(2-hydroxyethyl)ammonium mercaptoacetate

Endpoint summary

Description of key information

No substance-specific data on the repeated dose toxicity of MeaTG are available. However, according to Article 13 of the REACH legislation, in case no appropriate animal studies are available for assessment, information should be generated whenever possible by means other than vertebrate animal tests, i. e. applying alternative methods such as in vitro tests, QSARs, grouping and read-across.

There are reliable data available on the repeated dose toxicity of members of the category TGA and its salts, but not for MeaTG directly.

The subchronic toxicity of sodium thioglycolate was evaluated by oral and dermal administrations.

In an oral repeated dose toxicity study (OECD 408), sodium mercaptoacetate was administered by gavage, 7 days/week, for 13 weeks, to male and female Sprague-Dawley rats. Sporadic mortality and fully reversible effects on some haematological and biochemical parameters and histopathological changes in liver were observed at 60 mg/kg bw/d. These effects may be related to the inhibition of the β-oxidation of fatty acids a known mechanism of action of sodium mercaptoacetate. Consequently, based on the effects observed at 60 mg a. i. /kg/day, particularly mortality, hematological and significant blood chemistry changes associated with liver microscopic changes and the limited blood chemistry effects without microscopic adverse changes in the liver observed at 20 mg a. i. /kg/day, the NOAEL of sodium mercaptoacetate was 20 mg a. i. /kg/day, and the NOEL was 7 mg a. i. /kg/day given by daily oral administration (gavage) to rats for 13 weeks. Additional information on the oral repeated dose toxicity of sodium mercaptoacetate is provided by the 2-generation reprotoxicity study (OECD 416) study and the reproduction/developmental screening test (OECD 421). The results of both studies support the NOAEL 20 mg a. i. /kg/day defined in the 13-week toxicity study. In a repeated dose dermal toxicity (OECD 411), sodium mercaptoacetate was administered via dermal route, 5 days per week, for 13 weeks to male and female Fischer 344 rats and B6C3F1 mice. All animals survived the 13 weeks administration. The only treatment related effect was skin irritation at the site of application. The LOELs for skin irritation were 11.25 and 45 mg/kg bw/d and the NOAELs for systemic toxicity were higher than 180 and 360 mg/kg bw/d in rats and mice, respectively.

NOAEL = 20 mg/kg bw/d (13 wk repeated dose toxicity study, oral, rat, NaTG)

Key value for chemical safety assessment

Toxic effect type:

dose-dependent

Repeated dose toxicity: via oral route - systemic effects

Link to relevant study records

Reference

Endpoint:

sub-chronic toxicity: oral

Type of information:

read-across based on grouping of substances (category approach)

Adequacy of study:

weight of evidence

Justification for type of information:

HYPOTHESIS FOR THE CATEGORY APPROACH
The substances of this category have similar toxicological properties because:
- all substances are small organic molecules;
- they share structural similarities with common functional groups: one or more thiol and/or thioether group(s) and carboxylic acid (as free acid, salt or ester);
- the metabolism (i.e. ester hydrolysis) leads to comparable products (sulfur-containing core structure in its acid form and alcohols of differing chains lengths)

The substances were assigned to subgroups according to their main structural features (see Table 1); further justification for subgrouping based on toxicological properties is given below:
- TGA family: Thioglycolic acid, its salts and esters
- 3-MPA family: 3-Mercaptopropionic acid, its salts and esters
- TLA family: Thiolactic acid and its salts
- Intramolecular-S family: Thiodiglycolic acid or Dithiodiglycolic acid and its esters, Thiodipropionic acid or Dithiodipropionic acid and its esters, Methylene bis(butyl thioglycolate)
- Mercaptanes: Thioglycerol, Bis(2-mercaptoethyl) sulfide, 4-Mercaptomethyl-3,6-dithia-1,8-octanedithiol

The acids and salts will dissociate to the respective Thioglycolate or 3-Mercaptopropionate or Thiolactate and the corresponding cation. In case of the esters, the metabolism expected to occur is ester hydrolysis resulting in the corresponding acid and alcohol.

