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Link to relevant study record(s)

Reference
Endpoint:
basic toxicokinetics, other
Remarks:
expert statement
Type of information:
other: expert statement
Adequacy of study:
key study
Reliability:
1 (reliable without restriction)
Rationale for reliability incl. deficiencies:
comparable to guideline study
Details on absorption:
Based on molecular weight, physicochemical properties and experimental in vivo data, TBPEH is expected to become bioavailable after oral administration, either as parent and/or as breakdown products.

Oral absorption is favoured for molecular weights below 500 g/mol. TBPEH is not well soluble in water having a relatively high logPow values ranging between 4.79. It may therefore be taken up by micellular solubilisation due to its more lipophilic properties. When administered orally TBPEH might be expected to undergo hydrolysis, at least to some extent. However, it is noteworthy that acute toxicity of TBPEH is low, with an LD50-value of 10000 mg/kg bw in an acute study with rats. This indicates that the acutely more toxic hydrolysis products are probably not formed rapidly enough or to a sufficient amount following oral administration.
Oral absorption for the potential common biodegradation products and the carboxylic acids is well known.
The subacute (28 days) NOAEL was 316 mg/kg bw/day for males and 100 mg/kg bw/day for females, whereas a subchronic NOAEL in rats was determined to be 450 mg/kg bw/d. Based on findings at 300 mg/kg bw/d administered in an EOGRTS (OECD 443) the NOAEL for fertility was determined to be 100 mg/kg bw/d in female rats.
Dermal absorption might be low to moderate based on water solubility and lipophilic properties of TBPEH. Uptake will be limited to the stratum corneum cross the epidermis will be low. This was confirmed in an acute dermal study resulting in an LD50 value of 14142 mg/kg bw. However, TBPEH was shown to be a potential skin sensitizer, thus at least small amounts of the substance or its metabolites must have reached the subcutaneous layers when applied topically on the skin.
No local toxicity effects such as irritation or corrosion were observed in skin or eye.
Based on the low vapour pressure of 2 Pa as well as the considering the very limited use profile, inhalation exposure to TBPEH is not likely. Thus, the inhalation route is considered not relevant for humans.
Nevertheless, if the substance reaches the lung, TBPEH may be absorbed by micellular solubilisation (see above). The low water solubility may enhance penetration to the lower respiratory tract. TBPEH showed very low toxicity after inhalation administration, in an acute inhalation toxicity study revealing a LC50 of 42.2 mg/L. Together, this indicates low systemic availability after inhalation and if bioavailable, potential toxicity is considered to be low.
Details on distribution in tissues:
When reaching the body TBPEH may be distributed throughout the organism. Possible hydrolysis of TBPEH in the blood leads to formation of its degradation products. Both degradation products have low to moderate BCF values and are thus considered not bioaccumulative. Although TBPEH is expected to be distributed into the interior part of the cells due to its lipophilic properties, no bioaccumulation is expected since TBPEH is expected to be degraded to more hydrophilic metabolites when compared to the parent compound. TBPEH undergoes hydrolysis upon contact with water. Possible hydrolysis products are tert-butanol, tert-butyl hydroperoxide and 2-ethylhexanoic acid. All hydrolysis products have a lower log Pow value than TBPEH itself. Consequently, there is no bioaccumulation expected, neither for the parent nor for its degradation products.
Details on excretion:
Since TBPEH has a molecular weight below 300, renal excretion is expected to be the predominant route. This does also apply to the potential degradation products and /or metabolits. This is supported by findings after subchronic exposure to TBPEH. Increased liver and kidney weights were reported, in association with increased urine volume and decreased pH (in both sexes and different cohorts). This observation indicates an increased metabolizing and excretory activity of the kidney and liver induced by the test item exposure. No other histopathological lesions were seen in both organs, except increased findings of chronic progressive nephropathy in male animals.
Metabolites identified:
not measured
Details on metabolites:
The metabolism of this peroxyester is could possibly include the following pathways:
- enzymatic and non-enzymatic hydrolysis,
- direct reaction with biomolecules due the reactivity of the peroxyester group of the parent or the hydroperoxide formed during hydrolysis (e. g. TBHP)
Hydrolysis may already occur in the gastrointestinal tract. This might occur spontaneously or might be faciliated by an enzymatic process (e.g. esterases or other enzymatic systems of the intestinal microflora).
If the parent compound reaches the liver, enzymatic cleavage is also expected (e.g. by glutathione peroxidase). Unspecific reactions with biomolecules can take place either in the GI-tract or subsequent to absorption. It is assumed that the parent molecules can react directly or after cleavage to hydroperoxide. TBPEH is a skin sensitizer, reactions with macromolecules has been demonstrated to occur. Free radicals and reactive oxygen species which might be formed during metabolism can be deactivated by non-enzymatic antioxidants abundantly available in the organism (e. g. Vitamin E protecting lipid membranes from being peroxidised, Vitamin C and glutathione). Conjugates of these may be faciliated to become excreted or are re-cycled within the endogenous redox system.
Conclusions:
Based on physico-chemical characteristics, particularly water solubility, octanol-water partition coefficient and vapour pressure, no or only limited absorption by the dermal and inhalation routes is expected. This is supported by the dermal and inhalation acute toxicity studies results. For the oral route uptake of the hydrolysis products of TBPEH is also taken into account. Bioaccumulation of the parent as well as of the hydrolysis products is not likely to occur based on the physico-chemical properties. Excretion of the parent and/or its metabolites or hydrolysis products is expected to occur mainly via the urine.
Executive summary:

