Toxicity studies of acetoin and 2,3-pentanedione administered by inhalation to Wistar Han [Crl:WI(Han)] rats and B6C3F1/N mice

Acetoin and 2,3-pentanedione are highly volatile components of artificial butter flavoring (ABF). Concerns over the inhalation toxicity of these compounds originate from the association between occupational exposures to ABF and adverse fibrotic lung effects, specifically obliterative bronchiolitis (OB) in the distal airways. 2,3-Pentanedione has been used as a replacement for 2,3-butanedione (diacetyl) in some ABF due to concerns about the respiratory toxicity of 2,3-butanedione. However, 2,3-pentanedione is structurally similar to 2,3-butanedione and has been shown to exhibit potency similar to 2,3-butanedione regarding airway toxicity following acute inhalation (whole-body) exposure. This report describes a series of studies to evaluate the 2-week inhalation toxicity of acetoin and the 3-month inhalation toxicity of acetoin and 2,3-pentanedione. (Abstract Abridged).

Toxicity studies of sodium metavanadate and vanadyl sulfate administered in drinking water to Sprague Dawley (Hsd:Sprague Dawley SD) rats and B6C3F1/N mice

Oral human exposure to vanadium may occur due to its presence in food and drinking water and its use in dietary supplements. The most prevalent oxidation states of vanadium in food and drinking water have been characterized as tetravalent and pentavalent. Vanadyl sulfate and sodium metavanadate were selected as representative tetravalent (V4+) and pentavalent (V5+) test articles for these studies, respectively. To assess the potential for oral toxicity of vanadium compounds with differing oxidation states under similar test conditions, the 3-month National Toxicology Program (NTP) toxicity studies of sodium metavanadate and vanadyl sulfate were conducted in male and female Sprague Dawley (Hsd:Sprague Dawley SD) rats (including perinatal exposure) and in B6C3F1/N mice. Drinking water concentrations for sodium metavanadate (0, 31.3, 62.5, 125, 250, and 500 mg/L) and vanadyl sulfate (0, 21.0, 41.9, 83.8, 168, and 335 mg/L) were selected on the basis of previously published 14-day drinking water studies conducted as part of the NTP vanadium research program. (Abstract Abridged).

Toxicology and carcinogenesis studies of di(2-ethylhexyl) phthalate administered in feed to Sprague Dawley (Hsd:Sprague Dawley SD) rats

Di(2-ethylhexyl) phthalate (DEHP) is a member of the phthalate ester chemical class that occurs commonly in the environment and to which humans are widely exposed. Lifetime exposure to DEHP is likely to occur, including during the in utero and early postnatal windows of development. To date, no carcinogenicity assessments of DEHP have used a lifetime exposure paradigm that includes the perinatal period (gestation and lactation). The National Toxicology Program (NTP) tested the hypothesis that exposure during the perinatal period would alter the DEHP carcinogenic response quantitatively (more neoplasms) or qualitatively (different neoplasm types). Two chronic carcinogenicity assessments of DEHP were conducted in which Sprague Dawley (Hsd:Sprague Dawley SD) rats were exposed to dosed feed containing 0, 300, 1,000, 3,000, or 10,000 ppm DEHP for 2 years using different exposure paradigms. In Study 1, groups of 45 F0 time-mated females were provided dosed feed beginning on gestation day (GD) 6 through lactation. On postnatal day (PND) 21, groups of 50 F1 rats per sex continued on the study and were provided dosed feed containing the same DEHP concentration as their respective dam for 2 years. In Study 2, groups of 50 rats per sex, aged 6 to 7 weeks at study start, were provided dosed feed containing DEHP for 2 years. (Abstract Abridged).

