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Description of key information

In 2 valid oral toxicity studies with 2-mercaptomethylbenzimidazole a LD50 = 340 mg/kg bw (rat, male) (Loeser) and a LD50 = 330 mg/kg bw (rat, male/female) (Saitoh) was found. In an acute oral toxicity study with the corresponding zinc salt (2-mercaptomethylbenzimidazole, zinc salt CAS no 61617-00-3) a LD50 = 390 mg/kg bw was determined.
Acute inhalative toxicity was tested with methyl-2-mercaptobenzimidazole, zinc salt in male and female Sprague-Dawley rats. Rats were exposed nose only for 4 hours at room temperature to a concentration of 0 and 2120 mg/m³. No mortality was observed.
A valid study with methyl-2-mercaptobenzimidazole, zinc salt for dermal toxicity determined a LD50 > 2000 mg/kg bw. At 2000 mg/kg bw none of 10 animals died.
Justification for read across of 1,3-dihydro-4(or 5)-methyl-2H-benzimidazole-2-thione, zinc salt and 1,3-dihydro-4(or 5)-methyl-2H-benzimidazole-2-thione:
The toxicological properties of 1,3-dihydro-4(or 5)-methyl-2H-benzimidazole-2-thione (CAS n° 53988-10-6) and 1,3-dihydro-4(or 5)-methyl-2H-benzimidazole-2-thione, zinc salt (CAS n° 61617-00-3) are quite similar in toxicological studies available for both compounds (e.g. acute toxicity, irritation/corrosion, genotoxicity, repeated dose studies).
This result is assumed to be an effect of the of 1,3-dihydro-4(or 5)-methyl-2H-benzimidazole-2-thione anion rather than of the hydronium ion (which is a normal constituent of body fluids) or zinc cation, which is an essential element in humans.
Therefore a read across between 1,3-dihydro-4(or 5)-methyl-2H-benzimidazole-2-thione (CAS n° 53988-10-6) and its sodium salt (CAS n° 75045-07-7) and 1,3-dihydro-4(or 5)-methyl-2H-benzimidazole-2-thione, zinc salt (CAS n° 61617-00-3) is justified.

Key value for chemical safety assessment

Acute toxicity: via oral route

Link to relevant study records
Reference
Endpoint:
acute toxicity: oral
Type of information:
experimental study
Adequacy of study:
key study
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
other: well documented and scientifically acceptable
Principles of method if other than guideline:
Seven groups of 10 young adult male Wistar rats (160 -180 g) were dosed at 200, 300, 350, 380, 400, 450, or 500 mg/kg bw Vulkanox MB 2 (CAS 53988-10-6) in Lutrol. The animals were observed for mortality, body weights and clinical signs through day 14
GLP compliance:
no
Test type:
standard acute method
Species:
rat
Strain:
Wistar
Sex:
male
Route of administration:
oral: gavage
Vehicle:
polyethylene glycol
Doses:
200, 300, 350, 380, 400, 450, or 500 mg/kg bw
No. of animals per sex per dose:
10
Control animals:
no
Sex:
male
Dose descriptor:
LD50
Effect level:
340 mg/kg bw
Based on:
test mat.

Corresponding mortality was 0, 20, 70, 70, 80, 80 and 100% respectively. Sedation was the only clinical sign. Deaths occured on days 1 to 4. The acute oral LD50 for male rats is 340 mg/kg bw (95% confidence interval: 300 - 370 mg/kg bw).

Interpretation of results:
harmful
Remarks:
Migrated information
Executive summary:

Seven groups of 10 young adult male Wistar rats (160 -180 g) were dosed at 1 200, 300, 350, 380, 400, 450, or 500 mg/kg bw Vulkanox MB 2 (CAS 53988-10-6) in Lutrol. The animals were observed for mortality, body weights and clinical signs through day 14.

Corresponding mortality was 0, 20, 70, 70, 80, 80 and 100% respectively. Sedation was the only clinical sign. Deaths occured on days 1 to 4. The acute oral LD50 for male rats is 340 mg/kg bw (95% confidence interval: 300 - 370 mg/kg bw).

Endpoint conclusion
Endpoint conclusion:
adverse effect observed
Dose descriptor:
LD50
340 mg/kg bw
Quality of whole database:
The materials/methods and results are described in detail and are sufficient for evaluation

Acute toxicity: via inhalation route

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Reference
Endpoint:
acute toxicity: inhalation
Type of information:
migrated information: read-across from supporting substance (structural analogue or surrogate)
Adequacy of study:
key study
Study period:
October 2002
Reliability:
1 (reliable without restriction)
Rationale for reliability incl. deficiencies:
other: conducted according GLP
Qualifier:
according to
Guideline:
EPA OPPTS 870.1300 (Acute inhalation toxicity)
Deviations:
no
GLP compliance:
yes
Test type:
standard acute method
Limit test:
no
Species:
rat
Strain:
Sprague-Dawley
Sex:
male/female
Details on test animals and environmental conditions:
On receipt the animals were randomly allocated to cages. After an acclimation period of at least five days the animals were given a number unique within the study by ear punching and a number written on a colour coded cage card. At the start of the study the animals were approximately eight to twelve weeks old and within the weight range of 200g to 350g. The females were nulliparous and non-pregnant.
With the exception of the exposure period, free access to mains drinking water and food was allowed throughout the study.

