Fatty acids, montan-wax, stearyl esters

Endpoint summary

Description of key information

Fatty acids, montan-wax, stearyl esters (CAS 68308-30-5) is expected to be absorbed mainly via the oral route. The target substance may 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. The hydrolysis products, such as the fatty acids 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. The excretion will mainly be as CO2 in expired air; with a smaller fraction excreted as conjugated molecules in 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

There are no toxicokinetic studies available for Fatty acids, montan-wax, stearyl esters (CAS 68308-30-5). In accordance with Annex VIII, Column 1, 8.8.1, of Regulation (EC) No. 1907/2006 and ‘Guidance on information requirements and chemical safety assessment Chapter R.7c: Endpoint specific guidance’ (ECHA, 2017), an assessment of the toxicokinetic behaviour of the substance Fatty acids, montan-wax, stearyl esters (CAS 68308-30-5) 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).

Fatty acids, montan-wax, stearyl esters (CAS 68308-30-5) is a solid at 20 °C with a molecular weight ranging from 270.49 to 987.74 g/mol and a water solubility of < 0.539 mg/L. The calculated log Pow value is > 10 and the calculated vapour pressure is < 0.0001 Pa at 20 °C. Fatty acids, montan-wax, stearyl esters (CAS 68308-30-5) is an UVCB substance with the main constituents octadecyl hexacosanoate (in the range of 10 – 20%), octadecyl tetracosanoate (in the range of 10 – 20%), octadecyl octacosanoate (in the range of 10 – 20%), octadecyl triacontanoate (in the range of 5 – 15%) and octadecyl docosanoate (in the range of 5 - 15%).

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 (Aungst and Shen, 1986; ECHA, 2017).

The high log Pow and low water solubility are in a range that indicate poor absorption from the GI tract following oral ingestion. However, micellar solubilisation may have an effect on the overall absorption rate of the substance.

The indications that the target substance Fatty acids, montan-wax, stearyl esters (CAS 68308-30-5) has low to moderate oral absorption and/or low acute toxicity are supported by the available acute oral toxicity data on the target substance and additionally, data on source substances (CAS 17671-27-1, CAS 22393-85-7 and CAS 93803-87-3) covering repeated oral dose toxicity.

A single dose of 2000 mg/kg bw of the target substance Fatty acids, montan-wax, stearyl esters (CAS 68308-30-5), caused no effects in rats (key study, 2016).

In a combined repeated dose toxicity and reproduction/developmental toxicity study performed using the source substance docosyl docosanoate (CAS 17671-27-1), no toxicologically relevant effects were noted up to and including the highest dose level of 1000 mg/kg bw/day (key, 2014). In two short-term repeated dose toxicity studies conducted with the source substances 2-octyldodecyl isooctadecanoate (CAS 93803-87-3) and tetradecyl octadec-9-enoate (CAS 22393-85-7) no toxicologically relevant effects were noted up to and including the highest dose level of 1000 mg/kg bw/day, respectively (supporting study, 1998; supporting study, 2014).

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, by enzymes released into the GI tract or by hydrolysis. These changes will alter the physico-chemical characteristics of the substance and hence 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, alkyl esters are hydrolysed in the GI 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. Branching reduces the ester hydrolysis rate, compared with linear esters (Mattson and Volpenhein, 1969, 1972; WHO, 1999).

Based on the generic information on hydrolysis of alkyl esters, the target substance Fatty acids, montan-wax, stearyl esters (CAS 68308-30-5) is expected to be enzymatically hydrolysed to the C18-alcohol moiety and the respective C26-, C24-, C28-, C30- and C22-fatty acid moieties.

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 physico-chemical properties and molecular weight of Fatty acids, montan-wax, stearyl esters (CAS 68308-30-5) suggest that limited oral absorption is likely to occur. The substance is anticipated to undergo enzymatic hydrolysis in the GI tract and therefore absorption of the ester hydrolysis products is relevant.

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; low to moderate if it is between 1-100 mg/L; and moderate to high if it is between 100-10000 mg/L. Log Pow values in the range of 1 to 4 are favourable for dermal absorption (values between 2 and 3 are optimal), in particular if the 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).

Fatty acids, montan-wax, stearyl esters (CAS 68308-30-5) has a molecular weight ranging from 270.49 to 987.74 g/mol and a water solubility of < 0.539 mg/L, therefore a low dermal absorption potential might be assumed (ECHA, 2017). Due to the log Pow is > 10 the uptake into the stratum corneum is predicted to be slow and the rate of transfer between the stratum corneum and the epidermis will be slow (ECHA, 2017).

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).

Damage to the skin surface may enhance penetration of the test substance (ECHA, 2017). However, available in vivo skin irritation studies with the appropriate analogue substances 2-octyldodecyl isooctadecanoate (CAS 93803-87-3), hexadecanoic acid, -isooctadecyl ester (CAS 72576-80-8) and Fatty acids, montan-wax, stearyl esters (CAS 68308-30-5) revealed neither skin corrosive nor skin irritant properties of the test substance. Moreover, a local lymph node assay performed with Fatty acids, montan-wax, stearyl esters (CAS 68308-30-5) did not show a potential for skin sensitisation.