Toxicity studies of trimethylsilyldiazomethane administered by nose-only inhalation to Sprague Dawley (Hsd:Sprague Dawley SD) rats and B6C3F1/N mice

Trimethylsilyldiazomethane (TMSD) is a methylating reagent widely used in organic chemistry. TMSD is structurally related to the compound diazomethane, which is a known lethal respiratory toxicant in humans and in animal models. TMSD is less reactive (with lower explosive potential) than diazomethane and is considered a safer, less toxic alternative. Few toxicity data are available to support this claim, however, and TMSD is readily available commercially from chemical suppliers. Concern over the inhalation toxicity of TMSD originates from reports of the death of two chemists resulting from lung injury and acute respiratory distress syndrome following exposure to TMSD in the workplace. Other concerns include the known inhalation toxicity of diazomethane and the absence of inhalation toxicity data for TMSD. The National Toxicology Program (NTP) conducted this study to evaluate the acute inhalation toxicity of TMSD in vivo.(Abstract Abridged).

Toxicology and carcinogenesis studies of perfluorooctanoic acid administered in feed to Sprague Dawley (Hsd:Sprague Dawley SD) rats (revised)

Perfluorooctanoic acid (PFOA) is a perfluorinated alkyl substance (PFAS) with widespread exposure in the environment and human population. Lifetime exposure to this chemical is likely, which includes in utero and postnatal development. Previously conducted chronic carcinogenicity studies of PFOA began exposure after these critical periods of development, so it is unknown whether the carcinogenic response is altered if exposure during gestation and lactation is included. The current PFOA chronic studies were designed to assess the contribution of combined gestational and lactational exposure (herein referred to as perinatal exposure) to the chronic toxicity and carcinogenicity of PFOA. The hypothesis tested was that including exposure during gestation and lactation (perinatal exposure) with postweaning exposure would change the PFOA carcinogenic response quantitatively (more neoplasms) or qualitatively (different neoplasm types) compared to postweaning exposure alone. (Abstract Abridged).

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.

Aspartame administered in feed, beginning prenatally through life span, induces cancers of the liver and lung in male Swiss mice

BACKGROUND

Aspartame (APM) is a well-known intense artificial sweetener used in more than 6,000 products. Among the major users of aspartame are children and women of childbearing age. In previous lifespan experiments conducted on Sprague-Dawley rats we have shown that APM is a carcinogenic agent in multiple sites and that its effects are increased when exposure starts from prenatal life.

OBJECTIVE

The aim of this study is to evaluate the potential of APM to induce carcinogenic effects in mice.

METHODS

Six groups of 62-122 male and female Swiss mice were treated with APM in feed at doses of 32,000, 16,000, 8,000, 2,000, or 0  ppm from prenatal life (12 days of gestation) until death. At death each animal underwent complete necropsy and all tissues and organs of all animals in the experiment were microscopically examined.

RESULTS

APM in our experimental conditions induces in males a significant dose-related increased incidence of hepatocellular carcinomas (P < 0.01), and a significant increase at the dose levels of 32,000  ppm (P < 0.01) and 16,000  ppm (P < 0.05). Moreover, the results show a significant dose-related increased incidence of alveolar/bronchiolar carcinomas in males (P < 0.05), and a significant increase at 32,000  ppm (P < 0.05).

CONCLUSIONS

The results of the present study confirm that APM is a carcinogenic agent in multiple sites in rodents, and that this effect is induced in two species, rats (males and females) and mice (males). No carcinogenic effects were observed in female mice. Am. J. Ind. Med. 53:1197-1206, 2010. © 2010 Wiley-Liss, Inc.

Alternative mouse models for carcinogenicity assessment: industry use and issues with pathology interpretation

The Carcinogenicity Alternative Mouse Models (CAMM) Working Group of the Society of Toxicologic Pathology (STP) surveyed the membership to define current practices and opinions in industry regarding the use of alternative mouse models for carcinogenicity testing. The results of the survey indicated that CAMM are used most often to fulfill a regulatory requirement (e.g., to replace the two-year mouse bioassay) and are being accepted by regulatory agencies. Alternative models are also sometimes used for internal decision making or to address a mechanistic question. The CAMM most commonly used are the p53+/- and rasH2. The rasH2 appears to be the currently accepted model for general carcinogenicity testing. Problems with study interpretation included lack of historic background data, unexpected tumor finding, and tumor identification/characterization of early lesions. Problems with implementation or conduct of the study included extent of the pathology evaluation, numbers of animals, survival, and study duration. Recommendations were developed for, frequency and type of positive control testing, extent of histopathologic examination of test article-treated and positive control animals, current use and future development of diagnostic criteria; increased availability and use of historic data, and use of other genetically modified mice in carcinogenicity testing.

