2-ethyl-2-[(3-mercapto-1-oxopropoxy)methyl]propane-1,3-diyl bis[3-mercaptopropionate]

Endpoint summary

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:

key study

Justification for type of information:

Preliminary remark:
Currently, do data on toxicokinetics/metabolism is available for this category. Based on structural features (e.g. sterical hindrance) it is however assumed, that ester cleavage would not be fast and complete, especially since the substances contain up to 6 ester functions, which are in addition sterically shielded. Therefore, it seems more reasonable to base the category hypothesis on structural similarity.
In addition, it is not clear yet, whether the strength of the effects vary in a predictable manner, or if no relevant variations occur. However, there are variations in structure (number of ester bonds and consequently number of free -SH groups) and physicochemical properties (especially water solubility and log Kow). It is assumed that these variations will also be reflected by variations in effect levels. Therefore, scenario 4 is the working hypothesis for the time being.
More data points within the category are needed to further strengthen the category hypothesis (refer to Data matrix toxicity endpoints (systemic effects)). The scenario selection will be re-evaluated after the studies are finished.
This currently selected scenario covers the category approach for which the read-across hypothesis is based on structural similarity. For the REACH information requirement under consideration, the property investigated in studies conducted with different source substances is used to predict the property that would be observed in a study with the target substance if it were to be conducted. Similar properties are observed for the different source substances; this may include absence of effects for every member of the category.
There are expected to be differences in strength of the effects forming a regular pattern. The prediction will be based on a worst-case approach. The read-across is a category approach based on the hypothesis that the substances in this category share structural similarities with common functional groups. This approach serves to use existing data on acute toxicity, repeated-dose toxicity, and reproductive toxicity endpoints for substances in this category.
The hypothesis corresponds to Scenario 4 of the RAAF. The substances GDMP, TMPMP, PETMP, and Di-PETMP are esters of a common acid, 3-mercaptopropionic acid (3-MPA). The key functionality of the substances within this category is the presence of free SH-groups. It is hypothesised that the strength of effects correlates with the number of SH-groups. In addition, differences in bioavailability are expected to influence the strength of effects.
For details, please refer to the category document attached to Iuclid section 13.

Reason / purpose for cross-reference:

read-across: supporting information

Reason / purpose for cross-reference:

read-across source

Reason / purpose for cross-reference:

read-across source

Dose descriptor:

NOAEL

Effect level:

ca. 40 mg/kg bw/day (nominal)

Based on:

test mat.

Sex:

male/female

Basis for effect level:

clinical signs

Critical effects observed:

no

Conclusions:

Based on read-across from PETMP, the subchronic NOAEL of TMPMP is estimated to be ca 40 mg/kg bw/d

Endpoint conclusion

Endpoint conclusion:

adverse effect observed

Dose descriptor:

NOAEL

40 mg/kg bw/day

Study duration:

subchronic

Species:

rat

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 experimental data are available for TMPMP. However, a subchronic toxicity study was conducted with PETMP. A justification for read-across is attached to Iuclid section 13.

 

A 14 d dose range finding study as well as a subsequently conducted 90 d study is available for PETMP.

In a non-GLP dose range-finding toxicity study, PETMP was administered daily by oral gavage to SPF-bred Wistar rats of both sexes at dose levels of 50, 200 or 800 mg/kg body weight/day for a period of 14 days. A control group received a similar dose volume (5 mL/kg body weight) of the vehicle, corn oil. The study comprised five animals per group and sex which were sacrificed after 14 days of treatment.

At 800 mg/kg/day, all males and all females but one died before scheduled necropsy. Clinical signs of toxicological relevance included marked sedation in all rats, and weakened condition and ruffled fur were also noted in some females. Food consumption was markedly reduced at all measurement intervals. Males and females showed markedly lower mean body weights throughout the treatment period, but the body weight of the single female that survived until scheduled necropsy was similar to the controls on days 7 and 14. The organ weights of the males could not be assessed as all died before scheduled necropsy. The organ weights of the single surviving female (higher absolute liver, kidney and adrenal weights, and lower absolute thymus weight) were considered to result from metabolic adaptation and stress. The organ weight differences are clearly related to the treatment with the test item but not adverse in nature.

At 200 mg/kg/day, transient sedation was noted in three males and one female. There were no effects on food consumption or body weights. In males, the mean adrenal weights were marginally higher in males and the mean thymus weights were marginally lower in females: these findings are generally associated with stress. The higher relative testes weights were an artifact of the lower testes weights noted in two control males. There were no macroscopical findings.

At 50 mg/kg/day, no clinical signs were evident and there were no effects on food consumption or body weights. In males, higher relative testes weights were considered to be an artifact of the lower testes weights noted in two control males. There were no organ weight differences of toxicological relevance in females, and no macroscopical findings in either sex.

