Evaluation of the potential carcinogenicity and genetic toxicity to humans of the herbicide acetochlor

Abstract

<jats:p> Comprehensive toxicological studies of the herbicide acetochlor are presented and discussed. Although it gave a negative profile of responses in the many toxicity tests conducted there were some findings that prompted further investigation. First, although non-mutagenic in the Sal monella assay, acetochlor was clastogenic to mammalian cells treated in vitro. This clastogenic potential was not expressed in vivo in four rodent cytogenetic assays (bone marrow and germ cells). Second, although acetochlor gave a negative response in rat liver UDS assays when tested at the acute MTD, gavage administration of a single, supra- MTD dose (2000 mg/kg) gave a weak positive assay response. This dose-level (2000 mg/kg) was necrotic to the liver, depressed hepatic glutathione levels by up to ∼80%, altered the metabolism of acetochlor, and was associated with up to 33% lethality. In contrast, reference liver genotoxins such as DMN, DMH and 2AAF were shown to elicit UDS in the absence of such effects, and at ∼400 x lower dose-levels. Finally, microscopic nasal polypoid adenomas were induced in the rat when acetochlor was administered for two years at the maximum tolerated dose (MTD). The tumours were not life-threatening, they did not metastasize, and no DNA damage was induced in the nasal cells of rats maintained on a diet containing the MTD of acetochlor for either 1 or 18 weeks (comet assay). </jats:p><jats:p> In order to probe the mechanism of action of these high dose toxicities a series of chemical and genetic toxicity studies was conducted on acetochlor and a range of structural analogues. These revealed the chloroacetyl sub structure to be the clastogenic species in vitro. Although relatively inert, this substituent is preferentially reactive to sulphydryl groupings, most evidently, to glutathione (GSH). Similar chemical reactivity and clastogenicity in vitro was observed for two related chemicals bearing a chloroacetyl group, both of which have been defined as non-carcinogens in studies reported by the US NTP. These collective observations indicate that the source of the clastogenicity of acetochlor in vitro is also the source of its rapid detoxification in the rat in vivo, via reaction with GSH. Metabolic studies of acetochlor are described which reveal the formation of a series of GSH-associated biliary metabolites in the rat that were not produced in the mouse. The metabolism of acetochlor in the rat changes with increasing dose-levels, probably because of depletion of hepatic GSH. </jats:p><jats:p> It is most likely that a rat-specific metabolite is responsible for the rat nasal tumours observed uniquely at elevated dose-levels. The absence of genetic toxicity to the nasal epithelium of rats exposed acutely or sub- chronically to acetochlor favours a non-genotoxic me chanism for the induction of these adenomas. The observation of a time- and dose-related increase in S- phase cells in the nasal epithelium is consistent with this conclusion. </jats:p><jats:p> Despite some confusion caused by the early use of peri- lethal gavage administrations of acetochlor to rodents, and supra-MTD dietary concentrations in some of the chronic studies, the available MTD data are consistent with acetochlor not posing a genetic or carcinogenic hazard to humans. </jats:p>

Exposure to the herbicide acetochlor alters thyroid hormone-dependent gene expression and metamorphosis in Xenopus Laevis.

“…Despite a clear association between TH responsiveness and acetochlor action for many genes, not all T 3 -regulated genes are targets of acetochlor in X. laevis. In rats, studies have shown that acetochlor acts by increasing T 4 clearance from the liver, thereby reducing TH levels (Ashby et al 1996;Wilson et al 1996). This mechanism appears to be incompatible with our observations and those of others.…”

“…The preemergent herbicide acetochlor [2-chloro-N-(ethoxy-methyl)-N-(2-ethyl-6-methylphenyl) acetamide] is a persistent organic pollutant (POP) that can be detected in shallow ground water 1 year after field application, and there is evidence that it can act as an EDC (Ashby et al 1996;Barbash et al 1999;Cheek et al 1999a;Veldhoen and Helbing 2001;Wilson et al 1996). Acetochlor was introduced in 1994, and its use in agriculture has increased approximately 32% measured as the number of pounds applied annually (from 1994 to 2000).…”

Identification of human urinary metabolites of acetochlor in exposed herbicide applicators by high-performance liquid chromatography-tandem mass spectrometry

“…Acetochlor induces nasal tumors in rats at the maximumtolerated dose of 1000 parts per million and is classified as ''likely to be carcinogenic to humans'' by the US EPA (Ashby et al, 1996;Dearfield et al, 1999). In human and rat liver microsomes, acetochlor is bioactivated via a multistep pathway to the putative DNA-reactive metabolite, 2-methyl-6-ethylbenzoquinoneimine (Jefferies et al, 1998;Coleman et al, 2000;Green et al, 2000).…”

Nongenotoxic (epigenetic) carcinogens: Pesticides as an example. A critical review

“…In the rat, a series of GSH-associated biliary metabolites are formed which are not produced in the mouse, which are resistant to the nasal adenoma induction. Thyroid tumors were observed in rats exposed to the supra-MTD dose of acetochlor and developed by the mechanism described above, i.e., decrease of thyroid hormone and increased level of pituitary TSH [40].…”