Sensitivity analysis of biologically motivated model for formaldehyde-induced respiratory cancer in humans

Conolly et al. (2003, 2004) developed biologically motivated models of formaldehyde carcinogenicity in F344 rats and humans based on a two-stage clonal expansion model of cancer. Based on the human model, Conolly et al. (2004) claimed that cancer risks associated with inhaled formaldehyde are deminimis at relevant human exposure levels. However, they did not conduct a sensitivity analysis to evaluate the robustness of this conclusion. Here, we present a limited sensitivity analysis of the formaldehyde human model. We show that when the control animals from the National Toxicology Program (NTP) studies are replaced with control animals only from NTP inhalation studies, estimates of human risk are increased by 50-fold. When only concurrent control rats are used, the model does not provide any upper bound (UB) to human risk. No data went into the model on the effect of formaldehyde on the division rates and death rates of initiated cells. We show that slight numerical perturbations to the Conolly et al. assumptions regarding these rates can be made that are equally consistent with the underlying data used to construct the model, but produce estimates of human risk ranging anywhere from negative up to 10,000 times higher than those deemed by Conolly et al. to be 'conservative'. Thus, we conclude that estimates of human risk by Conolly et al. (2004) are extremely sensitive to modeling assumptions. This calls into question the basis for the Conolly et al. claim of de minimis human risk and suggests caution in using the model to derive human exposure standards for formaldehyde.

Life-span exposure to low doses of aspartame beginning during prenatal life increases cancer effects in rats

BACKGROUND

In a previous study conducted at the Cesare Maltoni Cancer Research Center of the European Ramazzini Foundation (CMCRC/ERF), we demonstrated for the first time that aspartame (APM) is a multipotent carcinogenic agent when various doses are administered with feed to Sprague-Dawley rats from 8 weeks of age throughout the life span.

OBJECTIVE

The aim of this second study is to better quantify the carcinogenic risk of APM, beginning treatment during fetal life.

METHODS

We studied groups of 70-95 male and female Sprague-Dawley rats administered APM (2,000, 400, or 0 ppm) with feed from the 12th day of fetal life until natural death.

RESULTS

Our results show a) a significant dose-related increase of malignant tumor-bearing animals in males (p < 0.01), particularly in the group treated with 2,000 ppm APM (p < 0.01); b) a significant increase in incidence of lymphomas/leukemias in males treated with 2,000 ppm (p < 0.05) and a significant dose-related increase in incidence of lymphomas/leukemias in females (p < 0.01), particularly in the 2,000-ppm group (p < 0.01); and c) a significant dose-related increase in incidence of mammary cancer in females (p < 0.05), particularly in the 2,000-ppm group (p < 0.05).

CONCLUSIONS

The results of this carcinogenicity bioassay confirm and reinforce the first experimental demonstration of APM's multipotential carcinogenicity at a dose level close to the acceptable daily intake for humans. Furthermore, the study demonstrates that when life-span exposure to APM begins during fetal life, its carcinogenic effects are increased.

