Ethylene dimethacrylate

Endpoint summary

Link to relevant study record(s)

Reference

Reference 1

Endpoint:

basic toxicokinetics in vitro / ex vivo

Type of information:

experimental study

Adequacy of study:

key study

Study period:

Sep. 2012 - March 2013

Reliability:

1 (reliable without restriction)

Rationale for reliability incl. deficiencies:

study well documented, meets generally accepted scientific principles, acceptable for assessment

Remarks:

State of the art metabolism/toxicokinetcs study, GLP

Objective of study:

metabolism

toxicokinetics

Principles of method if other than guideline:

In vitro metabolism study (rat liver microsomes, rat whole blood) in combination with toxicokinetic modelling (ADME model)

GLP compliance:

yes

Radiolabelling:

no

Details on test animals or test system and environmental conditions:

in vitro study

Details on metabolites:

By the action of the non-specific carboxylesterases in the microsomes and in whole blood the esters were hydrolysed quantitatively to methacrylic acid and the corresponding alcohols/diols.

Conclusions:

Interpretation of results (migrated information): no bioaccumulation potential based on study results
Seven methacrylate esters (including MMA, which served as a reference chemical) were initially chosen for experimental determination of metabolism rates in whole rat blood and rat liver enzymes at a single substrate concentration (Phase I). All seven methacrylates were quickly hydrolyzed to methacrylic acid (MAA) in, both, whole rat blood and rat liver microsomes. Hydrolysis half-lives of the esters in rat liver microsomes ranged 0.06 minutes to 4.95 minutes. Hydrolysis half-lives of the esters in whole rat blood ranged 1.56 minutes to 99 minutes.
Five methacrylate esters (including MMA, which served as a reference chemical) were chosen for further experiments to determine Km and Vmax values for these in rat liver microsomes. These values, along with QSAR-estimated partition coefficients were used for PBPK modeling to simulate in vivo blood concentrations of each molecule and its MAA hydrolysis product. Resulting blood concentrations were very similar between the five molecules. Differences in parent molecule blood concentrations (mg/L) varied by less than 2-fold and differences in MAA blood concentrations (mg/L) varied by less than 4-fold.
It is important to note that the PBPK model used for this effort was designed for methacrylate esters that have a single ester group, such that for each mol of parent ester hydrolyzed, one mol of methacrylic acid is formed. This is not the case for EGDMA and 1,4-BDDMA, which both contain two ester groups that can be hydrolyzed, resulting in two mol of methacrylic acid for every mol of parent ester. In the Phase II hydrolysis experiments, two mol of methacrylic acid were produced for every mol of methacrylate ester substrate introduced into the incubations. For these molecules, the PBPK model simulates only hydrolysis of the first ester group, resulting in one mol methacrylic acid per mol of parent ester.
Overall, these metabolism data and modeling results show that all five methacrylate esters studied in the definitive experiment are expected to be rapidly hydrolyzed in the rat, with greater than 86-99% cleavage by the oral route. Additionally, these simulated blood levels represent conservative estimates for those that would be expected to occur in the real world. In this study, only hydrolysis in the liver and blood has been considered. In reality, metabolism in other tissues would also be expected to occur (Brebner and Kalow, 1970; Fukami and Yokoi, 2012; Prusakiewicz et al., 2006; Satoh and Hosokawa, 1998; Zhu et al., 2000). Generally, exposures would be expected to occur via dermal, inhalation or oral ingestion. For any of these routes, pre-systemic hydrolysis would be expected to occur, significantly reducing the total amount of material reaching the systemic circulation. This has been illustrated by modeling of an oral dose route. However, even these simulations are expected to be highly conservative in terms of levels of parent ester and MAA metabolite present in the blood as the model assumes an oral bioavailability of 100 percent to the liver with no GI metabolism and no first-pass metabolism of MAA. In reality, esterase enzymes in the gut, as well as the lungs and skin in cases of dermal or inhalation exposure, would be expected to reduce the amount of methacrylate ester available to be absorbed (Brebner and Kalow, 1970; Fukami and Yokoi, 2012; Imai et al., 2003; Inoue et al., 1979; Li et al., 2007; Prusakiewicz et al., 2006). Additionally, further downstream metabolism of MAA, likely with significant first-pass metabolism, would be expected to reduce blood levels of this metabolite. Thus, real in vivo exposures are expected to result in lower blood levels of the methacrylate esters and their metabolic products than those simulated in this study.
-----------------------------
References:
Brebner, J., and Kalow, W. (1970). Soluble esterases of human lung. Canadian Journal of Biochemistry 48(9), 970-978.
Fukami, T. and Yokoi, T. (2012). The emerging role of human esterases. Drug Metabolism and Pharmacokinetics 27(5), 466-477.
Imai, T., Yoshigae, Y., Hosokawa, M., Chiba, K., and Otagiri, M. (2003). Evidence for the involvement of a pulmonary first-pass effect via carboxylesterase in the disposition of a propranolol ester derivative after intravenous administration. The Journal of Pharmacology and Experimental Therapeutics 307(3), 1234-1242.
Inoue, M., Morikawa, M., Tsuboi, M., and Sugiura, M. (1979). Species difference and characterization of intestinal esterase on the hydrolizing activity of ester-type drugs. The Japanese Journal of Pharmacology 29, 9-16.
Li, P., Callery, P. S., Gan, L-S., and Balani, S. K. (2007). Esterase inhibition attribute of grapefruit juice leading to a new drug interaction. Drug Metabolism and Disposition 35(7), 1023-1031.
Prusakiewicz, J. J., Ackermann, C., and Voorman, R. (2006). Comparison of skin esterase activities from different species. Pharmaceutical Research 23(7), 1517-1524.
Satoh, T. and Hosokawa, M. (1998). The mammalian carboxylesterases: from molecules to functions. Annual Review of Pharmacology and Toxicology 38, 257-288.
Zhu, W., Song, L., Zhang, H., Matoney, L., LeCluyse, E., and Yan, B. (2000). Dexamethasone differentially regulates expression of carboxylesterase genes in humans and rats. Drug Metabolism and Disposition 28(2), 186-191.

