Methyl methacrylate

Basic toxicokinetics

Administrative data

Endpoint:

basic toxicokinetics

Type of information:

experimental study

Adequacy of study:

key study

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

other: Generally accepted scientific standards.

Data source

Reference

Title:

Methyl methacrylate toxicity in rat nasal epithelium: Studies of the mechanism of action and comparisons between species.

Author:

Mainwaring G, Foster JR, Lund V, Green T

Year:

2001

Bibliographic source:

Toxicology 158: 109-118

Materials and methods

Objective of study:

metabolism

Test guideline

Qualifier:

no guideline followed

Principles of method if other than guideline:

Effects of bis-(p-nitrophenyl)phosphate, an inhibitor of carboxylesterase enzymes, were studied in rats after single inhalative exposure to methyl methacrylate. Additionally, the distribution of the carboxylesterases in nasal tissues has been investigated and the metabolism of methyl methacrylate to methacrylic acid has been compared in rat, hamster and human nasal tissue fractions in vitro.

GLP compliance:

not specified

Test material

Reference

Name:

Unnamed

Test animals

Species:

other: rats (in vivo and in vitro), hamsters and humans (in vitro)

Strain:

other: Fisher rats and Syrian hamsters

Sex:

male/female

Details on test animals and environmental conditions:

Female Fischer 344 rats and male Syrian hamsters

Administration / exposure

Route of administration:

inhalation

Vehicle:

unchanged (no vehicle)

Details on exposure:

Experiment 2: in vivo, rat: inhalation for 3 or 6 hours

Duration and frequency of treatment / exposure:

Experiment 2: in vivo: treatment was once

Doses / concentrations

Remarks:

Doses / Concentrations:
Experiment 2: in vivo -

No. of animals per sex per dose:

Experiment 2: in vivo - 5 females per group

Details on study design:

Experiment 1: Female Fischer 344 rats and male Syrian hamsters were used to collect respiratory and olfactory epithelium. Microsomes (purified S9 fraction) were prepared from pooled tissues by tissue type from groups of 20 animals. Liver was also removed and microsomes prepared in the same manner. Human nasal explants from five individuals were removed during a craniofacial procedure and, when quantity was adequate, microsomes prepared as for the animals. If human tissue quantity was limited, an S9 fraction was prepared and compared to a similarly prepared rat S9 fraction. The activity of carboxylesterase toward MMA was determined by measuring the formation of methacrylic acid in vitro using rat, hamster and human microsomal or S9 fractions. The carboxylesterase inhibitor, bis(4-nitrophenyl)phosphate (BNPP) was used to determine true carboxylesterase activity.
Experiment 2: Groups of five rats were dosed i.p. with either saline (10 ml/kg) or BNPP (100 mg/kg) in saline (10 ml/kg) 18 hours and again 1 hour before being exposed to MMA vapors via whole body inhalation for 3 or 6 hours. Animals were sacrificed either immediately after exposure (3 and 6 hours) or 18 hours after the 6-hour exposure. Histopathology of the nasal cavities was performed.

Results and discussion

Metabolite characterisation studies

Metabolites identified:

yes

Details on metabolites:

methacrylic acid

Any other information on results incl. tables

Experiment 1: The rate of MMA metabolism is summarized in the following table:

Species   Liver       Olf.      Resp.      Resp.    Olf.
        Microsomes  Microsomes Microsomes   S9       S9
        Km   Vmax    Km  Vmax   Km   Vmax  Vmax     Vmax
---------------------------------------------------------
Rat    0.10  46.5  0.14  38.6  0.15  14.3  3.5       12
Ham.   0.20  137   0.28  46    0.4    3.6  ----     ----
Human        494                     27    0.15     0.48
---------------------------------------------------------

The metabolism of MMA was completely inhibited by pretreatment of the microsomal preparations with BNPP at 0.1 mM.


Experiment 2: Female F344 rats exposed to 200 ppm MMA (ca. 0.82 mg/L) for 6 hours developed a lesion in the nasal olfactory epithelium 

which was characterized by degeneration and atrophy. The severity of the lesion was markedly reduced by pre-treatment 

of the rats with BNPP. Histocytochemisty showed that the carboxylesterases are heavily localized in the sustentacular 

cells and Bowman's glands of the rat olfactory region, but are more generally distributed in human olfactory epithelium.

Applicant's summary and conclusion

Conclusions:

The nasal lesions observed in MMA exposed rats are caused by the carboxylesterase mediated metabolism of MMA to methacrylic acid, an irritant and corrosive metabolite. The lesion occurred in the olfactory epithelium and, consistent with this, the enzyme activity in all 3 species was higher in fractions prepared from olfactory tissue than from respiratory tissue, 3-fold in rat and human and 12-fold in the hamster. The Vmax in rat and hamster olfactory tissue fractions were comparable, whereas those in human olfactory tissue fractions were at least 13-fold lower. The rate of metabolism in rat olfactory tissue was also comparable to that in rat liver whereas in humans, the rate in olfactory tissue was 500-fold lower than that in the liver. In respiratory tissues, the rate in humans was at least 6-fold lower than that in the rat. The authors stated that these results suggest that humans are less sensitive than rodents to the nasal toxicity of MMA.

Executive summary:

The nasal lesions observed in MMA exposed rats are caused by the carboxylesterase mediated metabolism of MMA to methacrylic acid, an irritant and corrosive metabolite. The lesion occurred in the olfactory epithelium and, consistent with this, the enzyme activity in all 3 species was higher in fractions prepared from olfactory tissue than from respiratory tissue, 3-fold in rat and human and 12-fold in the hamster. The Vmax in rat and hamster olfactory tissue fractions were comparable, whereas those in human olfactory tissue fractions were at least 13-fold lower. The rate of metabolism in rat olfactory tissue was also comparable to that in rat liver whereas in humans, the rate in olfactory tissue was 500-fold lower than that in the liver. In respiratory tissues, the rate in humans was at least 6-fold lower than that in the rat. The authors stated that these results suggest that humans are less sensitive than rodents to the nasal toxicity of MMA.