2-ethylhexyl methacrylate

Endpoint summary

Description of key information

Short description of key information on bioaccumulation potential result:
Methacrylate esters, including 2-ethyhexyl methacrylate, are readily absorbed by all

routes and rapidly hydrolyzed by carboxylesterases to methacrylic acid (MAA) and the respective alcohol. Clearance of the parent ester from the body is in the order of minutes. For 2-EHMA the half-life was 23.8 minutes and 99.9 % was removed by first-pass metabolism in the liver. Reliable data on the primary metabolites methacrylic acid and 2-ethyl hexanol are available and do not reveal critical properties.
Short description of key information on absorption rate:
2-EHMA readily absorbs through rat and human epidermis. Human epidermis appears to

be 10 times less permeable to 2-EHMA than rat epidermis.

Key value for chemical safety assessment

Absorption rate - dermal (%):

0.6

Additional information

 

Data availability:

There is some basic information available on 2-EHMA for metabolism in vitro and in vivo, as well as dermal absorption through rat and human skin. Furthermore, 2-EHMA is a member of a category of lower methacrylate esters, of which there are extensive data available for the methyl ester (methyl methacrylate, MMA) and this has been reviewed in the EU Risk Assessment (2002). Sufficient data is available to confirm applicability of this data across all members of the category and this has been reviewed in the OECD SIAR (2009). Data on methacrylic acid (MAA), the common metabolite, has been reviewed in the EU Risk Assessment (2002). The following text relies on these reviews with any addition to the original documents is italicised.Data on the corresponding alcohol metabolite 2-ethyl hexanol have been reviewed by the OECD SIAR process in 2005.

Trends/Results

The OECD SIAR on short chain methacrylate esters concluded that: “Other short chain alkyl-methacrylate esters, like MMA, are initially hydrolysed by non-specific carboxylesterases to methacrylic acid and the structurally corresponding alcohol in several tissues (ECETOC 1995, 1996b). In many regards, therefore, 2-EHMA behaves similarly to its close structural analogue MMA.

Taken from the EU Risk Assessment (RA) on MMA; “after oral or inhalation administration, methyl methacrylate is rapidly absorbed and distributed. In vitro skin absorption studies in human skin indicate that methyl methacrylate can be absorbed through human skin, absorption being enhanced under occluded conditions. However, only a very small amount of the applied dose (0.56%) penetrated the skin under unoccluded conditions (presumably due to evaporation of the ester from the skin surface (Syngenta/CEFIC, 1993)). After inhalation exposure to rats 10 to 20% of the substance is deposited in the upper respiratory tract where it is metabolized (by non-specific esterases to the acid, MAA (Morris, 1992)). Activities of local tissue esterases of the nasal epithelial cells appear to be lower in man than in rodents (Green, 1996 later published as Mainwaring, 2001). Toxicokinetics seem to be similar in man and experimental animal. After arthroplasty using methyl methacrylate-based cements, exhalation of unchanged ester occurs to a greater extent than after i. v., i. p. or oral administration. After oral or parenteral administration methyl methacrylate is further metabolised by physiological pathways with the majority of the administered dose being exhaled as CO2(Bratt and Hathway, 1977; ICI, 1977a). Conjugation with GSH or NPSH plays a minor role in methyl methacrylate metabolism and only occurs at high tissue concentrations (McCarthy and Witz, 1991; Elovaara et al., 1983) ”. While the basic metabolic pathway is very similar between MMA and 2-EHMA, inhalation exposure is much less significant due to an approximately 3 orders of magnitude lower vapour pressure and hence, the threshold of toxic effects in the respiratory tract is unlikely to be reached under normal use patterns.

Methacrylic acid and the corresponding alcohol are subsequently cleared predominantly via the liver (valine pathway and the TCA (TriCarboxylic Acid) cycle, respectively). The carboxylesterases are a group of non-specific enzymes that are widely distributed throughout the body and are known to show high activity within many tissues and organs, including the liver, blood, GI tract, nasal epithelium and skin (Satoh & Hosokawa, 1998; Junge & Krish, 1975; Bogdanffy et al., 1987; Frederick et al., 1994). Those organs and tissues that play an important role and/or contribute substantially to the primary metabolism of the short-chain, volatile, alkyl-methacrylate esters are the tissues at the primary point of exposure, namely the nasal epithelia and the skin, and systemically, the liver and blood.

