The stability of historical control data for common neoplasms in laboratory rats and the implications for carcinogenic risk assessment

Using historical control data in bioassays for regulatory toxicology

Historical control data (HCD) consist of pooled control group responses from bioassays. These data must be collected and are often used or reported in regulatory toxicology studies for multiple purposes: as quality assurance for the test system, to help identify toxicological effects and their effect-size relevance and to address the statistical multiple comparison problem. The current manuscript reviews the various classical and potential new approaches for using HCD. Issues in current practice are identified and recommendations for improved use and discussion are provided. Furthermore, stakeholders are invited to discuss whether it is necessary to consider uncertainty when using HCD formally and statistically in toxicological discussions and whether binary inclusion/exclusion criteria for HCD should be revised to a tiered information contribution to assessments. Overall, the critical value of HCD in toxicological bioassays is highlighted when used in a weight-of-evidence assessment.

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.

Human relevance of follicular thyroid tumors in rodents caused by non-genotoxic substances

Chronic stimulation of the thyroid gland of rodents by TSH leads to thyroid follicular hyperplasia and subsequently to thyroid follicular adenomas and carcinomas. However, the interpretations of rodent thyroid tumors are contradictory. The U.S. Food and Drug Administration (FDA) concluded that findings with drugs that lead to increased levels of thyroid-stimulating hormone (TSH) in rats are not relevant to humans, whereas the U.S. Environmental Protection Agency (US EPA) concluded that chemicals that produce rodent thyroid tumors may pose a carcinogenic hazard for humans although the thyroid of rodents appears to be more sensitive to a carcinogenic stimulus than that of humans. Meanwhile, based on the CLP Criteria of the European Chemicals Agency (ECHA), rodent thyroid tumors caused by the induction of uridine-diphosphate-glucuronosyl transferases (UDGT) were assessed as not relevant to humans. To clarify these discrepant positions, the function and regulation of the thyroid gland are described and the types of thyroid tumors and the causes of their development in humans and animals are examined. Based on these data and the evidence that so far, except radiation, no chemical is known to increase the incidence of thyroid tumors in humans, it is concluded that rodent thyroid tumors resulting from continuous stimulation of the thyroid gland by increased TSH levels are not relevant to humans. Consequently, compounds that induce such tumors do not warrant classification as carcinogenic.

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).

Evaluation of potential activity of luseogliflozin on vascular proliferation in the mesenteric lymph node with or without vascular tumors in Sprague-Dawley rats in a carcinogenicity study

The incidence of mesenteric lymph node vascular tumors can vary in rats, and appropriate assessment of potential risk of tumorigenicity is needed when the incidence is higher in treated groups than in a control group. In a 2-year rat carcinogenicity study of luseogliflozin, a selective sodium-dependent glucose co-transporter 2 inhibitor for the treatment of type 2 diabetes mellitus, there was a slight but statistically significant increase in the total number of hemangiomas and hemangiosarcomas in the mesenteric lymph nodes in males at a high-dose. As part of the risk assessment for luseogliflozin, its effect on the vascular proliferation potential in the mesenteric lymph nodes was examined in a rat carcinogenicity study by performing an image analysis using specimens with double immunohistochemical staining for PCNA and CD34 in control and high-dose males. In addition, immunohistochemical staining for VEGF was performed to detect enhanced angiogenesis. In the high-dose males that did not have a hemangioma/hemangiosarcoma, neither an increased number of PCNA/CD34-positive cells nor changes in the expression pattern of VEGF was observed. On the other hand, in the high-dose males that had a hemangioma/hemangiosarcoma, the number of PCNA-positive cells was increased in the tumor areas, and the number in the hemangioma/hemangiosarcoma was approximately one-half of that in the hemangiosarcoma in the control male. In conclusion, no potential change leading to vascular proliferation/tumors was detected in the mesenteric lymph nodes of high-dose males receiving luseogliflozin.

