Fatty acids, C16-18, esters with diethylene glycol

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 target substance Fatty acids, C16-18, esters with diethylene glycol (CAS 85116-97-8). However, the ester will be hydrolysed in the gastrointestinal tract to the respective fatty acids moieties (mainly C16 and C18) and diethylene 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 rises. The second hydrolysis product, diethylene glycol (DEG) is oxidised by alcohol dehydrogenases and aldehyde dehydrogenases leading to formation of CO2, 2-HEAA, and oxalic acid. However, the major part of absorbed DEG is expected to be excreted in the urine unchanged. The remaining fraction will most probably be excreted as 2-HEAA in the urine and hence, no bioaccumulation of DEG is expected.

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, esters with diethylene glycol (CAS 85116-97-8) has been investigated.

Therefore, in accordance with Annex VIII, Column 1, Section 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, 2014), assessment of the toxicokinetic behaviour of Fatty acids, C16-18, esters with diethylene 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, 2014) and taking into account further available information on structural analogue substances.

The substance Fatty acids, C16-18, esters with diethylene glycol is a multi-constituent substance specified by mainly C16 and C18 linear fatty acids esterified with diethylene glycol (DEG) resulting in mono- and diesters which meets the definition of an UVCB substance.

Fatty acids, C16-18, esters with diethylene glycol is a solid at 20°C which has a molecular weight ranging from 344.53 – 639.04 g/mol and a water solubility of < 1 µg/L (Frischmann, 2014). The calculated log Pow value is > 6 (Müller, 2014) and the vapour pressure is calculated to be < 1E-5 Pa at 20 °C (SPARC v4.6) (Nagel, 2014).

 

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, esters with diethylene glycol 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; National technical information service, 1973). 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 DEG and the corresponding fatty acids, mostly C16 and C18.

The low water solubility (< 1 µg/L) and the high log Pow value >6 of the parent compound indicate that 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). Regarding molecular weight, the breakdown products DEG (106.12 g/mol) and hexadecanoic or octadecanoic acid (256.4 or 284.5 g/mol, respectively) are generally favourable for absorption. The alcohol component DEG is rapidly and almost completely absorbed by laboratory animals via oral routes as would be expected from their total miscibility with water (NICNAS, 2009; SIAM, 2004, SCCP, 2008). The highly lipophilic fatty acids are absorbed by micellar solubilisation. Within the epithelial cells, fatty acids are (re)-esterified with glycerol to triglycerides.

Moreover, a study on acute oral toxicity of Fatty acids, C16-18, esters with diethylene glycol showed no signs of systemic toxicity resulting in a LD50 value > 2000 mg/kg bw (Dufour, 1994). Furthermore, available data on subacute oral toxicity of the analogue substance Fatty acids, C18 and C18 unsatd. epoxidized, ester with ethylene glycol (CAS 151661-88-0) showed no adverse systemic effects resulting in a NOAEL of 1000 mg/kg bw/day (Pitterman, 1991). The lack of systemic toxicity of the target substance and the structurally related analogue substance cannot be equated with a lack of absorption but rather with a low toxic potential of ethylene glycol esters and the breakdown products themselves.

Overall, limited absorption is considered for the parent substance whereas the breakdown products of hydrolysis are expected to be readily absorbed.

Dermal

There are no data available on dermal absorption or on acute dermal toxicity of Fatty acids, C16-18, esters with diethylene glycol. On the basis of the following considerations, the dermal absorption of the substance is considered to be low.

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 having an octanol/water partition coefficient above 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 solid, hindered dermal absorption has to be considered as dry particulates first have to dissolve into the surface moisture of the skin before uptake vie the skin is possible (ECHA, 2014).

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 (please refer to Table 1). QSAR calculations confirmed this assumption, as low dermal flux rates ranging from 2.8E-4 - 1.5E-9 mg/cm2 per h were calculated indicating only low dermal absorption potential for the components of Fatty acids, C16-18, esters with diethylene glycol (please refer to Table 1, Dermwin v2.02, EpiSuite 4.1; Müller, 2014).

