Fatty acids, C16-18 and C18-unsatd., mixed esters with adipic acid and trimethylolpropane

Endpoint summary

Key value for chemical safety assessment

Bioaccumulation potential:

low bioaccumulation potential

Additional information

Justification for grouping of substances and read-across

The category covers fatty acid polyesters of polyols (Propylidynetrimethanol (TMP) or Pentaerythritol (PE)) mixed with adipic acid. The category contains UVCB substances with fatty acid carbon chain lengths from C8-C18 (even-numbered, including linear saturated and unsaturated chains) building mono-, di-,tri- or higher esters with TMP or PE respectively in variable proportions.

Fatty acid esters are generally produced by chemical reaction of an alcohol (in this case Polyol e.g. Propylidynetrimethanol) with an organic fatty acid (e.g. oleic acid) in the presence of an acid catalyst (Radzi et al., 2005). The esterification reaction is started by the transfer of a proton from the acid catalyst to the acid to form an alkyloxonium ion. The carboxylic acid is protonated on its carbonyl oxygen followed by a nucleophilic addition of a molecule of the alcohol to the carbonyl carbon of the acid. An intermediate product is formed. This intermediate product loses a water molecule and proton to give an ester (Liu et al., 2006; Lilja et al., 2005; Gubicza et al., 2000; Zhao, 2000). The dicarboxylic acid adipic acid functions as a crosslinking agent forming diesters with polyols esterified with fatty acids, and thus building oligomeric structures with high molecular weights (MW >>500). Polyesters are the final products of esterification.

In accordance with Article 13 (1) of Regulation (EC) No 1907/2006, "information on intrinsic properties of substances may be generated by means other than tests, provided that the conditions set out in Annex XI are met.” In particular, information shall be generated whenever possible by means other than vertebrate animal tests, which includes the use of information from structurally related substances (grouping or read-across).

Having regard to the general rules for grouping of substances and read-across approach laid down in Annex XI, Item 1.5, of Regulation (EC) No 1907/2006, whereby substances may be considered as a category provided that their physicochemical, toxicological and ecotoxicological properties are likely to be similar or follow a regular pattern as a result of structural similarity, the substances listed below are allocated to the category of PFAE mixed and branched.

PFAE mixed and branched category members include:

ID#	CAS	EC	Chemical Name	Fatty acids	Di-carboxylic acid (cross-link)	Polyol	MW
1	95912-89-3 (a)	306-085-3	Fatty acids, C8-10, mixed esters with adipic acid and trimethylol-propane	C8-C10	Adipic acid	TMP	512.78 – 995.46
2	91001-61-5	292-832-8	Fatty acids, C16-18 and C18-unsatd., mixed esters with adipic acid and trimethylol-propane	C16, C18, C18 unsatd,	Adipic acid	TMP	849.46 – 1444.31
3	-	921-836-0	Mixed esters of fatty acid C16 and C16-C18-unsatd. with adipic acid and pentaerythritol	C16, C16- C18 unsatd,	Adipic acid	PE	1081.72 – 1969.08
4	78-16-0 (b, c)	201-089-0	2-ethyl-2-[[(1-oxoheptyl)oxy]methyl]propane-1,3-diyl bisheptanoate	C7	-	TMP	470.68
5	67762-53-2	267-022-2	Fatty acids, C5-9 tetraesters with pentaerythritol	C5, C7	-	PE	472.62 – 697.04
6	11138-60-6	234-392-1	Fatty acids, C8-10 (even numbered), di- and triesters with propylidynetrimethanol	C8-10	-	TMP	512.78 – 596.94
7	403507-18-6	442-150-3	Fatty acids, C16-18 and C18-unsatd., branched and linear ester with trimethylolpropane	C16-18, C18uns.	-	TMP	579.76 – 933.56
8	105-99-7	203-350-4	Dibutyl adipate	-	Adipic acid	Butanol	258.35

TMP = Propylidynetrimethanol / Trimethylolpropane; PE = Pentaerythritol

(a)  Category members subject to the REACh Phase-in registration deadline of 31 May 2013 are indicated in bold font.

(b)  Surrogate substances (indicated in normal font) are either chemicals forming part of a related category of structurally similar fatty acid esters or precursors/breakdown products of category members. Available data on these substances are used for assessment of toxicological properties by read-across on the same basis of structural similarity and/or mechanistic reasoning as described below for the present category.

