritz, Maureen R. Gwinn. Analysis of the relationship between experimentally induced rodent forestomach hyperplasia and tumorigenesis. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 4792. doi:10.1158/1538-7445.AM2013-4792

Abstract 1561: An evaluation of the use of genetically modified mouse models in human cancer risk assessment.

Abstract The development of short-term or accelerated cancer bioassays using genetically modified (GM) animals that exhibit high sensitivity to chemically induced cancers and that act through mode(s) of action relevant to humans continues to be an important focus of research. However, to date only a few GM mouse models such as the Trp53+/−, the Tg.AC and the rasH2 models have effectively been evaluated to substitute for the two-year rodent cancer bioassays. Whether these models provide an acceptable replacement for the conventional rodent cancer bioassay for assessing human cancer risks is the subject of ongoing debate. The objective of this study was to evaluate the current status of the use of GM mice for accelerated cancer bioassays in assessing the potential human health risks associated with exposure to carcinogenic agents. We compared the published data from the GM bioassays with results obtained using the National Toxicology Program's conventional 2-year mouse cancer bioassay presently used in human cancer risk assessment. The results demonstrate a moderate success for these GM models to distinguish carcinogens from non-carcinogens. Although the GM models are less efficient in detecting rodent carcinogens, they are more consistent in identifying non-carcinogens. Our analysis also shows that there continue to be concerns about the assay design and protocols used for the GM models, including issues related to sample size, study duration, genetic instability, and reproducibility. Additional issues of concern include specific toxicity pathway-dependent effects, and differences in the potential carcinogenic mechanisms operating in GM and non-GM animals, topics that still need to be investigated. In addition, the dose-responses of the GM models can vary substantially from those seen in conventional cancer bioassays. Overall, we conclude that the existing GM mouse models are of value for hazard identification, but are of limited use for dose-response analysis with currently used study designs. Hence, until these models are thoroughly validated using more robust study designs and the dose-response issues adequately addressed, caution should be exercised when using the current GM models to assess the carcinogenic risks of chemicals to humans. Disclaimer: The views expressed in this abstract are those of the authors and do not necessarily reflect the views or policies of the U.S. Environmental Protection Agency or the National Institute of Environmental Health Sciences. Citation Format: David A. Eastmond, Suryanarayana V. Vulimiri, John E. French, Babasaheb Sonawane. An evaluation of the use of genetically modified mouse models in human cancer risk assessment. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 1561. doi:10.1158/1538-7445.AM2013-1561

Transcriptomic responses generated by hepatocarcinogens in a battery of liver-based in vitro models

As the conventional approach to assess the potential of a chemical to cause cancer in humans still includes the 2-year rodent carcinogenicity bioassay, development of alternative methodologies is needed. In the present study, the transcriptomics responses following exposure to genotoxic (GTX) and non-genotoxic (NGTX) hepatocarcinogens and non-carcinogens (NC) in five liver-based in vitro models, namely conventional and epigenetically stabilized cultures of primary rat hepatocytes, the human hepatoma-derived cell lines HepaRG and HepG2 and human embryonic stem cell-derived hepatocyte-like cells, are examined. For full characterization of the systems, several bioinformatics approaches are employed including gene-based, ConsensusPathDB-based and classification analysis. They provide convincingly similar outcomes, namely that upon exposure to carcinogens, the HepaRG generates a gene classifier (a gene classifier is defined as a selected set of characteristic gene signatures capable of distinguishing GTX, NGTX carcinogens and NC) able to discriminate the GTX carcinogens from the NGTX carcinogens and NC. The other in vitro models also yield cancer-relevant characteristic gene groups for the GTX exposure, but some genes are also deregulated by the NGTX carcinogens and NC. Irrespective of the tested in vitro model, the most uniformly expressed pathways following GTX exposure are the p53 and those that are subsequently induced. The NGTX carcinogens triggered no characteristic cancer-relevant gene profiles in all liver-based in vitro systems. In conclusion, liver-based in vitro models coupled with transcriptomics techniques, especially in the case when the HepaRG cell line is used, represent valuable tools for obtaining insight into the mechanism of action and identification of GTX carcinogens.

