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Description of key information

Methyl methacrylate, allyl methacrylate and the other methacrylate esters 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. The one 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. The other primary metabolite is acrolein causes liver toxicity. Local effects resulting from the hydrolysis of the ester to MAA are only observed following inhalation exposure and this has been shown to be due to the localised concentration of non-specific esterases in nasal olfactory tissues. In summarising the available PBPK data on MMA SCOEL concluded that “Extensive PBPK modeling 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). 

Key value for chemical safety assessment

Bioaccumulation potential:
low bioaccumulation potential

Additional information

Data availability

Only one publication is available with allyl methacrylate. However, alkyl and glycol acrylates and methacrylates have been shown to hydrolyze to acrylic or methacrylic acid and the corresponding alcohol (McCarthy, 1997; cited in the OECD SIDS IUCLID data set of 2009). The affinity and turnover of this reaction is reduced with increasing chain length but the overall reaction is consistent and reasonably predictable.

In addition, Jones (2002) conducted an elaborate series of in vitro and in vivo studies on carboxylesterase activity with 7 methacrylates ranging from methyl methacrylate to octyl methacrylate (with increasing ester size) for several tissues (blood, liver, skin and nasal epithelium) from rats and humans (cited in the OECD SIDS IUCLID data set of 2009). It was concluded that methacrylate esters are 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.

Based on the results of these studies, allyl methacrylate can be expected to hydrolyze rapidly into allyl alcohol and methyl methacrylate.

Additionally Allyl acetate can be used for read across as the acetates will also rapidly be hydrolyzed by carboxylesterases and allyl acetate will be hydrolized into the common metabolite allyl alcohol and the ubiquitous metabolite acetic acid.

In conclusion available data of allyl alcohol and methyl methacrylate and other alkyl methacrylates can be used for assessment of the toxicokinetic parameters of allyl methacrylate. For methyl methacrylate and allyl alcohol (2 -propen-1 -ol) extensive data are available.

Allyl alcohol, Allyl acetate, Acrolein

- Auerbach SS et al, (2008) summarized as follows: Metabolism studies of allyl acetate and allyl alcohol strongly suggest these chemicals are protoxicants that are metabolized to acrolein, a highly reactive, unsaturated aldehyde. Upon oral administration allyl acetate is rapidly reduced by carboxyl esterases in the stomach, liver and blood to yield allyl alcohol and acetic acid. Allyl alcohol is then further oxidized to acrolein by alcohol dehydrogenase. Acrolein can subsequently be detoxified by aldehyde dehydrogenase to acrylic acid or be conjugated by glutathione.

The glutathione adducts can potentially be converted by cytochromes P450 to the hard electrophile and known mutagen/ carcinogen, glycidaldehyde (Barros et al., 1994; Feron et al., 1991). Alternatively, degradation products of glutathione adducts,S-(3-hydroxypropyl)mercapturic acid andS-(2-carboxyethyl) mercapturic acid, are found in the urine of rats administered acrolein, allyl alcohol, allyl chloride, allyl amine, or allyl bromide (Kaye, 1973; Sanduja et al., 1989). In support of this proposed mechanism of toxicity, pharmacological inhibition of carboxylesterase or alcohol dehydrogenase was shown to attenuate allyl ester mediated hepatotoxicity (Silver and Murphy, 1978). Similar studies with allyl alcohol demonstrated that inhibition of alcohol dehydrogenase leads to a reduction in hepatotoxicity (Reid, 1972). Furthermore,mice hypomorphic for alcohol dehydrogenase are resistant to allyl alcohol induced liver injury (Belinsky et al., 1985). Inhibition of aldehyde dehydrogenase enhances the toxicity of allyl alcohol suggesting that not only does the production of acrolein affect toxicity, but its metabolic breakdown also plays a role in liver injury (Rikans, 1987).

Collectively, the data from previous studies strongly support the role of acrolein as the primary reactive species and toxicant following allyl acetate or allyl alcohol exposure:

- In a publication on in vitro metabolism of α-β unsaturated esters glutathione depletion in rat hepatocytes was tested. Allyl methacrylate was found to cause higher GSH depletion than other acrylates and methacrylates. These results are in accordance with GSH depletion of acrolein which is a metabolite of allyl alcohol [Freiding A et al. , 2001]

- There are several studies cited in OECD SIDS report of Allyl alcohol (2 -propen-1 -ol):

2-Propen-1-ol is metabolized rapidly after administration in vivo. When rats were administered orally with 120 mg/kg bw of 2-propen-1-ol, the concentration in blood was 9-15 ug/mL between 15 and 120 minutes after administration. In the meantime, when rats were administered intravenously with 30 mg/kg bw of 2-propen-1-ol, the concentration in blood was 24 ug/mL at several minutes and decreased to 4 ug/mL at 15 minutes after administration. At one hour after administration, 2-propen-1-ol almost disappeared from the blood. When 2-propen-1-ol was administered intravenously on a continuous basis, it disappeared in blood at a rate of approximately 23 mg per hour [Kodama and Hine, 1958].

 

- Similarly, Penttila et al. (1988) reported that periportal and perivenous cells isolated from rat liver oxidized 2-propen-1-ol at rates of 3.4 and 3.1μmol/(g.min), respectively. Cellular GSH was rapidly depleted (95%) by oxidation of 700 uM 2-propen-1-ol.

