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

Endpoint summary

Description of key information

Short description of key information on bioaccumulation potential result: 
1) Toxicokinetic statement, Chemservice S.A., 2012
2) ADME study with the structural analogue ATBC (Hiser et al., 1992)
3) Prediction - Toxtree Chemservice S.A., 2011
Short description of key information on absorption rate: 
1) Toxicokinetic statement, Chemservice S.A., 2012

Key value for chemical safety assessment

Bioaccumulation potential:

no bioaccumulation potential

Absorption rate - oral (%):

100

Absorption rate - dermal (%):

100

Absorption rate - inhalation (%):

100

Additional information

Conclusion on toxicokinetics, absorption, distribution metabolism and excretion of tris(2-ethylhexyl) 2-(acetyloxy)propane-1,2,3-tricarboxylate

Based on physicochemical properties tris(2-ethylhexyl) 2-(acetyloxy)propane-1,2,3-tricarboxylate is expected to be poorly absorbed via the oral and dermal routes of exposure. It is in line with the acute oral and dermal toxicity studies in rats where the substance did not show any sign of systemic toxicity. Hydrolysis in the gastrointestinal tract is expected to a certain extent though the kinetic of this process is probably slow. This way the substance can be absorbed either unchanged or in form of its metabolites. Similar to its structural analogue acetyl tributyl citrate, unchanged tris(2-ethylhexyl) 2-(acetyloxy)propane-1,2,3-tricarboxylate is likely to be cleaved by esterases which are known to possess a broad substrate specificity. The intermediate metabolite products are expected therefore ethylhexanol, acetylated or not acetylated mono (di)triethylhexyl citrate. Further metabolic pathways like transformation through Phase I or Phase II enzymes are possible. The half-lives of metabolites are likely to be longer than those for its category members (for detailed discussion of toxicokinetics please refer to the read-across statement attached to the IUCLID file). Excretion is expected in form of conjugates via urine. Limited absorption and metabolism of tris(2-ethylhexyl) 2-(acetyloxy)propane-1,2,3-tricarboxylate point to absence of bioaccumulation potential.

Discussion on bioaccumulation potential result:

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

Tris(2-ethylhexyl) 2-(acetyloxy)propane-1,2,3-tricarboxylate was evaluated regarding its toxicokinetic behaviour. Due to its physico-chemical properties it is reasonable to assume, that the unchanged substance is absorbed poorly after oral exposure. The substance is expected to be poorly available after inhalation or after dermal exposure. It is expected to be distributed throughout the body in the circulatory system and into the inner cell compartments, due to its lipophilicity. No significant potential for accumulation was identified. 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 intermediate metabolite products are expected therefore ethylhexanol, acetylated or not acetylated mono (di)triethylhexyl citrate.Excretion (if not metabolised completely in beta-oxidation and citric cycle) is predicted to occur as metabolites (i.e. conjugates with glucuronic acid)via urine and to a lower extent via bile.

 

Read-across data

No experimental data is available to assess the toxicity of tris(2-ethylhexyl) 2 -(acetyloxy)propane-1,2,3-tricarboxylate (CAS 144 -15 -0). Therefore, available studies for the structural analogues as well as physicochemical properties of substance of interest were taken into account to conclude about its absorption, distribution, metabolism and excretion paths.

In a study on metabolism and disposition of acetyl tributyl citrate (ATBC) in male Sprague-Dawley rats (Hiser et al., 1992; also cited in US EPA (2003)) groups of 4 – 5 male Sprague-Dawley rats were dosed once via gavage with 70 mg [14C]ATBC/kg bw and urine, faeces, cage wash, expired organics and [14C]CO2, blood, tissues (including GI tract and contents) and carcass were analysed for [14C] and/or unchanged ATBC. Absorption of dosed [14C] was rapid (t_1/2= 1.0 h) and extensive (≥ 67 %). Absorbed [14C]ATBC was rapidly and completely metabolized and eliminated. More than 87 % of the administered radioactivity was excreted during the initial 24 hrs after dosing. For [14C] in blood an elimination half-life of 3.4 hrs was calculated during this interval. Less than 1 % of the dosed radioactivity remained in tissues and carcass 48 hrs post-dosing. The principle route of [14C] excretion was via urine (59 – 70 % of the [14C] dose), while 25 – 36 % were excreted via faeces and 2 % as [14C]CO₂. At least 9 radiolabeled metabolites were found in urine and 3 in faeces. Urinary metabolites positively identified were acetyl citrate, mono-butyl citrate (tentatively the major metabolite), acetyl mono-butyl citrate, dibutyl citrate, and acetyl dibutyl citrate. It was concluded that the low oral toxicity of ATBC is not due to poor absorption but is caused by an intrinsic property of ATBC and/or its metabolites or is due to rapid clearance in the rat.

