Di-tert-butyl 1,1,4,4-tetramethylbut-2-yn-1,4-ylene diperoxide

Basic toxicokinetics

Administrative data

Endpoint:

basic toxicokinetics, other

Type of information:

other: Expert Statement

Adequacy of study:

key study

Study period:

2018-03-07

Reliability:

1 (reliable without restriction)

Rationale for reliability incl. deficiencies:

other: Expert Statement in accordance with ECHA Guidance Document on IR & CSA Chapter R7c.

Data source

Reference

Reference Type:

other:

Title:

Unnamed

Year:

2018

Report date:

2018

Materials and methods

Test guideline

Qualifier:

no guideline followed

Principles of method if other than guideline:

Expert Statement in accordance with ECHA Guidance Document on Information Requirements and Chemical Safety Assessment Chapter R7c.

GLP compliance:

no

Test material

Test material information

Constituent 1

Reference substance name:

Di-tert-butyl 1,1,4,4-tetramethylbut-2-yn-1,4-ylene diperoxide

EC Number:

213-944-5

EC Name:

Di-tert-butyl 1,1,4,4-tetramethylbut-2-yn-1,4-ylene diperoxide

Cas Number:

1068-27-5

Molecular formula:

C16H30O4

IUPAC Name:

2,5-bis(tert-butylperoxy)-2,5-dimethylhex-3-yne

Results and discussion

Toxicokinetic / pharmacokinetic studies

Details on absorption:

As the hydrolysis rate was shown to be rather slow, absorption in the gastro intestinal tract (GIT) will mainly be limited to the parent compound. Generally, oral absorption is favored for molecular weights below 500 g/mol. However, based on the high logPow of > 6.5 DYBP can be regarded as lipophilic substance. This characteristic combined with the relatively low water solubility may limit oral absorption by the inability of this substance to dissolve in the gastro-intestinal fluids, which in turn hinders contact with the mucosal surface. However, absorption of such a lipophilic compound may be facilitated following possible micellular solubilisation by bile salts. With regards to the high lipophilicity and low water solubility, the mechanisms of micellular solubilisation may be of some importance for DYBP, as the substance would otherwise be poorly absorbed.
The above considerations are supported by experimental toxicity data. In an acute oral toxicity study performed with rats, the substance led to an LD50 value of > 2000 mg/kg bw, as no mortality occurred up to this dose level. However, clinical signs like lethargy and hunched posture were observed among the animals indicating that systemic absorption to at least some extent took place.
Due to its low vapour pressure (<0.1 Pa), DYBP is considered to be of low volatility. Therefore, exposure with the substance via the inhalation route is unlikely. Moreover, if the substance would reach the lungs in its vapour or gaseous state, the lipophilic character of DYBP hinders the direct absorption across the respiratory tract epithelium.
Similarly, based on its physico–chemical properties, DYBP is not likely to penetrate skin easily as the high logPow value and low water solubility do not favor dermal penetration. It is generally accepted that if a compound’s water solubility is between 1-100 mg/L, absorption can be anticipated to be low to moderate. Substances with a logPow value above 6 will be slowly absorbed through skin as the rate of transfer between the stratum corneum and the epidermis is slow. Nevertheless, there were some clinical signs noted, such as hunched posture and lethargy, in the acute dermal toxicity study with rats. Dermal absorption must therefore have taken place at least to a small extent.

Details on distribution in tissues:

Assuming that DYBP becomes bioavailable following oral intake, it may be distributed into the interior part of cells due to its lipophilic properties and in turn the intracellular concentration may be higher than extracellular concentration particularly in adipose tissues. It is expected that DYBP starts to hydrolyse to a low extent during and after absorption.
The results from the read-across 90-day repeated dose oral toxicity study in rats indicate that, following absorption, the thyroid gland and the kidney are the primary target organs to be effected in the rat. However, it has to be mentioned that the observed changes were considered to be limited to rats since the observed renal changes (increase in hyaline droplets associated with granular cast formation, increase in basophilic tubules and single cell necrosis of cortical tubules) were a consequence of the alpha-2µ-microglobulin nephropathy of the rat and the rat thyroid gland is more sensitive than the human thyroid gland. The read-across substance did also not cause developmental effects in an OECD 414 study.
Based on the logPow value of DYBP itself (6.5) a bioaccumulating potential cannot be excluded at first sight. However, the substance will hydrolyse the longer it is dissolved in body fluids resulting in smaller hydrolysis products being of lower logPow and higher water solubilities, respectively. Therefore, DYBP and its hydrolysis products are not considered to be of concern in regards to bioaccumulating potential.

