Goitrogens and thyroid follicular cell neoplasia: evidence for a threshold process

Toxicity of Pexacerfont, a Corticotropin-Releasing Factor Type 1 Receptor Antagonist, in Rats and Dogs

Pexacerfont is a corticotropin-releasing factor subtype 1 receptor antagonist that was developed for the treatment of anxiety- and stress-related disorders. This report describes the results of repeat-dose oral toxicity studies in rats (3 and 6 months) and dogs (3 months and 1 year). Pexacerfont was well tolerated in all of these studies at exposures equal to or greater than areas under the curve in humans (clinical dose of 100 mg). Microscopic changes in the liver (hepatocellular hypertrophy), thyroid glands (hypertrophy/hyperplasia and adenomas of follicular cells), and pituitary (hypertrophy/hyperplasia and vacuolation of thyrotrophs) were only observed in rats and were considered adaptive changes in response to hepatic enzyme induction and subsequent alterations in serum thyroid hormone levels. Evidence for hepatic enzyme induction in dogs was limited to increased liver weights and reduced thyroxine (T4) levels. Mammary gland hyperplasia and altered female estrous cycling were only observed in rats, whereas adverse testicular effects (consistent with minimal to moderate degeneration of the germinal epithelium) were only noted following chronic dosing in dogs. The testicular effects were reversible changes with exposure margins of 8× at the no observed adverse effect level. It is not clear whether the changes in mammary gland, estrous cycling, and testes represent secondary hormonal changes due to perturbation of the hypothalamic–pituitary–adrenal axis or are off-target effects. In conclusion, the results of chronic toxicity studies in rats and dogs show that pexacerfont has an acceptable safety profile to support further clinical testing.

Anti-amoebic properties of carbonyl thiourea derivatives

Thiourea derivatives display a broad spectrum of applications in chemistry, various industries, medicines and various other fields. Recently, different thiourea derivatives have been synthesized and explored for their anti-microbial properties. In this study, four carbonyl thiourea derivatives were synthesized and characterized, and then further tested for their anti-amoebic properties on two potential pathogenic species of Acanthamoeba, namely A. castellanii (CCAP 1501/2A) and A. polyphaga (CCAP 1501/3A). The results indicate that these newly-synthesized thiourea derivatives are active against both Acanthamoeba species. The IC50 values obtained were in the range of 2.39-8.77 µg·mL⁻¹ (9.47-30.46 µM) for A. castellanii and 3.74-9.30 µg·mL⁻¹ (14.84-31.91 µM) for A. polyphaga. Observations on the amoeba morphology indicated that the compounds caused the reduction of the amoeba size, shortening of their acanthopodia structures, and gave no distinct vacuolar and nuclear structures in the amoeba cells. Meanwhile, fluorescence microscopic observation using acridine orange and propidium iodide (AOPI) staining revealed that the synthesized compounds induced compromised-membrane in the amoeba cells. The results of this study proved that these new carbonyl thiourea derivatives, especially compounds M1 and M2 provide potent cytotoxic properties toward pathogenic Acanthamoeba to suggest that they can be developed as new anti-amoebic agents for the treatment of Acanthamoeba keratitis.

Risk assessment of chemical carcinogens and thresholds

The controversial arguments about the existence of "thresholds" for carcinogens are discussed and some conclusions are drawn: (1) The meaning of "threshold" has changed considerably during the last decades. Initially, the discussion focused on the genotoxic properties of chemicals. In dose-response studies the endpoint was tumor incidence. Later, DNA adducts represented the biologically active target dose and whether saturation of metabolic activation could lead to non-linear relationships was tested as a hypothesis. (2) In a next step, the implications of the initiation-promotion model were studied. Carcinogens with tumor-initiating properties showed linear dose-response relationships at low doses without a definable threshold, whereas those with tumor-promoting properties showed non-linear characteristics compatible with the existence of a threshold. However, the results are difficult to transfer to the human situation, and many critical endpoints are subject to other risk factors so that a meaningful value cannot be given. (3) Eventually, it turned out that most carcinogens exhibit genotoxic as well as non-genotoxic properties, and toxicity may be equally important as genotoxicity. (4) In view of the discussion for more than 60 years about the existence of thresholds for carcinogens, it is suggested that the threshold approach not be used to establish acceptable risk limits. (5) Instead of calculating an acceptable risk from cancer risk data, the recommended method is to assess the incremental contribution of exposure to the background of avoidable and unavoidable exposures by using biomonitoring data from human individuals. Such data could help in risk management, in order to reach acceptable limits of exposures on the basis of the "as low as reasonably achievable" or "ALARA" principle.

