Reaction products resulting from the esterification of Sorbitol with C8 – 18 (even) and C18 unsaturated fatty acids in the ratio of 1:1

Endpoint summary

Additional information

Justification for grouping of substances and read-across

The Sorbitan fatty acid esters category covers fatty series of analogous esters comprised of D-glucitol and natural fatty acids. The category contains UVCB substances, which exhibit differences in chain length (C8-C18), degree of esterification (mono-, di-, tri- and higher esters) and extent of unsaturation (saturated and mono unsaturated).

The category members are listed in the table below and their composition is defined in IUCLID Section 1.2. The naming of the substances is in accordance with the European Pharmacopeia (2011).

Sorbitan esters are produced generally in two stages. First, a D-glucitol solution is concentrated by heating to remove water, which results in the open chain D-glucitol cyclising with the loss of water to form a mixture of anhydrosorbitols and isosorbide. The mixture is reacted with fatty acid to give the respective mono-, di-, tri- and n-esters as the final products of esterification (JECFA 1973, Gennaro 1990; Canterbery 1997).

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, where by 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 Sorbitan fatty acid esters.

Sorbitan fatty acid esters include:

CAS	EC name	Molecular weight	Fatty acid chain length	Molecular formula	Degree of esterification
CAS 91844-53-0 (*)	Sorbitan octanoate (2:3)	MW 290.35 - 422.66	C8 C10	C14H26O6 C16H30O6 C22H40O7 C26H48O7	Mono- Mono- Di- Di-
CAS 1338-39-2 (*)	Sorbitan laurate	MW 346.46 – 528.79	C12 C18 C18:1	C18H34O6 C24H46O6 C24H44O6 C30H56O7	Mono- Mono- Mono- Di-
CAS 26266-57-9 (*)	Sorbitan palmitate	MW 402.57 – 879.38	C16	C22H42O6 C54H102O8	Mono- Tri-
CAS 1338-41-6 (*)	Sorbitan stearate	MW 402.57 – 981.56	C16 C18	C22H42O6 C24H46O6 C54H102O8 C60H116O9	Mono- Mono- Tri- Tri-
CAS 1338-43-8 (#)	Sorbitan oleate	MW 428.6	C18:1	C24H44O6	Mono-
CAS 71902-01-7 (*)	Sorbitan isooctadecanoate	MW 430.62 – 963.54	C18 iso	C24H46O6 C42H80O7 C60H114O8	Mono- Di- Tri-
CAS 8007-43-0 (*)	Sorbitan, (Z)-9-octadecenoate (2:3)	MW 679.04 – 957.49	C18 C18:1	C41H74O7 C60H108O8	Di- Tri-
CAS 26658-19-5 (*)	Sorbitan tristearate	MW 879.38 – 981.56	C16 C18	C54H102O8 C60H116O9	Tri- Tri-
CAS 26266-58-0 (*)	Anhydro-D-glucitol trioleate	MW 957.49	C18:1	C60H108O8	Tri-
CAS 50-70-4 (+)	D-glucitol	MW 182.17	--	--	--
CAS 124-07-2 (+)	Octanoic acid	MW 144.21	--	--	--
CAS 112-85-6 (+)	Docosanoic acid	MW 340.59	--	--	--

MW: molecular weight

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

(#) Substances that are either already registered under REACh or not subject to the REACh Phase-in registration deadline of 31 May 2013 are indicated in normal font.

(+) Surrogate substances are either chemicals forming part of a related category of structurally similar fatty acid esters or precursors/breakdown products of category members (i.e. alcohol and fatty acid moieties). 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.

