Docosyl docosanoate

Endpoint summary

Description of key information

Absorption: Absorption of parent substances in gastro-intestinal tract unlikely, hydrolysis is expected and hydrolysis products are anticipated to be readily absorbed; absorption following inhalation expected to be as high as oral absorption; high dermal absorption potential anticipated.

Distribution and accumulation: Parent substance not expected to be absorbed and distributed; hydrolysis products are expected to be systemically distributed; accumulation of fatty acids only if intake exceeds the caloric requirements of the organism.

Metabolism: Hydrolysis products subject to beta- and omega-oxidation via endogenous pathways.

Excretion: Unchanged parent substance excreted in the faeces; hydrolysis products mainly excreted as CO2 in exhaled air after oxidation processes.

Key value for chemical safety assessment

Bioaccumulation potential:

no bioaccumulation potential

Additional information

Basic toxicokinetics

There are no experimental studies available in which the toxicokinetic behaviour of docosyl docosanoate (CAS 17671-27-1) has been assessed.

In accordance with Annex VIII, Column 1, Item 8.8.1, of Regulation (EC) No. 1907/2006 and with 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 substance is 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 and taking into account available information on the analogue substances from which data was used for read-across to cover data gaps.

Docosyl docosanoate is a mono-constituent substance with a fully saturated and linear C22 alcohol moiety, a fully saturated linear C22 acid moiety, and a molecular weight of 649.17 g/mol. It is a white powder at room temperature (Kawashima, 2019), with an estimated water solubility of < 0.40 mg/L at 20 °C (flask method, OECD 105). The log Pow was estimated to be 20.51 (QSAR, EPI Suite v.411, Kowwin v1.68) and the vapour pressure was calculated to be 1.46E-19 Pa at 20 °C (QSAR, SPARC v4.6).

Absorption

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

Oral

The molecular weight of docosyl docosanoate is well above 500 g/mol, indicating that the substance is less available for absorption (ECHA, 2017). The high log Pow of 20.51 in combination with the low water solubility (< 0.40 mg/L) suggests that any absorption will happen via micellar solubilisation (ECHA, 2017).

The available acute oral toxicity study performed with the analogue substance fatty acids C20-22 (even numbered), C18-22 (even numbered) alkyl esters (EC 701-233-7) resulted in a LD50 > 2000 mg/kg bw and no systemic effects (NOF, 2008, rat). In a combined repeated dose toxicity study with the reproduction/developmental toxicity screening test with the same analogue substance, no toxicologically relevant effects were noted up to and including the highest dose level of 1000 mg/kg bw/day (Lasem, 2014). This indicates that docosyl docosanoate also has a low potential for toxicity, although no assumptions can be made regarding the absorption potential based on the experimental data.

The potential of a substance to be absorbed in the gastro-intestinal (GI) tract may be influenced by chemical changes taking place in GI fluids; as a result of metabolism by GI flora, by enzymes released into the GI tract or by ‘chemical’ hydrolysis at the extreme low pH values in the stomach (pH < 2). 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 no longer apply (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 enzymatic activity of ubiquitous carboxylesterases. There are indications that the hydrolysis rate in the intestine by pancreatic lipase is lower for alkyl esters than for triglycerides, the natural substrate of this enzyme. The hydrolysis rate of linear esters increases with increasing chain length of either the alcohol or acid moiety. Branching reduces the ester hydrolysis rate, compared with linear esters. (Mattson and Volpenhein, 1969, 1972; WHO, 1999). Docosyl docosanoate is therefore expected to be enzymatically hydrolysed to the C22 fatty acid and the linear C22 fatty alcohol.

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 increase with decreasing chain length (Greenberger et al., 1966; IOM, 2005; Mattson and Volpenhein, 1962, 1964; OECD, 2006; Sieber, 1974).

