TOXICOLOGY
Amitrole was first considered by the Joint Meeting in 1974. A conditional ADI of 0-0.00003 mg/kg bw was established at that time, which was extended by the 1977 Meeting after consideration of additional data on the basis of an NOAEL of 0.025 mg/kg bw per day in a three-month study in rats and a 1000-fold safety factor. In 1993, amitrole was re-evaluated within the CCPR periodic review programme, and the Meeting established a temporary ADI of 0-0.0005 mg/kg bw on the basis of an NOAEL of 0.5 mg/kg bw per day in a two-year study in rats and a 1000-fold safety factor because of the inadequacy of the database. The 1993 Meeting requested submission of the results of a two-generation study of reproductive toxicity in rats, a one-year study in dogs, a study of developmental toxicity after oral administration in rabbits, and a study of metabolism in rats. The results of these studies and of a study of effects on the thyroid gland of pregnant rabbits were reviewed at the present Meeting.
In rats, amitrole was rapidly absorbed, approximately 90% of the administered dose being eliminated within 48 h. The primary route of elimination was the urine, which accounted for over 87% of the administered dose; faecal elimination accounted for less than 6% of the dose and volatile metabolites represented only 0.1%. Forty-eight hours after dosing, the tissue residues amounted to less than 3% of the dose, with the majority found in the liver. Most of the radiolabel (62-90%) was excreted as the parent compound. Several metabolites were present at very low concentrations, a mercapturic acid derivative being predominant (2.1-7.3% of the administered 14C).
In a one-year study of toxicity, dogs were fed diets containing 0, 10, 500, or 1500 ppm amitrole. At 500 ppm (equal to 13 mg/kg bw per day) and above, the levels of tri-iodothyronine and thyroxine were reduced and there were increases in the incidence of rough coats, in thyroid weights, and in the frequency of lesions of the thyroid (follicular-cell hyperplasia, capsular fibrosis, vasculitis, and dilatation of the vasculature). In addition, ectopic thyroids with follicular hyperplasia and capsular fibrosis were noted, and changes in the pituitary (hyperplasia and hypertrophy) were evident in both males and females. At 1500 ppm (equal to 32 mg/kg bw per day), both males and females were anaemic and had increased platelet counts, decreased food intake, decreased brain weights (with histopathological lesions), and neurological effects. At this dose, males also had decreased weight gain, increased lactate dehydrogenase and cholesterol levels, decreased heart weights, and decreased P- and R-wave amplitudes. The NOAEL was 10 ppm, equal to 0.29 mg/kg bw per day.
In a two-generation study of reproductive toxicity, rats were fed diets containing 0, 0.5, 2, 15, or 110 ppm amitrole. At 15 ppm (equal to 0.9 mg/kg bw per day), the only effect observed was a slight increase in the severity of some histopathological changes in the thyroid, including small follicles, decreased colloid content, and follicular epithelial hypertrophy. Parental toxicity at 110 ppm was severe and was characterized by mortality, clinical signs, considerably decreased weight gain, thyroid effects (increased relative thyroid weights, follicular epithelial hyperplasia, and vascular ectasia), changes in relative organ weights (increased weights of pituitary, testes, epididymides, seminal vesicles, prostate, uterus, spleen, and kidneys and decreased weights of ovaries, adrenal glands, and liver), and histopathological lesions in the liver, adrenal glands, kidneys, and numerous reproductive tissues. In pups, the only observations were increased thyroid weights with accompanying histopathological lesions. The effects on reproduction included decreased mating and fertility indices, increased gestation length, and decreased prenatal survival, litter size, pup body weight, and viability. The NOAEL for systemic toxicity was 2 ppm, equal to 0.12 mg/kg bw per day. This value is supported by the findings of the range-finding study, in which enlarged, reddened thyroids and decreased colloid content were noted at 10 ppm (equal to 0.62 mg/kg bw per day). The NOAEL for reproductive toxicity was 15 ppm, equal to 0.9 mg/kg bw per day.
In a study of developmental toxicity, rabbits were treated with 0, 5, 20, or 80 mg/kg bw per day amitrole on days 6-18 of gestation. At 80 mg/kg bw per day, maternal food consumption and body-weight gain were reduced during the treatment period. Male fetal body weights and mean litter weights were reduced at 80 mg/kg bw per day. Clinical chemistry was not evaluated in this study. The NOAEL for maternal and fetal toxicity was 20 mg/kg bw per day.
In a special study to further characterize toxicity in pregnant rabbits, the animals were treated with 0, 5, 20, or 80 mg amitrole/kg bw per day on days 6-18 of gestation and were killed on day 19 of gestation. At 20 and 80 mg/kg bw per day, albumin and protein levels were decreased and absolute and relative liver weights were decreased. At 80 mg/kg bw per day, there were reductions in food consumption, increased creatine kinase levels, decreased tri-iodothyronine and thyroxine levels, and thyroid follicular-cell hypertrophy. The NOAEL for maternal toxicity was 5 mg/kg bw per day.
