Glycerol trioctanoate

Endpoint summary

Description of key information

Glycerol trioctanoate (CAS 538-23-8) is expected to be readily absorbed via the oral route after enzymatic hydrolysis in the gastrointestinal tract into glycerol and octanoic acid, as hydrolysis facilitates the absorption. The fatty acid will most likely be re-esterified to triglycerides after absorption and transported via chylomicrons; glycerol can be metabolised to dihydroxyacetone phosphate and glyceraldehyde-3-phosphate, which can then be incorporated in the standard metabolic pathways of glycolysis and gluconeogenesis. Absorption via the dermal and inhalation route of exposure is expected to be negligible. The excretion will mainly occur as carbon dioxide in expired air. Non-metabolised glycerol is a polar molecule and can readily be excreted via the urine. No bioaccumulation is expected to take place, as excess triglycerides are stored and used as the energy need rises.

Key value for chemical safety assessment

Bioaccumulation potential:

no bioaccumulation potential

Additional information

Basic toxicokinetics

In accordance with Annex VIII, Column 1, Item 8.8.1, of Regulation (EC) 1907/2006 and the Guidance on information requirements and chemical safety assessment Chapter R.7c: Endpoint specific guidance (ECHA, 2017), an assessment of the toxicokinetic behaviour of the target substance glycerol trioctanoate (CAS 538-23-8) was conducted to the extent that can be derived from the relevant available information. This comprises a qualitative assessment of the available substance specific data on physico-chemical and toxicological properties according to the Chapter R.7c Guidance document (ECHA, 2017) and taking into account available information from source substances. One study was available in which the toxicokinetic behaviour of glycerol trioctanoate (CAS 538-23-8) was investigated in new-born monkeys after single oral administration at 8.1 mg/kg bw via nasogastric tube.

Glycerol trioctanoate (CAS 538-23-8) is a mono-constituent substance with a glycerol backbone and three linear C8 acid moieties (triglyceride of octanoic acid esters). The substance has a molecular weight of 470.70 g/mol. It is a liquid at 20°C with a low water solubility of <0.532 µg/L. Freezing (i.e. solid-liquid transition) occurs at 5.8°C. The log Pow was estimated to be 9.2 at 20°C and the vapour pressure was found to be 9.3E-7 Pa at 25°C.

Absorption

Absorption is a function of the potential for a substance to diffuse across biological membranes. The most useful parameters providing information on this potential are the molecular weight, the octanol/water partition coefficient (log Pow) value and the water solubility. The log Pow value provides information on the relative solubility of the substance in water and lipids (ECHA, 2017).

Oral

In general, molecular weights below 500 and log Pow values between -1 and 4 are favourable for absorption via the gastrointestinal (GI-) tract, provided that the substance is sufficiently water soluble (> 1 mg/L). Lipophilic compounds may be taken up by micellar solubilisation by bile salts, but this mechanism may be of particular importance for highly lipophilic compounds (log Pow > 4), in particular for those that are poorly soluble in water (≤ 1 mg/L) as these would otherwise be poorly absorbed. Solids must be dissolved before absorption; the degree depends on the water solubility (Aungst and Shen, 1986; ECHA, 2017).

The molecular weight <500 g/moL favours uptake, while the low water solubility and high log Pow of 9.2 are in a range that indicate poor absorption from the GI-tract following oral ingestion. Micellar solubilisation may have an effect on the overall absorption rate of the substance.

The potential of a substance to be absorbed in the GI-tract may be influenced by several parameters, like: chemical changes taking place in GI-fluids as a result of metabolism by GI-flora and by enzymes released into the GI-tract. These changes will alter the physico-chemical characteristics of the substance and therefore predictions based on the physico-chemical characteristics of the parent substance may in some cases no longer apply or should be adjusted (ECHA, 2017).

In general, mono-, di- and triglycerides (e.g. from dietary fat) undergo hydrolysis by lipases (a class of ubiquitous carboxylesterases) prior to absorption (Lehninger et al., 1998). There is sufficient evidence to assume that mono-, di- and triglycerides in general will likewise undergo enzymatic hydrolysis in the gastrointestinal tract as the first step in their absorption, distribution, metabolism and excretion (ADME) pathways.