Grouping of substances into this category is based on:

(1) common functional groups: all members of the category are esters of an alcohol with one or more carboxylic (fatty) acid(s) chain(s). The alcohol moiety D-glucitol is common to all category members. The fatty acid moiety comprises carbon chain lengths from C8-C18 (even-numbered) and includes saturated and mono-unsaturated chains bound to the alcohol resulting in mono-, di-, tri-, n-esters; and

(2) common precursors and the likelihood of common breakdown products via biological processes, which result in structurally similar chemicals: Sorbitan fatty acid esters have common metabolic fate that involves stepwise hydrolysis to the respective fatty acid and D-glucitol (Stryer 1996). Fatty acids feed into physiological pathways like the β-oxidation (Stryer 1996) and D-glucitol is metabolized to D-glucose or D-fructose (Touster 1975). In general, hydrolysis of Sorbitan fatty acid esters in vitro and by lipolytic enzymes in the gastrointestinal tract occurs within a maximum of 48h for mono-, di- and triester but decreases with the number of esterified fatty acid so that no hydrolysis of hexa-ester occurs (Krantz 1951, Mattson and Nolen 1972, Treon 1967, Wick and Joseph 1953). Depending on the route of exposure, esterase-catalysed hydrolysis takes place at different places in the organism: After oral ingestion, esters of D-glucitol and fatty acids will undergo chemical changes already in the gastro-intestinal fluids as a result of enzymatic hydrolysis. 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 first cleavage product, the fatty acid, is stepwise degraded by beta-Oxidation based on enzymatic removal of C2 units in the matrix of the mitochondria in most vertebrate tissues. The C2 units are cleaved as acyl-CoA, the entry molecule for the citric acid cycle. For the complete catabolism of unsaturated fatty acids such as oleic acid, an additional isomerization reaction step is required. The alpha- and omega-oxidation, alternative pathways for oxidation, can be found in the liver and the brain, respectively (CIR, 1987). For the second cleavage product D-glucitol it was found, that it is relatively slowly absorbed from the gastro-intestinal tract compared with glucose and that it can be metabolized by the intestinal microflora (Senti 1986). Once absorbed, D-glucitol is primarily metabolized in the liver. The first step involves oxidation by L-iditol dehydrogenase to fructose which is metabolized by the fructose metabolic pathway (Touster 1975). D-glucitol does not enter tissues other than the liver and does not directly influence the metabolism of endogenous D-glucitol in other tissues (Allison 1979): 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 alcohol (mono-, di-, tri-, n-ester).

a) Physicochemical properties:

The molecular weight of the category members ranges from 290.35 to 981.56 g/mol. The physical appearance is related to the chain length of the fatty acid moiety, the degree of saturation and the number of ester bonds. Thus, monoesters of short-chain and unsaturated fatty acids (C8-C14 and C18:1) are liquid, while di- and triesters of short- and long-chain fatty acids are solids. All category members a non-volatile (vapour pressure: < 0.0001 kPa). The octanol/water partition coefficient increases with increasing fatty acid chain length and number of ester bonds, ranging from 0.97 (C8-monoester) to >10 (C16-18-triester). The water solubility decreases accordingly (750 mg/L for C8-mono ester to < 0.05 mg/L for C18-triester).

b) Environmental fate and ecotoxicological properties:

All substances in the Sorbitan esters category are readily biodegradable and are thus not expected to persist in the environment. Abiotic degradation via hydrolysis or phototransformation is not considered to be relevant for these substances. The water solubility is generally low, with the exception of the smallest substance sorbitan octanoate, which is highly soluble. The adsorption potential to organic soil and sediment particles increases with the size of the molecule, i.e. chain length and degree of esterification, following a clear trend. Generally, all sorbitan triesters, diesters from fatty acid chain length C12 and monoesters with C18 fatty acids show high adsorption potential (log Koc 3.3 - >10) and are expected to partition mainly in the compartments soil and sediment. Smaller sorbitan mono- and diesters (fatty acid chain length <C18 and <C12, respectively) have lower adsorption potential (log Koc 1.0 - 2.8) and may also be found in the water compartment. Due to the structure consisting of the polar sorbitan and the hydrophobic fatty acid carbon chain, the substances also have surface active properties. Nevertheless, since all category members are readily biodegradable, they are expected to be eliminated in sewage treatment plants to a high extent. Release to surface waters, and thereby exposure of aquatic and sediment organisms, is therefore very unlikely. In soil, the substances are expected to be rapidly degraded. Accumulation into organisms is not expected for members of the Sorbitan esters category, since they can be digested by common metabolic pathways. Evaporation into air and the transport through the atmospheric compartment is not expected since the category members are not volatile based on the low vapour pressure. Based on experimental data, all category members show very low toxicity to aquatic, sediment and terrestrial organisms in both acute and chronic tests.

