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Endpoint:
basic toxicokinetics, other
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:
other: Test procedure in accordance with generally accepted scientific standards and described in sufficient detail.
Objective of study:
metabolism
Principles of method if other than guideline:
Metabolism, ester hydrolysis, ADME
GLP compliance:
not specified
Species:
rat
Strain:
Fischer 344
Sex:
male
Route of administration:
other: in vitro and intavenous in vivo
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 n-BMA the half-life was 7.8 minutes and 99.7 % 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 n-BMA the half-life was 7.8 minutes and 99.7 % was removed by first-pass metabolism in the liver.

Endpoint:
basic toxicokinetics in vitro / ex vivo
Type of information:
experimental study
Adequacy of study:
supporting study
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
other: Study well documented, meets generally accepted scientific principles, acceptable for assessment.
Objective of study:
metabolism
Qualifier:
according to
Guideline:
other: not known
Principles of method if other than guideline:
Reactivity towards hydrolysis by carboxyl  esterase in vitro was studied 
GLP compliance:
not specified

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 butyl methacrylate; the second  order rate constant Km for the 

carboxylesterase hydrolysis of butyl  methacrylate was about 72 +/- 28* uM and 

Vmax was 1.84 ± 0.64* 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 reliable published study, the second order rate constant Km for the carboxylesterase hydrolysis of butyl methacrylate was about 72 +/- 28* µM and Vmax was 1.84 ± 0.64* nmole/min; increased alcohol chain length increases substrate affinity, yet decreases turnover for the enzymatic hydrolysis of these esters.
Executive summary:

In a reliable published study, the second order rate constant Km for the carboxylesterase hydrolysis of butyl methacrylate was about 72 +/- 28* µM and Vmax was 1.84 ± 0.64* nmole/min; increased alcohol chain length increases substrate affinity, yet decreases turnover for the enzymatic hydrolysis of these esters.

Endpoint:
basic toxicokinetics in vitro / ex vivo
Type of information:
experimental study
Adequacy of study:
supporting study
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
other: Study well documented, meets generally accepted scientific principles, accepted for assessment.
Objective of study:
metabolism
Principles of method if other than guideline:
Toxikokinetic study in vitro, reactivity of esters with glutathione
GLP compliance:
not specified
Details on study design:
Glutathione in phosphate buffered saline (PBS)  was mixed with an equal volume of the ester in PBS containing 5%(final)  propylene glycol; from the loss of free sulfhydryls (loss of optical  density at 412 nm over time) the rate of spontaneous reaction of ester  and glutathione was calculate

In a structure-activity relationship investigation of acrylate and methacrylate 

esters the reactivity towards glutathione and hydrolysis by  carboxyl esterase 

in vitro were studied in order to elucidate their  mechanism(s) of toxicity; 

glutathione in phosphate buffered saline (PBS)  was mixed with an equal volume of

the ester in PBS containing 5%(final)  propylene glycol; from the loss of free 

sulfhydryls (loss of optical  density at 412 nm over time) the rate of 

spontaneous reaction of ester  and glutathione was calculated; only the MMA value

has also been reported  in the 1991 paper.

Apparent Second-Order Rate Constants for the Reaction of Glutathione with  Acrylate and Methacrylate Esters
Ester               Apparent second-order rate constant
                    (K app, liter mol-1 min-1) a)
-------------------+-----------------------------------
Methyl acrylate      52.0 ± 5.0
Ethyl acrylate       26.6 ± 6.0
Butyl acrylate       38.7 ± 3.3
Tetraethyleneglycol 143.0 ± 4.0
diacrylate          (72.0 ± 6.4) b)
-------------------+-----------------------------------
Methyl methacrylate   0.325 ± 0.059 c)
Ethyl methacrylate    0.139 ± 0.022
Butyl methacrylate    No appreciable reaction rate
Tetraethyleneglycol   1.45 ±:0.17
dimethacrylate       (0.725 ± 0.087) b)
Ethyleneglycol        0.83 ± 0.12
dimethacrylate       (0.406 ± 0.059) b)c)
-------------------+-----------------------------------
a) Mean ± standard deviation of three experiments at 37°C and pH 7.4.
b) Bifunctional esters calculated as two independent esters.
c) The rate constant of each acrylate with the exception of methyl  methacrylate and ethyleneglycol dimethacrylate (as two independent  esters) is significantly different (p   0.05) from that of any other  acrylate.

Acrylates and methacrylates show a difference of roughly two orders of magnitude

in reactivity towards GSH, obviously due to the steric  hindrance of the 

nucleophilic reaction by the presence of the  alpha-methyl group in methacrylates. 

