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Endpoint:
basic toxicokinetics
Remarks:
in vitro and intavenous in vivo
Type of information:
experimental study
Adequacy of study:
key study
Reliability:
1 (reliable without restriction)
Rationale for reliability incl. deficiencies:
test procedure in accordance with generally accepted scientific standards and described in sufficient detail
Objective of study:
metabolism
Qualifier:
no guideline available
Principles of method if other than guideline:
A series of in vitro and PBPK models were used to determine and predict  the skin absorption and metabolism of a series of methacrylate monomers.  Initial studies were conducted using the rat epidermal membrane model. The results of these studies, when compared to the subsequent rat whole skin model in vitro experiments clearly indicated that the latter studies were more pertinent to the goals of the studies, particularly since the use of epidermal membranes appeared to remove the carboxylesterase activity from the skin samples.
Metabolism, ester hydrolysis, ADME
GLP compliance:
no
Species:
rat
Strain:
Wistar
Sex:
male
Details on test animals and environmental conditions:
Epidermal membrane absorption studies
Skin was used from male rats of the Wistar-derived strain (supplied by Charles River UK Ltd, Margate, Kent, UK.) aged 28 days ± 2 days

Whole skin absorption studies
Skin was taken from male Fischer F344 (supplied by Harlan Olac) rats weighing between 200 and 250 g.

Human epidermal membrane absorption studies
Extraneous tissue was removed from human abdominal whole skin samples obtained post mortem in accordance with local ethical guidelines.
Route of administration:
other: in vitro and intravenous in vivo
Details on study design:
A series of in vitro and in vivo studies with a series of methacrylates were used to develop PBPK models that accurately predict the metabolism and fate of these monomers. The studies confirmed that alkyl-methacrylate esters are rapidly hydrolyzed by ubiquitous carboxylesterases. First pass (local) hydrolysis of the parent ester has been shown to be significant for all routes of exposure. In vivo measurements of rat liver indicated this organ has the greatest esterase activity. Similar measurements for skin microsomes indicated approximately 20-fold lower activity than for liver. However, this activity was substantial and capable of almost complete first-pass metabolism of the alkyl-methacrylates. For example, no parent ester penetrated whole rat skin in vitro for n-butyl methacrylate, octyl methacrylate or lauryl methacrylate tested experimentally with only methacrylic acid identified in the receiving fluid. In addition, model predictions indicate that esters of ethyl methacrylate or larger would be completely hydrolyzed before entering the circulation via skin absorption. This pattern is consistent with a lower rate of absorption for these esters such that the rate is within the metabolic capacity of the skin. Parent ester also was hydrolyzed by S9 fractions from nasal epithelium and was predicted to be effectively hydrolyzed following inhalation exposure.
Type:
metabolism
Results:
Half-life of MMA after i.V. injection: 4.4 min (PBPK estimate)
Metabolites identified:
yes
Details on metabolites:
Methacrylic acid

A series of in vitro and in vivo studies with a series of methacrylates were 

used to develop PBPK models that accurately predict the metabolism  and fate of 

these monomers. The studies confirmed that alkyl-methacrylate  esters are 

rapidly hydrolyzed by ubiquitous carboxylesterases. First pass  (local) 

hydrolysis of the parent ester has been shown to be significant  for all routes 

of exposure. In vivo measurements of rat liver indicated  this organ has the 

greatest esterase activity.  Similar measurements for  skin microsomes indicated

 approximately 20-fold lower activity than for  liver.  However, this activity 

was substantial and capable of almost  complete first-pass metabolism of the 

alkyl-methacrylates. For example,  no parent ester penetrated whole rat skin in 

vitro for n-butyl  methacrylate, octyl methacrylate or lauryl methacrylate 

tested  experimentally with only methacrylic acid identified in the receiving  

fluid. In addition, model predictions indicate that esters of ethyl 

methacrylate or larger would be completely hydrolyzed before entering the 

circulation via skin absorption. This pattern is consistent with a lower rate of absorption for these esters 

such that the rate is within the metabolic capacity of the skin. Parent  ester 

also was hydrolyzed by S9 fractions from nasal epithelium and was  predicted to 

be effectively hydrolyzed following inhalation exposure. 

