1,3-diethyl-2-thiourea

Endpoint summary

Description of key information

Two oral studies of 7 weeks in rats and mice are available, there are two range finding tests for carcinogenicity tests. The aim of these studies was to establish the maximum tolerated concentrations of DETU for administration to dosed animals in the chronic studies. Animals were exposed to DETU in diet during seven weeks to 147, 215, 316 and 464 ppm for rats, and 680, 1000, 1470, 2160 and 3150 ppm for mice.
The target organ of DETU is the thyroid of rat. The LOAEL for thyroid toxicity is 6.25 mg/kg bw in rat (= 125 ppm).
No target organ is showed in the mice.

 

A Decision on Compliance Check was received requesting a Screening for reproductive/developmental toxicity on rats (OECD 422) followed by a Sub-chronic toxicity study (90-day) on rats (OECD 408) to be conducted on the registered substance. The deadline for providing the finalised study is: 20 February 2023.
Before initiating the OECD 422, it was considered appropriate to conduct a 14-day Dose Range Finding study. The aim was to generate appropriate experimental data on the registered substance in order to select the most appropriate dose-levels to be investigated during the OECD 422. Results of the OECD 422 were to be used to select the most appropriate dose-levels during the subsequent OECD 408, in accordance with the latest ECHA Guidance.

Due to unforeseen circumstances, delays were encountered resulting in the postponement of the studies. Consequently, the regulatory deadline cannot be met. Testing schedule is reported in the associated RSS.

Results will be provided as soon as they will be available.

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Key value for chemical safety assessment

Toxic effect type:

concentration-driven

Repeated dose toxicity: via oral route - systemic effects

Link to relevant study records

Reference

Endpoint:

sub-chronic toxicity: oral

Type of information:

experimental study

Adequacy of study:

key study

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

study well documented, meets generally accepted scientific principles, acceptable for assessment

Reason / purpose for cross-reference:

reference to same study

Qualifier:

no guideline followed

Principles of method if other than guideline:

It's a range finding test for carcinogenicity test. In order to establish the maximum tolerated concentrations of DETU for administration to dosed animals in the chronic studies, subchronic toxicity tests were conducted.
Rats were exposed to DETU during seven weeks.

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:

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

Duration of treatment / exposure:

7 weeks

Frequency of treatment:

daily

Dose / conc.:

147 ppm

Dose / conc.:

215 ppm

Dose / conc.:

316 ppm

Dose / conc.:

464 ppm

No. of animals per sex per dose:

5 males + 5 females per dose

Control animals:

yes, concurrent no treatment

Details on study design:

Post-exposure period: 1 week

Positive control:

no

Observations and examinations performed and frequency:

Individual body weights and food consumption data were recorded twice weekly throughout the study.

Sacrifice and pathology:

Upon termination of the study, all survivors were sacrified and necropsied.

Other examinations:

no

Clinical signs:

effects observed, treatment-related

Mortality:

mortality observed, treatment-related

Body weight and weight changes:

effects observed, treatment-related

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:

not examined

Neuropathological findings:

not examined

Histopathological findings: non-neoplastic:

not examined

Histopathological findings: neoplastic:

not examined

Other effects:

not examined

Details on results:

- At 316 ppm concentration: Mean body weight gain increased by 3 % among male rats compared to control group while it decreased by 11% in female rats. 1 female rat died and another had an arched back and rough coat.
- At 215 ppm concentration: Mean body weight gain decreased by 10 % among male rats compared to control group while it decreased by 1% in female rats.
=> The maximum tolerated concentration was around 250 ppm.

Key result

Dose descriptor:

other: Maximum tolerated dose

Effect level:

250 ppm

Sex:

male/female

Basis for effect level:

other: 250 ppm x 0.05 (assumed rat food consumption per bw) = 12.5 mg/kg bw/d

Key result

Dose descriptor:

NOAEL

Effect level:

147 ppm

Sex:

male/female

Basis for effect level:

other: Changes of bw were observed at 215 and 316 ppm. 147 ppm x 0.05 = 7.35 mg/kg bw

Critical effects observed:

no

Conclusions:

According to this study, the maximum tolerated concentration was 250 ppm of DETU in diet for the rats.

Executive summary:

It's a range finding test for carcinogenicity test. In order to establish the maximum tolerated concentrations of DETU for administration to dosed animals in the chronic studies, subchronic toxicity tests were conducted. Rats were exposed to DETU in diet during seven weeks to 147, 215, 316 and 464 ppm.