In conclusion, based on the available information, the physico-chemical properties and molecular weight of docosyl docosanoate suggest limited oral absorption. However, the substance is expected to undergo enzymatic hydrolysis in the GI tract and absorption of the ester hydrolysis products is also relevant. The absorption rate of the hydrolysis products is considered to be low to moderate, as the C-chain lengths of the fatty acid and the alcohol are both rather long and the absorption potential decreases with increasing C-chain length.

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 take place. 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 docosyl docosanoate is almost insoluble in water, indicating a low dermal absorption potential (ECHA, 2017). The molecular weight of 649.17 g/mol indicates the dermal absorption will be low. The log Pow is > 6, which means that the uptake into the stratum corneum is likely to be slow and the rate of transfer between the stratum corneum and the epidermis will also 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 is thus 1.52E+07 cm/h. Considering the water solubility (< 0.00040 mg/cm³), the dermal flux is estimated to be ca. 3.63E+05 mg/cm²/h. The dermal absorption potential is estimated to be high.

If the substance is a skin irritant or corrosive, damage to the skin surface may enhance penetration (ECHA, 2017).

The experimental data on the analogue substance fatty acids C20-22 (even numbered), C18-22 (even numbered) alkyl esters (EC 701-233 -7) show that no skin irritation occurred, which excludes enhanced penetration of the substance due to local skin damage (NOF, 2008, rabbit). This finding is also supported by a Guinea Pig Maximisation Test (GPMT) with the same analogue substance. In this study no skin irritation was observed in the animals (NOF, 2008, GPMT).

Overall, based on the available information, the dermal absorption potential of docosyl docosanoate is predicted to be high.

Inhalation

As the vapour pressure of docosyl docosanoate is very low (1.46E-19 Pa at 20 °C), the volatility is also low. Therefore, the potential for exposure and subsequent absorption via inhalation during normal use and handling is considered to be negligible.

If the substance is available as an aerosol, the potential for absorption via the inhalation route is increased. While particles with an aerodynamic diameter < 100 μm can be inhaled, in principle, only particles with an aerodynamic diameter < 50 μm can reach the bronchi and particles < 15 μm are likely to enter the alveolar region of the respiratory tract (ECHA, 2017).

As for oral absorption, the molecular weight, log Pow and water solubility suggest there will be limited absorption across the respiratory tract epithelium by micellar solubilisation.

Esterases present in the lung lining fluid may also hydrolyse the substance, hence making the resulting alcohol and acid available for inhalative absorption.

An acute inhalation toxicity study was performed with the analogue substance 2-ethylhexyl oleate (CAS 26399-02-0), in which rats were exposed nose-only to > 5.7 mg/L air of an aerosol for 4 h (SO.G.I.S., 2010). No mortality occurred and no toxicologically relevant effects were observed. Thus, the test substance is not acutely toxic by the inhalation route, but no firm conclusion can be drawn on respiratory absorption.

Due to the limited information available, absorption via inhalation is assumed to be as high as via the oral route in a worst-case approach.

Distribution and Accumulation

Distribution of a substance within the body depends on the physico-chemical properties of the substance; especially the molecular weight, the lipophilic character (log Pow) 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 its extracellular concentration, particularly in fatty tissues (ECHA, 2017).

Docosyl docosanoate will mainly be absorbed in the form of the hydrolysis products. The fraction of ester absorbed unchanged will undergo enzymatic hydrolysis by ubiquitous esterases, primarily in the liver (Fukami and Yokoi, 2012). Consequently, the hydrolysis products are the most relevant components to assess. Both hydrolysis products are expected to be distributed widely in the body.

After being absorbed, fatty acids are (re-)esterified along with other fatty acids into triglycerides and released in chylomicrons. Fatty acids of carbon chain length ≤ 12 may be transported as the free acid bound to albumin directly to the liver via the portal vein, instead of being re-esterified. Chylomicrons are transported in the lymph to the thoracic duct and eventually to the venous system. Upon 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 oxidised for energy or they are released into the systemic circulation and returned to the liver (IOM, 2005; Johnson, 1990; Johnson, 2001; Lehninger, 1993; Stryer, 1996).

