2-Butenedioic acid (2E)-, di-C12-14 (even numbered)-alkyl esters

Endpoint summary

Key value for chemical safety assessment

Bioaccumulation potential:

no bioaccumulation potential

Additional information

Basic toxicokinetics

There are no experimental studies available in which the toxicokinetic behaviour of 2-Butenedioic acid (2E)-, di-C12-14-alkyl esters (EINECS 938-575-3) has been assessed.

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, 2012), 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, 2012) and taking into account available information on the category members from which data was used for read-across to cover data gaps.

2-Butenedioic acid (2E)-, di-C12-14-alkyl esters is an unsaturated diester of fatty alcohols with carbon chain lengths ranging from C12-14 and fumaric acid, and meets the definition of a UVCB substance based on the analytical characterisation.

The substance is a white organic solid at room temperature with a molecular weight of 452.72 - 508.82 g/mol and a water solubility of > 0.01 mg/L and < 0.05 mg/L at 20 °C, pH=5.8 (Frischmann, 2014). The log Pow is calculated to be >10 (Nagel, 2014) and the vapour pressure is estimated to be < 0.0001 Pa at 20 °C (Nagel, 2014).

Absorption

Absorption is a function of the potential of 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, 2012).

Oral

The smaller the molecule, the more easily it will be taken up. In general, molecular weights below 500 are favourable for oral absorption (ECHA, 2012). As the molecular weight of 2-Butenedioic acid (2E)-, di-C12-14-alkyl esters is 452.72 - 508.82 g/mol, absorption of the molecule in the gastrointestinal tract is in general anticipated.

Absorption after oral administration of 2-Butenedioic acid (2E)-, di-C12-14-alkyl esters is considered limited when the “Lipinski Rule of Five” (Lipinski et al., 2001; Ghose et al., 1999) is applied. However, the log Pow of >10 suggests that 2-Butenedioic acid (2E)-, di-C12-14-alkyl esters is favourable for absorption by micellar solubilisation, as this mechanism is of importance for highly lipophilic substances (log Pow > 4), which are poorly soluble in water (< 1 mg/L).

After oral ingestion, dicarboxylic acid esters undergo stepwise hydrolysis of the ester bonds by gastrointestinal enzymes (Lehninger, 1970; Mattson and Volpenhein, 1972). The respective alcohols as well as the fatty acid are formed. The physicochemical characteristics of the cleavage products (e.g. physical form, water solubility, molecular weight, log Pow, vapour pressure, etc.) are likely to be different from those of the parent substance before absorption into the blood takes place, and hence the predictions based upon the physicochemical characteristics of the parent substance do no longer apply (ECHA, 2012). However, also for the resulting cleavage products with a high water solubility (i.e. fumaric acid), it is anticipated that they are absorbed in the gastrointestinal tract. In case of long carbon chains and thus rather low water solubility they are absorbed mainly by micellar solubilisation (Ramirez et al., 2001).

Overall, systemic bioavailability of 2-Butenedioic acid (2E)-, di-C12-14-alkyl esters and/or the respective cleavage products in humans is considered likely after oral uptake of the substance.

Dermal

The smaller the molecule, the more easily it may be taken up via the dermal route. In general, a molecular weight below 100 favours dermal absorption, above 500 g/mol the molecule may be too large to penetrate the skin (ECHA, 2012). As the molecular weight of 2-Butenedioic acid (2E)-, di-C12-14-alkyl esters is 452.72 - 508.82 g/mol, dermal absorption of the molecule cannot be excluded.

If the substance is a skin irritant or corrosive, damage to the skin surface may enhance penetration (ECHA, 2012). No data for skin irritation are available for the substance itself. However, data from the category members Didodecyl fumarate (CAS 2402-58-6) and Di-C12-15 Alkyl Fumarate are available, in which no skin irritating properties were observed using a validated human skin model (Remmele, 2013; Cantor, 1991). As the data for the structurally related substances are anticipated to be similar for 2-Butenedioic acid (2E)-, di-C12-14-alkyl esters, enhanced penetration due to local skin damage can be excluded for the substance.

