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Administrative data

Description of key information

subacute NOAEL = 29.5 mg/kg bw/d (rat) (read-across from 2 -methoxyethanol); BMD analysis and biomarker of testicular damage; not a full study, 10 d only, rather mechanistic study, not used for DNEL derivation

Key value for chemical safety assessment

Repeated dose toxicity: via oral route - systemic effects

Endpoint conclusion
Endpoint conclusion:
adverse effect observed
Dose descriptor:
NOAEL
29.5 mg/kg bw/day
Study duration:
subacute
Species:
rat

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 MTMA are available for the assessment of repeated dose toxicity. However, studies are available for the primary metabolites 2-methoxyethanol 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-methoxyethyl methacrylate (MTMA). 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-methoxyethanol.

 

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

 

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 MTMA (Heylings, 2013). For MTMA a low rate of dermal penetration is predicted (1.366 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 MTMA 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-methoxyethanol is mainly metabolized to methoxyacetic acid and excreted via the urine (Mebus et al, 1992; Miller RR, 1987).

 

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 MTMA. Thus, following systemic exposure to MTMA the organisms will be mainly exposed to the metabolites Methacrylic acid and 2-methoxyethanol.

 

On this basis the systemicbiological 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 MTMA 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-methoxyethanol 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 MTMA. However, based on the proposed hypothesis read-across from the metabolites methacrylic acid and 2 -methoxyethanol is proposed.

 

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

In the 13-week study of 2-methoxyethanol in rats, a no-observed-adverse-effect level (NOAEL) was not reached, since testicular degeneration in males and decreased thymus weights in males and females occurred at the lowest concentration administered (750 ppm, corresponding to 71 mg/kg bw/d).

 

For male mice treated with 2-methoxyethanol for 13 weeks, the NOAEL for testicular degeneration and increased haematopoiesis in the spleen was 2000 ppm (295 mg/kg bw/d). A NOAEL was not reached for female mice treated with 2-methoxyethanol, since adrenal gland hypertrophy and increased hematopoiesis in the spleen occurred at the lowest concentration administered (2000 ppm, corresponding to 492 mg/kg bw/d).

 

A sub-acute study in male rats was designed to establish if creatinuria (a non-invasive biomarker) was maintained as a result of testicular damage. Animals were dosed with methoxyethanol in drinking water up to nominal doses of 250 mg/kg bw/d. The study established a NOAEL of 43 mg/kg bw/ dbased on adverse testes histopathology and other gross effects at the next highest dose, but a NOAEL could not be established based on an elevated urinary creatine/creatinine ratio at the lowest dose tested (43 mg/kg bw/d).

Sufficient data is available in the paper to use the Benchmark dose approach to estimate a no effect level.  Using a continuous power model and a BMD level of 10%, the BMD for no elevation of creatine:creatinine ratio is 15.6 mg/kg bw/d. This study defines the lowest oral NOEC observed for methoxyethanol for available studies using oral treatment of rats.

 

For the second metabolite of MTMA, 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-methoxyethanol. Thus, the overall dose descriptor for the endpoint repeated dose toxicity is based on the BMDL10 of 15.6 mg/kg bw/d obtained in a subacute toxicity study in rat. The BMDL10 of MTMA for repeated dose toxicity is 29.5 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, MTMA 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.