The results of these supplemental studies correlate well with the results of the studies reviewed by the 1993 JMPR, showing that the thyroid is the primary target organ of amitrole. Contrary to the conclusions of the 1993 JMPR, the results demonstrate that amitrole is also goitrogenic in the dog. In the one-year study in dogs reviewed previously, no effects on the thyroid were noted at a dose of 12.5 mg/kg bw per day. In the current one-year study, however, there were extensive effects at 13 mg/kg bw per day.
The rat was the most sensitive species, the NOAELs being 2 ppm in both the two-generation study of reproductive toxicity (equal to 0.12 mg/kg bw per day) and the 90-day dietary study (equivalent to 0.1 mg/kg bw per day), on the basis of histopathological changes in the thyroid. The Meeting noted that the rat is more sensitive to the development of thyroid hyperplasia and subsequent neoplasia after exposure to goitrogenic compounds than are humans. Therefore, a smaller safety factor (50) to take into account the lower uncertainty of interspecies extrapolation was used, giving an ADI of 0-0.002 mg/kg bw. This ADI provides a 150-fold margin of safety over the NOAEL of 0.29 mg/kg bw per day in the one-year study in dogs.
An addendum to the toxicological monograph was prepared.
TOXICOLOGICAL EVALUATION
(based on studies from this and earlier JMPR evaluations)
Levels that cause no toxic effect
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Mouse: |
10 ppm, equivalent to 1.5 mg/kg bw per day (18-month study of toxicity and carcinogenicity) |
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Rat: |
10 ppm, equivalent to 0.5 mg/kg bw per day (two-year study of toxicity and carcinogenicity) |
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100 mg/kg bw per day (maternal and fetal toxicity in a study of developmental toxicity) |
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2 ppm, equal to 0.12 mg/kg bw per day (parental toxicity in a two-generation study of reproductive toxicity) |
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Rabbit: |
5 mg/kg bw per day (maternal toxicity in a study of developmental toxicity) |
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20 mg/kg bw per day (fetal toxicity in a study of developmental toxicity) |
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Hamster: |
10 ppm, equivalent to 1 mg/kg bw per day (18-month study of toxicity) |
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Dog: |
10 ppm, equal to 0.29 mg/kg bw per day (one-year study of toxicity) |
Estimate of acceptable daily intake for humans
0-0.002 mg/kg bw
Studies that would provide information useful for the continued evaluation of the compound
1. Clarification of the genotoxic potential of amitrole (e.g. DNA adducts in vivo)
2. Further observations in humans.
Toxicological criteria for estimating guidance values for dietary and non-dietary exposure to amitrole
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Human exposure |
Relevant route, study type, species |
Results, remarks |
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Short-term |
Oral, acute toxicity, rat |
LD50 > 2500 mg/kg bw |
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Dermal, acute toxicity, rat |
LD50 > 2500 mg/kg bw |
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Inhalation, acute toxicity, 4 h, rat |
LC50 > 439 mg/l |
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Dermal, irritation, rabbit |
Minimally irritating |
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Ocular, irritation, rabbit |
Mildly irritating |
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Dermal, sensitization, guinea-pig |
Moderately sensitizing (Magnusson-Kligman test) Non-sensitizing (Klecak open epicutaneous test) |
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Medium-term |
Repeated oral, 2-4 weeks, rat |
NOAEL = 3 mg/kg bw per day: effects on the thyroid, food consumption and body weight |
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Repeated oral, drinking-water, 4 weeks, rat |
NOAEL =10 mg/l water: decreased weight gain, enlarged thyroids |
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Repeated oral, 6-13 weeks, rat |
NOAEL = 0.1 mg/kg bw per day: effects on the thyroid |
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Repeated dermal, 3 weeks, rabbit |
NOAEL = 100 mg/kg bw per day (highest dose tested) |
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Repeated inhalation, 4 weeks, rat |
NOAEL = 0.1 mg/l: effects on the thyroid |
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Repeated oral, special maternal toxicity, rabbit |
NOAEL = 5 mg/kg bw per day: decreased liver weights and protein/albumin levels |
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Repeated oral, developmental toxicity, rabbit |
NOAEL = 20 mg/kg bw per day: developmental toxicity |
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Repeated dermal, developmental toxicity, rabbit |
NOAEL = 1500 mg/kg bw per day: maternal and developmental toxicity |
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Repeated oral, reproductive toxicity, rat |
NOAEL = 0.12 mg/kg bw per day (systemic): effects on the thyroid |
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NOAEL = 0.9 mg/kg bw per day: reproductive toxicity |
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Long-term |
Repeated oral, 1 year, dog |
NOAEL = 0.29 mg/kg bw per day: effects on the thyroid |
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Repeated inhalation, intermittent dosing, 2 years, carcinogenicity, rat |
LOAEL = 50 m g/l (lowest dose tested): effects on the thyroid |
