1,3-diethyl-2-thiourea

Carcinogenicity

Description of key information

A bioassay for the possible carcinogenicity of DETU was conducted using rats and mice (NTP 1978). DETU was carcinogenic to rats, causing follicular-cell carcinomas of the thyroid in males and follicular-cell neoplasms of the thyroid females. There was no evidence for the carcinogencicity of the compound in male and female mice.

Key value for chemical safety assessment

Carcinogenicity: via oral route

Link to relevant study records

Reference

Reference 1

Endpoint:

carcinogenicity: oral

Type of information:

experimental study

Adequacy of study:

key study

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

comparable to guideline study with acceptable restrictions

Reason / purpose for cross-reference:

reference to same study

Qualifier:

no guideline followed

Principles of method if other than guideline:

Study was performed to determine whether selected chemicals have the capacity to produce cancer in animals. Rats were exposed to DETU in diet during 103 weeks.

GLP compliance:

not specified

Species:

rat

Strain:

Fischer 344

Sex:

male/female

Details on test animals or test system and environmental conditions:

TEST ANIMALS
- Source: Frederick Cancer Research Center, Frederick, Maryland.
- Age at study initiation: around 6 weeks-old
- Weight at study initiation: no data
- Fasting period before study: no data
- Housing: in polycarbonate cages suspended from aluminium racks.
- Diet (e.g. ad libitum): Wayne Lab-Blox (Allied Mills, Inc. Chicago, Ill.), ad libitum
- Water (e.g. ad libitum): aciduled water (pH 2.5) was suplied to animals in water bottles filled in an automated metering device, ad libitum
- Acclimation period: 2 weeks

ENVIRONMENTAL CONDITIONS
- Temperature (°C): 22-26°C
- Humidity (%):45-55%
- Air changes (per hr):12-15
- Photoperiod (hrs dark / hrs light): fluorescent lighlting 8h/day (9.00 am to 5.00 pm)

Route of administration:

oral: feed

Vehicle:

unchanged (no vehicle)

Details on exposure:

DETU was administrated to the dosed animals as a component of the diet.

Analytical verification of doses or concentrations:

no

Duration of treatment / exposure:

103 weeks

Frequency of treatment:

daily

Post exposure period:

1-week

Dose / conc.:

250 ppm

Dose / conc.:

125 ppm

No. of animals per sex per dose:

- Number of animals per dose: 50 males + 50 females
- Control groups: 20 males + 20 females

Control animals:

yes, concurrent no treatment

Positive control:

no

Observations and examinations performed and frequency:

- Body weight: immediately prior to initiation of the experiment and then once monthly throughout the bioassay
- Food consumption: collected at monthly intervals from 20 of the animals of the animals in each group.
- Mortality: inspected twice daily

Sacrifice and pathology:

- Animal sacrifice: by CO2 asphyxiation, at the end of the bioassay or when moribund or when animals developed large, palpable masses that jeopardize their health.
- Necropsy: as soon as death occured.
- Gross and microscopic examination of all major tissues, organs and gross lesions.
- Tissues were preserved in a 10 percent neutral buffered formalin solution, embedded in paraffin, sectioned, and stained with hematoxylin and eosin prior to microscopic examination. Slides were prepared from the following tissues: skin, subcutaneous tissue, lungs and bronchi, trachea, bone marrow, spleen, lymph nodes, thymus, heart, salivary gland, liver, pancreas, esophagus, stomach, small intestine, large intestine, kidney, urinary bladder, pituitary, adrenal, thyroid, parathyroid, testis, prostate, brain, tunica vaginalis, uterus, mammary gland and ovary. - A few tissues were not examined for some animals, particularly for those that died early. Besides, some animals were missing, cannibalized, or judged to be in such an advanced state of autolysis as to preclude histopathologic interpretation.

