Fatty acids, C14-22, C16-24-alkyl esters

Endpoint summary

Description of key information

The target substance Fatty acids, C14-22, C16-24-alkyl esters (CAS 92797-30-3) is expected to be readily absorbed via the oral and inhalation route, and partly absorbed via the dermal route. The ester will be hydrolysed in the gastrointestinal tract and mucus membranes to the respective fatty acid and fatty alcohol, which facilitates the absorption. The absorbed ester will be hydrolysed mainly in the liver. The fatty acid will most likely be re-esterified to triglycerides after absorption and transported via chylomicrons; while the absorbed alcohol is mainly oxidised to the corresponding fatty acid and then to a triglyceride, as described above. The major metabolic pathway for linear and branched fatty acids is the beta-oxidation pathway for energy generation, while alternatives are the omega-pathway or direct conjugation to more polar products. The excretion will mainly be as CO2 in expired air; with a smaller fraction excreted as conjugated molecules in the urine. No bioaccumulation will 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 with 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 Fatty acids, C14-22, C16-24-alkyl esters (CAS 92797-30-3) 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 physicochemical and toxicological properties according to the Chapter R.7c Guidance document (ECHA, 2017) and taking into account further available information from source substances. There are no studies available in which the toxicokinetic behaviour of Fatty acids, C14-22, C16-24-alkyl esters was investigated.

Fatty acids, C14-22, C16-24-alkyl esters is a UVCB with the following main constituents:esters of C22 acid and C16 alcohol, esters of C20 acid and C18 alcohol and esters of C22 acid and C18 alcohol.

Fatty acids, C14-22, C16-24-alkyl esters has a molecular weight of 565.01 – 593.06 g/mol. It is a solid at 20 °C, with a water solubility of < 2.5 µg/L at 20 °C. The log Pow was estimated to be > 10 and the vapour pressure was calculated to be < 0.0001 Pa at 20 °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. 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 (Aungst and Chen, 1986; ECHA, 2017).

The physicochemical characteristics (molecular weight, log Pow and water solubility) of the substance are in a range that indicate poor absorption from the gastrointestinal tract following oral ingestion. Although it is unclear how much micellar solubilisation will increase the absorption rate of the substance, it is likely to affect the absorption.

The indications that the target substance Fatty acids, C14-22, C16-24-alkyl esters has low-medium oral absorption and/or low acute toxicity are supported by the available data on acute oral toxicity from suitable source substances. When rats were administered a single dose of 20 mL/kg bw (corresponding to 17400 mg/kg bw) of source substance Fatty acids, C16 and C18-22-unsatd., C16-18 and C18-unsatd. alkyl esters (CAS 90990-29-7) via gavage, there was no mortality, no effects on body weight, and no lesions were noted at necropsy (Key, 1982). Similarly, the administration of 2000 mg/kg bw of source substance Hexadecanoic acid, -isooctadecyl ester (CAS 72576-80-8) to rats via gavage did not cause mortality and no clinical signs or body weight changes were noted (1999). In a combined repeated dose toxicity and reproduction/developmental toxicity study performed using the source substance (Z)-octadec-9-enyl (Z)-docos-13-enoate (CAS 17673-56-2), no toxicologically relevant effects were noted up to and including the highest dose level of 1000 mg/kg bw/day (Key, 2014). This indicates that Fatty acids, C14-22, C16-24-alkyl esters also has a low potential for oral toxicity.

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

In general, alkyl esters are readily hydrolysed in the gastrointestinal tract, blood and liver to the corresponding alcohol and fatty acid by the ubiquitous carboxylesterases. There are indications that the hydrolysis rate in the intestine catalysed by pancreatic lipase is lower for alkyl esters than for triglycerides, which are the natural substrates of this enzyme. The hydrolysis rate of linear esters increases with increasing chain length of either the alcohol or acid (Mattson and Volpenhein, 1969, 1972; WHO, 1999).

Free fatty acids and alcohols are readily absorbed by the intestinal mucosa. Within the epithelial cells, fatty acids are (re-)esterified with glycerol to triglycerides. In general, short-chain or unsaturated fatty acids are more readily absorbed than long-chain, saturated fatty acids. As for fatty acids, the rate of absorption of alcohols is likely to decrease with increasing chain length (Greenberger et al., 1966; IOM, 2005; Mattson and Volpenhein, 1962, 1964; OECD, 2006; Sieber, 1974).

