2-hydroxyethyl palmitate

Endpoint summary

Additional information

Assessment of the Toxicokinetic Behaviour

of 2-Hydroxyethyl palmitate (CAS 4219-49-2)

 

Justification for grouping of substances and read-across

The Glycol ester category covers esters of an aliphatic diol (ethylene glycol (EG), propylene glycol (PG) or 1,3-butyleneglycol (1,3-BG)) and one or two carboxylic fatty acid chains. The fatty acid chains comprise carbon chain lengths ranging from C6 to C18, mainly saturated but also mono unsaturated C16 and C18, branched C18 and epoxidized C18.Fatty acid esters are generally produced by chemical reaction of an alcohol (e.g. ethylene glycol) with an organic acid (e.g. stearic acid) in the presence of an acid catalyst (Radzi et al., 2005). The esterification reaction is started by a transfer of a proton from the acid catalyst to the alcohol to form an alkyloxonium ion. The acid is protonated on its carbonyl oxygen followed by a nucleophilic addition of a molecule of the alcohol to a carbonyl carbon of acid. An intermediate product is formed. This intermediate product loses a water molecule and a proton to give an ester (Liu et al, 2006; Lilja et al., 2005; Gubicza et al., 2000; Zhao, 2000). Di- and/or monoesters are the final products of esterification of an aliphatic diol and fatty acids.

 

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 for human toxicity, 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 Glycol esters.

CAS	EC name	Molecular weight	Carbon number in Acid	Carbon number in dihydroxy alcohol	Total Carbons in Glycol Esters
CAS 111-60-4 (b)	Glycol stearate	MW 328.53	C18	C2	C20
CAS 624-03-3 (a)	Ethane-1,2-diyl palmitate	MW 538.89	C16	C2	C34
CAS 627-83-8	Ethylene distearate	MW 563.0	C18	C2	C38
CAS 91031-31-1	Fatty acids, C16-18, esters with ethylene glycol	MW 300.48 - 563.00	C16-18	C2	C18-38
CAS 151661-88-0	Fatty acids, C18 and C18 unsatd. epoxidized, ester with ethylene glycol	MW 328.54 - 622.97	C18	C2	C20-38
CAS 29059-24-3	Myristic acid, monoester with propane-1,2-diol	MW 286.45	C14	C3	C17
CAS 1323-39-3	Stearic acid, monoester with propane-1,2-diol	MW 342.55	C18	C3	C21
CAS 37321-62-3	Dodecanoic acid, ester with 1,2-propanediol	MW 258.40 - 440.71	C12	C3	C15-27
CAS 68958-54-3	1-methyl-1,2-ethanediyl diisooctadecanoate	MW 609.03	C18	C3	C39
CAS 31565-12-5	Octanoic acid ester with 1,2-propanediol, mono- and di-	MW 202.29 - 328.49	C8	C3	C11-19
CAS 85883-73-4	Fatty acids, C6-12, esters with propylene glycol	MW 202.29 - 440.71	C6-12	C3	C9-27
CAS 68583-51-7	Decanoic acid, mixed diesters with octanoic acid and propylene glycol	MW 328.49 - 384.59	C8-10	C3	C19-23
CAS 84988-75-0	Fatty acids, C14-18 and C16-18-unsatd., esters with propylene glycol	MW 286.46 - 609.02	C14-18	C3	C17-39
CAS 853947-59-8	Butylene glycol dicaprylate / dicaprate	MW 342.52 - 398.63	C8-10	C4	C20-24
CAS 4219-49 -2	2 -hydroxyethyl palmitate	MW 300.483 -538.89	C16	C2	C18 -34

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

(b) 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.

 

Grouping of substances into this category is based on:

 

(1) Common functional groups: The substances of the category are characterized by ester bond(s) between an aliphatic diol (ethylene glycol (EG), propylene glycol (PG) or 1,3-butyleneglycol (1,3-BG)) and one or two carboxylic fatty acid chains. The fatty acid chains comprise carbon chain lengths ranging from C6 to C18, mainly saturated but also mono unsaturated C16 and C18, branched C18 and epoxidized C18, are included into the category; and

 

