Ethylene thiourea is a white crystalline solid used extensively in the rubber industry as an accelerator in the vulcanization of elastomers. It is also a trace contaminant and metabolic degradation product of a widely used class of ethylene bisdithiocarbamate fungicides. Ethylene thiourea is known to produce thyroid neoplasms in rats and liver neoplasms in mice following long-term administration; thus, it was chosen by the National Toxicology Program in an investigation of the potential value of perinatal exposures in assessing chemical carcinogenicity.
Chronic toxicity and carcinogenicity studies of ethylene thiourea, 99% pure, were conducted in F344/N rats and B6C3F1 mice of each sex. The studies were designed to determine 1) the effects of ethylene thiourea in rats and mice receiving adult exposure only (a typical carcinogenicity study), 2) the toxic and carcinogenic effects of ethylene thiourea on rats and mice receiving perinatal exposure only (dietary exposure of dams prior to breeding and throughout gestation and lactation), and 3) the effects of combined perinatal and adult exposure to ethylene thiourea.
Studies in F344/N rats
In a preliminary study to determine the perinatal dietary concentrations for the 2-year studies, female F344/N rats were fed 0, 8, 25, 83, or 250 ppm ethylene thiourea in the feed beginning 2 weeks prior to breeding and continuing throughout gestation and lactation, and the pups were fed at these same concentrations up to 9 weeks postweaning. Based on decreased survival of rat pups between postnatal days 0 to 4 and reduction in body weight gains in male weanling rats receiving 250 ppm, dietary concentrations of 0, 9, 30, and 90 ppm were selected for the perinatal (F0) exposure levels in the 2-year studies. Groups of 10 male and 10 female rats, 8 to 9 weeks of age, were fed diets containing 0, 60, 125, 250, 500, or 750 ppm ethylene thiourea for 13 weeks to determine the adult dietary concentrations. Because of reduced weight gains and decreased feed consumption in rats receiving 500 or 750 ppm ethylene thiourea, dietary concentrations of 0, 25, 83, and 250 ppm were selected for the adult (F1) exposure during the 2-year studies.
In the 2-year studies, perinatal and adult exposures to ethylene thiourea were applied separately and together to groups of male or female rats as shown in the following table.
The principal toxic effects of ethylene thiourea involved the thyroid gland. Serum levels of thyroxine (T4) and/or triiodothyronine (T3) were significantly decreased in rats receiving adult concentrations of 83 or 250 ppm, and thyrotropin (thyroid-stimulating hormone, TSH) was significantly increased at these concentrations. In male and female rats receiving adult-only exposure of 83 or 250 ppm, the incidences of follicular cell hyperplasia or follicular cell adenoma of the thyroid gland were significantly increased relative to the controls. The incidences of follicular cell carcinoma were significantly increased in the 250 ppm groups, and carcinomas occurred more frequently in males than in females.
| F1 Concentration (ppm)[b] | ||||
|---|---|---|---|---|
| F0(ppm)[c] | 0 | 25 | 83 | 250 |
| 0 | 60 | -- | 60 | 60 |
| 9 | -- | 60 | -- | -- |
| 30 | -- | -- | 60 | -- |
| 90 | 60 | -- | 60 | 60 |
[a] Ten rats from each group were sacrificed and evaluated at 9 months
[b] Concentration of ethylene thiourea in feed given to rats beginning at 8 weeks of age for 24 months
[c] Concentration of ethylene thiourea in feed through breeding, gestation, and lactation until pups were 8 weeks of age
Perinatal-only exposure to 90 ppm had no effect on the incidence of thyroid neoplasms in these studies, although there was a marginal increase in follicular cell hyperplasia relative to the controls. However, for groups of rats receiving combined perinatal and adult exposure (F0:F1), males and females receiving concentrations of 90:250 ppm ethylene thiourea had significantly increased incidences of thyroid follicular cell neoplasms relative to those receiving adult-only exposure to 250 ppm. Further, groups of male rats receiving 90:83 ppm showed a significantly increased incidence of follicular cell hyperplasia. Final mean body weights of males and survival of males and females receiving combined perinatal (90 ppm) and adult (250 ppm) exposure were lower than those receiving adult-only exposure of 250 ppm.
Thus, in rats, combined perinatal and adult exposure slightly enhanced the toxicity and proliferative effects on the thyroid gland observed with adult-only exposure to ethylene thiourea.
Neoplasms of the Zymbal's gland were marginally increased in rats receiving 90:250 ppm (males - 0:0, 1/50; 90:250, 5/50; females - 0:0, 1/50; 90:250, 4/50). Mononuclear cell leukemia occurred with a significant trend in groups of male and female rats receiving perinatal exposure of 90 ppm and increasing adult concentrations (90:0, 90:83, and 90:250 ppm), and for female rats without perinatal exposure (0:0, 0:83, and 0:250 ppm). The incidences of mononuclear cell leukemia in males receiving 90:83 ppm and males and females receiving 90:250 ppm were statistically significant relative to the respective 0:0 ppm groups. Low incidences of renal tubule cell adenomas occurred in most dose groups of male rats, but not in the highest dose group or the controls.