Details on excretion:

As discussed above, DYBP will be metabolised either as parent or its hydrolysis products facilitating increased hydrophilicity. The first hydrolysis product tert-butyl alcohol has a low molecular weight of 74.12 g/mol, is miscible in water and thus may either be directly excreted or further metabolised by Phase II enzymes before excretion. Further potential but not confirmed hydrolysis products are 2,5-dimethylhex-3-yne-2,5-diol and tert-butylhydroperoxide. Both are of low molecular weight (146.23 and 90.1 g/mol) and well soluble in water and are therefore considered to be excreted via urine.

Metabolite characterisation studies

Details on metabolites:

Based on the structure of the molecule, DYBP may be metabolized by Phase I enzymes undergoing functionalization reactions aiming to increase the compound’s hydrophilicity. Here, metabolism to more toxic metabolites cannot completely be ruled out in the organism. However, available test data of studies conducted with and without metabolizing system do not give indication of increased toxicity due to metabolic activation. Further, Phase II conjugation reactions may covalently link an endogenous substrate to the parent compound or the Phase I metabolite in order to mask functional groups and ultimately facilitate excretion by elevated hydrophilicity.

Applicant's summary and conclusion

Conclusions:

Based on physicochemical properties of DYBP, particularly its water solubility, octanol-water partition coefficient and vapour pressure, some absorption via the oral and limited absorption via the dermal route is expected. This assumption is further supported by the results of the dermal and oral acute toxicity studies. DYBP is considered to be readily metabolized by Phase I and Phase II enzymes. Further it may partially undergo hydrolysis being dissolved in body fluids. Hydrolysis products are readily metabolised or excreted as such since their molecular weight and logPow is lower and water solubility is higher than for DYBP itself. Based on these considerations DYBP and its hydrolysis products are not considered to be of concern regarding bioaccumulation.

Executive summary:

Toxicological profile of 2,5-bis(tert-butylperoxy)-2,5-dimethylhex-3-yne (DYBP)

 

An acute oral toxicity study with the test substance revealed an LD50 > 2000 mg/kg bw in Wistar rats.In an acute dermal toxicity study conducted with Wistar rats an LD50 > 2000 mg/kg bw was observed. Furthermore, the substance was tested for its irritating properties to skin and eyes. The studies were performed with rabbits. As a result, DYBP was found to be neither irritating to eyes nor to skin. In a guinea pig maximization test the substance was further shown to not induce skin sensitisation. No adverse effects were observed in a 90-day oral repeated dose toxicity study with an analogue substance in Wistar rats up to the highest dose tested (150 mg/kg bw/day). Based on the results of the available HPRT-, Ames- and chromosome aberration test, DYBP can be considered as non-genotoxic.

 

Toxicokinetic analysis of of 2,5-bis(tert-butylperoxy)-2,5-dimethylhex-3-yne (DYBP)

DYBP is a colorless liquid at room temperature with a molecular weight of 286.41 g/mol. The substance is only slightly soluble in water (2.2 mg/L at 20°C). The logPow of DYBP was determined using the HPLC method according to the OECD guideline 117 and EU method A.8. The determined logPow (6.71) is outside the applicability domain of the HPLC method (0 < logPow < 6). Therefore, the logPow of DYBP can only be stated to be > 6.5. The substance has a low vapour pressure of 0.010855 Pa. Upon contact with water DYBP undergoes hydrolysis. One of the hydrolysis products formed is tert-butanol. The other potential hydrolysis products, 2,5-dimethylhex-3-yne-2,5-diol and tert-butylhydroperoxide, could not be detected by the techniques used. It is therefore anticipated that those undergo further degradation. Hydrolysis half-life of DYPB was determined to be between 7.6 and 9.0 days between pH 4 – 9 at 20 °C.

Absorption

As the hydrolysis rate was shown to be rather slow, absorption in the gastro intestinal tract (GIT) will mainly be limited to the parent compound. Generally, oral absorption is favored for molecular weights below 500 g/mol. However, based on the high logPow of > 6.5 DYBP can be regarded as lipophilic substance. This characteristic combined with the relatively low water solubility may limit oral absorption by the inability of this substance to dissolve in the gastro-intestinal fluids, which in turn hinders contact with the mucosal surface. However, absorption of such a lipophilic compound may be facilitated following possible micellular solubilisation by bile salts. With regards to the high lipophilicity and low water solubility, the mechanisms of micellular solubilisation may be of some importance for DYBP, as the substance would otherwise be poorly absorbed.

