Docosyl stearate

Endpoint summary

Description of key information

Docosyl stearate (CAS 22413-03-2) is expected to be readily absorbed via the oral 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 fraction of ester that is absorbed will be hydrolysed mainly in the liver. 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 major metabolic pathway for long chain linear 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 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

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 docosyl stearate (CAS 22413-03-2) 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 further available information from source substances. There are no studies available in which the toxicokinetic behaviour of docosyl stearate (CAS 22413-03-2) was investigated.

Docosyl stearate (CAS 22413-03-2) is a mono-constituent substance with a linear C22-alcohol moiety and linear C18 acid moiety. The substance has a molecular weight of 593.06 g/mol. It is a solid at 20 °C with melting point of 66.4°C, and a water solubility of < 0.504 mg/L at 20 °C (pH 6.0). The log Pow was estimated to be >10 and the vapour pressure was estimated to be 5.83E-17 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, 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 of the substance, the log Pow and water solubility is 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 indications that the target substance docosyl stearate (CAS 22413-03-2) has low-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 3687-46-5 and CAS 93803-87-3) covering repeated oral dose toxicity.

A single dose of 2000 mg/kg bw of the target substance docosyl stearate (CAS 22413-03-2), caused no adverse toxic effects on rats (Gotemba Laboratory, 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 (Harlan, 2014). In two short-term repeated dose toxicity studies conducted with the source substance 2-octyldodecyl isooctadecanoate (CAS 93803-87-3) and decyl oleate (CAS 3687-46-5) no toxicologically relevant effects were noted up to and including the highest dose level of 1000 mg/kg bw/day, respectively (Notox, 1998; Henkel, 1987).

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 readily 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 docosyl stearate (CAS 22413-03-2) is expected to be enzymatically hydrolysed to the C22-alcohol moiety and C18-acid moiety.

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 docosyl stearate (CAS 22413-03-2) 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 also relevant. Therefore the absorption rate of the hydrolysis products is expected to be high.

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 weightof 593.06 g/mol and the physico-chemical properties (low water solubility, high log Pow, solid form) 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

The Kp for docosyl stearate (CAS 22413-03-2) was calculated to be 3.25 cm/h (DermWin, 2012). The dermal flux was estimated to be 68.2 mg/cm²/h, indicating a medium-high dermal absorption potential.

An acute dermal toxicity study was performed with the source substance decyl oleate (CAS 3687-46-5), in which rats were exposed to 2000 mg/kg bw for 24 hours under occlusive conditions (Notox, 2010). No mortality occurred and no toxicologically relevant systemic effects were observed. Erythema (score 1) was noted during Day 3-7 in 4/10 animals, while scales or scabs were observed in 8/10 animals for up to 9 days during Day 7-15. No mortality occurred and no toxicologically relevant systemic effects were observed. An acute dermal toxicity study was performed with the source substance 2-octyldodecyl isooctadecanoate (CAS 93803-87-3) rats were exposed to 2000 mg/kg bw for 24 hours under occlusive conditions (Notox, 1998). No treatment related clinical signs were observed. 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 target substance docosyl stearate (CAS 22413-03-2) revealed no skin irritating effects in the rabbit (Kannami Laboratory, 2016). In addition, the source substance 2-octyldodecyl isooctadecanoate (CAS 93803-87-3) was assessed in an in vivo skin irritation study, which revealed no skin irritation in rabbits.

With regard to the skin sensitising potential, the available local lymph node assays with the target substance docosyl stearate (CAS 22413-03-2) and the source substance decyl oleate (CAS 3687-46-5) did not indicate a skin sensitising potential of both compounds, respectively.

Overall, based on the available information and using a worst-case approach, the dermal absorption potential of docosyl stearate (CAS 22413-03-2) is predicted to be moderate.

Inhalation

Docosyl stearate (CAS 22413-03-2) is a solid with a very low vapour pressure (5.83E-17 Pa at 20 °C), which indicates a low inhalation potential. Based on the granulometry analysis (D10: 100.1 µm; D50: 227.1 µm and D90: 399.3 µm), inhalation as dust is not very likely, since the particles do not have the potential to be inspired. 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 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. 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, docosyl stearate (CAS 22413-03-2) 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 for 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 C22-alcohol hydrolysis product of the target substance is likewise assumed to be oxidised to the corresponding fatty acid.

Metabolism

The metabolism of docosyl stearate (CAS 22413-03-2) initially occurs via enzymatic hydrolysis of the ester resulting in the corresponding linear C18 fatty acid and the linear C22 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 C22-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 metabolic 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.

The potential metabolites following enzymatic metabolism of the substance were predicted using the QSAR OECD toolbox (OECD, 2017). This 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. 65 metabolites in vivo (rat) and 12 hepatic metabolites were predicted for docosyl stearate (CAS 22413-03-2). Primarily, in vivo the ester bond is broken both in the liver and in general, and the hydrolysis products may be further metabolised. The resulting liver 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. Two skin metabolites were predicted: the C22-alcohol and the C18-acid, respectively. The metabolites formed in the skin are expected to enter the blood circulation and eventually meet the same fate as the hepatic metabolites. 122 metabolites were predicted to result from all kinds of microbiological metabolism of the ester 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.