Characteristics of the spectrum of proliferative lesions observed in the kidney and urinary bladder of Fischer 344 rats and B6C3F1 mice

Many rodent renal and bladder carcinogens rely upon epigenetic mechanisms of carcinogenesis; such mechanisms are likely to influence the spectrum of urinary tract tumors observed in control and treated animals. This is reflected in several features of chemically induced rodent urinary tract neoplasms, including a low overall tumor incidence, an increased prevalence of urinary tract tumors in rats compared to mice and males compared to females, the tendency for epithelial tumors to predominate over nonepithelial types, and demonstrated links to chronic progressive nephropathy and urolithiasis. Such tendencies are also characteristic of spontaneous urinary tract tumors in rodents. Data to support these observations can be derived from large historical databases such as the Toxicology Data Management System, maintained by National Toxicology Program.

Quality review procedures necessary for rodent pathology databases and toxicogenomic studies: the National Toxicology Program experience

Accuracy of the pathology data is crucial since rodent studies often provide critical data used for setting human chemical exposure standards. Diagnoses represent a judgment on the expected biological behavior of a lesion and peer review can improve diagnostic accuracy and consistency. With the conduct of 500 2-year rodent studies, the National Toxicology Program (NTP) has refined its process for comprehensive review of the pathology data and diagnoses. We have found that careful judgment can improve and simplify the review, whereas simply applying a set review procedure may not assure study quality. The use of reviewing pathologists and pathology peer review groups is a very effective procedure to increase study quality with minimal time and cost. New genomic technology to assess differential gene expression is being used to predict morphological phenotypes such as necrosis, hyperplasia, and neoplasia. The challenge for pathologists is to provide uniform pathology phenotypes that can be correlated with the gene expression changes. The lessons learned in assuring data quality in standard rodent studies also applies to the emerging field of toxicogenomics.

Impact of Helicobacter hepaticus infection in B6C3F1 mice from twelve National Toxicology Program two-year carcinogenesis studies

Male and female B6C3F1 mice from 12 National Toxicology Program (NTP) 2-yr carcinogenesis studies were found to be infected with Helicobacter hepaticus. Many of the male mice from 9 of these studies had an associated hepatitis (affected studies). Helicobacter hepaticus has been reported to be associated with an increased incidence of hepatitis and hepatocellular neoplasms in the A/JCr male mouse. We attempted to determine if the data from the Helicobacter-affected NTP B6C3F1 mouse studies were compromised and unsuitable for cancer hazard identification. The incidences of neoplasms of the liver (both hepatocellular and hemangiosarcoma) but not of other organs in control male B6C3F1 mice were increased in affected studies as compared with control males from unaffected studies. The increased incidence of hepatocellular neoplasms was observed in those males exhibiting H. hepaticus-associated hepatitis. Other observations further differentiated control male mice from affected and unaffected studies. H-ras codon 61 CAA to AAA mutations were less common in liver neoplasms from males from affected studies as compared with historical and study controls. In addition, increases in cell proliferation rates and apoptosis were observed in the livers of male mice with H. hepaticus-associated hepatitis. These data support the hypothesis that the increased incidence of liver neoplasms is associated with H. hepaticus and that hepatitis may be important in the pathogenesis. Therefore, interpretation of carcinogenic effects in the liver of B6C3F1 mice may be confounded if there is H. hepaticus-associated hepatitis.