Toxicokinetic analysis of tert.Butylperoxy-2-ethylhexanoate (TBPEH) 

Tert. Butylperoxy-2-ethylhexanoate (TBPEH) is a colorless liquid at room temperature with a molecular weight of 216.3 g/mol. The substance is only slightly soluble in water (46.3 mg/L). The logPow of TBPEH was measured to be 4.79. Based on this log Pow a BCF of 202.4 L/kg was calculated by QSAR estimation. TBPEH has a low vapour pressure of 2 Pa at 20 °C.

TBPEH undergoes hydrolysis upon contact with water. Possible hydrolysis products are tert-butanol, tert-butyl hydroperoxide and 2-ethylhexanoic acid. All hydrolysis products have a lower log Pow value than TBPEH itself. Consequently, there is no bioaccumulation expected, neither for the parent nor for its degradation products.

Based on its physico–chemical properties the substance is not likely to penetrate skin to a large extent as the high logPow value and low water solubility do not favour dermal penetration.

Uptake will be limited to the stratum corneum cross the epidermis will be low. This was confirmed in an acute dermal study resulting in an LD50 value of 14142 mg/kg bw. However, TBPEH was shown to be a potential skin sensitizer, thus at least some amounts of the substance or its metabolites must have reached the subcutaneous layers when applied topically on the skin.
No local toxicity effects such as irritation or corrosion were observed in skin or eye.

Oral absorption is favoured for molecular weights below 500 g/mol. Based on the high logPow of 4.79 and the low water solubility, TBPEH may be taken up by micellular solubilisation. When administered orally TBPEH might be expected to undergo hydrolysis, at least to some extent. However, it is noteworthy that acute toxicity of TBPEH is low, with an LD50-value of 10000 mg/kg bw in an acute study with rats. This indicates that the acutely more toxic hydrolysis products are probably not formed rapidly enough or to a sufficient amount following oral administration. The subacute (28 days) NOAEL was 316 mg/kg bw/day for males and 100 mg/kg bw/day for females, whereas a subchronic NOAEL in rats was determined to be 450 mg/kg bw/d. Based on findings at 300 mg/kg bw/d administered in an EOGRTS the NOAEL for fertility was determined to be 100 mg/kg bw/d in female rats.

