Methacrylic acid, monoester with propane-1,2-diol

Endpoint summary

Link to relevant study record(s)

Reference

Reference 1

Endpoint:

basic toxicokinetics in vivo

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

toxicokinetics

Qualifier:

no guideline followed

Principles of method if other than guideline:

This non-GLP pharmacokinetic study of both HEMA and HPMA in rats via intravenous (IV) administration was conducted to evaluate the potential quick hydrolysis of both HEMA and HPMA in vivo.

GLP compliance:

no

Radiolabelling:

no

Species:

rat

Strain:

Fischer 344/DuCrj

Details on species / strain selection:

F344/DuCrl rats were selected because of their use in previous toxicological studies for the two test materials (HEMA and HPMA). Rats are a suitable species for the analysis of metabolism of chemicals in vivo. The F344/DuCrl rats are also suitable due to the availability of historical background data, and the reliability of the commercial supplier.

Sex:

male

Details on test animals or test system and environmental conditions:

TEST ANIMALS
- Source: Charles River (Kingston, New York)
- Age at study initiation: 9 weeks
- Weight at study initiation: 191-208 g
- Housing: all animals were single housed in glass Roth-type metabolism cages for
acclimation purposes.
- Diet (e.g. ad libitum): LabDiet Certified Rodent Diet #5002 (PMI Nutrition International,
St. Louis, Missouri) in pelleted form. Feed was provided ad libitum.
- Water (e.g. ad libitum): Municipal water was supplied to all study animals ad libitum throughout the study.
- Acclimation period: Upon arrival, all animals were acclimated to the laboratory for at least two days prior to the study.

ENVIRONMENTAL CONDITIONS
- Temperature (°C): 22°C with a range of 20°C-26°C
- Humidity (%): 50% with a range of 38-78%
- Air changes (per hr):
- Photoperiod (hrs dark / hrs light): 12/12

Route of administration:

intravenous

Vehicle:

physiological saline

Details on exposure:

PREPARATION OF DOSING SOLUTIONS:
Appropriate amounts of HEMA or HPMA were added to sterile saline to obtain the appropriate dose of 5 mg/kg bw using aseptic techniques. The amount of dose solution administered was targeted at ~2.5 mL/kg bw and injected over a minimum of 45 seconds which corresponded to injection rates ranging from 0.7 to 0.8 mL/minute based on the averaged body weight of 0.2 kg.

Duration and frequency of treatment / exposure:

once

Dose / conc.:

5 mg/kg bw/day

No. of animals per sex per dose / concentration:

2 males

Control animals:

no

Positive control reference chemical:

no

Details on study design:

- Dose selection rationale: A dose level of 5 mg/kg body weight (bw) for HEMA and 5 mg/kg bw for HPMA was used in this study. These two dose levels are equimolar based on the study design.

Details on dosing and sampling:

TOXICOKINETIC STUDY
- Tissues and body fluids sampled (delete / add / specify): bloodpling:
- Time and frequency of sampling: 5-10-30-60-180 min
- From how many animals: 2, not pooled
- Method type(s) for identification: GC-MS
- Limits of detection and quantification: LOQ: 48.8 ng/mL (HPMA)

Statistics:

Descriptive statistics were used, i.e., mean ± standard deviation, when applicable. All calculations in the database were conducted using Microsoft Excel (Microsoft Corporation, Redmond, Washington) spreadsheets and databases in full precision mode
(15 digits of accuracy). Certain pharmacokinetic parameters were calculated for blood, including AUC (area-under-the-curve), using a pharmacokinetic computer modeling program PK Solutions (v.2.0.6., Summit Research Services, Montrose, Colorado).

Type:

metabolism

Results:

Rapid hydrolysis after intavenous administration in rats

Key result

Test no.:

#1

Toxicokinetic parameters:

half-life 1st: ca. 1 min

Remarks:

mean

mean T1/2 around 1 min (0.69 and 0.95 min for each animal, respectively)

Conclusions:

After i.v. administration in rats, HPMA quickly hydrolyses in the order of a few minutes. The estimated half-lives for HPMAwas near 1 minute, indicating that the current study results support the assumption that HPMA was quickly hydrolyzed to its primary metabolites.

