Tris(2-ethylhexyl) 2-(acetyloxy)propane-1,2,3-tricarboxylate

Basic toxicokinetics

Administrative data

Endpoint:

basic toxicokinetics

Type of information:

other: expert statement

Adequacy of study:

key study

Study period:

2011

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

other: see 'Remark'

Remarks:

An extensive Assessment of the toxicological behaviour of tris(2-ethylhexyl) 2-(acetyloxy)propane-1,2,3-tricarboxylate was performed, taking into account the chemical structure, the available physico-chemical-data and the available (eco-)toxicological data.

Data source

Reference

Reference Type:

study report

Title:

Unnamed

Year:

2012

Report date:

2012

Materials and methods

Test guideline

Qualifier:

no guideline required

Test material

Test material information

Constituent 1

Reference substance name:

Tris(2-ethylhexyl) 2-(acetyloxy)propane-1,2,3-tricarboxylate

EC Number:

205-617-0

EC Name:

Tris(2-ethylhexyl) 2-(acetyloxy)propane-1,2,3-tricarboxylate

Cas Number:

144-15-0

Molecular formula:

C32H58O8

IUPAC Name:

1,2,3-tris(2-ethylhexyl) 2-(acetyloxy)propane-1,2,3-tricarboxylate

Test material form:

other: liquid

Details on test material:

not applicable

Radiolabelling:

other: not applicable in this expert statement

Test animals

Species:

other: not applicable

Strain:

other: not applicable

Details on test animals or test system and environmental conditions:

not applicable

Administration / exposure

Route of administration:

other: all routes of administration are discussed in the expert statement

Vehicle:

other: not applicable

Details on exposure:

all routes of administration are discussed in the expert statement

Details on study design:

not applicable

Details on dosing and sampling:

not applicable

Results and discussion

Toxicokinetic / pharmacokinetic studies

Details on absorption:

The molecular weight of tris(2-ethylhexyl) 2-(acetyloxy)propane-1,2,3-tricarboxylate (570.8 g/mol) is not favoured for absorption. Its water solubility is very low , this implies that absorption is presumed to be hindered. In addition, the substance is not favoured for absorption as its LogPow is above 4. For unchanged tris(2-ethylhexyl) 2-(acetyloxy)propane-1,2,3-tricarboxylate the absorption after oral administration is expected to be low. It is expected, that tris(2-ethylhexyl) 2-(acetyloxy)propane-1,2,3-tricarboxylate is marginally available in the air for inhalation, due to its low vapour pressure. The molecular weight of the substance is above 500, which indicates practically no potential to penetrate the skin. For tris(2-ethylhexyl) 2-(acetyloxy)propane-1,2,3-tricarboxylate the water solubility will prevent significant uptake via the skin. The LogPow value for this substances is not optimal, its LogPow (5.7) does not favour dermal absorption. The systemic toxicity via the skin is assumed to be low and this has been proven with the results of the acute dermal toxicity study, which showed no mortality after dermal application of 2000 mg/kg bw in rats.

Details on distribution in tissues:

The distribution is expected to be more extensive in fat tissues than in other tissues, due to its better solubility in octanol than in water (LogPow of 5.7).

Details on excretion:

Based on its chemical structure and its molecular weight, the substance is assumed to be excreted (if not metabolised completely in beta-oxidation and citric cycle) as metabolites (i.e. conjugates with glucuronic acid) via urine and to a lower extent via bile.

Metabolite characterisation studies

Details on metabolites:

It is most likely that the substance will be subject to extensive metabolism by esterases and cytochrome P450 enzymes and break-down in the beta-oxidation or citric acid cycle or in cases subsequent glucuronidation.

Any other information on results incl. tables

Background

There is data available on the physico-chemical properties. The substance is a colourless liquid. The molecular weight of tris(2-ethylhexyl) 2-(acetyloxy)propane-1,2,3-tricarboxylate is 570.8 g/mol. The melting point is reported as -55°C. It has a high boiling point of 267°C. Tris(2-ethylhexyl) 2-(acetyloxy)propane-1,2,3-tricarboxylate is little soluble in water (< 0.05 mg/L at 20°C). The value for LogPow has been reported to be 5.7. The vapour pressure of calculated 0.0318 Pa is considered to be very low. Hydrolysis (as a function of pH) has not been determined.

The citrate substances are not acutely toxic, when administered to rats orally or dermally (oral LD₅₀>2000 mg/kg bw, dermal LD₅₀ >2000 mg/kg bw, respectively). It is not a skin irritant and not eye irritating. Reliable data indicate that this substance to not bear a potential to cause allergic reactions. Additionally, the available data on genetic toxicity do not indicate a potential for mutagenicity in studies according to OECD 471.