It was demonstrated, that PETMP and 3-MPA strongly bind to plasma proteins (e.g. via S-S bond to cysteine) in vitro, which is well known for substances containing free SH-groups (Bruno Bock, 2014). Strong protein binding is also expected to occur with the other substances assessed within this paper. The members of the intramolecular-S family are an exception, as they do not contain free SH-groups – protein binding may be less relevant for this family.

This read-across hypothesis corresponds to scenario 4 of the Read-Across Assessment Framework (RAAF), ECHA, March 2017 - different compounds have qualitatively similar properties - of the read-across assessment framework i.e. variations in the properties are observed among the source substances; the prediction is based on a worst-case approach.

Overall, based on close structural similarities, a read-across from the existing repeated dose and reproduction toxicity studies is considered as an appropriate adaptation to the standard information requirements of the REACH Regulation in accordance with the provisions of Annex XI, 1.5 of the REACH Regulation.

A detailed justification for this category approach is attached to Iuclid section 13.

Reason / purpose:

read-across: supporting information

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Dose descriptor:

NOEL

Effect level:

20 mg/kg bw/day (actual dose received)

Based on:

act. ingr.

Sex:

male/female

Basis for effect level:

histopathology: non-neoplastic

Critical effects observed:

not specified

Executive summary:

Across the whole group of substances, there were some differences in the type of effects and effect levels: Mainly unspecific toxicity was observed in the 3-MPA, TLA, intramolecular-S and mercaptan family.

 

In the oral repeated dose toxicity studies conducted with NaTG, 2-EHTG, and Di-2-EHDTDG, mild liver toxicity was observed consisting of e.g. hypertrophy, vacuolation, and lipidosis.

 

Comparing these effects to the literature, in vivo and in vitro studies with liver mitochondria suggested that the most probable mechanism of toxicity of TGA and 3-MPA is the inhibition of the β‑oxidation of fatty acids. TGA (syn. 2-mercaptoacetate) is a substrate for acetyl-CoA synthase, the resulting compound, 2-mercaptoacetyl-CoA may inhibit the long chain acyl-CoA dehydrogenase. Consequently, the concentration of long-chain fatty acids or acyl-CoA will increase in the mitochondria. Consequently, they will be esterified in the liver to form triglycerides leading to lipidosis (Bauché, 1981; Bauché, 1982; Bauché, 1983).This is in line with the effects observed in the available studies.

 

Nevertheless, the effects observed in the studies described above were not sufficient to explain the mortality observed at dose levels of 40 mg/kg bw/d and higher (NaTG).

The changes in blood fatty acid levels and lipidosis in liver were likely to be the result of changes in the biochemistry of the animals and reflected physiological, rather than pathological changes induced by the test material. Changes were observed in the clinical chemistry with high transaminases which also indicated toxicity to the liver, but these changes were very high in only one animal.

 

Thus, as no specific target organ with evidence of severe toxicity has been identified, a classification for specific target organ toxicity after repeated exposure is not warranted.

 

For better comparison, the NO(A)ELs have been recalculated on the basis of S-content, which is assumed to be the main driver for toxicity. Additionally, an adjustment for differences in study duration was made (subacute and chronic studies were normalized to subchronic):

 

Overall comparison of NO(A)ELs(general toxicity)

Family	Substance (study)	NO(A)EL [mg/kg bw/d]	% S in molecule	Related to S-content	Adjustment for study duration
TGA	NaTG (13 wk NOAEL, OECD TG 408, oral: gavage)	20	30.7	6.1	6.1
	NaTG (ca 90 - 111 d NOAEL, OECD TG 421, oral: gavage, general toxicity)	20	30.7	6.1	6.1
	NaTG (NOAEL, OECD TG 416, oral: gavage)	20	30.7	6.1	6.1
	GMT (4 wk NOEL, OECD TG 422, oral: gavage)	50	21.1	10.5	3.5
	2-EHTG (6-7 wk NOAEL, OECD TG 421, oral: gavage, general toxicity)	50	17.1	8.6	2.9
	2-EHTG (4 wk NOAEL, OECD TG 407, oral: diet, general toxicity)	170	17.1	29.1	9.7
3-MPA	MMP (4 wk NOAEL, OECD TG 422, oral: gavage)	50	29.1	14.6	4.9
	PETMP (13 wk NOAEL, OECD TG 408, oral: gavage)	50	7.2	3.6	3.6
TLA	TLA (4 wk NOAEL, OECD TG 407, oral:gavage)	150	33.0	49.5	16.5
Intra-molecular-S	E12 (13 wk NOAEL, OECD TG 408, oral: gavage)	350	6.8	23.8	23.8
	E18 (2 yr NOAEL, oral: diet)	1125	5.1	57.6	115.2
	Di-2-EHDTDG (4 wk NOAEL, OECD TG 407, oral: gavage)	200	9.3	18.7	6.2
Mercaptans	DMPT (4 wk NOAEL, OECD TG 407, oral: gavage)	50	13.5	6.7	3.3