Hepatocellular carcinomas in B6C3F1 mice treated with Ginkgo biloba extract for two years differ from spontaneous liver tumors in cancer gene mutations and genomic pathways.

Ginkgo biloba leaf extract (GBE) has been used for centuries in traditional Chinese medicine and today is used as an herbal supplement touted for improving neural function and for its antioxidant and anticancer effects. Herbal supplements have the potential for consumption over extended periods of time, with a general lack of sufficient data on long-term carcinogenicity risk. Exposure of B6C3F1 mice to GBE in the 2-year National Toxicology Program carcinogenicity bioassay resulted in a dose-dependent increase in hepatocellular tumors, including hepatocellular carcinoma (HCC). We show that the mechanism of hepatocarcinogenesis in GBE exposed animals is complex, involving alterations in H-ras and Ctnnb1 mutation spectra, WNT pathway dysregulation, and significantly altered gene expression associated with oncogenesis, HCC development, and chronic xenobiotic and oxidative stress compared to spontaneous HCC. This study provides a molecular context for the genetic changes associated with hepatocarcinogenesis in GBE exposed mice and illustrates the marked differences between these tumors and those arising spontaneously in the B6C3F1 mouse. The molecular changes observed in HCC from GBE-treated animals may be of relevance to those seen in human HCC and other types of cancer, and provide important data on potential mechanisms of GBE hepatocarcinogenesis.

Benzo[a]pyrene-induced transcriptomic responses in primary hepatocytes and in vivo liver: Toxicokinetics is essential for in vivo–in vitro comparisons

The traditional 2-year cancer bioassay needs replacement by more cost-effective and predictive tests. The use of toxicogenomics in an in vitro system may provide a more high-throughput method to investigate early alterations induced by carcinogens. Recently, the differential gene expression response in wild-type and cancer-prone Xpa (-/-) p53 (+/-) primary mouse hepatocytes after exposure to benzo[a]pyrene (B[a]P) revealed downregulation of cancer-related pathways in Xpa (-/-) p53 (+/-) hepatocytes only. Here, we investigated pathway regulation upon in vivo B[a]P exposure of wild-type and Xpa (-/-) p53 (+/-) mice. In vivo transcriptomics analysis revealed a limited gene expression response in mouse livers, but with a significant induction of DNA replication and apoptotic/anti-apoptotic cellular responses in Xpa (-/-) p53 (+/-) livers only. In order to be able to make a meaningful in vivo-in vitro comparison we estimated internal in vivo B[a]P concentrations using DNA adduct levels and physiologically based kinetic modeling. Based on these results, the in vitro concentration that corresponded best with the internal in vivo dose was chosen. Comparison of in vivo and in vitro data demonstrated similarities in transcriptomics response: xenobiotic metabolism, lipid metabolism and oxidative stress. However, we were unable to detect cancer-related pathways in either wild-type or Xpa (-/-) p53 (+/-) exposed livers, which were previously found to be induced by B[a]P in Xpa (-/-) p53 (+/-) primary hepatocytes. In conclusion, we showed parallels in gene expression responses between livers and primary hepatocytes upon exposure to equivalent concentrations of B[a]P. Furthermore, we recommend considering toxicokinetics when modeling a complex in vivo endpoint with in vitro models.

Statistical Comparison of Carcinogenic Effects and Dose–Response Relationships in Rats and Mice for 2,4-Toluene Diamine to those Ascribed to Toluene Diisocyanate

ABSTRACT The U.S. National Toxicology Program (NTP) conducted 2-year bioassays of commercial grade toluene diisocyanate (TDI) (80% 2,4-TDI and 20% 2,6-TDI) and 2,4-toluene diamine (TDA) and concluded that both were carcinogenic in rodents. In the TDI study, there was an unproven but likely formation of TDA either because of flawed test-substance handling and storage conditions and/or the atypical exposure conditions employed. Although the carcinogenic responses in both studies were qualitatively similar, several statistical analyses were performed to substantiate this possibility more rigorously. Seven different statistical approaches combine to yield a robust and consistent conclusion that, if only a small fraction (approximately 5%) of the dose of TDI were hydrolyzed to TDA in the TDI study, then that would be sufficient to explain the observed carcinogenic responses in the TDI study.

