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Basic toxicokinetics

There are no studies available in which the toxicokinetic behaviour of Ethylene diformate has been investigated.

Therefore, in accordance with Annex VIII, Column 1, Item 8.8, of Regulation (EC) No 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 Ethylene diformate was conducted to the extent that can be derived from the relevant available information on physicochemical and toxicological characteristics. This comprises a qualitative assessment of the available substance specific data on physico-chemical and toxicological properties and taking into account further available information on the breakdown products of ester hydrolysis.

The substance Ethylene diformate is a diester of ethylene glycol and formic acid and meets the definition of a mono-constituent substance based on the analytical characterization.

Ethylene diformate is a liquid at 20 °C and has a molecular weight of 118.1 g/mol and a water solubility of 200 g/L at 25°C (Barwell, 2011). The log Pow is calculated to be -0.69 (Müller, 2011) and the vapour pressure is 172 Pa at 25°C (SRC database, 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

When assessing the potential of Ethylene diformate to be absorbed in the gastrointestinal (GI) tract, it has to be considered that esters like Ethylene diformate can undergo to a high extent hydrolysis by ubiquitous expressed GI enzymes (Long, 1958; Lehninger, 1970; Mattson and Nolen, 1972). In addition, an in vitro hydrolysis test with Ethylene diformate confirmed a rapid hydrolysis using simulated intestinal fluid to the degradation products Ethylene glycol and Formic acid (Butler, 2013). Thus, due to the hydrolysis the predictions based upon the physico-chemical characteristics of the intact parent substance alone may no longer apply but the physico-chemical characteristics of the breakdown products of the ester; the alcohol Ethylene glycol and Formic acid.

As Ethylene diformate is highly water-soluble, the substance will readily dissolve into the GI fluids. The molecular weight of the parent substance Ethylene diformate (118.1 g/mol) does suggest absorption. When considering the hydrolysis products Ethylene glycol and Formic acid, both are highly water-soluble and have low molecular weights and can therefore dissolve into GI fluids, as well. The respective molecular weights of the hydrolysis products Ethylene glycol (62.07 g/mol) and Formic acid (46.03 g/mol) favour absorption. Due to their low molecular weights, the hydrolysis products and the intact parent substance may pass through aqueous pores or may be carried through the epithelial barrier by the bulk passage of water. Furthermore, the moderate log Pow of the substances (Ethylene glycol: log Pow =1.36; Formic acid: log Pow= -0.54; SRC database) favours absorption by passive diffusion. Thus, Ethylene diformate, Ethylene glycol and Formic acid will be readily absorbed through the GI tract (ATSDR, 2010; ICPS, 2001; NTP, 1992; Hanzlik, 2005).

Studies investigating the acute oral toxicity of Ethylene diformate are available showing acute oral toxicity resulting in a LD50 value of 390 mg/kg bw in guinea pigs showing absorption as well. Furthermore, available data on the subchronic oral toxicity of the breakdown product Ethylene glycol showed systemic effects in animals resulting in a NOAEL of 200 mg/kg bw/day (DePass, 1986). Thus, the breakdown products of the parent substance Ethylene diformate; Ethylene glycol and Formic acid will be absorbed rapidly after oral administration.

Dermal

There are no data available on dermal absorption or on acute dermal toxicity of Ethylene diformate. On the basis of the following considerations, the dermal absorption of Ethylene diformate is considered to be high. Regarding the molecular weight of both the parent substance (118.1 g/mol) and the breakdown products of hydrolysis (62.07 and 46.03 g/mol) dermal uptake is anticipated to be high. However, regarding the calculated octanol/water partition coefficient of the parent substance of -0.69 (Müller, 2011) and the high water solubility, the intact substance may be too hydrophilic to cross the lipid rich environment of the stratum corneum. Nevertheless, QSAR calculation using EPIwebv4.1 resulted in a Dermal Flux of 4.58E-2 mg/cm² per h and QSAR calculation using DERMWIN showed a high dermal absorption potential with a permeability constant of 1.21E-04 cm/h of Ethylene diformate. However, it is important to note that Ethylene diformate has shown corrosive properties and thus the damage to the skin surface may enhance penetration of the breakdown products.

Overall, taking into account the physico-chemical properties of Ethylene diformate, Ethylene glycol and Formic acid, the QSAR calculation and available toxicological data, the dermal absorption potential of the substance and the hydrolysis products is anticipated to be high.

Inhalation

Ethylene diformate has a low vapour pressure of 172.0 Pa at 20 °C thus being of low volatility (Neely, 1985 as cited in SRC database). 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, 2008).

