The bleomycin-induced rodent lung fibrosis model is commonly

Thiamethoxam is a neonicotinoid insecticide that is not a mutagen, but it did cause a significant increase in liver cancer in mice, but not rats, in chronic dietary feeding studies. Previous studies in mice have characterized a carcinogenicity mode of action that involved depletion of plasma cholesterol, cell death, both as single cell necrosis and as apoptosis, and sustained increases in cell replication rates. In a study reported in this article, female rats have been exposed to thiamethoxam in their diet at concentrations of 0, 1000, and 3000 ppm for 50 weeks, a study design directly comparable to the mouse study in which the mode of action changes were characterized. In rats, thiamethoxam had no adverse effects on either the biochemistry or histopathology of the liver at any time point during the study. Cell replication rates were not increased, in fact they were significantly decreased at several time points. The lack of effect on the rat liver is entirely consistent with the lack of liver tumor formation in the two-year cancer bioassay. Comparisons of the metabolism of thiamethoxam in rats and mice have shown that concentrations of the parent chemical were either similar or higher in rat blood than in mouse blood in both single dose and the dietary studies strongly indicating that thiamethoxam itself is unlikely to play a role in the development of liver tumors. In contrast, the concentrations of the two metabolites, CGA265307 and CGA330050, shown to play a role in the development of liver damage in the mouse, were 140- (CGA265307) and 15- (CGA330050) fold lower in rats than in mice following either a single oral dose, or dietary administration of thiamethoxam for up to 50 weeks. Comparisons of the major metabolic pathways of thiamethoxam in vitro using mouse, rat, and human liver fractions have shown that metabolic rates in humans are lower than those in the rat suggesting that thiamethoxam is unlikely to pose a hazard to humans exposed to this chemical at the low concentrations found in the environment or during its use as an insecticide.

Teased-fiber technique is the best approach for studying peripheral myelinated nerve fibers in their continuity. It

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Thiamethoxam, a neonicotinoid insecticide, which is not mutagenic either in vitro or in vivo, caused an increased incidence of liver tumors in mice when fed in the diet for 18 months at concentrations in the range 500 to 2500 ppm. A number of dietary studies of up to 50 weeks duration have been conducted in order to identify the mode of action for the development of the liver tumors seen at the end of the cancer bioassay. Both thiamethoxam and its major metabolites have been tested in these studies. Over the duration of a 50-week thiamethoxam dietary feeding study in mice, the earliest change, within one week, is a marked reduction (by up to 40%) in plasma cholesterol. This was followed 10 weeks later by evidence of liver toxicity including single cell necrosis and an increase in apoptosis. After 20 weeks there was a significant increase in hepatic cell replication rates. All of these changes persisted from the time they were first observed until the end of the study at 50 weeks. They occurred in a dose-dependent manner and were only observed at doses (500, 1250, 2500 ppm) where liver tumors were increased in the cancer bioassay. There was a clear no-effect level of 200 ppm. The changes seen in this study are consistent with the development of liver cancer in mice and form the basis of the mode of action. When the major metabolites of thiamethoxam, CGA322704, CGA265307, and CGA330050 were tested in dietary feeding studies of up to 20 weeks duration, only metabolite CGA330050 induced the same changes as those seen in the liver in the thiamethoxam feeding study. It was concluded that thiamethoxam is hepatotoxic and hepatocarcinogenic as a result of its metabolism to CGA330050. Metabolite CGA265307 was also shown to be an inhibitor of inducible nitric oxide synthase and to increase the hepatotoxicity of carbon tetrachloride. It is proposed that CGA265307, through its effects on nitric oxide synthase, exacerbates the toxicity of CGA330050 in thiamethoxam treated mice.

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The dose and time dependence of the cellular phenotype in preneoplastic and neoplastic liver lesions was evaluated quantitatively in groups of male Sprague-Dawley rats exposed for 7 weeks to 0, 12 and 24 mg/kg body wt of N-nitrosomorpholine (NNM) and studied at different time points up to 80 weeks after withdrawal of NNM (stop model). NNM-treated rats showed a dose- and time-dependent increase in the total number and volume of preneoplastic foci of altered hepatocytes (FAH) and in the incidence of hepatocellular adenomas (HCA) and carcinomas (HCC) at both dose levels, compared with the untreated controls. After stopping treatment with 12 mg/kg body wt, the well-known sequence of cellular changes leading from glycogenotic clear and clear/acidophilic cell foci to mixed and diffusely basophilic cell populations poor in glycogen was found. In contrast, at the higher NNM dose level (24 mg/kg) predominantly mixed and diffusely basophilic cell foci appeared immediately after cessation of treatment, but their number rapidly declined up to 13 weeks after withdrawal. At the same time, there was a reciprocal increase in the number of the less altered clear/acidophilic cell foci, indicating an early reversion-linked phenotypic instability of FAH. However, in spite of this reversion higher numbers of mixed and diffusely basophilic cell foci were retained after treatment with 24 compared to 12 mg/kg of NNM at all time points studied, and there was even a slow additional increase in the number of these types of FAH 20 weeks after withdrawal of NNM. At both dose levels, the volume fraction of the persistent mixed cell foci correlated positively with the incidence of HCA and HCC, suggesting that this phenotype of FAH represents a direct precursor of the neoplastic lesions. Tigroid cell foci, which appeared most frequently after treatment with the lower dose of NNM, were not integrated into the predominant sequence of cellular changes leading to HCC, but they may represent an intermediate stage in a side lineage of this sequence, endowed with the potential to progress at least to HCA. Our results show that reversion-linked phenotypic instability of FAH occurs mainly after high dose treatment, possibly resulting from rapid adaptive cellular responses to the primary carcinogenic lesion(s) which may be fixed by genetic or epigenetic mechanisms. In contrast, progression-linked phenotypic instability of FAH is a slow process developing in a dose- and time-dependent manner at all dose levels leading to hepatic neoplasia.

A variety of phenotypic cellular changes emerge in the liver of different species prior to the appearance of hepatocellular adenomas and carcinomas induced by carcinogenic agents (chemicals, radiation, hepadna viruses) or develop "spontaneously." Foci of altered hepatocytes have been studied most extensively in rats treated with chemical carcinogens; they are considered preneoplastic lesions and have been used in several laboratories as endpoints in carcinogenicity testing. The principles and problems of the morphological classification of foci of altered hepatocytes are presented. In addition to the 4 types of foci generally accepted (clear, acidophilic, basophilic and mixed cell foci), further subtypes (intermediate cell foci) or other types of foci, namely tigroid cell foci and amphophilic cell foci, have more recently been separated as distinct pathomorphological entities. Whereas the amphophilic foci might result from a modulation of clear and acidophilic cell foci, the tigroid cell foci apparently represent a stage in a separate cell lineage leading to hepatocellular adenomas. It remains open whether the tigroid cell foci may also progress to carcinomas. Extrafocal phenotypic changes of hepatocytes might also be involved in hepatocarcinogenesis. The cellular phenotypes within foci also depend strongly, among many other factors, on the dose and duration of the carcinogenic treatment. Cytomorphological, cytochemical, microbiochemical and stereological studies suggest that the predominant sequence of cellular changes during hepatocarcinogenesis leads from the clear and acidophilic cell foci storing glycogen in excess through mixed cell foci and nodules to basophilic cell populations prevailing in hepatocellular carcinomas. A multitude of metabolic aberrations is associated with the sequential cellular changes. Aberrations in carbohydrate metabolism are particularly prominent and might be causally related to the neoplastic transformation of the hepatocytes.