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Based onphysico-chemical characteristics, particularly water solubility andoctanol-water partition coefficient absorption via oral, dermal and inhalation route is likely to occur. Intracellular concentration is likely to be higher than extracellular due to the moderatelipophilicityand water solubility of TBPIB. Hydrolytic and metabolic conversion intotert-butylhydroperoxideandisobutyricacid is expected and further oxidation and conjugation are likely to occur as supported by experimental data. Excretion via urine and exhalation are demonstrated to be the main excretion pathways of metabolites formed due to their molecular weight (< 300 g/molin rat) and metabolic pathway. Bioaccumulation of TBPIB itself and of its hydrolysis products is not likely to occur based on theirphysico-chemical properties.

Key value for chemical safety assessment

Bioaccumulation potential:
no bioaccumulation potential

Additional information

Toxicological profile of tert-butyl peroxyisobutyrate (TBPIB)


An acute oral toxicity study conducted with TBPIB with a purity of 75 % using rats revealed a LD50 value of >2000 mg/kg bw. No acute dermal toxicity study was conducted with TBPIB. The 4 hour LC50 value determined in an acute inhalation toxicity study was considered to be greater than 2.53 mg/L and below 6.8 mg/L. In a read-across approach to tert-butyl peroxypivalate (TBPPI, purity 75 %) a LD50 of 2500 mg/kg bw was determined. It is assumed that TBPIB also shows a LD50 value >2000 mg/kg bw due to structure similarity.

In anin vivoskin irritation study conducted with TBPIB with a purity of 75 % no skin irritation was observed when applied on rabbit’s skin. An eye irritation test performed with 75 % TBPIB on rabbits showed that the substance caused slight effects. However, it was not considered to be an eye irritant. A guinea pig maximisation test revealed that TBPIB has no potential for skin sensitisation.

TBPIBdid induce reverse mutations in a bacterial reverse mutation test (Ames test) with fiveSalmonella typhimuriumstrains. The mutagenic response was depended on the bacterial strain and on the presence of a metabolic activation system (S9 liver homogenate). The chemical caused an increase in the reverse mutation frequency in the tester strains TA 98, TA 100, TA 1537 and TA 102 in the presence of Aroclor-induced rat liver S9 and in TA 1535 in the absence of a metabolic activation system. A secondin vitromutation assay was performed. The chemical induced a mutagenic response in anin vitro mammalian cell gene mutation test (HPRT assay) performed on CHO-K1 cells both in the absence and presence of metabolic activation. In a further in vivo micronucleous test TBPIB did not cause an increase in the frequency of micronucleated polychromatic erythrocytes in mice. Also an in vivo Comet assay showed no genotoxic activity in the examined tissues.

A 14 day dose range finding study using oral administration of TBPIB was performed in male and female Wistar rats in order to obtain first information on the toxic potential of the test item after long-term administration to allow a dose-setting for a combined repeated dose toxicity study with the reproduction/developmental toxicity screening test. The chemical was administered orally (by gavage) once a day for a total of 14 days at 0 (vehicle control), 50, 250 and 750 mg/kg bw/day. No mortality was observed through this study. The test substance caused a reduced food intake and, in turn, a reduced body weight particularly in male rats treated with the high dose of 750 mg/kg bw/day. Furthermore,changes in hematology parameters (elevated percentage of reticulocytes along with a slight decrease in the red blood cell parameters) and changes in organ pathology (hyperemia in the mesenteric lymph nodes, higher spleen weights) were observed in male and female animals at 750 mg/kg bw/day. At 250 mg/kg bw/day, salivation, reduced body weight gain in male animals, elevated percentage of reticulocytes in female animals were observed. At 50 mg/kg bw/day, no test item or treatment related changes were present. Based on these results the following three doses were selected for the aforementioned combined repeated dose toxicity study with the reproduction/developmental toxicity screening study: 60, 200 and 600 mg/kg bw/day.


