Ethylene distearate

Endpoint summary

Additional information

The hazard assessment is based on the data currently available. New studies with the registered substance and/or other member substances of the glycol esters category will be conducted in the future. The finalised studies will be included in the technical dossier as soon as they become available and the hazard assessment will be re-evaluated accordingly.

For further details, please refer to the category concept document attached to the category object (linked under IUCLID section 0.2) showing an overview of the strategy for all substances within the glycol esters category.

 

Basic toxicokinetics

There are no studies available in which the toxicokinetic behaviour of ethylene distearate 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, 2008), assessment of the toxicokinetic behavior of the substance ethylene distearate 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, 2008) and taking into account further available information on the Glycol Ester category.

The substance ethylene distearate is a diester of ethylene glycol and stearic acid and meets the definition of a mono-constituent substance based on the analytical characterization. The chemical structure of ethylene distearate is shown in Figure 1 (see attached document).

Ethylene distearate is a solid and has a molecular weight of 594.99 g/mol and a water solubility of 4.24E-12 mg/L at 25°C (SRC database, 2011). The log Pow is calculated to be > 10 (Müller, 2011) and the vapour pressure is calculated to be 2.36E-15 Pa (Nagel, 2011).

 

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

When assessing the potential of ethylene distearate to be absorbed in the gastrointestinal (GI) tract, it has to be considered that fatty acid esters will undergo to a high extent hydrolysis by ubiquitous expressed GI enzymes (Long, 1958; Lehninger, 1970; Mattson and Volpenhein, 1972). Thus, due to the hydrolysis the predictions based upon the physico-chemical characteristics of the intact parent substance alone may no longer apply but also the physico-chemical characteristics of the breakdown products of the ester; the alcohol ethylene glycol and the fatty acid stearic acid and the monoester 2-hydroxyethyl stearate (see Figure 2 in attached document). The molecular weight of ethylene distearate (594.99 g/mol) does not favour absorption. Furthermore, the low water solubility and the high log Pow value of the parent compound indicate that the absorption may be limited by the inability to dissolve into GI fluids. However, micellular solubilisation by bile salts may enhance absorption, a mechanism which is especially of importance for highly lipophilic substances with log Pow > 4 and low water solubility (Aungst and Shen, 1986).

When considering the hydrolysis products, the respective molecular weights of 2-hydroxyethyl stearate (328.53 g/mol), ethylene glycol (62.07 g/mol) and stearic acid (284.48 g/mol) favour absorption. Furthermore, highly lipophilic long chain fatty acids like stearic acid will be absorbed into the walls of the intestine villi due to their role as nutritional energy source (Lehninger, 1970). The alcohol component ethylene glycol is highly water-soluble and has a low molecular weight and can therefore dissolve into GI fluids. Thus, ethylene glycol will be readily absorbed through the GI tract (ATSDR, 2010; ICPS, 2001).

In addition, in-vivo studies with 14C-labelled propylene glycol distearate (PGDS), a structurally similar glycol ester, have shown that absorption was similar to a labeled stearic acid mixture of glyceride esters (Long, 1958).

 

Studies with ethylene distearate after oral administration to rats, showed no signs of systemic toxicity in acute oral toxicity tests, resulting in LD50 values greater than 2000 mg/kg bw (Wnorowski, 1991a,b; Bouffechoux, 1995; Elder, 1982).

Furthermore, available data on the subchronic oral toxicity of three substances of the Glycol ester Category (Stearic acid, monoester with propane-1,2-diol, Fatty acids, C18 and C18 unsatd. epoxidized, ester with ethylene glycol and Decanoic acid, mixed diesters with octanoic acid and propylene glycol consistently showed no adverse systemic effects in animals resulting in NOAELs of 1000 mg/kg bw/day (Pittermann, 1991,1993; Saatman, 1967). The lack of short- and long-term systemic toxicity of ethylene distearate and further category members cannot be equated with a lack of absorption or with absorption but rather with a low toxic potential of the test substance and the breakdown products themselves.

 

Dermal

There are no data available on dermal absorption or on acute dermal toxicity of ethylene distearate. On the basis of the following considerations, the dermal absorption of ethylene distearate is considered to be low. Regarding the molecular weight of 594.99 g/mol and a calculated octanol/water partition coefficient of > 10 (Müller, 2011) in combination with the low water solubility, a low dermal absorption rate is anticipated. Log Pow values above 6, like for ethylene distearate, will slow the rate of transfer between the stratum corneum and the epidermis and therefore absorption across the skin will be limited and uptake into the stratum corneum itself is slow. Furthermore, QSAR calculation using EPIwebv4.1 confirmed this assumption, resulting in a very low Dermal Flux of 4.63E-9 mg/cm2 per h.