c) Toxicological properties:

All available experimental data indicate that the members of the Sorbitan fatty acid esters category are not acutely toxic, are not irritating to the skin or to the eyes and do not have sensitizing properties. In addition, no hazard was identified for any category member regarding repeated dose, genetic and reproductive/developmental toxicity.

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 data for source substance(s) within the group by interpolation to the target substances in the group (read-across approach) applying the group concept in accordance with Annex XI, Item 1.5, of Regulation (EC) No 1907/2006. In particular, for each specific endpoint the source substance(s) structurally closest to the target substance is/are chosen for read-across, with due regard to the requirements of adequacy and reliability of the available data. Structural similarities and similarities in properties and/or activities of the source and target substance are the basis of read-across.

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

 

Basic toxicokinetics

In accordance with Annex VIII, Column 1, Item 8.8 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 behavior of the substance Sorbitan laurate (CAS No. 1338-39-2) was conducted to the extent that can be derived from the relevant available information on physicochemical and toxicological characteristics. One in vivo study focusing on absorption and distribution after oral ingestion of the radiolabeled test substance is available (Giovanetto et al. 1984).

The substance Sorbitan laurate (molecular weight of 346.46 to 528.79 g/mol) is a liquid, which is slightly soluble in water (<3 mg/L at 20 °C). The log Po/w is 3.15 – 6.10 and the vapour pressure < 0.0001 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, 2008).

Oral

The smaller the molecule, the more easily it will be taken up. In general, molecular weights below 500 are favorable for oral absorption (ECHA, 2008). As the molecular weight of Sorbitan laurate ranges between 346.46 and 528.79 g/mol, an absorption of the molecule in the gastrointestinal tract is in general probable.

Absorption after oral administration is also expected when the “Lipinski Rule of Five” (Lipinski et al. (2001), refined by Ghose et al. (1999)) is applied to the substance Sorbitan laurate.

After oral ingestion, esters of D-glucitol and fatty acids will undergo stepwise chemical changes in the gastro-intestinal fluids as a result of enzymatic hydrolysis.The hydrolysis of Sorbitan fatty acid esters occurs within a maximum of 48h for mono-, di- and tri-ester but decreases with the number of esterified fatty acid so that no hydrolysis of hexa-ester occurs (Krantz 1951, Mattson and Nolen 1972, Treon 1967, Wick and Joseph 1953). The physico-chemical characteristics of the cleavage products (e.g. physical form, water solubility, molecular weight, log Pow, vapour pressure, etc.) will 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, 2008). However, also for both cleavage products, it is anticipated that they will be absorbed in the gastro-intestinal tract. The highly lipophilic fatty acid will be absorbed by micellar solubilisation (Ramirez et al., 2001), whereas D-glucitol, being a highly water-soluble substance, will dissolve into the gastrointestinal fluids and slowly be absorbed with a subsequent metabolism in the liver (Senti 1986, Touster 1975).

Experimental data with orally administered¹⁴C-labelled Sorbitan laurate (2100 mg/kg bw) showed a significant initial accumulation of radioactivity (not further specified) in different organs indicating bioavailability of the test substance and/or its cleavage products after oral ingestion (Giovanetto et al. 1984). Furthermore, oral acute and repeated dose toxicity studies indicated diarrhea as main clinical sign in the. This effect might me due to the laxative properties of D-glucitol which is described after excessive consumption (Peters and Lock 1958).