In whole blood the difference in reactivity between acrylates and  methacrylates 

appeared to be considerably smaller.

In both subgroups the reactivity towards GSH decreases with increasing  length of 

the alcohol side chain. Bifunctional esters show a reactivity  which is higher 

than that of the monoesters, absolutely and relatively  (more than the proportional 

effect expected by the presence of two double  bonds).

Conclusions:
In a reliable published study, acrylates and methacrylates show a difference of roughly two orders of magnitude in reactivity towards GSH, obviously due to the steric hindrance of the nucleophilic reaction by the presence of the alpha-methyl group in methacrylates. In whole blood the difference in reactivity between acrylates and methacrylates appeared to be considerably smaller. In both subgroups the reactivity towards GSH decreases with increasing length of the alcohol side chain. Bifunctional esters show a reactivity which is higher than that of the monoesters, absolutely and relatively (more than the proportional effect expected by the presence of two double bonds). For n-BMA no reactivity towards glutathione could be measured.
Executive summary:

In a reliable published study, acrylates and methacrylates show a difference of roughly two orders of magnitude in reactivity towards GSH, obviously due to the steric hindrance of the nucleophilic reaction by the presence of the alpha-methyl group in methacrylates. In whole blood the difference in reactivity between acrylates and methacrylates appeared to be considerably smaller. In both subgroups the reactivity towards GSH decreases with increasing length of the alcohol side chain. Bifunctional esters show a reactivity which is higher than that of the monoesters, absolutely and relatively (more than the proportional effect expected by the presence of two double bonds).For n-BMA no reactivity towards glutathione could be measured.

Endpoint:
dermal absorption in vitro / ex vivo
Type of information:
experimental study
Adequacy of study:
key study
Reliability:
1 (reliable without restriction)
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 nBMA was evaluated through rat and human epidermis and through rat whole skinin an in vitro system.
Signs and symptoms of toxicity:
not examined
Dermal irritation:
not examined
Absorption in different matrices:
Absorption of nBMA through rat epidermis:
The fastest rate of absorption (mean) of n-BMA through rat epidermis was measured to be 1543 µg cm-2 hr-1, which occurred between 0 and 6 hrs following the application of the chemical. The rate of absorption diminished from this time onwards, as indicated by a flattening of the curve. The total amount absorbed was calculated as 11% by 6 hours and 18.3% after 24 hours, at which point no more samples were taken.

Absorption of n-BMA through human epidermis:
The rate of absorption of n-butyl methacrylate was linear over the duration of the experiment and was calculated to be 76.7 µg cm-2 hr-1. Just over 2% of the applied dose was absorbed over the exposure period.

Absorption of n-BMA through whole (viable) rat skin
Only methacrylic acid (MAA) appeared in the receptor chambers of skin that had had n-butyl methacrylate applied to the surface. This would imply that all of the n-BMA that is absorbed through the skin is hydrolysed by carboxylesterases that are present in this tissue. The peak rate of appearance of MAA, which occurred between 2 and 10 hrs, was calculated to be 40.9 µg cm-2 hr-1. There is a lag-time present between 0 and 2 hrs. This experiment did not extend beyond ten hours, and the percentage dose removed from the donor reservoir was 0.4%.
Dose:
100 µl/cm²
Parameter:
percentage
Absorption:
18 %
Remarks on result:
other: 24 hours
Remarks:
Rat epidermis
Dose:
100 µl/cm²
Parameter:
percentage
Absorption:
2 %
Remarks on result:
other: 24 hours
Remarks:
Human epidermis
Dose:
100 µl/cm²
Parameter:
percentage
Absorption:
0.4 %
Remarks on result:
other: 10 hours
Remarks:
Whole (viable) rat skin
Conversion factor human vs. animal skin:
Human epidermis appears to be 20 times less permeable to n-BMA 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

22.7.7±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.
Conclusions:
n-BMA readily absorbs through rat and human epidermis and through whole rat skin. Human epidermis appears to be 20 times less permeable to 2-EHMA than rat epidermis.
Executive summary:

The absorption of n-BMA was evaluated through rat and human epidermis and through whole (viable) rat skin in an in vitro system. Glass diffusion cells are employed to measure the amount of n-BMA that is received into a receptor chamber with respect to time, following the application of 100 µl/cm² of n-BMA to the epidermal surface. The mean rate of absorption was 1540, 76.7 and 40.9 (appearance of MAA) µg cm-2 hr-1 and the total amount of chemical that was absorbed during the time of exposure was 18 (over 24 hours), 2 (over 24 hours) and 0.4% (over 10 hours), respectively. n-BMA appears readily absorbed through rat and human epidermis, but human epideremis is 20 times less permeable to n-BMA than rat epidermis.