Furthermore, available data on acute dermal toxicity of three substances of the Glycol Ester category (Fatty acids, C18 and C18 unsatd. epoxidized, ester with ethylene glycol (CAS 151661-88-0); Butylene glycol dicaprylate / dicaprate (CAS 853947-59-8) and Octanoic acid ester with 1,2-propanediol, mono- and di– did not show signs of systemic toxicity, resulting in LD50 values > 2000 mg/kg bw (CAS 31565-12-5, Potokar, 1989; Mürmann, 1992a,b). In addition, irritation and sensitisation studies with ethylene distearate showed no irritating effects to skin and eyes and no signs of systemic toxicity were determined in respective studies (Wnorowski, 1991a,b; Bouffecoux, 1995; Elder, 1982; Jones, 1984; Müller, 1982; Parcell, 1990; Elder, 1982).

Overall, taking into account the physico-chemical properties of ethylene distearate, the QSAR calculation and available toxicological data on ethylene distearate, the dermal absorption potential of the test substance is anticipated to be very low.

 

Inhalation

Ethylene distearate has a very low vapour pressure of 2.36E-15 Pa at 20 °C thus being of low volatility (Nagel, 2011). 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 significant.

However, the substance may be available for respiratory absorption in the lung after inhalation of aerosols, if the formulated 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).

As discussed above, absorption after oral administration of ethylene distearate is driven by enzymatic hydrolysis of the ester bond to the respective metabolites and subsequent absorption of the breakdown products. However, in the respiratory tract the required enzymes for hydrolysis are not expressed as high as in the GI tract.

In addition, the acute inhalation studies with the category member Decanoic acid, mixed esters with octanoic acid and propylene glycol in rats and guinea pigs did not show any mortality or systemic toxicity after inhalative exposure (Re, 1978a,b).

Therefore, inhalative absorption of ethylene distearate is considered to be not higher than through the intestinal epithelium.

Based on the physicochemical properties of ethylene distearate and data on acute inhalation toxicity of the category member Decanoic acid, mixed esters with octanoic acid and propylene glycol (CAS 68583-51-7) the absorption via the lung is expected to be not higher than after oral absorption.

 

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).

As the parent compound ethylene distearate will be hydrolysed before absorption as discussed above, the distribution of the intact ethylene distearate is not relevant but rather the distribution of the breakdown products of intestinal hydrolysis. The absorbed products of hydrolysis, stearic acid and ethylene glycol can both be distributed within the body.

The alcohol ethylene glycol has a low molecular weight and high water solubility. Based on the physico-chemical properties, ethylene glycol will be distributed within the body (ATSDR, 2010; ICPS, 2001). Substances with high water solubility like ethylene glycol do not have the potential to accumulate in adipose tissue due to its low log Pow.

Like all medium and long chain fatty acids, stearic acid may be re-esterified with glycerol into triacylglycerides (TAGs) and transported via chylomicrons. Via these transport vehicles, fatty acids are transported via the lymphatic system and the blood stream to the liver and to extrahepatic tissue for storage e.g. in adipose tissue (Stryer, 1996).

Therefore, the intact parent compound ethylene distearate is not assumed to be accumulated as hydrolysis takes place before absorption and distribution. However, accumulation of the fatty acid stearic acid in triglycerides in adipose tissue or the incorporation into cell membranes is possible as further described in the metabolism section below. At the same time, stearic acid may also be used for energy generation. Thus, stored fatty acids underlie a continuous turnover as they are permanently metabolised and excreted. Bioaccumulation of fatty acids only takes place, if their intake exceeds the caloric requirements of the organism.

In summary, the available information on ethylene distearate indicate that no significant bioaccumulation of the parent substance in adipose tissue is expected. The breakdown products of hydrolysis, ethylene glycol and stearic acid will be distributed in the organism.

 

Metabolism

Metabolism of ethylene distearate occurs initially via enzymatic hydrolysis of the ester resulting in 2-hydroxyethyl stearate and steric acid. 2-Hydroxyethyl stearate can be subsequently hydrolysed into the corresponding free fatty acid stearic acid and ethylene glycol (Elder, 1983; see Figure 2 in attached document).

The hydrolysis of fatty acid esters with ethylene glycol was also confirmed by in-vitro studies using a pancreatic lipase preparation (Noda et al., 1977). In the study, the fatty acid release from ethylene dioleate was comparable to those from the triglyceride trioleoylglycerol, which is the natural substrate of the ubiquitously expressed GI lipases. Furthermore, in-vivo studies in rats with fatty acid esters containing one, two (like ethylene glycol esters) or three ester groups showed that they are rapidly hydrolysed by ubiquitously expressed esterases and almost completely absorbed (Mattson and Volpenheim, 1968; 1972). Furthermore, the in-vivo hydrolysis of propylene glycol distearate (PGDS), a structurally related glycol ester, was studied using isotopically labeled PGDS (Long et al., 1958). Oral administration of PGDS showed intestinal hydrolysis into propylene glycol monostearate, propylene glycol and stearic acid confirming the metabolism of ethylene distearate, as well.