Based on the low vapour pressure of 2 Pa inhalation exposure is not likely. Nevertheless, if the substance reaches the lung, TBPEH may be absorbed by micellular solubilisation (see above). The low water solubility may enhance penetration to the lower respiratory tract. TBPEH showed very low toxicity after inhalation administration, in an acute inhalation toxicity study revealing a LC50 of 42.2 mg/L. Together, this indicates low systemic availability after inhalation and if bioavailable, potential toxicity is considered to be low.

When reaching the body, TBPEH may be distributed throughout the organism. Hydrolysis of TBPEH in the blood may lead to formation of its degradation products. Both degradation products have low to moderate BCF values (1.09 L/kg and 66.37 L/kg respectively) and are thus considered not bioaccumulative.

Metabolism of parent compound and hydrolysis products may occur. Enzymatic transformation products include the cytochrome P450 system. Phase I reactions are typically followed by phase II reactions resulting in molecules more polar for extraction.

Excretion is likely to occur mainly via the urine and bile. This is supported by findings after subchronic exposure to TBPEH. Increased liver and kidney weights were reported, in association with increased urine volume and decreased pH (in both sexes and different cohorts). This observation indicates an increased metabolizing and excretory activity of the kidney and liver induced by the test item exposure. No other histopathological lesions were seen in both organs, except increased findings of chronic progressive nephropathy in male animals.

Description of key information

Key value for chemical safety assessment

Bioaccumulation potential:
no bioaccumulation potential

Additional information

General

No experimental data on absorption, distribution and excretion is available for TBPEH. Therefore, the toxicokinetic assessment is based on physicochemical properties of the substance and existing toxicological data. Since it cannot finally be concluded if the parent compound or its respective breakdown products are the relevant agents both possibilities are taken into consideration.

 

Tert. Butylperoxy-2-ethylhexanoate (TBPEH) is a colorless liquid at room temperature with a molecular weight of 216.3 g/mol. The substance is only slightly soluble in water (46.3 mg/L). The logPow of TBPEH was measured to be 4.79. Based on this log Pow a BCF of 202.4 L/kg was calculated by QSAR estimation. TBPEH has a low vapour pressure of 2 Pa at 20 °C.

 

Toxicological profile of tert. Butylperoxy-2-ethylhexanoate (TBPEH)

An acute oral toxicity study with rats revealed a LD50-value of > 10000 mg/kg bw. An acute inhalation toxicity study with rats revealed a LC50-value of 42.2 mg/L. In an acute dermal toxicity study with rabbits a LD50 of 16818 mg/kg bw was determined. A skin irritation study with rabbits revealed no skin irritating or corrosive effects leading to classification according to GHS. An eye irritation test also showed that TBPEH is not irritating to the rabbit’s eye. A guinea pig maximisation test revealed that TBPEH has the potential to cause skin sensitisation, based on which, TBPEH was classified as skin sensitizer Cat 1 according to CLP.

TBPEH was not mutagenic in one bacterial mutagenicity test (a reverse mutation test - Ames test). Another Ames test showed positive results after metabolic activation. In addition, a HPRT test with V79 cells showed positive results with and without metabolic activation. However, in an in vivo micronucleus test in mice no mutagenic effects were detected. Thus, based on the results obtained TBPEH was considered not mutagenic. A reliable and suitable in vivo TRM assay according to OECD 488 showed that the test item did not induce gene mutation in the liver, forestomach, kidney and small intestine of transgenic mice after an exposure time of 28 days.

A 28-day oral (gavage) toxicity study was performed with TBPEH in male and female Fischer, F344/DuCrj rats. The test item was administered at 100, 316 and 1000 mg/kg bw dose levels. The vehicle was corn oil. The test substance caused a decrease in the number of platelets in the blood of the mid and the high dose females and some minor liver changes (higher organ weights in the high dose groups of both sexes, elevated plasma alkaline phosphatase level in the high dose females). There was also a borderline indication for renal tubular alterations in the high dose females. There was a sex difference in the response to the test substance with different effects in both sexes and a somewhat higher susceptibility of the females. The NOAEL of TBPEH was 316 mg/kg bw/day in males and 100 mg/kg bw/day females.