Executive summary:

To support the REACH registration for hydroxyethyl methacrylate (HEMA) and hydroxypropyl methacrylate (HPMA), a Read-across approach can be applied if test materials can be quickly hydrolyzed to the methacrylic acid and the corresponding alcohols (glycols) in vivo. This non-GLP pharmacokinetic study of both HEMA and HPMA in rats via intravenous administration was conducted to evaluate the potential quick hydrolysis of both HEMA and HPMA in vivo.

Two male rats per compound were intravenously administered HEMA or HPMA individually at 5.0 mg/kg dose level with saline as the dose vehicle. After dose administration, blood samples (200 μl) were collected at 5, 10, 30, 60, and 180 minutes into individual glass vials containing ethyl acetate (600 μL) acidified with 1% formic acid. After vortexing, the levels of HEMA and HPMA in the blood samples were quantitatively analyzed by GC/MS-MS.

The results showed that levels of both HEMA and HPMA dropped rapidly after administration and were not quantifiable by 60 minutes with limit of quantitation (LOQ) of 48.8 ng/mL (HPMA) and 45.0 ng/mL (HEMA). The estimated half-lives for HEMA and HPMA were less than or near 1 minute, indicating that the current study results support the assumption that both HEMA and HPMA were quickly hydrolyzed after intravenous administration in rats.

Reference 2

Endpoint:

basic toxicokinetics in vitro / ex vivo

Type of information:

experimental study

Adequacy of study:

key study

Reliability:

2 (reliable with restrictions)

Objective of study:

metabolism

Principles of method if other than guideline:

Identification and measurement of monomers and methacrylic acid were performed by high pressure liquid chromatography.

GLP compliance:

not specified

Radiolabelling:

no

Details on test animals or test system and environmental conditions:

Porcine liver esterase obtained from Sigma Chemical Company, St. Louis, USA

Details on study design:

20.0 mg polymer powder was suspended in 10.0 mL 0.01 M phosphate, pH 6.5 and 10 U esterase/mL was added. The suspension was slowly stirred at 37 deg C, and aliquots taken after various periods of time were filtered to separate the polymer from the solution.

HPMA is hydrolyzed to methacrylic acid and 1, 2-propanediol at pH 6.5 and 37°C catalyzed by an unspecific esterase (Porcine liver esterase)in vitro.

Conclusions:

Hydroxypropyl methacrylate is hydrolyzed to methacrylic acid and  1,2-propanediol at pH 6.5 and 37 deg. C catalyzed by an unspecific  esterase (Porcine liver esterase). Methacrylic acid and alcohol formation  were determined by HPLC analysis. The substance is absorbable through the skin and is hydrolysed in the body.

Reference 3

Endpoint:

basic toxicokinetics, other

Remarks:

(Q)SAR

Type of information:

(Q)SAR

Adequacy of study:

supporting study

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

other: Acceptable calculation method

Justification for type of information:

QSAR prediction

Principles of method if other than guideline:

Protein/GSH reactivity modelling with OECD Toolbox

GLP compliance:

no

The algorithm of the OECD toolbox has been used to predict GSH/protein reactiviry.