Absence of carcinogenic response to multiwall carbon nanotubes in a 2-year bioassay in the peritoneal cavity of the rat

Absence of carcinogenic response to multiwall carbon nanotubes in a 2-year bioassay in the peritoneal cavity of the rat

Dammar resin, a non-mutagen, inducts oxidative stress and metabolic enzymes in the liver of gpt delta transgenic mouse which is different from a mutagen, 2-amino-3-methylimidazo[4,5-f]quinoline

Dammar resin has long been used in foods as either a clouding or a glazing agent. In a recent study, 2% Dammar resin showed significant hepatocarcinogenicity in a rat 2-year bioassay. Therefore, for an accurate estimate of human risk, it is necessary to understand whether Dammar resin induces liver genotoxicity and the underlying mechanisms of its hepatocarcinogenicity. Modifying effects of 2-amino-3-methylimidazo[4,5-f]quinoline (IQ), a typical genotoxic carcinogen produced during cooking of protein-rich foods, was also studied in the present study. Exposure of gpt delta mice to Dammar resin at a dose of 2% for 12 weeks did not induce any obvious mutagenicity in the liver. However, the index of cell proliferation, the level of 8-OHdG, and bax, bcl-2, p53, cyp1a2, cyp2e1, gpx1 and gstm2 gene expression were all significantly increased when compared with the control group. In the IQ treatment group, at a dose of 300ppm, mutagenicity was readily detected, the index of cell proliferation increased, and p53, cyp2e1 and gpx1 gene expression was down-regulated in the liver. Down-regulation of p53, P450s, and gpx1 in the livers of IQ treated mice are consistent with its genotoxic mechanism of carcinogenicity observed in a 675-day study. In contrast, our results using gpt delta mice suggest that Dammar resin is not genotoxic. Instead, the Dammar resin-induced hepatocarcinogenicity seen in our previous 2-year study with rats may have been mediated by non-genotoxic mechanisms, including increased P450 enzyme activity, increased oxidative stress, altered gene expression, and promotion of cell proliferation.

Nonclinical Assessment of Carcinogenic Risk and Tumor Growth Enhancement Potential of Prasugrel, a Platelet-Inhibiting Therapeutic Agent

Prasugrel, a thienopyridine ADP receptor antagonist, is an orally administered prodrug requiring in vivo metabolism to form the active metabolite that irreversibly inhibits platelet activation and aggregation mediated by the P2Y12[sub 12] receptor. A comprehensive nonclinical safety assessment including genotoxicity and carcinogenicity studies supported the chronic use of prasugrel in patients with atherothrombotic disease. In addition, a special assessment of the potential for prasugrel to enhance tumor growth was undertaken to address regulatory concerns relating to increases in human cancers. Prasugrel demonstrated no evidence of genotoxicity and was not oncogenic in a 2-year rat carcinogenicity study. In the 2-year mouse study, an increase in hepatocellular adenomas was considered secondary to enzyme induction and not relevant to human safety. Further, the absence of any increase in common background tumors at any other organ site in either rodent study indicated a lack of tumor promoting activity (apart from the CYP450 induction-related increase in mouse liver tumors). Cell culture studies with 3 human tumor cell lines (lung, colon, prostate) demonstrated that exposure of serum-starved cells to prasugrel's active and major circulating human metabolites does not increase cell proliferation relative to starved cells stimulated to proliferate by addition of 10% FBS. Prasugrel also did not increase tumor growth relative to vehicle controls in nude mice implanted with 3 human tumor cell lines. Thus, traditional genotoxicity and 2-year bioassays as well as specially designed tumor growth enhancement studies in human tumor cell lines and mouse xenograft models clearly demonstrated prasugrel's lack of tumorigenic potential.