 

After adjustment to S-content and study duration, the NOAELs of most of the substances were within the same order of magnitude, with the exception of TLA, E12 and E18, which were less toxic. Using the lowest NOAEL obtained in the 13 wk repeated dose toxicity study conducted with NaTG is considered to be an appropriate starting point for DNEL derivation. Remaining uncertainties due to read-across will be taken care of by applying appropriate assessment factors.

Endpoint conclusion

Endpoint conclusion:

adverse effect observed

Dose descriptor:

NOAEL

20 mg/kg bw/day

Study duration:

subchronic

Species:

rat

Quality of whole database:

The available key studies are reliable or reliable with restrictions (Klimisch 1 – 2) and were conducted according to or similar to guidelines.

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

Additional information

For the assessment of repeated dose toxicity, data are available from NaTG, GMT, 2-EHTG, MMP, PETMP, TLA, E12, E18, Di-2-EHDTDG and DMPT. A justification for read-across is attached to IUCLID section 13.

 

Studies with NaTG

NaTG was administered daily, for 14 days, by the oral route (gavage), to Sprague-Dawley rats at dose-levels of 15/100/150, 30, 60 or 75 mg/kg/day (Arkema, 2010b). Following absence of toxicity, the 15 mg/kg/day dose-level was increased, on day 8 of dosing, to 100 mg/kg/day and was further increased, on day 11 of dosing, to 150 mg/kg/day. Due to mortalities at 150 mg/kg/day, the remaining animals in the group were not dosed on day 14. The dose-level of 150 mg/kg/day resulted in mortality, reduced body weight gain (males) or body weight loss (females) and reduced food consumption. There were no group mean effects of treatment on body weight performance or food consumption at dose-levels of 15/100, 30, 60 or 75 mg/kg/day. Four animals given 75 mg/kg/day had ptyalism at the end of the study and one male given 30 mg/kg/day and one male given 75 mg/kg/day had low body weight gains between day 7 to day 11 or day 1 to day 4, respectively. Macroscopic abnormalities were observed in the liver at 30 (females), 60 (males), 75 and 15/100/150 (females) mg/kg/day and in the kidneys in females given 60 or 75 mg/kg/day and in males given 60 or 15/100/150 mg/kg/day. 150 mg/kg/day is considered to exceed the maximum tolerated dose because mortality occurred after 3 days of treatment.

 

In a 13 wk oral study with NaTG (0, 7, 20, 60 mg/kg bw/d) in rat (Bruno Bock, 2010) 2 animals (1 m, 1 f) of the high dose group (60 mg/kg bw/d) died or were sacrificed prematurely. In the high dose increased absolute and relative liver weights were noted correlating with minimal centrilobular hepatocellular hypertrophy. Further, higher values of liver transaminases (ALAT and ASAT) were observed, periportal hepatocellular vacuolation, lipidosis, tubular vacuolation in the kidneys correlated with increased urea and creatinine values.

LOAEL=60 mg/kg bw/d; NOAEL = 20 mg/kg bw/d; NOEL=7 mg/kg bw/d

 

In a 13 wk dermal study with NaTG (0, 11.25, 22.5, 45, 90, 180 mg/kg bw/d in rat; 0, 22.5, 45, 90, 180, 360 mg/kg bw/d in mice) in either mouse or rat no mortality occurred (NTP, 2003). Increases in absolute and relative liver weights and relative kidney, spleen and testes weights were observed in rat, which were without histopathological correlates. In mice absolute and relative liver and heart weights were increased without histopathological correlates. The only histopathogical observation was at the site of application (dermal hypertrophy or hyperplasia).