Executive summary:

All seven methacrylate esters were rapidly converted to MAA in whole rat blood and rat liver microsomes. Hydrolysis half-lives ranged from 1.56 to 99 minutes, and from 0.06 to 4.95 minutes for blood and liver microsomes, respectively. The incubations in whole rat blood and rat liver microsomes were performed on three separate days with MMA included as a positive control on each day. Table 6 shows elimination rates (ke), intrinsic clearance (Cl_int) and half-life values for each molecule in whole rat blood and rat liver microsomes at 0.25 mM starting concentrations. 

Rat liver microsome hydrolysis rates for the positive control (MMA) were somewhat variable between days. This was likely due to the rapidity of hydrolysis of MMA. Often, measurable levels of MAA were present even in the zero minute samples and the substrate was completely hydrolyzed by 2 minutes. This made it difficult to accurately calculate hydrolysis rates for MMA in these experiments. However, generally the calculated rates were similar to rates for hydrolysis for MMA reported previously (Jones, 2002; Mainwaring et al., 2001) and confirmed that the in vitro test systems were enzymatically active for each day of incubation experiments. The remaining six molecules exhibited rat liver microsome hydrolysis rates approximately 10 fold lower than MMA. However, all seven molecules were completely, or nearly completely, hydrolyzed to MAA within 15 minutes incubation.

----------------------------

References:

Jones, R. D .O. (2002). Using physiologically based pharmacokinetic modeling to

predict the pharmacokinetics and toxicity of methacrylate esters. Thesis

submitted to theoffor the degree of Doctor of Philosophy

in the Faculty of Medicine, Dentistry, Nursing and Pharmacy.