These studies showed that any systemically absorbed parent ester will be  

effectively removed during the first pass through the liver (CL as % LBF,  

see table). In addition, removal of methacrylic acid from the blood also  

occurs rapidly (T50%; see table).  


Table:
Rate constants for ester hydrolysis by rat-liver microsomes and predicted 

 systemic fate kinetics for methacrylates following i.v. administration:

 Ester    Vmax       Km        CL    T50%    Cmax    Tmax
----------------------------------------------------------
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
----------------------------------------------------------

Vmax (nM/min/mg) and Km (µM) from rat-liver microsome (100 µg/ml)  determinations;  
CL = clearance as % removed from liver blood flow, T50% = Body  elimination time

(min) for 50% parent ester, Cmax = maximum concentration  (mg/L) of MAA in blood, 

Tmax = time (min) to peak MAA concentration in  blood from model predictions.

Table 2:
Rate constants for ester hydrolysis by human-liver microsome samples:

 Ester    Vmax (nM/min*mg) Km (mM) CL (µL/min*mg)    
-----------------------------------------------
MMA       1721      4103     419   
EMA        936      1601     584  
i-BMA       80       441     181
n-BMA      211       158    1332
HMA        229       66    3465
2EHMA      53        48     1109
OMA        243       38    6403

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

CL is calculated from the mean Vmax and Km

Conclusions:
Using a reliable experimental method, the in vivo and in vitro investigations as well as the PBPK models developed from the data showed that alkyl-methacrylate esters are rapidly absorbed and are hydrolyzed at exceptionally high rates to methacrylic acid by high capacity, ubiquitous carboxylesterases. Further, the removal of the hydrolysis product, methacrylic acid, also is very rapid (minutes). For OMA the half-life was 27.2 minutes and 99.9 % was removed by first-pass metabolism in the liver.
Executive summary:

Using a reliable experimental method, the in vivo and in vitro investigations as well as the PBPK models developed from the data showed that alkyl-methacrylate esters are rapidly absorbed and are hydrolyzed at exceptionally high rates to methacrylic acid by high capacity, ubiquitous carboxylesterases. Further, the removal of the hydrolysis product, methacrylic acid, also is very rapid (minutes). For OMA the half-life was 27.2 minutes and 99.9 % was removed by first-pass metabolism in the liver.

Endpoint:
dermal absorption in vitro / ex vivo
Type of information:
experimental study
Adequacy of study:
key study
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
other: Test procedure in accordance with generally accepted scientific standards and described in sufficient detail
Qualifier:
equivalent or similar to
Guideline:
OECD Guideline 428 (Skin Absorption: In Vitro Method)
GLP compliance:
no
Radiolabelling:
no
Species:
rat
Strain:
Wistar
Sex:
male
Type of coverage:
open
Vehicle:
unchanged (no vehicle)
Duration of exposure:
48 hours
Doses:
100 µl/cm²
Details on in vitro test system (if applicable):
The absorption of n-OMA was evaluated through rat and human epidermis in an in vitro system.
Signs and symptoms of toxicity:
not examined
Dermal irritation:
not examined
Absorption in different matrices:
Absorption of OMA through rat epidermis:
Octyl methacrylate readily absorbed through rat epidermis at a constant mean rate of 159 µg cm-2 hr-1. This rate of absorption was constant over the whole 24 hour exposure/sampling period. The total amount of chemical that was absorbed during the time of exposure was 4.2% of the donor reservoir.

Dose:
100 µl/cm²
Parameter:
percentage
Absorption:
4.2 %
Remarks on result:
other: 24 hours
Remarks:
Rat epidermis
Conversion factor human vs. animal skin:
Human epidermis appears to be several times less permeable to n-OMA than rat epidermis.

The results of the whole-skin penetration studies and the model predictions for 

other methacrylate esters are presented in the table.

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 ±3.7

3-24

0.6% / 24h

OMA

159±15

0-24

-

7.8

-

-


Ester Peak = rate of appearance of the parent ester (µg/cm2/hr)
 
MAA Peak = rate of appearance of the hydrolysis product, MAA (µg/cm2/hr)
Period Peak Absorp. = Time (hours) after application for peak absorption
% Applied Dose = total % absorbed
** Predicted rates of MAA from model estimates.