 

- Patel et al. (1980) investigated the biotransformation of 2-propen-1-ol (Allyl alcohol) in rat liver and lung preparations. 2-Propen-1-ol was metabolized to acrolein by alcohol dehydrogenase in liver and cytosolic fractions but not in liver microsomes or in lung preparations (where alcohol dehydrogenase is absent). They showed that the reaction is alcohol dehydrogenase-dependent as it was significantly inhibited by pyrazole, a known inhibitor of alcohol dehydrogenase. Acrolein was oxidized to acrylic acid by liver aldehyde dehydrogenase in the presence of NAD+ or NADP+ in liver, cytosolic and microsomal fractions. Incubation of 2-propen-1-ol and acrolein with liver and lung microsomes in the presence of NADPH resulted in the formation of glycidol and glycidaldehyde, respectively. Epoxide production proceeded rapidly in the first few minutes but levels then fell. This may be explained by epoxide hydrase activity in lung and liver microsomes which converts the epoxides to glycerol and glyceraldehydes, respectively.

 

- Reid (1972) reported that pretreatment of rats with 4-methylpyrazole blocked the hepatic necrosis due to 14C-2-propen-1-ol and reduced the amount of radiolabeled material bound in the liver by 80%. He concluded that the binding of 14C-2-propen-1-ol to macromolecules (via sulfhydryl groups) and the subsequent necrosis were dependent on the oxidation of 2-propen-1-ol to acrolein.

 

- Kaye (1973) reported the isolation of 3-hydroxypropyl mercapturic acid (the end product of conjugation to glutathione) from the urine of rats after subcutaneous injection of either 2-propen-1- ol or acrolein (6.3% and 10.5% conversion, respectively). Sanduja et al. (1989) investigated the excretion in urine of 3-hydroxypropyl mercapturic acid in rats given 2-propen-1-ol (64 mg/kg bw) or acrolein (13 mg/kg bw) by gavage. Recoveries were 28.3% and 78.5%, respectively.

 

Hormann et al. (1989) investigated the time course of 2-propen-1-ol induced toxicity in isolated rat hepatocytes. They observed an initial rapid depletion of glutathione (GSH), a subsequent increase in malondialdehyde (MDA) and decrease in protein sulfhydryl groups (PSH) and the eventual loss of membrane integrity. Addition of sulfhydryl compounds (N-acetylcysteine and dithiothreitol) markedly delayed the depletion of GSH, prevented significant loss of PSH and protected the cells against viability loss. In contrast, antioxidants (butylated hydroxytoluene and Trolox C) and the iron chelating agent desferoxamine suppressed 2-propen-1-ol induced MDA production without affecting the depletion of cellular thiols or the loss of viability. These results suggest that the inactivation of protein thiol groups is critical for 2-propen-1-ol toxicity, whereas lipid peroxidation is not essential to the toxic process.

Belinsky et al (1985) devoided alcohol dehydrogense activity in deer mice due to genetic defect in the ADH gene. No detectable response has been measured by histopathology and serum sorbitol dehydrogenase and serum glutamic oxaloacetic transaminase activities after mice received doses of allyl alcohol that caused marked increases in serum enzyme activity and periportal necrosis of the liver in a strain of deer mice that expresses normal levels of ADH activity. Moreover, the age-associated increase in ADH activity observed in male F344 rats correlates well with the age-associated increase in allyl hepatoxicity. The lack of age associated increase in ADH activity in female F344 rats also correlates with the lack of of an age associated increase in allyl alcohol hepatoxicity in females (Riskans and Moore, 1987).

Methyl methacrylate (MMA)

There are extensive data available for the methyl ester (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 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.

Trends/Results

The toxicokinetic behaviour of MMA is described in the EU Risk Assessment as follows: “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 (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)”.

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).

 

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). 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.

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, 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; 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 (100mg ml-1)

Vmax           Km(nM min-1mg-1) (mM)

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. 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”.

 

In terms of MAA, the common metabolite for these esters, the following can be taken from the EU ESR: “Methacrylic acid is rapidly absorbed in rats after oral and inhalative administration (Bereznowski et al., 1994).A high dose orally administered methyl methacrylate is rapidly hydrolyzed by esterases and the methacrylic acid concentration in the blood serum reached a very low level after one hour. In an inhalation study deposition efficiency of 95% was measured in the surgically isolated upper respiratory tract of anaesthetized rats (Morris and Frederick, 1995). However, the degree of penetration to underlying cells could not be derived from this experiment.There are no studies which specifically address the metabolism of exogenously applied methacrylic acid.

Studies completed after the RA on MAA indicate rapid absorption through skin and subsequent clearance from blood. Topically applied MAA absorbs rapidly through intact rat epidermis and viable whole skin in-vitro (Jones, 2002). In another study intravenous injection of MAA in rats demonstrated very rapid clearance from the blood (half-life <5mins), suggestive of rapid subsequent metabolism (Jones, 2002).

Trends

Short chain esters and MAA are absorbed by all routes. The rate of absorption decreases with increasing ester chain length. All esters are rapidly hydrolysed in local tissues as well as in blood by non-specific esterases. There is a trend towards increasing half-life of the ester in blood with increasing ester chain length. The primary metabolite, MAA, is cleared rapidly from blood in all cases.

Conclusions

Methyl methacrylate, allyl methacrylate and the other methacrylate esters 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. The one 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. The other primary metabolite is acrolein causes liver toxicity.

Local effects resulting from the hydrolysis of the ester to MAA are only observed following inhalation exposure and this has been shown to be due to the localised concentration of non-specific esterases in nasal olfactory tissues. In summarising the available PBPK data on MMA SCOEL concluded that “Extensive PBPK modeling 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).