Based on a QSAR calculated dermal absorption a value of < 0.00001 mg/cm²/event (very low) was predicted for 2-Butenedioic acid (2E)-, di-C12-14-alkyl esters (Danish EPA, 2010). Based on this value the substance has a very low potential for dermal absorption.

For substances with a log Pow above 4, the rate of dermal penetration is limited by the rate of transfer between the stratum corneum and the epidermis, but uptake into the stratum corneum will be high. For substances with a log Pow above 6, the rate of transfer between the stratum corneum and the epidermis will be slow and will limit absorption across the skin, and the uptake into the stratum corneum itself is also slow. The substance must be sufficiently soluble in water to partition from the stratum corneum into the epidermis (ECHA, 2012). As the water solubility of 2-Butenedioic acid (2E)-, di-C12-14-alkyl esters is less than 0.1 mg/L, dermal uptake is likely to be (very) low.

Overall, the calculated low dermal absorption potential, the low water solubility, the molecular weight (>100 g/mol), the high log Pow value and the fact that the substance is not irritating to skin implies that dermal uptake of 2-Butenedioic acid (2E)-, di-C12-14-alkyl esters is considered very limited. This is also reflected in an acute dermal toxicity study with the structurally related substance Didodecyl fumarate according to OECD 402, in which application of a limit dose of 5000 mg/kg bw on the skin of rabbits did not result in adverse systemic effects throughout the study period (Höger, 2013).

Inhalation

In general, particles with an aerodynamic diameter < 100 μm have the potential to be inhaled, whereas only particles with an aerodynamic diameter < 50 μm can reach the thoracic region and those < 15 μm may enter the alveolar region of the respiratory tract (ECHA, 2012).

2-Butenedioic acid (2E)-, di-C12-14-alkyl esters has a very low vapour pressure of < 0.0001 Pa at 20 °C thus being of low volatility. Therefore, the potential for exposure and subsequent absorption via inhalation as vapour can be excluded. Due to its physical appearance as a waxy solid, dust formation of the substance and a subsequent exposure of particles via the inhalation route can be considered negligible under the identified use conditions. Therefore, the potential of the test substance to induce systemic effects after inhalation is not considered to be likely.

Bioaccumulation

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 >10 implies 2-Butenedioic acid (2E)-, di-C12-14-alkyl esters may have the potential to accumulate in adipose tissue (ECHA, 2012).

However, as further described in the section metabolism below, esters of alcohols and fatty acids undergo esterase-catalysed hydrolysis, leading to the cleavage products fumaric acid (CAS 110-17-8) and fatty alcohols within carbon chains ranging from C12 to C14.

The first cleavage product, fumaric acid, has a log Pow of 0.46 and a water solubility of 7 g/L (HSDB, 2011; ESIS, 2000). As it is an intermediate of the citric acid cycle, it will not accumulate in adipose tissue, but will be rapidly metabolised (HSDB, 2011; Lehninger, 1970). The other cleavage products are fatty alcohols with C12-14 chain length which are only slightly water-soluble (OECD SIDS, 2006). As fatty alcohols are efficiently metabolised, they have limited potential for retention or bioaccumulation (OECD SIDS, 2006).

This assumption is supported by results from studies performed with the structurally similar substance Bis(2-ethylhexyl) adipate (CAS 103-23-1) indicating no potential for bioaccumulation (Elcombe, 1981; Takahashi et al., 1981).

Overall, the available information indicates that no significant bioaccumulation of the parent substance and/or the cleavage products in adipose tissue is expected.

Distribution

Distribution within the body through the circulatory system depends on the molecular weight, the lipophilic character and water solubility of a substance. 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).

The substance 2-Butenedioic acid (2E)-, di-C12-14-alkyl esters undergoes chemical changes as a result of enzymatic hydrolysis, leading to the cleavage products fumaric acid and fatty alcohols within carbon chains ranging from C12 to C14.