Test Chemical / Compound Identity	Acronym	SMILES	Molecular Weight	Protein Binding Potency
2-ethylhexyl methacrylate	EHMA	O=C(OCC(CCCC)CC)\C(=C)C	198.3	Slightly reactive (GSH) >> Methacrylates (MA)
ethyl methacrylate	EMA	CCOC(=O)C(=C)C	114.14	Slightly reactive (GSH) >> Methacrylates (MA)
isobutyl methacrylate	iBMA	CC(C)COC(=O)C(=C)C	142.2	Slightly reactive (GSH) >> Methacrylates (MA)
Methacrylic acid	MAA	CC(=C)C(=O)O	100.12	No alert found
Methyl methacrylate	MMA	CC(=C)C(=O)OC	100.12	Slightly reactive (GSH) >> Methacrylates (MA)
n-butyl methacrylate	nBMA	CCCCOC(=O)C(=C)C	142.2	Slightly reactive (GSH) >> Methacrylates (MA)
n-Hexyl methacrylate	n-HMA	CCCCCCOC(=O)C(=C)C	170.25	Slightly reactive (GSH) >> Methacrylates (MA)
ter-Butyl Methacrylate	tBMA	CC(=C)C(=O)OC(C)(C)C	142.2	Slightly reactive (GSH) >> Methacrylates (MA)
Diethylaminoethyl methacrylate	DEAEMA	O=C(OCCN(CC)CC)C(=C)C	185.263	Slightly reactive (GSH) >> Methacrylates (MA)
2-tert-Butylaminoethyl methacrylate	TBAEMA	O=C(OCCNC(C)(C)C)C(=C)C	185.26	Slightly reactive (GSH) >> Methacrylates (MA)
2-Dimethylaminoethyl methacrylate	MADAME	CC(=C)C(=O)OCCN(C)C	157.22	Slightly reactive (GSH) >> Methacrylates (MA)
2-(2-Butoxyethoxy) ethyl methacrylate, Butyldiglycol methacrylate	BDGMA	CCCCOCCOCCOC(=O)C(=C)C	230.3	Slightly reactive (GSH) >> Methacrylates (MA)
2-(2-(2-Ethoxy ethoxy)-thoxyethyl methacrylate	ET3EGMA	O=C(OCCOCCOCCOCC)C(=C)C	246.3	Slightly reactive (GSH) >> Methacrylates (MA)
2-Methoxyethyl methacrylate	MTMA	CC(=C)C(=O)OCCOC	144.08	Slightly reactive (GSH) >> Methacrylates (MA)
2-Ethoxyethyl methacrylate	ETMA	CCOCCOC(=O)C(=C)C	158.09	Slightly reactive (GSH) >> Methacrylates (MA)
Phenoxyethyl Methacrylate	PTMA	CC(=C)C(=O)OCCOC1=CC=CC=C1	206.24	Slightly reactive (GSH) >> Methacrylates (MA)
Allyl methacrylate	AMA	CC(=C)C(=O)OCC=C	126.15	Slightly reactive (GSH) >> Methacrylates (MA)
Benzyl methacrylate	BNMA	CC(=C)C(=O)OCC1=CC=CC=C1	176.21	Slightly reactive (GSH) >> Methacrylates (MA)
Cyclohexyl methacrylate	c-HMA	O=C(OC(CCCC1)C1)C(=C)C	168.23	Slightly reactive (GSH) >> Methacrylates (MA)
Isobornyl methacrylate	IBOMA	CC(=C)C(=O)OC1CC2CCC1(C2(C)C)C	222.32	Slightly reactive (GSH) >> Methacrylates (MA)
Phenyethyl methacrylate	Phenylethyl MA	CC(=C)C(=O)OCCC1=CC=CC=C1	190.24	Slightly reactive (GSH) >> Methacrylates (MA)
Phenyl methacrylate	PHMA	CC(=C)C(=O)OC1=CC=CC=C1	162.19	Slightly reactive (GSH) >> Methacrylates (MA)
3,3,5-Trimethylcyclohexyl methacrylate	TMCHMA	CC1CC(CC(C1)(C)C)OC(=O)C(=C)C	210.31	Slightly reactive (GSH) >> Methacrylates (MA)