 

Mainwaring, G., Foster, J. R.,, V., and Green, T. (2001). Methyl methacrylate

toxicity in rat nasal epithelium: studies of the mechanism of action and

comparisons between species.Toxicology158,109 -118.

Reference 2

Endpoint:

basic toxicokinetics in vitro / ex vivo

Type of information:

experimental study

Adequacy of study:

key study

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

study well documented, meets generally accepted scientific principles, acceptable for assessment

Objective of study:

metabolism

Qualifier:

no guideline followed

Principles of method if other than guideline:

In a structure-activity relationship investigation of acrylate and  methacrylate esters the reactivity towards hydrolysis by carboxyl esterase in vitro was studied in order to elucidate their mechanism(s) of toxicity.

GLP compliance:

not specified

Specific details on test material used for the study:

- Supplier: Sartomer Company
- Analytical purity: 85 %
- Inhibitor: 40 - 150 ppm HQ
- Purity test date: no data
- Lot/batch No.: GSC 288
- Expiration date of the lot/batch: no data

Radiolabelling:

no

Species:

other: not applicable

Metabolites identified:

yes

Details on metabolites:

Methacrylic acid and Ethyleneglycol

In a structure-activity relationship investigation of acrylate and  methacrylate esters 

the reactivity towards hydrolysis by carboxyl esterase in vitro was studied in order to 

elucidate their mechanism(s) of toxicity; the compounds tested included ethyleneglycol dimethacrylate, the

second order rate constant Km for the carbioxylesterase hydrolysis of ethyleneglycol dimethacrylate was about

64 ±24* µM and Vmax was 6.9 ± 2.4# nmole/min; 

from comparison with the reaction rate of other esters the authors  concluded that 

increased alcohol chain length increases substrate affinity, yet decreases turnover 

for the enzymatic hydrolysis of these  esters.

Hydrolysis of acrylate esters by porcine liver carboxylesterase in vitro
------+------ -+---+----------+--------------+--------
Ester | Conc.  | n |    Km**  |     Vmax**   | Vmax/Km
      | [µM]   |   |   [µM]   | [nmol/min]   | [1/min]
------+------ -+---+----------+--------------+--------
EA     25-2500   4   134 ± 16     8.9 ± 2.0      66
EMA    25-2500   7   159 ± 90     5.2 ± 2.5*     33
BA      5-250    3  33.3 ± 8.5*  1.49 ± 0.83*    45
BMA     5-250    5    72 ± 28*   1.84 ± 0.64*    26
EGDMA   5-250    4    64 ± 24*    6.9 ± 2.4#    108
TEGDMA  5-250    4    39 ± 15*    2.9 ± l.0*°    74
------+------ -+---+----------+--------------+--------
EA = Ethyl acrylate, EMA = Ethyl methacrylate, BA = Butyl acrylate, 

BMA =  Butyl methacrylate, EGDMA = Ethyleneglycol dimethacrylate, 

TEGDMA =  Tetraethyleneglycol dimethacrylate      
**Mean + standard deviation.
*Significantly different at P 0.05 compared with ethyl acrylate.
#Significantly different at P 0.05 compared with butyl acrylate.
°Significantly different at P 0.05 compared with ethyleneglycol dimethacrylate.

Conclusions:

In a valid scientific study, it was shown that increased alcohol chain length increases substrate affinity, yet decreases turnover for the enzymatic hydrolysis of these esters. For the reaction with porcine liver carboxylesterase the following values were determined for EGDMA: Km 64 ±24 µM and
Vmax was 6.9 ± 2.4 nmole/min; Vmax/Km 108 1/min

Executive summary:

In a valid scientific study, it was shown that increased alcohol chain length increases substrate affinity, yet decreases turnover for the enzymatic hydrolysis of these esters. For the reaction with porcine liver carboxylesterase the following values were determined for EGDMA: Km 64 ±24 µM and Vmax was 6.9 ± 2.4 nmole/min; Vmax/Km 108 1/min

Reference 3

Endpoint:

dermal absorption

Type of information:

(Q)SAR

Adequacy of study:

key study

Reliability:

2 (reliable with restrictions)

Justification for type of information:

QSAR prediction

1. SOFTWARE
no software used

2. MODEL (incl. version number)
The prediction model used in this investigation for a set of methacrylate chemicals is based on an established model (Potts and Guy, 1992).