Effects of larval-juvenile treatment with perchlorate and co-treatment with thyroxine on zebrafish sex ratios

The objective of this study was to determine the effect of larval-juvenile exposure to perchlorate, a thyroid hormone synthesis inhibitor, on the establishment of gonadal sex ratios in zebrafish. Zebrafish were exposed to untreated water or water containing perchlorate at 100 or 250 ppm for a period of 30 days starting at 3 days postfertilization (dpf). Recovery treatments consisted of a combination of perchlorate and exogenous thyroxine (T4; 10 nM). Thyroid histology was assessed at the end of the treatment period (33 dpf), and gonadal histology and sex ratios were determined in fish that were allowed an additional 10-day period of growth in untreated water. As expected, exposure to perchlorate caused changes in thyroid histology consistent with hypothyroidism and these effects were reversed by co-treatment with exogenous T4. Perchlorate did not affect fish survival but co-treatment with T4 induced higher mortality. However, relative to the corresponding perchlorate concentration, co-treatment with T4 caused increased mortality only at a perchlorate concentration of 100 ppm. Perchlorate alone or in the presence of T4 suppressed body length at 43 dpf relative to control values. Perchlorate exposure skewed the sex ratio toward female in a concentration-dependent manner, and co-treatment with T4 not only blocked the feminizing effect of perchlorate but also overcompensated by skewing the sex ratio towards male. Moreover, co-treatment with T4 advanced the onset of spermatogenesis in males. There was no clear association between sex ratios and larval survival or growth. We conclude that endogenous thyroid hormone plays a role in the establishment of gonadal sex phenotype during early development in zebrafish.

Induction of thyroid lesions in 14-week toxicity studies of 2 and 4-methylimidazole in Fischer 344/N rats and B6C3F1 mice

Fifteen-day and 14-week studies of 2-methylimidazole (2MI) and 4-methylimidazole (4MI) were conducted because of widespread human exposure via ingestion of food products containing the compounds and lack of toxicity data. Groups of five male and five female Fischer rats and B6C3F1 mice were administered 2MI by dosed feed at 0, 1,200, 3,300, or 10,000 ppm or 4MI at 0, 300, 800, or 2,500 ppm for 15 days, and groups of 10 male and 10 female Fischer rats and B6C3F1 mice were administered 2MI or 4MI at 0, 625, 1,250, 2,500, 5,000 or 10,000 ppm for 14 weeks. In the 15-day studies, 2MI induced thyroid follicular-cell hyperplasia and pituitary pars-distalis hypertrophy in rats and thyroid follicular-cell hypertrophy and spleen hematopoietic-cell proliferation in mice; 4MI induced no histopathological changes in rats and mice. In the 14-week studies, 2MI increased concentrations of thyroid-stimulating hormone (TSH) and decreased those of thyroxine (T(4)) and triiodothyroxine (T(3)) in male and female rats according to the dosage. Incidences of diffuse follicular-cell hyperplasia of the thyroid gland increased significantly in male rats exposed to 1,250 ppm or greater and female rats exposed to 2,500 ppm or greater. Thyroid follicular-cell adenoma was diagnosed in two males in the 10,000-ppm group. A dose-related anemia occurred in female rats. In mice, follicular-cell hypertrophy of the thyroid gland, anemia, splenic hematopoietic-cell proliferation, and hemosiderin in kidney tubules appeared. In rats, 4MI induced tremors and ataxia in the high-dose groups. Serum T(3), T(4), and TSH levels were not altered, and no thyroid lesions occurred. Anemia, hepatocytic vacuolation, testicular degeneration, and prostatic atrophy were observed. In mice, anemia, liver cytoplasmic vacuolization, and renal degeneration and dilation occurred. Our studies demonstrated that, in rats and mice, 2MI induces thyroid hyperplasia and hypertrophy, and both 2MI and 4MI induce anemia; 2MI induces thyroid follicular-cell adenoma in male rats.

Interspecies differences in susceptibility to perturbation of thyroid homeostasis: a case study with perchlorate

Despite many physiological similarities, humans and rats exhibit notably different susceptibilities to thyroid perturbation. Considerable research has recently been conducted on the thyroid-active chemical perchlorate, a chemical of emerging environmental and regulatory interest. While the data indicate humans and rats exhibit similar dose-response relationships in terms of acute inhibition of thyroidal iodide uptake, the two species appear to exhibit notable differences in terms of thyroid hormone response, the toxicologically significant consequence of iodide uptake inhibition. We analyzed dose-response data for changes in serum T(3), T(4), and TSH levels from studies in humans, rats, mice, and rabbits. We found that thyroid homeostasis in the rat appears to be strikingly more sensitive to perchlorate than any of the other species. Rats exhibited an increase in serum TSH at 0.1mg/kg-day whereas other species remained unresponsive even at doses of 10mg/kg-day. Less pronounced but consistent effects were seen with serum T(3) and T(4). These cross-species comparisons provide strong evidence that data obtained from rat studies should be critically evaluated for their relevance to humans. If rat data are used to develop toxicity criteria for perchlorate, we propose that this is an instance where an inter-species uncertainty factor less than one is supportable. DISCLOSURE STATEMENT: One of the authors (BDB) has been hired by Lockheed Martin Corporation as an expert in litigation involving perchlorate. A portion of the initial research presented in this paper was conducted in conjunction with her role in that matter.

Risk communication--the perceptions and realities

This paper defines risk communication and puts it into the perspective of risk analysis as a whole. Case studies originating for the food industry are described in the areas of food colours, pesticides and genetically, modified foods to exemplify both the difficulties of risk communication and the lessons that can be learned. This paper concludes by suggesting ways in which successful risk communication should be managed.

Pathogenesis of thyroid nodules: histological classification?