Occurrence of Pineal Gland Tumors in Combined Chronic Toxicity/Carcinogenicity Studies in Wistar Rats

Pineal gland tumors are very rare brain lesions in rats as well as in other species including humans. A total of 8 (out of 1,360 examined) Wistar rats from 3 different combined chronic toxicity/carcinogenicity or mere carcinogenicity studies revealed pineal gland tumors. The tumors were regarded to be spontaneous and unrelated to treatment. The morphology and immunohistochemical evaluation led to the diagnosis malignant pinealoma. The main characteristics that were variably developed within the tumors were the following: cellular atypia, high mitotic index, giant cells, necrosis, Homer Wright rosettes, Flexner-Wintersteiner rosettes and pseudorosettes, positive immunohistochemical reaction for synaptophysin, and neuron-specific enolase. The pineal gland is not a protocol organ for histopathological examination in carcinogenicity studies. Nevertheless, the pineal gland can occasionally be encountered on the routine brain section or if it is the origin of a tumor protruding into the brain, the finding will be recorded. Therefore, although known to be a rare tumor in rats, pineal neoplasms should be included in the list of possible differential diagnoses for brain tumors, especially when the tumor is located in the region of the pineal body.

Proceedings of the 2014 National Toxicology Program Satellite Symposium

The 2014 annual National Toxicology Program (NTP) Satellite Symposium, entitled "Pathology Potpourri" was held in Washington, D.C., in advance of the Society of Toxicologic Pathology's 33rd annual meeting. The goal of this annual NTP Symposium is to present current diagnostic pathology or nomenclature issues to the toxicologic pathology community. This article presents summaries of the speakers' presentations, including diagnostic or nomenclature issues that were presented, along with select images that were used for audience voting and discussion. Some lesions and topics covered during the symposium included a pulmonary mucinous adenocarcinoma in a male B6C3F1 mouse; plexiform vasculopathy in Wistar Han (Crl:WI[Han]) rats; staging of the estrous cycle in rats and mice; peri-islet fibrosis, hemorrhage, lobular atrophy and inflammation in male Sprague-Dawley (SD) rats; retinal dysplasia in Crl:WI[Han] rats and B6C3F1 mice; multicentric lymphoma with intravascular microemboli and tumor lysis syndrome, and 2 cases of myopathy and vascular anomaly in Tg.rasH2 mice; benign thymomas in Crl:WI[Han] rats; angiomatous lesions in the mesenteric lymph nodes of Crl:WI[Han] rats; an unusual foveal lesion in a cynomolgous monkey; and finally a series of nomenclatures challenges from the endocrine International Harmonization of Nomenclature and Diagnostic Criteria (INHAND) Organ Working Group (OWG).

Nonclinical Safety of the Sodium-Glucose Cotransporter 2 Inhibitor Empagliflozin

Empagliflozin, a selective inhibitor of the renal tubular sodium-glucose cotransporter 2, was developed for treatment of type 2 diabetes mellitus. Nonclinical safety of empagliflozin was studied in a battery of tests to support global market authorization. Safety pharmacology studies indicated no effect of empagliflozin on measures of respiratory or central nervous system function in rats or cardiovascular safety in telemeterized dogs. In CD-1 mouse, Wistar Han rat, or beagle dogs up to 13, 26, or 52 weeks of treatment, respectively, empagliflozin exhibited a toxicity profile consistent with secondary supratherapeutic pharmacology related to glucose loss and included decreased body weight and body fat, increased food consumption, diarrhea, dehydration, decreased serum glucose and increases in other serum parameters reflective of increased protein catabolism, gluconeogenesis, and electrolyte imbalances, and urinary changes such as polyuria and glucosuria. Microscopic changes were consistently observed in kidney and included tubular nephropathy and interstitial nephritis (dog), renal mineralization (rat) and tubular epithelial cell karyomegaly, single cell necrosis, cystic hyperplasia, and hypertrophy (mouse). Empagliflozin was not genotoxic. Empagliflozin was not carcinogenic in female mice or female rats. Renal adenoma and carcinoma were induced in male mice only at exposures 45 times the maximum clinical dose. These tumors were associated with a spectrum of nonneoplastic changes suggestive of a nongenotoxic, cytotoxic, and cellular proliferation-driven mechanism. In male rats, testicular interstitial cell tumors and hemangiomas of the mesenteric lymph node were observed; both tumors are common in rats and are unlikely to be relevant to humans. These studies demonstrate the nonclinical safety of empagliflozin.

