Glycerides, mixed C8-10 and succinyl

Endpoint summary

Additional information

Basic toxicokinetics

In accordance with Annex VIII, Column 1, Item 8.8.1, of Regulation (EC) 1907/2006 and with Guidance on information requirements and chemical safety assessment Chapter R.7c: Endpoint specific guidance (ECHA, 2008), assessment of the toxicokinetic behaviour of the substance 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 physicochemical and toxicological properties according to the relevant Guidance (ECHA, 2008) and taking into account further available information from members of the Glycerides category. There are no studies available in which the toxicokinetic behaviour of Glycerides, mixed C8-C10 and succinyl (CAS No. 91744-56-8) has been investigated.

The substance Glycerides, mixed C8-C10 and succinyl (UVCB) is a mixed ester of glycerol, fatty acids with chain lengths of C8 (octanoic acid) and C10 (decanoic acid) and succinic acid.

The substance Glycerides, mixed C8-C10 and succinyl has a molecular weight ranging from 470.68 - 1540.07 g/mol. The substance is a light yellow liquid (characteristic odour) at 20 °C with a melting point of < -40 °C at normal pressure (Spilker, 2011), water solubility of 1 - 2 mg/L at 20 °C (Spilker, 2011), determined log Pow of 10.7 (Spilker, 2012) and determined vapour pressure of < 5 Pa at 20 °C and 50 °C (Conte, 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

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) which would otherwise be poorly absorbed (Aungst and Chen, 1986; ECHA, 2008).

The physicochemical characteristics (log Pow and water solubility) of the substance and the molecular weight are in a range suggestive of low absorption from the gastrointestinal tract subsequent to oral ingestion.

This assumption of poor oral absorption is supported by the available data on acute oral toxicity in rats and mice, consistently showing no mortality and no systemic effects after single administration of at least 5000 mg/kg bw (Consultox Laboratories Ltd., 1979; Dufour, 1992).

The potential of a substance to be absorbed in the (GI) tract may be influenced by chemical changes taking place in GI fluids 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, 2008).

It is well-accepted knowledge that triglycerides (e.g. from dietary fat) undergo hydrolysis by lipases (a class of ubiquitous carboxylesterases) prior to absorption; and there is sufficient evidence to assume that Glycerides, mixed C8-C10 and succinyl will likewise undergo enzymatic hydrolysis in the GI tract as the first step in their absorption, distribution, metabolism and excretion (ADME) pathways as summarised below.

In the gastrointestinal tract, gastric and intestinal (pancreatic) lipase activities are the most important. 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 substance Glycerides, castor-oil-mono, hydrogenated, acetates (CAS No. 736150-63-3), rapid ester hydrolysis in intestinal fluid simulant was confirmed (Jensen, 2002).

Triesters of glycerol with succinic acid have been shown to be readily hydrolysed to glycerol and succinic acid in cultured pancreatic islets, entering pathways of glycolysis and citric cycle, respectively. This was shown by the high incidence of radiolabelled CO2 and acidic metabolites resulting from the metabolic degradation of these esters (Belfiore, 1998).

The substance Glycerides, mixed C8-C10 and succinyl is therefore anticipated to be enzymatically hydrolysed to glycerol, octanoic acid (C8), decanoic acid (C10) and succinic acid.

Following hydrolysis, the resulting products free glycerol, free succinic acid, free fatty acids 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 little of it is re-esterified. As for hydrolysis, the absorption rate of free fatty acids is chain length-dependent. The absorption of short-chain fatty acids can therefore already begin in the stomach. In general, for intestinal absorption short-chain or unsaturated fatty 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). Recently a study was 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 No. 736150-63-3) to investigate the pharmacokinetics, tissue distribution, excretion and mass balance of radioactivity in rats after a single oral dose of the test material (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%.

Succinic acid occurs endogenously as a component of the tricarboxylic acid cycle, and is thus anticipated to be readily absorbed and metabolised (EFSA, 2009; Lehninger, 1998; Stryer, 1996).

In conclusion, based on the available information, the physicochemical properties and molecular weight of Glycerides, mixed C8-C10 and succinyl suggest low oral absorption. However, the substance is anticipated 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. 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. 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, thus limiting dermal absorption (ECHA, 2008).

The physicochemical properties (log Pow and water solubility) of the substance and the molecular weight are in a range suggestive of low to moderate absorption through the skin.

If a substance shows skin irritating or corrosive properties, damage to the skin surface may enhance penetration. If the substance has been identified as a skin sensitizer then some uptake must have occurred although it may only have been a small fraction of the applied dose (ECHA, 2008).

The available data on Glycerides, mixed C8-C10 and succinyl provide no indication that the substance is skin irritating or skin sensitising (Dufour, 1992; Consultox Laboratories Ltd., 1979). Therefore, no enhanced penetration of the substance due to skin damage is expected.

Overall, taking all available information into account, the dermal absorption potential is considered to be low.

Inhalation

Glycerides, mixed C8-C10 and succinyl is a liquid with low vapour pressure < 5 Pa at 20 °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 substance is sprayed (e.g. as a formulated product). 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 for oral absorption, the molecular weight, log Pow and water solubility are suggestive of low absorption across the respiratory tract epithelium but may occur by micellar solubilisation.

Overall, systemic bioavailability is considered likely after inhalation of aerosols with aerodynamic diameters below 15 µm.

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 discussed under oral absorption, mono-di- and triesters of glycerol undergo enzymatic hydrolysis in the gastrointestinal tract prior to absorption. Therefore, assessment of distribution and accumulation of the hydrolysis products is considered more relevant.

Absorbed glycerol and succinic acid are readily distributed throughout the organism and can be re-esterified to form endogenous triglycerides (in the case of glycerol), 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. 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).

Stored fatty acids underlie a continuous turnover as they are permanently metabolised for energy and excreted as CO2. Bioaccumulation of fatty acids 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 No. 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.

Succinic acid is a small molecule with high water solubility, thus being easily distributed in the body and absorbed by either passive diffusion or active transport via cell membranes.

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 the 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 thus 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). Thus 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 naturally 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).

Succinic acid is an important biochemical intermediate of the citric acid cycle, in which it is oxidised to fumaric acid by the removal of 2 hydrogen atoms in a reaction catalysed by succinic acid dehydrogenase in complex with FAD. The two hydrogen atoms removed from the succinic acid molecule are accepted by FAD, which is reduced to FADH2 by the uptake of two electrons. The two electrons from FADH2 are then released to the electron transport chain for energy generation by reversion of FADH2 to FAD. Fumaric acid is further metabolised in the citric cycle, finally resulting in the formation of the energy rich products like ATP (Lehninger, 1998).

Excretion

As far as Glycerides are not hydrolysed in the gastrointestinal tract, they are excreted in the faeces.

In general, the hydrolysis products glycerol and fatty acids are catabolised entirely by oxidative physiologic pathways ultimately leading to the production of carbon dioxide and water. Glycerol, being a polar molecule 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 thus confirm that Glycerides are mainly excreted as CO2 in the expired air as a result of metabolism.

Succinic acid is metabolised during the citrus acid cycle for energy generation, resulting in the formation of CO2 (Lehninger, 1998), which is excreted via exhalation.

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