Differential Transcriptomic Analysis of Spontaneous Lung Tumors in B6C3F1 Mice: Comparison to Human Non–Small Cell Lung Cancer

Lung cancer is the leading cause of cancer-related death in people and is mainly due to environmental factors such as smoking and radon. The National Toxicology Program (NTP) tests various chemicals and mixtures for their carcinogenic hazard potential. In the NTP chronic bioassay using B6C3F1 mice, the incidence of lung tumors in treated and control animals is second only to the liver tumors. In order to study the molecular mechanisms of chemically induced lung tumors, an understanding of the genetic changes that occur in spontaneous lung (SL) tumors from untreated control animals is needed. The authors have evaluated the differential transcriptomic changes within SL tumors compared to normal lungs from untreated age-matched animals. Within SL tumors, several canonical pathways associated with cancer (eukaryotic initiation factor 2 signaling, RhoA signaling, PTEN signaling, and mammalian target of rapamycin signaling), metabolism (Inositol phosphate metabolism, mitochondrial dysfunction, and purine and pyramidine metabolism), and immune responses (FcγR-mediated phagocytosis, clathrin-mediated endocytosis, interleukin 8 signaling, and CXCR4 signaling) were altered. Meta-analysis of murine SL tumors and human non-small cell lung cancer transcriptomic data sets revealed a high concordance. These data provide important information on the differential transcriptomic changes in murine SL tumors that will be critical to our understanding of chemically induced lung tumors and will aid in hazard analysis in the NTP 2-year carcinogenicity bioassays.

Mode of Action of Pulegone on the Urinary Bladder of F344 Rats

Essential oils from mint plants, including peppermint and pennyroyal oils, are used at low levels as flavoring agents in various foods and beverages. Pulegone is a component of these oils. In a 2-year bioassay, oral administration of pulegone slightly increased the urothelial tumor incidence in female rats. We hypothesized that its mode of action (MOA) involved urothelial cytotoxicity and increased cell proliferation, ultimately leading to tumors. Pulegone was administered by gavage at 0, 75, or 150 mg/kg body weight to female rats for 4 and 6 weeks. Fresh void urine and 18-h urine were collected for crystal and metabolite analyses. Urinary bladders were evaluated by light microscopy and scanning electron microscopy (SEM) and bromodeoxyuridine (BrdU) labeling index. Pulegone and its metabolites, piperitenone, piperitone, menthofuran, and menthone, were tested for cytotoxicity in rat (MYP3) and human (1T1) urothelial cells by the 3-(4,5-dimethythiazol-2-yl)-2,5-diphenyl tetrazolium bromide assay. No abnormal urinary crystals were observed by light microscopy. Urine samples (18-h) showed the presence of pulegone, piperitone, piperitenone, and menthofuran in both treated groups. By SEM, bladders from treated rats showed superficial necrosis and exfoliation. There was a significant increase in the BrdU labeling index in the high-dose group. In vitro studies indicated that pulegone and its metabolites, especially piperitenone, are excreted and concentrated in the urine at cytotoxic levels when pulegone is administered at high doses to female rats. The present study supports the hypothesis that cytotoxicity followed by regenerative cell proliferation is the MOA for pulegone-induced urothelial tumors in female rats.

Comparison of hepatocarcinogen-induced gene expression profiles in conventional primary rat hepatocytes with in vivo rat liver