Endpoint:
dermal absorption, other
Remarks:
calculation (not (Q)SAR)
Type of information:
calculation (if not (Q)SAR)
Remarks:
Migrated phrase: estimated by calculation
Adequacy of study:
weight of evidence
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
other: QSAR model based on several hundred chemicals tested in the same human skin model at the Central Toxicology laboratory and dermal Technology Laboratory .
Principles of method if other than guideline:
In silico prediction of skin permeation (human skin)
Absorption in different matrices:
The absorption rate of n-Butyl methacrylate through human skin was predicted to be 12.458 µg/cm²/h (moderate absorption).

In silico prediction of dermal absorption of higher methacrylates

In a QSAR model based on the physico-chemical properties (MW, logPow and satutared aqueous solubility) of chemicals the permeability of dermal absorption of a group of higher methacrylates was calculated. QSARs, when applied to estimating dermal permeability coefficients are also known as quantitative structure-permeability relationships (QSPeRs or QSPRs). The prediction model used in this investigation for a set of 54 methacrylate chemicals was based on an established model [Potts and Guy, (1992). Predicting Skin Permeability, Pharm. Res. 9(5): 663-669], using data derived with human epidermal membranes. The QSPeR approach can be used to identify compounds that are more likely to cross the stratum corneum barrier. Well dermal absorbed compounds include a low molecular weight, a general tendency towards being lipophilic in order to partition into the lipid rich stratum corneum/epidermis, but having sufficient aqueous solubility to cross the more polar regions of the dermis, prior to resorption into the blood circulation.

 

The dermal absorption was expressed using the permeability coefficient Kp which characterises the steady-state permeation rate of a chemical from a specific vehicle through a given membrane.

 

Terms used for categorising absorption of chemicals through human skin

Kp

(cm/h)

Absorption Rate

(µg/cm²/h)

Relative Absorption Rate Category

Predicted Absorption fromExposure

1 x 10-2– 10-1

> 500

Fast

very high

1 x 10-3– 10-2

100-500

rapid –fast

high

1 x 10-4– 10-3

10-50

50-100

slow – moderate

moderate – rapid

moderate

1 x 10-5– 10-4

0.1-10

very slow – slow

low

1 x 10-6– 10-5

0.001-0.1

extremely – very slow

minimal

 < 1 x 10-6

< 0.001

extremely slow

negligible

 

The absorption rate of n-Butyl methacrylate was predicted to be 12.458 µg/cm²/h. Therefore the dermal absorption is predicted to be moderate.

Conclusions:
In a QSAR model based on the physico-chemical properties (MW, logPow and satutared aqueous solubility) of chemicals the permeability of dermal absorption of a group of higher methacrylates was calculated. The absorption rate of n-Butyl methacrylate through human skin was predicted to be 12.458 µg/cm²/h. Therefore the dermal absorption is predicted to be moderate.
Executive summary:

In a QSAR model based on the physico-chemical properties (MW, logPow and satutared aqueous solubility) of chemicals the permeability of dermal absorption of a group of higher methacrylates was calculated.

The dermal absorption was expressed using the permeability coefficient Kp which characterises the steady-state permeation rate of a chemical from a specific vehicle through a given membrane.

The absorption rate of n-Butyl methacrylate was predicted to be 12.458 µg/cm²/h. Therefore the dermal absorption is predicted to be moderate.

Description of key information

Short description of key information on bioaccumulation potential result: 
Methacrylate esters, including n-butyl 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.
Short description of key information on absorption rate:
n-butyl methacrylate appears to be readily absorbed through rat and human epidermis, but human epidermis is 20 times less permeable to nBMA than rat epidermis.

Key value for chemical safety assessment

Bioaccumulation potential:
low bioaccumulation potential
Absorption rate - dermal (%):
2

Additional information

Absorption

N-BMA, like all members of the Lower Alkyl (C1-C8) Methacrylates category can be absorbed following ingestion with the shorter esters being absorbed more rapidly through the skin than the longer esters. The volatile esters in the category are efficiently scrubbed from the inhaled air in the upper respiratory tract after inhalation.

MMA was reviewed under the EU Risk Assessment Program while the other esters were addressed under the OECD SIDS program, but this did not address absorption. The EU Risk Assessment on MMA concluded that; “after oral or inhalation administration, methyl methacrylate is rapidly absorbed and distributed.In vitroskin 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 (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)).