3. SMILES OR OTHER IDENTIFIERS USED AS INPUT FOR THE MODEL
not stated in report

4. SCIENTIFIC VALIDITY OF THE (Q)SAR MODEL
The “Relative Dermal Absorption” potential assigned to the predicted skin flux for methacrylate data is an arbitrary estimation of skin penetration potential, and is not a regulatory or OECD approved classification. It is based on several hundred chemicals tested in the same human skin model at the Central Toxicology Laboratory and Dermal Technology Laboratory. This database includes a wide variety of pharmaceutical, agrochemical and industrial chemicals tested over a 20 year period. The dermal absorption potential of a particular chemical substance is placed into one of six categories based on its skin permeability coefficient or its predicted (or actual) absorption rate

5. APPLICABILITY DOMAIN
not explicitely stated in the report

6. ADEQUACY OF THE RESULT
not explicitely stated in the report

Principles of method if other than guideline:

Prediction of the skin penetration characteristics using the physicochemical properties used as a first level assessment of the ability of chemical to cross the human epidermis based on: Potts RO and Guy RH (1992). Predicting Skin Permeability. Pharm. Res. 9(5): 663-669

GLP compliance:

no

Species:

other: human skin QSAR

Details on test animals or test system and environmental conditions:

Prediction of the skin penetration characteristics using the physicochemical properties used as a first level assessment of the ability of chemical to cross the human epidermis based on: Potts RO and Guy RH (1992). Predicting Skin Permeability. Pharm. Res. 9(5): 663-669
Assumed skin temperature: 32 °C

Chemical Class	Test Chemical / Compound Identity	Acronym	Molecular Weight	Log P	Predicted Flux (µg/cm^2/h)	Relative Dermal Absorption
Multi-func aliph/hydrophob DMA	Ethylene glycol dimethacrylate	EGDMA	198.22	2.4	6.109	Low

 

Conclusions:

Based on the dermal penetration model for human skin the predicted skin penetration for EGDMA is low (6.109 µg/cm²/h).

Executive summary:

Based on the dermal penetration model for human skin the predicted skin penetration for EGDMA is low (6.109 µg/cm²/h).

Description of key information

EGDMA is likely to be absorbed by all routes. Due to the low vapour pressure, the dermal route is the primary route of exposure, since inhalation is unlikely. The dermal absorption rate however is calculated to be low.

The ester is rapidly within a few minutes hydrolysed by carboxylesterases to methacrylic acid (MAA) and ethylene glycol. Ethylene glycol will be transformed to glycolic acid and, subsequently, to glyoxylic acid. Metabolites can be degraded to formate, glycine or malate which can be broken down to generate respiratory CO2, or to oxalic acid, which is excreted in the urine. These metabolic steps are partially rate-limited. MAA is metabolized to succinic acid which will be degraded through the tricarboxylic acid cycle and primarily excreted as CO2.

Key value for chemical safety assessment

Bioaccumulation potential:

no bioaccumulation potential

Absorption rate - oral (%):

50

Absorption rate - dermal (%):

50

Absorption rate - inhalation (%):

100

Additional information

Oral absorption

The physicochemical properties of EGDMA (log P = 2.4), the relatively high water solubility of > 1 g/L and the molecular weight of 198.22 g/mol are in a range suggestive of absorption from the gastro-intestinal tract subsequent to oral ingestion (ECHA guidance 7c, 2017). For chemical safety assessment an oral absorption rate of 50% is assumed as a worst case default value (ECHA R.8 guidance, 2012).