Due to the high hydrophilicity of Ethylene diformate, deposition in the mucus is possible. It is important to note, that a hydrolysis test in vitro is available indicating rapid hydrolysis of the ester. In this study, an immediate degradation about 32% of Ethylene diformate in aqueous solution was observed indicating, that the parent compound may only be present in the respiratory tract for a limited period of time (Butler, 2013). Due to the moderate log Pow value of Ethylene diformate and the products of hydrolysis Ethylene glycol and Formic acid, direct absorption across the respiratory tract epithelium by passive diffusion is favoured. Absorption of deposited material is anticipated to be high, due to the low molecular weight and the moderate log Pow of the parent substance and the hydrolysis products as discussed in the oral absorption section. In addition, one subchronic toxicity study via inhalation in rats with the breakdown product Formic acid is available (NTP, 1992). The resulting systemic NOAEC was equivalent to the highest tested dose of 244.7 mg/m³ air (nominal). However it cannot be concluded that Formic acid is not absorbed from the respiratory tract. In contrast, Formic acid is anticipated to be rapidly absorbed through the lung epithelium (NTP, 1992; Bouchard, 2001). Furthermore, data of one acute inhalation toxicity study with Ethylene diformate did not show systemic toxicity, however the lack of systemic toxicity 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 after acute administration via the inhalation route. As systemic absorption after oral administration is high, it is likely that Ethylene diformate and the breakdown products will be absorbed via inhalation, as well.

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 (ECHA, 2008). As the parent compound Ethylene diformate will be hydrolysed to a substantial extent before absorption as discussed above, the distribution of intact Ethylene diformate is not solely relevant but also the distribution of the breakdown products of hydrolysis. Ethylene diformate and the absorbed products of hydrolysis, Formic acid and Ethylene glycol can be distributed within the body. Both the parent substance and Ethylene glycol and Formic acid have low molecular weights and high water solubilities. Based on the physico-chemical properties, Ethylene diformate, Ethylene glycol and Formic acid will be distributed within the body (ATSDR, 2010; ICPS, 2001; NTP 1992; Hanzlik, 2005). Substances with high water solubility like Ethylene diformate, Ethylene glycol and Formic acid do not have the potential to accumulate in adipose tissue due to their low log Pow. In addition, the intact parent compound Ethylene diformate is not assumed to be accumulated as hydrolysis to Ethylene glycol and Formic acid is anticipated to take place to a high extent before absorption or during metabolism (see below).

In summary, the available information on Ethylene diformate indicate that no significant bioaccumulation of the parent substance or the breakdown products of hydrolysis in adipose tissue is expected. The parent substance and the breakdown products of hydrolysis, Ethylene glycol and Formic acid will be distributed in the organism.

Metabolism

Metabolism of Ethylene diformate occurs initially via enzymatic hydrolysis of the ester resulting in Ethylene monoformate which will be further cleaved into Ethylene glycol and Formic acid. The hydrolysis of the ester was also confirmed by an in-vitro study using simulated intestinal fluid (Butler, 2013). In the study, rapid ester hydrolysis was confirmed with a half live of the parent substance of 0.67 h in vitro. Moreover, an immediate degradation about 32% of Ethylene diformate in aqueous solution was observed. Furthermore, in-vivo studies in rats with 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; Mattson and Nolen, 1972). In addition, simulation of intestinal metabolism of Ethylene diformate, using the OECD QSAR ToolBox v.2.3.0, resulted in 8 intestinal metabolites including Ethylene monoformate supporting the metabolism pathway of a stepwise ester hydrolysis, as well.

Regarding the intact substance Ethylene diformate, metabolism in the liver will likewise lead to ester cleavage. This is supported by QSAR calculations (OECD ToolBox v.2.3.0) showing 7 liver metabolites of Ethylene diformate including the oxidized alcohol component (e.g. Glyoxylic acid) and Formic acid.

The resulting alcohol component of ester hydrolysis, 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 metabolised 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 CO2, 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). The substance Ethylene diformate has shown acute toxicity after oral administration resulting in a LD50 value of 390 mg/kg bw and a category 4 classification according to Regulation (EC) No 1272/2008, Annex VI (CLP), as well. Following absorption into the intestinal lumen, Formic acid is primarily oxidized in the liver (NTP, 1992). Under physiological conditions, Formic acid dissociates to Formate which is further metabolised to the excretion products CO2 and H2O (Bouchard, 2001). Oxidation of Formate is driven by mainly by a tetrahydrofolate-dependent multistep pathway to carbon dioxide and a small percentage of Formate is excreted directly with the urine (Bouchard, 2001). Available genotoxicity data from the parent substance Ethylene diformate and the degradation products of hydrolysis do not show any genotoxic properties. In particular, an Ames test with Ethylene diformate (Thompson, 2012), an in-vitro chromosomal aberration test with Ethylene glycol and Formic acid (Morita, 1989; NTP, 1993), an in-vitro mammalian gene mutation assay with Ethylene glycol and Formic acid (NTP, 1993; Engelhardt, 2002) were consistently negative and therefore no indication of a genotoxic reactivity of the parent substance Ethylene diformate and the degradation products Ethylene glycol and Formic acid under the test conditions is indicated.

Excretion

Based on the metabolism described above, the breakdown products of Ethylene diformate will be metabolised in the body to a high extent. Formic acid will be metabolised and mainly excreted via exhaled air as CO2 (Bouchard, 2001; NTP 1992). 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).

 

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