Registration Dossier

Administrative data

Link to relevant study record(s)

Reference
Endpoint:
basic toxicokinetics, other
Type of information:
experimental study
Adequacy of study:
key study
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
study well documented, meets generally accepted scientific principles, acceptable for assessment
Objective of study:
metabolism
Qualifier:
no guideline 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.
GLP compliance:
not specified
Radiolabelling:
no
Species:
rat
Strain:
Wistar
Sex:
male
Route of administration:
other: in vitro and intravenous in vivo
Type:
metabolism
Results:
The studies confirmed that alkyl-methacrylate esters are rapidly hydrolyzed by ubiquitous carboxylesterases. 

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 firstpass metabolism of the alkylmethacrylates. 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.

Description of key information

Bioaccumulation potential:

As indicated in a dermal absorption study, the structural analogue Dodecyl methacrylate is metabolised during penetration of the skin and not expected to enter the circulation as the parent ester.

Gastro intestinal-, respiratory- and dermal absorption are practically not expected due to physico-chemical properties and in vitro dermal absorption studies,  where methacrylate esters of molecular weights equal to or greater than butyl methacrylate were not detected in the receptor fluid and are not expected to enter the circulation as the parent ester. Due to structural analogies the same behaviour can be expected from C9 -11 methacrylates.

Absorption rate:

The structural analogue Dodecyl methacrylate appears to be absorbed through rat skin and epidermis to a very low extent of 0.26% in 24 hrs. It is fully metabolized to methacrylic acid during the passage (first-pass effect). As indicated by a PB-PK model used in this study, human skin is 14 times less permeable to Dodecyl methacrylate than rat skin. Due to structural analogies the same behaviour can be expected from C9 -11 methacrylates.

Key value for chemical safety assessment

Bioaccumulation potential:
no bioaccumulation potential

Additional information

Physical chemical properties

The test substance is a liquid UVCB containing isomer with a molecular weight of 230.0 g/mol. The calculated log Pow value is > 6.5 at 23 °C and the solubility in water is 40 μg/L ± 20µg/l at 20 °C. There is a hydrolysis study available. Because of the low solubility in the buffer solutions, the determination of the hydrolysis as a function of pH according to OECD Guideline is not feasible and sensible.The vapour pressure of the substance is 9.3 hPa at 20 °C. These physico-chemical properties of the substance will enable qualitative judgements of the toxicokinetic behaviour (Guidance on information requirements and chemical safety assessment Chapter R.7.c, R.7.12 Guidance on Toxicokinetics).

GI absorption

No experimental data are available for GI absorption. Substances with a molecular weight below 500, high water solubility and a log Pow between -1 and 4 are favourable for absorption. With a log Pow > 4 passive diffusion through membranes is not expected but the substance may form micelles and be absorbed into the lymphatic system. No signs of systemic toxicity indicating that absorption has occurred were seen in an acute oral toxicity test up to 2000 mg/kg bw.

But in an oral repeated dose study (OECD 422) theadministration by gavage of C9-11 Methacrylate to Wistar rats revealed adverse signs of systemic toxicity in male animals at a dose level of 1200 mg/kg bw/d and in females at a dose level of 360 mg/kg bw/d. The target organ was the liver showing increased incidence and severity of centrilobular single cell necrosis. Local adverse effects in the forestomach were observed at 360 and 1200 mg/kg bw/d. Thus, the no observed adverse effect level (NOAEL) for general systemic toxicity was 360 mg/kg bw/d for male and 120 mg/kg bw/d for female Wistar rats.

 

Respiratory absorption – Inhalation

No experimental data are available for respiratory absorption. The vapour pressure of C9-11 Methacrylate is 9.3 hPa and therefore inhalation is not a relevant route of absorption.

 

Dermal absorption

C9-11 Methacrylate is a liquid with a molecular weight below 500 g/mol which would favour dermal uptake, but with a low water solubility 40 μg/L ± 20µg/l at 20 °C. Dermal uptake from the stratum corneum into the epidermis is likely to be very low. With log Pow > 6.5 the rate of transfer between the stratum corneum and the epidermis will be slow and will limit absorption across the skin. 