Thyroid nodule genesis may be considered as an amplification of thyroid heterogeneity due to genetic and/or epigenetic mechanisms. We classified the thyroid nodules in five types with distinct histological features: hyperplastic, neoplastic, colloid, cystic and thyroiditic nodules. Hyperplastic: Thyrocyte proliferation is under the control of TSH but several other paracrine and autocrine factors are secreted by follicular cells, the stromal apparatus and the lymphocytes, which are implicated in initiation and perpetuation of thyroid hyperplasia. Growth occurs mainly through TSHR, cAMP and PKA. Constitutive cAMP overproduction has been shown to be due to point mutation of the TSHR or Gs protein, producing overgrowth and hyperfunction. Neoplastic: Several activated oncogenes have been identified in thyroid malignancies. Oncogenes relevant to the thyroid carcinogenesis are: mutated TSHR and gsp (constitutive activation of cAMP); TRK (receptor for NGF); RET/PTC (phosphorylation of tyrosine kinase receptor)--an isoform of this oncogene is induced by radiation: ras (it encodes Gs proteins transducing mitogenic signals); and c-MET (receptor for hepatocyte growth factor). The evolution of a differentiated thyroid cancer towards an undifferentiated cancer is due to a mutation of a family of proteins (i.e., p53), which acts as a brake, preventing the genomic instability of cancer. It is suggested that a tumor initiates by RET or ras and possibly progresses--as a result of additional mutations and by p53 mutation--to anaplastic carcinoma. Colloid: Flattening of the epithelium and dilatation of follicles containing viscous material--made up by a concentrated solution of thyroglobulin (hTg)--is the characteristic of the colloid nodule. A defect of intraluminal reabsorption of hTg has been suggested but not proven. Experimentally, a load of iodine is able to change thyroid hyperplasia to a colloid feature; however, a load of iodine is rarely found in the clinical history of patients. A new clue to the pathogenesis comes from the finding that a relevant part of the colloid (10-20%) is made up of insoluble globules, where hTg is compacted in a polymeric form. It is suggested that stocking hTg into globules is defective in colloid nodules, leading to enormous enlargement of the follicle. Cystic: It is estimated that between 15 and 40% of thyroid nodules are partly or entirely cystic. The 'true cyst' is rare; most of the so-called cystic nodules are 'pseudocysts', which follow necrosis and colliquation. Necrosis issues as an imbalance between growth and the precisely regulated process of angiogenesis. More recently, the VEGF/VPF has been found to be at the origin of recent and recurrent cysts. Immunotoxic and apoptotic mechanisms have also been suggested. Chemical analysis of cystic fluid showed a 'denatured' and 'serum-like' pattern suggesting different mechanisms in the pathogenesis of the pseudocystic thyroid nodules. Thyroiditic: Nodular lymphocytic thyroiditis (NLT) includes two different entities: 1) lymphocyte thyroiditis growing as a nodule in a hyperplastic or normal gland, and 2) lymphocyte thyroiditis associated in the same nodule with other nodular diseases of the thyroid: papillary thyroid carcinoma and lymphoma have been found to be associated to chronic lymphocytic thyroiditis.

3-Cloro-4-(dichloromethyl)-5-hydroxy-2(5H)-furanone (MX), a rat thyroid gland carcinogen, does not affect serum levels of TSH and thyroid hormones

3-Chloro-4-(dichloromethyl)-5-hydroxy-2(5H)-furanone (MX), a chlorine disinfection by-product in drinking water, causes follicular adenomas and carcinomas in thyroid glands of Wistar rats with an unknown mechanism. We evaluated effects of MX on blood thyroid stimulating hormone (TSH), thyroxine (T(4)), triiodothyronine (T(3)), prolactin (PRL) and growth hormone (GH) levels in male and female Wistar rats to assess their role in the tumorigenesis. The levels of TSH, PRL and GH in serum of male rats were not significantly affected by a single dose of 1, 10 or 60 mg/kg of MX administered by gavage 2 h before sampling. In repeated dose experiments MX was administered at dose levels of 1, 10 or 60 mg/kg of MX (40 mg/kg for females) in water by gavage daily for 1 or 3 weeks. Thyroid glands, adrenal glands and the liver were evaluated for morphological changes and cell proliferation activity after staining with proliferating cell nuclear antigen (PCNA). The dose of 60 mg/kg MX was toxic upon repeated administration. Nevertheless, MX did not affect blood TSH and T(4) levels at any time point in either sex. T(3) concentration increased transiently in males (by 37% after week 1) at the highest MX dose but not in females. MX did not change the weights of thyroid glands, their morphology and cell proliferation activity by the end of the week 3. MX did not affect blood PRL levels but decreased GH levels in males at all doses after the first week of MX treatment. The results indicate that MX does not alter blood TSH and thyroid hormone levels in rats, and imply that MX may not cause thyroid follicular cell tumors by TSH-mediated hormonal promotion.

Thyroid hyperactivity induced by methimazole, spironolactone and phenobarbital in marmosets (Callithrix jacchus): histopathology, plasma thyroid hormone levels and hepatic T4 metabolism

To determine drug-induced hyperfunction of marmoset thyroids due to inhibition of synthesis or enhancement of metabolic elimination of thyroid hormones, males were orally administered 10 and 30 mg/kg/day methimazole (MMI), 30 and 100 mg/kg/day spironolactone (SPL), or 50 mg/kg/day phenobarbital (PB) for 4 weeks. MMI caused marked hypertrophy of follicular epithelial cells in accordance with a significant decrease in the plasma thyroxin (T4) level. Hypertrophied epithelial cells were filled with dilated rough endoplasmic reticulum and reabsorbed intracellular colloids, and the luminal surface was covered with abundant microvilli. The colloid included vacuoles positive to anti T4 immuno-staining. SPL and PB also caused similar histomorphological changes, although they were less severe than those due to MMI and were not clearly associated with decrease in the plasma T4 levels. Hepatic T4 UDPGT activities tended to increase due to SPL and PB treatment, however, which were not so significant as increases in microsomal cytochrome P-450 contents. Some animals treated with SPL and PB showed marked increases in thyroid weights due to inactive dilated follicles. In conclusion, hyperactivity of thyroid follicles was induced in marmosets not only due to inhibition of T4 synthesis produced by MMI but also because of enhancement of hepatic T4 elimination produced by SPL and PB. However, hypertrophic effects of SPL and PB were less severe than MMI, because plasma T4 levels were maintained at almost pretreatment or control levels after SPL or PB treatment.