In the gastrointestinal tract, gastric and intestinal (pancreatic) lipase activities are the most important. Triglycerides are hydrolysed by gastric and pancreatic lipases with high specificity for the sn1- and sn3-positions. For the remaining monoester at the sn2-position (2-monoacylglycerol), there is evidence that it can either be absorbed as such by the intestinal mucosa or isomerize to 1-monoacylglycerol, which can then be hydrolysed. (Cohen, 1971; Greenberger, 1966; IOM, 2005; Mattson and Volpenhein, 1964, 1968). The speed of hydrolysis of esters of primary n-alcohols containing from 1 to 18 carbon atoms with fatty acids containing from 2 to 18 carbon atoms was found to depend on both, the chain length of either the alcohol or acid. With respect to fatty acid moiety, esters of C12 and C4 were hydrolysed at the most rapid rate. With respect to alcohol moiety C7 was hydrolysed most rapidly (Mattson and Volpenhein, 1969).

Following hydrolysis, the resulting products (free glycerol, free fatty acids and (in the case of di- and triglycerides) 2-monoacylglycerols) are absorbed by the intestinal mucosa. Within the epithelial cells, triglycerides will be reassembled, primarily by re-esterification of absorbed 2-monoacylglycerols. The free glycerol is readily absorbed and little of it is re-esterified. The absorption of short-chain fatty acids can begin already in the stomach. This is because, in general, for intestinal absorption short-chain or unsaturated fatty acids are more readily absorbed than long-chain, saturated fatty acids. The absorption rate of saturated long-chain fatty acids is increased if they are esterified at the sn2-position of glycerol (Greenberger et al., 1966; IOM, 2005; Mattson and Volpenhein, 1962, 1964).

Low toxicity after oral absorption can be anticipated from the available experimental data. A single dose of ≥ 29600 mg/kg bw of the target substance glycerol trioctanoate caused no adverse toxic effects on rats and mice (WoE, 1970). In a two-year oral carcinogenicity study no toxicologically relevant effects were noted up to a dose level of 2390 mg/kg bw/day (key study, 1994). In patients with a diagnosis of probable Alzheimer's disease glycerol trioctanoate administered orally daily at 20 g per patient (equivalent to 333 mg/kg bw/day for a body weight of 60 kg) over a period of 6 to 12 months, no adverse effects on the health status was seen.

Glycerol trioctanoate is predicted to undergo enzymatic hydrolysis in the gastrointestinal tract and absorption of the ester hydrolysis products, glycerol and octanoic acid, rather than the parent substance. The absorption rate of the hydrolysis products is expected to be high. In conclusion, the available information indicates a high oral absorption rate of the hydrolysis products of glycerol trioctanoate.

Dermal

The dermal uptake of liquids and substances in solution is higher than that of dry particulates, since dry particulates need to dissolve into the surface moisture of the skin before uptake can begin. Molecular weights below 100 g/mol favour dermal uptake, while for those above 500 g/mol the molecule may be too large. Dermal uptake is anticipated to be low if the water solubility is < 1 mg/L; Log Pow values in the range of 1 to 4 (values between 2 and 3 are optimal) are favourable for dermal absorption, in particular if water solubility is high. For substances with a log Pow 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. Log Pow values above 6 reduce the uptake into the stratum corneum and decrease the rate of transfer from the stratum corneum to the epidermis, thus limiting dermal absorption (ECHA, 2017).

The molecular weight of glycerol trioctanoate of 470.70 g/mol favours dermal absorption. However, other physico-chemical properties (low water solubility, high log Pow) indicate a limited dermal absorption, as the uptake into the stratum corneum is predicted to be slow and the rate of transfer between the stratum corneum and the epidermis is considered to be slow as well (ECHA, 2017).

The dermal permeability coefficient (Kp) can be calculated from log Pow and molecular weight (MW) applying the following equation described in US EPA (2012), using the Epi Suite software:

log(Kp) = -2.80 + 0.66 log Pow – 0.0056 MW

Using the log Pow of 9.2 and water solubility on 0.532 µg/L as data entered into DermWin, the Kp for glycerol trioctanoate (CAS 538-23-8) was calculated to be 4.65 cm/h (US EPA, 2012). The dermal flux was estimated to be 4.67e-005 mg/cm²/h, indicating a very low dermal absorption potential.

An acute dermal toxicity study was performed with the source substance propane-1,2,3-triyl triheptanoate (CAS 620-67-7), in which rats were exposed to 2000 mg/kg bw for 24 hours under semi-occlusive conditions (key study, 1993). No mortality occurred and no clinical signs of toxicity were observed. In a second acute dermal toxicity study performed with the source substance glycerides, C8-18 and C18-unsatd. mono- and di-, acetates (CAS 91031-13-0) rats were exposed to 2000 mg/kg bw for 24 hours under occlusive conditions (supporting study, 2010). No mortality occurred. No toxicologically relevant clinical signs of toxicity were recorded. Based on the results from the source substances, the target substance is not expected to be acutely toxic via the dermal route.

