Environmental Project, 944 – Evaluation of Health Hazards by exposure to Triazines and Degradation Products

Evaluation of Health Hazards by exposure to Triazines and Degradation Products

2 Toxicokinetics

2.1 Absorption, distribution and excretion
2.2 Metabolism
2.3 Mode of action

2.1 Absorption, distribution and excretion

2.1.1 Inhalation

No data were found.

2.1.2 Oral intake

Rats dosed orally with radioactively marked atrazine excreted 65-67% of the dose in urine within 72 hours indicating an extensive absorption by this route. About 20% of the dose was found in faeces, 4-16% was found in tissues, and only 0.1% was found in expired air at 72 hours following dosing. In tissues, the highest concentrations were found in erythrocytes, liver, spleen, kidneys and lungs. The plasma concentration of atrazine in rats peaked 8-10 hours after dosing. The elimination half-time was 11 hours. (Studies quoted from ACGIH 1991, ATSDR 2001, IARC 1999a, US-EPA 2002b, WHO 1996a).

Rats dosed orally with radioactively marked simazine excreted 49% of the dose in urine within 96 hours. The highest tissue concentrations were found in spleen, liver, and kidney in mice and rats. (Studies quoted from US-EPA 2002b, WHO 1996b).

At least 60% of an oral dose of terbutylazine was absorbed in rats, completely metabolised, and excreted via the urine and bile with a half-life of 16-17 hours. The highest tissue concentrations were found in kidney, liver and blood. (Studies quoted from WHO 1998b).

Cyanazine was rapidly absorbed from the gastrointestinal tract of rats, dogs and cows. In rats, 80-88% of the administered dose was eliminated within 4 days with almost equal amounts in urine and faeces. Only 3% was found in tissues after 4 days. Cyanazine was detected in cows' milk. (Studies quoted from ACGIH 1991, WHO 1998a).

2.1.3 Dermal contact

In humans, 90-94% of an applied dose of technical atrazine (by dermal patches) remained on the skin 24 hours following application, but only 0.3-5.1% of the applied dose was recovered in the urine and faeces within 7 days after application. An in vitro study using human skin samples showed that about 16% of atrazine was absorbed in a 24-hour period but most of the absorbed atrazine (12% of the applied dose) remained in the skin. (Studies quoted from ATSDR 2001).

In one study, absorption of atrazine through rat skin was limited amounting to less than 2% after a 10 hour exposure. In other rat studies, an inversely dose-dependent absorption (3-26%) of atrazine through skin was demonstrated. In some of these studies atrazine was in an aqueous formulation. In one of the studies it was stated that the majority of the "absorbed" dose was found in the skin application site after washing and up to 5% was detected as systemically absorbed. More than 50% of atrazine was absorbed through rat skin when it was administered as a solution in ethanol or tetrahydrofuran. (Studies quoted from ATSDR 2001, IARC 1999a, EC 1996a, WHO 1996a).

Following dermal application of simazine to rats, less than 1% of the applied dose was absorbed through skin after a 24 hour exposure period. 20-40% of the applied dose was found in the skin application site. (Study quoted from EC 1996b).

Following dermal application of terbutylazine to rats, 30% of the applied dose was found in urine and faeces (Study quoted from WHO 1998b).

2.2 Metabolism

The main biotransformation pathways for atrazine, cyanazine, simazine and terbutylazine in rats are N-dealkylation by the hepatic cytochrome P450 system, and glutathione conjugation of either the parent compound or the N-dealkylated metabolite to the ultimately excreted mercapturic acid conjugate (US-EPA 2002b).

Atrazine is extensively metabolised. In urine, unchanged atrazine accounted for less than 2% of all atrazine-related compounds after dermal exposure in humans or oral exposure in rats. Figure 1 shows the metabolism for atrazine. Several studies have shown that the major urinary metabolite in rats dosed orally with radioactively labelled atrazine was desethyldesisopropyl atrazine (DACT), which accounted for more than half of the total urinary radioactivity. The other reported urinary metabolites in rats were desethyl atrazine mercapturate, desethyldesisopropyl atrazine mercapturate (diaminotriazine mercapturate in Figure 1), desethyl atrazine (DEA), desisopropyl atrazine (DIA), and ammeline. In humans, the same urinary metabolites have been detected except for ammeline following dermal exposure. In addition, desisopropyl atrazine mercapturate and atrazine mercapturate have been found in human urine. In humans, desethyldesisopropyl atrazine (DACT) and desisopropyl atrazine (DIA) seems to be the major metabolites. Rats and humans produced the same type of metabolites following exposure to atrazine, but species-specific differences in the metabolite ratios were found. (Several studies quoted from ATSDR 2001, IARC 1999a, US-EPA 2002b).

Desethyldesisopropyl atrazine (DACT) and desisopropyl atrazine (DIA) has been detected as urinary metabolites in rats following oral dosing with simazine. Like for atrazine, conjugated mercapturates have also been detected (Studies quoted from US-EPA 2002b, WHO 1996b).