Summary of the peak rates of absorption of MAA & alkyl-methacrylate esters through whole rat and human skin

Substance

Molecular volume

Rat whole rat

Human whole skin

Peak rate of appearance (µg*cm-2*h-1)+- SEM

Period of

peak

absorption

rate (hours)

% age of

applied

dose

absorbed

over x hours

Rate of

absorption of

ester/MAA

(μg*cm-2 *hr-1)

Ester

MAA

MAA

78.96

 

4584 ±

344

5-8

70%/24

327

MMA

93.1978

360±

20.9

108±

4.59

2.5-24

11.3%/24

33.4**

EMA

107.436

 

190**

 

 

13.6**

iBMA

135.646

 

56**

 

 

4**

nBMA

135.856

 

40±

9.4

2-10

0.4%/10

2.9**

6HMA

164.277

 

20**

 

 

1.4**

2EHMA

191.66

 

9**

 

 

0.6**

OMA

192.696

 

10.3 ±

0.65

8-24

0.24%/24

0.7**

12LMA

249.536

 

11.8 ±

2.11

8-24

0.26%/24

0.8**

The values in normal type were obtained experimentally, whilst those in italics are predicted values.

** Values are predicted rates of appearance of total chemical including parent ester and metabolite

Conclusions:
Prediction due to the experimental results of analogue substances (n-BMA and 2-EHMA): OMA readily absorbs through rat and human epidermis. Human epidermis appears to be several times less permeable to OMA than rat epidermis.
Human epidermis appears to be 10 times less permeable to 2-EHMA than rat epidermis and 20 times less permeable to n-BMA than rat epidermis.
Executive summary:

The absorption of Octyl methacrylate (OMA) was evaluated through rat and human epidermis in an in vitro system. The technique measures the rate of absorption of OMA across the epidermis. Glass diffusion cells are employed to measure the amount of OMA that is received into a receptor chamber with respect to time, following the application of 100 µl/cm² of OMA to the epidermal surface. OMA absorbed at a constant rate throughoutr the period of exposure/sampling (0 -24 hours). The mean rate of absorption was calculated 159 µg cm-2 hr-1 and the total amount of chemical that was absorbed during the time of exposure was 4.2% (over 24 hours), respectively. OMA appears readily absorbed through rat and human epidermis, but human epideremisc is several times less permeable to OMA 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.

Description of key information

Short description of key information on bioaccumulation potential result:

Methacrylate esters, including octyl methacrylate (n-OMA), 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 n-OMA the half-life was 27.2 minutes and 99.9 % was removed by first-pass metabolism in the liver. Reliable data on the primary metabolites methacrylic acid are available and do not reveal critical properties.

Short description of key information on absorption rate of 2-Ethylhexyl methacrylate (2 -EHMA) structural analogue substance and member of the category (C1 -C8 Lower Alkyl Methacrylates):

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

Additional information

Read across to stuctural analogue substance 2-Ethylhexyl methacrylate (2 -EHMA) also a member of category (C1 -C8 Lower Alkyl Methacrylates)

In the case of the two octyl esters there is a high degree of confidence that data on the linear, n-octyl ester (n-OMA) can be read across to the branched, 2-Ethylhexyl ester based upon the very similar physical chemical properties, Michael addition reactivity, half-life within the body and ultimate metabolic fate, both of the parent ester as well as the alcohol metabolites. There is only very limited toxicokinetic data for the n-octyl methacrylate ester and the 2-ethyl hexyl methacrylate ester other than for dermal absorption. However, the low volatility of this ester indicates that this is not critical to the overall assessment.

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 as well as n-OMA 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, 2 -EHMA and n-OMA, 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 and n-OMA 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 and n-OMA.

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; T50%- time taken for 50% of parent ester to have been eliminated from the body; Cmax– maximum concentration of MAA in circulating blood; Tmax– 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) for n-OMA (27.2) . 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 involving aldehyde 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 ωq(Schwöbel et al., 2010) and the13C chemical shift, of the β-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 ωq(Schwöbel et al., 2010, Cronin, 2012, 2015), or13C chemical shift, of the β-Carbon atom (Fujisawa and Kadoma, 2012; Tindale, 2015) when compared with other esters and unsaturated ketones and aldehydes (McCarthy & Witz, 1991). This is because the 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 25 ppm 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

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Discussion on absorption rate:

The absorption of 2-EHMA was evaluated through rat and human epidermis in an in vitro system. 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.