The environmental controls were set to achieve values of 21 +/- 2 deg C and 55 +/- 15% relative humidity. The rate of air exchange was at least fifteen changes per hour and the lighting was controlled to give twelve hours continuous light and twelve hours darkness. The animals were retained in this accomidation at all times except during the exposure period.
Route of administration:
inhalation: dust
Type of inhalation exposure:
nose only
Vehicle:
air
Details on inhalation exposure:
GENERATION OF TEST ATMOSPHERE / CHAMBER DESCRIPTION
A dust atmosphere was produced from the test material using a SAG 410 Solid Aerosol Generator located adjacent to the exposure chamber. The SAG 410 was connected to a metered compressed air supply.
Compressed air was supplied by means of an oil free compressor and passed through a water trap and respiratory quality filters before it was introduced to the SAG 410.
The cylindrical exposure chamber had a volume of approximately 30 liters (dimensions: 28 cm diameter x 50 cm high). The concentration within the chamber was controlled by adjusting the test material feed rate from the SAG 410. The extract from the exposure chamber passed through a 'scrubber' trap and was connected with a high efficiency filter to a metered exhaust system. The chamber was maintained under negative pressure.
Homogeneity of the test atmosphere within the chamber was not specifically determined during this study. Chambers of the same design (ADG Developments Ltd) have been fully validated and shown to produce evenly distributed atmospheres in the animals' breathing zone with a wide variety of test materials.
Prior to the start of the study, test material atmospheres were generated within the exposure chamber. During this characterisation period, test material input rates were varied to achieve the required atmospheric concentrations.

TEST ATMOSPHERE
The temperature and relative humidity inside the exposure chamber were measured by an electronic thermometer/humidity meter located in a vacant port in the animals' breathing zone of the chamber and recorded every thirty minutes throughout the four-hour exposure period.
Oxygen levels within the exposure chamber were measured by an electronic oxygen analyser located in a sampling port in the animals breathing zone during each exposure period. The test atmosphere was generated to contain at least 19% oxygen.
The actual chamber concentration was measured at regular intervals during the exposure period. The gravimetric method used employed glass fibre filters placed in a filter holder. The holder was temporarily sealed in a vacant port in the exposure chamber in the animals' breathing zone and a suitable, known volume of exposure chamber air was drawn through the filter using a vacuum pump.
Each filter was weighed before and after sampling in order to calculate the weight of collected test material. The difference in the two weights, divided by the volume of atmosphere sampled, gave the actual chamber concentration.
The nominal chamber concentration was calculated by dividing the mass of test material used by the total volume of air passed through the chamber.

PARTICLE SIZE DISTRIBUTION
The particle size of the generated atmosphere inside the exposure chamber was determined three times during the exposure period using a Marple Personal Cascade Impactor. This device consisted of six impactor stages (9.8, 6.0, 3.5, 1.55, 0.93 and 0.52 microM cut points) with stainless steel collection substrates and a back up glass fibre filter, housed in an aluminium sampler. The sampler was temporarily sealed in a sampling port in the animals' breathing zone and a suitable, known volume of exposure chamber air was drawn through it using a vacuum pump.
The collection substrates and backup filter were weighed before and after sampling and the weight of test material, collected at each stage, claculated by difference.
The mean amount for each stage was used to determine the cumulative amount below each cut-off point size. In this way, the proportion (%) of aerosol less than 9.8, 6.0, 3.5, 1.55, 0.93 and 0.52 microM was calculated.
The resulting values were converted to probits and plotted against Log10 cut-point size. From this plot, the Mass Median Aerodynamic Diameter (MMAD) was determined (as the 50% point) and the geometric standard deviation was calculated. In addition the proportion (%) of aerosol less than 4 microM (considered to be the respirable portion) was determined.
Analytical verification of test atmosphere concentrations:
no
Duration of exposure:
4 h
Concentrations:
A target concentration of 2.0 mg/L was used for exposure.
No. of animals per sex per dose:
5
Control animals:
no
Details on study design:
All animals were observed for clinical signs at hourly intervals during exposure, immediately on removal from the restraining tubes at the end of exposure, one hour after termination of exposure and subsequently once daily for fourteen days. Any evidence of overt toxicity was recorded at each observation.
Sex:
male/female
Dose descriptor:
LC50
Effect level:
> 2.12 mg/L air
Based on:
test mat.
Exp. duration:
4 h
Mortality:
No mortality
Clinical signs:
Signs of hunched posture, pilo-erection and red/brown staining around snout are commonly seen in animals for short periods on removal from the chamber following 4-hour inhalation studies. Wet fur is commonly recorded both during and for a short period after exposure. These observations are considered to be associated with the restraint procedure and, in isolation, are not indicative of toxicity.
In addition to the observations considered to be due to the restraint procedure, the following abnormalities were detected:
During exposure, sporadic instances of increased respiratory rate were noted. On removal from the chamber after four hours, this observation was recorded for all animals and there was an isolated instance of lethargy. One hour after removal, increased respiratory rate was still observed and another two instances of lethargy were recorded.
One day after exposure, increased respiratory rate and hunched posture was noted in all animals and pilo-erection was frequently recorded. These observations receded steadily to leave the animals appearing normal three or four days post-exposure.
Body weight:
Reduced bodyweight gain was noted in one male during week 1 of the study. Normal development was noted during week 2. Four females showed reduced bodyweight gain or a slight bodyweight loss during week 1 and/or 2 of the study but such variations are not uncommon in female rats of this strain or age and are considered not to be significant.
Gross pathology:
No macroscopic abnormalities were detected at necropsy.
Interpretation of results:
Toxicity Category IV
Remarks:
Migrated information Criteria used for interpretation of results: EU
Conclusions:
No deaths occured in a group of ten rats exposed to a mean achieved atmosphere concentration of 2.12 mg/L. It was therefore considered that the acute inhalation median lethal concentration of the test substance in the Sprague-Dawley Crl:CD (SD) IGS BR strain rat, was greater than 2.12 mg/L.
Endpoint conclusion
Dose descriptor:
discriminating conc.
2 120 mg/m³
Quality of whole database:
The materials/methods and results are described in detail and are sufficient for evaluation