 

Category specific similarities/trends:

Grouping of substances into this category is based on:

(1) common functional groups: all members of the respective category are esters of a polyfunctional alcohol with one or more carboxylic fatty acid(s) chain(s) and adipic acid (cross-linking agent) resulting in oligomeric structures. The alcohol moiety is either TMP or PE, which only differ by one hydroxyl-group. The carboxylic acid moiety comprises on one hand fatty acids with carbon chain lengths from C8-C18 (even-numbered) including saturated and unsaturated chains and on the other hand the dicarboxylic acid adipic acid; both acid components are bound to the alcohol with the latter acting as cross-linking agent, resulting in polyol polyesters. These Polyol polyesters may have multiple ester linkages and may include mixed esters derived from different carbon-length fatty acid mixtures; and

(2) common precursors and the likelihood of common breakdown products via biological processes, which result in structurally similar chemicals: all members of the category result from esterification of the alcohol with the respective fatty acids and adipic acid. Esterification is, in principle, a reversible reaction (hydrolysis). Thus, in theory the alcohol, dicarboxylic acid and fatty acid moieties are simultaneously precursors and breakdown products of the category members. Taken into account the high MW resulting in a very complex structure, the high log Pow and the very limited water solubility absorption of the substance is highly unlikely for any category member. Enzymatic hydrolysis in the gastrointestinal tract and/or liver - usually identified as the biological process, by which the breakdown of esters takes place - is highly unlikely for PFAE mixed and branched category members. The category members have a complex structure consisting of several mixed polyols linked with adipic acid that is not anticipated to fit into the binding pocket of e.g. pancreatic lipase which hydrolyses emulsified triacylglycerols in the diet. The ester bond is thus not available to catalytic center of the enzyme. (refer to Toxicokinetics for details); and

(3) constant pattern in the changing of the potency of the properties across the category: the available data show similarities and trends within the category in regard to physicochemical, environmental fate, ecotoxicological and toxicological properties. For those individual endpoints showing a trend, the pattern in the changing of potency is clearly and expectedly related to the length of the fatty acid chains and the degree of substitution of the polyol:

a) Physicochemical properties:

The molecular weight of the category members ranges from 512.77 to 1969.08 g/mol. This range represents structures completely esterified and cross linked with one adipic acid unit and is taken as the lower limit of molecular weight and size for the different substances. The physical appearance is related to the chain length of the fatty acid moiety, the degree of saturation and the number of ester bonds. All substances are liquid. All category members a non-volatile (vapour pressure: <0.01 Pa). The octanol/water partition coefficient increases with increasing fatty acid chain length and number of ester bonds, ranging from log Pow = 10.64 for the C8 TMP triester component for CAS 95912-89-3 to log Pow = 28.35 for the C16 PE tetraester component of EC 921-836-0 (calculated values). These are values for the smallest components of all substances in the category. As log Pow >10 exceeds the applicability domain for the (Q)SAR model, log Pow is >10 is given for all category members. The water solubility for all substances is <1 mg/L, for Fatty acids, C8-10, mixed esters with adipic acid and trimethylolpropane (CAS 95912‑89‑3) the water solubility is determined to be even lower (<0.05 mg/L).

b) Environmental fate and ecotoxicological properties:

Considering the low water solubility (<1 mg/L) and the potential for adsorption to organic soil and sediment particles (log Koc >5), the main compartments for environmental distribution are expected to be the soil and sediment. Nevertheless, persistency in these compartments is not expected since all members of the category are readily biodegradable and are thus expected to be eliminated in sewage treatment plants to a high extent. Release to surface waters, and thereby exposure of sediment, is very unlikely. Thus, the soil is expected to be the major compartment of concern. Nevertheless, the category members are expected to be metabolised by soil microorganisms. Evaporation into air and the transport through the atmosphere to other environmental compartments is not expected since the category members are not volatile based on the low vapour pressure (<0.01 Pa).

