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Endpoint summary

Description of key information

Based on the available information, the physico-chemical properties and molecular weight of glycerides, C16-18 (even numbered) mono- and di- and their citrates suggest limited systemic absorption via oral, dermal and inhalation routes. However, the substance is expected to undergo enzymatic hydrolysis in the gastrointestinal tract and absorption of the ester hydrolysis products rather than the parent substance is likely. The absorption rate of the hydrolysis products is considered to be moderate/high. No significant bioaccumulation of the parent substance or its hydrolysis products in adipose tissue is expected.  The distribution of the intact parent compound within the body is expected to be low, but the cleavage products, diglycerol, citric acid and fatty acids, will be distributed in the organism. The fraction that is not absorbed in the gastrointestinal tract, will be excreted in the faeces. The hydrolysis product glycerol is a polar molecule and can readily be excreted in the urine. The fatty acid components will be metabolised in the body for energy generation, and on the basis of the extensive metabolism, the primary route of excretion will be in the form of CO2.

Key value for chemical safety assessment

Bioaccumulation potential:

no bioaccumulation potential

Additional information

Basic toxicokinetics

No toxicokinetic studies are available for glycerides, C16-18 (even numbered) mono- and di- and their citrates (EC 701 -358 -7). 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, 2017), assessment of the toxicokinetic behaviour of glycerides, C16-18 (even numbered) mono- and di- and their citrates was conducted to the extent that can be derived from the relevant available information on physicochemical and toxicological characteristics. There are no studies evaluating the toxicokinetic properties of the substance available.

 

Physico-chemical properties

Glycerides, C16-18 (even numbered) mono- and di- and their citrates is a UVCB substance. It is a white solid with poor water solubility (129 µg/L at 20°C and pH 6.3) and a molecular weight range of 330.5 – 817.1 g/mol, a log Pow range of 5.57 to > 10 (QSAR, EPI Suite v4.11, Kowwin v1.68) and a vapour pressure of < 0.00001 Pa at 20 °C (QSAR, SPARC v4.6).

 

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, 2017).

 

Oral

The absorption of a substance can occur at different sites and with different mechanisms along the gastrointestinal (GI) tract, and depends primarily on the physico-chemical characteristics of the substance. A molecular weight below 500 g/mol is favourable for absorption while a molecular weight above 1000 g/mol does not favour absorption(ECHA, 2017). The molecular weight ofthe substanceis in the range330.5 – 817.1 g/mol, indicating a moderate to low potential for oral absorption.

Micellar solubilisation is a mechanism of importance for highly lipophilic substances (with a log Pow > 4), that are poorly soluble in water (1 mg/L or less), as the substances are unlikely to dissolve into GI fluids.The low water solubility (0.129 mg/L) and the high log Pow (5.57 to > 10) of the registered substance indicate that micellar solubilisation is the most likely mechanism for oral absorption of the ester.

The potential of a substance to be absorbed in the GI tract may be influenced by chemical changes as a result of metabolism by GI flora, by enzymes released into the GI tract or by hydrolysis. These changes will alter the physicochemical characteristics of the substance and hence predictions based upon the physico-chemical characteristics of the parent substance may no longer apply (ECHA, 2014).

Triglycerides (e.g. from dietary fat) can undergo hydrolysis by lipases (a class of ubiquitous carboxylesterases) prior to absorption in the GI tract, as can the mono- and diglycerides (Lehninger, 1998)

In the gastrointestinal tract, gastric and intestinal (pancreatic) lipase activities are the most important for the hydrolysis of esters. Triglycerides are hydrolysed by gastric and pancreatic lipases with high specificity for the sn1- and sn3-positions. For the remaining monoester at the sn2-position (2-monoacylglycerol), there is evidence that it can either be absorbed as such by the intestinal mucosa or isomerize to 1-monoacylglycerol, which can then be hydrolysed. The rate of hydrolysis by gastric and intestinal lipases depends on the carbon chain length of the fatty acid moiety. Thus, triesters of short-chain fatty acids are hydrolysed more rapidly and to a larger extent than triesters of long-chain fatty acids. (Barry et al., 1966; Cohen et al.,1971; Greenberger et al., 1966; IOM, 2005; Mattson and Volpenhein, 1964, 1966, 1968; WHO, 1967, 1975). In a recent study conducted with the structurally related substance glycerides, castor-oil-mono, hydrogenated, acetates (CAS 736150-63-3), rapid ester hydrolysis in intestinal fluid simulant was confirmed (Jensen, 2002).