Dermal absorption

Based on a QSAR Prediction of Dermal Absorption (extract from Heylings JR, 2013) EGDMA is predicted on the basis of their molecular weight and lipophilicity to have a relatively low ability to be absorbed through the skin. The predicted flux was 6.109 μg/cm²/h. However, for chemical safety assessment, a dermal absorption rate of 50% is assumed as a worst case default value (ECHA R.8 guidance, 2012).

Inhalative absorption

Due to the low vapour pressure of EGDMA (1.0 Pa at 20°C), exposure via inhalation is unlikely. For chemical safety assessment an inhalative absorption rate of 100% is assumed as a worst case default value (R8 guidance, 2012)

Distribution

As a small, water-soluble molecule with a logP > 0, a wide distribution can be expected (ECHA guidance 7c, 2017). No information on potential target organs is available.

 

Metabolism of Methacrylic esters

Di- and mono-ester hydrolysis

Ester hydrolysis has been established as the primary step in the metabolism of methacrylate esters. In the case of diol di-methacrylate esters the first step would be hydrolysis of one of the ester bonds to produce the corresponding mono-ester followed by subsequent hydrolysis of the second ester bond to produce methacrylic acid (MAA) and the corresponding alcohol Ethylene glycol. The metabolic pathway is shown in the category document.

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. For multifunctional methacrylates mostly the same would be the case except, because of the much lower vapour pressure, inhalation is generally not a relevant route of exposure and there is a very low likelihood of a relevant exposure of the nasal epithelium to vapour.

Kinetics data have been reported for the hydrolysis of two multifunctional methacrylates (EGDMA and TREGDMA) by porcine liver carboxylesterase in vitro. For comparison reasons, the results from two lower alkyl methacrylates (EMA and BMA), are also presented in the table below (McCarthy and Witz, 1997). The four studied substances showed comparable hydrolysis rates in vitro.

Table: Hydrolysis of Acrylate Esters by Porcine Liver Carboxylesterasein vitro (extract from McCarthy and Witz., 1997); supporting data from lower alkyl methacrylates in grey

Ester	Km (mM)	Vmax (nmol/min)
Tetraethyleneglycol dimethacrylate (TREGDMA)	39±15*	2.9±1.0
Ethyleneglycol dimethacrylate (EGDMA)	64±24*	6.9±2.4
Ethyl methacrylate (EMA)	159±90	5.2±2.5
n-Butyl methacrylate (n-BMA)	72±28*	1.8±0.6

*Significantly different (p < 0.05) from ethyl acrylate

A recent study, designed to extend an earlier work on lower alkyl methacrylates (Jones 2002, see below) to higher and more complex methacrylate esters, studied the in vitro metabolism of higher and more complex methacrylate esters in rat blood and liver microsomes. This study included three esters of the multifunctional methacrylate category (TREGDMA, EGDMA and 1,4-BDDMA) (DOW, 2013). The results of those studies are summarized in below table.

Table: Elimination Rates, Intrinsic Clearance and Half-life in Rat Liver Microsomes and Whole Rat Blood for Five Methacrylate Esters at 0.25 mM Substrate Concentration (DOW, 2013); data from supporting substances in grey.

	Liver Microsomes	Liver Microsomes	Liver Microsomes	Whole Blood	Whole Blood	Whole Blood
Molecule	Clint (μl/min/mg)	ke	Half‐Life (min)	Clint(μl/min)	ke	Half‐Life (min)
TREGDMA	116	0.23	3.01	219	0.12	5.68
EGDMA	142	0.28	2.45	796	0.44	1.56
1,4-BDDMA	78	0.16	4.46	304	0.17	4.10
MMA	1192	2.38	0.29	19	0.01	63.00
HEMA	74	0.15	4.62	12	0.01	99.00

ke: elimination rate
Cl_int: intrinsic clearance (ke x volume of incubation / mg/mL microsomal protein)
MMA: Methyl methacrylate (supporting substance); HEMA: Hydroxyethyl methacrylate (supporting substance)

All studied methacrylate esters were rapidly converted to MAA in whole rat blood and rat liver microsomes. Hydrolysis half-lives of the studied category members were in the order of minutes for blood and liver microsomes, respectively.