All members of the category did not show any effects on aquatic organisms in the available acute and chronic tests representing the category members up to the limit of water solubility. Moreover, bioaccumulation is assumed to be low since due to the molecular weight, size and structural complexity of the molecules of the category members absorption is not likely and ingested molecules will be excreted unchanged.  

c) Toxicological properties:

The toxicological properties show that all category members have similar toxicokinetic behaviour (very high MW, complex structure, limited water solubility, high log Pow) and that the constant pattern consists in a lack of potency change of properties across the category, explained by the very low potential for absorption and thus systemic availability of all mixed Polyol polyesters with adipic acid independently of the fatty acid chain length. Therefore, considering all available evidence and expert judgement no category member showed acute oral, dermal or inhalation toxicity, no skin irritation, eye irritation or sensitizing properties and are not mutagenic. Based on the very limited potential for absorption and the lack of toxicity of characteristic surrogate substances, repeated dose toxicity, reproductive toxicity and developmental toxicity are not expected and further tests are not considered necessary.  

The available data allows for an accurate hazard and risk assessment of the category and the category concept is applied for the assessment of environmental fate, environmental and human health hazards. Thus where applicable, environmental and human health effects are predicted from adequate and reliable read across data within the group applying the group concept in accordance with Annex XI, Item 1.5, of Regulation (EC) No 1907/2006.

A detailed justification for the grouping of chemicals and read-across is provided in the technical dossier (see IUCLID Section 13).

 

Toxicokinetic metabolism and distribution:

Basic toxicokinetics

There are no studies available in which the toxicokinetic behaviour of Fatty acids, C16-18 and C18-unsatd., mixed esters with adipic acid and trimethylolpropane (CAS 91001-61-5) has been investigated.

Therefore, in accordance with Annex VIII, Column 1, Item 8.8.1, of Regulation (EC) No 1907/2006 and with Guidance on information requirements and chemical safety assessment Chapter R.7c: Endpoint specific guidance (ECHA, 2012), assessment of the toxicokinetic behaviour of the substance Fatty acids, C16-18 and C18-unsatd., mixed esters with adipic acid and trimethylolpropane 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 physico-chemical and toxicological properties according to Guidance on information requirements and chemical safety assessment Chapter R.7c: Endpoint specific guidance (ECHA, 2012) and taking into account further available information on the PFAE mixed and branched category.

The substance Fatty acids, C16-18 and C18-unsatd., mixed esters with adipic acid and trimethylolpropane consists of mixed esters (mainly triester) of trimethylolpropane (TMP) with fatty acids (C16-18, C18 unsaturated) and/or adipic acid and meets the definition of an UVCB substance based on the analytical characterization. It is a liquid at room temperature and has a molecular weight between 849.46 and 1444.31 g/mol (this range represents structures completely esterified and cross linked with one adipic acid unit and is taken as the lower limit of molecular weight and size for the different substances) and a water solubility of < 1 mg/L at 20 °C (Undesa Italia srl, 2011). The log Pow is calculated to be > 10 (Hopp, 2011) and the vapour pressure is estimated to be < 0.0001 Pa at 20 °C (Nagel, 2012).

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, 2012).

Oral:

The smaller the molecule, the more easily it will be taken up. In general, molecular weights below 500 g/mol are favourable for oral absorption (ECHA, 2012). As the molecular weight of Fatty acids, C16-18 and C18-unsatd., mixed esters with adipic acid and trimethylolpropane is between 849.46 and 1444.31 g/mol, absorption of the molecule in the gastrointestinal tract is considered limited.

Applying the “Lipinsky Rule of Five” (Lipinski et al., 2001; Ghose et al., 1999), the potential for absorption after oral administration can be assessed. When Fatty acids, C16-18 and C18-unsatd., mixed esters with adipic acid and trimethylolpropane is considered, three of the rules are not fulfilled; the molecular weight, the log Pow as well as the total number of atoms are above the given ranges. Thus, based on this method, oral absorption is not expected to be high either.

If absorption occurs, the favourable mechanism will be absorption by micellar solubilisation, as this mechanism is of importance for highly lipophilic substances (log Pow > 4), which are poorly soluble in water (1 mg/L or less) like Fatty acids, C16-18 and C18-unsatd., mixed esters with adipic acid and trimethylolpropane with a log Pow > 10 and a water solubility < 1 mg/L.