Toxicity study of a rubber antioxidant, mixture of 2-mercaptomethylbenzimidazoles, by repeated oral administration to rats

2-Mercaptobenzimidazole (2-MBI), a rubber antioxidant, is known to exhibit potent antithyroid toxicity in rats and is a candidate as an environmental endocrine disrupter. 2-Mercaptomethylbenzimidazoles (a 1:1 mixture of 4-methyl and 5-methyl isomers, MMBIs), are also employed industrially as rubber antioxidants and are suspected to exert antithyroid toxicity such as 2-MBI. In this investigation, acute and subacute oral toxicity studies of MMBIs in Wistar rats were conducted. The clinical signs of acute oral toxicity were observed including decreased spontaneous movement, a paralytic gait, salivation and lacrimation, and adoption of prone and lateral positions. The LD50 was estimated to be 330 mg/kg. In the subacute oral toxicity study, male and female rats were treated with MMBIs by gavage at doses of 0 (corn oil), 4, 20 and 100 mg/kg for 28 consecutive days followed by a 2-week recovery period for the control and highest dose groups. Body weight and food consumption, clinical signs, organ weights, clinical biochemistry and haematological parameters including clotting times and micronuclei induction in bone marrow erythropoeitic cells, and histopathology were examined. Relative organ weights of lung, liver and kidney, and serum cholesterol and phospholipid significantly increased in male rats treated with MMBIs at doses of 20 and 100 mg/kg. Male rats administered 100 mg/kg MMBIs exhibited a 1.8-fold increase in thyroid weight associated with histopathological changes but not altered serum thyroid hormone levels. Female rats administered 100 mg MMBIs/kg exhibited significant increases of liver and kidney but not thyroid weights, and serum cholesterol level. The antithyroid toxicity of MMBIs in rats was estimated to be one-tenth that of 2-MBI. No-observed-effect levels for male and female rats were found to be 4 and 20 mg/kg, respectively, in this subacute oral toxicity study.

Mechanistic data and risk assessment of selected toxic end points of the thyroid gland

Many goitrogenic xenobiotics that increase the incidence of thyroid tumors in rodents exert a direct effect on the thyroid gland to disrupt one of several possible steps in the biosynthesis, secretion, and metabolism of thyroid hormones. This includes (a) inhibition of the iodine trapping mechanism, (b) blockage of organic binding of iodine and coupling of iodothyronines to form thyroxine (T4) and triiodothyronine (T3), and (c) inhibition of thyroid hormone secretion by an effect on proteolysis of active hormone from the colloid. Another large group of goitrogenic chemicals disrupts thyroid hormone economy by increasing the peripheral metabolism of thyroid hormones through an induction of hepatic microsomal enzymes. This group includes central nervous system-acting drugs, calcium channel blockers, steroids, retinoids, chlorinated hydrocarbons, polyhalogenated biphenyls, and enzyme inducers. Thyroid hormone economy also can be disrupted by xenobiotics that inhibit the 5'-monodeiodinase that converts T4 in peripheral sites to biologically active T3. Inhibition of this enzyme by FD&C Red No. 3 lowers circulating T3 levels, which results in a compensatory increased secretion of thyroid-stimulating hormone (TSH), follicular cell hypertrophy and hyperplasia, and an increased incidence of follicular cell tumors in 2-yr or lifetime studies in rats. Physiologic perturbations alone, such as the feeding of an iodine-deficient diet, partial thyroidectomy, natural goitrogens in certain foods, and transplantation of TSH-secreting pituitary tumors in rodents also can disrupt thyroid hormone economy and, if sustained, increase the development of thyroid tumors in rats. A consistent finding with all of these goitrogens, be they either physiologic perturbations or xenobiotics, is the chronic hypersecretion of TSH, which places the rodent thyroid gland at greater risk to develop tumors through a secondary (indirect) mechanism of thyroid oncogenesis associated with hormonal imbalances.

Neoplastic lesions of questionable significance to humans

Many compounds giving a positive result in animal carcinogenicity studies through mechanisms involving secondary carcinogenesis pose little or no risk to humans. This article provides an overview of current understanding, with particular reference to renal tumors in male rats with alpha 2mu-globulin nephropathy, urinary bladder neoplasia in rodents, mesovarian leiomyomas induced in rats by beta 2-receptor stimulants, carcinoid tumors in the rodent stomach induced by prolonged suppression of acid secretion, thyroid follicular cell tumors in rodents, canine mammary neoplasia due to administration of progestagens, rodent mammary neoplasia induced by estrogens, uterine endometrial carcinomas of rats induced by dopamine agonists, Leydig cell tumors in the testis of rats, and ovarian tubulostromal adenomas in mice. A positive result on a rodent carcinogenicity study should not automatically preclude further development of a compound; future progress in this field should increase the accuracy of the rodent carcinogenicity study as a tool in human safety assessment.