Dermal

Jones (2002) studied the permeability of separated rat and human skin to alkyl methacrylate esters as part of a PhD thesis on development of a PBPK model to predict the pharmacokinetics and toxicity of methacrylate esters. Data on MMA, nBMA and 2-EHMA indicate that rat skin is more permeable to the absorption of these esters than human skin and there is a steep decline in the rate of penetration across the category from MMA to the larger ester, 2-EHMA. The rate of penetration of EMA was predicted to be between that of MMA and nBMA and that of iBMA comparable to nBMA.

Table: Summary of the results for the peak rates of absorption of alkyl methacrylate esters through rat & human epidermis(adapted from Jones, 2002)

Ester

Rat epidermis

Human epidermis

 

 

Peak rate of absorption (μg/cm²/hr) ±SEM

Period of peak absorption rate (hours)

% age of applied dose absorbed over x hours

Peak rate of absorption (μg/cm²/hr) ±SEM

Period of peak absorption rate (hours)

% age of applied dose absorbed over x hours

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

The in vitro measured data of Jones is very comparable to the predictions made later by Heylings who 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 members of the lower alkyl category (Table 10). Quantitatively the trend of decreased skin penetration rate across the Lower Alkyl (C1-C8) Methacrylatesategory was confirmed.

Table: QSAR prediction of absorption of methacrylate esters through whole human skin (extract from Heylings, 2013)

Ester

Molecular

Weight

Log P

Predicted

Flux

(μg/cm2/h)

Relative

Dermal

Absorption

MMA

100.12

1.38

64.422

Moderate

EMA

114.14

1.87

36.132

Moderate

iBMA

142.2

2.95

14.271

Moderate

nBMA

142.2

3.03

12.458

Moderate

n-HMA

170.25

4.34

5.940

Low

n-OMA

198.3

5.24

3.093

Low

2EHMA

198.3

4.95

1.126

Low

 

 

Metabolism

Taken from the EU Risk Assessment on MMA; “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 athigh tissue concentrations (McCarthy and Witz, 1991; Elovaara et al., 1983)”.

Taken from the OECD SIAR: “Other short chain alkyl-methacrylate esters, like MMA, are initially hydrolyzed by non-specific carboxylesterases to methacrylic acid and the structurally corresponding alcohol in several tissues (ECETOC 1995, 1996b).

 

Studies completed after the MMA RA have confirmed that all lower alkyl-methacrylate esters are rapidly hydrolysed by ubiquitous carboxylesterases (Table 11, 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, 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 (%LBF; Table below) resulting in their relatively rapid elimination from the body (T50%; Table below).

 

Table: Rate Constants for ester hydrolysis by rat-liver microsomes and predicted systemic fate kinetics following i.v. administration(extract from Jones, 2002)

Ester

Rat liver microsomes (100mg ml-1)

Rat liver microsomes (100mg ml-1)              

CL

(%LBF)

T50%(min)

Cmax(MAA)

(mg L-1)

Tmax(MAA)

(min)

 

Vmax(nM min-1mg-1)      

Km(mM)

 

 

 

 

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; OMAoctyl methacrylate. Fate kinetics determined using the “well-stirred” model; CL%LBFClearance as percentage removed from liver blood flow i.e. first pass clearance; T50%- time taken for 50% of parent ester to have been eliminatedfrom the body; Cmaxmaximum concentration of MAA in circulating blood; Tmaxtime in minutes to peak MAA concentration in blood “Jones, 2002”.

Subsequent metabolism of the primary metabolites within the body

Again, taken from the OECD SIAR:Methacrylic acid and the corresponding alcohol are subsequently cleared predominantly via the liver (valine pathway and the TCA (TriCarboxylic Acid) cycle, respectively). The carboxylesterasesare 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.

In terms of MAA, the common metabolite for these estersa comparison of measured blood concentration data after i. v. administration of 10 and 20 mg/kg MAA was made and a simulation based on a one-compartment model shows good agreement (Jones, 2002). Based on that information, the following kinetic parameters were determined from a simultaneous fit of the in vivo data to a one-compartment model with non-linear elimination (Vss = 0.039 L/SRW; Vmax = 19.8 mg/hr x SRW; Km = 20.3 mg/L; SRW: standard rat weight = 250 g) the half-life of MAA in blood was calculated as 1.7 min.

Interms of the corresponding alcohol metabolite for these esters, they are all 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. In the case of the more volatile alcohols a significant portion is excreted un-metabolised in urine and/or exhaled air.

 

Conjugation

The C=C double bond of methacrylate esters makes these chemicals potential 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 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 Lower Alkyl (C1-C8) Methacrylatescategory (Tindale, 2015).The rate of reaction is low for all members of this category (see table below) whether based upon the local charge-limited electrophilicity index ωq(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.