The main study revealed no mortality of male and female animals/dams exposed to TBPIB. Test item related salivation appeared in all dose groups. A slightly decreased food consumption and, in turn, a reduced body weight in males were observed in the high dose group.Slightly less number of red blood cell count and higher reticulocyte countas well asslightly higher mean weights of spleen and adrenal gland (absolute and relative to body and brain weights) were indicative of test item influence on function of these organs in male animals at 600 mg/kg bw/day dose. No test item related changes were observed in clinical chemistry parameters and in macroscopic and histopathological examinations.There were no differences between the control and test item treated groups in the reproductive performance of male and female animals and in delivery data of dams. No test item related adverse effect on the offspring development was observed. Based on these observations the respective NOAEL was set to 200 mg/kg bw/day for male rats and to 600 mg/kg bw/day for female rats. The NOAEL for reproductive performance of the male and female rats was evaluated to be 600 mg/kg bw/day and the NOAEL for the offspring was determined to be 600 mg/kg bw/day.

In order to assess reversibility, persistence or delayed occurrence of potential toxicological effects a 90-day toxicity study followed by a 28-day recovery period was performed with tert-butyl 3,5,5-trimethylperoxyhexanoate (TBPIN), a structural analogous substance of TBPIB. The test item was administered orally (by gavage) to Hsd.Brl.Han: Wistar rats once a day at 0 (vehicle control), 160, 40 and 10 mg/kg bw/day doses. There was no test item related mortality. Toxic signs related to the test item were not detected at any dose level at the daily and detailed weekly clinical observations and in the course of the functional observation battery. Salivation was observed in the male and female animals of the 160 or 40 mg/kg bw/day groups with variable frequency within a group but in a dose related manner regarding the incidence and onset. The body weight development and the daily mean food consumption of male and female animals was not affected by the test item. A test item influence on the estrous cycle was not detected. No test item-related changes were observed in investigated hematology or blood coagulation parameters. Clinical chemistry examinations did not reveal any pathologic changes in the examined parameters.

Although statistical significant increases of mean liver weights in male animals of all dose groups andof mean kidney weights in male animals of 160 and 40 mg/kg bw/day group were noted, all values remained within the historical control ranges. Sperm analysis did not reveal test item influence on the sperm cells (count, motility and morphology) at 160 mg/kg bw/day dose. Histopathology investigations revealed test item related hyaline–like droplets in the epithel cells of some proximal convoluted tubules in the kidney of male animals treated with 160, 40 and 10 mg/kg bw/day, which was accompanied by dilatation of tubuli in the distal area, at the border of cortical – medullary region at 160 and 40 mg/kg bw/day.The renal lesion was not present at the end of the recovery period and no additional renal lesions or increased cell turnover were determined. Based on immunohistochemistry examinations hyaline-like droplets at 160, 40 and 10 mg/kg bw/day doses in male rats could not be caused by a2u-globulin. However, in accordance with literature data (ECETOC, 2002)the isolated and reversible finding hyaline-like droplets in male rats has to be considered as a toxicological relevant, but not adverse effect.A NOEL of < 10 mg/kg bw/day for male animals and 10 mg/kg bw/day for female animals and aNOAEL of 160 mg/kg bw/day for male and female animals was determined for TBPIN.


The same structural analogue of TBPIB was used in a prenatal development toxicity study. Groups of 24 sperm-positive female Hsd. Brl. Han: Wistar rats were treated with TBPIN by oral administration daily at three dose levels of 20, 50 and 150 mg/kg bw/day from day 6 up to and including day 19 post coitum. A control group of 25 sperm positive females was included and the animals were given the vehicle sunflower oil. A Caesarean section and gross pathology were performed on gestational day 20.

In total, there were 19, 20, 21 and 22 evaluated litters in the control, 20, 50 and 150 mg/kg bw/day groups, respectively. Two pregnant females died in the course of the study, one female in the 50 mg/kg bw/day group on gestational day (GD) 17 and one in the 150 mg/kg bw/day group on GD 9. The death and clinical signs were considered to be likely without a relationship with an effect of the test item.