In conclusion, the physicochemical properties and molecular weight of Fatty acids, C14-22, C16-24-alkyl esters suggest some oral absorption may occur. However, the substance is anticipated to undergo enzymatic hydrolysis in the GI-tract and therefore absorption of the ester hydrolysis products is also relevant. The absorption rate of the hydrolysis products is expected to be high.

Dermal

The dermal absorption 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 absorption 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; low to moderate if it is between 1-100 mg/L; and moderate to high if it is between 100-10000 mg/L. Dermal uptake of substances with a water solubility > 10000 mg/L (and log Pow < 0) will be low, as the substance may be too hydrophilic to cross the stratum corneum. 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 substance Fatty acids, C14-22, C16-24-alkyl esters is insoluble in water, indicating a low dermal absorption potential (ECHA, 2017). The molecular weight of 565.01 – 593.06 g/mol is in the lowest end of the range indicating a low potential for dermal absorption. The log Pow is > 10, which means that the uptake into the stratum corneum is limited and the rate of transfer between the stratum corneum and the epidermis will be slow (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 (2004):

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

The Kp calculated for Fatty acids, C14-22, C16-24-alkyl esters was 3.01- 4.33 cm/h. Considering the water solubility (<2.5 µg/L), the dermal flux is estimated to be 0.009 – 0.013 µg/cm²/h for the main constituents, indicating a very low to low dermal absorption potential.

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 data on three source substances provide no indications for skin irritating effects in the rabbit (Key, 1998; 1982 and 1999). No skin effects were noted in the acute dermal toxicity study at the limit dose of 2000 mg/kg bw, performed with two source substances (Key, 2010; 1998). The results of the skin sensitisation tests performed in guinea pigs with two source substances were both negative (WoE, 1998 and WoE, 1995). Therefore, no penetration of the target substance due to skin damage is expected.

Overall, based on the available information, the dermal absorption potential of Fatty acids, C14-22, C16-24-alkyl esters is predicted to be low.

 

Inhalation

In humans, particles with aerodynamic diameters below 100 μm have the potential to be inhaled. Particles with aerodynamic diameters below 50 μm may reach the thoracic region and those below 15 μm the alveolar region of the respiratory tract (ECHA 2017). Fatty acids, C14-22, C16-24-alkyl esters is a solid with particle size distribution values of 0.5% (< 100 µm), 7.3% (< 200 µm), 22.7% (< 300 µm), 36.4% (< 400 µm), 23% (< 500 µm) and 10.1% (> 500 µm). Since approx. 82% of the target substance particels have a diamater > 200 - 500 µm and only 0.5% have a diameter < 100 µm, the potential to be inhaled is very low.

The absorption of very large particles, several hundreds of micrometers in diameter, that were administered dry (e.g. in the diet) or in a suspension may be reduced because of the time taken for the particle to dissolve. For poorly water-soluble dusts, the rate at which the particles dissolve into the mucus will limit the amount that can be absorbed directly. Poorly water-soluble dusts depositing in the nasopharyngeal region could be coughed or sneezed out of the body or swallowed (Schlesinger, 1995). Such dusts depositing in the tracheo-bronchial region would mainly be cleared from the lungs by the mucocilliary mechanism and swallowed. However, a small amount may be taken up by phagocytosis and transported to the blood via the lymphatic system. Poorly water-soluble dusts depositing in the alveolar region would mainly be engulfed by alveolar macrophages. The macrophages will then either translocate particles to the ciliated airways or carry particles into the pulmonary interstitium and lymphoid tissues.

An acute inhalation toxicity study was performed with the liquid source substance 2-ethylhexyl oleate (CAS 26399-02-0), in which rats were exposed nose-only to 5.7 mg/L of an aerosol for 4 hours (Key, 2010). No mortality occurred and no toxicologically relevant effects were observed. Thus, the source substance is not acutely toxic by the inhalation route, but no firm conclusion can be drawn on respiratory absorption of target substance, since it is a solid and not liquid.