A Reproduction/Developmental Toxicity Screening Test with the test item TBPEH (in corn oil) was performed according to OECD 421. TBPEH was administered once daily orally (by gavage) at doses of 0, 100, 300, and 1000 mg/kg/day to male rats for 41 days in total and to female rats throughout the pre-mating, mating, gestation and lactation periods until day 3 post partum (last dosing). The 1000 mg/kg bw/day dose group showed decreased food consumption in male and female animals during the first week of the pre-pairing period. Some dams were noted with ruffled fur after parturition. No test item-related effects were noted during necropsy and for macroscopic findings. Treatment at 1000 mg/kg bw/d was associated with an increase of pre-implantation-, post-implantation-, and post-natal loss, and a reduction of live pups. The mean body weight of pups also was reduced at this dose. The NOEL was 300 mg/kg body weight/day for the parent generation as well as for the offspring.

The test item was further assessed in a subchronic toxicity study with an exposure period of 90 days followed by a 28-day recovery period. The test item administered orally (by gavage, in sunflower oil) at 450, 150 and 70 mg/kg bw/day did not induce any mortalities nor were any other toxic signs observed related to the test item treatment. Salivation was observed in the male and female animals of the 450, 150 and 70 mg/kg bw/day groups with variable frequency within a group but in a dose related manner regarding the incidence and onset. Therefore, the No Observed Adverse Effect Level (NOAEL) was determined to be 450 mg/kg bw/day for male and female animals.

In an Extended one-generation study (EOGRTS) according to OECD TG 443 male and female rats (Wistar) were orally administered once a day with TBPEH in sunflower oil at dose levels of 100, 300, and 1000 mg/kg bw/d. In the P0 generation, exposure duration for males was up to 153-156 days (10 weeks pre-mating, mating, 2ndmating and post mating period) for females up to 114-129 days (10 weeks pre-mating, (prolonged) mating, gestation and lactation period). In the F1 generation, oral exposure to the test item by gavage started (at the same dosing levels as for P0 generation) at the age of three weeks and lasted until week 20 (i .e. overall 17 weeks) for cohort 1B (as this cohort was mated to produce F2 generation). At the high dose level of 1000 mg/kg bw/d reduced body weight and gain was observed in male animals in both generations, despite unremarkable food consumption throughout the course of the study. In females no adverse effects on body weight / gain was observed. Clinical signs included salivation and nuzzling up the bedding material in all animals at the high dose and some animals in the mid dose. Pup and litter weights were statistically significantly reduced at birth. While female offspring tended to catch up with the control in body weight development from PND 40 onwards, male offspring remained at a lower body weight level when compared to the control (about – 12 %) until the end of the study. Liver and kidney weight increases were observed. Both organs revealed higher organ weight (absolute and relative to body weight) in males in both generations (not evaluated in the F2) and females in the F1 generation. No histopathological findings were seen in the liver, however, in the kidney chronic progressive nephropathy (CPN) was observed in male animals. In addition, increased urine volume (with a lower pH) in both sexes and both generations was recorded. Regarding fertility, at the highest dose level, a decreased pregnancy rate of females was observed (P0: 16 pregnant out of 24 mated; F1 Cohort 1B: 10 pregnant out of 20 mated), and in the F1 generation also at mid dose level (16 pregnant out of 20 mated). The reduced pregnancy rate was considered related with decreased number of developing follicles and increased atresia observed in the ovaries of P0 females. However, this finding could not be confirmed in the F1 generation animals. Based on the reduced fertility effects seen the NOAEL with TBPEH was determined to be 100 mg/kg bw/d in this study. Based on the findings of this study the test item was classified as toxic to reproduction category 1B according to CLP.