Tridecyl methacrylate	TDMA C13MA	CCCCCCCCCCCCCOC(=O)C(=C)C	268.43	Slightly reactive (GSH) >> Methacrylates (MA)
Isodecyl methacrylate	IDMA	CC(C)CCCCCCCOC(=O)C(=C)C	226.36	Slightly reactive (GSH) >> Methacrylates (MA)
Dodecyl methacrylate	LMA	CCCCCCCCCCCCOC(=O)C(=C)C	254.41	Slightly reactive (GSH) >> Methacrylates (MA)
n-Octyl methacrylate	n-OMA	CCCCCCCCOC(=O)C(=C)C	198.3	Slightly reactive (GSH) >> Methacrylates (MA)
2-Hydroxyethyl methacrylate	HEMA	CC(=C)C(=O)OCCO	130.1	Slightly reactive (GSH) >> Methacrylates (MA)
Hydroxypropyl methacrylate (propyleneglycol monoester) = isomer mixture of < 80 % 2-Hydroxypropyl methacrylate > 20% 2-Hydroxy-1-methylethyl methacrylate	HPMA	CC(COC(=O)C(=C)C)O CC(CO)OC(=O)C(=C)C	144.17	Slightly reactive (GSH) >> Methacrylates (MA)
Hydroxypropyl methacrylate (propyleneglycol monoester) = isomer mixture of < 80 % 2-Hydroxypropyl methacrylate > 20% 2-Hydroxy-1-methylethyl methacrylate	HPMA	CC(CO)OC(=O)C(=C)C	144.17	Slightly reactive (GSH) >> Methacrylates (MA)
N-butoxymethyl methacrylamide	N-BMMA	CCCCOCNC(=O)C(=C)C	157.21	Moderately reactive (GSH) >> 2-Vinyl carboxamides (MA)
N-Methylol methacrylamide	N-MMAA	CC(=C)C(=O)NCO	115.13	Moderately reactive (GSH) >> 2-Vinyl carboxamides (MA)
N,N'-methylenbis(methacrylamide)		O=C(NCNC(=O)\C(=C)C)\C(=C)C	154.19	Moderately reactive (GSH) >> 2-Vinyl carboxamides (MA)
N-Dimethylaminopropyl methacrylamide	DMAPMA	CC(=C)C(=O)NCCCN(C)C	170.25	Moderately reactive (GSH) >> 2-Vinyl carboxamides (MA)
Methacrylamide	MAA	CC(=C)C(=O)N	85.1	Moderately reactive (GSH) >> 2-Vinyl carboxamides (MA)
1,12-Dodecanediol dimethacrylate	1,12 DDDMA	O=C(OCCCCCCCCCCCCOC(=O)\C(=C)C)\C(=C)C	338.48	Slightly reactive (GSH) >> Methacrylates (MA)
1,3-Butandiol dimethacrylate	1,3-BDDMA	CC(CCOC(=O)C(=C)C)OC(=O)C(=C)C	226.27	Slightly reactive (GSH) >> Methacrylates (MA)
1,4-Butandiol dimethacrylate	1,4-BDDMA	CC(=C)C(=O)OCCCCOC(=O)C(=C)C	226.27	Slightly reactive (GSH) >> Methacrylates (MA)
1,6-Hexanediol dimethacrylate	1,6 HDDMA	O=C(OCCCCCCOC(=O)\C(=C)C)\C(=C)C	254.32	Slightly reactive (GSH) >> Methacrylates (MA)
Ethylene glycol dimethacrylate	EGDMA	CC(=C)C(=O)OCCOC(=O)C(=C)C	198.22	Slightly reactive (GSH) >> Methacrylates (MA)
Trimethylpropane trimethacrylate	TMPTMA	CCC(COC(=O)C(=C)C)(COC(=O)C(=C)C)COC(=O)C(=C)C	338.4	Slightly reactive (GSH) >> Methacrylates (MA)
