Brain tumours--problems and perspectives

Differences reported in the lifespan and aging of male Wistar rats maintained on diets containing fat with different fatty acid profiles (virgin olive, sunflower or fish oils) are not reflected by histopathological lesions found at death in central nervous and endocrine systems

The present study was designed to examine if dietary fat sources that have shown differences in lifespan and if some aging-related aspects can modulate the range of histopathologic changes in central nervous and endocrine systems that occur during the lifespan of Wistar rats. Moreover, it was attempted to gain insight into the relationship between longevity and the development of the different pathological changes, as well as possible interaction with diet. In order to achieve this, male Wistar rats were randomly assigned to three experimental groups fed semisynthetic and isoenergetic diets from weaning until death with different dietary fat sources, namely virgin olive, sunflower, or fish oil. An individual follow-up until death of each animal was performed. Incidence, severity, and burden of specific or group (i.e., neoplastic or non-neoplastic proliferative and non-proliferative) of lesions was calculated along with individual's disease and individual organ lesion burden. Most of the histopathological lesions found have been described in previous studies. Neoplasms, and in particular pituitary adenomas followed by brain tumors, were the most prevalent lesions found in the rats and the main cause of death involving both systems. Incidence of brain lesions was associated with age-at-death. Assayed dietary fats did not present differential effects on pathological changes occurring in endocrine and central nervous systems throughout rat lifespan.

Central nervous system tumors in 2-year rat carcinogenicity studies: perspectives on human risk assessment

Rodent in vivo carcinogenicity bioassays are required for human risk assessment and have been utilized in this capacity for decades. Accordingly, there is an abundance of data that could be accessed and analyzed to better understand the translatability of xenobiotic-induced rodent tumors to human risk assessment. In the past decade, various groups have published assessments of the value garnered by these life-time rodent studies. Results and recommendations from the International Council for Harmonization Expert Working Group (ICH-S1 EWG) on the predictability of the current testing paradigm and proposal for an integrated approach to human carcinogenicity risk assessment are pending. Central nervous system (CNS) tumors in rats are rare and translatability to human remains unknown. This review focuses on microglial cell tumors (MCT) of the CNS in rats including its classification, nomenclature, incidence and translatability to human risk assessment. Based on emerging immunohistochemistry (IHC) characterization, glial tumors previously thought of astrocytic origin are more likely MCTs. These may be considered rodent specific and glucose dysregulation may be one component contributing to their formation. Based on review of the literature, MCTs are rarely diagnosed in humans, thus this tumor type may be rat-specific. We propose to include MCTs as a tumor type in revised International Harmonization of Nomenclature and Diagnostic Criteria (INHAND) classification and all glial tumors to be classified as MCTs unless proven otherwise by IHC.

A Comprehensive, Integrated Review and Evaluation of the Scientific Evidence Relating to the Safety of the Herbicide 2,4-D

The safety of 2,4-D to farm and forestry workers, commercial applicators and the general public has been of continuing concern because certain epidemiological studies of groups potentially exposed to 2,4-D have suggested a relationship between 2,4-D use and increased risk of soft tissue sarcoma, Hodgkin's disease or non-Hodgkin's lymphoma. This review on 2,4-D is unique in that the approach taken was to integrate data from worker exposure studies, whole animals, metabolic and other relevant laboratory studies with the epidemiological findings to assess the extent to which there is scientific support for the hypothesis that 2,4-D exposure is associated with any increased risk of human cancer.

The case-control epidemiological studies that have been the source of the cancer risk hypothesis are inconclusive. Problems in assessing exposure based on patients' memories make these studies difficult to interpret. Cohort studies of exposed workers do not generally support the specific hypothesis that 2,4-D causes cancer. Taken together, the epidemiological studies provide, at best, only weak evidence of an association between 2,4-D and the risk of cancer.

Importantly, the cancer hypothesis is not supported by other data. A critical evaluation of the exposure data indicates that exposure to 2,4-D in user groups is intermittent and much lower than the doses tested chronically in long-term animal studies that have not shown evidence of tumor induction. Moreover, the structure of 2,4-D does not suggest it would be a carcinogen. 2,4-D is a simple organic acid, that is largely excreted unchanged, and there is no evidence that it is metabolized to critically reactive metabolites or accumulates in tissues. This evidence is supported by a large body of negative studies on genotoxicity, which taken together with the metabolic studies, clearly indicates that 2,4-D is highly unlikely to be a genotoxic carcinogen. Furthermore, 2,4-D has no known hormonal activity and does not induce proliferative changes in any tissue or organ, indicating that it does not possess any of the characteristics of non-genotoxic animal carcinogens. Thus the available mechanistic studies provide no plausible basis for a hypothesis of carcinogenicity.

In this review, data relating to potential neurotoxicity, immunotoxicity and reproductive toxicity also were evaluated. There is no evidence that 2,4-D adversely affects the immune system and neurotoxic and reproductive effects only have been associated with high toxic doses that would not be encountered by 2,4-D users.