Overall, a systemic bioavailability of Sorbitan laurate and/or the respective cleavage products in humans is considered possible after oral uptake of the substance.

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 the molecule may be too large (ECHA, 2008). As the molecular weight of Sorbitan laurate ranges between346.46 to 528.79 g/mol, a dermal absorption of the molecule remains rather unlikely.

If the substance is a skin irritant or corrosive, damage to the skin surface may enhance penetration (ECHA, 2008). As Sorbitan laurate is not considered as skin irritating in humans, an enhanced penetration of the substance due to local skin damage can be excluded.

Based on QSAR, a dermal absorption value of 1.89E-04 mg/cm²/event was calculated (molecular weight of 346.5 g/mol, Dermwin 2012), which indicates 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, 2008). As Sorbitan laurate has a low water solubility and the log Pow ranges from 3.15 to 6.10, dermal uptake is likely to be low.

Overall, the calculated very low dermal absorption potential, the low water solubility, the relatively high molecular weight (>100), the log Pow value and the fact that the substance is not irritating to skin implies that dermal uptake of Sorbitan laurate in humans is considered as limited.

Inhalation

Sorbitan laurate has a low vapour pressure of< 0.0001 Pa at 20 °Cthus 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 not expected to be significant.

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, 2008). Due to its lipophilic character, Sorbitan laurate can be taken up by micellar solubilisation.

Overall, a systemic bioavailability of Sorbitan laurate in humans is considered likely after inhalation of aerosols with aerodynamic diameters below 15 µm.

However, experimental data investigating acute toxicity after inhalation revealed an LC50 of >=5000 mg/m³ (van Huygewvoort 2010).

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 relatively high log Pow of 3.15 -6.10 implies that Sorbitan laurate may have the potential to accumulate in adipose tissue (ECHA, 2008).

However, as further described in the section metabolism below, Sorbitan fatty acid esters will undergo esterase-catalysed hydrolysis, leading to the cleavage productsD-glucitoland fatty acids.

The log Pow of the first cleavage productD-glucitolis -2.2, indicating a high solubility in water. Consequently, there is no potential for D-glucitol to accumulate in adipose tissue. The second cleavage product, the 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.

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

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, 2008).

Sorbitan fatty acid esters will undergo chemical changes as a result of enzymatic hydrolysis, leading to the cleavage productsD-glucitoland fatty acids.

D-glucitol, a small (MW 182.2 g/mol), polar water-soluble substance (log Pow -2.2), will be distributed in aqueous fluids by diffusion through aqueous channels and poresand oxidized by L-iditol dehydrogenase to fructose which is subsequently metabolized by the fructose metabolic pathway (Touster 1975).

The 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).

Experimental data with orally administered¹⁴C-labelled Sorbitan laurate to rats showed aradioactivitypeak of 4.01 ± 0.71% per 100 mL within 30 min after test substance application with a decline to background level within 24 hours (the compartment was not further specified) (Giovanetto et al. 1984). A maximum of the radioactive labeled substance was recovered in the liver, kidneys, lung, heart and spleen with 1.7 and 0.81% in the liver and kidneys of after 4 hours, respectively, indicating that the test substance and/or its metabolites reached the internal organs. In view of the log Po/w, a general accumulation of the test substance itself in fat tissue may normally be unlikely with repeated intermittent exposure, but may accumulate if exposure is continuous.

Overall, the available information indicates that the cleavage products,D-glucitoland fatty acids distribute in the organism.