There is no acute dermal study for C9-11 Methacrylate available. In addition, no signs of systemic toxicity indicating absorption were observed in an acute dermal toxicity study on Isodecy methacrylate with doses up to 3000 mg/kg bw.

 

Metabolism

No data are available on the metabolism of C9-11 Methacrylate in vivo.

 

The prominent pathway for the metabolism of higher methacrylate esters starts with ester hydrolysis resulting in methacrylic acid and the corresponding alcohol (Jones, 2002; McCarthy and Witz, 1997). While the acid is further metabolised via the valine pathway of the citric acid cycle (ECETOC, 1995; European Union, 2002), the alcohol may be further metabolised by the two standard metabolic pathways for fatty alcohols (1. oxidation: fatty alcohol -> aldehyde -> acid, and subsequently CoA-mediated fatty acid metabolism - or - 2.: glucuronidation of the alcohol and excretion). Alkyl esters of methacrylic acid up to C8 (2-ethylhexyl methacrylate) showed rapid metabolism with half lives in rat blood of less than

30 min (Jones, 2002): A series of in vitro and in vivo studies with a series of methacrylates were used to develop a PBPK model that accurately predicts the metabolism and fate of these monomers. The studies confirmed that alkyl methacrylate esters are rapidly hydrolysed in the organism by ubiquitous carboxylesterases.

First pass (local) hydrolysis of the parent esters has been shown to be significant for all routes of exposure. In vivo measurements of rat liver metabolism or activity indicated this organ as the one with the greatest esterase activity.Similar measurements for skin microsomes indicated an approximately 20-fold lower activity than for liver. Nevertheless, this activity was substantial and capable of almost complete first-pass metabolism of the alkyl methacrylates applied on skin. For example, no parent ester penetrated whole rat skin in vitro for n-butyl methacrylate, octyl methacrylate or lauryl methacrylate. When related methacrylates tested, only methacrylic acid was identified in the receiving fluid. In addition, model predictions indicate that esters of ethyl methacrylate or larger would be completely hydrolysed before entering the circulation via skin absorption. This pattern is consistent with a lower rate of absorption for these esters indicating that the rate of metabolism 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 hydrolysed following inhalation exposure. These studies showed that any systematically absorbed parent ester will be effectively removed either upon the site of contact or 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 2).

 

Table 2: Rate constants for the ester hydrolysis by rat-liver microsomes and predicted systemic fate kinetics from methacrylates following i.v. administration:

 

Ester

Vmax

Km

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

 

MAA = Methacrylic acid (CAS 79-41-4); MMA = Methyl methacrylate (CAS 80-62-6); EMA =

Ethyl methacrylate (CAS 97-63-5); i-BMA = Isobutyl methacrylate (CAS 97-86-9); n-BMA = n-Butyl methacrylate (CAS 97-88-1); HMA = Hexyl methacrylate (CAS 142-09-6); 2EHMA = 2-Ethylhexyl methacrylate (CAS 688-84-6); OMA = Octyl methacrylate (CAS 2157-01-9)

 

Vmax (nM/min/mg) and Km (μM) from rat-liver microsomes (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.

 

GSH conjugation, the second potential pathway, has only been observed with small alkyl methacrylates (methyl methacrylate/MMA, ethyl methacrylate/EMA) but was no longer measurable with butyl methacrylate. Moreover, GSH conjugation was only detectable with MMA and EMA at high concentrations which are only achievable under laboratory conditions (Elovaara et al., 1983, Mc Carthy et al., 1994).

 

Distribution

As the expected bioavailability is very low, neither GI- and respiratory absorption nor dermal absorption are expected to a more than minimal extent, and complete metabolism is predicted. Only a very low amount of the substance comes into consideration for distribution in blood or plasma and accumulation in organs and tissues. In theory the lipophilic molecule is likely to distribute into cells and then the intracellular concentration may be higher than extracellular concentration particular in fatty tissues, but this is of secondary importance as the bioavailability of the substance is very low.

 

Accumulation

In principle, C9-11 Methacrylate accumulation in adipose tissue could be expected as the calculated log Pow is > 6.5, but before this can occur, the substance is expected to be completely metabolized due to rapid cleavage byesterasesas described above.

 

Excretion

As absorption is very low and complete metabolism is very fast, excretion of the parent compound is not to be expected.