Historical exposures to 2,4-D by user groups, particularly farmers, forestry workers and commercial applicators, would be higher than those sustained under present rigorous standards for application which involve the use of protective clothing and other measures to reduce exposure. Proposed label changes indicate that in the future exposures will be even further reduced. Viewed in this context, the available data indicate that the potential public health impact of 2,4-D, including the risk of human cancer, was negligible in the past and would be expected to be even smaller in the present and future.

Immunohistochemical characterization of spontaneous and acrylonitrile-induced brain tumors in the rat

Twenty-eight spontaneously occurring glial tumors (previously diagnosed as astrocytomas, oligodendrogliomas, and gliomas) and eleven granular cell tumors (GCTs) were selected for evaluation using a panel of immunohistochemistry (IHC) stains (Ricinus communis agglutinin type 1 [RCA-1], ionized calcium-binding adapter molecule 1 [Iba-1], OX-6/major immunohistocompatibility complex class II, oligodendrocytes transcription factor 2 [Olig2], glial fibrillary acidic protein [GFAP], S100 beta, glutamine synthetase, neurofilament, proliferating cell nuclear antigen). In addition, nine brain tumors from a 2-year drinking water study for acrylonitrile were obtained from the Acrylonitrile Group, Inc. Based on IHC staining characteristics, Olig2+ oligodendrogliomas were the most commonly diagnosed spontaneous tumor in these animals. Many of the spontaneous tumors previously diagnosed as astrocytomas were RCA-1+, Iba-1+ and negative for GFAP, S100beta, and glutamine synthetase; the diagnosis of malignant microglial tumor is proposed for these neoplasms. Three mixed tumors were identified with Olig2+ (oligodendrocytes) and Iba-1+ (macrophage/microglia) cell populations. The term mixed glioma is not recommended for these tumors, as it is generally used to refer to oligoastrocytomas, which were not observed in this study. GCT were positive for RCA-1 and Iba-1. All acrylonitrile tumors were identified as malignant microglial tumors. These results may indicate that oligodendrogliomas are more common as spontaneous tumors, while acrylonitrile-induced neoplasms are microglial/histiocytic in origin. No astrocytomas (GFAP, S100 beta, and/or glutamine synthetase-positive neoplasms) were observed.

Aspartame: review of safety

Over 20 years have elapsed since aspartame was approved by regulatory agencies as a sweetener and flavor enhancer. The safety of aspartame and its metabolic constituents was established through extensive toxicology studies in laboratory animals, using much greater doses than people could possibly consume. Its safety was further confirmed through studies in several human subpopulations, including healthy infants, children, adolescents, and adults; obese individuals; diabetics; lactating women; and individuals heterozygous (PKUH) for the genetic disease phenylketonuria (PKU) who have a decreased ability to metabolize the essential amino acid, phenylalanine. Several scientific issues continued to be raised after approval, largely as a concern for theoretical toxicity from its metabolic components--the amino acids, aspartate and phenylalanine, and methanol--even though dietary exposure to these components is much greater than from aspartame. Nonetheless, additional research, including evaluations of possible associations between aspartame and headaches, seizures, behavior, cognition, and mood as well as allergic-type reactions and use by potentially sensitive subpopulations, has continued after approval. These findings are reviewed here. The safety testing of aspartame has gone well beyond that required to evaluate the safety of a food additive. When all the research on aspartame, including evaluations in both the premarketing and postmarketing periods, is examined as a whole, it is clear that aspartame is safe, and there are no unresolved questions regarding its safety under conditions of intended use.

The role of chemical, physical, or viral exposures and health factors in neurocarcinogenesis: implications for epidemiologic studies of brain tumors

This review highlights some future prospects and implications for epidemiologic research on the etiology of nervous system tumors. It reviews some points regarding physiology of the nervous system, in connection with mechanisms of neurocarcinogenesis, and experimental studies in animals. The results of epidemiologic studies are summarized in the light of the biological and experimental observations. The following aspects are particularly emphasized: (i) higher susceptibility of the developing nervous system to neurocarcinogenic agents (in the fetus and after birth); (ii) possible implications of knowledge about mechanisms of neurocarcinogenesis regarding crossing of the blood-brain barrier, activation of oncogenes and inactivation of anti-oncogenes, relationship between chemical structure and neurocarcinogenic action; (iii) necessity of further investigation concerning the occurrence of nitrosoureas and their precursors in the environment, and the potential role of nitroso compounds in the development of human brain tumors; (iv) lack of information about promoting or inhibiting neurocarcinogenic effects, and co-carcinogenesis--among others, interaction between X-irradiation and exposure to neurocarcinogenic nitrosoureas; (v) need for studying the potential neurocarcinogenic risk of polyomaviruses BKV, JCV, and SV40 to humans.