Ethoxylated bisphenol A dimethacrylate	2EBADMA	CC(C)(C1=CC=C(C=C1)O)C2=CC=C(C=C2)O.C=CC(=O)O.C(CO)O ID of reference isomer: CC(=C)C(=O)OC1=CC=C(C=C1)C(C)(C)C1=CC=C(OC(=O)C(C)=C)C=C1	452.5394	Slightly reactive (GSH) >> Methacrylates (MA)
2,2-bis-[4-(3'-methacryloyloxy-2'-hydroxy)propoxyphenyl] propane	bis-GMA	O=C(OCC(O)COc1ccc(cc1)C(c2ccc(OCC(O)COC(=O)\C(=C)C)cc2)(C)C)\C(=C)C	512.61	Slightly reactive (GSH) >> Methacrylates (MA)
4,4'-Isopropylidenediphenol, oligomeric reaction products with 1-chloro-2,3-epoxy propane, reaction products with methacrylic acid	Bis-GMA NLP (DSM/Akzo/+others)		n.d	Slightly reactive (GSH) >> Methacrylates (MA)
Diethyleneglycol dimethacrylate	DEGDMA	CC(=C)C(=O)OCCOCCOC(=O)C(=C)C	242.27	Slightly reactive (GSH) >> Methacrylates (MA)
Glycerol dimethacrylate	GDMA	O=C(OCC(O)COC(=O)\C(=C)C)\C(=C)C	228.24	Slightly reactive (GSH) >> Methacrylates (MA)
7,7,9-(resp. 7,9,9-)trimethyl-4,13-dioxo-3,14-dioxa-5,12-diazahexadecane-1,16-diol-dimethacrylate	HEMATMDI	O=C(OCCOC(=O)NCCC(C)CC(C)(C)CNC(=O)OCCOC(=O)\C(=C)C)\C(=C)C	470,57	Slightly reactive (GSH) >> Methacrylates (MA)
Triethyleneglycol dimethacrylate	TREGDMA	O=C(OCCOCCOCCOC(=O)\C(=C)C)\C(=C)C	286.32	Slightly reactive (GSH) >> Methacrylates (MA)
Tetraethyleneglycol dimethacrylate	TTEGDMA/4EDMA	CC(=C)C(=O)OCCOCCOCCOCCOC(=O)C(=C)C	198.22	Slightly reactive (GSH) >> Methacrylates (MA)
2-hydroxyethyl methacrylate phosphate	HEMA phosphate	CC(=C)C(=O)OCCOP(=O)(O)O	228.14	Slightly reactive (GSH) >> Methacrylates (MA)
Methacrylic anhydride	MAAH	CC(=C)C(=O)OC(=O)C(=C)C	154.16	Slightly reactive (GSH) >> Methacrylates (MA)
Hydroxyethyl ethylene urea methacrylate	MEEUW	O=C1NCCN1CCOC(=O)\C(=C)C	129.16	Slightly reactive (GSH) >> Methacrylates (MA)
Tetrahydrofurfurylmethacrylat	THFMA	O=C(OCC1OCCC1)\C(=C)C	170.21	Slightly reactive (GSH) >> Methacrylates (MA)
2,2-dimethyl-1,3-dioxolan-4-ylmethyl methacrylate		CC(=C)C(=O)OCC1COC(O1)(C)C	200.23	Slightly reactive (GSH) >> Methacrylates (MA)
Polyethylengeglycol-200-dimethacrylate	PEG200DMA	for approximation see Tetraethyleneglycol dimathacrylate	no data	Slightly reactive (GSH) >> Methacrylates (MA)
2,2,2-Trifluoroethyl methacrylate	TFMEA 3FM	CC(=C)C(=O)OCC(F)(F)F	336	Slightly reactive (GSH) >> Methacrylates (MA)
N-Trimethylammoniumpropyl methacrylamide-chloride	MAPTAC	CC(=C)C(=O)NCCC[N+](C)(C)C.[Cl-]	220.74	Moderately reactive (GSH) >> 2-Vinyl carboxamides (MA)
2-Trimethylammoniummethyl methacrylate-chloride 2-(Methacryloyloxy)-N,N,N-trimethylethanaminium chloride	TMAEMC DMCMA	CC(=C)C(=O)OCC[N+](C)(C)C.[Cl-]	207.7	Slightly reactive (GSH) >> Methacrylates (MA)