At present, substantial efforts are focused on the development of in vitro assays coupled with "omics" technologies for the identification of carcinogenic substances as an alternative to the classical 2-year rodent carcinogenicity bioassay. A prerequisite for the eventual regulatory acceptance of such assays, however, is the in vivo relevance of the observed in vitro findings. In the current study, hepatocarcinogen-induced gene expression profiles generated after the exposure of conventional cultures of primary rat hepatocytes to three non-genotoxic carcinogens (methapyrilene hydrochloride, piperonyl butoxide, and Wy-14643), three genotoxic carcinogens (aflatoxin B1, 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone, and 2-nitrofluorene), and two non-carcinogens (nifedipine and clonidine) are compared with previously obtained in vivo data after oral administration for up to 14 days of the same hepatocarcinogens to rats. In addition to the comparison of deregulated genes and functions per compound between in vivo and in vitro models, the major discriminating cellular pathways found in vivo in livers of exposed rats were examined for deregulation in vitro. Further, in vivo-derived gene signatures for the identification of genotoxic versus non-genotoxic carcinogens are used to classify in vitro-tested hepatocarcinogens and non-carcinogens. In the primary hepatocyte cultures, two out of the three tested genotoxic carcinogens mimicked the in vivo-relevant DNA damage response and were correctly assessed. Exposure to the non-genotoxic hepatocarcinogens, however, triggered a relatively weak response in the in vitro system, with no clear similarities to in vivo. This study contributes to the further optimization of toxicogenomics predictive tools when applied in in vitro settings.

In vivo genotoxicity of furan in F344 rats at cancer bioassay doses

Furan, a potent rodent liver carcinogen, is found in many cooked food items and thus represents a human cancer risk. Mechanisms for furan carcinogenicity were investigated in male F344 rats using the in vivo Comet and micronucleus assays, combined with analysis of histopathological and gene expression changes. In addition, formamidopyrimidine DNA glycosylase (Fpg) and endonuclease III (EndoIII)-sensitive DNA damage was monitored as a measure of oxidative DNA damage. Rats were treated by gavage on four consecutive days with 2, 4, and 8mg/kg bw furan, doses that were tumorigenic in 2-year cancer bioassays, and with two higher doses, 12 and 16mg/kg. Rats were killed 3h after the last dose, a time established as producing maximum levels of DNA damage in livers of furan-treated rats. Liver Comet assays indicated that both DNA strand breaks and oxidized purines and pyrimidines increased in a near-linear dose-responsive fashion, with statistically significant increases detected at cancer bioassay doses. No DNA damage was detected in bone marrow, a non-target tissue for cancer, and peripheral blood micronucleus assays were negative. Histopathological evaluation of liver from furan-exposed animals produced evidence of inflammation, single-cell necrosis, apoptosis, and cell proliferation. In addition, genes related to apoptosis, cell-cycle checkpoints, and DNA-repair were expressed at a slightly lower level in the furan-treated livers. Although a mixed mode of action involving direct DNA binding cannot be ruled out, the data suggest that furan induces cancer in rat livers mainly through a secondary genotoxic mechanism involving oxidative stress, accompanied by inflammation, cell proliferation, and toxicity.

Association of Renal Tubular Hyaline Droplets with Lymphoma in CD-1 Mice

In mice, hyaline droplets in renal proximal tubules have been associated with histiocytic sarcoma but have not been reported with lymphoma. Tissues from CD-1 mice in a 2-year carcinogenicity bioassay were examined microscopically. Twenty-five mice with hyaline droplets in renal tubules were identified. Immunohistochemistry to detect IgA, IgG, IgM, lysozyme, albumin, CD3, and CD79a was performed on kidneys of 21 affected mice. Hyaline droplets were present in the kidneys of 11 mice with lymphoma (1 male, 10 female), of which 1 female also had histiocytic sarcoma. Hyaline droplets were also present in 7 other mice with histiocytic sarcoma, 2 with chronic progressive nephropathy, 3 with renal cortical tubular necrosis, and 2 with granulocytic leukemia. Five of the 11 lymphomas were CD3+, indicating a T lymphocyte origin. Hyaline droplets in mice with lymphoma did not stain for IgA, IgG, or IgM, except in one questionable case. Results of other immunohistochemical stains were inconclusive. Although the droplet composition could not be determined immunohistochemically, the study findings indicate that renal tubular hyaline droplets may be associated with lymphoma in mice.