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.

Pathophysiology of chemical injury of the thyroid gland

Many goitrogenic xenobiotics that increase the incidence of thyroid tumors in rodents exert a direct effect on the thyroid gland to disrupt one of several possible steps in the biosynthesis and secretion of thyroid hormones. This includes: (1) inhibition of the iodine trapping mechanism (thiocyanate or perchlorate); (2) blockage of organic binding of iodine and coupling of iodothyronines to form thyroxine (T4) and triiodothyronine (T3) (e.g. sulfonamides, thiourea, methimazole, and aminotriazole, amongst others); (3) inhibition of thyroid hormone secretion by an effect on proteolysis of active hormone from the colloid (lithium or an excess of iodide). Another large group of goitrogenic chemicals disrupts thyroid hormone economy by increasing the peripheral metabolism of thyroid hormones through an induction of hepatic microsomal enzymes. This group includes CNS-acting drugs (phenobarbital, benzodiazepines), calcium channel blockers (nicardipine, nifedipine), steroids (spironolactone), retinoids, chlorinated hydrocarbons (chlordane, DDT, TCDD), polyhalogenated biphenyls (PCB, PBB), and enzyme inducers. Thyroid hormone economy also can be disrupted by xenobiotics that inhibit the 5'-monodeiodinase which converts T4 in peripheral sites (e.g. liver and kidney) to biologically active T3. Inhibition of this enzyme by FD&C Red No. 3, amiodarone, and iopanoic acid lowers circulating T3 levels which results in a compensatory increased secretion of thyroid stimulating hormone (TSH), follicular cell hypertrophy and hyperplasia, and an increased incidence of follicular cell tumors in 2-year or lifetime studies in rats. Physiologic perturbations alone, such as the feeding of an iodine-deficient diet, partial thyroidectomy, natural goitrogens in certain foods, and transplantation of TSH-secreting pituitary tumors in rodents also can disrupt thyroid hormone economy and, if sustained, increase the development of thyroid tumors in rats. A consistent finding with all of these goitrogens, be they either physiologic perturbations or xenobiotics, is the chronic hypersecretion of TSH which places the rodent thyroid gland at greater risk to develop tumors through a secondary mechanism of thyroid oncogenesis.

Thyroid follicular cell carcinogenesis: results from 343 2-year carcinogenicity studies conducted by the NCI/NTP

The National Toxicology Program data base on 343 mouse and rat carcinogenesis studies was reviewed to determine the frequency of and relationship between hyperplastic and neoplastic follicular lesions of the thyroid gland. The frequency of chemically related lesions in the thyroid was also compared to neoplastic lesions in the liver to investigate a possible correlation. The percentage of studies observed to have positive or equivocal chemically related thyroid proliferative lesions was rats: male, 14%, female, 11%; mice; male, 8%; female, 9%. When positive in one sex for a given chemical, there was a 60-80% chance of it being positive in the other sex of the same species, although interspecies correlation was not as strong. Thyroid follicular cell neoplasia without hyperplasia was uncommon in mice but was common in rats. Chemicals that caused thyroid proliferative changes were more likely (P less than 0.05) to produce liver neoplasms (both within and between species) than were chemicals causing no thyroid changes. However, this correlation was far from perfect, with many chemicals producing thyroid proliferative lesions, but not liver neoplasms and vice versa. This suggests that universal correlations are not supportable by the data and that individual chemicals should be evaluated on a case-by-case basis.

Classification of carcinogens: polemics, pedantics, or progress?

Rodent carcinogens may, for physiological or other reasons, induce cancer by a variety of mechanisms which vary in their ability to affect humans. While the current approach of some regulatory agencies to carcinogen risk assessment and regulation may possibly be justified with most genotoxic carcinogens, this is not true with all nongenotoxic carcinogens. Mechanisms attributable to high dose toxicity occasioned by misuse of the maximum tolerated dose concept, imbalancing of homeostasis, unphysiological conditions, and induced cellular proliferation are reviewed. The greatest present need for meaningful regulation of carcinogens is to obtain public acceptance of the fact that some carcinogens are species specific and probably will not exert their effects in humans.

Early indicators of potential neoplasia produced in the rat forestomach by non-genotoxic agents: the importance of induced cellular proliferation

Forestomach neoplasia induced by the apparently non-genotoxic carcinogens, butylated hydroxyanisole and propionic acid, appears to arise by way of sustained high levels of cellular proliferation. Several other inducers of enhanced cellular proliferation, or the consequential incidence of hyperplastic lesions, have been identified in the rodent forestomach but the requisite carcinogenicity bioassays remain undone. In other tissues, such as the male rat kidney, the rodent thyroid follicular cell and the bladder epithelium, there is also evidence supporting the concept that sustained enhanced cellular proliferation may be an important early marker for non-genotoxic carcinogens. This reaction is, however, not likely to be the only marker necessary for the identification of non-genotoxic carcinogens.