The incubations in whole rat blood and rat liver microsomes were performed on three separate days with MMA included as a positive control on each day. Rat liver microsome hydrolysis rates for the positive control (MMA) were somewhat variable between days. This was likely due to the rapidity of hydrolysis of MMA. Often, measurable levels of MAA were present even in the zero minute samples and the substrate was completely hydrolyzed by 2 minutes. This made it difficult to accurately calculate hydrolysis rates for MMA in these experiments. However, generally the calculated rates were similar to rates for hydrolysis for MMA reported previously (Jones, 2002; Mainwaring et al., 2001) and confirmed that the in vitro test systems were enzymatically active for each day of incubation experiments. The other studied methacrylates exhibited rat liver microsome hydrolysis rates approximately 10 fold lower than MMA. From its very rapid degradation to MAA, MMA can be understood as suitable donor substance for MAA as common primary metabolite of all category members.

Supporting information on Alkyl methacrylates

The above mentioned EMA and n-BMA were also studied in an elaborate series of in vitrostudies on carboxylesterase activity with 7 alkyl methacrylates ranging from methyl methacrylate to octyl methacrylate (with increasing ester size) (Jones, 2002). This was used to establish a PB-PK model of in vivoclearance for several tissues (blood, liver, skin and nasal epithelium) from rats and humans, which showed thatmethacrylate mono-esters are rapidly hydrolyzed by carboxylesterases to methacrylic acid (MAA) and the respective alcohol. The validity of the model was verified with targeted in vivo experiments. Whilst there was a trend of increasing half-life of alkyl methacrylates with increasing chain length (up to octyl), clearance of the parent ester from the body was always in the order of minutes.

Although the absolute rate measurements obtained by Jones differ slightly to those determined by McCarthy and Witz, presumably due to differences in experimental conditions such as protein content etc., the rates obtained for the two lower alkyl methacrylates (EMA and BMA) can be used to draw parallels between the work of the two researchers indicating that the kinetics for the hydrolysis of EGDMA and TREGDMA fall within the range observed by Jones for lower alkyl methacrylates. On this basis the parent ester would be expected to have a short systemic half–life within the body being effectively cleared from the blood within the first or second pass through the liver.

Hydrolysis of the di- and monoester would yield the common metabolite methacrylic acid and the respective alcohol.

Subsequent metabolism

Methacrylic acid (MAA, CAS 79-41-4)

From the available extensive toxicokinetic data on lower alkyl methacrylates it has been established that the common primary metabolite methacrylic acid is subsequently cleared predominantly via the liver (valine pathway and the TCA (TriCarboxylic Acid) cycle, respectively; ECB, 2002; OECD SIAR, 2009). Methyl methacrylate (MMA) is rapidly degraded in the body to MAA and can thus be understood as metabolite donor for MAA. The metabolic pathway is shown in the category document.

Ethylene glycol (EG, CAS 107 -21 -1)

As described in the ATSDR review on ethylene glycol (2010), “Ethylene glycol is converted to glycolaldehyde by nicotinamide adenine dinucleotide (NAD)-dependent alcohol dehydrogenase. Subsequent reduction of NAD results in the formation of lactic acid from pyruvate. Glycolaldehyde has a brief half-life and is rapidly converted to glycolic acid (and to a lesser extent glyoxal) by aldehyde dehydrogenase and aldehyde oxidase, respectively. Glycolic acid is oxidized to glyoxylic acid by glycolic acid oxidase or lactic dehydrogenase. Glyoxylic acid can be metabolized to formate, glycine, or malate, all of which may be further broken down to generate respiratory CO2, or to oxalic acid, which is excreted in the urine. In excess, oxalic acid can form calcium oxalate crystals. Rate-limiting steps in the metabolism of ethylene glycol include the initial formation of glycolaldehyde and the conversion of glycolic acid to glyoxylic acid, both of which are saturable processes. The conversion of glycolic acid to glyoxylic acid is the most rate-limiting step in ethylene glycol metabolism.” The metabolic pathway is shown in the category document.