Application of in vitro cell transformation assays in regulatory toxicology for pharmaceuticals, chemicals, food products and cosmetics

Two year rodent bioassays play a key role in the assessment of carcinogenic potential of chemicals to humans. The seventh amendment to the European Cosmetics Directive will ban in 2013 the marketing of cosmetic and personal care products that contain ingredients that have been tested in animal models. Thus 2-year rodent bioassays will not be available for cosmetics/personal care products. Furthermore, for large testing programs like REACH, in vivo carcinogenicity testing is impractical. Alternative ways to carcinogenicity assessment are urgently required. In terms of standardization and validation, the most advanced in vitro tests for carcinogenicity are the cell transformation assays (CTAs). Although CTAs do not mimic the whole carcinogenesis process in vivo, they represent a valuable support in identifying transforming potential of chemicals. CTAs have been shown to detect genotoxic as well as non-genotoxic carcinogens and are helpful in the determination of thresholds for genotoxic and non-genotoxic carcinogens. The extensive review on CTAs by the OECD (OECD (2007) Environmental Health and Safety Publications, Series on Testing and Assessment, No. 31) and the proven within- and between-laboratories reproducibility of the SHE CTAs justifies broader use of these methods to assess carcinogenic potential of chemicals.

Intestinal Lymphangiectasis and Lipidosis in Rats Following Subchronic Exposure to Indole-3-Carbinol via Oral Gavage

To investigate the toxicity and carcinogenic potential of indole-3-carbinol (I3C), the National Toxicology Program has conducted 13-week subchronic studies in Fisher 344 rats and B6C3F1 mice, and chronic 2-year bioassays in Sprague-Dawley rats and B6C3F1 mice. While the chronic study results are not yet available, subchronic study results and short-term special evaluations of interim sacrifices in the 2-year rat bioassay are presented. F344 rats were orally gavaged ≤300 mg I3C/kg body weight 5 days a week for 13 weeks. Rats treated with ≥150 mg/kg demonstrated a dose-related dilation of lymphatics (lymphangiectasis) of the duodenum, jejunum, and mesenteric lymph nodes. Material within dilated lacteals stained positively for Oil Red O and Sudan Black, consistent with lipid. Electron microscopic evaluation confirmed extracellular lipid accumulation within the villar lamina propria, lacteals, and within villar macrophages. Analyses of hepatic and pulmonary CYP1A enzymes demonstrated dose-dependent I3C induction of CYP1A1 and 1A2. B6C3F1 mice orally gavaged ≤250 mg I3C/kg body weight did not demonstrate histopathological changes; however, hepatic CYP induction was similar to that in rats. The histopathologic changes of intestinal lymphangiectasis and lipidosis in this study share similarities with intestinal lymphangiectasia as observed in humans and dogs. However, the resultant clinical spectrum of protein-losing enteropathy was not present.

Spontaneous Tumor Incidence in rasH2 Mice

Alternate transgenic mouse models are accepted as replacements for the standard carcinogenicity mouse bioassay by regulatory agencies with a companion 2-year rat bioassay. The slower rate of industry acceptance of these shorter transgenic mouse cancer bioassays has been due to lack of historical data and diagnostic criteria, and the use of nonstandardized terminologies in published data. To address these issues, especially that of generating a large historical database, a retrospective analysis of the spontaneous tumor incidences in rasH2 mice from internally sponsored 6-month carcinogenicity studies was compared to the published literature. Incidences of common spontaneous tumors (incidences > 1%) observed in these studies were lung bronchiolo-alveolar adenomas (mean 3.9-9.9%; range 0-18%), lung bronchiolo-alveolar adenocarcinomas (mean 1.4-2.4%; range 0-5%), splenic hemangiosarcomas (mean 3.0-3.9%; range 0-17%), cutaneous squamous cell papillomas (mean 1.1-1.2%; range 0-4%), Harderian gland adenoma (mean 0.8-1.2%; range 0-4%), and hepatocellular adenomas (mean 1.8%; 0-9% in males only). The remarkable similarity in the tumor incidences in multiple rasH2 studies over a decade and the observed stability of the inserted human gene are important indicators of the minimal drift in this model. Overall, the historical control data for spontaneous neoplasms should assist in the interpretation of future rasH2 mouse studies.