NOAEL(rat, systemic) >/= 180 mg/kg bw/d; LOAEL(local) =11.25 mg/kg bw/d, no NOAEL determined

NOAEL(mouse, systemic) >/= 360 mg/kg bw/d; NOAEL(local) =22.5 mg/kg bw/d

 

In the reproduction/developmental toxicity screening study with NaTG (Bruno Bock, 2010b) (0, 20, 40, 80 mg/kg bw/d) increased liver and kidney weights were observed in males dosed with 80 mg/kg bw/d (high dose), as well as an increased glycogen content. The NOAEL for general parental toxicity was 20 mg/kg bw/d based on mortality at 40 and 80 mg/kg bw/d.

 

In the 2-generation reproduction toxicity test with NaTG (Arkema, 2010) (0, 10, 20, 40 mg/kg bw/d) minimal to moderate periportal hepatocellular microvacuolation was observed at 40 mg/kg bw/d (high dose) in 2/25 males and 6/25 females, and in 4/6 prematurely sacrificed or found dead females suggesting mild liver toxicity at this dose-level. Furthermore, female plasma fatty acid concentration was statistically significantly decreased. The NOEL for general parental toxicity was 20 mg/kg bw/d.

 

Study with GMT

GMT was tested in a combined repeated dose/reproduction/developmental toxicity screening test (Thioesters Association, 2007a) (0, 15, 50, 150 mg/kg bw/d). In the high dose group (150 mg/kg bw/d) 5 females died or were killed in extremis. The cause of death was, however, not evident from histopathological examination. Females of the high dose group showed reduced food consumption, body weight and body weight gain (not statistically significant). No other treatment related effects were noted. The NOEL for systemic toxicity was 50 mg/kg bw/d.

 

Studies with 2-EHTG

2-EHTG was tested in a 7 d (Bibra, 1992) (0, 150, 200, 250 mg/kg bw/d) and in a 28 d (Bibra, 1988) (0, 0.05, 0.1, 0.2% in diet) repeated dose toxicity study in rats. In the 7 d study, 6/10 of the high dose animals (250 mg/kg bw/d) and 4/10 of the mid dose animals (200 mg/kg bw/d) died. The main histopathological effect was microvesiculation in the liver, possibly due to fat accumulation. The 7 d NOAEL was 150 mg/kg bw/d.

No adverse effects were noted in the 28 d dietary study (0, 0.05, 0.1 or 0.2% 2-EHTG in diet) up to the highest dose level. NOAEL >/= 168 (males) / 173 (females) mg/kg bw/d

 

2-EHTG in corn oil was administered orally by gavage to three groups of five Crl:CD (SD) rats/sex/dose once daily for 14 days (Thioesters Association, 2005b). Dosage levels were 10, 50 and 150 mg/kg/day administered at a dosage volume of 4 mL/kg. A concurrent control group composed of five animals/sex received the vehicle on a comparable regimen. All animals were observed twice daily for mortality and moribundity. Clinical observations, body weights and food consumption were recorded daily. On study day 14, each surviving animal was subjected to a gross necropsy and selected organs were weighed.

One female in the 150 mg/kg/day group was found dead on study day 2. Decreased defecation was observed in three females in the 150 mg/kg/day group.

Body weight losses and associated slight decrease in feed consumption were observed in the 150 mg/kg/day group males and females during the first week of dose administration. Nevertheless, when the overall treatment period (study days 0-14) was evaluated, mean male and female body weight gains in the 150 mg/kg/day groups were found to be only slightly lower (not statistically significant) than the respective control groups.

Increased absolute and relative (to final body weight) liver weights were observed in the 50 mg/kg/day (males) and the 150 mg/kg/day (males and females) groups. Absolute and relative kidney weights were increased in the 50 and 150 mg/kg/day males compared to control group values. Absolute and relative thymus gland and thyroid/parathyroid gland weights in the 150 mg/kg/day males and females were slightly reduced compared to the control animals.

 

In the reproduction/developmental toxicity screening study with 2-EHTG (0, 10, 50, 150 mg/kg bw/d) (Thioesters Association, 2005) 3 males and 3 females were found dead or were euthanized in extremis in the high dose (150 mg/kg bw/d).

