Relevance of Carcinogen-Induced Preclinical Cancer Models

Chemical agents can cause cancer in animals by damaging their DNA, mutating their genes, and modifying their epigenetic signatures. Carcinogen-induced preclinical cancer models are useful for understanding carcinogen-induced human cancers, as they can reproduce the diversity and complexity of tumor types, as well as the interactions with the host environment. However, these models also have some drawbacks that limit their applicability and validity. For instance, some chemicals may be more effective or toxic in animals than in humans, and the tumors may differ in their genetics and phenotypes. Some chemicals may also affect normal cells and tissues, such as by causing oxidative stress, inflammation, and cell death, which may alter the tumor behavior and response to therapy. Furthermore, some chemicals may have variable effects depending on the exposure conditions, such as dose, route, and duration, as well as the animal characteristics, such as genetics and hormones. Therefore, these models should be carefully chosen, validated, and standardized, and the results should be cautiously interpreted and compared with other models. This review covers the main features of chemically induced cancer models, such as genetic and epigenetic changes, tumor environment, angiogenesis, invasion and metastasis, and immune response. We also address the pros and cons of these models and the current and future challenges for their improvement. This review offers a comprehensive overview of the state of the art of carcinogen-induced cancer models and provides new perspectives for cancer research.

Proposal to Eliminate Urethane-Treated Positive Control Dose Groups in 26-Week Tg.rasH2 Carcinogenicity Studies

Short-term (26 weeks) Tg.rasH2 mouse carcinogenicity studies have been conducted as an alternative model to the conventional 2-year mouse carcinogenicity studies, using urethane as a positive control material. In these studies, urethane was used at a dose of 1,000 mg/kg/dose, administered intraperitoneally on days 1, 3, and 5. Urethane consistently produces lung adenomas and carcinomas and hemangiosarcomas of the spleen, proving validity of the assay. We conducted 3 pilot studies at 3 different sites of Charles River Laboratories using a lower dose of urethane (500 mg/kg/dose), administered on days 1, 3, and 5, followed by a 12-week observation period. Our results demonstrate that a lower dose can be used successfully with fewer number of animals per sex to prove the validity of the assay. However, based on our cumulative experience with this model, we propose to eliminate positive control dose groups in future Tg.rasH2 carcinogenicity studies.

The safety evaluation of food flavoring substances: the role of genotoxicity studies

The Flavor and Extract Manufacturers Association (FEMA) Expert Panel relies on the weight of evidence from all available data in the safety evaluation of flavoring substances. This process includes data from genotoxicity studies designed to assess the potential of a chemical agent to react with DNA or otherwise cause changes to DNA, either in vitro or in vivo. The Panel has reviewed a large number of in vitro and in vivo genotoxicity studies during the course of its ongoing safety evaluations of flavorings. The adherence of genotoxicity studies to standardized protocols and guidelines, the biological relevance of the results from those studies, and the human relevance of these studies are all important considerations in assessing whether the results raise specific concerns for genotoxic potential. The Panel evaluates genotoxicity studies not only for evidence of genotoxicity hazard, but also for the probability of risk to the consumer in the context of exposure from their use as flavoring substances. The majority of flavoring substances have given no indication of genotoxic potential in studies evaluated by the FEMA Expert Panel. Examples illustrating the assessment of genotoxicity data for flavoring substances and the consideration of the factors noted above are provided. The weight of evidence approach adopted by the FEMA Expert Panel leads to a rational assessment of risk associated with consumer intake of flavoring substances under the conditions of use.

The Antitussive Benzonatate Is Not Tumorigenic in Rodent Carcinogenicity Studies

Benzonatate is a peripheral oral antitussive that dampens the activity of cough stretch receptors. Rodent carcinogenicity studies were performed in Tg.rasH2 mice and Wistar Han rats. Mice were orally gavaged benzonatate at 10, 30, 75, and 100 mg/kg/day for males and 5, 15, and 50 mg/kg/day for females. Rats were gavaged at 10, 30, and 90 mg/kg/day for males and 5, 15, and 50 mg/kg/day for females. Higher doses in males were due to differences in maximum tolerated doses in dose-ranging studies. In both species, benzonatate was not detected in plasma because of rapid ester hydrolysis producing 4-(butylamino) benzoic acid (BBA) and methylated polyethylene glycol polymer. This metabolism was similar in human plasma; therefore, plasma BBA was used to show systemic exposure. Both species had no evidence of a benzonatate-related increase in any neoplasm. A slight increase in nasal cavity exudative inflammation was present in benzonatate-dosed male mice. Retinal atrophy was observed in male rats at ≥30 mg/kg/day, but the incidence was within historical control data range and not related to benzonatate. In conclusion, benzonatate and its 2 major metabolites were not carcinogenic in rodent carcinogenicity studies at BBA exposures of ≥32 and 70 times a 200 mg human benzonatate dose, respectively.