Oral treatment of pregnant Hsd. Brl. Han: WISTAR rats from gestation day 5 up to day 19 with the test item at the dose levels of 200, 400 and 1000 mg/kg bw/day did not cause death or any necropsy findings. TBPEH did not reveal any adverse effect on the pregnancy and the intrauterine mortality of the conceptuses, the number of viable fetuses and their sex distribution. Further the test substance did not increase significantly the incidence of external and visceral variations, and caused no skeletal malformations. The slight delay in ossification in fetuses of the 1000 mg/kg bw/day dose group is considered to be non-adverse. Based on these observations the No Observed Adverse Effect Level (NOAEL) was determined as follows: NOAEL maternal toxicity: 1000 mg/kg bw/day NOAEL developmental toxicity: 1000 mg/kg bw/day Based on these observations the No Observed Effect Level (NOEL) was determined as follows: NOEL maternal toxicity: 400 mg/kg bw/day; NOEL developmental toxicity: 400 mg/kg bw/day.

Oral treatment of inseminated New Zealand White rabbits with TBPEH at dose levels of 30, 100 and 300 mg/kg bw/day respectively from day 6 up to and including day 27 post insemination revealed pronounced maternal toxicity in the high and mid dose. Four does got into moribund state at 300 mg/kg bw/day after the food consumption was extremely reduced and weight loss, reduced activity, lying, weakness was observed as clear indication of severe toxicity. In association with the treatment eight does aborted in the 300 mg/kg bw/day group and two in the 100 mg/kg bw/day group. Increase of early embryonic death and post-implantation loss/total intrauterine mortality as well as decrease of mean number of viable fetuses was recorded in the 300 mg/kg bw/day dose group. Also, the incidence of females with total post-implantation loss was higher in the high dose group. There was no significant difference in incidence of overall fetal malformations among the experimental groups. Significantly lower fetal weight and crownrump length as well as increase of growth retarded fetuses were observed in the 300 mg/kg bw/day dose group as well as increasing skeletal variations mainly due to delayed ossification (e.g. phalanges) associated to the lower body weights. Multiple malformations of ribs and vertebrae in three fetuses were considered as likely to be secondary due to the severe maternal toxicity. The visceral development of the fetuses was not affected. Based on these observations the No Observed Adverse Effect Level (NOAEL) was determined as follows: NOAEL (maternal toxicity): 30 mg/kg bw/day; NOAEL (developmental toxicity): 100 mg/kg bw/day.

 

Toxicokinetic analysis of tert. Butylperoxy-2-ethylhexanoate (TBPEH)

 

Absorption

Based on molecular weight, physicochemical properties and experimentalin vivodata, TBPEH is expected to become bioavailable after oral administration, either as parent and/or as breakdown products.

 

Oral absorption is favoured for molecular weights below 500 g/mol. TBPEH is not well soluble in water having a relatively high logPow values ranging between 4.79. It may therefore be taken up by micellular solubilisation due to its more lipophilic properties. When administered orally TBPEH might be expected to undergo hydrolysis, at least to some extent. However, it is noteworthy that acute toxicity of TBPEH is low, with an LD50-value of 10000 mg/kg bw in an acute study with rats. This indicates that the acutely more toxic hydrolysis products are probably not formed rapidly enough or to a sufficient amount following oral administration.

Oral absorption for the potential common biodegradation products and the carboxylic acids is well known.

The subacute (28 days) NOAEL was 316 mg/kg bw/day for males and 100 mg/kg bw/day for females, whereas a subchronic NOAEL in rats was determined to be 450 mg/kg bw/d. Based on findings at 300 mg/kg bw/d administered in an EOGRTS (OECD 443) the NOAEL for fertility was determined to be 100 mg/kg bw/d in female rats.