In general fatty acid esters undergo stepwise hydrolysis of the ester bonds by gastrointestinal enzymes after oral ingestion (Lehninger, 1970; Mattson and Volpenhein, 1972). The respective alcohol as well as the fatty acid is formed. However, in case of the PFAE mixed and branched category, it is not anticipated that enzymatic hydrolysis of the parent substance is taking place in significant quantities. In a digestive fluid simulant study according to EFSA guidance no hydrolysis could be detected for the surrogate substance TMP-triheptanoate, which is a representative subunit of a PFAE mixed and branched molecule (Severac, 2012). The PFAE mixed and branched category members present a much more complex structure than TMP-triheptanoate alone, as they are forming polyesters of similar molecules due to the crosslinking function of the adipic acid content. Hence, taken the results on enzymatic hydrolysis from TMP-triheptanoate into consideration, it is anticipated that for the members of the PFAE mixed and branched category, an enzymatic hydrolysis in significant quantities, is even less likely.

Overall, a systemic bioavailability of Fatty acids, C16-18 and C18-unsatd., mixed esters with adipic acid and trimethylolpropane and/or the respective cleavage products in humans is considered very limited after oral uptake of the substance due to its high molecular weight.

Dermal:

The smaller the molecule, the more easily it may be taken up. In general, a molecular weight below 100 favours dermal absorption, above 500 g/mol the molecule may be too large (ECHA, 2012). As the molecular weight of Fatty acids, C16-18 and C18-unsatd., mixed esters with adipic acid and trimethylolpropane is between 849.46 – 1444.31 g/mol, dermal absorption of the molecule is not likely.

If the substance is a skin irritant or corrosive, damage to the skin surface may enhance penetration (ECHA, 2012). As Fatty acids, C16-18 and C18-unsatd., mixed esters with adipic acid and trimethylolpropane is not skin irritating in humans, enhanced penetration of the substance due to local skin damage can be excluded.

Based on a QSAR calculated dermal absorption a value in the range of 1.16E-05 to 2.25E-22 mg/cm2/event (very low) was predicted for Fatty acids, C16-18 and C18-unsatd., mixed esters with adipic acid and trimethylolpropane (Dermwin v.2.01, EPI Suite). Based on this value the substance has a very low potential for dermal absorption.

For substances with a log Pow above 4, the rate of dermal penetration is limited by the rate of transfer between the stratum corneum and the epidermis, but uptake into the stratum corneum will be high. For substances with a log Pow above 6, the rate of transfer between the stratum corneum and the epidermis will be slow and will limit absorption across the skin, and the uptake into the stratum corneum itself is also slow. The substance must be sufficiently soluble in water to partition from the stratum corneum into the epidermis (ECHA, 2012). As the water solubility of Fatty acids, C16-18 and C18-unsatd., mixed esters with adipic acid and trimethylolpropane is estimated to be less than 1 mg/L, dermal uptake is likely to be very low.

Overall, the calculated low dermal absorption potential, the low water solubility, the molecular weight (>100 g/mol), and the fact that the substance is not irritating to skin implies that dermal uptake of Fatty acids, C16-18 and C18-unsatd., mixed esters with adipic acid and trimethylolpropane in humans is considered as very limited.

Inhalation:

Fatty acids, C16-18 and C18-unsatd., mixed esters with adipic acid and trimethylolpropane has a low vapour pressure below 0.0001 Pa thus being of low volatility. Therefore, under normal use and handling conditions, inhalation exposure and thus availability for respiratory absorption of the substance in the form of vapours, gases, or mists is considered negligible.

However, the substance may be available for respiratory absorption in the lung after inhalation of aerosols, if the substance is sprayed. 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, 2012). Lipophilic compounds with a log Pow > 4, that are poorly soluble in water (1 mg/L or less) like Fatty acids, C16-18 and C18-unsatd., mixed esters with adipic acid and trimethylolpropane can be taken up by micellar solubilisation.

Overall, a systemic bioavailability of Fatty acids, C16-18 and C18-unsatd., mixed esters with adipic acid and trimethylolpropane in humans is considered possible after inhalation of aerosols with aerodynamic diameters below 15 μm.

Accumulation

Highly lipophilic substances tend in general to concentrate in adipose tissue, and depending on the conditions of exposure may accumulate. Although there is no direct correlation between the lipophilicity of a substance and its biological half-life, it is generally the case that substances with high log Pow values have long biological half-lives. The high log Pow of >> 5 implies that Fatty acids, C16-18 and C18-unsatd., mixed esters with adipic acid and trimethylolpropane may have the potential to accumulate in adipose tissue (ECHA, 2012).

