Fatty acids, C16-18, 2-hydroxyethyl esters

Endpoint summary

Description of key information

Dermal and inhalation absorption of the parent compound is considered to be low. Limited absorption is considered after oral ingestion for the non-metabolised substance Fatty acids, C16-18, 2-hydroxyethyl esters (CAS 97281-23-7). However, the ester will be hydrolysed in the gastrointestinal tract to the respective fatty acids moieties (mainly C16 and C18) and ethylene glycol. The breakdown products of hydrolysis are expected to be readily absorbed in the gastrointestinal tract after oral administration. The fatty acids will most likely be re-esterified to triglycerides after absorption and transported via chylomicrons. The major metabolic pathway for linear and branched fatty acids is the beta-oxidation pathway for energy generation, while alternatives are the omega-pathway or direct conjugation to more polar products. The excretion will mainly be as CO₂ in expired air; with a smaller fraction excreted in the urine. No bioaccumulation will take place, as excess triglycerides are stored and used as the energy need arises. The second hydrolysis product ethylene glycol will be distributed within the body, but it does not have the potential to accumulate in adipose tissue. Ethylene glycol will be metabolised primary in the liver mainly to carbon dioxide, glycolic acid and oxalic acid. Non-metabolised ethylene glycol and glycolic acid are excreted in urine.

Key value for chemical safety assessment

Bioaccumulation potential:

no bioaccumulation potential

Additional information

Basic toxicokinetics

 

There are no studies available in which the toxicokinetic behaviour of Fatty acids, C16-18, 2-hydroxyethyl esters (CAS 97281-23-7) has been investigated.

 

Therefore, 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, 2014), an assessment of the toxicokinetic behaviour of the substance Fatty acids, C16-18, 2-hydroxyethyl esters 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 the Guidance on information requirements and chemical safety assessment Chapter R.7c: Endpoint specific guidance (ECHA, 2014) and taking into account further available information on the target and source substances.

 

The substance Fatty acids, C16-18, 2-hydroxyethyl esters is a UVCB substance specified by mainly C16 and C18 linear fatty acids esterified with ethylene glycol resulting in mono- and diesters.

 

Fatty acids, C16-18, 2-hydroxyethyl esters is a solid and has a molecular weight of 300.48 – 594.99 g/mol and a water solubility of ≤ 5 µg/L at 20°C (Schwarzkopf, 2015). The log Pow is calculated to be > 6 (Adaktylou, 2016) and the vapour pressure is calculated to be < 0.0001 Pa at 20 °C (Adaktylou, 2016).

 

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, 2014).

 

Oral

In general, molecular weights below 500 and log Pow values between -1 and 4 are favourable for absorption via the gastrointestinal (GI) tract, provided that the substance is sufficiently water soluble (> 1 mg/L). Lipophilic compounds may be taken up by micellar solubilisation by bile salts, but this mechanism may be of particular importance for highly lipophilic compounds (log Pow > 4), in particular for those that are poorly soluble in water (≤ 1 mg/L) as these would otherwise be poorly absorbed (Aungst and Shen, 1986; ECHA, 2014).

 

When assessing the potential of Fatty acids, C16-18, 2-hydroxyethyl esters to be absorbed in the gastrointestinal (GI) tract, it has to be considered that fatty acid esters will undergo extensive hydrolysis by ubiquitous expressed GI enzymes (Long, 1958; Lehninger, 1970; Mattson and Volpenhein, 1972). Thus, due to the hydrolysis, not only the predictions based on the physico-chemical characteristics of the intact parent substance will apply, but also the physico-chemical characteristics of the breakdown products of the ester; the alcohol and the fatty acid moieties.

The molecular weight range of Fatty acids, C16-18, 2-hydroxyethyl esters indicates that absorption of the parent substance, especially the smaller components with a molecular weight < 500 g/mol, is possible. However, due to the low water solubility and the high log Pow, absorption in the gastrointestinal tract may be limited by the inability of the parent substance to dissolve into GI fluids. Thus, 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 the alcohol moiety (ethylene glycol, 62.07 g/mol) and the respective fatty acids (C16 (256.44 g/mol) or C18 (284.48 g/mol) fatty acids) suggest facilitated absorption in the gastrointestinal tract. Furthermore, highly lipophilic long chain fatty acids 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. Therefore, solution in the gastrointestinal fluids is expected to occur, and ethylene glycol will be readily absorbed through the gastrointestinal tract (ATSDR, 2010; IPCS, 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 labelled stearic acid mixture of glyceride esters (Long, 1958).

