2-ethoxyethyl methacrylate

Endpoint summary

Description of key information

subchronic NOAEL = 163 mg/kg bw/d (rat) (read-across from 2-ethoxyethanol)

Key value for chemical safety assessment

Repeated dose toxicity: via oral route - systemic effects

Link to relevant study records

Reference

Endpoint:

sub-chronic toxicity: oral

Type of information:

read-across from supporting substance (structural analogue or surrogate)

Adequacy of study:

weight of evidence

Justification for type of information:

REPORTING FORMAT FOR THE ANALOGUE APPROACH

1. HYPOTHESIS FOR THE ANALOGUE APPROACH
This read-across is based on the hypothesis that source substances are both metabolites of the target substance and fast metabolism occurs.
Therefore, read-across from the existing toxicity studies on the source substances is considered as an appropriate adaptation to the standard information requirements of REACH regulation

2. SOURCE AND TARGET CHEMICAL(S) (INCLUDING INFORMATION ON PURITY AND IMPURITIES)
see attached “Justification for read-across”

3. ANALOGUE APPROACH JUSTIFICATION
see attached “Justification for read-across”

4. DATA MATRIX
see attached “Justification for read-across”

Reason / purpose for cross-reference:

read-across source

Reason / purpose for cross-reference:

read-across source

Reason / purpose for cross-reference:

read-across source

Reason / purpose for cross-reference:

read-across: supporting information

Details on oral exposure:

VEHICLE
- Justification for use and choice of vehicle (if other than water): water
- Concentration in vehicle: 5 mL/kg

Dose descriptor:

NOAEL

Remarks:

rat

Effect level:

163 mg/kg bw/day (nominal)

Based on:

act. ingr.

Sex:

male/female

Basis for effect level:

histopathology: non-neoplastic

Dose descriptor:

NOAEL

Remarks:

dog

Effect level:

46 mg/kg bw/day (actual dose received)

Based on:

act. ingr.

Sex:

male/female

Basis for effect level:

histopathology: non-neoplastic

Critical effects observed:

yes

Lowest effective dose / conc.:

326 mg/kg bw/day (actual dose received)

System:

immune system

Organ:

thymus

Treatment related:

yes

Dose response relationship:

yes

Relevant for humans:

yes

Endpoint conclusion

Endpoint conclusion:

adverse effect observed

Dose descriptor:

NOAEL

163 mg/kg bw/day

Study duration:

subchronic

Species:

rat

System:

other: male reproductive system, immune system

Organ:

testes

thymus

Repeated dose toxicity: inhalation - systemic effects

Endpoint conclusion

Endpoint conclusion:

no study available

Repeated dose toxicity: inhalation - local effects

Endpoint conclusion

Endpoint conclusion:

no study available

Repeated dose toxicity: dermal - systemic effects

Endpoint conclusion

Endpoint conclusion:

no study available

Repeated dose toxicity: dermal - local effects

Endpoint conclusion

Endpoint conclusion:

no study available

Additional information

No experimental data on ETMA are available for the assessment of repeated dose toxicity. However, studies are available for the source substances 2-ethoxyethanol and methacrylic acid. A detailed justification for read-across is attached to IUCLID section 13.

Hypothesis for the analogue approach

The read across hypothesis relies on the observation that alkyl esters are rapidly hydrolysed by carboxylesterase enzymes within the body to release methacrylic acid (MAA) and free alcohol. Local effects, including genotoxicity and sensitisation, if they occur are likely to be due to electrophilic reactivity of the parent ester 2-Ethoxyethyl methacrylate (ETMA). Due to the short half-life of the parent ester within the body systemic exposure to parent ester is extremely unlikely so the observed systemic toxicity profile is determined by the systemic toxicity profile of the primary metabolites Methacrylic acid (MAA) and 2-Ethoxyethanol.

 

This read-across hypothesis corresponds to scenario 1 – biotransformation to common compounds – of the read-across assessment framework) i.e. Property of the target substance is predicted to be quantitatively equal to those of the source substance. Namely, the metabolites Methacrylic acid and 2-Ethoxyethanol predict the toxicological properties of the parent compound ETMA.

 

Based on the available experimental data, including data from acute toxicity and genotoxicity studies, the read-across hypothesis is supported by close structural analogy and similar toxicological profile of the substances.

Toxicokinetics

AE 1.1 Formation of common (identical) compound(s)

The focus of this AE is on the scientific explanation and documentation on how the (bio)transformation from source and target substances to the common compound(s) occur. It will be shown that biotransformation from parent ester to primary metabolite occurs rapidly within the body and that the ensuing metabolism of these primary metabolites is well understood thereby providing a high confidence in the assertion that the metabolites alone influence systemic toxicity alone.