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

Spontaneous neoplasm rates were determined for control Fischer 344 (F344) rats and B6C3F1 mice from 2-yr rodent carcinogenicity studies carried out by the National Toxicology Program (NTP). The most frequently occurring neoplasms in untreated male F344 rats were testicular adenoma (89.1%), mononuclear cell leukemia (50.5%), adrenal gland pheochromocytoma (31.9%), and pituitary gland neoplasms (30.4%). For untreated female F344 rats, the most frequently occurring neoplasms were pituitary gland neoplasms (54.2%), mammary gland fibroadenoma (41.2%), and mononuclear cell leukemia (28.1%). The most frequently occurring neoplasms in untreated male B6C3F1 mice were liver adenoma/carcinoma (42.2%), lung adenoma/carcinoma (20.5%), and malignant lymphoma (8.3%). For untreated female B6C3F1 mice, the most frequently occurring neoplasms were liver adenoma/carcinoma (23.6%), malignant lymphoma (20.9%), and pituitary gland adenoma/carcinoma (14.8%). The tumor rates observed in feeding study (untreated) and inhalation study (chamber) control rats were generally similar. The major exceptions were pituitary gland tumors and testicular adenoma in male F344 rats. The overall incidence of testicular adenoma was much lower in chamber controls (69.4%) than in feeding study controls (89.1%), whereas pituitary gland neoplasm showed the opposite trend (60.7% vs 30.4%). The most likely explanation for this difference is related to the individual housing of chamber controls and the group housing of feeding study controls. Differences in diagnostic criteria may influence reported tumor rates. To ensure consistency and comparability of tumor diagnosis from study to study, the NTP uses rigorous histopathology quality assurance and peer review procedures. Biological factors such as body weight may also affect tumor incidence. For example, increased body weights are associated with increased incidences of certain site-specific neoplasms, especially pituitary gland and mammary gland neoplasms in rats and liver tumors in mice. The presence of Helicobacter hepaticus has been associated with an increased incidence of liver neoplasms in male B6C3F1 mice. Other factors that may produce differences in control tumor rates from study to study include diet, environmental factors, genetic drift, study duration, and survival differences. The NTP database provides historical control data that may be useful in the evaluation of possible chemically related changes in tumor incidence. However, it is essential that the study being evaluated be comparable to those in the NTP database with respect to those factors that are known to influence tumor occurrence.

Effect of dietary optimization on growth, survival, tumor incidences and clinical pathology parameters in CD Sprague-Dawley and Fischer-344 rats: a 104-week study

Controversy regarding the use of ad libitum feeding in chronic rodent toxicity studies will soon result in issue of a FDA Points to Consider document. Caloric intakes are now recognized to be important uncontrolled variables in bioassays because rodents chronically fed ad libitum become obese, reproductively senile and have increased incidences of age-related diseases, higher tumor burdens and decreased survival. The available literature suggests that ad libitum feeding neither optimizes the health and well-being of rodents nor provides the best model for use in evaluation of pharmacological and toxicological profiles. Use of an optimized diet, restricted in terms of caloric intakes, has been proposed for chronic toxicity and carcinogenicity studies in rodents. It is suggested that limiting caloric intakes to 50-80% of ad libitum consumption would result in lower body weights, decreased tumor incidences and prolonged survival in the controls. To evaluate the influence of diet on chronic toxicity and carcinogenicity studies in rats, two 104-week studies were conducted. These studies consisted of 280 CD Sprague-Dawley and 280 Fischer-344 rats fed ad libitum, and 140 CD Sprague-Dawley and 140 Fischer-344 rats fed a diet that was optimized by limiting caloric intakes by 15-35%. Both diets consisted of certified commercial diet in meal form. The optimized diet reduced weight gain approximately 50% after 100 weeks. Clinical chemistry and hematology parameters showed negligible effects of reduced diet, with the exception that serum triglycerides were lower in males and females in both strains at weeks 52 and 104. The ad libitum-fed animals had a higher incidence of pseudopregnancy, aggressiveness, foot sores and abscesses than the animals fed an optimized diet. These effects were more pronounced in the CD Sprague-Dawley rats than in the Fischer-344 rats. At the completion of the 104-week study, survival in the ad libitum fed CD Sprague-Dawley rats was approximately one-half that of the animals fed an optimized diet (39% versus 76%). The difference in survival between Fischer-344 rats fed ad libitum and those fed an optimized diet was less pronounced (78% versus 89%). A reduced incidence of palpable tissue masses in the ad libitum-fed CD Sprague-Dawley rats versus the animals fed an optimized diet reflected inability to detect small masses in the obese ad libitum-fed animals. In contrast, the leaner Fischer-344 ad libitum-fed animals had an increased incidence of palpable tissue masses. After 52 weeks, 40 animals from each strain and feeding regimen were killed and subjected to complete necropsy and histopathological examination; the remainder of the survivors was examined at the completion of the study (104 weeks). Use of an optimized diet substantially reduced the incidences of endocrine-mediated tumors in both rat strains and delayed the onset of leukemia in Fischer-344 rats. These results indicate the need to further investigate the relationship of increased caloric intakes and endocrine-mediated or strain specific tumors and support FDA's and others' positions that use of diet optimization in chronic toxicity and carcinogenicity rodent bioassays has the potential to remarkably improve the scientific quality and relevance of these studies. It also identified that the small increases in cost associated with diet optimization are far exceeded by the advantages of increased survival of animals, reduced intercurrent disease and rumor burdens, and increased ease of histopathological processing and evaluation.