In rabbits, glucuronic acid conjugation was not observed after application of 4 mmol/kg bw (Gessner et al., 1960). Enzymatic key parameters of alcohol dehydrogenase were 1000±60 mM k_mand 64±1 nmol/min V_maxfor EG (Herold et al., 1989).

Glutathione reactivity

A QSAR model for the methacrylates in the category predicts only slight reactivity with glutathione for the category members and no reactivity for the primary metabolite of the methacrylic moiety, methacrylic acid (Cronin, 2012).

Table: QSAR prediction of GSH reactivity (Protein Binding Potency; Cronin, 2012)

Abbreviation	SMILES	Molecular Weight	Log P	Sat. Water Sol. (µg/mL)	Protein Binding Potency
EGDMA	CC(=C)C(=O)OCCOC(=O)C(=C)C	198.22	2.4	1086 mg/l	Slightly reactive

Studies with methacrylates in vitro confirm low reactivity with GSH, in particular compared to the corresponding acrylates, and have proposed that this is due to steric hindrance of the addition of a nucleophile at the double bond by the alpha-methyl side-group (McCarthy & Witz, 1991, McCarthy et al., 1994, Tanii and Hashimoto, 1982). For example, HEMA as the primary metabolite of EGDMA affected intracellular GSH depletion only in concentrations of 5 or 10 mM in two different human cell types (Chang et al. 2005).

Table: Apparent Second-Order Rate Constants for the Reaction of Glutathione with Methacrylate Esters (extract from McCarthy et al., 1994); data from the supporting substances in grey

Ester	App. 2^ndorder rate const. Kapp[L/mol/min]
Tetraethyleneglycol dimethacrylate (TREGDMA)	1.45±1.0 (0.725±0087)*
Ethyleneglycol dimethacrylate (EGDMA)	0.83±0.12 (0.406±0.059)*
Methyl methacrylate (MMA)	0.325±0.059

*Bifunctional esters calculated as two independent esters.

In vivo data on category members are absent. However, in an inhalation study with MMA at an overtly cytotoxic exposure level off 566 ppm and absolute deposition rates of 35-42μg/min under unidirectional flow, a 20% lowering of nasal non-protein sulfhydryl (NPSH) content was observed, indicative of direct protein reactivity, whereas methacrylic acid exposures had no effect, even at higher delivered dose rates. Around the local (nasal) LOAEL, at an exposure concentration of 109 ppm, MMA had no effect on nasal NPSH levels (Morris and Frederick, 1995). 

Hence, ester hydrolysis is considered to be the major metabolic pathway for alkyl and multifunctional methacrylate esters, with GSH conjugation only playing a minor role in their metabolism.

Excretion

As the ester will not survive first pass metabolism in the liver, excretion of the parent compound is of no relevance. The primary metabolite, MAA, is cleared rapidly from blood by standard physiological pathways, with the majority of the administered dose being exhaled as CO2.

In summary, the metabolism data and modelling results show that EGDMA would be rapidly hydrolysed in the rat.

Dermal absorption

Table: QSAR prediction of dermal absorption (extract from Heylings, 2013)

Substance	Molecular Weight	Log P	Predicted Flux (μg/cm^2/h)	Relative Dermal Absorption
TREGDMA	286.32	2.3	4.989	Low
DEGDMA	242.27	2.2	5.997	Low
EGDMA	198.22	2.4	6.109	Low
GDMA	228.24	2.05	24.986	Moderate
1,3-BDDMA	226.27	3.1	2.895	Low
1,4-BDDMA	226.27	3.1	2.895	Low
1,6-HDDMA	254.32	4.08	0.917	Low
TMPTMA	338.4	4.193	0.296	Low

 

All members of the MfMA category, but GDMA, are predicted on the basis of their molecular weight and lipophilicity to have a relatively low ability to be absorbed through the skin (Heylings, 2013).