- At 316 ppm concentration: Mean body weight gain increased by 3 % among male rats compared to control group while it decreased by 11% in female rats. 1 female rat died and another had an arched back and rough coat.

- At 215 ppm concentration: Mean body weight gain decreased by 10 % among male rats compared to control group while it decreased by 1% in female rats.

=> The maximum tolerated concentration was around 250 ppm.

Endpoint conclusion

Endpoint conclusion:

adverse effect observed

Dose descriptor:

LOAEL

6.25 mg/kg bw/day

Study duration:

subchronic

Species:

rat

Quality of whole database:

Reliable study (klimish score of 2)

System:

endocrine system

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

Range finding test for carcinogenicity test (rat and mice).

Rats were exposed to DETU in diet during seven weeks to 147, 215, 316 and 464 ppm.

- At 316 ppm concentration: Mean body weight gain increased by 3 % among male rats compared to control group while it decreased by 11% in female rats. 1 female rat died and another had an arched back and rough coat.

- At 215 ppm concentration: Mean body weight gain decreased by 10 % among male rats compared to control group while it decreased by 1% in female rats.

=> The maximum tolerated concentration was around 250 ppm, and the NOAEL is 147 ppm (7.35 mg/kg bw/d).

Mice were exposed to DETU during seven weeks to 680, 1000, 1470, 2160 and 3150 ppm in diet.

At 680 ppm, the mean body weight gain was decreased among males (-10%) and females (-8%) compared to control group.

=> The high concentration selected for administration to dosed mice in the main study was 500 ppm (=NOAEL).

Carcinogenicity tests on rats and mice.

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.

This study shows that thyroid is a target organ of DETU at the higher dose (250 ppm). 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 LOAEL for carcinogenicity and thyroid toxicity is 125 ppm (6.25 mg.kg bw/d).

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.

No effects on thyroid were observed in this study at any dose.

The NOAEL for general toxicity is smaller than 12.5 mg/kg bw/d.

Thyroid toxicity in rats of DETU

In the carcinogenicity test, a thyroid toxicity was observed on rats :

1/ the incidence of follicular-cell carcinomas was significantly increased in male rats exposed to high dose (11 animals among 48 examined histologically),

2/ 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),

3/ 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.

=>The LOAEL of thyroid toxicity was 125 ppm (6.25 mg.kg bw/d) in rat.

This thyroid toxicity was confirmed in the study of Hasegawa (1991) :

DETU was administered in the diet to male rats at 200 ppm for 52 weeks. The incidence of liver tumors was observed.

No significant changes were observed for mortality, clinical signs, food consumption or bodyweight in the treated group (DETU) in comparison to the control group. Thyroid weight was significantly increased as compared to control group (p < 0.05: 0.16 ± 0.01g vs 0.13 ± 0.01g). Thyroid follicular cell carcinomas were only induced in 1 of 21 DETU-treated rats (5%). A significant increase of T4 level serum was observed after 52 weeks of exposure (3.67 ± 0.58 µg/dl in treated group vs 2.93 ± 0.55 µg/dl in control group). An apparent correlation between the T4 levels and thyroid weight was noted: the lower the serum T4 level with the greater the thyroid weight.

=> The NOAEL of thyroid toxicity was smaller than 200 ppm.

Study of Astwood (1945) : Effects of DETU on thyroid function (section 7.9.3.)

DETU was tested in rats for capacity to interfere with the endocrine function of the thyroid gland. Test substance was administered to young rats by mixing it with the food at 37 mg/kg bw. At the end of ten days the thyroid glands were examined grossly and microscopically, and the relative activities of the compounds were then roughly compared on the basis on the minimal dose required to produce a noticeable effect.

The weight of thyroid was increased in rat treated with DETU compared to control rats. Thyroid iodine concentration in treated rats was smaller than concentration of control rats. Therefore, an antithyroid activity of DETU was observed in the rats treated with DETU compared to the control rats (treated with thiouracil) at 37 mg/kg bw.

Justification for classification or non-classification

Proposed self-classification (Regulation (EC) No 1272/2008):

STOT RE 1 (thyroid) - H372 "Causes damage to organs through prolonged or repeated exposure".

Justification: the LOAEL for thyroid toxicity (hyperplasia, antithyroid activity) is 6.25 mg/kg bw/d (<10 mg/kg bw/d).

Current conclusion based on experimental data generated before receiving the ECHA Decision on Compliance Check.

This conclusion will be revised upon receiving the results from the currently ongoing studies.