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EC number: 201-071-2 | CAS number: 77-94-1
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.
In order to assess the toxicological behaviour of tributyl citrate, the available experimental and predicted physico-chemical data have been evaluated. The substance is expected to be absorbed well (based on molecular weight of 360.442 g/mol, water solubility and LogPow value of 3.5 and experimentally derived data). The absorption of any metabolite of hydrolysis 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 3.5) - if absorbed - it is expected to be absorbed directly across the respiratory tract epithelium. Tributyl citrate 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 of 360.442 g/mol and its LogPow of 3.5. In addition, the systemic toxicity via the skin is assumed to be low. Concerning the distribution in the body, tributyl citrate is expected to be available in the circulatory system and to be distributed into the cells, since it is a more lipophilic substance. The experimentally determined or calculated LogPow values, the water solubility and predicted behaviour concerning absorption, the substance does not indicate a potential for accumulation. 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. 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.
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