The substance glycerides, C16-18 (even numbered) mono- and di- and their citrates is therefore expected to be enzymatically hydrolysed to glycerol, citric acid, hexadecanoic acid and octadecanoic acid.

Following hydrolysis, the resulting products free glycerol, free fatty acids, citric acid and (in the case of di and triglycerides) 2-monoacylglycerols are absorbed by the intestinal mucosa. Within the epithelial cells, triglycerides are reassembled, primarily by re-esterification of absorbed 2-monoacylglycerols. Thus, free glycerol is readily absorbed independently of the fatty acids and a certain extent of it is re-esterified. The absorption of short-chain carboxylic acids can begin already in the stomach. In general, for intestinal absorption short-chain or unsaturated carboxylic acids are more readily absorbed than long-chain, saturated fatty acids. However, the absorption of saturated long-chain fatty acids is increased if they are esterified at the sn2-position of glycerol (Greenberger et al., 1966; IOM, 2005; Mattson and Volpenhein, 1962, 1964). In a study conducted with 12-[1-14C]acetoxy-octadecanoic acid-2,3-diacetoxy-propyl ester, serving as surrogate for the substance glycerides, castor-oil-mono, hydrogenated, acetates (CAS 736150-63-3), the pharmacokinetics, tissue distribution, excretion and mass balance of radioactivity in rats after a single oral dose of the test material was assessed (St-Pierre, 2004). The results of the study showed that the test material, specifically the fatty acid moiety, was readily absorbed from the gastrointestinal tract, systemically distributed and metabolised. Based on the reported data on mass balance of radioactivity, absorption was higher than 80%.

In conclusion, based on the available information, the physicochemical properties and molecular weight of glycerides, C16-18 (even numbered) mono- and di- and their citrates suggest low oral absorption. However, the substance is expected to undergo enzymatic hydrolysis in the gastrointestinal tract and absorption of the ester hydrolysis products rather than the parent substance is likely. The absorption rate of the hydrolysis products is considered to be high.

 

Dermal

The dermal uptake of liquids and substances in solution is higher than that of dry particulates, since dry particulates need to dissolve into the surface moisture of the skin before uptake can begin. Molecular weights below 100 g/mol favour dermal uptake, while for those above 500 g/mol the molecule may be too large (ECHA, 2017). Dermal uptake is anticipated to be low, if the water solubility is < 1 mg/L; low to moderate if it is between 1-100 mg/L; and moderate to high if it is between 100-10000 mg/L. Dermal uptake of substances with a water solubility > 10000 mg/L (and log Pow < 0) will be low, as the substance may be too hydrophilic to cross the stratum corneum (ECHA, 2017). Log Pow values in the range of 1 to 4 (values between 2 and 3 are optimal) are favourable for dermal absorption, in particular if water solubility is high. For substances with a log Pow above 4, the rate of penetration may be limited by the rate of transfer between the stratum corneum and the epidermis, but uptake into the stratum corneum will be high. Log Pow values above 6 reduce the uptake into the stratum corneum and decrease the rate of transfer from the stratum corneum to the epidermis, limiting dermal absorption. The substance must be sufficiently soluble in water to partition from the stratum corneum into the epidermis (ECHA, 2017).

As the molecular weight range of glycerides, C16-18 (even numbered) mono- and di- and their citrates is 330.5 – 817.1 g/mol, the water solubility is low (0.129 mg/L) and the log Pow is rather high (5.57 –to > 10), the physico-chemical characteristics indicate that dermal absorption of the molecule will be low.