Table: GSH reactivity of methacrylate esters

Ester

GSH reactivity*(meas.)
logKGSH

GSH reactivity* (predicted.)
logKGSH

GSH reactivity assessment**

GSH reactivity*** (predicted.)
logKGSH

MMA

-1.14

-0.73

slight

-0.62

EMA

-1.24

-0.88

slight

-0.79

i-BMA

-0.44

-0.82

slight

-0.74

n-BMA

 

 

slight

-0.78

HMA

 

 

slight

 

2EHMA

 

 

slight

 

Metabolites

 

 

 

 

MAA

 

 

No alert found

 

*as quoted by Schwöbel et al., 2010
**as calculated by Cronin, 2012 using the method of Schwöbel et al., 2010

***as calculated by Tindale, 2015 according to the method of (Fujisawa and Kadoma, 2012)

Trends

Particularly in the case of skin absorption there is trend of decreasing rate of absorption of the parent ester C1-C8with increasing ester chain length. This trend is also likely to extend to some extent to absorption across all barrier membranes including the gut and respiratory epithelium. All esters are rapidly hydrolysed within the body whether in local tissues, the blood or ultimately within the liver by non-specific esterases. There is an observed trend towards increasing half-life of the parent ester with increasing ester chain length. The common primary metabolite, MAA, as well as the corresponding alcohol primary metabolites are cleared rapidly in all cases.

Summary and discussion on toxicokinetics

N-BMA and the other methacrylate esters are absorbed to a greater or lesser extent by all routes and are subsequently rapidly hydrolysed 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 the short chain esters to tens of minutes for the longer esters. The primary metabolites, MAA and the corresponding alcohol, are also subsequently cleared rapidly from blood by standard physiological pathways, with the majority of the administered dose being exhaled as CO2. In the case of MAA the systemic half-life in rats is only 1.7 minutes. On the basis of the rapid metabolism and short half-lives a systemic accumulation of the esters and their metabolites is not expected.

Local effects resulting from the hydrolysis of the ester to the irritant and corrosive metabolite MAA are only observed following inhalation exposure in rodents and this has been shown to be due to the localised concentration of high levels of non-specific esterases in the Bowman’s glands of the nasal olfactory tissues. This combination of highly efficient mechanisms of absorption/deposition and localised enzyme activity does not occur in the case of the dermal and oral routes so localised tissue corrosion would not be expected to occur. In summarising the available PBPK data on MMA SCOEL concluded that “Extensive PBPK modelling work has predicted that on kinetic grounds for a given level of exposure to MMA human nasal olfactory epithelium will be at least 3 times less sensitive than that of rats to the toxicity of MMA” (SCOEL, 2005). Local effects are not anticipated as a result of the localised concentration of the corresponding alcohol since the alcohols themselves do not produce local effects.

Overall there is a high level of confidence in the toxicokinetic and toxicodynamic assessment for these chemicals based upon in vitro and in vivo studies in rodents and human tissues. This is further supported by clear trends across the category consistent with predicted trends based on recognised QSAR based models. In terms of the overall relevance of the findings in animals to humans there is a high degree of confidence since the same toxicokinetic/dynamic processes are known to occur in humans. In the case of dermal exposure there is robust in vitro data based upon measurements in animal and human skin supported by an established QSAR model that shows that dermal absorption, and therefore risk, of these esters is lower in humans than in rodents. In the case of inhalation exposure well recognised morphological and physiological differences between rodents and humans have been confirmed for the methyl ester to indicate a lower sensitivity of humans than rodents to the local effects in the upper respiratory tract. This is consistent with findings in limited studies in clinical volunteers and by cross-sectional studies in workers with long-term exposure to concentrations of MMA vapour well in excess of the effect concentration in rodents. There is a high level of confidence that the findings for MMA can be equally applied to the ethyl ester. This is based upon the close structural similarity and physico-chemical properties supported by robust PBPK observations and effect data in animals.

In the case of the two butyl esters there is a high degree of confidence that data on the linear, n- butyl ester can be read across to the branched, iso-butyl 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 ethyl hexyl ester other than for dermal absorption. However, the low volatility of this ester indicates that this is not critical to the overall assessment. Overall, therefore, it is considered that the available toxicodynamic data is sufficiently adequate, particularly since this is consistent with the general trend within the category and the predicted limited opportunity for exposure. In conclusion therefore, there is a high level of confidence in the toxicokinetic assessment for the category of Lower Alkyl (C1-C8) Methacrylates.

References quoted from the EU ESR 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