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

Component	Structural formula	Flux (mg/cm2/h)
Hexadecanoic acid, monoester	C20 H40 O4	1.8E-4
Hexadecanoic acid, diester	C36 H70 O5	1.2E-8
Octadecanoic acid, monoester	C22 H44 O4	6.7E-5
Octadecanoic acid, diester	C40 H78 O5	1.5E-9

 

Moreover, the irritation studies performed with Fatty acids, C16-18, esters with diethylene glycol (CAS 85116-97-8) and ethylene distearate (CAS 627-83-8) showed no or only mild irritating effects (Dufour, 1994).

Overall, taking into account the physico-chemical properties of Fatty acids, C16-18, esters with diethylene glycol, the QSAR calculations and available toxicological data, the dermal absorption potential of the substance is anticipated to be low.

 

Inhalation

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).

Inhalation of Fatty acids, C16-18, esters with diethylene glycol is considered as negligible as the test substance is used only as pastilles with intentional diameters of 5-8 mm. Thus, the contained particles are far above the inhalable size. Moreover, the test substance has a very low calculated vapour pressure of <1E-5 Pa thus being of low volatility (Nagel, 2014). 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.

Based on the physical state and the physico-chemical properties of Fatty acids, C16-18, esters with diethylene glycol, absorption via the lung is expected as negligible.

 

Distribution and accumulation

Distribution of a compound within the body depends on the physico-chemical 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, esters with diethylene glycol will be hydrolysed prior to absorption (as discussed above); the distribution of the intact substance is not relevant but rather the distribution of the breakdown products of hydrolysis. The absorbed products of hydrolysis, DEG and the respective fatty acid moieties can be distributed within the body.

The alcohol DEG has a low molecular weight and high water solubility. Based on the physico-chemical properties, DEG will be distributed within the body (ICPS, 1997). Upon absorption, DEG is well distributed throughout the aqueous tissues of the body with lower concentrations in adipose tissues. After gavage dosing of 14C-DEG in rats, radioactivity was rapidly distributed from the blood into the organs and tissues in the order kidneys > brain > spleen > liver > muscle > fat (i.e. the same order as the blood flow) (NICNAS, 2009; SCCP, 2008).

Like all medium and long chain fatty acids, the fatty acids may be re-esterified with glycerol into triacylglycerides (TAGs) and transported via chylomicrons or absorbed from the small intestine directly into the bloodstream and transported to the liver. Via chylomicrons, 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, 1994).

Therefore, the intact parent compound is not assumed to accumulate as hydrolysis takes place before absorption and distribution. However, accumulation of the fatty acids 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, fatty acids 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 Fatty acids, C16-18, esters with diethylene glycol indicates that no significant bioaccumulation of the parent substance is expected. The breakdown products of hydrolysis, DEG and the respective fatty acids will be distributed within the organism. Bioaccumulation of the breakdown products is considered to be limited (pleased refer to “Metabolism” and “Excretion”).

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, esters with diethylene glycol is expected to be readily hydrolysed at the ester bond resulting in release of DEG and the respective fatty acid moieties (mainly C16 and C18).

The hydrolysis of fatty acid esters in-vivo was studied in rats dosed with fatty acid esters containing one, two (like propylene glycol esters) or three ester groups. The studies showed that fatty acid esters with two ester groups are rapidly hydrolysed by ubiquitously expressed esterases and almost completely absorbed (Mattson and Volpenheim, 1968; 1972). 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 Fatty acids, C16-18, esters with ethylene glycol, as well.