Methacrylate esters can conjugate with glutathione (GSH) in vitro, although they show a low reactivity, since the addition of a nucleophile at the double bond is hindered by the alpha-methyl side-group (McCarthy & Witz, 1991, McCarthy et al., 1994, Tanii and Hashimoto, 1982). There are no data for 2-EHMA itself, although data with MMA, ethyl methacrylate (EMA) and n-butyl methacrylate (n-BMA) indicate that GSH conjugation decreases with increasing chain length and is already negligible with n-BMA (McCarthy et al. 1994). Hence, ester hydrolysis is considered to be the only significant metabolic pathway for 2-EHMA.

Studies completed after the MMA RA have confirmed that all short chain alkyl-methacrylate esters are rapidly hydrolysed by ubiquitous carboxylesterases (see table below, adapted from Jones; 2002). First pass (local) hydrolysis of the parent ester has been shown to be significant for all routes of exposure. For example, no parent ester can be measured systemically following skin exposure to EMA and larger esters (including 2-EHMA), as the lower rate of absorption for these esters is within the metabolic capacity of the skin (Jones, 2002). Parent ester will also be effectively hydrolysed within the G. I. tract and within the tissues of the upper respiratory tract (particularly the olfactory tissue). Systemically absorbed parent ester will be effectively removed during the first pass through the liver (% Liver Blood Flow, LBF; see table below) resulting in their relatively rapid elimination from the body (T50%; see table below).

 

Table: Rate Constants for ester hydrolysis by rat-liver microsomes and predicted systemic fate kinetics following i.v. administration

Ester	Rat liver microsomes (100µg ml^-1) Vmax               Km    (nM min^-1mg^-1)  (µM)		CL (%LBF)	T50%(min)	Cmax(MAA) (mg L^-1)	Tmax(MAA) (min)
MAA	-	-	51.6%	-	-	-
MMA	445.8	164.3	98.8%	4.4	14.7	1.7
EMA	699.2	106.2	99.5%	4.5	12.0	1.8
i-BMA	832.9	127.4	99.5%	11.6	7.4	1.6
n-BMA	875.7	77.3	99.7%	7.8	7.9	1.8
HMA	376.4	34.4	99.7%	18.5	5.9	1.2
2EHMA	393.0	17.7	99.9%	23.8	5.0	1.2
OMA	224.8	11.0	99.9%	27.2	5.0	1.2

 

HMA – hexyl methacrylate; OMA – octyl methacrylate;; i-BMA - iso-butyl methacrylate.Fate kinetics determined using the “well-stirred” model; CL%LBF – Clearance as percentage removed from liver blood flow i.e. first pass clearance; T_50%- time taken for 50% of parent ester to have been eliminated from the body; C_max– maximum concentration of MAA in circulating blood; T_max– time in minutes to peak MAA concentration in blood “Jones, 2002”.

 

Table: Summary of the results for the peak rates of absorption of MAA & alkylmethacrylate esters through rat & human epidermis

	Rat epidermis			Human epidermis
Ester	Peak rate of absorption (μg cm-2hr-1) ±SEM	Period of peak absorption rate (hours)	% age of applied dose absorbed over x hours	Peak rate of absorption (μg cm-2hr-1) ±SEM	Period of peak absorption rate (hours)	% age of applied dose absorbed over x hours
MAA	23825±2839	0.5-4	93% / 24h	812	-	-
MMA	5888±223	2-8	46% / 16h	453±44.5	4-24	10% / 24h
EMA	4421	-	-	253	-	-
i-BMA	1418	-	-	80	-	-
n-BMA	1540±69	0-6	18% / 24h	76.7±9.8	0-24	2% / 24h
HMA	147	-	-	25	-	-
2EHMA	234±4.8	0-30	7.8% / 30h	22.7.7±3.7	3-24	0.6% / 24h
OMA	159±15	0-24	-	7.8	-	-

Key: The values in normal type were obtained experimentally, whilst those in italics, are predicted values based on statistical analysis (single exponential fit) of the experimental data

In terms of MAA, the common metabolite for these esters, another study using intravenous injection of MAA in rats demonstrated very rapid clearance from the blood (half-life <5mins), suggestive of rapid subsequent metabolism (Jones, 2002).

Conclusions

2-EHMA, like other, shorter chain esters and MAA are absorbed by all routes. The rate of absorption decreases with increasing ester chain length so it will be absorbed less rapidly than MMA. All esters are rapidly hydrolysed in local tissues as well as in blood by non-specific esterases to methacrylic acid (MAA) and the respective alcohol.. There is a trend towards increasing half-life of the ester in blood with increasing ester chain length (see table above). The half life of 2-EHMA is still, however, in the order of minutes (23.8). The primary metabolite, MAA, is subsequently cleared rapidly from blood and, as indicated by studies with MMA, this metabolism is by standard physiological pathways, with the majority of the administered dose being exhaled as CO2.