(2) common precursors and the likelihood of common breakdown products via biological processes, which result in structurally similar chemicals: glycol esters are expected to be initially metabolized via enzymatic hydrolysis in the corresponding free fatty acids and the free glycol alcohols such as ethylene glycol and propylene glycol. The hydrolysis represents the first chemical step in the absorption, distribution, metabolism and excretion (ADME) pathways expected to be similarly followed by all glycol esters. The hydrolysis is catalyzed by classes of enzymes known as carboxylesterases or esterases (Heymann, 1980). Ethylene and propylene glycol are rapidly absorbed from the gastrointestinal tract and subsequently undergo rapid biotransformation in liver and kidney (ATSDR, 1997; ICPS, 2001; WHO, 2002; ATSDR, 2010). Propylene glycol will be further metabolized in liver by alcohol dehydrogenase to lactic acid and pyruvic acid which are endogenous substances naturally occurring in mammals (Miller & Bazzano, 1965, Ritchie, 1927). Ethylene glycol is first metabolised by alcohol dehydrogenase to glycoaldehyde, which is then further oxidized successively to glycolic acid, glyoxylic acid, oxalic acids by mitochondrial aldehyde dehydrogenase and cytosolic aldehyde oxidase (ATSDR, 2010; WHO, 2002). The anabolism of fatty acids occurs in the cytosol, where fatty acids esterified into cellular lipids that are the most important storage form of fatty acids (Stryer, 1994). The catabolism of fatty acids occurs in the cellular organelles, mitochondria and peroxisomes via a completely different set of enzymes. The process is termed ß-oxidation and involves the sequential cleavage of two-carbon units, released as acetyl-CoA through a cyclic series of reaction catalyzed by several distinct enzyme activities rather than a multienzyme complex (Tocher, 2003); and

 

(3) constant pattern in the changing of the potency of the properties across the category:

(a) Physico-chemical properties: The physico-chemical properties of the category members are similar or follow a regular pattern over the category. The pattern observed depends on the fatty acid chain length and the degree of esterification (mono- or diesters). The molecular weight of the category members ranges from 202.29 to 622.97 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, mono- and diesters of short-chain fatty acids and unsaturated fatty acids (C6-14 and C16:1, C18:1) as well as diesters of branched fatty acids (C18iso) are liquid, while mono- and diesters of long-chain fatty acids are waxy solids. All category members are non-volatile (vapour pressure: ≤ 16 Pa). The octanol/water partition coefficient increases with increasing fatty acid chain length and number of ester bonds, ranging from log Kow = 1.78 (C6 PG monoester component) to log Kow >10 (C12 PG diester component). The water solubility decreases accordingly (624.3 mg/L for C6 PG monoester component to <1 mg/L for C18 PG diester component); and

 

(b) Environmental fate and ecotoxicological properties: Considering the low water solubility and the potential for adsorption to organic soil and sediment particles, the main compartment for environmental distribution is expected to be the soil and sediment. Nevertheless, persistency in these compartments is not expected since the members of the Glycol Esters Category are rapidly biodegradable. 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. All members of the category are rapidly biodegradable and did not show any effects on aquatic organisms in acute and chronic tests representing the category members up to the limit of water solubility. Moreover, bioaccumulation is assumed to be low based on metabolism data.

 

(c) Toxicological properties: The toxicological properties show that all category members have a similar toxicokinetic behaviour (hydrolysis of the ester bond before absorption followed by absorption and metabolism of the breakdown products) and that the constant pattern consists in a lack of potency change of properties across the category, explained by the common metabolic fate of glycol esters independently of the fatty acid chain length and degree of glycol substitution. Thus, no category member showed acute oral, dermal or inhalative toxicity, no skin or eye irritation properties, no skin sensitisation, are of low toxicity after repeated oral exposure and are not mutagenic or clastogenic and have shown no indications for reproduction toxicity and have no effect on intrauterine development.

 

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 and 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

There are no studies available in which the toxicokinetic behaviour of 2 -hydroxyethyl pamitate 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, 2012c), assessment of the toxicokinetic behavior of the substance Fatty acids, C16-18, esters with ethylene glycol 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, 2012c) and taking into account further available information on the Glycol Ester category.

The substance 2-hydroxyethyl palmitate is a multiconstituent substance including mono- and diesters of ethylene glycol with palmitic acid. The chemical structure of

2-hydroxyethyl palmitate is

shown in the category justification document attached to IUCLID (chapter 13).

2-hydroxyethyl palmitate

is a solid and has a molecular weight of 300.48 – 538.89 g/mol and a water solubility of < 1 mg/L at 20°C. The log Pow is calculated to be 7 - 16 and the vapour pressure is 14 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, 2012c).