Immunotoxicologic effects of cyclosporine on tumor progression in models of squamous cell carcinoma and B-cell lymphoma in C3H mice

Many immunosuppressive drugs are associated with an increased risk of neoplasia, principally non-melanoma skin cancers and B-cell lymphomas. However, only 6 of the 13 immunosuppressive drugs tested in 2 year bioassays increased the incidence of neoplasia. For example, the 2-year bioassays conducted with cyclosporine (CSA), an International Agency for Research on Cancer (IARC) Group 1 human carcinogen, were negative. The purpose of these investigations was to use transplanted tumor models in immunocompetent, syngeneic mice to gain insight into the failure of the 2-year bioassay to show an increased incidence of neoplasia with CSA. C3H HeN mice were used in a battery of assays with a transplanted squamous cell carcinoma (SCC VII cells) or a B-cell, lymphoma (38C13 cells) cells to study effects of CSA on local growth and metastases, experimental metastases, and progression of established metastases. Mice received CSA twice weekly by subcutaneous (SC) injection at doses of 0.5, 5, or 50 mg/kg; controls received the CSA vehicle. CSA had a modest inhibitory effect on SC tumors initiated by 38C13 cells and on intramuscular tumors initiated by SCC VII cells. CSA also decreased the number of lung colonies and decreased the size, growth fraction and vascularity of established lung metastases initiated by SCC VII cells. In contrast, CSA increased progressive growth of metastases to the sentinel lymph node from an intramuscular SCC VII tumor, but had no effect cellular traffic to the node. In conclusion, CSA at doses up to 50 mg/kg did not facilitate tumor progression and it partially inhibited tumor growth, suggesting that suppression of tumor progression may partially explain the failure of CSA to act as a carcinogen in 2 year bioassays.

Monitoring Cyp2b10 mRNA expression at cessation of 2-year carcinogenesis bioassay in mouse liver provides evidence for a carcinogenic mechanism devoid of human relevance: The dalcetrapib experience

IntroductionDalcetrapib is a cholesteryl ester transfer protein (CETP) modulator in clinical assessment for cardiovascular outcome benefits. In compliance with regulatory requirements, dalcetrapib was evaluated in rodent 2-year carcinogenesis bioassays. In the mouse bioassay, male mice demonstrated increased liver weight and statistically increased incidences of hepatocellular adenoma/carcinoma. Hepatic cytochrome p450 (Cyp) 2b10 mRNA induction and increased Cyp2b10 enzyme activity signify activation of hepatic nuclear receptor constitutive androstane receptor (CAR), a widely established promoter of rodent-specific hepatic tumors. We therefore monitored hepatic Cyp2b10 mRNA and its enzyme activity in a subset of dalcetrapib-treated male mice from the bioassay. MethodsLiver samples were obtained from ~1/3 of male mice from each dose group including vehicle-controls (mean and earliest study day of death 678 and 459 respectively). Quantitative real time PCR (qRT-PCR) was performed to determine Cyp2b10 mRNA expression and Cyp1a-, Cyp2b10- and Cyp3a-selective activities were monitored. ResultsCyp2b10 mRNA was strongly induced by dalcetrapib with an expected wide inter-individual variation (5–1421-fold). Group average fold-induction versus vehicle-controls showed a dose-related increase from 48-fold (250mg/kg/day) to 160-fold (750mg/kg/day), which declined slightly at 2000mg/kg/day (97-fold). Cyp enzyme activities showed approximate doubling of total Cyp P450 content per milligram protein and a 9-fold increase in Cyp2b10-selective pentoxyresorufin O-dealkylase activity (750mg/kg/day). DiscussionThese data from hepatic Cyp2b10 monitoring are strongly suggestive of CAR activation by dalcetrapib, a mechanism devoid of relevance towards hepatocarcinogenesis in humans; results show feasibility of Cyp2b10 as a surrogate marker for this mechanism at cessation of a carcinogenesis bioassay.