Evidence for and possible mechanisms of non-genotoxic carcinogenesis in the rodent thyroid

Thyroid tumours are a common finding in toxicity tests in rodents. It is known that prolonged administration of antithyroid drugs leads to the development of multiple thyroid tumours, and the role of genotoxic and non-genotoxic mechanisms in this needs definition. The role of drugs with an antithyroid action in thyroid carcinogenesis requires a knowledge of thyroid physiology. This review briefly discusses the anatomy and physiology of the thyroid before concentrating on the cellular pathology of the changes that take place in the transition from a normal to a neoplastic thyroid cell. The malignant cell is characterised by excess growth and invasiveness. The normal thyroid cell does not possess an unlimited growth potential because of a growth-desensitising mechanism (GDM) of the antioncogene type. Spontaneous thyroid carcinogenesis requires three key steps which are presumed to arise by mutation and clonal selection: the loss of the GDM, the acquisition of TSH-independent growth, and the acquisition of invasiveness. The sequence of the cell biological changes involved is not fully understood, but it has been shown that IGF-1 is a necessary co-factor for the growth-stimulating effect of TSH in the normal cell, and that autocrine production of IGF-1 is a feature of spontaneous thyroid adenomas. Another early change that has been shown in both experimental and human thyroid tumours is mutation of one of the ras oncogenes. In carcinogenesis due to the prolonged administration of an agent known to interfere with thyroid hormone metabolism and to induce a high TSH, two rather than three key steps will be required for carcinogenesis, as the development of TSH independent growth will not confer any selective advantage. We have shown that monoclonal lesions induced in this way regress when the goitrogen is withdrawn and therefore retain TSH dependence. The development of the other two key changes--the loss of the GDM and the acquisition of invasiveness--may be due to genotoxic or non-genotoxic mechanisms. They can occur in man in the absence of any known mutagenic agent. In patients with dyshormonogenesis a congenital defect in one of the steps of thyroid hormone synthesis is associated with multiple tumour production. It is reassuring that in these patients, exposed to decades of high TSH levels, benign lesions are common, but malignant thyroid tumours are very rare. The occurrence of thyroid tumours following the use of substances known to interfere with thyroid hormone metabolism does not itself exclude a genotoxic component to the carcinogenesis.(ABSTRACT TRUNCATED AT 400 WORDS)

An appreciation of the maximum tolerated dose: an inadequately precise decision point in designing a carcinogenesis bioassay?

Cancers arise in specific tissues. One difficulty with the present definitions of the Maximum Tolerated Dose (MTD), as they pertain to the rodent cancer bioassay, is that they base MTD on relatively crude parameters associated with the well-being of the entire animal rather than with the lack of specific tissue toxicity. Additional factors that could be included in the MTD definition, or could be separately determined, are addressed. Many of these factors refer to toxic behavior in one or a few tissues and, if used in setting the MTD, may mask more relevant events occurring at higher dose levels in other tissues. Reducing the MTD to a level that fails to take into account pesticide or drug-related toxicity may lead to the loss of relevant information in the bioassay. It is concluded, therefore, that there are two possible approaches to a more appropriate use of the MTD. The highest dose of the test agent (MTD) may be chosen (i) to lie below the thresholds of carcinogenicity-related non-genotoxic toxicity or (ii) the present high level MTD may continue to be used and tumors that arise may be classified as being irrelvant to humans at some or all exposure levels. The latter approach is to be preferred. It has the potential to avoid missing high level effects of the test agent that may be relevant to the human population.

International Commission for Protection Against Environmental Mutagens and Carcinogens. ICPEMC publication No. 19. The classification of carcinogens identified in the rodent bioassay as potential risks to humans: what type of substance should be tested next?

The relevance of rodent cancer bioassay data to humans is discussed in relation to the needs of regulatory agencies. The usefulness of in vivo and in vitro genotoxicity testing in this connection is also discussed. In the case of rodent carcinogens that do not elicit genotoxicity, it is suggested that homeostatic imbalance, cell proliferation, and other processes may play a major role in tumor development and its importance to the possible ability of the test agent to induce human cancer. These possibilities need to be evaluated on a case by case basis. The methods by which chemicals are selected for the rodent cancer bioassay are also discussed and it is pointed out that naturally-occurring constituents of human foods should in future receive greater priority as a consequence of anticipated changes resulting from biotechnology.

Consideration of both genotoxic and nongenotoxic mechanisms in predicting carcinogenic potential

Bacterial and cell culture genotoxicity assays have proven to be valuable in the identification of DNA reactive carcinogens because mutational events that alter the activity or expression of growth control genes are a key step in carcinogenesis. The addition of metabolizing enzymes to these assays have expanded the ability to identify agents that require metabolic activation. However, chemical carcinogenesis is a complex process dependent on toxicokinetics and involving at least steps of initiation, promotion and progression. Identification of those carcinogens that are activated in a manner unique to the whole animal, such as 2,6-dinitrotoluene, require in vivo genotoxicity assays. There are many different classes of non-DNA reactive carcinogens ranging from the potent promoter 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) that acts through a specific receptor, to compounds that alter growth control, such as phenobarbital. Many compounds, such as saccharin, appear to exhibit initiating, promotional and/or carcinogenic activity as events secondary to induced cytotoxicity and cell proliferation seen only at the chronic lifetime maximum tolerated doses mandated in rodent bioassays. Simple plus/minus vs. carcinogen/noncarcinogen comparisons used to validate the predictivity of bacterial and cell culture genotoxicity assays have revealed that a more comprehensive analysis will be required to account for the carcinogenicity of so many diverse chemical agents. Predictive assays and risk assessments for the numerous types of nongenotoxic carcinogens will require understanding of their mechanism of action, reasons for target organ and species specificity, and the quantitative dose-response relationships between endpoints such as induced cell proliferation and carcinogenic potential.