At the same dose level reduced body weight and/or body weight gain was observed in males, though food consumption was similar to controls. In the prematurely deceased animals pale liver correlated with hepatocellular vacuolization was observed. Relative liver and kidney weights were increased at this dose level in the surviving animals. The NOAEL for general parental toxicity was 50 mg/kg bw/d.

 

Study with MMP

MMP was tested in a combined repeated dose/reproduction/developmental toxicity screening test (0, 25, 50, 100 mg/kg bw/d) (Thioesters Association, 2007b). In the mid and high dose, relative liver weights were dose dependently increased in males. In the absence of a histopathological correlation this was considered to be of no adverse character. Furthermore at 100 mg/kg/day, a minimal to slight hyperplasia of the forestomach squamous epithelium was noted in males and females. No other test-item related adverse effects were noted. The NOAEL for general toxicity was 50 mg/kg bw/d.

 

Studies with PETMP

In the 14 d dose range finding study with PETMP (0, 50, 200, 800 mg/kg bw/d) (Bruno Bock, 2015a), all males and all females but one of the high dose group (800 mg/kg bw/d) died before scheduled necropsy. Clinical signs included sedation, weakened condition and ruffled fur. Reduced food consumption and lower mean body weights were noted. In the mid dose, transient sedation was noted. There were no effects on food consumption or body weights. There were no substance related adverse effects on organ weights or macroscopical findings. In the low dose, no clinical signs were evident and there were no effects on food consumption or body weights. There were no organ weight differences of toxicological relevance or macroscopical findings.

 

In the 13 wk repeated dose study with PETMP (Bruno Bock, 2015b) (0, 12.5, 50, 200 mg/kg bw/d) no test item-related deaths were noted, no differences in mean food consumption or body weights, weekly observations (weeks 1 - 13) or functional observational battery (week 13), no differences of toxicological relevance in the fore- and hind limb grip strength values, no test item-related differences in the ophthalmoscopy, no test item related effects on hematology or urine parameters. There were no test item-related macroscopic findings. There were differences in blood biochemistry (higher levels of sodium, potassium and chloride in mid and high dose males, and higher potassium and chloride levels in high dose females). Test item-related microscopic findings were recorded in the stomach of high dose animals (slight forestomach erosion, slight forestomach ulceration, minimal or moderate forestomach squamous hyperplasia). Minimal forestomach squamous hyperplasia was also noted in one low dose female and two mid dose females. The NOAEL was 50 mg/kg bw/d.

 

Study with TLA

TLA was tested in a subacute toxicity study (Bruno Bock, 2000) according to OECD TG 407 (28 d + 14 d recovery) (0, 15, 150, 500/250 mg/kg bw/d). Five treatment-related deaths occurred in the high dose (500 mg/kg/day, reduced to 250 mg/kg/day from day 7), clinical signs consisted of clonic convulsion, distended abdomen, dehydration, pallor of the extremities, hunched posture, lethargy, pilo-erection, decreased respiratory rate, gasping, laboured and noisy respiration and tiptoe gait, which persisted also after reduction to 250 mg/kg bw/d. Laboured and noisy respiration were still observed during the first week of the treatment-free period.

Reduced bodyweight gain with individual weight loss and reduced food consumption was observed in the 500 mg/kg bw/d group; after reduction to 250 mg/kg/d, bodyweight normalised and was similar to controls. Effects on hematology and blood biochemistry were noted in high dose animals, which were at least partially reversible. Absolute and relative liver and kidney weight were increased in high dose animals and still present in recovery females

Gross pathology and histopathology revealed changes in the gastro intestinal tract:

Stomach: Mucosal erosion / submucosal inflammatory cell infiltrates in the stomach as well as acanthosis, hyperkeratosis, inflammatory cell infiltrates and focal ulceration in the forestomach.

The effects observed were most likely a result of gastric irritation which in turn produced blood chemical changes resulting in a so-called protein losing enteropathy

No treatment-related effects were observed in the low and mid dose. NOEL = 150 mg/kg bw/d

 

Studies with E12, E18 and DTDPA

Oral gavage administration of E12 to rat at dose levels of between 125 and 1000 mg/kg/day for four weeks was not associated with any signs of toxicity. Consequently, dose levels of 125, 350 and 1000 mg/kg/day were recommended for the 13 week study (BASF, 1993).