Immunohistochemical characterization of urethane-induced lung tumors in CB6F1-Tg rasH2 mice

The purpose of this study was to define the histopathological characteristics of pulmonary neoplastic lesions, especially focusing on the origin of tumor cells, in urethane-treated Tg rasH2 mice. Bronchiolar-alveolar adenomas/adenocarcinomas were observed in the lungs from all of the urethane-treated animals. Immunohistochemically, these tumors showed an alveolar epithelial type II (AE2) cell phenotype demonstrating positive staining of surfactant protein C (SP-C). Cells expressing Clara cell 10 (CC10), a Clara cell marker, were also observed in a scattered manner in some tumors. Several SP-C and CC10 double-positive cells were observed in these tumors. Most of the urethane-induced pulmonary tumors were considered to have an AE2 cell phenotype, but the presence of SP-C and CC10 double positive cells in the tumors of Tg rasH2 mice suggests that some tumors arose from bronchioalveolar stem cells, which are known to express both SP-C and CC10.

Integration of Pig-a, micronucleus, chromosome aberration and comet assay endpoints in a 28-day rodent toxicity study with urethane

As part of the international Pig-a validation trials, we examined the induction of Pig-a mutant reticulocytes and red blood cells (RET(CD59-) and RBC(CD59-), respectively) in peripheral blood of male Sprague Dawley(®) rats treated with urethane (25, 100 and 250mg/kg/day) or saline by oral gavage for 29 days. Additional endpoints integrated into this study were: micronucleated reticulocytes (MN-RET) in peripheral blood; chromosome aberrations (CAb) and DNA damage (%tail intensity via the comet assay) in peripheral blood lymphocytes (PBL); micronucleated polychromatic erythrocytes (MN-PCE) in bone marrow; and DNA damage (comet) in various organs at termination (the 29th dose was added for the comet endpoint at sacrifice). Ethyl methanesulfonate (EMS; 200mg/kg/day on Days 3, 4, 13, 14, 15, 27, 28 and 29) was evaluated as the concurrent positive control (PC). All animals survived to termination and none exhibited overt toxicity, but there were significant differences in body weight and body weight gain in the 250-mg/kg/day urethane group, as compared with the saline control animals. Statistically significant, dose-dependent increases were observed for urethane for: RET(CD59-) and RBC(CD59-) (on Days 15 and 29); MN-RET (on Days 4, 15 and 29); and MN-PCE (on Day 29). The comet assay yielded positive results in PBL (Day 15) and liver (Day 29), but negative results for PBL (Days 4 and 29) and brain, kidney and lung (Day 29). No significant increases in PBL CAb were observed at any sample time. Except for PBL CAb (likely due to excessive cytotoxicity), EMS-induced significant increases in all endpoints/tissues. These results compare favorably with earlier in vivo observations and demonstrate the utility and sensitivity of the Pig-a in vivo gene mutation assay, and its ability to be easily integrated, along with other standard genotoxicity endpoints, into 28-day rodent toxicity studies.

Regulatory Forum Opinion Piece*

High doses in Tg.rasH2 carcinogenicity studies are usually set at the maximum tolerated dose (MTD), although this dose selection strategy has not been critically evaluated. We analyzed the body weight gains (BWGs), mortality, and tumor response in control and treated groups of 29 Tg.rasH2 studies conducted at BioReliance. Based on our analysis, it is evident that the MTD was exceeded at the high and/or mid-doses in several studies. The incidence of tumors in high doses was lower when compared to the low and mid-doses of both sexes. Thus, we recommend that the high dose in male mice should not exceed one-half of the estimated MTD (EMTD), as it is currently chosen, and the next dose should be one-fourth of the EMTD. Because females were less sensitive to decrements in BWG, the high dose in female mice should not exceed two-third of EMTD and the next dose group should be one-third of EMTD. If needed, a third dose group should be set at one-eighth EMTD in males and one-sixth EMTD in females. In addition, for compounds that do not show toxicity in the range finding studies, a limit dose should be applied for the 26-week carcinogenicity studies.