The aliphatic C12-14 fatty alcohols are widely distributed within the body and efficiently eliminated (OECD SIDS, 2006). Due to its low molecular weight and moderate log Pow, fumaric acid will also be distributed within the body (HSDB, 2011).

As described in the following chapter, the distribution of Bis(2-ethylhexyl) adipate (DEHA) (CAS 103-23-1), a structurally similar substance, was assessed in rats treated with the radioactive labelled substance. Relatively high levels of radioactivity appeared in the liver, kidney, blood, muscle and adipose tissue apart from the stomach and intestine. All other tissues contained very little residual radioactivity. In liver, kidney, testicle and muscle, the amount of residual radioactivity reached a maximum in the first 6 - 12 h and reduced to less than 50% of administered radioactivity at 24 h. In other tissues the radioactivity declined with time after 6 h. The blood contained about 1% of the administered radioactivity after 6-12 h which then decreased to undetectable levels by the end of 2 days. It was also evident that total radioactivity in the tissues examined was about 10% of the administered radioactivity after 24 h of dosing and it decreased to about 2% and 0.5% of administered radioactivity after 48 h and 96 h, respectively. From these results, it can be concluded that the elimination of radioactivity from tissues and organs is very rapid and there is no specific organ affinity under these experimental conditions (Takahashi et al., 1981).

Overall, the available information indicates that 2-Butenedioic acid (2E)-, di-C12-14-alkyl esters and its cleavage products, C12-14 fatty alcohols and fumaric acid, will be distributed within the organism.

Metabolism

Dicarboxylic acid esters are expected have the same metabolic fate as fatty acid esters. Esters of carboxylic acids are hydrolysed to the corresponding alcohol and carboxylic acid by esterases (Fukami and Yokoi, 2012; Lehninger, 1970). Depending on the route of exposure, esterase-catalysed hydrolysis takes place at different places in the organism: After oral ingestion, esters of alcohols and fatty acids undergo stepwise enzymatic hydrolysis already in the gastrointestinal fluids. In contrast, substances absorbed through the pulmonary alveolar membrane or through the skin enter the systemic circulation directly before entering the liver where hydrolysis will basically take place.

In the first step of hydrolysis, the monoester is produced that is further hydrolysed to the alcohol and the dicarboxylic acid. During the first step of biotransformation the alcohols are oxidised to the corresponding carboxylic acids, followed by a stepwise elimination of C2-units in the mitochondrial beta-oxidation process (OECD SIDS, 2006). The second cleavage product fumaric acid is , as it is also an endogenous metabolite, incorporated into the citric acid cycle and rapidly degraded to CO2 (ESIS, 2000).

Experimental data of the structurally similar Bis(2-ethylhexyl) adipate (DEHA) (CAS 103-23-1) are regarded exemplarily. The elimination, distribution and metabolism were assessed in rats according to a protocol similar to OECD Guideline 417 (Takahashi et al., 1981). 14C-DEHA in DMSO was administered to male Wistar rats by oral gavage. Adipic acid was found as main metabolite in urine in a short time and its excretion reached 20-30% of the administered dose within 6 h. In blood it was found at 1% and in liver at 2-3% of the administered dose; mono-(2-ethylhexyl) adipate (MEHA) was the second metabolite found, but to a very little extent. Thus, cleavage of parent substance was shown in vivo within 6 hours into adipic acid (20-30% of the administered dose in urine, 1% of the administered dose in blood, 2-3% of the administered dose in liver) and MEHA to a lower extent. From these results, it is clear that orally ingested DEHA is rapidly hydrolysed to MEHA and adipic acid which is the main intermediate metabolite.

