Potassium carbamoylcarbamate

Endpoint summary

Description of key information

According to the read-across strategy document as provided as an attachment to this Chemical Safety Report (CSR), potassium allophonate quickly hydrolyses to urea under physiological conditions. Therefore, the repeated-dose systemic toxicity of potassium allophonate was evaluated based on read-across from appropriate studies on urea. The highest quality repeated-dose study available on urea is a 12-month oral carcinogenicity/chronic toxicity screening assay, in which F-344 rats and C57BL/6 mice (50/sex/group) were exposed to urea in the diet at concentrations of 4500, 9000 or 45,000 ppm for 12 months. Chronic administration of urea to rats and mice in their feed produced no dose-dependent, biologically significant chronic or carcinogenic effects that could be clearly attributable to the test substance. For purposes of derivation of DNELs for potassium allophonate, the lowest NOAEL from the one-year urea study is used (3786 mg/kg/day for male rats).

Key value for chemical safety assessment

Repeated dose toxicity: via oral route - systemic effects

Link to relevant study records

Reference

Endpoint:

chronic toxicity: oral

Type of information:

migrated information: read-across from supporting substance (structural analogue or surrogate)

Adequacy of study:

weight of evidence

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

other: Published U.S. National Cancer Institute screening study for chronic toxicity and carcinogenicity: 12-month exposure with 4-month recovery period; however, limited toxicological investigations.

Principles of method if other than guideline:

12-month screening carcinogenesis/chronic toxicity study followed by a 4-month recovery period with limited assessment of toxicological endpoints.

GLP compliance:

not specified

Limit test:

no

Species:

rat

Strain:

Fischer 344

Sex:

male/female

Details on test animals or test system and environmental conditions:

Animals were obtained from Charles River and randomly assigned to dose groups. Animals were 6 weeks old when assigned to the study and were group housed (5/sex/cage). Food and water were available ad libitum. The mean ambient air temperature was 23 degrees C and a 12-hour light/dark cycle was maintained. Diets were prepared weekly.

Route of administration:

oral: feed

Vehicle:

unchanged (no vehicle)

Details on oral exposure:

DIET PREPARATION
- Rate of preparation of diet (frequency): once each week
- Mixing appropriate amounts with (Type of food): The test substance was initially formulated/prepared as a concentrated hand-blended premix. The mixture and remainder of the stock diet were then thoroughly blended in a Patterson-Kelley stainless steel Twin Shell V-Blender for 20-30 minutes.
- Storage temperature of food: Test diets were stored at 4 degrees C.

Stability studies on the test substance admixed with feed were performed on day 1 and day 14 using a colorimetric urea nitrogen method. The analytical results indicated the test substance was stable over a period of 14 days. Diet mixes older than two weeks were discarded.

Analytical verification of doses or concentrations:

not specified

Duration of treatment / exposure:

Animals were exposed to urea for 12 months, followed by a 4-month recovery period.

Frequency of treatment:

Continuous (ad libitum)

Remarks:

Doses / Concentrations:
4500, 9000, 45000 ppm
Basis:
nominal in diet

No. of animals per sex per dose:

50/sex

Details on study design:

- Dose selection rationale: no data
- Rationale for animal assignment (if not random): the random casual method was used
- Rationale for selecting satellite groups: no satellite groups were used
- Post-exposure recovery period in satellite groups: not applicable
- Section schedule rationale (if not random): not applicable

Observations and examinations performed and frequency:

CAGE SIDE OBSERVATIONS: No data

DETAILED CLINICAL OBSERVATIONS: Yes
- Time schedule: Twice daily

BODY WEIGHT: Yes
- Time schedule for examinations: Individual bodyweights were recorded pre-test and at study termination. Cage weights were recorded weekly during the study.