If the substance is a skin irritant or corrosive, damage to the skin surface may enhance penetration (ECHA, 2017). The primary skin irritation studies conducted with structurally related substances glycerides, C16-18 and C18-hydroxy mono- and di- (CAS 91845-19-1) and glycerides, C16-C18 (even numbered) mono-, di- and tri-, hydrogenated, citrates, potassium salts (CAS 91744-38-6) showed no sign of skin irritation.

Furthermore, skin sensitisation studies in guinea pigs did not show any skin sensitisation reactions for the structurally related substances glycerides, C16-18 and C18-hydroxy mono- and di- (CAS 91845-19-1) and glycerides, C16-18 mono-, di- and tri-, hydrogenated, citrates, potassium salts (CAS 91744-38-6). Based on the available studies on the structural analogue substances, glycerides, C16-18 (even numbered) mono- and di- and their citrates is not expected to cause any skin irritation and sensitisation reactions, which might enhanced penetration of the substance due to local skin damage and thus increase dermal absorption.

The dermal permeability coefficient (Kp) can be calculated with the DERMWIN tool which is part of the EpiSuite software from the (estimated) log Pow and molecular weight (MW) applying the following equation described in US EPA (2012):

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

The dermal absorption potential – the permeability constant (Kp) – of the registration substance was calculated (Dermwin v2.01, 2013). With a molecular weight range of 330.5 – 817.1 and a log Pow ranging from 5.57 to > 10, the Kp value range was determined to be in the range of 2.00E-04 (very low) to 8.90E+01 (high).

 

Overall, the dermal absorption potential glycerides, C16-18 (even numbered) mono- and di- and their citrates in humans is considered to be low for the main constituents.

 

Inhalation

Glycerides, C16-18 (even numbered) mono- and di- and their citrates has a negligible vapour pressure of < 0.00001 Pa at 20 °C.Therefore, under normal use and handling conditions, inhalation exposure and therefore an availability for respiratory absorption of the substance in the form of vapours, gases, or mists is not expected.

 

Accumulation and distribution

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. 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 high log Pow of glycerides, C16-18 (even numbered) mono- and di- and their citrates indicates that the substance may have the potential to accumulate in adipose tissue (ECHA, 2017). However as the absorption of the substance is considered to be very low, the potential of its bioaccumulation is very low as well.

 

As discussed under oral absorption, esters of glycerol undergo enzymatic hydrolysis in the gastrointestinal tract prior to absorption. Therefore, an assessment of distribution and accumulation of the hydrolysis products is considered more relevant.

Absorbed glycerol is readily distributed throughout the organism and can be re-esterified to form endogenous triglycerides, be metabolised and incorporated into physiological pathways or be excreted in the urine. After being absorbed, fatty acids are (re-)esterified along with other fatty acids into triglycerides and released in chylomicrons into the lymphatic system (Bergström, 1951). Fatty acids of carbon chain length ≤ 12 may be transported as the free acid bound to albumin directly to the liver via the portal vein, instead of being re-esterified. Chylomicrons are transported in the lymph to the thoracic duct and eventually to the venous system. Upon contact with the capillaries, enzymatic hydrolysis of chylomicron triacylglycerol fatty acids by lipoprotein lipase takes place. Most of the resulting fatty acids are taken up by adipose tissue and re-esterified into triglycerides for storage. Triacylglycerol fatty acids are likewise taken up by muscle and oxidized for energy or they are released into the systemic circulation and returned to the liver (IOM, 2005; Johnson, 1990; Johnson, 2001; Lehninger, 1998; NTP, 1994; Stryer, 1996; WHO, 2001; Matulka, 2009).

There is a continuous turnover of stored fatty acids, as they are constantly metabolised to generate energy and then excreted as CO2. Accumulation of fatty acids only takes place, if their intake exceeds the caloric requirements of the organism.