Absorbed alcohols are likewise transported via the lymphatic system. 24 h after intraduodenal administration of a single dose of radiolabelled octadecanol to rats, the percent absorbed radioactivity in the lymph was 56.6 ± 14. Thereof, more than half (52 - 73%) was found in the triglyceride fraction, 6 - 13% as phospholipids, 2 - 3% as cholesterol esters and 4 - 10% as unchanged octadecanol. Almost all radioactivity recovered in the lymph was localised in the chylomicron fraction. This shows the alcohol is oxidised to the corresponding fatty acid and esterified in the intestine as described above (Sieber, 1974).

Taken together, the hydrolysis products of docosyl docosanoate are predicted to distribute systemically. The fatty alcohols are rapidly converted into the corresponding fatty acids by oxidation and distributed in form of triglycerides, which can be used as energy source or stored in adipose tissue. Stored fatty acids underlie a continuous turnover as they are permanently metabolised for energy and excreted as CO2. Bioaccumulation of fatty acids takes place, if their intake exceeds the caloric requirements of the organism.

Metabolism

The metabolism of docosyl docosanoate initially occurs via enzymatic hydrolysis of the ester resulting in the corresponding C22 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 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 undergo enzymatic hydrolysis already 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 linear C22 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 the corresponding carboxylic acid, catalysed by aldehyde dehydrogenase. Both the alcohol and the aldehyde may also be conjugated with e.g. glutathione and excreted directly, without further metabolism steps (WHO, 1999).

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 C2 units from the aliphatic acyl-CoA molecule. Further oxidation via the citric acid cycle leads to the formation of H2O and CO2 (Lehninger, 1993). 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 docosyl docosanoate were predicted using the OECD QSAR Toolbox version 4.4 (OECD, 2020). This QSAR tool predicts which metabolites may result from enzymatic activity in the liver, in the skin and by intestinal bacteria in the GI tract. While in the skin only two metabolites resulting from the hydrolysis of docosyl docosanoate is predicted, 61 hepatic metabolites are predicted for the substance. Primarily, the ester bond is broken and the hydrolysis products are subject to oxidative reactions in various parts of the fatty acid and alcohol alkyl chains, i.e. the insertion of OH groups and the subsequent oxidation of the OH groups to aldehydes and carboxylic acids. The predicted metabolites are fully in line with the expected metabolic pattern of the beta- and omega-oxidation processes as described above. In general, the hydroxyl groups make the metabolites 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. 130 metabolites were predicted to result from all kinds of microbiological metabolism. Most of the metabolites were found to be a consequence of fatty acid oxidation and associated chain degradation of the molecule. In conclusion, the results of the OECD QSAR Toolbox simulation support the information retrieved in the literature.

There is no indication that docosyl docosanoate is activated to reactive intermediates under the relevant test conditions. The experimental studies performed on genotoxicity (Ames test, gene mutation in mammalian cells in vitro, chromosome aberration assay in mammalian cells in vitro) using the analogue substances fatty acids C20-22 (even numbered), C18-22 (even numbered) alkyl esters (EC 701-233-7) and (Z)-octadec-9-enyl oleate (CAS 3687-45-4) were negative, with and without metabolic activation (NOF, 2008, Ames; Emery, 1994, CA; Emery, 1994, HPRT). The result of the GPMT skin sensitisation study performed with the analogue substance fatty acids C20-22 (even numbered), C18-22 (even numbered) alkyl esters was likewise negative (NOF, 2008, GMPT).

Excretion

The linear C22 fatty acid from the oxidation of the corresponding alcohol as well as the 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 e.g. 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 excreted via exhaled air as CO2 or stored as described above. Experimental data with ethyl oleate (CAS 111-62-6) 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. At sacrifice (72 h 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 expired air as CO2. 12 h 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 (Bookstaff et al., 2003).

In an alternative pathway, the alcohol may be conjugated with e.g. glutathione and excreted directly, bypassing further metabolism steps (WHO, 1999).

References

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