The main metabolic degradation of terbutylazine occurs through N-dealkylation (creating desethyl terbutylazine), oxidation of one methyl group of the tert-butyl group, and subsequent conjugation of the alcohol with glucuronic acid. Minor pathways have been described with glutathione conjugation like for atrazine and simazine, with the formation of hydroxytriazine metabolites, and with the formation of sulphate esters of the alcohol derivative. (Study quoted from WHO 1998b).

The major route of metabolic degradation of cyanazine occurs through N-dealkylation (creating desethyl cyanazine) followed by conjugation with glutathione (Studies quoted from WHO 1998a).

Figure 1. Metabolism for atrazine (Copied from US-EPA 2002).

Click here to see Figure 1.

2.3 Mode of action

Treatment of laboratory animals with atrazine (and some of the other triazines and metabolites) results in toxic effects such as attenuation of the luteinizing hormone (LH) surge, and as a consequence alteration of the oestrous cycle, altered pregnancy outcome, delayed pubertal development, and mammary gland tumours. The mammary gland tumours are only found in female Sprague-Dawley rats (and not in Fischer 344 rats, CD-1 mice or ovariectomised Sprague-Dawley rats). Several non-guideline studies have been performed to elucidate the mechanism behind those effects. The primary proposed mode of action for these effects involves disruption of the hypothalamic-pituitary-gonadal axis in a manner very similar to the known mechanism of reproductive senescence in some strains of rats. According to this hypothesis, atrazine affects the hypothalamus leading to a decreased secretion of hypothalamic norepinephrine (NE). Decreased NE levels result in decreased release of gonadotrophin releasing hormone (GnRH) from the hypothalamus. GnRH stimulates release of follicle stimulating hormone (FSH) and LH from the anterior pituitary. Thus, a decreased GnRH level leads to a low serum level of LH (and FSH). LH normally provides a signal to the ovaries promoting ovulation, but low serum levels leads to anovulation. Under these circumstances, the ovarian follicles continue to secrete oestrogen leading to a physiological state of prolonged or persistent oestrus. Increased oestrogen stimulates the pituitary leading to hypertrophy and consequently an increased secretion of prolactin. A constant elevated serum level of prolactin and oestrogen may result in tumour formation in sensitive tissues such as the mammary glands. (Several studies quoted from ATSDR 2001, IARC 1999a, US-EPA 2002b).

Alternative modes of action for the neuroendocrine effects following exposure to triazines have been suggested. Although several studies have found that the estrogenic effects associated with the triazines are not oestrogen receptor-mediated, these effects may be explained partly by their ability to induce aromatase, the enzyme responsible for converting androgens to estrogens. Recent studies demonstrated that atrazine and simazine and the metabolites desethyl atrazine (DEA) and desisopropyl atrazine (DIA) but not desethyldesisopropyl atrazine (DACT) induced aromatase activity in various cell lines. It has also been suggested that the anorexic effects of atrazine could account for most of atrazine's effects on LH since reduced food intake and weight loss is a potent stimulus for reduced LH. However, in pair-fed studies in both males and females, decreased food consumption and body weight could not account for the adverse effects of atrazine on the oestrous cycle and pubertal development. (Sanderson et al. 2001, several studies quoted from US-EPA 2002b).

IARC and US-EPA have concluded that the mechanism by which atrazine increases the incidence of mammary gland tumours in Sprague-Dawley rats is not relevant to humans because of critical interspecies differences in hormonal changes associated with reproductive senescence. In women, reproductive senescence is characterized by ovarian depletion, declining oestrogen levels, and, eventually, dioestrus. While the pattern of reproductive senescence in female Fischer 344 rats is not identical to that of women, Fischer 344 rats share the following features with women, in contrast to female Sprague-Dawley rats: later onset of senescence, low oestrogen concentrations during late life and an ability to control the luteinizing hormone secretion during reproductive senescence. (US-EPA 2002b, several studies quoted from IARC 1999a).

Nevertheless, US-EPA has also concluded that it is not unreasonable to expect that atrazine might cause adverse effects on hypothalamic-pituitary function in humans and that the same endocrine perturbations that induce tumours also appear to play a role in at least some reproductive developmental effects which may be relevant to humans. (US-EPA 2002b).

Because of a common mode of action and for purposes of cumulative risk assessment, US-EPA has grouped atrazine, simazine, propazine and their degradation products desethyl atrazine (DEA), desisopropyl atrazine (DIA), and desethyldesisopropyl atrazine (DACT). Although hydroxyatrazine has been shown to alter pregnancy and delay puberty in males it was not included in this group based on the absence of mammary gland tumour induction and inconclusive data on its effect on the LH surge and/or LH-dependent events. Terbutylazine and cyanazine was not considered for grouping because they did not have uses that resulted in exposure to the general public. (US-EPA 2002b).