The above considerations are supported by experimental toxicity data. In an acute oral toxicity study performed with rats, the substance led to an LD50 value of > 2000 mg/kg bw, as no mortality occurred up to this dose level. However, clinical signs like lethargy and hunched posture were observed among the animals indicating that systemic absorption to at least some extent took place.

Due to its low vapour pressure (<0.1 Pa), DYBP is considered to be of low volatility. Therefore, exposure with the substance via the inhalation route is unlikely. Moreover, if the substance would reach the lungs in its vapour or gaseous state, the lipophilic character of DYBP hinders the direct absorption across the respiratory tract epithelium.

Similarly, based on its physico–chemical properties, DYBP is not likely to penetrate skin easily as the high logPow value and low water solubility do not favor dermal penetration. It is generally accepted that if a compound’s water solubility is between 1-100 mg/L, absorption can be anticipated to be low to moderate. Substances with a logPow value above 6 will be slowly absorbed through skin as the rate of transfer between the stratum corneum and the epidermis is slow. Nevertheless, there were some clinical signs noted, such as hunched posture and lethargy, in the acute dermal toxicity study with rats. Dermal absorption must therefore have taken place at least to a small extent.

Distribution

Assuming that DYBP becomes bioavailable following oral intake, it may be distributed into the interior part of cells due to its lipophilic properties and in turn the intracellular concentration may be higher than extracellular concentration particularly in adipose tissues. It is expected that DYBP starts to hydrolyse to a low extent during and after absorption.

The results from the read-across 90-day repeated dose oral toxicity study in rats indicate that, following absorption, the thyroid gland and the kidney are the primary target organs to be effected in the rat. However, it has to be mentioned that the observed changes were considered to be limited to rats since the observed renal changes (increase in hyaline droplets associated with granular cast formation, increase in basophilic tubules and single cell necrosis of cortical tubules) were a consequence of the alpha-2µ-microglobulin nephropathy of the rat and the rat thyroid gland is more sensitive than the human thyroid gland. The read-across substance did also not cause developmental effects in an OECD 414 study.

Based on the logPow value of DYBP itself (6.5) a bioaccumulating potential cannot be excluded at first sight. However, the substance will hydrolyse the longer it is dissolved in body fluids resulting in smaller hydrolysis products being of lower logPow and higher water solubilities, respectively. Therefore, DYBP and its hydrolysis products are not considered to be of concern in regards to bioaccumulating potential.

Metabolism

Based on the structure of the molecule, DYBP may be metabolized by Phase I enzymes undergoing functionalization reactions aiming to increase the compound’s hydrophilicity. Here, metabolism to more toxic metabolites cannot completely be ruled out in the organism. However, available test data of studies conducted with and without metabolizing system do not give indication of increased toxicity due to metabolic activation. Further, Phase II conjugation reactions may covalently link an endogenous substrate to the parent compound or the Phase I metabolite in order to mask functional groups and ultimately facilitate excretion by elevated hydrophilicity.

Excretion

As discussed above, DYBP will be metabolised either as parent or its hydrolysis products facilitating increased hydrophilicity. The first hydrolysis product tert-butyl alcohol has a low molecular weight of 74.12 g/mol, is miscible in water and thus may either be directly excreted or further metabolised by Phase II enzymes before excretion. Further potential but not confirmed hydrolysis products are 2,5-dimethylhex-3-yne-2,5-diol and tert-butylhydroperoxide. Both are of low molecular weight (146.23 and 90.1 g/mol) and well soluble in water and are therefore considered to be excreted via urine.

Summary

Based on physicochemical properties of DYBP, particularly its water solubility, octanol-water partition coefficient and vapour pressure, some absorption via the oral and limited absorption via the dermal route is expected. This assumption is further supported by the results of the dermal and oral acute toxicity studies. DYBP is considered to be readily metabolized by Phase I and Phase II enzymes. Further it may partially undergo hydrolysis being dissolved in body fluids. Hydrolysis products are readily metabolised or excreted as such since their molecular weight and logPow is lower and water solubility is higher than for DYBP itself. Based on these considerations DYBP and its hydrolysis products are not considered to be of concern regarding bioaccumulation.