Conclusions:

Interpretation of results (migrated information): other: low reactivity with GSH
A dataset with methacrylic acid derivatives has been assessed using the reactivity profiler in the OECD QSAR Toolbox. This profiler contains structural alerts derived from an analysis of experimental reactivity data as measured using glutathione (the Schultz assay). The profiler assigns chemicals to one of five potency classes (non-reactive, slightly reactive, moderately reactive, highly reactive and extremely reactive) based on the experimental results.

The majority of the chemicals, including HPMA, are slightly reactive. For methacrylates it is well known that the addition of an alkyl group on the alpha-carbon significantly reduces reactivity in the Michael addition reaction.

There are also a group of vinyl carboxamides (methacrylamide derivatives) that have been flagged as being moderately reactive. It should be noted, however, that in the underlying QSAR data in the TB there is no information regarding the effect of an alpha substituent for this class of chemical. In reality this means that no chemicals were tested with glutathione that contained an alpha alkyl substituent thus the prediction is being made based on the un-substituted parent (acrylamide; thus the over-cautious prediction). The investigator indicated that they are likely to be pretty unreactive in reality.

Executive summary:

A dataset with methacrylic acid derivatives has been assessed using the reactivity profiler in the OECD QSAR Toolbox. This profiler contains structural alerts derived from an analysis of experimental reactivity data as measured using glutathione (the Schultz assay). The profiler assigns chemicals to one of five potency classes (non-reactive, slightly reactive, moderately reactive, highly reactive and extremely reactive) based on the experimental results.

 

The majority of the chemicals, including HPMA, are slightly reactive. For methacrylates it is well known that the addition of an alkyl group on the alpha-carbon significantly reduces reactivity in the Michael addition reaction.

 

There are also a group of vinyl carboxamides (methacrylamide derivatives) that have been flagged as being moderately reactive. It should be noted, however, that inthe underlying QSAR data in the TB there is no information regarding the effect of an alpha substituent for this class of chemical. In reality this means that no chemicals were tested with glutathione that contained an alpha alkyl substituent thus the prediction is being made based on the un-substituted parent (acrylamide; thus the over-cautious prediction). The investigator indicated that they are likely to be pretty unreactive in reality.

Reference 4

Endpoint:

dermal absorption

Type of information:

other: QSPR

Remarks:

Based on an established human skin model by Potts and Guy (Potts RO and Guy RH (1992). Predicting Skin Permeability. Pharm. Res. 9(5): 663-669)

Adequacy of study:

key study

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

other: Generally accepted calculation method

Principles of method if other than guideline:

The physicochemical parameters of MW, Log P and saturated aqueous solubility have been used in the evaluation of 56 methacrylate compounds. An output of predicted steady-state flux was calculated using the principles defined in the Potts and Guy prediction model. (Potts RO and Guy RH (1992). Predicting Skin Permeability. Pharm. Res. 9(5): 663- 669)

GLP compliance:

no

Species:

other: human skin model

Details on test animals or test system and environmental conditions:

not applicable; in silico modelling

Type of coverage:

other: not applicable; in silico modelling

No. of animals per group:

not applicable; in silico modelling

Absorption in different matrices:

predicted flux 150.8 μg/cm²/h; the relative dermal absorption is high

Based on a molecular weight of 144.17 g/mol and a g Kow of 0.97, the predicted flux of HPMA is 150.8 μg/cm²/h; the relative dermal absorption is high.

Conclusions:

The dermal absorption of HPMA is predicted to be high; the predicted flux is 150.8 μg/cm²/h.

Executive summary:

The dermal absorption (steady-state flux) of HPMA has been estimated by calculation using the principles defined in the Potts and Guy prediction model.

Based on a molecular weight of 144.17 g/mol and a logKow of 0.97, the predicted flux of HEMA is 150.8μg/cm²/h; the relative dermal absorption is high.

Description of key information

Reliable experimental data exists to demonstrate that HPMA is rapidly metabolized by a ubiquitous metabolic pathway within the body. HPMA is metabolised to propyl glycol and methacrylic acid. There is a high level of confidence based upon experimental data showing that the half-live of HPMA is in the order of a few minutes.

Key value for chemical safety assessment

Bioaccumulation potential:

no bioaccumulation potential

Additional information

Reliable data on the toxicokinetics of HPMA are available. These data show that HPMA is rapidly hydrolyzed by carboxylesterases to Methacrylic Acid (MAA) and the respective alcohol, Propylene Glycol (PG).

Further, studies in guinea pigs and in mice with HPMA indicate that most of the administered material is rapidly cleared from the body following metabolism to CO2. These results are consistent with the conclusion that HPMA is rapidly metabolized by tissue esterases to the respective glycol, propylene glycol (PG) and methacrylic acid (MAA). Subsequently, both glycol and methacrylic acid are further oxidized before excretion.

 

ADME and Ester Hydrolysis

Methacrylate esters in general

For the understanding of the toxikokinetic of the hydroxyalkyl-methacrylate esters HEMA and HPMA it is important to understand the general metabolism of methacrylate esters in mammals. For MMA and other short-chain alkyl-methacrylate esters extensive toxicokinetic data are available. These data have been reviewed and summarized in the EU Risk Assessment for MMA as well as the OECD SIAR for short-chain alkyl-methacrylate esters. A brief summary follows:

After oral or inhalation administration, methacrylate esters are expected to be rapidly absorbed and distributed. Dermal absorption of esters is extensive only with occlusion of the site. Toxicokinetics seem to be similar in man and experimental animals. MMA and other short chain alkyl-methacrylate esters are initially hydrolyzed by non-specific carboxylesterases to methacrylic acid and the structurally corresponding alcohol in several tissues. Methacrylic acid (MAA) 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. 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.