In addition, simulation of intestinal metabolism of ethylene distearate, using the OECD QSAR ToolBox v.2.3.0, resulted in 136 intestinal metabolites including the free fatty acid stearic acid and the monoester 2-hydroxyethyl stearate supporting the metabolism pathway, as well.

Following hydrolysis, absorption and distribution of the alcohol component, ethylene glycol will be metabolised primary in the liver. Ethylene glycol is oxidized in experimental animals and in humans in successive steps, first to glycoaldehyde, catalysed by alcohol dehydrogenase), then to glycolic acid, glyoxylic acid, and oxalic acid. Glyoxylic acid is metabolized in intermediary metabolism to malate, formate, and glycine. Ethylene glycol, glycolic acid, calcium oxalate, glycine and its conjugate, hippurate are excreted in urine. The metabolites of ethylene glycol that have been typically detected are carbon dioxide, glycolic acid, and oxalic acid (WHO, 2002). It has to be considered, that the predicted metabolite ethylene glycol is classified as acutely toxic (oral), category 4, according to Regulation (EC) No 1272/2008, Annex VI (CLP). The effects observed in laboratory animals and humans are due primarily to the actions of one or more of its metabolites, rather than to the parent compound Ethylene glycol (WHO, 2002). Considering the available data on acute toxicity of Ethylene distearate, where doses of 2000 mg/kg bw were administered, and assuming a 100% release of ethylene glycol as a result of the ester hydrolysis; a maximal released dose of 209 mg/kg bw ethylene glycol can be calculated. Published values for the minimum lethal oral dose in humans have ranged from approximately 400 mg/kg body weight to 1300 mg/kg body weight (WHO, 2002). However, the hypothetical maximum available dose of ethylene glycol from the release of the intact ester is lower than the minimum lethal oral dose in humans. Furthermore, respective animal data of the intact esters have shown no acute oral toxicity up to the limit dose of 2000 mg/kg bw.

Following absorption into the intestinal lumen, fatty acids like stearic acid are re-esterified with glycerol to triacylglycerides (TAGs) and included into chylomicrons for transportation via the lymphatic system and the blood stream to the liver. In the liver, fatty acids can be metabolised in phase I and II metabolism. Using the OECD QSAR ToolBox 2.3.0, liver metabolism simulation resulted in 31 metabolites.

An important metabolic pathway for fatty acids is the beta-oxidation for energy generation. In this multi-step process, the fatty acids are at first esterificated 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 (see Figure 3 in attached document). Further oxidation via the citric acid cycle leads to the formation of H2O and CO2 (Lehninger, 1970; Stryer, 1996).

Available genotoxicity data for the parent compound ethylene distearate and structurally related substances of the category do not show any genotoxic properties. An Ames-Test with ethylene distearate (Grötsch, 1996), an in-vitro chromosomal aberration test with Butylene glycol dicaprylate / dicaprate (CAS 853947-59-8; Dechert, 1997), an in-vitro mammalian gene mutation assay with Fatty acids, C16-18, esters with ethylene glycol (CAS 91031-31-1; Verspeek-Rip, 2010) and a micronucleus assay in-vivo with Fatty acids, C18 and C18 unsatd. epoxidized, ester with ethylene glycol (CAS 151661-88-0; Banduhn, 1990) were consistently negative and therefore no indication of a reactivity of Glycol Esters under the test conditions is indicated.

 

Excretion

Based on the metabolism described above, ethylene distearate and its breakdown products will be metabolised in the body to a high extent. In-vivo studies with propylene glycol distearate showed, that 94% of the labeled PGDS was recovered from 14CO2 excretion and only ~ 0.4% of the total dose of PGDS were excreted in the urine after 72 h supporting this notion as well (Long et al., 1958).

The fatty acid component stearic acid, will be metabolised for energy generation or stored as lipid in adipose tissue or used for further physiological properties e.g. incorporation into cell membranes (Lehninger, 1970; Stryer, 1996). Therefore, the fatty acid component is not expected to be excreted to a significant degree via the urine or faeces. As ethylene glycol will be highly metabolised as well, the primary route of excretion will be via exhaled air as CO2 and as parent compound and glycolic acid in the urine. Higher doses of ethylene glycol lead to the excretion of the metabolite oxalate via the urine (ATSDR, 2010).

 

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