There is no indication that docosyl stearate (CAS 22413-03-2) 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) using target and source substances were negative, with and without metabolic activation (Ames, 2012; HPRT 1994, CA 1994, CA 1998).

Excretion

The linear C22-fatty acid derived from the oxidation of the corresponding alcohol as well as the C18-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. 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 docosyl stearate (CAS 22413-03-2) that is not absorbed in the GI-tract will be excreted via the faeces.

 

References

Aungst B. and Shen D.D. (1986). Gastrointestinal absorption of toxic agents. In Rozman K.K. and Hanninen O. Gastrointestinal Toxicology. Elsevier, New York, US.

Bookstaff et al. (2003). The safety of the use of ethyl oleate in food is supported by metabolism data in rats and clinical safety data in humans. Regul Toxicol Pharm 37: 133-148.

Dermwin v2.02 (Sep 2012), EPI Suite, US Environmental Protection Agency. Calculation performed 04 May 2018.

ECHA (2017). Guidance on information requirements and chemical safety assessment, Chapter R.7c: Endpoint specific guidance. Version 3.0, June 2017.

Fukami, T. and Yokoi, T. (2012). The Emerging Role of Human Esterases. Drug Metab Pharmacokinet 27(5): 466-477

Greenberger et al. (1966). Absorption of medium and long chain triglycerides: factors influencing their hydrolysis and transport. J Clin Invest. 45(2):217-27.

IOM (2005). Dietary Reference Intakes for Energy, Carbohydrate, Fiber, Fat, Fatty Acids, Cholesterol, Protein, and Amino Acids (Macronutrients). Institute of the National Academies. The National Academies Press. http://www.nap.edu/openbook.php?record_id=10490&page=R1

Johnson, R.C. et al. (1990). Medium-chain-triglyceride lipid emulsion: metabolism and tissue distribution. Am J Clin Nutr 52(3):502-8.

Johnson W. Jr; Cosmetic Ingredient Review Expert Panel. (2001). Final report on the safety assessment of trilaurin, triarachidin, tribehenin, tricaprin, tricaprylin, trierucin, triheptanoin, triheptylundecanoin, triisononanoin, triisopalmitin, triisostearin, trilinolein, trimyristin, trioctanoin, triolein, tripalmitin, tripalmitolein, triricinolein, tristearin, triundecanoin, glyceryl triacetyl hydroxystearate, glyceryl triacetyl ricinoleate, and glyceryl stearate diacetate. Int J Toxicol. 2001;20 Suppl 4:61-94.

Lehninger, A.L., Nelson, D.L. and Cox, M.M. (1993). Principles of Biochemistry. Second Edition. Worth Publishers, Inc., New York, USA. ISBN 0-87901-500-4.

Mattson, F.H. and Volpenhein, R.A. (1962). Rearrangement of glyceride fatty acids during digestion and absorption. J Biol Chem. 237:53-5.

Mattson, F.H. and Volpenhein, R.A. (1969). Relative rates of hydrolysis by rat pancreatic lipase of esters of C2 - C18 fatty acids with C1 – C18 primary n-alcohols.J Lipid Res Vol(10): 271-276.

Mukherji M. et al. (2003). The chemical biology of branched-chain lipid metabolism. Progress in Lipid Research 42: 359-376.

National Toxicology Program (NTP) (1994) Comparative toxicology studies of Corn Oil, Safflower Oil, and Tricaprylin (CAS Nos. 8001-30-7, 8001-23-8, and 538-23-8) in Male F344/N Rats as vehicles for gavage. http://ntp.niehs.nih.gov/ntp/htdocs/LT_rpts/tr426.pdf (2011-12-19). Report No.: C62215. Owner company: U.S. Department of Health and Human Services, Public Health Services, National Institutes of Health.

OECD (2017). (Q)SAR Toolbox v4.1 Developed by Laboratory of Mathematical Chemistry, Bulgaria for the Organisation for Economic Co-operation and Development (OECD). Calculation performed 04 May 2018. http://toolbox.oasis-lmc.org/?section=overview

Sieber, S.M., Cohn, V.H., and Wynn, W.T. (1974). The entry of foreign compounds into the thoracic duct lymph of the rat.Xenobiotica 4(5), 265.

Stryer, L. (1996). Biochemie. 4. Auflage. Heidelberg Berlin Oxford: Spektrum Akademischer Verlag.

US EPA (2012).Estimation Programs Interface Suite™ for Microsoft® Windows, v 4.11. United States Environmental Protection Agency, Washington, DC, USA.Downloaded from: http://www.epa.gov/oppt/exposure/pubs/episuite.htm

WHO (1999). Evaluation of certain food additives and contaminants. Forty-ninth report of the joint FAO/WHO Expert Committee on Food Additives. WHO Technical Report Series 884. ISBN 92 4 120884 8.

WHO (2001). Safety Evaluation of Certain Food Additives and Contaminants: Aliphatic Acyclic Diols, Triols, and Related Substances. WHO Food Additives Series No. 48.