Other examinations:

no

Statistics:

- Statistical analyses for a possible dose-related effect on survival used the method of Cox (1972) when testing two groups for equality and used Tarone's (1975) extensions of Cox's methods when testing a dose-related trend.
- To determine whether animals receiving the test substance developed a significantly higher proportion of tumors than did the control animals, the one-tailed Fisher exact test was used to compare the tumor incidence of a control group to that of a group of treated animals at each dose level. In practice, the incidence of lesions was analyzed by a Cochran-Armitage test which was used when comparing the dosed groups to the control and which was supported by a Fisher exact test, comparing the high dose group to the control.

Clinical signs:

no effects observed

Description (incidence and severity):

No abnormal clinical signs were recorded.

Dermal irritation (if dermal study):

not examined

Mortality:

no mortality observed

Description (incidence):

There were no significant positive associations between the dosages of N,N'-diethylthiourea administered and mortality of either sex.
There were adequate numbers of animals in all dose groups survived sufficiently long to be at risk from late-developing tumors.

Body weight and weight changes:

no effects observed

Description (incidence and severity):

No evidence of mean body weight depression was associated with compound administration in either male or female rats.

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

Immunological findings:

not examined

Organ weight findings including organ / body weight ratios:

not examined

Gross pathological findings:

effects observed, treatment-related

Description (incidence and severity):

Please find the summary of the gross pathological findings in the section "histopathological findings: neoplastic".

Neuropathological findings:

not examined

Histopathological findings: non-neoplastic:

effects observed, treatment-related

Description (incidence and severity):

Please find the summary of the non-neoplastic histopathological findings in the section "histopathological findings: neoplastic".

Histopathological findings: neoplastic:

effects observed, treatment-related

Description (incidence and severity):

A relatively high incidence of thyroid tumors (in particular thyroid follicular-cell carcinomas and follicular-cell adenomas) was noted and appeared to be related to the dietary administration of N,N'-diethylthiourea:
. the incidence of follicular-cell carcinomas was significantly increased in male rats exposed to high dose (11 animals among 48 examined histologically),
. the incidence of follicular-cell adenomas and carcinomas was significantly increased too in female rats exposed to high dose (follicular-cell adenomas: 9/46, follicular-cell carcinomas: 8/48),
. moreover few C-cell carcinomas and C-cell adenomas were observed too in males and females exposed to high dose.
Thyroid hyperplasia (cystic and follicular-cell) was commonly recognized and appeared to be related to dietary administration and dosage compound.
A few neoplasms were found in other organs (in particular in pituitary, mammary gland and uterus) but there were no other significant association between administration of test substance and increased incidences of tumors at any site in either male or female rats.
Other non-neoplastic lesions were observed but their incidence did not increase significantly in comparison with control groups (ex: pneumonia, myocardium fibrosis, liver hyperplasia, parasitism of colon chronic inflammation of kidney...).

Other effects:

not examined

Relevance of carcinogenic effects / potential:

Under the conditions of the bioassay, DETU was carcinogenic in Fischer 344 rats, inducing thyroid neoplasms and hyperplasia.

Dose descriptor:

NOAEL

Effect level:

< 125 ppm

Sex:

male/female

Basis for effect level:

other: based on thyroid toxicity (hyperplasia) in rats at this dose 125 ppm x 0.05 (assumed rat food consumption per bw) = 6.25 mg/kg bw/d.

Remarks on result:

other: general systemic toxicity

Dose descriptor:

NOAEL

Effect level:

< 125 ppm

Sex:

male/female

Basis for effect level:

other: Hyperplasia and atrophy of thyroid were observed at 125 ppm in male (15%) and female (15%) rats. 125 ppm x 0.05 (assumed rat food consumption per bw) = 6.25 mg/kg bw/d.