Dermal absorption might be low to moderate based on water solubility and lipophilic properties of TBPEH. Uptake will be limited to the stratum corneum cross the epidermis will be low. This was confirmed in an acute dermal study resulting in an LD50 value of 14142 mg/kg bw. However, TBPEH was shown to be a potential skin sensitizer, thus at least small amounts of the substance or its metabolites must have reached the subcutaneous layers when applied topically on the skin. No local toxicity effects such as irritation or corrosion were observed in skin or eye.

Based on the low vapour pressure of 2 Pa as well as the considering the very limited use profile, inhalation exposure to TBPEH is not likely. Thus, the inhalation route is considered not relevant for humans.

Nevertheless, if the substance reaches the lung, TBPEH may be absorbed by micellular solubilisation (see above). The low water solubility may enhance penetration to the lower respiratory tract. TBPEH showed very low toxicity after inhalation administration, in an acute inhalation toxicity study revealing a LC50 of 42.2 mg/L. Together, this indicates low systemic availability after inhalation and if bioavailable, potential toxicity is considered to be low.

 

 

Distribution

When reaching the body TBPEH may be distributed throughout the organism. Possible hydrolysis of TBPEH in the blood leads to formation of its degradation products. Both degradation products have low to moderate BCF values and are thus considered not bioaccumulative. Although TBPEH is expected to be distributed into the interior part of the cells due to its lipophilic properties, no bioaccumulation is expected since TBPEH is expected to be degraded to more hydrophilic metabolites when compared to the parent compound. TBPEH undergoes hydrolysis upon contact with water. Possible hydrolysis products are tert-butanol, tert-butyl hydroperoxide and 2-ethylhexanoic acid. All hydrolysis products have a lower log Pow value than TBPEH itself. Consequently, there is no bioaccumulation expected, neither for the parent nor for its degradation products. 

 

Metabolism

The metabolism of this peroxyester is could possibly include the following pathways:

-      enzymatic and non-enzymatic hydrolysis,

-      direct reaction with biomolecules due the reactivity of the peroxyester group of the parent or the hydroperoxide formed during hydrolysis (e. g. TBHP)

Hydrolysis may already occur in the gastrointestinal tract. This might occur spontaneously or might be faciliated by an enzymatic process (e.g. esterases or other enzymatic systems of the intestinal microflora).

If the parent compound reaches the liver, enzymatic cleavage is also expected (e.g. by glutathione peroxidase). Unspecific reactions with biomolecules can take place either in the GI-tract or subsequent to absorption. It is assumed that the parent molecules can react directly or after cleavage to hydroperoxide. TBPEH is a skin sensitizer, reactions with macromolecules has been demonstrated to occur. Free radicals and reactive oxygen species which might be formed during metabolism can be deactivated by non-enzymatic antioxidants abundantly available in the organism (e. g. Vitamin E protecting lipid membranes from being peroxidised, Vitamin C and glutathione). Conjugates of these may be faciliated to become excreted or are re-cycled within the endogenous redox system.

 

Excretion

Since TBPEH has a molecular weight below 300, renal excretion is expected to be the predominant route. This does also apply to the potential degradation products and /or metabolits. This is supported by findings after subchronic exposure to TBPEH. Increased liver and kidney weights were reported, in association with increased urine volume and decreased pH (in both sexes and different cohorts). This observation indicates an increased metabolizing and excretory activity of the kidney and liver induced by the test item exposure. No other histopathological lesions were seen in both organs, except increased findings of chronic progressive nephropathy in male animals.

 

Summary

Based on TBPEH's substance properties, uptake via the dermal and inhalation route is not very likely. However, following oral uptake TBPEH becomes systemically available. Bioaccumulation of the parent as well as of the hydrolysis products is not likely to occur based on the physico-chemical properties. Excretion of the parent and/or its metabolites or hydrolysis products is expected to occur mainly via the urine.