Trends

MfMA esters are typically predicted to have a relatively low potential for skin absorption. There is a suggestion of trend for predicted absorption decreasing with increasing ester chain length and increasing lipophilicity. The larger members of the category, like TMPTMA, are extremely unlikely to be absorbed through the skin to any significant extent.

Human information

There are no relevant toxicokinetic data for MfMA in humans.

For lower alkyl methacrylates there is information indicating that skin absorption rates are lower in human skin compared to rat skin, while for MMA it has been demonstrated that human fate kinetics is similar to those in rats (Jones, 2002).

 

Summary and discussion on toxicokinetics

Methacrylate esters are absorbed by all routes while the dermal absorption is limited with the larger members of the category. Due to the low vapour pressure of the multifunctional methacrylates, the dermal route is the primary route of exposure, since inhalation is unlikely. The rate of dermal absorption decreases with increasing ester chain length. All esters are rapidly hydrolyzed by carboxylesterases to methacrylic acid (MAA) and the respective alcohol. In the case of di- and triesters the apparent rate of hydrolysis is highest for the parent ester, but this likely reflects the higher number of hydrolysable target sites instead as opposed to any greater specific activity. Ester hydrolysis can occur in local tissues at the site of contact as well as in blood and other organs by non-specific carboxylesterases. By far the highest enzyme activity has been shown in liver microsomes indicating that the parent ester will be fully metabolized in the liver. Clearance of the parent ester from the body is in the order of minutes. There is a trend towards increasing half-life of the ester in blood with increasing ester chain length, however, none of the esters will survive first pass metabolism in the liver to any significant extent. The primary methacrylic metabolite, MAA, is subsequently cleared rapidly from blood by standard physiological pathways, with the majority of the administered dose being exhaled as CO2. The respective alcohol moieties will undergo further metabolism in the liver.

 

Ethylene glycol dimethacrylate will rapidly be hydrolyzed by unspecific carboxyl esterases in the liver into methacrylic acid and ethylene glycol. Ethylene glycol will be transformed rapidly by alcohol dehydrogenases and aldehyde dehydrogenase and aldehyde oxidase, respectively into glycolic acid (and to a lesser extent glyoxal) and will be converted to glycolic acid. Glycolic acid is oxidized to glyoxylic acid by glycolic acid oxidase or lactic dehydrogenase. Glyoxylic acid can be metabolized to formate, glycine, or malate, all of which may be further broken down to generate respiratory CO2, or to oxalic acid, which is excreted in the urine.

 

 

Compliance to REACh requirements

The information requirement is covered with reliable in vitro studies on the primary metabolism, reliable in vitro/ in vivo studies on the metabolism of the methacrylic metabolite MAA as well as reliable publication data on the metabolism of the alcohol metabolite EG. All mentioned sources are reliable (Reliability 1 or 2) so that the category/ read across approach can be done with a high level of confidence.

 

References

ATSDR 2010. Toxicological Profile For Ethylene Glycol. ATSDR TP No. 96 .S. Department of Health and Human Services; Public Health Service Agency for Toxic Substances and Disease Registry

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Gessner PK, ParkeDV, Williams RT (1960). Studies in Detoxication. 80. The metabolism of glycols. Biochemical Journal, 74: 1-5

McCarthy TJ, Witz G (1997). Structure-activity relationships in the hydrolysis of acrylate and methacrylate esters by carboxylesterase in vitro. Toxicology 116: 153-158. Owner company: Published.

Satoh T, Hosokawa M (1998). The Mammalian carboxylesterases: From models to functions. Annual Review of Pharmacology and Toxicology 38, 257-288. Medicine and Biology 283, 333-335

Tanii H., Hashimoto K.(1982); Structure-Toxicity Relationship of Acrylates and Methacrylates; Toxicol. Lett. 11: 125-129