 

In a 13 week repeated dose toxicity study with E12 (0, 125, 350, 1000 mg/kg bw/d) (Thioesters Association, 2001a)no unscheduled deaths, no treatment related clinical signs, no effects on body weight gain and food consumption and hematology were observed. A reversible elevation in serum cholesterol in the high dose females and a reversible elevation of ALAT + ASAT activities in all high dose animals were noted.

Minor differences in the weight of the major organs were considered of no toxicological significance in the absence of microscopic lesions.

Treatment related microscopic lesions were seen in the heart of high dose animals. The lesion was described as small foci of degenerated or necrotic fibers associated with minimal to moderate mononuclear cell infiltration. This association suggested early or ongoing myocarditis. These lesions were not present in recovery animals.

NOAEL = 350 mg/kg bw/d, NOEL = 125 mg/kg bw/d

 

In a 2 yr chronic toxicity study (Thioesters Association, 2001b) with E18 (0, 0.5, 1, 3% in diet) only minor effects on the weight development were observed. At the end of the study, 3, 2, 7 and 2 of 20 rats died from the control, 0.5, 1.0 and 3.0% groups, respectively. No characteristic gross pathology was evident from the autopsies performed on the respective experimental groups. NOAEL = 3% in diet, corresponding to approx. 1125 mg/kg bw/d.

 

E12,E18 andDTDPA have been evaluated as antioxidants for fats (WHO, 1962): up to 3% in diet (corresponding to approx. 1500 mg/kg bw/d) did not cause adverse effects in the rat.

 

Study with Di-2-EHDTDG

In a 28 d toxicity study + 2 week recovery period (0, 10, 50, 200, 800 mg/kg bw/d) with Di-2-EHDTDG (Ciba-Geigy, 1992) 2/10 males and 5/10 females died in the 800 mg/kg bw/d dose group. Due to mortality and marked signs of toxicity, the 800 mg/kg bw/d dose group was terminated. Body weight, food and water consumption were reduced in this group. At necropsy fatty change of perilobular region in liver, hyperplasia/ulceration of non-glandular stomach, atrophy of testis and thymus were observed. Minor reversible hematological findings (minimal increase in white blood cell count in males and females) at 200 mg/kg bw/d. No other adverse effects were noted. NOEL = 50 mg/kg bw/d, NOAEL = 200 mg/kg bw/d.

 

Study with DMPT

DMPT was tested in a 4 week toxicity study + 2 week recovery in rats (0, 10, 50, 200 mg/kg bw/d) (Mitsui Toatsu Chemicals Incorporated, 1991). No mortality occurred. Clinical signs including salivation, stained fur, hair-loss, lack of grooming were present in high dose animals.Body weight gain and food efficiency were low. Hematologiocal changes were observed in high dose animals:slightly high erythrocyte count, low mean cell volume and mean cell hemoglobin in males; high platelet count in males and females; longer activated partial thromboplastin time in males and longer prothrombin time in males and females. Prothrombin time was also longer in mid dose males. At the end of the recovery period high erythrocyte count, low mean cell volume and mean cell haemoglobin were still evident. Effects to blood biochemistry parameters were noted in high dose animals: low alkaline phosphatase and high erythrocyte acetylcholinesterase activities and low total cholesterol concentration in males and high alanine amino-transferase activity in males and females. Alkaline phosphatase activity was also low in males receiving 50 mg/kg/day. At the end of the recovery period high erythrocyte acetylcholinesterase and alkaline phosphatase activities were evident.

Absolute and relative liver weights were increased in high dose females. There were no toxicologically relevant macroscopic changes. Histopathology revealed slight centriacinar hepatocytic fatty vacuolation in 3/10 females. NOAEL = 50 mg/kg bw/d.

 

Summary

Across the whole group of substances, there were some differences in the type of effects and effect levels: Mainly unspecific toxicity was observed in the 3-MPA, TLA, intramolecular-S and mercaptan family.

 

In the oral repeated dose toxicity studies conducted with NaTG, 2-EHTG, and Di-2-EHDTDG, mild liver toxicity was observed consisting of e.g. hypertrophy,vacuolation, and lipidosis.