Trend Analysis of Body Weight Parameters, Mortality, and Incidence of Spontaneous Tumors in Tg.rasH2 Mice

Carcinogenicity studies have been performed in conventional 2-year rodent studies for at least 3 decades, whereas the short-term carcinogenicity studies in transgenic mice, such as Tg.rasH2, have only been performed over the last decade. In the 2-year conventional rodent studies, interlinked problems, such as increasing trends in the initial body weights, increased body weight gains, high incidence of spontaneous tumors, and low survival, that complicate the interpretation of findings have been well established. However, these end points have not been evaluated in the short-term carcinogenicity studies involving the Tg.rasH2 mice. In this article, we present retrospective analysis of data obtained from control groups in 26-week carcinogenicity studies conducted in Tg.rasH2 mice since 2004. Our analysis showed statistically significant decreasing trends in initial body weights of both sexes. Although the terminal body weights did not show any significant trends, there was a statistically significant increasing trend toward body weight gains, more so in males than in females, which correlated with increasing trends in the food consumption. There were no statistically significant alterations in mortality trends. In addition, the incidence of all common spontaneous tumors remained fairly constant with no statistically significant differences in trends.

The 26-Week Tg.Rash2 Mice Carcinogenicity Studies

A typical 26-week Tg.rasH2 mouse carcinogenicity study usually has 4 dose groups, composed of 25 mice/sex, which include 1 control and 3 test article–treated groups. In every study, there is a protocol required full tissue list of 49 tissues which is examined microscopically in all animals of these 4 dose groups. Based on retrospective analysis of the historical control data collected from studies conducted in Tg.rasH2 mice from 2004 to 2012, we propose that a full tissue list be examined as usual in the control and high-dose groups; however, in the low- and mid-dose groups, only select tissues should be examined. The select tissue list is generated after analyzing common tumors, uncommon tumors seen grossly, uncommon tumors not seen grossly, organ weight variations with accountable microscopic lesions, and target organs identified in the high-dose groups. The proposed changes to the International Conference on Harmonization S1 guidance may lead to an increased need for 26-week Tg.rasH2 mice studies. The time savings resulting from processing and evaluating a select tissue list rather than a full tissue list from low- and mid-dose groups of Tg.rasH2 mouse studies will further accelerate early completion of these studies without compromising the quality and integrity.

Historical control data of spontaneous tumors in transgenic CByB6F1-Tg(HRAS)2Jic (Tg.rasH2) mice

The Tg.rasH2 mouse is a hemizygous transgenic mouse, approved by regulatory agencies for carcinogenicity assessment. However, the absence of a historical database for the incidence of spontaneous neoplasms has subsequently led to reluctance by some pharmaceutical companies to adopt the use of this short-term carcinogenicity assay. Our laboratory has generated a database summarizing the mortality, body weights, and the incidence of spontaneous tumors in 1420 male and female mice assigned to 26 studies conducted at our facility. In addition, we present the incidence of tumors in positive control mice treated with urethane from these studies. Mortality in the vehicle-treated Tg.rasH2 mouse was low (average of 1% in each study). The most common spontaneous tumors in the Tg.rasH2 mice were alveolar bronchiolar adenoma of the lungs (10.14% in males and 5.77% in females) and hemangiosarcoma of the spleen (3.66% in both males and females). The incidence of all other tumors was generally very low. In the positive control, urethane-treated animals, the incidence of alveolar bronchiolar adenomas and alveolar bronchiolar carcinomas in the lungs was 93.69% and 42.88% in males and 92.43% and 72.79% in females, respectively. In addition, the incidence of splenic hemangiosarcomas in urethane-treated males was 89.18% and 92.25% in females. The 6-month Tg.rasH2 assay is more precise, faster, and more economical than the conventional 2-year mouse assays because of the low incidence of background tumors, very high survival, shorter duration, and the lower number of animals used.