 

Studies with the analogue substances 2-hydroxyethyl stearate and Ethylene distearate showed no signs of systemic toxicity after oral ingestion (Dufour, 1994, Gloxhuber, 1982, Wnorowski, 1991).The lack of short- and long-term systemic toxicity cannot be equated with a lack of absorption or with absorption but it indicates a low toxic potential of the test substance and the breakdown products themselves.

 

Overall, limited absorption of the parent compound is expected whereas rapid absorption is expected for the products of hydrolysis.

 

Dermal

 

To partition from the stratum corneum into the epidermis, a substance must be sufficiently soluble in water. Thus, with a water solubility < 1 µg/L, dermal uptake of the substance is likely to be low. In addition, for substances with an octanol/water partition coefficient > 6, the rate of transfer between the stratum corneum and the epidermis will be slow and thus limit absorption across the skin. Furthermore, uptake into the stratum corneum itself may be slow. In addition, as the test substance is a waxy solid, limited dermal absorption has to be considered as dry particulates first have to dissolve into the surface moisture of the skin before uptake via the skin is possible (ECHA, 2014).

 

There are no data available on dermal absorption or on acute dermal toxicity of Fatty acids, C16-18, 2-hydroxyethyl esters. Regarding the molecular weight range of 300.48 – 594.99 g/mol and a calculated octanol/water partition coefficient > 6 (Adaktylou, 2016) in combination with the low water solubility, a low dermal absorption rate is anticipated. As log Pow values > 6 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.

 

The dermal permeability coefficient (Kp) can be calculated from log Pow and molecular weight (MW) applying the following equation described in US EPA (2014):

log(Kp) = -2.80 + 0.66 log Pow – 0.0056 MW

 

The Kp was calculated for the 4 main constituents of the target substance. QSAR calculations confirmed low dermal flux rates ranging from 1.5E-4 - 3.5E-9 mg/cm2 per h indicating only low dermal absorption potential (please refer to Table 1, Dermwin v2.02, EpiSuite 4.1; Adaktylou, 2016).

 

Table 1: Dermal absorption values for the components of Fatty acids, C16-18, 2-hydroxyethyl esters (calculated with Dermwin v 2.02, Epiweb 4.1)

Component	Structural formula	Flux (mg/cm2/h)
C16, monoester	C18 H36 O3	3.9E-4
C16, diester	C34H66O2	2.8E-8
C18, monoester	C2 H40 O3	1.5E-4
C18, diester	C38H74O2	3.5E-9

 

 

Furthermore, the structural analogue substances Fatty acids, C16-18, esters with ethylene glycol and 2-hydroxyethyl stearate did not exhibit irritating properties towards the skin which might enhance dermal absorption (Coguet, 1976, Dufour, 1994).

 

Overall, taking into account the physico-chemical properties of Fatty acids, C16-18, 2-hydroxyethyl esters, the QSAR calculation and available toxicological data, the dermal absorption potential of the test substance is anticipated to be low.

 

Inhalation

 

Fatty acids, C16-18, 2-hydroxyethyl esters has a very low vapour pressure of < 0.0001 Pa at 20 °C (calculated) and therefore a low volatility (Adaktylou, 2016). 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, 2014).

 

As discussed above, absorption after oral administration of Fatty acids, C16-18, 2-hydroxyethyl esters is driven by enzymatic hydrolysis of the ester bond to the respective metabolites and subsequent absorption of the breakdown products. The presence of esterases and lipases in the mucus lining fluid of the respiratory tract would therefore be essential for absorption. However, due to the physiological function of esterases and lipases in the context of nutrient absorption, the esterase and lipase activity in the lung is expected to be lower in comparison to the gastrointestinal tract. Thus, hydrolysis and subsequent absorption in the respiratory tract is considered to be less effective, compared with that in the gastrointestinal tract.