 

Comparing these effects to the literature,in vivoandin vitrostudies with liver mitochondria suggested that the most probable mechanism of toxicity of TGA and 3-MPA is the inhibition of the β‑oxidation of fatty acids. The thioglycolate also known as 2-mercaptoacetate anion is a substrate for acetyl-CoA synthase, the resulting compound, 2-mercaptoacetyl-CoA may inhibit the long chain acyl-CoA dehydrogenase. Consequently, the concentration of long-chain fatty acids or acyl-CoA will increase in the mitochondria. Consequently, they will be esterified in the liver to form triglycerides leading to lipidosis (Bauché, 1981; Bauché, 1982; Bauché, 1983).This is in line with the effects observed in the available studies.

 

Nevertheless, the effects observed in the studies described above were not sufficient to explain the mortality observed at dose levels of 40 mg/kg bw/d and higher (NaTG).

The changes in blood fatty acid levels and lipidosis in liver were likely to be the result of changes in the biochemistry of the animals and reflected physiological, rather than pathological changes induced by the test material. Changes were observed in the clinical chemistry with high transaminases which also indicated toxicity to the liver, but these changes were very high in only one animal.

 

Thus, as no specific target organ with evidence of severe toxicity has been identified, a classification for specific target organ toxicity after repeated exposure is not warranted.

 

For better comparison, the NO(A)ELs have been recalculated on the basis of S-content, which is assumed to be the main driver for toxicity. Additionally, an adjustment for differences in study duration was made (subacute and chronic studies were normalized to subchronic).

 

Overall comparison of NO(A)ELs (general toxicity)

Family	Substance (study)	NO(A)EL [mg/kg bw/d]	% S in molecule	Related to S-content [mg/kg bw/d]	Adjustment for study duration
TGA	NaTG (13 wk NOAEL, OECD TG 408, oral: gavage)	20	30.7	6.1	6.1
	NaTG (ca 90 - 111 d NOAEL, OECD TG 421, oral: gavage, general toxicity)	20	30.7	6.1	6.1
	NaTG (NOAEL, OECD TG 416, oral: gavage)	20	30.7	6.1	6.1
	GMT (4 wk NOEL, OECD TG 422, oral)	50	21.1	10.5	3.5
	2-EHTG (6-7 wk NOAEL, OECD TG 421, oral: gavage, general toxicity)	50	17.1	8.6	2.9
3-MPA	MMP (4 wk NOAEL, OECD TG 422, oral: gavage)	50	29.1	14.6	4.9
	PETMP (13 wk NOAEL, OECD TG 408, oral: gavage)	50	7.2	3.6	3.6
TLA	TLA (4 wk NOAEL, OECD TG 407, oral:gavage)	150	33.0	49.5	16.5
Intra-molecular-S	E12 (13 wk NOAEL, OECD TG 408, oral: gavage)	350	6.8	23.8	23.8
	E18 (2 yr NOAEL, oral: feed)	1125	5.1	57.6	115.2
	Di-2-EHDTDG (4 wk NOAEL, OECD TG 407, oral: gavage)	200	9.3	18.7	6.2
Mercaptans	DMPT (4 wk NOAEL, OECD TG 407, oral: gavage)	50	13.5	6.7	3.3

 

After adjustment to S-content and study duration, the NOAELs of most of the substances were within the same order of magnitude, with the exception of TLA, E12 and E18, which were less toxic. Therefore, using the lowest NOAEL obtained in the 13 wk repeated dose toxicity study conducted with NaTG is considered to be an appropriate starting point for DNEL derivation for the whole group of substances described in Table 1. Remaining uncertainties due to read-across will be taken care of by applying appropriate assessment factors.

References

 

Arkema, 2010. Two-generation reproduction toxicity study by oral route (gavage) in rats. Unpublished report. CIT 35047 RSR

 

Arkema, 2010b.Sodium Thioglycolate, 2-week range-finding toxicity study by oral route (gavage) in rats. Unpublished report. CIT 30720 TSR

 

Bauché F. et al. 1981. 2-Mercaptoacetate administration depresses the β-oxidation pathway through an inhibition of long-chain acyl-CoA dehydrogenase activity. Biochem. J. (1981) 196, 803-809

 

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Justification for classification or non-classification

As no specific target organ with evidence of severe toxicity has been identified, a classification for specific target organ toxicity after repeated exposure is not warranted in accordance with regulation (EC) 1272/2008. Thus, no labelling is required.