 

HC=C(CH3)H-C=OOR + H2O ---(carboxylesterase)--->  HC=C(CH3)H-C=OOH  +  R-OH

 

Figure: Ester hydrolysis by carboxylesterases, “R” is a placeholder for any alkyl or hydroxyalkyl group  

 

As supporting evidence, metabolism and half-live data from a range of lower alkyl-methacrylate esters indicate that this category of methacrylate esters of comparable molecular weight (HEMA MW 130.1; HPMA MW 144.2) are rapidly hydrolysed by ubiquitous carboxylesterases. First pass (local) hydrolysis of the parent ester has been shown to be significant for all routes of exposure (see Category Justification, chapter 5.2).

Conjugation

The reactivity towards glutathion of more than 50 methacrylates and other chemicals with related structures has been estimated with a QSPR model by Cronin (2012). For both hydroxyalkyl methacrylates it is predicted that they are slightly reactive towards glutathione (GSH). This is consistent with experimental data by Freidig et al. (1999) who investigated and compared the reactivity withglutathionof a series of acrylate and methacrylate esters.

Methacrylate esters can conjugate with 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. 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.

 

HPMA

A non-GLP toxicokinetic study of both HEMA and HPMA in rats via intravenous administration was conducted to evaluate the potential quick hydrolysis of both HEMA and HPMA in vivo (Dow, 2017 (see also above).

Two male rats were intravenously administered HPMA individually at 5.0 mg/kg dose level with saline as the dose vehicle. After dose administration, blood samples (200 μl) were collected at 5, 10, 30, 60, and 180 minutes into individual glass vials containing ethyl acetate (600 μL) acidified with 1% formic acid. After vortexing, the levels of HPMA in the blood samples were quantitatively analyzed by gas chromatography tandem mass spectrometry (GC/MS-MS).

The results showed that HPMA dropped rapidly after administration and were not quantifiable by 60 minutes with limit of quantitation (LOQ) of 48.8 ng/mL. The estimated half-live for HEMA was around one minute (i.e. 0.69 and 0.94 minutes for the two tested animals), indicating that the current study results support the assumption that HPMA was quickly hydrolyzed after intravenous administration in rats.

HPMA is hydrolyzed to methacrylic acid and 1, 2-propanediol at pH 6.5 and 37°C catalyzed by an unspecific esterase (Porcine liver esterase)in vitro[Munksgaard and Freund, 1990]. It can be concluded that this substance is absorbable through the skin and is hydrolysed in the body.

 

 

Absorption

Heylings (2013) used a QSPeR model for whole human skinbased on that described by Potts and Guy(1992)to predict the dermal penetration rate of a large number of methacrylate esters, including the hydroxyalkyl methacrylates (Table below).

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

Substance	Molecular Weight	Log P	Predicted Flux (μg/cm^2/h)	Relative Dermal Absorption
HEMA	130.1	0.42	151.293	High
HPMA	144.2	0.97	150.827	High

 

As relatively small, hydrophilic esters the hydroxyalkyl methacrylates are predicted to be rapidly absorbed. However, as indicated by the studies above, they will be subject to hydrolysis by local esterases in the stratum corneum.

 

Primary Metabolites

MAA

For MAA, the common metabolite for these esters,a comparison of measured blood concentration data after i v. administration of 10 and 20 mg/kg MAA was made and a simulation was performed based on a one-compartment model. This shows good agreement with the measured data in vivo (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/hrx 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.

 

PG

The ATSDR review (1997; attached to this summary) describes the PG metabolism as follows: “The major route of metabolism for propylene glycol is via alcohol dehydrogenase tolactaldehyde, then to lactate, via aldehyde dehydrogenase, and on to glucose through gluconeogenic pathways (as summarized in Christopher et al. 1990; Huff 1961; Miller and Bazzano1965; Morshedet al. 1989; Ruddick 1972). Conversion to methylglyoxal is an alternate route via alcohol dehydrogenase, ending in metabolism to D-lactate through glyoxalase.” 