Metabolism

Sorbitan laurate represents a Sorbitan fatty acid ester. Esters are known to hydrolyse into carboxylic acids and alcohols by esterases (Fukami and Yokoi, 2012). Therefore it is expected that the test substance hydrolyses toD-glucitoland the respective fatty acids under physiological conditions. Depending on the route of exposure, esterase-catalysed hydrolysis takes place at different places in the organism: After oral ingestion, Sorbitan fatty acid esters will undergo chemical changes already in the gastro-intestinal fluids as a result of enzymatic hydrolysis. In contrast, substances which 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 first cleavage product, the fatty acid, is stepwise degraded by beta-Oxidation based on enzymatic removal of C2 units in the matrix of the mitochondria in most vertebrate tissues. The C2 units are cleaved as acyl-CoA, the entry molecule for the citric acid cycle. For the complete catabolism of unsaturated fatty acids such as oleic acid, an additional isomerization reaction step is required. The alpha- and omega-oxidation, alternative pathways for oxidation, can be found in the liver and the brain, respectively (CIR, 1987).

For the second cleavage productD-glucitolit was found, that it is relatively slowly absorbed from the gastro-intestinal tract compared with glucose and that it can be metabolized by the intestinal microflora (Senti 1986). Once absorbed,D-glucitolis primarily metabolized in the liver. The first step involves oxidation by L-iditol dehydrogenase to fructose which is metabolized by the fructose metabolic pathway (Touster 1975).D-glucitoldoes not enter tissues other than the liver and does not directly influence the metabolism of endogenousD-glucitolin other tissues (Allison 1979).

However, using the OECD toolbox Vs. 2.3, the liver metabolism simulator provided 26 potential metabolites. Studies on genotoxicity (chromosomal aberration and gene mutation in mammalian cells) were negative, indicating no reactivity of the test substance or its metabolites under the test conditions. The GI metabolism simulator provided 144 potential metabolites.

Excretion

Characteristics favourable for urinary excretion are low molecular weight (below 300 in the rat), good water solubility, and ionization of the molecule at the pH of urine. In the rat, molecules that are excreted in the bile are amphipathic (containing both polar and nonpolar regions), hydrophobic/strongly polar and have a high molecular weight. In general, for organic cations with a molecular weight below 300 it is unlikely that more than 5-10% will be excreted in the bile. For organic anions this cut off value may be lower. Substances excreted via bile may potentially undergo enterohepatic circulation. Little is known about the determinants of biliary excretion in humans. Highly lipophilic substances that have penetrated the stratum corneum but not penetrated the viable epidermis may be sloughed off with skin cells (ECHA, 2008).

After oral ingestion, Sorbitan fatty acid esters will undergo stepwise chemical changes in the gastro-intestinal fluids as a result of enzymatic hydrolysis. As the physico-chemical characteristics of the cleavage products (e.g. physical form, water solubility, molecular weight, log Pow, vapour pressure, etc.) will be different from those of the parent substance, the predictions based upon the physico-chemical characteristics of the parent substance do no longer apply (ECHA, 2008). However, also for both cleavage products, it is anticipated that they will be absorbed in the gastro-intestinal tract. The highly lipophilic fatty acids will be readily absorbed by micelullar solubilisation und undergo beta-Oxidation or will be stored in fat tissue (Ramirez et al., 2001). The D-glucitol, being a highly water-soluble substance, will dissolve into the gastrointestinal fluids and slowly be absorbed with a subsequent metabolism to fructose in the liverby L-iditol dehydrogenase(Senti 1986, Touster 1975). Absorbed and non-metabolized D-glucitol will mainly be excreted via urine, due the low molecular weight and the high water solubility of the substance whereas non-absorbed amounts will be excreted via faeces. High amounts of D-glucitol in the intestine trigger diarrhea (Peters 1958) and de facto, diarrhea was observed in nearly all acute oral toxicity and repeated dose toxicity studies with Sorbitan fatty acid esters.

Experimental in vivo data on excretion of Sorbitan laurate is available. In rats, urinary excretion was observed to be the major route of elimination (Giovanetto et al. 1984). 48 hours after the treatment, 87.2 ± 0.8 and 5.6 ± 1% of the dose were excreted in urine and faeces, respectively, indicating a rapid absorption and elimination of the test substance and/or its metabolites after oral administration. These data also indicate hydrolysis of the test substance since substances with a good water solubility are predominantly excreted via the urine and the parent substance exhibits only slight water solubility.

 

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