 

After oral or inhalation administration, methacrylate esters are expected to be rapidly absorbed via all routes and distributed. Dermal absorption of esters is extensive only with occlusion of the site. Heylings (2013) used a QSPeR model for whole human skin based on that described by Potts and Guy (1992) to predict the dermal penetration rate of a large number of methacrylate esters, including ETMA (Heylings, 2013). For ETMA a low rate of dermal penetration is predicted (8.261 µg/cm²/h).

Toxicokinetics seem to be similar in man and experimental animals. MMA and other short chain alkyl-methacrylate esters are initially hydrolyzed by non-specific carboxylesterases to methacrylic acid and the structurally corresponding alcohol in several tissues, including but not limited to liver, olfactory epithelium, stratum corneum and blood. This has been shown for linear alkyl esters, several ether methacrylates, diesters as well as cycloalkyl and -aryl esters (Jones 2002, DOW 2013, McCarthy and Witz, 1997). Because of the structural similarity of ETMA to the other esters rapid hydrolysis is expected in the order of minutes.

Methacrylic acid (MAA) is subsequently cleared predominantly via the liver (valine pathway and the TCA (Tricarboxylic Acid) cycle).

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. 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.

 

2-ethoxyethanol is mainly metabolized toethoxyacetic acid and ethylene glycol. Exhalation of unmetabolized glycol ether is a minor route of elimination (Medinsky MA et al., 1990).

 

Alternative(minor) pathway: GSH Conjugation

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 (Cronin, 2012, Freidig et al. 1999). Hence, 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.

 

The fast hydrolysis observed for other Methacrylic acid esters is predicted to occur also for ETMA. Thus, following systemic exposure to ETMA the organisms will be mainly exposed to the metabolites Methacrylic acid and 2-Ethoxyethanol.

 

On this basis the systemic biological targets for the common compound(s)(AE 1.2) and the exposure of these systemic biological target(s) to the common compound(s) (AE 1.3) will be the same for ETMA as they are for the primary metabolites.

 

Furthermore, since carboxylesterases are widely distributed throughout the body and the half-life of the parent ester is very short the impact of parent compound (AE 1.4) is unlikely to be significant other than at the site of initial contact. Indeed, local hydrolysis at the site of contact is likely to be very rapid thereby minimising exposure to parent ester even at local targets. Since the source and target compounds are monoconstituents of high purity there are no impurities worthy of consideration. Finally, since the hydrolysis of the parent ester to Methacrylic acid and 2-Ethoxyethanol is equimolar and does not involve the formation of non-common compounds (AE 1.5) (including possible intermediates) their possible impact on the property under consideration does not have been considered.

Data availability

No experimental data are available for the target substance ETMA. However, based on the proposed hypothesis read-across from the metabolites methacrylic acid and 2-ethoxyethanol is proposed.

 

Several oral studies with repeated administration are available for the source substance 2-ethoxyethanol:

The test substance 2-ethoxyethanol was administered over 13 weeks (subchronic) to rats and dogs. The NOELs for rat and dog were determined to be 100 µL/kg bw/d (93 mg/kg bw/d) and 50 µL/kg bw/d (46 mg/kg bw/d).

 

For male mice treated with 2-ethoxyethanol for 13 weeks, the NOAEL for testicular degeneration and increased hematopoiesis in the spleen was 20,000 ppm (approx. 5123 mg/kg bw/d). For female mice in the 13-week study of 2-ethoxyethanol, the NOAEL for adrenal gland hypertrophy and increased hematopoiesis in the spleen was 5000 ppm (approx. 1300 mg/kg bw/d.

 

In the 13-week study of 2-ethoxyethanol in rats, the NOAEL for decreased thymus weights in males was 1250 ppm (approx. 100 mg/kg bw/d); for female rats treated with 2-ethoxyethanol for 13 weeks, the NOAEL for all histopathologic and hematologic effects was 5000 ppm (approx. 466 mg/kg bw/d).

 

For the second metabolite of ETMA, methacrylic acid, a subchronic inhalation toxicity study is available, which resulted in a NOAEL of 100 ppm for male and female rats exposed by whole body inhalation for 90 days.

 

The metabolite mainly determining toxicity is 2-ethoxyethanol. Thus, the overall NOAEL for the endpoint repeated dose toxicity is based on the NOAEL of 93 mg/kg bw/d obtained in a subchronic toxicity study in rat. Due to the allometric scaling factor of 1.4 for dog and 4 for rat, the DNEL derived from the rat study would be more conservative and should be used as key value for this endpoint.

The NOAEL of ETMA for repeated dose toxicity is 163 mg/kg bw/d (extrapolated based on molecular weight).

 

There are no data gaps for the endpoint repeated dose toxicity. There is no reason to believe that the results would not be relevant to humans. 

Justification for classification or non-classification

Based on the available data, ETMA does not need to be classified for repeated dose toxicity according to the criteria given in regulation (EC) 1272/2008. Thus, no labelling is required.