There were no clinical signs and necropsy findings recorded for the survived dams in the experimental groups. There was no indication of an effect of the test item on body weight, corrected body weight and food consumption of the dams in the 20, 50, 150 mg/kg bw/day dose groups. There was no effect indicated related to the administration of TBPIN in the intrauterine mortality of the conceptuses, the number of implantations, viable fetuses and their sex distribution. The mean fetal weight was similar in the control and 20 mg/kg bw/day groups. The slight but statistically significant reduction in the bodyweight of the fetuses in the 50 and 150 mg/kg bw/day dose groups might be attributed to an effect of the test item. Relative placental weight was similar in all experimental groups.

The visceral malformations were considered to be incidental and were not attributed to the administration of the test item to the dams. The incidence of visceral variations was similar to the vehicle control level.

Skeletal malformations in two fetuses of the low dose group and in one fetus of the control group were without a relationship to the test item. Statistically significant skeletal variations were seen in the 20 and in the 150 mg/kg bw/day dose group when the ossification of sternum (3 or less ossified sternebra) was evaluated. Considering that there was no dose response indicated between the low and mid dose group and that the mean percent of the high dose group was slightly above the laboratory´s historical control level, this variation was judged to be in the biological variations. Incomplete ossification of the skull-bones (marked and less marked) was evaluated as variation during skeletal examination. Incomplete ossification of the skull bones correlated with the slightly lower mean fetal weight observed at 50 and 150 mg/kg bw/day dose groups, which was below the mean value seen in the laboratory´s historical control data. Overall, incomplete ossification of the skull bones (marked and less marked) seen at 50 mg/kg bw/day and at 150 mg/kg bw/day is considered to be non-adverse. Based on these observations a No Observed Effect Level (NOEL) for maternal toxicity of 150 mg/kg bw/day and developmental toxicity of 20 mg/kg bw/day and aNo Observed Adverse Effect Level (NOAEL) for maternal toxicity of 150 mg/kg bw/day and for developmental toxicity of 150 mg/kg bw/day was determined.


A read-across justification is provided in IUCLID section 13 demonstrating the reliable use of data from TBPIN (source substance) for the hazard assessment of TBPIB.



Toxicokinetic analysis of tert-butyl peroxypivalate (TBPIB)


Tert-butyl peroxyisobutyrate (TBPIB) is a colourless liquid at room temperature with a molecular weight of 160.2108 g/mol. The substance is soluble in water (4 g/L at 20°C). The log Pow of TBPIB was measured and determined to be 2.7 at 25°C. Based on this log Pow, a BCF of 9.7 L/kg wet-wt was calculated. The vapour pressure of TBPIB is approximately 508 Pa at 38°C. In an aqueous solution, TBPIB is degraded hydrolytically totert-butyl hydroperoxide and isobutyric acid. The half-life of TBPIB in aqueous solution at pH = 4, 7 and 9 at 20 °C are 1224 hours, 8092 hours and 96 hours, respectively.

Both hydrolysis substances have a lower log Pow value than TBPIB itself (approximately 0.846 fortert-butyl hydroperoxide and 1.1 for isobutyric acid at pH 3). Also the BCF values are lower as compared to TBPIB (approximately 3.16 L/kg wet-wt for both hydrolysis products). Water solubility of both hydrolysis products is higher than of TBPIB itself (>691 g/L for tert-butyl hydroperoxide and 618 g/L for isobutyric acid at a pH of 1.7).



Generally, oral absorption is favoured for molecular weights below 500 g/mol. This characteristic combined with the moderate lipophilic log Pow value and water solubility allow dissolution of TBPIB in the gastro-intestinal fluids and contact with the mucosal surface.

Administered without a vehicle in an acute oral toxicity study performed on rats, TBPIB (75 % solution) lead to a LD50 of >2000 mg/kg bw/day. However, long-term administration of TBPIB in a combined repeated dose toxicity study with the reproduction/developmental toxicity screening study (OECD 422) indicate that the compound, and to a lower amount, its hydrolysis products became bioavailable.

In this regards, as indicated by the half-life values from the hydrolysis test, a small fraction of TBPIB will hydrolyze to tert-butyl hydroperoxide and isobutyric acid following oral administration which is indicated by the half-live in an aqueous solution at basic conditions. The results of the hydrolysis tests at a pH range of 4 to 9 are somewhat representative for the conditions found in the GIT with the stomach having an acidic milieu (~ pH 1.4 to 4.5) and the intestine a slightly acidic to slightly alkaline milieu (~ pH 5 to 8).