Absorption

In general, absorption of a chemical is possible, if the substance crosses biological membranes. This process requires a substance to be soluble, both in lipid and in water and, is also dependent on its molecular weight (substances with molecular weights below 500 are favourable for absorption). Generally, the absorption of chemicals which are surfactants or irritants may be enhanced, because of damage to cell membranes.

Tris(2 -ethylhexyl)2 -acetyloxy)propane-1,2,3 -tricaboxylate is not favourable for absorption taking its molecular weight (570.8 g/mol) into account. The substance is also nearly insoluble in water (< 0.05 mg/l), so its absorption is hindered,as well. The value of the LogPow (5.7) demonstrates that the substance has likely a better solubility in octanol than in water (positive LogPow for lipophilic substances, negative LogPow for hydrophilic substances). Considering its LogPow, the absorption into the body will not be favoured (LogPow between 0 and 4 are favourable for absorption). The substance is not irritating to skin and not irritating to eyes. Consequently, the above mentioned enhancement of absorption for irritants, does not apply.

In conclusion it can be stated, that tris(2-ethylhexyl) 2-(acetyloxy)propane-1,2,3-tricarboxylate is expected to be poorly absorbed, considering its molecular weight, water solubility and LogPow value.

Absorption from the gastrointestinal tract

Regarding oral absorption, in the stomach, a substance will most likely be hydrolysed, because this is a favoured reaction in the acidic environment of the stomach. In accordance with the above mentioned principles, it is likely for this substance to be hydrolysed in the stomach.

In the small intestine absorption occurs mainly via passive diffusion or lipophilic compounds may form micelles and be taken into the lymphatic system. Additionally, metabolism may occur by gut microflora or by enzymes in the gastrointestinal mucosa. However, the absorption of highly lipophilic substances (LogPow of 4 or above) may be limited by the inability of such substances to dissolve into gastrointestinal fluids and hence make contact with the mucosal surface. The absorption of such substances will be enhanced if they undergo micellular solubilisation by bile salts. Substances absorbed as micelles enter the circulation via the lymphatic system, bypassing the liver.

In conclusion, it is stated that the substance is expected to be poorly absorbed after oral exposure, either as the unchanged substance or after hydrolysis as the corresponding metabolites.

Absorption from the respiratory tract

Concerning absorption in the respiratory tract, any gas or vapour has to be sufficiently lipophilic to cross the alveolar and capillary membranes (moderate LogPow values between 0-4 favourable for absorption). The rate of systemic uptake of very hydrophilic gases or vapours may be limited by the rate at which they partition out of the aqueous fluids (mucus) lining the respiratory tract and into the blood. Such substances may be transported out of the lungs with the mucus and swallowed or pass across the respiratory epithelium via aqueous membrane pores. Lipophilic substances (LogPow >0) would have the potential to be absorbed directly across the respiratory tract epithelium. Very hydrophilic substances might be absorbed through aqueous pores (for substances with molecular weights below and around 200) or be retained in the mucus.

The substance has a low vapour pressure, which indicates only slight availability for inhalation. However, due to its LogPow, the amount available is assumed to be absorbed directly across the respiratory tract epithelium. Based on this data, it can be expected that tris(2-ethylhexyl) 2-(acetyloxy)propane-1,2,3-tricarboxylate is marginally available in the air for inhalation, due to its low vapour pressure.

Absorption following dermal exposure

In order to cross the skin, a compound must first penetrate into the stratum corneum and may subsequently reach the epidermis, the dermis and the vascular network. The stratum corneum provides its greatest barrier function against hydrophilic compounds, whereas the viable epidermis is most resistant to penetration by highly lipophilic compounds. Substances with a molecular weight (MG) below 100 are favourable for penetration of the skin and substances with a MG > 500 are normally not able to penetrate. The substance must be sufficiently soluble in water to partition from the stratum corneum into the epidermis. Therefore if the water solubility is below 1 mg/l, dermal uptake is likely to be low. Additionally LogPow values between 1 and 4 favour dermal absorption (values between 2 and 3 are optimal). Above 4, the rate of penetration may be limited by the rate of transfer between the stratum corneum and the epidermis, but uptake into the stratum corneum will be high. Above 6, the rate of transfer between the stratum corneum and the epidermis will be slow and will limit absorption across the skin. Uptake into the stratum corneum itself may be slow. Moreover vapours of substances with vapour pressures below 100 Pa are likely to be well absorbed and the amount absorbed dermally may be more than 10% of the amount that would be absorbed by inhalation. If the substance is a skin irritant or corrosive, damage to the skin surface may enhance penetration. During the whole absorption process into the skin, the compound may be subject to biotransformation.