Interms of the corresponding alcohol metabolite for these esters, they are subsequently rapidly metabolised, primarily in the liver, by oxidative pathways involvingaldehyde dehydrogenase (ALDH), alcohol dehydrogenase (ADH), cytochrome P450 (CYP2E1), and catalase enzymes to the corresponding aldehyde and acid before ultimately being converted to CO₂.

Conjugation:

The C=C double bond of methacrylate esters render these chemicals as Michael acceptors capable of electrophilic attack of protein and other cellular macromolecules. This is the mode of action through which a wide range of toxicities including allergic contact dermatitis is thought to be mediated. This reactivity also means, however, that methacrylate esters are capable of conjugating with cellular glutathione (GSH). Quantitative structure activity relationships (QSAR) have been developed for the reaction with glutathione based upon the local charge-limited electrophilicity index ω^q(Schwöbel et al., 2010) and the¹³C chemical shift, of theβ-Carbon atom (Fujisawa and Kadoma, 2012)i.e. the unsubstituted end of the double bond in these molecules. The most recent of these models from 2012 is claimed to have a reliability factor (r²) of 0.99 for the prediction of the rate of the Michael addition reaction with GSH for a range of acrylate and methacrylate monomers. This publication includes calculation of the rate constants for EMA and n-BMA. This model was used by Tindale to calculate the rates for the remaining esters in the C1-C8 category (Tindale, 2015).The rate of reaction is low for all members of the C1-C8 methacrylate category (table 10) whether based upon the local charge-limited electrophilicity index ω^q(Schwöbel et al., 2010, Cronin, 2012, 2015), or¹³C chemical shift, of theβ-Carbon atom (Fujisawa and Kadoma, 2012; Tindale, 2015)when compared with other esters and unsaturated ketones and aldehydes (McCarthy & Witz, 1991).This is becausethe addition of a nucleophile at the double bond is hindered by the alpha-methyl side-group in the case of the methacrylates (McCarthy & Witz, 1991; McCarthy et al., 1994; Tanii and Hashimoto, 1982).A minor impact is also exerted by the +I-effect of the alcohol subgroup, but the incremental impact on electrophilicity rapidly decreases with increasing alcohol chain length. Therefore for direct electrophilic reactions the alcohol group will only have a minor, rather monotonic influence with increasing chain length. This is reflected in the very similar GSH reactivity constants for all members of the category. On this basis, ester hydrolysis is considered to be the major metabolic pathway for alkyl-methacrylate esters, with GSH conjugation only playing a minor role in their metabolism, and then possibly only when very high tissue concentrations are achieved. Furthermore, since there is no structural alert for Michael addition reactivity in the case of MAA (Cronin, 2012) or the alcohol metabolites, hydrolysis of the ester is essentially a detoxification process with regard to this MoA.

 

Like with MMA, local effects resulting from the hydrolysis of the ester to MAA might be expected following inhalation exposure. In the case of MMA this has been shown to be due to the localised concentration of non-specific esterases in nasal olfactory tissues. Data with MMA and n-BMA indicate that the NOEC for this local effect increases with increasing ester chain length within the category. For MMA the NOAEC in rats is 25ppm while for n-BMA the NOAEC for this lesion is already 310 ppm (LOAEC was 952 ppm) in a subacute study. With 2-EHMA with a saturated vapour density of 50-100 ppm at ambient temperature (64 ppm at 20 °C) it is unlikely that concentrations could ever be reached which cause this effect. In the case of systemic effects the profile of effects is comparable between 2-EHMA and n-BMA. As the systemic NOAEC for n-BMA was 1891 ppm, a concentration approaching the saturated vapour density, in a 28 day study the 30fold lower saturated vapour density of 2-EHMA ensures that systemic effects would not be seen following repeated inhalation exposure to vapour. Hence, based on phys.-chem- properties and toxicokinetic information, the inhalation pathway is not considered a relevant route of exposure.

References quoted from the EU RA on MMA and other documents, but not copied into the IUCLID dataset:

Bogdanffy MS, Randall HW, Morgan KT (1987) Biochemical quantitation and histochemical localization of carboxylesterase in the nasal passages of the Fischer-344 rat and B6C3F1 mouse. Toxicology and Applied Pharmacology 88: 183-194

Bratt H,(1977). Fate of methyl methacrylate in rats. Brit. J. Cancer 36, 114-119.