Remarks on result:

other: carcinogenicity

Tables of results: Carcinogenicity study in rats with DETU

 

1/ Analyses of the incidence of primary tumors at specific sites in female rats treated with DETU

 

Topography: morphology	control	Low dose (250 ppm)	High dose (500 ppm)
Hematopoietic system : Leukemia or malignant lymphoma weeks to first observed tumor	3/20 (15%)   104	7/50 (14%)   89	4/49 (8%)   83
Pituitary: Chromophobe adenoma or chromophobe carcinoma weeks to first observed tumor	5/18 (28%)   87	18/47 (38%)   95	22/45 (49%)   64
Adrenal: Phechromocytoma weeks to first observed tumor	1/18 (6%) 104	1/47 (2%) 104	2/45 (4%) 104
Thyroid: Follicular-cell carcinoma weeks to first observed tumor	0/18 (0%) /	1/46 (2%) 104	8/46 (17%) 100
Thyroid: Follicular cell-carcinoma or Follicular-cell adenoma weeks to first observed tumor	0/18 (0%)   /	5/46 (11%)   104	17/46 (37%)*   91
Mammary gland: Fibroadenoma weeks to first observed tumor	0/20 (0%) /	6/50 (12%) 101	6/49 (12%) 84
Uterus: Endometrial stromal polyp weeks to first observed tumor	4/19 (21%) 86	6/49 (12%) 104	4/48 (8%) 76

 

* Significative difference between treated and control groups (cochran-Armitage est supported by a Fischer exact test)

 

2/ Analyses of the incidence of primary tumors at specific sites in male rats treated with DETU

 

Topography: morphology	control	Low dose (250 ppm)	High dose (500 ppm)
Subcutaneous tissue: Fibrosarcoma weeks to first observed tumor	0/20 (0%) /	3/50 (6%) 87	1/50 (2%) 104
Skin and subcutaneous tissue: Fibrosarcoma or Neurofibrosarcoma weeks to first observed tumor	0/20 (0%)     /	3/50 (6%)     87	2/50 (4%)     104
Hematopoietic system : Leukemia or malignant lymphoma weeks to first observed tumor	2/20 (10%)   99	8/50 (16%)   89	2/50 (4%)   88
Pituitary: Chromophobe adenoma or chromophobe carcinoma weeks to first observed tumor	0/17 (0%)   /	6/46 (13%)   87	6/48 (13%)   101
Adrenal: Phechromocytoma weeks to first observed tumor	1/18 (6%) 104	4/50 (8%) 104	3/50 (6%) 104
Thyroid: Follicular-cell carcinoma weeks to first observed tumor	0.18 (0%) /	1/45 (2%) 104	11/48 (23%)* 91
Thyroid: Follicular cell-carcinoma or Follicular-cell adenoma weeks to first observed tumor	0.18 (0%)   /	1/45 (2%)   104	15/48 (31%)*   91
Thyroid C-cell adenoma or C-cell carcinoma weeks to first observed tumor	1/18 (6%)   104	0/45 (0%)   /	3/48 (6%)   104
Testis : Interstitial-cell tumor weeks to first observed tumor	14/20 (70%) 94	37/49 (76%) 88	36/50 (72%) 88

 

* Significative difference between treated and control groups (cochran-Armitage est supported by a Fischer exact test)

 

 

3/ Male and female rat thyroid tumors (incidence)

 

Thyroid	Males			Females
	control	Low dose	High dose	control	Low dose	High dose
Number of animals with tissues examined histopathologically	(18)	(45)	(48)	(18)	(46)	(46)
C-cell adenoma	0	0	2	0	1	1
C-cell carcinoma	1	0	1	0	0	1
Follicular-cell adenoma (all types)	0	0	6	0	4	9
Follicular-cell carcinoma (all types)	0	1	11	0	1	8

 

Executive summary:

Study was performed to determine whether DETU have the capacity to produce cancer in animals.

Rats were exposed to DETU in diet during 103 weeks at 125 and 250 ppm (6.25 and 12.50 mg/kg bw/d respectively).

No mortality, no clinical signs and no change of body weight gain were observed during this study.