Due to the lower log Pow values of the hydrolysis products, readily absorption through the GIT epithelium is assumed. Furthermore, molecular weights of the hydrolysis productscombined with their relatively high water solubility (> 10 g/L) may allow the direct uptake into the systemic circulation through aqueous pores or via carriage of the molecules across the membrane with the bulk passage of water. 

Based on the vapour pressure of approximately 504 Pa at 37°C TBPIB might become available for inhalation to a certain extend. If the substance would reach the lungs in its vapour or gaseous state, absorption directly across the respiratory tract epithelium by passive diffusion is likely to occur due to its log Pow value and water solubility.

Similarly, based on physico–chemical properties ofTBPIBthe substance is likely to penetrate skin as the log Pow value and water solubility favour dermal penetration. It is general accepted that if a compound’s water solubility falls between 100-10000 mg/L, absorption can be anticipated to be moderate to high. Moreover, for substances with a log Pow between 1 and 4, both penetration into stratum corneum and partition into the epidermis are likely to occur. These assumptions based on the physico-chemical properties of TBPIB are further supported by the results achieved from an acute dermal toxicity study performed on rabbits with the structure analogue TBPPI (CAS 927-07-1). During this study test item related mortality and specific effects of systemic toxicity were observed at high doses. The LD50 was 2500 mg/kg bw. No increased penetration is expected after long-term application as the chemical does not induce dermal irritation.

Taken together, physico-chemical properties and experimental data indicate bioavailability of TBPIB via oral, dermal and inhalation route.



Assuming that TBPIB is absorbed into the body following oral intake, it may be distributed into the interior part of cells due to its lipophilic properties (log Pow >0). Due to the water solubility of TBPIB diffusion through aqueous channels and pores is an alternative pathway of distribution which is limited by the transfer rate. Consequently, intracellular concentration may be higher than extracellular concentration particularly in adipose tissues. However, slow hydrolysis of TBPIB into tert-butyl hydroperoxide and isobutyric acid is likely to occur. As mentioned above, the physico-chemical properties especially the lower molecular weight and the higher water solubility of the hydrolysis products favour systemic absorption. Direct transport through aqueous pores is likely to be an entry route to the systemic circulation. The results from the combined repeated dose toxicity study with the reproduction/developmental toxicity screening test revealed no indications of a specific target organ. No embryotoxicity/teratogenicity was observed in the reproduction and developmental performance. However, penetration through the placenta could not entirely be excluded. Based on their BCF values both, the parent molecule TBPIB and its hydrolysis products have no potential to bioaccumulate in the human body.



Based on the structure of the molecule, TBPIB may be hydrolysed after being in contact with an aqueous solution as well as enzymatically metabolised. The first degradation product tert-butyl hydroperoxide may be converted by glutathione peroxidase into tert-butanol which in turn might be conjugated with glucuronic acid or sulfate to increase the compound’s hydrophilicity (Chance, B. et al. 1979). Oxidation of tert-butanol by ADH and AlDH is an alternative metabolic pathway resulting in 2,2-dimethyl-propionic acid (pivalic acid). Pivalic acid was found to be completely conjugated with carnitine in the human body (Nakashima et al., 1992). Isobutyric acid was demonstrated to be metabolised to propionic acid which in turn enter the citric acid cycle where it is oxidised to carbon dioxide (DiVincenzo and Hamilton, 1979).


As discussed above, TBPIB will be hydrolysed both after being in contact with an aqueous solution or enzymatically metabolised and will probably not be excreted in its unchanged form. Pivalic acid, an oxidation product of a degradation product of TBPIB, was shown to be conjugated with carnitine and excreted via urine (Nakashima et al., 1992).For other conjugation products of tert-butanol renal excretion is also expected due to their molecular weight and water solubility. Since isobutyric acid was shown to be oxidised to carbon dioxide, exhalation is the preferred excretion pathway for this degradation product of TBPIB (DiVincenzo and Hamilton, 1979).