Spontaneous brain and spinal cord/nerve neoplasms in aged Sprague-Dawley rats

Primary malignant neoplasms of the brain and spinal cord occurred in 20/718 male (2.8%) and in 13/717 female (1.8%) Crl:CD Br strain Sprague-Dawley rats. Of 33 neoplasms, 30 were found in brain while 3 were in the spinal cord. In males and females, the most common brain neoplasm was astrocytoma (13 males, 9 females). Other neoplasms, granular cell tumor (1 male), mixed glioma (2 males, 1 female), reticulosis (1 male, 2 females), and oligodendroglioma (2 males), were especially uncommon. Spinal cord neoplasms included 2 schwannomas (1 male, 1 female) and an astrocytoma (1 male). The overall brain neoplasm incidence was similar for males (2.8%) compared to data compiled for this strain, and there was a 2-fold increase for females (1.8% vs 0.9%) compared to available incidence data.

Weight of the evidence on the human carcinogenicity of 2,4-D

The phenoxy herbicide 2,4-dichlorophenoxyacetic acid (2,4-D) is widely used to control the growth of weeds and broadleaf plants. We convened a panel of 13 scientists to weigh the evidence on the human carcinogenicity of 2,4-D. The panel based its findings on a review of the toxicological and epidemiological literature on 2,4-D and related phenoxy herbicides. The toxicological data do not provide a strong basis for predicting that 2,4-D is a human carcinogen. Although a cause-effect relationship is far from being established, the epidemiological evidence for an association between exposure to 2,4-D and non-Hodgkin's lymphoma is suggestive and requires further investigation. There is little evidence of an association between use of 2,4-D and soft-tissue sarcoma or Hodgkin's disease, and no evidence of an association between 2,4-D use and any other form of cancer. Scientists on the panel were asked to categorize 2,4-D as a "known," "probable," "possible," or "unlikely" carcinogen or as a noncarcinogen in humans. The predominant opinion among the panel members was that the weight of the evidence indicates that it is possible that exposure to 2,4-D can cause cancer in humans, although not all of the panelists believed the possibility was equally likely: one thought the possibility was strong, leaning toward probable, and five thought the possibility was remote, leaning toward unlikely. Two panelists believed it unlikely that 2,4-D can cause cancer in humans.

Review of lead toxicology relevant to the safety assessment of lead acetate as a hair colouring

The literature on lead toxicology has been critically reviewed to provide a safety assessment of lead acetate as a hair colouring. The main objectives were: (i) to determine the additional lead contribution from hair-colouring use to the total daily environmental lead intake; and (ii) to assess the toxicological significance of this additional contribution. The review also focuses attention on newer issues of concern over the effects of environmental lead on human health. Data available in animals and humans (including occupational exposure), mainly on lead acetate and other inorganic lead salts, have been presented and evaluated in respect of the following: absorption, distribution and excretion following ingestion; percutaneous absorption; carcinogenicity; genotoxicity; reproductive toxicity; neurological/behavioural status with particular reference to neuropsychological effects in children; and effects on other systems (e.g. cardiovascular). It is concluded that the absorption of lead from hair-colouring use represents about 0.5% of the lead absorption from the current average daily environmental lead intake. No convincing evidence could be found of any deleterious effect of current environmental lead levels on human health and thus the tiny contribution of lead acetate exposure from hair-colouring use can be regarded unequivocally as being toxicologically insignificant.

Two-year carcinogenicity study of 6-mercaptopurine in F344 rats

The carcinogenicity of 6-mercaptopurine (6-MP), an anticancer drug, was examined in F344 rats of both sexes, administered the chemical at dietary levels of 0 (control), 25 ppm or 50 ppm for 2 years. Many tumors developed in all groups including the control group, the organ distribution and histological types being similar to those reported for spontaneous lesions. In males, there was no significant increase in the incidence of any tumor in the treated groups over that in the control group. In females, however, positive trends were noted in the occurrence of C-cell tumors, pheochromocytomas, uterine adenocarcinomas and gliomas, and the incidences of C-cell tumors and pheochromocytomas in the 50 ppm group were significantly higher than the values in the respective control group. In addition, the total numbers of malignant tumors increased significantly in the female 50 ppm group. However, most of the tumors demonstrating increase are frequently observed spontaneous lesions in this strain of rats, and their incidences in the present female control group were lower than in our historical data. In addition, there were no significant differences in the incidences of preneoplastic changes and induction times for the above-listed tumors between the female control and the 50 ppm groups. These results thus indicated that while the carcinogenic potential of 6-MP can not be precluded, it can be only very weak or marginal, after continuous administration in the diet at the 50 ppm level for 2 years. The leukemogenic action of 6-MP was negative under the present experimental conditions.

What is the proper experimental unit for long-term rodent studies? An examination of the NTP benzyl acetate study

The National Toxicology Program (NTP) rat bioassay on benzyl acetate shows an apparent increase in acinar-cell adenomas of the pancreas in male rats [1]. The statistical significance of the apparent trend is examined using the animal, cage, or rack/shelf location as the statistical independent experimental (sampling) unit. Experimental, statistical, and empirical considerations indicate that, in this experiment, if there is a proper experimental unit, it is the shelf of animals on a rack rather than the individual animal or cage of animals. Using the shelf as the experimental unit, the apparent increase in acinar-cell adenomas is not statistically significant. There is strong evidence for pancreatic tumor rates to differ among shelves which implies that toxicologists should consider cage randomization.