But, based upon statistical results, the administration of DETU was associated with the increased incidence of follicular-cell carcinomas of the thyroid in male and follicular-cell neoplams of the thyroid in female rats (at 125 and 250 ppm).

Under the conditions of the bioassay, DETU was carcinogenic in Fischer 344 rats, inducing thyroid neoplasms and hyperplasia.

Reference 2

Endpoint:

carcinogenicity: oral

Type of information:

experimental study

Adequacy of study:

key study

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

comparable to guideline study with acceptable restrictions

Reason / purpose for cross-reference:

reference to same study

Qualifier:

no guideline followed

Principles of method if other than guideline:

Study was performed to determine whether selected chemicals have the capacity to produce cancer in animals. Mice were exposed to DETU in diet during 103 weeks.

GLP compliance:

not specified

Species:

mouse

Strain:

B6C3F1

Sex:

male/female

Details on test animals or test system and environmental conditions:

TEST ANIMALS
- Source: Charles River breeding Laboratories, Inc., Wilmington, Massachussets
- Age at study initiation: around 6 weeks-old
- Weight at study initiation: no data
- Fasting period before study: no data
- Housing: in polycarbonate cages suspended from aluminium racks.
- Diet (e.g. ad libitum): Wayne Lab-Blox (Allied Mills, Inc. Chicago, Ill.), ad libitum
- Water (e.g. ad libitum): aciduled water (pH 2.5) was suplied to animals in water bottles filled in an automated metering device, ad libitum
- Acclimation period: 2 weeks

ENVIRONMENTAL CONDITIONS
- Temperature (°C): 22-26°C
- Humidity (%):45-55%
- Air changes (per hr):12-15
- Photoperiod (hrs dark / hrs light): fluorescent lighlting 8h/day (9.00 am to 5.00 pm)

Route of administration:

oral: feed

Vehicle:

unchanged (no vehicle)

Details on exposure:

- Concentration: 250 and 500 ppm.
- Positive control: non

Analytical verification of doses or concentrations:

no

Duration of treatment / exposure:

103 weeks

Frequency of treatment:

daily

Post exposure period:

1 week

Dose / conc.:

500 ppm

Dose / conc.:

250 ppm

No. of animals per sex per dose:

- Number of animals per dose: 50 males + 50 females
- Control groups: 20 males + 19 females

Control animals:

yes, concurrent no treatment

Positive control:

no

Observations and examinations performed and frequency:

- Body weight: immediately prior to initiation of the experiment and then once monthly throughout the bioassay
- Food consumption: collected at monthly intervals from 20 of the animals of the animals in each group.
- Mortality: inspected twice daily

Sacrifice and pathology:

- Animal sacrifice: by CO2 asphyxiation, at the end of the bioassay or when moribund or when animals developed large, palpable masses that jeopardize their health.
- Necropsy: as soon as death occured.
- Gross and microscopic examination of all major tissues, organs and gross lesions.
- Tissues were preserved in a 10 percent neutral buffered formalin solution, embedded in paraffin, sectioned, and stained with hematoxylin and eosin prior to microscopic examination. Slides were prepared from the following tissues: skin, subcutaneous tissue, lungs and bronchi, trachea, bone marrow, spleen, lymph nodes, thymus, heart, salivary gland, liver, pancreas, esophagus, stomach, small intestine, large intestine, kidney, urinary bladder, pituitary, adrenal, thyroid, parathyroid, testis, prostate, brain, tunica vaginalis, uterus, mammary gland and ovary. - A few tissues were not examined for some animals, particularly for those that died early. Besides, some animals were missing, cannibalized, or judged to be in such an advanced state of autolysis as to preclude histopathologic interpretation.