Based on physicochemical properties of the target substance, absorption via inhalation is assumed to be low but cannot be excluded.

Distribution and Accumulation

Distribution of a compound within the body depends on the physicochemical 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, Fatty acids, C14-22, C16-24-alkyl esters will mainly undergo enzymatic hydrolysis in the gastrointestinal tract prior to absorption. The fraction of ester absorbed unchanged will undergo enzymatic hydrolysis by ubiquitous esterases, primarily in the liver (Fukami and Yokoi, 2012). The distribution and accumulation of the hydrolysis products is considered the most relevant.

After being absorbed, fatty acids are (re-)esterified along with other fatty acids into triglycerides and released in chylomicrons into the lymphatic system. 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 also 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, 1993; 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

The metabolism of Fatty acids, C14-22, C16-24-alkyl esters initially occurs via enzymatic hydrolysis of the ester resulting in the corresponding fatty acid and fatty alcohol. The esterases catalysing the reaction are present in most tissues and organs, with particularly high concentrations in the GI-tract and in the liver (Fukami and Yokoi, 2012). Depending on the route of exposure, esterase-catalysed hydrolysis takes place at different places in the body. After oral ingestion, esters of alcohols and fatty acids can undergo enzymatic hydrolysis in the GI-tract. In contrast, substances which are absorbed through the pulmonary alveolar membrane or through the skin may enter the systemic circulation directly before entering the liver where hydrolysis will generally take place.

The fatty alcohol will mainly be metabolised to the corresponding carboxylic acid via the aldehyde as a transient intermediate (Lehninger, 1993). The stepwise process starts with the oxidation of the alcohol by alcohol dehydrogenase to the corresponding aldehyde, where the rate of oxidation increases with increased chain-length. Subsequently, the aldehyde is oxidised to carboxylic acid, catalysed by aldehyde dehydrogenase. Both the alcohol and the aldehyde may also be conjugated with e.g. glutathione and excreted directly, bypassing additional metabolism steps (WHO, 1999).

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 (Lehninger, 1993).

The potential metabolites following enzymatic metabolism of the substance were predicted using the QSAR OECD toolbox v4.1 (OECD, 2017). This QSAR tool predicts which metabolites may result from enzymatic activity in the liver and in the skin, and by intestinal bacteria in the gastrointestinal tract. For the three main constituents of Fatty acids, C14-22, C16-24-alkyl esters 11 - 12 hepatic metabolites and 2 dermal metabolites were predicted. Primarily, the ester bond is broken both in the liver and in the skin and the hydrolysis products may be further metabolised. The resulting liver and skin metabolites are the product of alpha-, beta- or omega-oxidation (= addition of hydroxyl group). In the case of omega-oxidation, it is followed by further oxidation to the aldehyde, which is then oxidised to the corresponding carboxylic acid. In general, the hydroxyl groups make the substances more water-soluble and susceptible to metabolism by phase II-enzymes. The metabolites formed in the skin are expected to enter the blood circulation and have the same fate as the hepatic metabolites. Up to 122 metabolites were predicted to result from all kinds of microbiological metabolism in the GI-tract, including hydrolysis of the ester bond, aldehyde formation and fatty acid chain degradation of the molecule. The results of the OECD Toolbox simulation support the information retrieved in the literature on metabolism.

The experimental studies performed on genotoxicity (Ames test, gene mutation in mammalian cells in vitro, chromosome aberration assay in mammalian cells in vitro) using source substances were negative, with and without metabolic activation (WoE, 2007; WoE, 1998; WoE, 1994; WoE, 1998). The results of the skin sensitisation studies performed with a source substance were likewise negative (WoE, 1998 and WoE, 1995).

Excretion

The fatty acid resulting from hydrolysis of the ester will be metabolised for energy generation or stored as lipid in adipose tissue or used for further physiological functions, like incorporation into cell membranes (Lehninger, 1993). Therefore, the fatty acid metabolites are not expected to be excreted to a significant degree via the urine or faeces but to be excreted via exhaled air as CO2 or stored as described above. The branched alcohol component may be oxidised to the corresponding acid as described above. The fraction of Fatty acids, C14-22, C16-24-alkyl esters that is not absorbed in the GI-tract will be excreted via the faeces.

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