Absorption is a prerequisite for accumulation within the body. Due to its MW and high log Pow, absorption is expected to be minimal for the registered substance, therefore accumulation is not favoured as well. In the exceptional case of esterase-catalysed hydrolysis, the cleavage products TMP and C16-18, C18 unsaturated fatty acids or adipic acid are produced. The log Pow of the ultimate cleavage product TMP is < 0 indicating a high water solubility (OECD SIDS, 1991). Consequently, there is no potential for TMP to accumulate in adipose tissue. The second ultimate cleavage product, the (di)carboxylic/fatty acids, can be stored as triglycerides in adipose tissue depots or be incorporated into cell membranes. At the same time, fatty acids are also required as a source of energy. Thus, stored fatty acids underlie a continuous turnover as they are permanently metabolized and excreted. Bioaccumulation of fatty acids only takes place, if their intake exceeds the caloric requirements of the organism. The third cleavage product adipic acid has a log Pow of 0.08 and is water-soluble, and thus not likely to accumulate in the body.

Overall, the available information indicates that no significant bioaccumulation of the parent substance in adipose tissue is anticipated.

Distribution

Distribution within the body through the circulatory system depends on the molecular weight, the lipophilic character and water solubility of a substance. 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, 2012).

Distribution of the parent substance is not expected as only very limited absorption will occur. Only the potential ultimate cleavage products of Fatty acids, C16-18 and C18-unsatd., mixed esters with adipic acid and trimethylolpropane after chemical changes as a result of enzymatic hydrolysis, namely TMP and the respective acids, might be distributed within the body.

TMP, a rather small (MW = 134.20 g/mol) substance of moderate water solubility will mainly be distributed in aqueous compartments of the organism and may also be taken up by different tissues (OECD SIDS, 1991). Fatty acids are also distributed in the organism and can be taken up by different tissues. They can be stored as triglycerides in adipose tissue depots or they can be incorporated into cell membranes (Masoro, 1977). Adipic acid will be distributed in the aqueous compartments of the body.

Overall, the available information indicates that the ultimate cleavage products, TMP and the respective acids will be distributed in the organism.

Metabolism

In general, esters of fatty acids are hydrolysed to the corresponding alcohol and fatty acid by esterases (Fukami and Yokoi, 2012). Depending on the route of exposure, esterase-catalysed hydrolysis takes place at different places in the organism: After oral ingestion, esters of alcohols and fatty acids may undergo enzymatic hydrolysis already in the gastro-intestinal fluids. In contrast, substances that are absorbed through the pulmonary alveolar membrane or through the skin enter the systemic circulation directly before entering the liver where hydrolysis will basically take place. The members of the PFAE mixed and branched category are of more complex structure than the simple fatty acid esters, therefore, due to steric hindrance, ester bond hydrolysis is expected to occur to a minor extent. This has also been shown for the surrogate substance TMP-triheptanoate (Polyol), for which no hydrolysis in a digestive fluid simulant could be detected in a study according to EFSA guidance (Severac, 2012). The Polyol TMP-triheptanoate can be regarded as a representative subunit of a PFAE mixed and branched. In the PFAE mixed and branched category are Polyols crosslinked with adipic acid, thus forming polyol polyester, an even more complex structure. Hence, taken the results on enzymatic hydrolysis from TMP-triheptanoate into consideration, it is anticipated that for the members of the PFAE mixed and branched category, an enzymatic hydrolysis in significant quantities, is even less likely.

Nevertheless possible ultimate cleavage products should be briefly discussed here.

In general, the physico-chemical characteristics of the cleavage products (e.g. physical form, water solubility, molecular weight, log Pow, vapour pressure, etc.) are likely to be different from those of the parent substance before absorption into the blood takes place, and hence the predictions based upon the physico-chemical characteristics of the parent substance do no longer apply (ECHA, 2012). However, for the cleavage products, it is anticipated that they have a higher potential for being absorbed in the gastro-intestinal tract. In case of long carbon chain substances and thus rather low water solubility by micellar solubilisation (Ramirez et al., 2001), and for small and water soluble cleavage products by dissolution into the gastrointestinal fluids (ECHA, 2012).

Excretion

The main route of excretion of Fatty acids, C16-18 and C18-unsatd., mixed esters with adipic acid and trimethylolpropane is expected to be excretion of unabsorbed substance with the faeces. The second route of excretion is expected to be by expired air as CO2 after metabolic degradation (beta-oxidation). The potential ultimate cleavage products TMP and adipic acid might also be excreted via the urine, unchanged or metabolised and exhaled (OECD SIDS, 2004).

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