Statistics:

- A Tarone test was performed to evaluate association between dosage and mortality. - Statistical analyses for a possible dose-related effect on survival used the method of Cox (1972) when testing two groups for equality and used Tarone's (1975) extensions of Cox's methods when testing a dose-related trend.
- To determine whether animals receiving the test substance developed a significantly higher proportion of tumors than did the control animals, the one-tailed Fisher exact test was used to compare the tumor incidence of a control group to that of a group of treated animals at each dose level. In practice, the incidence of lesions was analyzed by a Cochran-Armitage test which was used when comparing the dosed groups to the control and which was supported by a Fisher exact test, comparing the high dose group to the control.

Clinical signs:

no effects observed

Description (incidence and severity):

No abnormal clinical signs were recorded.

Dermal irritation (if dermal study):

not examined

Mortality:

no mortality observed

Description (incidence):

There were no significant positive associations between the dosages of N,N'-diethylthiourea administered and mortality of either sex.
There were adequate numbers of animals in all dose groups survived sufficiently long to be at risk from late-developing tumors.

Body weight and weight changes:

effects observed, treatment-related

Description (incidence and severity):

Dose related mean body weight depression was apparent in both male and female mice after week 30, indicating that the concentrations of N,N'-diethylthiourea administered to mice may have approximated the maximum tolerated dosages (250 and 500 ppm)..

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

Immunological findings:

not examined

Organ weight findings including organ / body weight ratios:

not examined

Gross pathological findings:

no effects observed

Neuropathological findings:

not examined

Histopathological findings: non-neoplastic:

no effects observed

Description (incidence and severity):

The severity and incidence of non-neoplastic lesions were also not unusual (ex: pneumonia, spleen hyperplasia...).

Histopathological findings: neoplastic:

no effects observed

Description (incidence and severity):

In both sexes, the neoplasms observed were similar in type and distribution in dosed and control mice (ex: alveolar/bronchiolar adenoma, hepatocellular adenoma and carcinoma, leukemia or malignant lymphoma...). None of statistical tests for any site revealed a significant positive association between administration of compound and increased tumor incidence.

Other effects:

not examined

Relevance of carcinogenic effects / potential:

The test substance was considered as not carcinogenic in B6C3F1 mice.

Dose descriptor:

NOAEL

Effect level:

< 250 ppm

Sex:

male/female

Basis for effect level:

other: Decrease of body weight gain was observed at 250 and 500 ppm in mice.

Remarks on result:

other: Systemic toxicity

Dose descriptor:

NOAEL

Effect level:

> 500 ppm

Sex:

male/female

Basis for effect level:

other: No tumors was observed in mice after exposure of 250 and 500 ppm of DETU during 103 weeks.

Remarks on result:

other: carcinogenicity

Tables of results: Carcinogenicity study in mice with DETU

 

1/ Analyses of the incidence of primary tumors at specific sites in female mice treated with DETU

 

Topography: morphology	control	Low dose (250 ppm)	High dose (500 ppm)
Hematopoietic system : Leukemia or malignant lymphoma weeks to first observed tumor	8/19 (42%)   93	15/48 (31%)   78	9/41 (22%)   92
Uterus: Endometrial stromal polyp weeks to first observed tumor	0/17 (0%) /	3/45 (7%) 62	2/38 (5%) 104

 

 

2/ Analyses of the incidence of primary tumors at specific sites in male mice treated with DETU

 

Topography: morphology	control	Low dose (250 ppm)	High dose (500 ppm)
Lung: Alveolar/Bronchiloar adenoma or alveolar/bronchiolar carcinoma weeks to first observed tumor	2/13 (15%)     99	4/46 (9%)     104	6/46 (13%)     99
Hematopoietic system : Leukemia or malignant lymphoma weeks to first observed tumor	3/15 (20%)   93	11/48 (23%)   90	12/49 (24%)   76
Circulatory system: Hemangioma or hemangiosarcoma weeks to first observed tumor	1/15 (7%)   104	1/48 (2%)   104	3/49 (6%)   99
Liver: Hepatocellular carcinoma weeks to first observed tumor	2/14 (14%) 104	5/48 (10%) 71	2/49 (4%) 104
Liver: Hepatocellular carcinoma or hepatocellular adenoma weeks to first observed tumor	5/14 (36%)*   /	7/48 (15%)   104	3/49 (6%)*   91

 

* Significative difference between treated and control groups (cochran-Armitage est supported by a Fischer exact test)

 

Executive summary:

Study was performed to determine whether DETU have the capacity to produce cancer in animals.