Tris(2-ethylhexyl) 2-(acetyloxy)propane-1,2,3-tricarboxylate is expected not to be absorbed via the skin, as its molecular weight is above 500 g/mol. Its water solubility will prevent significant uptake via the skin. The LogPow value for this substances is not optimal (values 2-3), and does not favour absorption via the skin (LogPow between 1 and 4 would be favoured). Tris(2-ethylhexyl) 2-(acetyloxy)propane-1,2,3-tricarboxylate has a high LogPow (5.7), which does not favour dermal absorption. The amount of substance, which is absorbed following dermal exposure into the stratum corneum is however unlikely to be transferred into the epidermis, due to its molecular weight and LogPow. The systemic toxicity via the skin is assumed to be low and this has been proven with the results of the acute dermal toxicity study, which showed no mortality after dermal application of 2000 mg/kg bw in rats (tris(2-ethylhexyl) 2-(acetyloxy)propane-1,2,3-tricarboxylate).

Distribution

In general, the following principle applies: the smaller the molecule, the wider the distribution. A lipophilic molecule (LogPow >0) is likely to distribute into cells and the intracellular concentration may be higher than extracellular concentration particularly in fatty tissues. It’s not possible to foresee protein binding, which can limit the amount of a substance available for distribution. Furthermore, if a substance undergoes extensive first-pass metabolism, predictions made on the basis of the physico-chemical characteristics of the parent substance may not be applicable.

In case of tris(2 -ethylhexyl)2 -acetyloxy)propane-1,2,3 -tricarboxylate, no data is available for distribution patterns. The distribution is expected to be more extensive in fat tissues than in other tissues, due to its better solubility in octanol than in water (LogPow of 5.7).

Accumulation

It is also important to consider the potential for a substance to accumulate or to be retained within the body. Lipophilic substances have the potential to accumulate within the body (mainly in the adipose tissue), if the dosing interval is shorter than 4 times the whole body half-life. Although there is no direct correlation between the lipophilicity of a substance and its biological half-life, substances with high LogPow values tend to have longer half-lives. On this basis, there is the potential for highly lipophilic substances (LogPow >4) to accumulate in biota which are frequently exposed. Highly lipophilic substances (LogPow between 4 and 6) that come into contact with the skin can readily penetrate the lipid rich stratum corneum but are not well absorbed systemically. Although they may persist in the stratum corneum, they will eventually be cleared as the stratum corneum is sloughed off. A turnover time of 12 days has been quoted for skin epithelial cells.

Accordingly, the experimentally determined LogPow value, the water solubility and the predicted behaviour concerning absorption, the logPow value of tris(2-ethylhexyl) 2-(acetyloxy)propane-1,2,3-tricarboxylate might indicate a potential for accumulation, but this is unlikely because of the limited absorption of tris(2-ethylhexyl) 2-(acetyloxy)propane-1,2,3-tricarboxylate.

Metabolism

Route specific toxicity may result from several phenomena, such as hydrolysis within the gastrointestinal or respiratory tracts, also metabolism by gastrointestinal flora or within the gastrointestinal tract epithelia (mainly in the small intestine), respiratory tract epithelia (sites include the nasal cavity, tracheo-bronchial mucosa [Clara cells] and alveoli [type 2 cells]) and skin.

Hydrolysis does apply for tris(2 -ethylhexyl)2 -(acetyloxy)propane-1,2,3 -tricarboxylate. Esterases are enzymes known for their broad substrate specificity and therefore tris(2-ethylhexyl) 2-(acetyloxy)propane-1,2,3-tricarboxylate is expected to be cleaved releasing ethylhexanol as a major intermediate metabolism product. Further metabolites should be acetyl mono (di) ethylhexyl citrate, acetyl citrate, citric and acetic acid.

Metabolism is very likely to occur via the Cytochrome P450 group of metabolising enzymes, as it has been predicted with the TOXTREE modelling tool (Chemservice S.A., 2011, report-no. CSL-DCD-178). The chemicals have been identified to bear primary, secondary and tertiary sites and more than 4 sites for metabolism by the Cytochrome P450 group of metabolising enzymes. The primary, secondary and tertiary sites of metabolism are predicted to be subject to aliphatic hydroxylation.

To identify all possible sites for phase 1 reactions, the molecular structures were investigated in detail. The acetyl- and the methyl groups of ethyl- or butyl-rests are possibly subject to oxidative desalkylation and as a consequence the molecular weight and the length of the molecule would decrease. Additionally, as specified above, the carbon atoms in the chain (in the center and in periphery) are possibly subject to aliphatic hydroxylation, possibly yielding multiple hydroxyl groups, which is facilitating the elimination. The introduced hydroxyl-groups can, however, also be subject to further phase I metabolism by alcohol dehydrogenase to a ketone, which in turn might be metabolised to a carboxylic acid.

In conclusion, it is most likely that the substance of interest will be subject to extensive metabolism by esterases and cytochrome P450 enzymes and break-down in the beta-oxidation or citric acid cycle or in cases subsequent glucuronidation.