FOOD CONSUMPTION AND COMPOUND INTAKE (if feeding study):
- Food consumption for each animal determined and mean daily diet consumption calculated as g food/kg body weight/day: No
- Compound intake calculated as time-weighted averages from the consumption and body weight gain data: No

FOOD EFFICIENCY:
- Body weight gain in kg/food consumption in kg per unit time X 100 calculated as time-weighted averages from the consumption and body weight gain data: No

OPHTHALMOSCOPIC EXAMINATION: No

HAEMATOLOGY: No

CLINICAL CHEMISTRY: No

URINALYSIS: No

NEUROBEHAVIOURAL EXAMINATION: No

Sacrifice and pathology:

Five animals/sex/group were sacrificed at the end of the 365-day exposure period and a comprehensive list of tissues were investigated histopathologically; interim deaths were similarly investigated. Brain, lung, trachea, heart, thymus, pituitary, thyroids, parathyroids, adrenals, esophagus, stomach, duodenum, jejunum, ileum, colon, liver, spleen, lymph nodes, bone and bone marrow, skin, salivary and mammary glands were collected and fixed in 10% neutral buffered formalin. Tissues were paraffin embedded, cut at 4-6 um, stained with H&E, and examined microscopically to determine presence of neoplastic/preneoplastic changes or signs or any toxic syndrome.

All remaining animals were sacrificed after the 4-month recovery period and investigated histopathologically.

Statistics:

The purpose of the statistical analysis of tumors was to determine whether animals receiving test substance developed a significantly (P<0.05) different proportion of tumors than control animals. Statistical analyses of the incidences of specific types of tumors were made using the Fisher exact test to compare each control group with each group of treated animals at each dose. If more than one dose level of test substance was used, the Cochran-Armitage test for linear trend in proportions with continuity correction was used.

Clinical signs:

effects observed, treatment-related

Description (incidence and severity):

The mid-dose group of male rats showed decreased survival (89%) relative to diet controls (95%).

Mortality:

mortality observed, treatment-related

Description (incidence):

The mid-dose group of male rats showed decreased survival (89%) relative to diet controls (95%).

Body weight and weight changes:

no effects observed

Food consumption and compound intake (if feeding study):

not examined

Food efficiency:

not examined

Water consumption and compound intake (if drinking water study):

not examined

Ophthalmological findings:

not examined

Haematological findings:

not examined

Clinical biochemistry findings:

not examined

Urinalysis findings:

not examined

Behaviour (functional findings):

not examined

Organ weight findings including organ / body weight ratios:

not examined

Gross pathological findings:

no effects observed

Histopathological findings: non-neoplastic:

no effects observed

Histopathological findings: neoplastic:

effects observed, treatment-related

Description (incidence and severity):

Among the high dosed rats, there was a statistically significant increase in incidence of interstitial cell adenomas of the testes.

Details on results:

The lesion of interstitial cell adenomas of the testes found in the high dose group males may occur in 100% of controls (historically); thus, its biological significance is questionable according to the study authors.

There were no bodyweight effects. The decreased survival of the mid-dose group males had no dose-relationship as survival of all other test groups remained unaffected by treatment.

Dose descriptor:

NOAEL

Effect level:

45 000 ppm

Sex:

male/female

Basis for effect level:

other: No dose-related, biologically significant adverse effects were attributable to the test substance up to the highest dose level (4.5% in the diet)

Critical effects observed:

not specified

Based on the limited toxicological endpoints evaluated in this study, there were no dose-dependent, biologically significant carcinogenic or toxic effects at dose levels of up to 45000 ppm.

Conclusions:

In this study with limited evaluation of toxicological endpoints, chronic administration of urea to the rat produced no dose-dependent, biologically significant chronic or carcinogenic effects that could be clearly attributable to the test substance.

Executive summary:

In a 12-month carcinogenicity/chronic screening assay, Fischer 344 rats (50/sex/group) were exposed to urea in the diet at concentrations of 4500, 9000 or 45000 ppm for 12 months. Five animals/sex/group were sacrificed at the end of the 365-day exposure period and a comprehensive list of tissues was investigated histopathologically; interim deaths were similarly investigated. All remaining animals were sacrificed after the 4-month recovery period and investigated histopathologically. There were no clear dose-dependent signs of chronic toxicity based on the limited toxicological assessment; bodyweights were unaffected by treatment, while only the mid-dose group of males had reduced survival. Gross and microscopic pathology did not reveal dose-dependent, biologically significant adverse effects (chronic or carcinogenic). This study concluded that urea is non-carcinogenic and of low chronic toxicity.