 

Figure (see "attachments"): Propylene glycol metabolism in mammals (according to Christopher et al. 1990)

 

In addition, the pharmacokinetics of propylene glycol are reasonably well understood in humans as well as animals according to NTP-CERHR Expert Panel (2004b): “The rate-determining stepin the PG metabolismis alcohol dehydrogenase which, when saturated, switches from a first order process into a zero order process. Saturation of metabolism appears to occur in rats and rabbits at a dose of about 1.6 to 2 g/kgbw, whereas in humans this seems to happen at a dose of about 0.2 g/kgbw. … In accordance with a zero order process, the half-life of propylene glycol in humans and rats increases from about 1.5 hours to more than 5 hours with increasing doses above metabolic saturation. Alcohol dehydrogenase converts propylene glycol tolactaldehyde, which is further metabolized to lactate. Since propylene glycol has a chiral center, technical grade propylene glycol results in the formation of 50/50 D, L-lactate. L-lactate is indistinguishable from endogenous lactate, which is a good substrate for gluconeogenesis. D-lactate is less readily converted to glucose than L-lactate, which prolongs its half-life leading, under conditions of prolonged exposure, to D-lactic acidosis. It is difficult to cause L-lactic acidosis even with very high doses of propylene glycol because of its efficient detoxification via gluconeogenesis.

The excretion of propylene glycol is species-dependent.

Since propylene glycol has very low intrinsic toxicity, saturation of metabolism plays a protective role in its toxicity since the conversion of propylene glycol to the more toxic lactate (particularly D-lactate) is slowed.” As the protective saturation of metabolism in humans occurs at 10 fold lower dosages as in rats or rabbits, an additional margin of safety can be anticipated.

 

Finally it has to be mentioned that “in mammals, part of the propylene glycol dose is eliminated unchanged by the kidney and part is metabolized by the liver to lactic acid and further metabolized to pyruvic acid; in mammals, with the exception of cats, the remainder is conjugated with glucuronic acid and eliminated in the urine. The amount of propylene glycol eliminated by the kidneys has been estimated for humans at 45%, for dogs at 55−88%, and for rabbits at 24−14.2%.” (NTP-CERHR 2004b). Available data indicate that human metabolism is rather like dogs than cats.

 

Conclusions

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 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. For PG, the primary glycolic metabolite of HPMA, a comprehensive knowledge on its metabolism is also given. Here, PG is metabolized to glucose or it is eliminated in the urine either unchanged or as glucuronic acid conjugate. The similarity of metabolism between HEMA/HPMA and MMA also implies that systemic toxicity data on MMA and other lower chain alkyl methacrylic acid esters are relevant to the potential toxicity profile of HEMA/HPMA. Thus, following absorption of MMA or HEMA/HPMA into the body the metabolic disposition of all three materials are likely to be similar differing substantively only on the alcohol/glycol moiety released and toxicity data for MMA may also be of relevance to HEMA and HPMA.

Local effects (irritation) 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 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). Similar to EHMA, the lower alkyl methacrylate category member with the highest molecular weight, it is unlikely that this is a relevant mode of action for the hydroxyalkyl methacrylates, since the vapour pressure is too low so that toxic, local MAA levels cannot be reached in the respective tissues.

 

Summary and discussion on toxicokinetics

The read across hypothesis and the satisfaction of the higher tier data requirements for HPMA relies on the observation that HPMA is metabolized very rapidly within the body to their respective alcohol (PG) and methacrylic acid (MAA) and as a consequence repeated dose systemic toxicity reflect the combined toxicities of the primary metabolites, which have already been studied extensively. Toxicokinetics therefore is a key element in the mode of action for many endpoints as well as in the read across hypothesis.

Experimental data exists to demonstrate that HPMA is metabolized by a ubiquitous metabolic pathway for all esters, including methacrylates within the body. HPMA is metabolised to PG and methacrylic acid. There is a high level of confidence based upon experimental data showing that the half-live of HPMA is in the order of a few minutes (Dow, 2017). Supporting evidence for the expimental data on HPMA is coming from data on HEMA and a number of other, structurally related methacrylate esters in the same range of molecular weight and polarity (Jones, 2002; Dow, 2013).