Excretion

The major routes of excretion for substances from the systemic circulation are the urine and/or the faeces (via bile and directly from the gastrointestinal mucosa). For non-polar volatile substances and metabolites exhaled air is an important route of excretion. Substances that are excreted favourable in the urine tend to be water-soluble and of low molecular weight (below 300 in the rat) and be ionized at the pH of urine. Most will have been filtered out of the blood by the kidneys though a small amount may enter the urine directly by passive diffusion and there is the potential for reabsorption into the systemic circulation across the tubular epithelium. Substances that are excreted in the bile tend to be amphipathic (containing both polar and nonpolar regions), hydrophobic/strongly polar and have higher molecular weights and pass through the intestines before they are excreted in the faeces and as a result may undergo entero-hepatic recycling which will prolong their biological half-life. This is particularly a problem for conjugated molecules that are hydrolysed by gastrointestinal bacteria to form smaller more lipid soluble molecules that can then be reabsorbed from the GI tract. Those substances less likely to recirculate are substances having strong polarity and high molecular weight of their own accord. Other substances excreted in the faeces are those that have diffused out of the systemic circulation into the GIT directly, substances which have been removed from the gastrointestinal mucosa by efflux mechanisms and non-absorbed substances that have been ingested or inhaled and subsequently swallowed. Non-ionized and lipid soluble molecules may be excreted in the saliva (where they may be swallowed again) or in the sweat. Highly lipophilic substances that have penetrated the stratum corneum but not penetrated the viable epidermis may be sloughed off with skin cells.

Concerning the above mentioned behaviour predicted for its metabolic fate, it is not very likely that the parent substance will be excreted unchanged.

Based on its chemical structure and its molecular weight, the substance is assumed to be excreted (if not metabolised completely in beta-oxidation and citric cycle) as metabolites (i.e. conjugates with glucuronic acid) via urine and to a lower extent via bile. However, it has to be kept in mind, that metabolites conjugated to glucuronic acid, can be subject to entero-hepatic recycling, and therefore might re-enter the system.

Applicant's summary and conclusion

Conclusions:

Interpretation of results (migrated information): no bioaccumulation potential based on study results
An extensive Assessment of the toxicological behaviour of tris(2-ethylhexyl) 2-(acetyloxy)propane-1,2,3-tricarboxylate was performed (expert statement), taking into account the chemical structure, the available physico-chemical-data and the available (eco-)toxicological data.

Executive summary:

In order to assess the toxicological behaviour of tris(2-ethylhexyl) 2-(acetyloxy)propane-1,2,3-tricarboxylate, the available experimental and predicted physico-chemical data have been evaluated. Unchanged tris(2-ethylhexyl) 2-(acetyloxy)propane-1,2,3-tricarboxylate is expected to be poorly absorbed after oral exposure (based on molecular weight, water solubility and LogPow value and experimentally derived data). This prediction for tris(2-ethylhexyl) 2-(acetyloxy)propane-1,2,3-tricarboxylate can not be reconsidered based on the acute toxicity data available for this substance, as the results do not clearly indicate that the rapid absorption noted is based on absorption of the unchanged compound. The absorption of any metabolite of the substances of interest is, however, fast and complete. Concerning the absorption after exposure via inhalation, as the chemical has a low vapour pressure, it is clear, that the substance is poorly available after inhalation. Given its lipophilicity (LogPow 5.7) - if absorbed - it is expected to be absorbed directly across the respiratory tract epithelium. The substance is expected to be also poorly absorbed following dermal exposure into the stratum corneum and to a certain extent into the epidermis, due to its molecular weight and its LogPow. In addition, the systemic toxicity via the skin is assumed to be low and this has been proven with the results of the acute dermal toxicity study with tris(2-ethylhexyl) 2-(acetyloxy)propane- 1,2,3-tricarboxylate, which showed no mortality after dermal application of 2000 mg/kg bw in rats. Concerning the distribution in the body, the chemical is expected to be better distributed into the cells, since it is a lipophilic substance. The experimentally determined LogPow value, the water solubility and predicted behaviour concerning absorption,. the LogPow value of tris(2-ethylhexyl) 2-(acetyloxy)propane-1,2,3-tricarboxylate might indicate a potential for accumulation, but this is unlikely because of the limited absorption of unchanged tris(2-ethylhexyl) 2-(acetyloxy)propane-1,2,3-tricarboxylate. The substance is expected to be extensively metabolised by esterases and cytochrome P450 enzymes and break-down in the beta-oxidation or citric acid cycle or in cases subsequent glucuronidation.The substance is assumed to be excreted (if not metabolised completely in beta-oxidation and citric cycle) as metabolites (i.e. conjugates with glucuronic acid)via urine and to a lower extent via bile.