Following hydrolysis, absorption and distribution of the alcohol component, the postulated pathway for metabolism of DEG is oxidation via alcohol dehydrogenases and aldehyde dehydrogenases (ADH/ALD). Identified DEG metabolites include CO2, 2-(hydroxyethoxy)acetic acid (2-HEAA), and oxalic acid. In rats, oxalic acid is not a significant metabolite (NICNAS; 2009; SIAM, 2004). Instead, 2-hydroxyethoxyacetic acid (HEAA) was the primary metabolite in the urine, with only minor amounts of urinary diglycolic acid (DGA). Small amounts of ethylene glycol (EG), but not oxalate or glycolate, were observed in the urine. About 70–80% of the radiolabeled DEG was excreted in the urine unchanged and another 10–30% was determined to be urinary 2-hydroxyethoxyacetic acid (HEAA). Ethylene glycol (EG) and its metabolites, glycolic acid, glyoxylic acid and oxalic acid, were not detected in any of these studies. Further evidence that DEG does not undergo ether cleavage to become two EG molecules has been reported in DEG-intoxicated patients because oxalate crystals, a hallmark of EG poisoning, are not found in the urine or kidney tissues (as summarized in Besenhofer et al., 2010)

Although the predicted metabolite DEG is classified as acutely toxic (oral), category 4, according to Regulation (EC) No 1272/2008, Annex VI, the available data on acute toxicity do not indicate intrinsic hazardous properties of Fatty acids, C16-18, esters with diethylene glycol after single exposure up to the limit dose of 2000 mg/kg bw. Therefore, the anticipated formation of the hydrolysis product DEG does not account for displayed acute toxicity of the parent substance.

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 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, 1994).

Available genotoxicity data from structural related analogue substances do not show any genotoxic properties. In particular, Ames-tests with Fatty acids, C16-18, esters with ethylene glycol (Banduhn, 1991) and Octadecanoic acid, monoester with 1,2-propanediol (CAS 1323-39-3) and Palmitic acid, monoester with propane-1,2-diol (CAS 29013-28-3) (Spruth, 2015), an in-vitro chromosomal aberration test with C8-C10-1,3-Butandiolester (CAS 853947-59-8; Dechert, 1997) and an in-vitro mammalian gene mutation assay with Fatty acids, C16-18, esters with ethylene glycol (CAS 91031-31-1; Verspeek-Rip, 2010) were consistently negative and therefore no indication of a genotoxic reactivity of structurally related glycol esters is indicated.

 

Excretion

Based on the metabolism described above, Fatty acids, C16-18, esters with diethylene glycol and its breakdown products will be metabolised in the body. In-vivo studies with propylene glycol distearate (PGDS) 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). A similar observation was made for propylene glycol, which was excreted in substantial amounts as 14CO2 during the first 24 h after administration of radioactive label (National technical information service, 1973).

The fatty acid components 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, 1994). Therefore, the fatty acid component is not expected to be excreted to a significant degree via the urine or faeces but excreted via exhaled air as CO2 or stored as described above.

In contrast, approximately 40-70% of the total DEG dose is excreted unchanged in the urine within 48 hours, depending on the dose administered. Approximately 11-37% are excreted as 2-HEAA. Excretion in the faeces accounts for minor amounts, between 0.7%-2.2% of the total dose (NICNAS, 2009; SCCP, 2008).

References

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Besenhofer, et al. 2010. Inhibition of Metabolism of Diethylene Glycol Prevents Target Organ Toxicity in Rats. Toxicological Sciences, 117 (1): 25 – 35.

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Mattson, F.H. and Volpenheim, R.A. (1972). Absorbability by rats of compounds containing from one to eight ester groups. J Nutrition, 102: 1171 -1176

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Stryer, L. (1994): Biochemie. 2nd revised reprint, Heidelberg; Berlin; Oxford: Spektrum Akad. Verlag.

Müller,T. (2014). Fatty acids, C16-18, esters with diethylene glycol (CAS 85116-97-8). EPIsuite 4.11 calculation with Fatty acids, C16-18, esters with diethylene glycol. Dr. Knoell Consult GmbH. Report Number: 10124-mue-20140515.