In rats and mice, DEHA is rapidly converted to primarily 2-Ethylhexanoic acid (EHA), 2-ethyl-5-hydroxyhexanoic acid and 2-ethylhexan-1,6-dioic acid and their glucuronides. However in monkeys, large amounts of MEHA glucuronide and 2-ethylhexanol glucuronide are excreted and only a very small proportion of the dose is converted to EHA and other downstream metabolites. Apart from MEHA, monoester metabolites have not been identified (Elcombe, 1981).

In a publication on fumaric acid esters, depletion of glutathione (GSH) after administration of dimethyl fumaric ester is described indicating conjugation with GSH catalysed by GSH-transferases (GST) as Phase-II metabolism (Rostami Yazdi and Mrowietz, 2008). The reason for this pathway is the double bond of the α,β-unsaturated carbonyl group of fumaric acid as substrate for GST whereas saturated dicarboxylic acid esters like adipic acid diester are glucuronidated directly at the carbonylgroup (Takahashi et al., 1981).

Overall, 2-Butenedioic acid (2E)-, di-C12-14-alkyl esters is hydrolysed and the cleavage products are metabolized by beta oxidation and/or conjugation with GSH.

Excretion

For 2-Butenedioic acid (2E)-, di-C12-14-alkyl esters and its cleavage products, the main route of excretion is expected to be via expired air as CO2 after metabolic degradation. Further routes of excretion might be via faeces and renal excretion after conjugation with GSH of the substance itself or its metabolites.

Experimental data of the structurally similar Bis(2-ethylhexyl) adipate (CAS 103-23-1) indicate rapid excretion via urine or expired air, too (Takahashi et al., 1981).

Thus, renal excretion after conjugation with GSH and exhalation as CO2 are the most relevant routes of excretion of the parent substance or its metabolites.

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

 

References:

Danish EPA (2010). Danish (Q)SAR Database Report powered by OASIS Database. http://qsar.food.dtu.dk/oasis/dbstart.html?enterButton=Start+search

ECHA (2012). Guidance on information requirements and chemical safety assessment, Chapter R.7c: Endpoint specific guidance.

Elcombe, 1981: Di(2-Ethylhexyl)Adipate (DEHA): Carcinogenicity and Possible Relevance to Man.

ESIS (2000), IUCLID data sheet (fumaric acid)   http://esis.jrc.ec.europa.eu/doc/IUCLID/datasheet/110178.pdf

Fukami, T. and Yokoi, T. (2012). The Emerging Role of Human Esterases. Drug Metabolism and Pharmacokinetics, Advance publication July 17th, 2012.

Ghose et al. (1999). A Knowledge-Based Approach in Designing Combinatorial or Medicinal Chemistry Libraries for Drug Discovery. J. Comb. Chem. 1 (1): 55-68.

HSDB (2011). Hazardous Substances Data Bank, Toxnet Home, National Library of Medicinehttp://toxnet.nlm.nih.gov/cgi-bin/sis/htmlgen?HSDB

Lehninger, A.L. (1970). Biochemistry. Worth Publishers, Inc.

Lipinski et al. (2001). Experimental and computational approaches to estimate solubility and permeability in drug discovery and development settings. Adv. Drug Del. Rev. 46: 3-26.

Mattson F.H. and Volpenhein R.A., 1972: Hydrolysis of fully esterified alcohols containing from one to eight hydroxyl groups by the lipolytic enzymes of rat pancreatic juice. J Lip Res 13, 325-328

OECD SIDS (2006): SIDS Initial Assessment Profile: Long Chain Alcohols (C6-22 primary aliphatic alcohols).

http://webnet.oecd.org/hpv/UI/handler.axd?id=03441f78-d135-4cab-b832-edfb1d0d677e

Ramirez et al. (2001). Absorption and distribution of dietary fatty acids from different sources. Early Human Development 65 Suppl.: 95-101.

Rostami Yazdi, M. and Mrowietz, U. (2008) Fumaric acid ester. Clinical Dermatol. 26(5): 552-556

Takahashi T. et al., 1981: Elimination , distribution and metabolism of di(2-ethylhexyl)adipate (DEHA) in rats. Toxcology 22: 223-233