Studies in B6C3F1 mice
In a preliminary study to determine the perinatal dietary concentrations for the 2-year studies, adult female C57BL/6N mice were fed 0, 33, 100, 330, or 1,000 ppm ethylene thiourea in the feed beginning 2 weeks prior to breeding and continuing throughout gestation and lactation and up to 9 weeks postweaning. Because of reduced survival of mouse pups at postnatal day 28 and lower final mean body weights in weanlings receiving perinatal exposure of 1,000 ppm, dietary concentrations of 0, 33, 110, and 330 ppm were selected for the perinatal exposure levels in the 2-year studies. Groups of 10 male and 10 female mice, 8 to 9 weeks of age, were fed diets containing 0, 125, 250, 500, 1,000, or 2,000 ppm ethylene thiourea for 13 weeks to determine the adult dietary concentrations. Moderately severe diffuse follicular cell hyperplasia in the thyroid gland and centrilobular cytomegaly of the liver occurred in mice receiving 2,000 ppm. Because the severity of the thyroid lesion (and degree of hypothyroidism) at this concentration was considered potentially life threatening in 2-year studies, dietary concentrations of 0, 100, 330, and 1,000 ppm ethylene thiourea were selected for adult exposure during the 2-year studies.
In the 2-year studies, perinatal and adult exposures to ethylene thiourea were applied separately and together to groups of male or female mice as shown in the following table.
| F1 Concentration (ppm)[b] | ||||
|---|---|---|---|---|
| F0(ppm)[c] | 0 | 100 | 330 | 1,000 |
| 0 | 69 | -- | 60 | 60 |
| 33 | -- | 34/29[d] | -- | -- |
| 110 | -- | -- | 60 | -- |
| 330 | 60 | -- | 60 | 60 |
[a] Ten mice from each group except the 33:100 ppm group were sacrificed and evaluated at 9 months
[b] Concentration of ethylene thiourea in feed given to mice beginning at 8 weeks of age for 24 months
[c] Concentration of ethylene thiourea in feed through breeding, gestation, and lactation until pups were 8 weeks of age
[d] 34 males and 29 females assigned to group
The principal toxic effects of ethylene thiourea in mice occurred in the thyroid gland, liver, and pituitary gland. Serum levels of T3 were significantly decreased in groups of mice receiving adult concentrations of 1,000 ppm; TSH was significantly increased in mice receiving 330 and 1,000 ppm. The incidences of follicular cell hyperplasia and neoplasia increased principally in males receiving 1,000 ppm and in females receiving 330 or 1,000 ppm. Follicular cell carcinomas were significantly increased in mice receiving 1,000 ppm. Incidences of centrilobular hepatocellular cytomegaly (males and females), hepatocellular adenoma (females), hepatocellular carcinoma (males and females), and adenoma or carcinoma combined (males and females) also were significantly increased in mice receiving F1 concentrations of 330 or 1,000 ppm. In the pituitary gland, incidences of focal hyperplasia (males) or adenoma (males and females) of the pars distalis were significantly increased in groups of mice receiving 1,000 ppm ethylene thiourea.
Perinatal exposure to concentrations of 330 ppm had no effect on the incidences of nonneoplastic lesions or neoplasms in mice. For groups of mice receiving combined perinatal and adult exposure, females receiving F0:F1 concentrations of 330:330 ppm had significantly increased incidences of follicular cell adenoma relative to those receiving adult-only exposure to 330 ppm. Similarly, male mice receiving F0:F1 concentrations of 330:330 ppm had significantly increased incidences of follicular cell hyperplasia. Thus, in mice, perinatal exposure slightly enhanced the proliferative effects on the thyroid gland of adult exposure. There were no effects of perinatal exposure in mice at sites other than in the thyroid gland.
Conclusions
2-Year Adult-Only Exposure: Under the conditions of these 2-year adult-only dietary exposures, there was clear evidence of carcinogenic activity of ethylene thiourea in male and female F344/N rats, as shown by increased incidences of thyroid follicular cell neoplasms. There was clear evidence of carcinogenic activity of ethylene thiourea in male and female B6C3F1 mice as shown by increased incidences of thyroid follicular cell neoplasms, hepatocellular neoplasms, and adenomas of the pars distalis of the pituitary gland.
Nonneoplastic lesions associated with the administration of ethylene thiourea included follicular cell hyperplasia in rats and mice and follicular cell cytoplasmic vacuolation, centrilobular hepatocellular cytomegaly, and focal hyperplasia of the pars distalis of the pituitary gland in mice. Other effects associated with the administration of ethylene thiourea included decreased serum levels of T4 and/or T3 in rats and increased serum levels of TSH in rats and mice.
Perinatal-Only Exposure: Perinatal exposure alone had no effect on the incidences of neoplasms in rats or mice after 2 years. Animals may have been able to tolerate higher perinatal exposure concentrations.
Combined Perinatal and 2-Year Adult Exposures: Combined perinatal and 2-year adult dietary exposure to ethylene thiourea confirmed the findings of the 2-year adult-only exposures for the incidences of neoplasms in the thyroid gland of rats and mice and the liver and pituitary gland of mice. In male and female rats, combined perinatal and adult exposure to 90:250 ppm was associated with marginal increases, relative to the untreated (0:0 ppm) controls, in Zymbal's gland neoplasms and mononuclear cell leukemia, which may have been related to chemical administration. In rats receiving adult exposure to 250 ppm ethylene thiourea, perinatal exposure to 90 ppm was associated with a slightly enhanced incidence of thyroid neoplasms compared to adult-only exposure. However, increasing perinatal exposure from 0 to 90 ppm had no effect on incidences of thyroid neoplasms in rats receiving adult exposure to 83 ppm. Increasing perinatal exposure from 0 to 330 ppm was associated with a marginally increased incidence of thyroid neoplasms in female mice receiving adult exposure to 330 ppm, but there were no enhancing effects of perinatal exposure in mice receiving adult exposure to 1,000 ppm.