In the study by St-Pierre (2004) with 12-[1-14C]acetoxy-octadecanoic acid-2,3-diacetoxy-propyl ester (surrogate of glycerides, castor-oil-mono, hydrogenated, acetates (CAS 736150-63-3)), systemic distribution of the radiolabelled material was confirmed in rats. Radioactivity was detected in all tissues and organs sampled (adipose tissue, gastrointestinal tract and content, kidneys and adrenals, liver, thymus and the remaining carcass) with highest levels recovered in the gastrointestinal tract, liver and the remaining carcass. Due to excretion and absorption of the radiolabelled material, the radioactivity content in the gastrointestinal tract decreased rapidly over the course of the study (168 h). This was similar for the radioactivity recovered in liver, whereas the radioactivity found in the carcasses was nearly constant at the selected time points, indicating that the radiolabelled material may have been distributed to other tissues than the ones selected for analyses. Based on the results of this study, no bioaccumulation potential was observed for 12-acetoxy-octadecanoic acid-2,3-diacetoxy-propyl ester.

Overall, the available information indicates that the cleavage products, diglycerol and fatty acids, will be distributed in the organism. No significant bioaccumulation of the parent substance or hydrolysis products in adipose tissue is expected.

 

Metabolism

Glycerol can be metabolised to dihydroxyacetone phosphate and glyceraldehyde-3-phosphate, which can then be incorporated in the standard metabolic pathways of glycolysis and gluconeogenesis. Fatty acids are degraded by mitochondrial β-oxidation, which takes place in most animal tissues and uses an enzyme complex for a series of oxidation and hydration reactions resulting in the cleavage of acetate groups in form of acetyl CoA. The alkyl chain length is reduced by 2 carbon atoms in each β-oxidation cycle. The complete oxidation of unsaturated fatty acids such as oleic acid requires an additional isomerisation step. Alternative pathways for oxidation can be found in the liver (ω-oxidation) and the brain (α-oxidation). Iso fatty acids such as isooctadecanoic acid have been found to be activated by acyl coenzyme A synthetase of rat liver homogenates and to be metabolised to a large extent by ω-oxidation. Each two-carbon unit resulting from β-oxidation enters the citric acid cycle as acetyl CoA, through which they are completely oxidized to CO2. Acetate, resulting from hydrolysis of acetylated glycerides, is readily absorbed and feeds into physiological pathways of the body and can be utilized in oxidative metabolism or in anabolic syntheses (CIR, 1983, 1987; IOM, 2005; Lehninger, 1998; Lippel, 1973; Stryer, 1996; WHO, 1967, 1974, 1975, 2001; Adolph, 1999). Furthermore, a similar path as the acetate is expected for citric acid in the body. Citric acid, resulting from the hydrolysis of glycerides, C16-18 (even numbered) mono- and di- and their citrates, will be readily absorbed and will feed into the metabolic pathways of the body and will be utilized in the citric acid cycle (tricarboxylic acid cycle (TCA)).

 

 Excretion

The fraction of the parent compound that is not absorbed in the gastrointestinal tract, will be excreted in the faeces. In general, the fatty acid components will be metabolised in the body for energy generation, and following extensive metabolism, the primary route of excretion will be exhalation in the form of CO₂. Glycerol is a polar molecule that can readily be excreted in the urine. Small amounts of ketone bodies resulting from the oxidation of fatty acids are excreted via the urine (Lehninger, 1998; IOM, 2005; Stryer, 1996).

In rats given a single dose of 12-[1-14C]acetoxy-octadecanoic acid-2,3-diacetoxy-propyl ester at 5000 mg/kg bw, the mean total recovery of radioactivity in the excreta of the 72 h period post-dose was 108.5% of the dose (urine, 6.5%; faeces, 24.5%; CO2, 77%; and cage wash, 0.5%). Most of the recovered radioactivity (97.5%) was excreted by 24 h post dose (St-Pierre, 2004). The results suggest that the hydrolysis products of glycerides are metabolised as described above.

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

 

 

References

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