Impact of non-constant concentration exposure on lethality of inhaled hydrogen cyanide

The ten Berge model, also known as the toxic load model, is an empirical approach in hazard assessment modeling for estimating the relationship between the inhalation toxicity of a chemical and the exposure duration. The toxic load (TL) is normally expressed as a function of vapor concentration (C) and duration (t), with TL equaling C(n) × t being a typical form. Hypothetically, any combination of concentration and time that yields the same "toxic load" will give a constant biological response. These formulas have been developed and tested using controlled, constant concentration animal studies, but the validity of applying these assumptions to time-varying concentration profiles has not been tested. Experiments were designed to test the validity of the model under conditions of non-constant acute exposure. Male Sprague-Dawley rats inhaled constant or pulsed concentrations of hydrogen cyanide (HCN) generated in a nose-only exposure system for 5, 15, or 30 min. The observed lethality of HCN for the 11 different C versus t profiles was used to evaluate the ability of the model to adequately describe the lethality of HCN under the conditions of non-constant inhalation exposure. The model was found to be applicable under the tested conditions, with the exception of the median lethality of very brief, high concentration, discontinuous exposures.

Transition of historical control data for high incidence tumors in f344 rats

Historical control data of tumor incidence were collected from the control groups (215 animals of each sex) in four recent carcinogenicity studies that were started between 2005 to 2009 (terminally sacrificed between 2007 and 2011) at BoZo Research Center Inc. (Gotemba, Shizuoka, Japan) using Fischer 344 rats (F344/DuCrlCrlj). These data were compared to the previous historical control data (from 1990 to 2004, previously reported) in the same facility. In the results, the incidence of C-cell adenoma in the thyroid tended to increase in both sexes in recent years (30.8% for males and 24.4% for females in 2005-2009) as compared with the previous data (17.4% and 20.1% for males and 11.5% and 11.8% for females in 1990–1999 and 2000–2004, respectively). In addition, the incidences of pancreatic islet cell adenoma in males and uterine adenocarcinoma tended to increase from around 2000 and remained high in recent years (incidences of islet cell adenoma in males of 10.5%, 17.1% and 20.5% in 1990–1999, 2000–2004 and 2005–2009; incidences of uterine adenocarcinoma of 3.3%, 12.0% and 13.5% in 1990–1999, 2000–2004 and 2005–2009, respectively). There was no apparent difference in the incidence of other tumors.

Comparing radiation toxicities across species: an examination of radiation effects in Mus musculus and Peromyscus leucopus

PURPOSE

Life shortening and pathological complications in similarly irradiated cohorts of the laboratory mouse Mus musculus and the white-footed mouse Peromyscus leucopus were recorded in the course of the Janus studies conducted at Argonne National Laboratory from 1970-1992. This study examines how lifespan, tumor and non-tumor disease incidence, and tumor multiplicity are differentially affected by gamma-rays and neutron radiation exposure in two different animal species.

MATERIALS AND METHODS

Survival analyses examined differences in lifespan across species, while decision tree analyses examined statistically significant associations between lifespan, radiation exposure, and specific diseases. Logistic regression models were generated to examine the likelihood of disease incidence in these two species following gamma-ray or neutron radiation exposure.

RESULTS

Life shortening in response to radiation was more significant in Peromyscus leucopus than in Mus musculus, irrespective of radiation quality. Many types of tumor and non-tumor diseases were found to be consistently species specific. Tumor multiplicity was observed in both species in response to radiation, although more pronounced in Mus musculus.

CONCLUSION

The response to radiation was highly species specific, highlighting the difficulty in extrapolating conclusions from one species to another, irrespective of their phenotypic similarities and ecologic niches.