Cell kinetics of a rat thyroid transplantable tumour

Changes in morphology and cell kinetics are described in a rat thyroid transplantable tumour (TTT) during the first few transplant generations. The growth of TTT in animals was possible only with an increased circulation level of the thyroid stimulating hormone (TSH). With serial transplantation subcutaneously in isologous animals, the morphology of TTT changed dramatically from that of a follicular tumour in the 3rd passage to become, by the 9th generation, a poorly differentiated tumour with a trabecular arrangement of cells. This change in tumour morphology was accompanied by an increase in the number of proliferating cells--mitotic index (MI), [3H]thymidine labelling index (LI), growth fraction (GF)--and cell loss factor (O) as well as a decrease in the cell cycle time (Tc) and potential population doubling time (TPD). TTT belongs to the class of tumours with a low proliferative activity and might be used in a variety of cell kinetic, radiobiological and chemotherapy studies.

Thyroid carcinoma mimicking a toxic adenoma

A young woman with a thyroid papillary carcinoma behaving as an autonomously hyperfunctioning nodule is described. Only 17 similar patients have been seen in the past 25 years. It is emphasized that hyperthyroidism does not exclude malignant disease in hot nodules. This possibility suggests that all thyroid nodules, either cold or hot, require careful management. Therefore, in "at risk" cases, surgery could be the most useful treatment.

Risk assessment of carcinogenic and noncarcinogenic chemicals

"Risk Assessment" is a general term used with increasing frequency by both scientists and regulators. Scientifically based risk assessments consider available toxicologic data when judging which agents pose a significant risk to the human population. The science of toxicology focuses on identifying potential hazards to human health using surrogate animal studies. Margins of Safety and establishment of ADIs (Acceptable Daily Intakes) are methods applied to animal test data to set "safe" levels of potential exposure. While the use of Safety Factors in development of the ADI can support a pragmatic conclusion of safety, this approach cannot provide estimates of the probability of harm or the degree of safety. Therefore, Quantitative Risk Assessment (QRA) methods using mathematical models have been advanced to extrapolate from animal exposures which are usually high to much lower human exposure levels where experimental response is absent. Such methodology has been applied primarily by U.S. regulatory agencies to experimental oncogenic responses to estimate the risks of chemical exposure. The present manuscript considers both methods for evaluation of chemical safety and focuses on the scientific merits and limitations of each.

Important recent advances in the practice of health risk assessment: implications for the 1990s

Health risk assessments have been so widely adopted in the United States that their conclusions are a major factor in many environmental decisions. The procedure by which these assessments are conducted is one which has evolved over the past 10-15 years and a number of short-comings have been widely recognized. Unfortunately, improvements in the process have often occurred more slowly than advancements in technology or scientific knowledge. Recent significant advances for more accurately estimating the risks posed by environmental chemicals are likely to have a dramatic effect on the regulation of many substances. Each of the four portions of risk assessment (hazard identification, dose-response assessment, exposure assessment, and risk characterization) has undergone significant refinement since 1985. This paper reviews some of the specific changes and explains the likely benefits as well as the implications. Emphasis is placed on the improved techniques for (a) identifying those chemicals which may pose a human cancer or developmental hazard, (b) using statistical approaches which account for the distribution of interindividual biological differences, (c) using lognormal statistics when interpreting environmental data, (d) using physiologically based pharmacokinetic models for estimating delivered dose and for scaling up rodent data, (e) using biologically based cancer models to account for the seven or more apparently different mechanisms of chemical carcinogenesis, (f) describing the severity of the public health risks by considering those portions of the population exposed to various concentrations of a contaminant, and (g) reviewing how criteria for acceptable risk have been influenced by the number of exposed persons. The net benefit of these improvements should be a reduction in the uncertainty inherent in current estimates of the health risks posed by low level exposure to carcinogens and developmental toxicants.

International Commission for Protection Against Environmental Mutagens and Carcinogens. ICPEMC publication No. 17. Can a mechanistic rationale be provided for non-genotoxic carcinogens identified in rodent bioassays?

In a recent survey of the results of the National Cancer Institute/National Toxicology Program's Carcinogenesis Bioassay Program, Ashby and Tennant (1988) drew attention to the high proportion of carcinogens that were non-genotoxic insofar as their response to the Salmonella-microsome test was concerned. The present review contrasts these findings with what is known mechanistically about non-genotoxic carcinogens that affect the tissues which are considered to be particularly prone to non-genotoxic tumor induction. Excessive and often thresholded increases in cellular proliferation in the affected tissues appear to be one common feature in tumor induction by these agents, which act either through cytotoxicity followed by regeneration or through hormone-mimetic action. It is suggested that a weight of the evidence approach on a chemical by chemical basis is necessary to decide the relevance of these agents to the human situation.

Short-term effects of butylated hydroxytoluene on the Wistar rat liver, urinary bladder and thyroid gland

Long-term feeding of butylated hydroxytoluene (BHT) to rats and mice has been linked to the enhancement of the incidence of liver tumors. It is shown in this paper that in the liver, urinary bladder and thyroid of the male Wistar rat, feeding the highest tolerated doses of BHT for 30 days does not lead to detectable increases in [3H]thymidine labeling. On the other hand, treatment of rats with 0.5% dietary BHT leads to a time-limited increase in liver cell [3H]thymidine labeling that subsided to control values within 8 days. This increase in [3H]thymidine labeling in the liver is accompanied by an unexpectedly large increase in the mitotic index. These results are discussed in the light of the behavior of certain rodent liver tumorigens.