Mice were exposed to DETU in diet during 103 weeks at 250 and 500 ppm (12.5 and 25 mg/kg bw/d respectively).

No mortality, no clinical signs were observed during this study, but decrease of body weight gain was observed at each dose.

In both sexes, the neoplasms observed were similar in type and distribution in dosed and control mice (ex: alveolar/bronchiolar adenoma, hepatocellular adenoma and carcinoma, leukemia or malignant lymphoma...). None of statistical tests for any site revealed a significant positive association between administration of compound and increased tumor incidence.

The severity and incidence of non-neoplastic lesions were also not unusual (ex: pneumonia, spleen hyperplasia...).

The test substance was considered as not carcinogenic in B6C3F1 mice.

Endpoint conclusion

Endpoint conclusion:

adverse effect observed

Dose descriptor:

LOAEL

6.25 mg/kg bw/day

Study duration:

chronic

Species:

rat

Quality of whole database:

The study is a reliable chronic study with a klimisch score of 2.

Carcinogenicity: via inhalation route

Endpoint conclusion

Endpoint conclusion:

no study available

Carcinogenicity: via dermal route

Endpoint conclusion

Endpoint conclusion:

no study available

Justification for classification or non-classification

IARC Classification and justification :

According to the IARC (2001), 1,3-diethyl-2-thiourea is not classifiable as to its carcinogenicity to humans (Group 3) based on inadequate evidence in humans and limited evidence in experimental animals for the carcinogenicity of DETU.

DETU was tested for carcinogenicity by dietary administration in one experiment each mice and rats. Thyroid follicular-cell adenomas and carcinomas were induced in rats of each sex, but no increase in the incidence of tumours at any site was seen in mice.

No information was available on the mechanism of action of DETU, but some data were available at this time on its genotoxic potential.

The indication of a potential carcinogenic risk to human is questionable because DETU produced thyroid-follicular cell tumours in rats and no tumours in mice and an equivocal response in terms of DNA repair induction in human thyroid cells.

Considering the limited evidence of carcinogenicity in animal studies and the questionable genotoxic effects on the target organ (thyroid, genotoxicity study of Mattioli 2006), according to Regulation EC no.1272/2008, no classification as carcinogenic is warranted for DETU.

- Regulation (EC) No 1272/2008 :Not classified

Additional information

Summary of the carcinogencicity study (NTP 1978):

DETU was administered in the feed to groups of 50 males and 50 females of rats and mice. Twenty animals of each sex and species were placed on test as controls. The high and low dietary concentrations of DETU were respectively, 250 and 125 ppm for rats and 500 and 250 ppm for mice. The compound was administered during 103 weeks, followed by an observation period of 1 week for all dosed groups.

There were no significant positive associations between te dosages of DETU administered and mortality in rats of mice of either sex. Adequate numbers of animals in all groups survived sufficiently long to be at risk from late-developing tumors. Compound-related mean bodyweight depression was apparent among dosed male and female mice when compared to their respective controls, indicating that concentrations of DETU administered to mice may have approximated the maximum tolerated dosages.

There were statistically significant elevated incidences of follicular-cells carcinomas of the thyroid in high dose male rats. In addition, there were statistically significant elevated incidences of a combinaison of thyroid follicular-cell carcinomas and follicular-cell adenomas in high dose male and female rats.

Under the conditions of this bioassay, DETU was carcinogenic to rats, causing follicular-cell carcinomas of the thyroid in males and follicular-cell neoplasms of the thyroid females. There was no evidence for the carcinogencicity of the compound in male and female mice.