Registration Dossier

Administrative data

Link to relevant study record(s)

Description of key information

Short description of key information on bioaccumulation potential result: 
1) Publication on the metabolism of the test substance and its structural analogue ATBC in Human Serum and Rat Liver Homogenates (Davis, 1991)
2) Toxicokinetic statement, Chemservice S.A., 2012
3) ADME study with the structural analogue ATBC (Hiser et al., 1992)
4) 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 absorption, distribution, metabolism and excretion of tributyl citrate

Based on physicochemical properties and on the experimental data for tributyl citrate and its structural analogues, tributyl citrate is expected to be well absorbed via oral and dermal exposure routes (for details please refer to the read-across statement and the toxicokinetik statement, attached to the IUCLID file). Due to its low vapour pressure tributyl citrate is marginally available in the air for inhalation. As evident from oral acute and subacute studies the substance is hydrolysed slowly in the gastrointestinal tract demonstrating its very low toxicity. It is very likely to be distributed in circulatory system and in tissues where it is expected to be rapidly metabolised to citric acid and butanol and further to butanoic acid, metabolites which can be whether excreted or involved in the body metabolism and therefore no potential for bioaccumulation is foreseen.

 

Discussion on bioaccumulation potential result:

Toxicokinetic statement for tributyl citrate

Tributyl citrate was evaluated regarding its toxicokinetic behaviour. Due to its physico-chemical properties it is reasonable to assume, that the substance is absorbed well. The substance is expected to be poorly available after inhalation or after dermal exposure. Tributyl citrate is expected to be distributed throughout the body in the circulatory system and also into the inner cell compartments, due to its lipophilicity. No significant potential for accumulation was identified. Tributyl citrate 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. 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.

Data for tributyl citrate

There is experimental data available on metabolism of tributyl citrate (TBC), which was tested together with its precursor acetyl tributyl citrate (ATBC) in human serum and in rat liver homogenates (Davis, 1991). It was shown that human serum and rat liver homogenates are capable of the metabolism of ATBC and TBC and that butanol is a stoichiometric metabolite of both. Dose levels of test material were as follows: 100 μg ATBC/mL (248 nmoles/mL), 100 μg TBC/mL (252 nmoles/mL) 14.8 μg n-butanol/mL (200 nmoles/mL). Both ATBC and the intermediate metabolite TBC undergo rapid metabolism in both human serum and rat liver homogenates. The half-lives were: 32 hours for ATBC, 4 hours for TBC and only seconds for n-butanol. It would be expected yielding the principal metabolites acetic acid, citric acid and butanol. The butanol would then be expected to further oxidize to butanoic acid and assimilated by ß-oxidation. Although a direct stoichiometry of butanol formed from ATBC and TBC was not observed, these results are partially explained based on the fact that butanol also is metabolised in the rat liver homogenate at a rate of 37 nmoles/mL/hr. It also may be suggested that an initial single or double debutylation may yield products which are less readily hydrolysed in the system; products which would be, as fully ionisable carboxylic acids, readily excreted in vivo.

Data for analogues substances

In a study on metabolism and disposition of acetyl tributyl citrate (ATBC) in male Sprague-Dawley rats 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 (Hiser et al., 1992, cited also in US EPA, 2003). Absorption of dosed [14C] was rapid (t1/2= 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 rat liver homogenate esterase activity was determined to predict the metabolism of acetyl-tributylcitrate (ATBC) in vivo (Edlund and Ostelius, 1991) . ATBC was hydrolysed relatively slowly in human serum (t1/2: 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 (t1/2: < 30 min) and about 2.3 moles of n-butanol were recovered.

The structurally similar substances triethyl citrate (TEC) and triethyl-O-acetylcitrate (ATEC) were subject to a detailed investigation of their toxicological properties (Finkelstein and Gold, 1959). The acute and repeated oral toxicity were investigated in rats and cats. Doses ranging from 5 to 15 cc per kilogram were administered by stomach tube to 165 rats and 1 - 9.5 cc per kilogram were given by stomach tube 24 hours after the last feeding to fifty-two cats. The effects of the two compounds were indistinguishable. The toxicological signs in rats included weakness, depression, ataxia, hyperexcitability, unrest, urinary dribbling, irregular and laboured respiration, and in the advanced phase of poisoning convulsions appeared in some of the animals. Their absorption in rats 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 an hour or so and terminating either in death approximately 2 hours to 3 days after dosing or in apparent recovery about 15 hours to 4 days after administration. The toxicological signs in cats consisted of nausea, ataxia, weakness, muscle twitching, tremors, reflex hyperexcitability, lowering of body temperature, gasping and shallow respiration, prostration, convulsions, respiratory failure, and death. The absorption of both compounds in cats was fairly rapid, signs usually appearing within a few minutes. The course of the poisoning, as judged by the manifest signs in the intact animals, was also fairly rapid, progressing to advanced stages within approximately one hour and terminating in death about 2 hours to 2 days after administration, or apparent recovery about 4 hours to 3 days after dosing. As will be seen later, the disappearance of manifest signs of poisoning can be deceptive, since the poisoning persists for a much longer time. TEC appears to be approximately twice as potent as ATKC in the cat.

In addition, it is speculated that the toxic effects and the course of TEC and ATEC poisoning, studied here in greater detail in the cat, resemble those of the citrate ion introduced into the circulation, resulting in deionization of calcium and producing effects of hypocalcaemia.

The structurally similar substances triethyl citrate (TEC) and triethyl-O-acetylcitrate (ATEC) were subject to an investigation of their potential to hydrolyse in vitro (Bruns and Werners, 1959). It was investigated, whether triethyl citrate and triethyl-O-acetylcitrate are hydrolysed by a) liver homogenates and b) blood serum. The esters triethyl citrate and triethyl-O-acetyl-citrate, used as plasticisers, are hydrolysed by liver homogenate of human, rat and mouse origin, and by blood serum to citric acid and ethanol or acetic acid, citric acid and ethanol, respectively. Per mole of triethyl citrate 1 mol if 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 appear in intermediary metabolism, where they are oxidized in well-defined paths in the organism to CO2 and H2O.

Prediction using TOXTREE

The chemical structure of tributyl citrate 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.

Tributyl citrate, containing the structural alerts: carbonyl compound: aldehyde or ketone, alcohol, tertiary alcohol, carboxylic acid derivative, carbonic acid ester and carbonic acid diester, is expected to be well metabolized by the Cytochrome P450 group of drugs metabolizing enzymes. The primary, secondary and tertiary sites of metabolism are predicted to be subject to aliphatic hydroxylation. The primary sites are predicted to be the centred carbon atoms; the secondary and tertiary sites of metabolism are predicted to be the carbon-atoms of the collateral chains.

Discussion on absorption rate:

Toxicokinetic statement

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 tributyl citrate, the molecular weight is above 100 and below 500, which indicates a low potential to penetrate the skin. Even though the water solubility of tributyl citrate is intermediate, absorption through the lipophilic stratum corneum is possible. The LogPow value is not optimal (values 2-3), but still favour absorption via the skin (between 1 and 4). The amount of tributyl citrate, 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.

Conclusion

In order to assess the toxicological behaviour of tributyl citrate, the available experimental and predicted physico-chemical data have been evaluated. Tributyl citrate 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 360.45 g/mol and its LogPow of 3.5. In addition, the systemic toxicity via the skin is assumed to be low.