Oral

When assessing the potential of 2 -hydroxyethyl palmitate 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 acids palmitic and the monoester 2-hydroxyethyl palmitate. The molecular weight range of diesters of 2 -hydroxyethyl palmitate indicates that absorption may be possible, at least for the smaller components with a molecular weight smaller than 500 g/mol. Furthermore, the low water solubility and the high log Pow values of the components 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 palmitate (300.49 g/mol), ethylene glycol (62.07 g/mol) and palmitic (256.44 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 Fatty acids, C16-18, esters with ethylene glycol, which is structurally very close to 2-hydroxyethyl palmitate, after oral administration to rats, showed no signs of systemic toxicity in a prenatal developmental toxicity study, resulting in a NOAEL greater than 900 mg/kg bw/day (Pittermann, 1997).The lack of short- and long-term systemic toxicity of 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 2-hydroxyethyl palmitate. On the basis of the following considerations, the dermal absorption of 2 -hydroxyethyl palmitate is considered to be low. Regarding the molecular weight range of 300.48 – 538.89 g/mol and a calculated octanol/water partition coefficient of 7-16 in combination with the low water solubility, a low dermal absorption rate is anticipated. Log Pow values above 7, 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 for the analogue substance ethylene distearate (CAS 627-83-8) using EPIwebv4.1 confirmed this assumption, resulting in a very low Dermal Flux of 4.63E-9 mg/cm2 per h.In addition, 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- (CAS 31565-12-5, Potokar, 1989; Mürmann, 1992a,b) showed no systemic toxicity.

Furthermore, available data from irritation studies with Fatty acids, C16-18, esters with ethylene glycol showed no no signs of systemic toxicity (Gattefosse, 1976).

Overall, taking into account the physico-chemical properties of 2-hydroxyethyl palmitate, the QSAR calculation and available toxicological data on 2-hydroxyethyl palmitate and further category members, the dermal absorption potential of the test substance is anticipated to be low.

 

Inhalation

2-hydroxyethyl palmitate has a low vapour pressure of 16 Pa at 20 °C and a high boiling point above 300°C, 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 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, 2012).

As discussed above, absorption after oral administration of 2-hydroxyethyl palmitate 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. However, due to the physiological function in the context of nutrient absorption, esterase and lipase activity in the lung is expected to be lower in comparison to the gastrointestinal tract. Thus, hydrolysis comparable to that in the gastrointestinal tract and subsequent absorption in the respiratory tract is considered to be less effective.

 

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 2-hydroxyethyl palmitate is considered to be not higher than through the intestinal epithelium.

Based on the physicochemical properties of 2-hydroxyethyl palmitate 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, 2012).

As the parent compound 2-hydroxyethyl palmitate will be hydrolysed before absorption as discussed above, the distribution of the intact 2-hydroxyethyl palmitate is not relevant but rather the distribution of the breakdown products of intestinal hydrolysis. The absorbed products of hydrolysis can all 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, palmitic 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 2-hydroxyethyl palmitate is not assumed to be accumulated as hydrolysis takes place before absorption and distribution. However, accumulation of the fatty acid palmitic 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, palmitic 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 2-hydroxyethyl palmitate indicates that no significant bioaccumulation of the parent substance in adipose tissue is expected. The breakdown products of hydrolysis will be distributed in the organism.

 

Metabolism

Metabolism of 2-hydroxyethyl palmitate occurs initially via enzymatic hydrolysis of the ester resulting in 2-hydroxyethyl palmitate and palmitic acid. The monoester can be subsequently hydrolysed into the corresponding free fatty acid and ethylene glycol (Elder, 1983).

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 2-hydroxyethyl palmitate, 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 Fatty acids, C16-18, esters with ethylene glycol (structurally very close to 2-hydroxyethyl palmitate), 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 palmitic 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. 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. 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 substances of the category do not show any genotoxic properties. An Ames-Test with 2-hydroxyethyl palmitate, an Ames test and a Mouse Lymphoma Assay with Fatty acids, C16-18, esters with ethylene glycol (Banduhn,1991; Verspeek-Rip, 2010)), an in-vitro chromosomal aberration test with Butylene glycol dicaprylate / dicaprate (CAS 853947-59-8; Dechert, 1997) 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, 2-hydroxyethyl palmitate 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 palmitic 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).

 

For a detailed reference list please refer to the CSR or IUCLID section 13.