Selection of dose levels for the subsequent 13-week study (Harlan Laboratories study D89724) with PETMP was based upon the results attained in this study as well as the dose range-finding study for a pre-natal development studies performed at Harlan Laboratories in Shardlow UK. In that study, the high dose level of 450 mg/kg/day produced marked clinical signs (tonic/clonic convulsions) that required reduction of this dose to 300 mg/kg/day. In view of the marked clinical signs noted in this 14-day dose range-finding study and the dose range-finding study for the pre-natal development study, dose levels of 12.5, 50 or 200 mg/kg body weight/day are proposed for the subsequent 13-week study (Harlan Laboratories study D89724) with PETMP.

 

In the follow-up subchronic toxicity study according to OECD Guideline 408, PETMP (97.4% a.i.) was administered to 10 RccHan: WIST(SPF) rats/sex/dose in corn oil by gavage at dose levels of 0, 12.5, 50 and 200 mg a.i./kg bw/day (corrections were made for purity) for 91 or 92 days. Additional animals in satellite groups (control and high dose, 5 males and 5 females, each) were kept for further 28 days without treatment to detect recovery from, or persistence of, toxic effects.

Test item-related findings were noted in some animals on the first day of treatment: tachypnea, convulsions and/or shivering, prostration, hunched posture, ruffled fur and/or stiff gait. Pica, a typical protective behavior for rats, was observed at this dose. These signs were generally seen only on day 1. The findings were of limited incidence and transient, and were considered to indicate acute toxicity from a metabolic overload which triggered a subsequent adaptive) if nonspecific) metabolic response. Findings of toxicological relevance were not seen at 50 mg/kg bw/day or 12.5 mg/kg bw/day.

There were no test item-related deaths, 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.

Differences noted in mean levels of sodium, potassium and chloride of male rats treated with 50 mg/kg bw/day and 200 mg/kg bw/day were considered to be test item-related. Females treated with 200 mg/kg bw/day had higher mean potassium and chloride levels. The potassium levels exceeded the upper range of the historical control data. The females treated with 50 mg/kg bw/day and both sexes at 12.5 mg/kg bw/day compared favorably with the control values. All other parameters were considered to be unaffected by treatment.

Test item-related differences in organ weights included higher mean relative kidney weights in males at 50 mg/kg bw/day and 200 mg/kg bw/day, and lower mean absolute thymus weight, mean thymus-to-body weight ratio and mean thymus-to-brain weight ratio in females at 200 mg/kg bw/day. However, these differences did not correlate with microscopical changes of toxicological relevance.

Test item-related microscopic findings were recorded in the stomach. At the end of the main study and in the two unscheduled deaths, these findings consisted of forestomach erosion (slight in degree) in one male at 200 mg/kg bw/day male and in one female at 200 mg/kg bw/day, forestomach ulceration (minimal in degree) in one female at 200 mg/kg bw/day, and forestomach squamous hyperplasia (minimal or moderate in degree) in two males at 200 mg/kg bw/day. This latter finding (minimal in degree) was also observed in one female at 12.5 mg/kg bw/day, two females at 50 mg/kg/day) and seven females at 200 mg/kg bw/day. Minimal forestomach squamous hyperplasia persisted after the recovery period in one female previously treated with 200 mg/kg bw/day. These findings are indicative of local irritation in the forestomach.

Based on these results, a no-observed-effect level (NOEL) could not be identified, and a specific target organ was not apparent. The LOEL for local effects (forestomach squamous hyperplasia) was 12.5 mg/kg bw/day. The LOEL for local effects, based on local irritation at the site of application (forestomach), is judged as not relevant to humans due to significant different anatomic situation and exposure probability in humans.

The no-observed adverse effect level (NOAEL) relevant to human DNEL calculation was considered to be 50 mg/kg bw/day.

 

After correction for molecular weight differences, the subchronic NOAEL for TMPMP is estimated to be 40 mg/kg bw/d.

 

Based on the considerations concerning bioavailability as well as on the available acute toxicity studies, there is reason to believe that GDMP is the worst-case within this category. Therefore, a combined Repeated Dose Toxicity Study with the Reproduction/Developmental Toxicity Screening Test according to OECD TG 422 was initiated with GDMP. However, the results are not available, yet. When the results of this study are available, it has to be re-evaluated whether the worst-case assumption can be verified. In this case, the data obtained with GDMP will be used to cover the endpoint short-term-repeated dose toxicity for TMPMP.

Justification for classification or non-classification

Based on the available data, no classification for specific organ toxicity after repeated exposure is required.