CEFIC (1993). Methyl methacrylate: in vitro absorption through human epidermis: Ward RJ and Heylings JR, Zeneca Central Toxicology Lab., Unpublished study on behalf of CEFIC Methacrylates Toxicology Committee, Brussels

European Chemicals Bureau (2002). European Union - Risk Assessment Report on Methacrylic Acid.European Union - Risk Assessment Report, Vol. 25, Doc. No. EUR 19837 EN

European Chemicals Bureau (2002). European Union - Risk Assessment Report on Methyl methacrylate. European Union - Risk Assessment Report, Vol. 22. Doc. No. EUR 19832 EN

ECETOC (1995) Joint Assessment of Commodity Chemicals No 30 ; Methyl Methacrylate, CAS No. 80-62-6. European Centre for Ecotoxicology and Toxicology of Chemicals, Avenue Van Nieuwenhuyse 4, (Bte 6) B-1160,. ISSN-0773-6339-30

ECETOC (1996a) Joint Assessment of Commodity Chemicals No 35 ; Methacrylic acid, CAS No. 79-41-4. European Centre for Ecotoxicology and Toxicology of Chemicals, Avenue Van Nieuwenhuyse 4, (Bte 6) B-1160,. ISSN-0773-6339-35

Elovaara E, Kivistoe H, Vainio H (1983).Effects of methyl Methacrylate on non-protein thiols and drug metabolizing enzymes in rat liver and kidneys. Arch. Toxicol. 52, 109-121.Hext PM., Pinto PJ, Gaskell BA. (2001) Methyl methacrylate toxicity in rat nasal epithelium: investigation of the time course of lesion development and recovery from short term vapour inhalation.Toxicology 156: 119-128

FrederickCB (1998). Interim report on interspecies dosimetry comparisons with a hybrid computational fluid dynamics and physiologically-based pharmacokinetic inhalation model. Rohm and Haas Co.,.

ICI (1977). The biological fate of methylmethacrylate in rats; Rep. CTL/R/396 by Hathaway DE and Bratt H; Zeneca, Alderly Park, Macclesfield, Cheshire.

Junge W, Krisch K (1975) The carboxylesterases/amidases of mammalian liver and their possible significance. Critical Reviews in Food Science and Nutrition, 371-434.Mainwaring G, Foster JR, Lund V, Green T (2001) Methyl methacrylate toxicity in rat nasal epithelium: Studies of the mechanism of action and comparisons between species. Toxicology 158, 109-118

Mainwaring G, Foster JR, Lund V, Green T (2001) Methyl methacrylate toxicity in rat nasal epithelium: Studies of the mechanism of action and comparisons between species. Toxicology 158, 109-118

Morris JB (1992). Uptake of Inspired Methyl Methacrylate and Methacrylic Acid Vapors in the Upper Respiratory Tract of the F344 Rat. Prepared byof, Univ.for US Methacrylate Producers Association (MPA).Washington, DC

OECD (2004) SIDS Initial Assessment Report for18: Short-Chain Alkyl Methacrylates;http://webnet.oecd.org/hpv/UI/handler.axd?id=1da0a7af-3b11-45b7-865f-acad8b19c6c3

Tanii H, Hashimoto K (1982). Structure-toxicity relationship of acrylates and methacrylates. Toxicol Lett. 11: 125 129

Discussion on absorption rate:

The absorption of 2-EHMA was evaluated through rat and human epidermis in anin vitrosystem. The technique measures the rate of absorption of 2 -EHMA across the epidermis. Glass diffusion cells are employed to measure the amount of 2 -EHMA that is received into a receptor chamber with respect to time, following the application of 100 µl/cm² of 2 -EHMA to the epidermal surface. The mean rate of absorption through rat and human separated epidermis was 234 and 7.72 µg/cm²/hr and the total amount of chemical that was absorbed during the time of exposure was 7.8 % (over 30 hours) and 0.56 % (over 24 hours), respectively. 2-EHMA appears to be readily absorbed through rat and human epidermis, but human epidermis is at least 10 times less permeable to 2-EHMA than rat epidermis. However, measuring the rate of absorption through rat and human epidermis provides a quantitative estimate for inter-species differences; however, because only the epidermal layer is used, no measure of metabolism during skin absorption is possible.