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Basic toxicokinetics

There are no experimental studies available in which the toxicokinetic behaviour of isononanoic acid, C16-18 alkyl esters (CAS 111937-03-2) has been assessed.

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, 2008), 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 according to the relevant Guidance (ECHA, 2008) (ECHA, 2008) and taking into account available information on the analogue substances from which data was used for read-across to cover data gaps.

The substance isononanoic acid, C16-18 alkyl esters is an UVCB substance with a linear C16-18-alcohol moiety and a highly branched C9 (isononanoic)-acid moiety. The molecular weight is 382.66-410.72 g/mol. It is a liquid at 20 °C (Stearinerie Dubois Fils, 2011), with a water solubility of < 0.05 mg/L (20 °C) (Frischmann, 2010). The log Pow was calculated to be 11.07-12.05 (Müller, 2011) and the vapour pressure is 0.0095 Pa at 20 °C (Kintrup, 2010).

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, 2008). The log Pow value provides information on the relative solubility of the substance in water and lipids (ECHA, 2008). 

Oral

The molecular weight of isononanoic acid, C16-18 alkyl esters is lower than 500 g/mol, indicating that the substance is available for absorption (ECHA, 2008). The high log Pow in combination with the low water solubility suggests that any absorption will likely happen via micellar solubilisation (ECHA, 2008).

The available acute oral toxicity data on isononanoic acid, C16-18 alkyl esters consistently showed LD50 > 5000 mg/kg bw and no systemic effects (Dufour, 1991). In the 28-day repeated dose toxicity study performed with the test substance, no toxicologically relevant effects were noted up to and including the highest dose level of 1000 mg/kg bw/day (Pitterman, 1993). This indicates that isononanoic acid, C16-18 alkyl esters 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 (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 hydrolysis. These changes will alter the physicochemical characteristics of the substance and hence predictions based upon the physico-chemical characteristics of the parent substance may no longer apply (ECHA, 2008).

In general, alkyl esters are readily hydrolysed in the gastrointestinal 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 action of 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. Branching reduces the ester hydrolysis rate, compared with linear esters. (Mattson and Volpenhein, 1969, 1972; WHO, 1999).

The substance isononanoic acid, C16-18 alkyl esters is therefore anticipated to be enzymatically hydrolysed to hexadecanol (C16), octadecanol (C18) and isononanoic acid (isoC9).

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. In rats given a single dose of radiolabelled octadecanol via duodenal cannula, 56.6 ± 14% of the administered material was absorbed within 24 h. As for fatty acids, the rate of absorption 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 physicochemical properties and molecular weight of isononanoic acid, C16-18 alkyl esters suggest oral absorption. However, the substance is anticipated to undergo enzymatic hydrolysis in the gastrointestinal tract and absorption of the ester hydrolysis products is also relevant. The absorption rate of the hydrolysis products is considered 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 favour dermal uptake, while for those above 500 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, 2008).

Isononanoic acid, C16-18 alkyl esters is almost insoluble in water; indicating a low dermal absorption potential (ECHA, 2008). The molecular weight of 382.66-410.72 g/mol is relatively close to the 500 g/mol limit above which dermal absorption is 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 be slow (ECHA, 2008).

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 252-789 cm/h. Considering the water solubility (0.0005 mg/cm³), the dermal flux is estimated to be ca. 1.3E-02 – 3.9E-02 mg/cm²/h.

If the substance is a skin irritant or corrosive, damage to the skin surface may enhance penetration (ECHA, 2008).

The experimental animal and human data on isononanoic acid, C16-18 alkyl esters shows that no significant skin irritation occurred, which excludes enhanced penetration of the substance due to local skin damage (Dufour, 1991; Häntschel, 1999).

Overall, based on the available information, the dermal absorption potential of isononanoic acid, C16-18 alkyl esters is predicted to be low.

Inhalation

As the vapour pressure of isononanoic acid, C16-18 alkyl esters is very low (0.0095 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 droplets with an aerodynamic diameter < 100 μm can be inhaled, in principle, only droplets with an aerodynamic diameter < 50 μm can reach the bronchi and droplets < 15 μm may enter the alveolar region of the respiratory tract (ECHA, 2008).

As for oral absorption, the molecular weight, log Pow and water solubility are suggestive of absorption across the respiratory tract epithelium either 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 read-across 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 (Van Huygevoort, 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 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, 2008).

The substance isononanoic acid, C16-18 alkyl esters 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. Twenty-four hours 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 of the radioactivity recovered in the lymph was localized in the chylomicron fraction. Thus, 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 isononanoic acid, C16-18 alkyl esters are anticipated to distribute systemically. Long-chain 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 isononanoic acid, C16-18 alkyl esters initially occurs via enzymatic hydrolysis of the ester resulting in isononanoic acid and C16-18 alcohols. 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 gastrointestinal 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 C16-18 alcohols 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, by passing 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 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). 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 two main constituents of the substance were predicted using the QSAR OECD toolbox (OECD, 2011). 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. Nine hepatic metabolites and 9 dermal metabolites were predicted for both the esters of isononanoic acid with the C16 alcohol and C18 alcohol. Primarily, the ester bond is broken both in the liver and in the skin and the hydrolysis products may be further metabolised. Besides hydrolysis, the resulting liver and skin metabolites are all 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 a few cases the ester bond remains intact, and only fatty acid oxidation products are found, which result in the addition of one hydroxyl group to the molecule. 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. Between 114 and 122 metabolites were predicted to result from all kinds of microbiological metabolism for the ester with C16 alcohol and C18 alcohol, respectively. Most of the metabolites were found to be a consequence of fatty acid oxidation and associated chain degradation of the molecule. The results of the OECD Toolbox simulation support the information retrieved in the literature.

There is no indication that the substance isononanoic acid, C16-18 alkyl esters 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 isononanoic acid, C16-18 alkyl esters and the read-across substances were negative, with and without metabolic activation (Banduhn, 1988; Buskens, 2010; Haddouk, 2007; Paika, 1991; Poth, 1994; Verspeek-Rip, 2010; Völkner, 1994). The result of the skin sensitisation studies performed with isononanoic acid, C16-18 alkyl esters and Fatty acids, C16-18, 2-ethylhexyl esters (CAS 91031-48-0) were likewise negative (Clouzeau, 1991; Potokar, 1972).

Excretion

The C16 and C18-fatty acids from the oxidation of the corresponding alcohols 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, 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. 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 β -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 addition, the alcohol components may also be conjugated to form a more water-soluble molecule and excreted via the urine (WHO, 1999). In an alternative pathway, the alcohol may be conjugated with e.g. glutathione and excreted directly, bypassing further metabolism steps.

In contrast, the isononanoic acid is unlikely to be used for energy generation and storage, since saturated aliphatic, branched-chain acids are described to be subjected to omega-oxidation due to steric hindrance by the methyl groups at uneven position, which results in the formation of various diols, hydroxyl acids, ketoacids or dicarbonic acids. In contrast to the products of beta-oxidation, these metabolites may be conjugated to glucuronides or sulphates, which subsequently can be excreted via urine or bile or cleaved in the gut with the possibility of reabsorption (entero-hepatic circulation) (WHO, 1998).

 

A detailed reference list is provided in the technical dossier (see IUCLID, section 13) and within CSR.