If a substance shows skin irritating or corrosive properties, damage to the skin surface may enhance penetration. If the substance has been identified as a skin sensitizer then some uptake must have occurred although it may only have been a small fraction of the applied dose (ECHA, 2017).

The available skin irritation data on the source substances propane-1,2,3-triyl triheptanoate (CAS 620-67-7), glycerides, mixed decanoyl and octanoyl (CAS 73398-61-5), and dodecanoic acid, ester with 1,2,3-propanetrial, acetylated (CAS 97593-30-1) showed no or limited, reversible skin irritating effects in the rabbit not leading to classification as skin irritant according to CLP/EU GHS criteria (WoE, 1993, 1988, and 2008). In the acute dermal toxicity study with the source substance propane-1,2,3-triyl triheptanoate (CAS 620-67-7) no skin irritation was observed and in the acute dermal toxicity study with the source substance glycerides, C8-18 and C18-unsatd. mono- and di-, acetates (CAS 91031-13-0) scales and scabs of the right flank and/or treated skin were observed in 3/5 males and 1/5 female between Days 4 and 15 (last day of observation).

No erythema or oedema was seen during the preliminary testing for an in vivo skin sensitisation (Buehler) study in guinea pigs with the source substance propane-1,2,3-triyl triheptanoate (CAS 620-67-7) after 6 hours of exposure to the undiluted test substance under occlusive conditions.

Overall, based on the available information and using a worst-case approach, the dermal absorption potential of glycerol trioctanoate is predicted to be low.

Inhalation

Glycerol trioctanoate is a liquid with a very low vapour pressure (9.3E-7 Pa at 25°C), and therefore of low volatility. Under normal use and handling conditions, inhalation exposure and availability for respiratory absorption of the substance in the form of vapours, gases or liquid aerosols (both liquid substances and solid substances in solution) is considered to be limited (ECHA, 2017).

No toxicity was found in an acute inhalation toxicity study conducted with the source substance triglycerides, mixed decanoyl and octanoyl (CAS 73398 -61-5) in male rats (key study, 1976). The inhalation LC50 value for male rats was found to be > 1.86 mg/L (aerosol, maximum attainable concentration of respirable particles).

Based on the available data, the acute toxicity potential via the inhalation route of exposure is considered to be negligible.

Distribution and accumulation

Distribution of a compound within the body depends on the physico-chemical properties of the substance; especially the molecular weight, the lipophilic character and the water solubility. In general, the smaller the molecule, the wider is the distribution. If the molecule is lipophilic, it is likely to distribute into cells and the intracellular concentration may be higher than extracellular concentration, particularly in fatty tissues (ECHA, 2017).

As discussed under oral absorption, mono-, di- and triesters of glycerol undergo enzymatic hydrolysis in the gastrointestinal tract prior to absorption. Therefore, an assessment of distribution and accumulation of the hydrolysis products is considered more relevant.

Absorbed glycerol is readily distributed throughout the body and it can be re-esterified to form endogenous triglycerides, be metabolised and enter physiological pathways, like the glycolysis pathway (Lehninger, 1998). After being absorbed, fatty acids are (re-)esterified along with other fatty acids into triglycerides and released in chylomicrons into the lymphatic system. Fatty acids of carbon chain length ≤ 12, like the C8-fatty acid hydrolysis product, may be transported directly to the liver via the portal vein as the free acid bound to albumin, instead of being re-esterified. Chylomicrons are transported in the lymph to the thoracic duct and subsequently to the venous system. On contact with the capillaries, enzymatic hydrolysis of chylomicron triacylglycerol fatty acids by lipoprotein lipase takes place. Most of the resulting fatty acids are taken up by adipose tissue and re-esterified into triglycerides for storage. Triacylglycerol fatty acids are likewise taken up by muscle and oxidized to derive energy or they are released into the systemic circulation and returned to the liver, where they are metabolised, stored or re-enter the circulation (IOM, 2005; Johnson, 1990; Johnson, 2001; Lehninger, 1998; NTP, 1994; Stryer, 1996; WHO, 2001).