Endpoint conclusion

Endpoint conclusion:

no adverse effect observed

Dose descriptor:

NOAEL

3 786 mg/kg bw/day

Study duration:

chronic

Species:

rat

Quality of whole database:

Reliable with restrictions

Repeated dose toxicity: inhalation - systemic effects

Endpoint conclusion

Endpoint conclusion:

no study available

Repeated dose toxicity: inhalation - local effects

Endpoint conclusion

Endpoint conclusion:

no study available

Repeated dose toxicity: dermal - systemic effects

Endpoint conclusion

Endpoint conclusion:

no study available

Repeated dose toxicity: dermal - local effects

Endpoint conclusion

Endpoint conclusion:

no study available

Additional information

No repeat dose toxicity data are currently available on potassium allophonate; however, repeat dose toxicity data are available for urea. Based on the read-across strategy developed, the repeated-dose oral toxicity data on urea was used to support potassium allophonate (Fleischman et al., 1980). In a 12-month carcinogenicity/chronic screening assay, Fischer 344 rats and C57BL/6 mice (50/sex/group) were exposed to urea in the diet at concentrations of 4500, 9000 or 45000 ppm for 12 months. The feed consumption and dose per kg bw were values were not reported in the original study; however, they were calculated in U.S. EPA (2011) as described herein. Feed consumption was calculated in using the formula feed intake (kg/day) = 0.065 × (body weight)^0.7919. The default body weights for male and female F344 rats in a 1-year study are 0.29 and 0.175 kg, respectively, and for mice 0.0238 and 0.0206 kg, respectively. The resulting feed intakes were calculated to be 0.0244 kg/day for male rats, 0.0163 kg/day for female rats, 0.0034 kg/day for male mice, and 0.0030 kg/day for female mice. Approximate dose levels for the various groups of animals are thus 0, 379, 757 or 3786 mg/kg bw/day for male rats; 0, 419, 838 or 4191 mg/kg bw/day for female rats; 0, 644, 1288 or 6442 mg/kg bw/day for male mice; and 0, 655, 1311 or 6553 mg/kg bw/day for female mice. Five animals/sex/group were sacrificed at the end of the 365-day exposure period and a comprehensive list of tissues was investigated histopathologically; interim deaths were similarly investigated. All remaining animals were sacrificed after the 4-month recovery period and investigated histopathologically. There were no clear dose-dependent signs of chronic toxicity based on the limited toxicological assessment in either rats or mice. Gross and microscopic pathology did not reveal any dose-dependent, biologically significant adverse effects.

In addition, large quantities of urea are formed naturally in the human body as a consequence of normal protein catabolism. The level of any primary or occupational exposure to urea or any secondary exposure (i.e., via exposure to potassium allophonate which hydrolyses to urea) is likely to be insignificant compared to the quantities (20-35 g/day) produced by normal metabolism and present at high concentrations in the blood.

References:

Fleischman, R. W., Baker, J. R., Hagopian, M., Wade, G. G., Hayden, D. W., Smith, E. R., Weisburger, J. H. and Weisburger, E. K. 1980. Carcinogenesis bioassay of acetamide, hexanamide, adipamide, urea and P-tolylurea in mice and rats. Journal of Environmental Pathology and Toxicology, 3(5-6): 149-70.

United States Environmental Protection Agency’s (U.S. EPA). 2011. Toxicological Review of Urea (CAS No. 57-13-6): In Support of Summary Information on the Integrated Risk Information System (IRIS). July 2011. Available at: http://www.epa.gov/iris/toxreviews/1022tr.pdf

Justification for selection of repeated dose toxicity via oral route - systemic effects endpoint:
No repeated-dose studies with potassium allophonate were identified. However, it has been documented that potassium allophonate hydrolyses to urea under physiological conditions (see the read-across strategy document attached to this Chemical Safety Report [CSR]). A few repeated-dose oral toxicity studies with urea are available for read across to potassium allophonate. The highest quality repeat-dose study available on urea is a 12 -month carcinogenicity/chronic toxicity screening assay, in which F-344 rats and C57BL/6 mice (50/sex/group) were exposed to urea in the diet at concentrations of 4500, 9000 or 45,000 ppm for 12 months. For purposes of derivation of Derived No Effect Levels (DNEL) for potassium allophonate, the lowest No Observable Adverse Effect Level (NOAEL) of 3786 mg/kg/day from the one-year urea study is used.

Justification for classification or non-classification

The repeated-dose systemic toxicity of potassium allophonate was evaluated based on read-across from appropriate studies on urea, since it has been documented that potassium allophonate hydrolyses to urea under physiological conditions. Based on the one-year repeat-dose oral toxicity study on urea, urea would not warrant classification. Therefore, based on read-across, potassium allophonate does not warrant classification for the repeat-dose toxicity endpoint.