Overall, taking into account the physico-chemical and toxicological properties of Fatty acids, montan-wax, stearyl esters (CAS 68308-30-5), the dermal absorption potential of the substance is anticipated to be low.

Inhalation

Inhalation of Fatty acids, montan-wax, stearyl esters (CAS 68308-30-5) is considered negligible as the substance has a very low vapour pressure < 0.0001 Pa at 20 °C and a very high boiling point > 300 °C, thus being of low volatility. Moreover, granulometric analysis showed particle size distribution of D10: 206.7 µm, D50: 346.9 µm and D90: 517.0 µm. Most of the inhaled particles will therefore remain in the upper airways and be coughed out or swallowed, with few or no particles reaching the pulmonary alveoli. Therefore, inhalation of the pure substance is not considered to be a significant route of exposure. However, to the minor extent if absorption via inhalation occurs, it is assumed to be as high as via the oral route (micellar solubilisation) in a worst case approach.

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. Small water-soluble molecules and ions will diffuse through aqueous channels and pores. 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, Fatty acids, montan-wax, stearyl esters (CAS 68308-30-5) may undergo enzymatic hydrolysis in the GI 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. This is also relevant to the C18-fatty acid hydrolysis product of the target substance. This route of absorption and metabolism of a fatty acid was shown in an in vivo study performed by Sieber (1974). Twenty-four hours after intraduodenal administration of a single dose of [1-14C]-radiolabelled octadecanoic acid to rats, 52.5 ± 26% of the radiolabelled carbon was recovered in the lymph. A large majority (68 - 80%) of the recovered radioactive label was incorporated in triglycerides, 13 - 24% in phospholipids and 0.7 - 1% in cholesterol esters. No octadecanoic acid was recovered. Almost all the radioactivity recovered in the lymph was localized in the chylomicron fraction. 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.

Absorbed alcohols are mainly oxidised to the corresponding fatty acid and then follow the same metabolism route as described above for fatty acids, to form triglycerides. The absorption and metabolism of a fatty alcohol was assessed in an in vivo study performed by Sieber (1974). Twenty-four hours after intraduodenal administration of a single dose of [1-14C]-radiolabelled octadecanol to rats, 56.6 ± 14% of the radiolabelled carbon was recovered in the lymph. More than half (52-73%) of the recovered radioactive label was incorporated in triglycerides, 6-13% in phospholipids, 2-3% in cholesterol esters and 4-10% in unmetabolised octadecanol. Almost all the radioactivity recovered in the lymph was localized in the chylomicron fraction. The conversion into the corresponding fatty acids by oxidation and distribution in the form of triglycerides, means that the metabolites of fatty alcohols are also used as an energy source or stored in adipose tissue. The C18-alcohol hydrolysis product of the target substance is likewise assumed to be oxidised to the corresponding fatty acid.

Metabolism

The metabolism of Fatty acids, montan-wax, stearyl esters (CAS 68308-30-5) initially occurs via enzymatic hydrolysis of the ester resulting in the corresponding C18-alcohol moiety and the respective C26-, C24-, C28-, C30- and C22-fatty acid moieties. 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 as such enter the systemic circulation directly before entering the liver where hydrolysis will generally take place.

The C18-alcohol that is the hydrolysis product of the target substance, as well as the fatty alcohols of the source substances, 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. 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). The fatty acid can also be conjugated (by e.g. glucuronides, sulfates) to more polar products that are excreted in the urine.

Given die available genetic toxicity as well as skin sensitization data, there is no indication that Fatty acids, montan-wax, stearyl esters (CAS 68308-30-5) is metabolised to reactive species which are interacting with DNA or proteins.

Excretion

The linear C18-fatty acid derived from the oxidation of the corresponding alcohol as well as the respective C26-, C24-, C28-, C30- and C22-fatty acids 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. Experimental data with ethyl oleate (CAS 111-62-6, ethyl ester of oleic acid) support this principle. The absorption, distribution, and excretion of ¹⁴C-labelled ethyl oleate was studied in Sprague Dawley rats after a single, oral dose of 1.7 or 3.4 g/kg bw (Bookstaff et al., 2003). At sacrifice (72 hours post-dose), mesenteric fat was the tissue with the highest concentration of radioactivity. The other organs and tissues had very low concentrations of test material-derived radioactivity. The main route of excretion of radioactivity in the groups was via the expired air as CO₂. 12 hours after dosing, 40-70% of the administered dose was excreted in expired air (consistent with beta-oxidation of fatty acids). 7-20% of the radioactivity was eliminated via the faeces, and approximately 2% via the urine.

The alcohol component may be conjugated with e.g. glutathione to form a more water-soluble molecule and excreted via the urine, bypassing further metabolism steps (WHO, 1999). The fraction of Fatty acids, montan-wax, stearyl esters (CAS 68308-30-5) that is not absorbed in the GI tract will be excreted via the faeces.

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