Endometrial stromal polyps in rodents: biology, etiology, and relevance to disease in women

Endometrial stromal polyps (ESP) are a common spontaneous reproductive tract lesion in the female rat. However, there is limited information concerning the etiology, biology, and significance of these polyps as an end point in toxicology and carcinogenicity studies. This paper reviews relevant literature to address these aspects of ESP with respect to potential relevance to human uterine tumors. Endometrial stromal polyps in rodents appear as age-related lesions. There are only a few chemicals tested for carcinogenicity in rat and mouse cancer bioassays associated with increased incidence of ESP with no common characteristics or mechanism of action. Uterine endometrial polyps that occur in women and the uterine stromal polyps that occur in rodents have distinct characteristics, although both types of uterine lesions are common, benign, and noncancerous. Human endometrial polyps develop from both endometrial and stromal components, whereas rodent polyps develop from the stromal component of the uterus. Endometrial polyps in women are hormone sensitive, but there is no scientific or experimental evidence to date that suggests that uterine stromal polyps in rodents are hormone sensitive. Therefore, based on differences in their etiology and biology, endometrial stromal polyps observed in rodent toxicity and carcinogenicity studies appear to have limited relevance to human endometrial polyps occurring in women.

Laboratory models and their role in assessing teratogenesis

Laboratory models have an important role in identifying exposures with teratogenic potential, determining mechanisms of abnormal development, and supporting or refuting the biological plausibility of associations identified in human studies. Laboratory animals are the most widely used models, but are rapidly being supplemented by in vitro tools. Testing paradigms that have been in place since the mid-1960s for pharmaceuticals, and soon thereafter for industrial chemicals and pesticides, have been used to evaluate the potential developmental toxicity of thousands of agents. These models have served as the principal basis for regulatory decisions about acceptable exposure levels and restrictions on use of certain drugs during pregnancy.

Investigation of carcinogenicity for levamisole administered in the diet to F344 rats

A two year carcinogenicity study of anthelmintic drug levamisole (LV) was performed using 50 male and 50 female F344 rats at dietary drug concentrations of 0, 60, or 300 ppm. The daily intakes of LV were calculated to be 2.6, 12.9 mg/kg b.w./day for males and 2.9, 14.1mg/kg b.w./day for females, respectively. No significant differences in general condition and survival rate (82%, 74%, 80% in males and 84%, 84%, 84% in females, respectively) were observed. In the 300 ppm group, suppression of body weight gain was observed from the onset of treatment and reduction in final body weights was 6% in males and 11% in females. Significant increases in the absolute and/or relative weights of the lungs, heart, spleen, liver, kidneys, and adrenals were observed in males and/or females treated with 300 ppm. Some of high incidences neoplasms were observed, and there were also tendencies to increase for mammary gland fibroma and thoracic/abdominal cavity mesothelioma in males. However, there were no significant inter-group differences in incidences, histopathological types or differences compared with historical control data. Thus, it was concluded that LV was not carcinogenic to male and female F344 rats under the experimental conditions.

Apigenin and naringenin suppress colon carcinogenesis through the aberrant crypt stage in azoxymethane-treated rats