 

In addition, the acute inhalation studies with the analogue substance 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 Fatty acids, C16-18, 2-hydroxyethyl esters is considered not to be higher than through the intestinal epithelium.

 

Based on the physicochemical properties of Fatty acids, C16-18, 2-hydroxyethyl esters and data on acute inhalation toxicity of the structural analogue Decanoic acid, mixed esters with octanoic acid and propylene glycol, absorption via the lung is not expected to be 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, 2014).

 

As the parent compound Fatty acids, C16-18, 2-hydroxyethyl esters will mainly be hydrolysed prior to absorption, as discussed above, the distribution of the parent compound is less relevant than that of the breakdown products of intestinal hydrolysis.

 

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 their low log Pow.

 

Like all medium- and long chain fatty acids, palmitic and 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).

 

The intact parent compound Fatty acids, C16-18, 2-hydroxyethyl esters is not assumed to accumulate as hydrolysis takes place before absorption and distribution. However, accumulation of the fatty acid moieties 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, the fatty acids may also be used for energy generation. Thus, there is a continuous turnover of stored fatty acids, 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 Fatty acids, C16-18, 2-hydroxyethyl esters indicates that no significant bioaccumulation of the parent substance is expected. The breakdown products of hydrolysis will be distributed in the organism.

 

Metabolism

 

In general, glycol fatty acid esters are stepwise hydrolysed at the ester bonds by gastrointestinal enzymes resulting in the release of the fatty acid component and the respective alcohol moiety (Long, 1958; Lehninger, 1970; Mattson and Volpenhein, 1972). Therefore, Fatty acids, C16-18, 2-hydroxyethyl esters is expected to be readily hydrolysed at the ester bond resulting in release of ethylene glycol and the respective fatty acid moieties (mainly C16 and C18).

 

The hydrolysis of fatty acid esters with ethylene glycol was also confirmed by in-vitro studies using a pancreatic lipase preparation (Noda et al., 1978). 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). The in-vivo hydrolysis of propylene glycol distearate (PGDS), a structurally related glycol ester, was studied using isotopically labelled PGDS (Long et al., 1958). Oral administration of PGDS was shown to result in intestinal hydrolysis into propylene glycol monostearate, propylene glycol and stearic acid, indicating thatFatty acids, C16-18, 2-hydroxyethyl esters will be similarly metabolised.

 

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 2-hydroxyethyl stearate and 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 414 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 bw to 1300 mg/kg bw (WHO, 2002). However, although the hypothetical maximum available dose of ethylene glycol from the release of the intact ester is close to the minimum lethal oral dose in humans, the respective animal data of the intact esters including ethylene glycol have shown no acute oral toxicity up to the limit dose of 2000 mg/kg bw.

 

Following absorption into the intestinal lumen, fatty acids 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. 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 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, 1970; Stryer, 1996).

 

Available genotoxicity data for the parent compound and structurally related analogue substances do consistently not show any genotoxic properties. The Ames Test performed with Fatty acids, C16-18, 2-hydroxyethyl esters (Verbaan, 2016), the Mouse Lymphoma Assay with Fatty acids, C16-18, esters with ethylene glycol (Verspeek-Rip, 2010), and the in-vitro chromosomal aberration test with Butylene glycol dicaprylate / dicaprate (Dechert, 1997) were consistently negative and therefore no indication of a reactivity is indicated.

 

Excretion

 

Based on the metabolism described above, Fatty acids, C16-18, 2-hydroxyethyl esters and its breakdown products will be extensively metabolised in the body. This view is supported by in-vivo studies with propylene glycol distearate, which showed that 94% of the labelled PGDS was recovered from 14CO2 excretion and only ~ 0.4% of the total dose of PGDS was excreted in the urine after 72 h (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).

 

 

References

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

 

Stryer, L. (1996). Biochemistry. Fourth edition. Spektrum Akad. Verl.

 

WHO (2002a): Ethylene Glycol: Human Health Aspects.Concise International Chemical Assessment Document 45.