Nongenotoxic carcinogens in the regulatory environment

The biological activity of many carcinogens is to directly induce mutational events, thereby altering the information encoded in the DNA. Short-term tests for potential carcinogens and risk assessment models generally rely on the assumption that the agent in question will operate through a genotoxic mechanism. However, carcinogenesis is a multistep process, and it is increasingly clear that the primary biological effect for many carcinogenic chemicals involves events other than direct DNA reactivity. For many experimental rodent models as well as human cancers, nongenotoxic mechanisms appear to be the driving force in the formation of tumors. Many of these nongenotoxic mechanisms are highly species-specific. Thus, it is increasingly important to ask if the rodent model applies to the human situation at all, in addition to the examination of appropriate, hypothetical, mathematical risk assessment models. More research is now being focused to better define the mechanisms by which the many distinctly different classes of nongenotoxic carcinogens are acting. This understanding will become the basis for new predictive assays and more realistic risk assessment models. If specific conditions are met, then a no observed effect level with a safety factor may be the most appropriate risk model for some carcinogens.

The significance of hepatic microsomal enzyme induction and altered thyroid function in rats: implications for thyroid gland neoplasia

Hepatic microsomal enzymes play an important role in thyroid hormone homeostasis. Glucuronidation of thyroxine is the rate limiting step in the biliary excretion of thyroxine in the rat; the monodeiodinases are important in the conversion of T4 to T3 and reverse T3 and subsequent deiodinations. Phenobarbital is known to affect thyroid function in rats due to an alteration of hormone disposition. We have further characterized these effects and have demonstrated that phenobarbital increases the biliary excretion of thyroxine glucuronide primarily as a result of an induction of hepatic thyroxine glucuronyltransferase. Studies on the mode of action for phenobarbital promotion of thyroid follicular neoplasia were conducted using an initiation-promotion model established by Hiasa et al (35). In this model, we demonstrated that supplemental administration of thyroxine blocked the promoting effect of phenobarbital and furthermore, using various dosages of thyroxine, we observed that the tumor promoting effect of phenobarbital was directly proportional to the level of plasma TSH. The results of these studies support the hypothesis that the tumor promoting effect of phenobarbital in the thyroid gland is mediated via increased secretion of pituitary TSH as a compensatory response to the known effects of phenobarbital on peripheral thyroid hormone disposition. Since a number of microsomal enzyme inducing agents have increased the incidence of thyroid follicular neoplasia in rat carcinogenicity studies, thyroid function should be assessed and a secondary mechanism of hormone imbalance should be considered in the interpretation of the significance of these findings in rodents.

The effects of xenobiotics on the structure and function of thyroid follicular and C-cells

The mammalian thyroid gland is composed of 2 distinct endocrine cell populations concerned with the synthesis of 2 different classes of hormones. Follicular cells secrete the metabolically active iodothyronines whereas the C-(parafollicular) cells are concerned with the production of calcitonin, a hormone that influences blood levels of calcium and phosphorus, and bone cell metabolism. The synthesis of metabolic thyroid hormones is different than in other endocrine glands because the final assembly of hormone occurs within the follicular lumen. This extracellular synthesis of thyroid hormones is made possible by thyroglobulin, a glycoprotein synthesized by follicular cells. The secretion of thyroid hormones under the influence of pituitary thyrotrophin (TSH) from stores in the luminal colloid is initiated by elongation of microvilli and formation of pseudopods. FD&C Red No. 3 is a tetraiodinated derivative of fluorescein which in lifetime studies increases the incidence of thyroid follicular cell adenomas in male Sprague-Dawley rats. The striking changes in circulating levels of thyroid hormones and morphologic evidence of follicular cell stimulation are the result of alterations in the peripheral metabolism of thyroxine. An inhibition by FD&C Red No. 3 of 5'-deiodinase in the liver and kidney would explain the lower serum triiodothyronine (T3) levels. The pituitary, sensing the lowered circulating levels of T3, increased the secretion of thyroid stimulating hormone which resulted in the morphologic evidence of follicular cell stimulation in the long-term studies. Other xenobiotics increase the incidence of thyroid tumors in rodents by a direct effect on the thyroid gland to disrupt 1 of 3 or more possible steps in the biosynthesis of thyroid hormones. Physiologic perturbations alone, such as iodine deficiency or partial thyroidectomy, can disrupt thyroid hormone economy in rodents and, if sustained, increase the development of thyroid tumors. The wide variety of drugs, chemicals, and physiologic perturbations which increase thyroid tumor development appear to act through a secondary (indirect) mechanism to promote tumor development by causing a long-standing hypersecretion of thyroid stimulating hormone. Nodular and/or diffuse hyperplasia of C-cells occurs with advancing age in many strains of laboratory rats and in response to long-term hypercalcemia in certain animal species and human beings. Focal or diffuse hyperplasia often precedes the development of C-cell neoplasms. Radiation and the feeding of diets high in vitamin D resulting in hypercalcemia have been reported to increase the incidence of C-cell tumors in rats.