Evaluation of false positive and false negative outcomes in NTP long-term rodent carcinogenicity studies

The decision-making process used by the National Toxicology Program (NTP) in its evaluation of long-term rodent carcinogenicity studies was investigated to determine whether or not this procedure resulted in an excessive number of false positive or false negative outcomes. All site-specific tumor incidences that were found to be significantly (p < 0.05) increased either by a trend test or by pairwise comparisons of each dosed group against the controls in 218 NTP 2-year studies with Fischer 344 rats and/or B6C3F1 mice were tabulated and compared to the number of statistically significant tumor increases expected to occur by chance. Our evaluation suggests that false positive rates are fairly low in NTP long-term studies. Assessing false negative rates is more difficult because of the limited sensitivity of the bioassay for detecting subtle carcinogenic effects. Moreover, reduced body weights frequently occur in dosed animals, and the positive correlation between the incidences of certain site-specific tumors and body weight may mask the detection of carcinogenic effects. Despite these difficulties, our analysis did identify one tumor showing evidence of false negative outcomes: interstitial cell tumors of the testis in male Fischer 344 (F344) rats. This tumor showed considerably more significant (p < 0.05) increased incidences than expected by chance, yet none were considered to be chemically-related. However, the biological significance of interstitial cell tumor increases in F344 rats is uncertain because of the high background rate of neoplasia (> 90%) for this target site.

Rationale for the use of dietary control in toxicity studies--B6C3F1 mouse

Significant variability in critical study parameters such as tumor incidences and survival, increasing tumor incidence and decreasing survival in common toxicity test models, and agent-induced changes in body weight (BW) and BW distribution all generate concern about the reproducibility, consistency, and equity of chronic toxicity tests used in regulation. These concerns have led to suggestions to control BW in chronic tests by the modulation of dietary intake without inducing malnutrition [dietary control (DC)] thereby minimizing tumor and survival variability both between and within studies. Evaluating the reports of the best controlled set of chronic experiments, the National Toxicology Program bioassay series, from studies initiated from 1981 to 1990, there is an increase in tumor incidence, especially liver tumors, with a consistent increase in BW. The studies are classified as to whether normal or aberrant BW growth curves occur. When the studies with normal growth curves are considered, the variance in the BW at 12 mo on test (BW12) can account for over 50% of the variance in liver tumor incidence. Additional stratification by study type, which alter tumor prevalences, as well as appreciation of housing effects [group housing decreases survival (in male mice) and induces tumors in males and females when compared to individual housing], further increase the strength of the correlations, accounting for up to 90% of the variance seen in tumor incidences. These updated analyses further support the hypothesis that it is the BW variation that is resulting in much of the variability seen in tumor incidences and refine the suggestions for the BW curves used as the desired targets for DC.