Epidemiological evidence suggests that a diet abundant in fruits and vegetables may protect against colon cancer. Bioactive compounds, including flavonoids and limonoids, have been shown to possess antiproliferative and antitumorigenic effects in various cancer models. This experiment investigated the effects of four citrus flavonoids and one limonoid mixture at the promotion stage of chemically induced colon cancer in rats. Male Sprague-Dawley rats (n = 10 rats/group) were randomly allocated to one of six diets formulated to contain 0.1% apigenin, 0.02% naringenin, 0.1% hesperidin, 0.01% nobiletin, 0.035% limonin glucoside/obacunone glucoside mixture or a control diet (0% flavonoid/limonoid). Rats received experimental diets for 10 weeks and were injected with azoxymethane (15 mg/kg) at weeks 3 and 4. Excised colons were evaluated for aberrant crypt foci (ACF) formation, colonocyte proliferation (proliferating cell nuclear antigen assay), apoptosis (terminal deoxynucleotidyl transferase dUTP nick end labeling assay) and expression of inducible nitric oxide synthase (iNOS) and cyclooxygenase-2 (COX-2) (immunoblotting). When compared with the control diet, apigenin lowered the number of high multiplicity ACF (HMACF >4 aberrant crypts/focus) by 57% (P < 0.05), while naringenin lowered both the number of HMACF by 51% (P < 0.05) and the proliferative index by 32% (P < 0.05). Both apigenin and naringenin increased apoptosis of luminal surface colonocytes (78% and 97%, respectively; P < 0.05) when compared with the control diet. Hesperidin, nobiletin and the limonin glucoside/obacunone glucoside mixture did not affect these variables. The colonic mucosal protein levels of iNOS or COX-2 were not different among the six diet groups. The ability of dietary apigenin and naringenin to reduce HMACF, lower proliferation (naringenin only) and increase apoptosis may contribute toward colon cancer prevention. However, these effects were not due to mitigation of iNOS and COX-2 protein levels at the ACF stage of colon cancer.

Hepatoid carcinoma of the skin: spontaneous rat skin hepatoid carcinoma with eosinophilic globules and crystals immunoreactive to alpha-1-antitrypsin

We present a case of hepatoid carcinoma of the abdominal skin in a male Wistar rat. Histopathologically, this carcinoma resembled human hepatocellular carcinoma with respect to trabecular-sinusoidal structures. Carcinoma tissues contain numerous eosinophilic globules and crystals, and in this case, we found the characteristic eosinophilic globules in the hepatoid carcinoma cells and the crystals in the extracellular portions. Vivid carcinoma cells full of eosinophilic globules were present near the necrotic areas in tumor tissue, wherein quadrate crystals unstained with eosin were observed. PAS staining after diastase digestion revealed that the globules were PAS positive and diastase resistant. In addition, we found that the hepatoid carcinoma cells were immunoreactive for alpha-1-antitrypsin (anti-A1AT) antibody with the globules and crystals staining peripherally, and a central unstained region. Ultrastructural study of intracytoplasmic globules and extracellular crystals revealed that the fringe of each globule and crystal had no limiting membrane and showed the same level of electron density. These findings suggest that the characteristic crystals in this tumor may have originated from the globules that were emitted from the carcinoma cells after their death as a result of saturation with intracytoplasmic globules.

Testicular cancer and hormonally active agents

Testicular cancer (TC) is a rare form of cancer, accounting for 1% of all new cancer cases in Canadian males. TC is the most common malignancy among young men, aged 25-34 yr old. Over previous decades, the incidence of TC has increased in many Western countries. Countries with a sufficiently long period of cancer registration, such as Denmark, document this trend back to the first half of the 20th century. The etiology of TC remains poorly understood. Most of the established risk factors are likely related to in utero events, including some factors that are purported to be surrogate measures for exposure to endogenous estrogens. The correlation of TC with other testicular abnormalities and with pregnancy factors led to the proposal that these conditions are a constellation of sequelae of impairment of testicular development called testis dysgenesis syndrome. There is some limited evidence suggesting that exposure to pharmacological estrogens may contribute to some cases of TC. There is currently no compelling evidence that exposure to environmental estrogenic or other hormonally active substances is contributing to the rise in TC incidence observed in Western nations over the last several decades; however, this question has not been extensively studied. The (1) rarity of this condition in the population, (2) long lag time between the presumed sensitive period during fetal development and clinical appearance of the condition, and (3) lack of a good animal model to study the progression of the disease have greatly hindered the understanding of environmental influences on TC risk.

Cancer in rodents: does it tell us about cancer in humans?

Information obtained from animal models (mostly mice and rats) has contributed substantially to the development of treatments for human cancers. However, important interspecies differences have to be taken into account when considering the mechanisms of cancer development and extrapolating the results from mice to humans. Comparative studies of cancer in humans and animal models mostly focus on genetic factors. This review discusses the bio-epidemiological aspects of cancer manifestation in humans and rodents that have been underrepresented in the literature.