Acute toxicity: via dermal route

Link to relevant study records
Reference
Endpoint:
acute toxicity: dermal
Type of information:
migrated information: read-across from supporting substance (structural analogue or surrogate)
Adequacy of study:
key study
Reliability:
4 (not assignable)
Rationale for reliability incl. deficiencies:
other: secondary literature - U.S. Environmental Protection Agency, Hazard characterisation document sponsored Chemical: 2H-Benzimidazole-2-thione, 1,3-dihydro-, 4(or 5)-methyl-,zinc salt
Qualifier:
according to
Guideline:
OECD Guideline 402 (Acute Dermal Toxicity)
Principles of method if other than guideline:
Test material was moistened with arachis oil and applied to an area of shorn skin. All test animals received a single dermal exposure of 2,000 mg/kg b.w. The test material was held in place by surgical gauze and self-adhesive bandage. The semi-occlusive wrap was removed after 24 hours and the excess material was wiped from the test animal.
Animals were examined for mortality, systemic toxicity, signs of dermal irritation, weight gain and necropsy.
GLP compliance:
yes
Test type:
standard acute method
Limit test:
yes
Species:
rat
Strain:
Sprague-Dawley
Sex:
male/female
Type of coverage:
semiocclusive
Vehicle:
arachis oil
Duration of exposure:
24 hours
Doses:
2000 mg/kg bw
No. of animals per sex per dose:
10 male/female animals
Control animals:
not required
Sex:
male/female
Dose descriptor:
LD50
Effect level:
> 2 000 mg/kg bw
Based on:
test mat.
Interpretation of results:
practically nontoxic
Remarks:
Migrated information
Executive summary:

Test material was moistened with arachis oil and applied to an area of shorn skin. All test animals received a single dermal exposure of 2,000 mg/kg b.w. The test material was held in place by surgical gauze and self-adhesive bandage. The semi-occlusive wrap was removed after 24 hours and the excess material was wiped from the test animal.

LD50 > 2000 mg/kg bw. There were no deaths, no signs of systemic toxicity, no signs of dermal irritation and all animals showed expected weight gain. No abnormalities were noted at necropsy.

Endpoint conclusion
Endpoint conclusion:
no adverse effect observed
Dose descriptor:
discriminating dose
2 000 mg/kg bw
Quality of whole database:
The materials/methods and results are described sufficient for evaluation

Additional information

In 3 valid acute oral toxicity studies a LD50 > 330 mg/kg bw for male (and female) rats was found

In a study on rats with 2120 mg/m³ methyl 2-mercaptotobenzimidazole, zinc salt (nose-only exposure) for 4 hours, no mortality was observed.

Ten male/female animals rats were exposed in an acute dermal toxicity study semicoocusively to methyl 2-mercaptotobentimidazole, zinc salt) for 24 hours. None of the rats died.


Justification for selection of acute toxicity – oral endpoint
key study used

Justification for selection of acute toxicity – inhalation endpoint
key study used

Justification for selection of acute toxicity – dermal endpoint
only available study used

Justification for classification or non-classification

The acute oral toxicity studies revealed a LD50 > 340 mg/kg bw. In the acute inhalation study and a LC50 > 2.12 mg/l was determined.

Therefore a classification as Xn; R20/22 (GHS: Acute Tox 4, H 332; Acute Tox 4, H 302) is justified. In an acute dermal toxicity study,

none of 10 animals died at 2000 mg/kg bw. Therefore a classification is not necessary.