FDA points-to-consider documents: the need for dietary control for the reduction of experimental variability within animal assays and the use of dietary restriction to achieve dietary control

Standard protocols for conducting chronic toxicity and carcinogenicity studies have been refined over the years to carefully control for many variables. Nevertheless, over the last 2 decades, there has been a steady increase in variability, a decrease in survival, an increase in tumor incidence rates, and an increase in the average body weight of control animals among the various rodent species and strains used for toxicity testing. These observations have prompted an evaluation of chronic study designs to determine what factor(s) may be responsible for such confounding changes. Ad libitum feeding and the selection of successful breeders with rapid offspring growth is believed to be at least partially responsible for the heavier, obese rodents with which many laboratories are coping today. As a result of these changes, some studies used for the evaluation of safety have been deemed inconclusive or inadequate for regulatory purposes and either additional supportive studies have been requested and/or studies per se have been repeated. Research on the molecular mechanisms of caloric restriction and agent-induced toxicity at the Food and Drug Administration (FDA) National Center for Toxicological Research stimulated the first international conference on the biological effects of dietary restriction in 1989; this was followed in 1993 by an FDA workshop exploring the utility of dietary restriction in controlling reduced survival in chronic tests and an international conference in 1994 exploring the implications for the regulatory community of using dietary restriction in toxicity and carcinogenicity studies used in support of a sponsor's submission or in risk assessments. The outcome of that conference was the FDA's commitment to develop Points-to-Consider documents that address the issue of dietary control in chronic toxicity and carcinogenicity studies.

Analysis of National Toxicology Program rodent bioassay data for anticarcinogenic effects

We reanalyzed data from 218 two-year rodent carcinogenicity studies carried out by the National Toxicology Program (NTP). These data were originally collected for the purpose of identifying potential human carcinogens. However, the objective of our analysis was to investigate the frequency of possible anticarcinogenic effects in these data, since recurring cases of chemical-associated tumor reductions have been noted in the course of these studies over time. Our analysis reveals that most (>90%) NTP-tested chemicals show at least one statistically significant (p<0.05) decrease in site-specific tumor incidence. Because of the large number of statistical comparisons made in a long-term bioassay, random variability can account for many of these tumor decreases. However, we found that certain tumors (predominantly those with a high spontaneous incidence) show chemically related decreases far more frequently than chance expectation. Many of these decreases, particularly those for pituitary and mammary gland tumors, adrenal pheochromocytoma and uterine polyps in rats and liver and lung tumors in mice, are associated with the reduced body weights frequently observed in the dosed groups. The chemically related decreased incidences of leukemia in rats appear to be related to spleen damage, i.e., chemically related splenic toxicity is evident for most chemicals showing decreased incidences of leukemia. While random variability, associations with body weight and splenic toxicity can account for most of the decreased tumor incidences observed in NTP studies, there are other tumor decreases that could not be totally explained by these factors. Further investigations of possible mechanisms of action are underway. These data are relevant to the concept of chemoprevention as well as to the task of using long-term laboratory animal studies to predict enhanced human environmental-cancer risk for regulatory purposes.

Rodent carcinogenicity bioassay: past, present, and future

Toxicity/carcinogenicity studies in rodents have played a pivotal role in identifying chemicals that are potentially hazardous to humans. In fact, nearly all of the known human carcinogens are also carcinogenic in 1 or more rodent species. During the past 20 yr the quality and consistency of rodent studies has improved considerably, and much has been learned about mechanisms whereby chemicals initiate or promote the carcinogenic process in rats and mice. The process of identifying chemicals that cause toxicity or carcinogenicity in rodents is quite well established, but the procedures for extrapolating this data for risk management decisions in the protection of human health have lagged far behind. While many would accept the assumptions that genotoxic chemicals that cause cancer in animals pose a cancer risk to humans and that genotoxic chemicals causing cancer at high doses pose a risk at lower doses, there is much less certainty with respect to nongenotoxic chemicals. The confusion about risk extrapolation for nongenotoxic chemicals has often lead to criticism of the hazard identification process for chemicals in general. There is increasing awareness of the complexity of the carcinogenic process that has made species extrapolation and dose extrapolation from rodent studies to humans more complex. Although newer molecular biological techniques and cell kinetic measurements offer exciting possibilities for better risk assessment, it is the combination of well-designed rodent studies with appropriate mechanistic studies that offers the best hope for regulatory decisions based on sound scientific principles.