There is a continuous turnover of stored fatty acids, as they are constantly metabolised to generate energy and then excreted as CO₂. Accumulation of fatty acids takes place only if their intake exceeds the caloric requirements of the organism.

Metabolism

Glycerol can be metabolised to dihydroxyacetone phosphate and glyceraldehyde-3-phosphate, which can then be incorporated in the standard metabolic pathways of glycolysis and gluconeogenesis.

The fatty acids can be further metabolised directly following absorption, following oxidation from an alcohol or following de-esterification of triglycerides. A major metabolic pathway for linear and branched fatty acids is the beta-oxidation for energy generation. In this multi-step process, the fatty acids are at first esterified into acyl-CoA derivatives and subsequently transported into cells and mitochondria by specific transport systems. In the next step, the acyl-CoA derivatives are broken down into acetyl-CoA molecules by sequential removal of 2-carbon units from the aliphatic acyl-CoA molecule. Further oxidation via the citric acid cycle leads to the formation of H₂O and CO₂. The complete oxidation of unsaturated fatty acids such as oleic acid requires an additional isomerisation step (Lehninger, 1993). Branched-chain acids can be metabolised via the same beta-oxidation pathway as linear, depending on the steric position of the branch, but at lower rates (WHO, 1999). The alpha-oxidation pathway is a major metabolic pathway for branched-chain fatty acids where a methyl substituent at the beta-position blocks certain steps in the beta-oxidation (Mukherji, 2003). Generally, a single carbon unit is cleaved off the branched acid in an additional step before the removal of 2-carbon units continues. Alternative pathways for long-chain fatty acids include the omega-oxidation at high dose levels (WHO, 1999). Acetate, resulting from hydrolysis of acetylated glycerides, is readily absorbed and will enter into the physiological pathways of the body and can be utilized in oxidative metabolism or in anabolic syntheses (CIR, 1983, 1987; IOM, 2005; Lehninger, 1998; Lippel, 1973; Stryer, 1996; WHO, 1967, 1974, 1975, 2001). The fatty acid can also be conjugated (by e.g. glucuronides, sulfates) to more polar products that are excreted in the urine.

The metabolites following enzymatic metabolism of the substance were assessed using the QSAR OECD toolbox (OECD, 2017). The QSAR tool predicts which metabolites may result from enzymatic activity in vivo (rat), in (rat) liver and in the skin, and by intestinal bacteria in the GI-tract. Three skin metabolites were predicted: octanoic acid (C8:0), 3-hydroxypropane-1,2-diyl dioctanoate, and 2-hydroxypropane-1,3-diyl dioctanoate. Sixteen hepatic metabolites (rat liver S9 metabolite simulator) and fifty-three (53) metabolites were predicted to result from all kinds of microbiological metabolism. The results of the OECD Toolbox simulation support the information retrieved in the literature on metabolism.

In a study with 5 new-born monkeys (Macaca mulatta) glycerol trioctanoate was administered as single nasogastric dose at 8.1 mg/kg bw (Tetrick, 2010). The metabolites octanoic acid and 3-hydroxybutyrate were detectable at higher concentrations in blood sampled 1 and 3 hours after administration than in blood sampled prior to administration. Due to the observed metabolism in monkeys, no bioaccumulation potential is expected for glycerol trioctanoate.

There is no indication that Glycerol trioctanoate is metabolized to reactive intermediates. The experimental studies performed on genotoxicity (Ames test, gene mutation in mammalian cells in vitro, chromosome aberration assay in mammalian cells in vitro, genotoxicity in vivo) using the target and source substances were negative, except for one Salmonella typhimurium strain when exposed to very high concentrations of the target substance in the presence of metabolic activation (Ames, 1989). The result of the skin sensitisation studies performed with source substance was likewise negative (WoE studies 1993 and 2008, QSAR 2017).

Excretion

Any non-absorbed fraction of glycerol trioctanoate that is not hydrolysed in the gastrointestinal tract will be excreted via the faeces.

In general, the hydrolysis products glycerol and fatty acids are catabolised entirely by oxidative physiologic pathways, ultimately leading to the formation of carbon dioxide and water. Non-metabolised glycerol is a polar molecule and can readily be excreted via the urine. Small amounts of ketone bodies resulting from the oxidation of fatty acids may be excreted via the urine, however, the major part of the fatty acids will enter an oxidative pathway as described above under ‘Metabolism’ (Lehninger, 1998; IOM, 2005; Stryer, 1996).

References

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