In another study on in vitro hydrolysis of acetyl-tributylcitrate in human serum and in rat liver homogenate (Edlund and Ostelius, 1991; also cited in US EPA (2003)) the esterase activity was determined to predict the metabolism of acetyl-tributylcitrate (ATBC) in vivo. ATBC was hydrolysed relatively slowly in human serum (t_1/2: ca. 7 h) into the equivalent of 2 moles of n-butanol. One butyl ester group of ATBC showed no hydrolysis (most probably due to the lower affinity for the butyl group at the 2 position). In rat liver homogenate, hydrolysis was faster (t_1/2: < 30 min) and about 2.3 moles of n-butanol were recovered.

Triethyl citrate (TEC) and triethyl-O-acetylcitrate (ATEC) were subject to a detailed investigation of their toxicological properties (Finkelstein and Gold, 1959). Poisoning pattern in treated animals during acute and repeated oral toxicity studies elucidated mechanism of toxic action of the substance. Doses ranging from 5 to 15 grams per kilogram were administered by stomach tube to rats and 1 - 9.5 grams per kilogram were given by stomach tube to cats. The effects of the two compounds were indistinguishable in treated animal species. The absorption of two substances was fairly rapid; signs, depending on the dose, appeared within a few minutes. The course of their poisoning was also fairly rapid, progressing to advanced stages within approximately one hour and terminating either in death about 2 hours to 2-3 days after dosing or in apparent recovery. The disappearance of manifest signs of poisoning can be deceptive, since the poisoning persisted for a much longer time. TEC appeared to be approximately twice as potent as ATEC in the cat. According to the authors the toxic effects and the course of TEC and ATEC poisoning resemble those of the citrate ion introduced into the circulation, resulting in deionization of calcium and inducing effects of hypocalcaemia. In this way, a rapid hydrolysis of citrates forming citric acid and ethanol were suggested (Finkelstein and Gold, 1959).

Bruns and Werners verified the assumptions of Finkelstein and Gold in an in vitro study where TEC and ATEC were investigated in their potential to hydrolyse by liver homogenates and by blood serum (Bruns and Werners, 1962, also cited in TNO BIBRA, 1998). The substances were hydrolysed by liver homogenate of human, rat and mouse origin, and by blood serum to citric acid and ethanol (in case of TEC) or acetic acid, citric acid and ethanol (in case of ATEC). Per mole of triethyl citrate 1 mol of citric acid and 3 moles of ethanol are formed during complete hydrolysis. In case of triethyl-O-acetylcitrate a further 1 mol of acetic acid is build. These three metabolites are known to be of low toxic potential and appear in intermediary metabolism, where they are oxidized in well-defined paths in the organism to CO2 and H2O (Bruns and Werners, 1962; CSTEE, 1999).

Further, triethyl citrate was reported to be rapidly hydrolysed (within 15 minutes) by freshly collected rat serum, and at a much slower rate (hydrolyses was not complete after 4 hrs) by freshly collected human serum (Figdor & Ballinger, 1981, cited in TNO BIBRA, 1998; CSTEE, 1999).

Prediction using TOXTREE

The chemical structure of tris(2-ethylhexyl) 2-(acetyloxy)propane-1,2,3-tricarboxylate was assessed by Toxtree (v.2.5.0) modelling tool for possible metabolism. SMART Cyp is a prediction model, included in the tool, which identifies sites in a molecule that are labile for the metabolism by Cytochromes P450.

Tris(2-ethylhexyl) 2-(acetyloxy)propane-1,2,3-tricarboxylate, containing the structural alerts: carbonyl compound: aldehyde or ketone, ether, dialkylether, carboxylic acid derivative, carboxylic acid ester and carbonic acid diester, is expected to be well metabolized by the Cytochrome P450 group of metabolizing enzymes. The primary, secondary and tertiary sites of metabolism are predicted to be subject to aliphatic hydroxylation. The primary site is predicted to be the carbon-atom at the acetylated oxygen-atom; the secondary sites are the centred carbon atoms and the tertiary sites of metabolism are predicted to be the carbon-atoms of the collateral chains.

 

Discussion on absorption rate:

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

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.

In case of tris(2-ethylhexyl) 2-(acetyloxy)propane-1,2,3-tricarboxylate, the molecular weight 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 substance 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 (tris(2-ethylhexyl) 2-(acetyloxy)propane-1,2,3-tricarboxylate).

Conclusion

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. Tris(2-ethylhexyl) 2-(acetyloxy)propane-1,2,3-tricarboxylate is expected to be poorly absorbed following dermal exposure into the stratum corneum and to a certain extent into the epidermis, due to its molecular weight of 570.8 g/mol and its LogPow of 5.7. 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.