Reaction mass of methyl acetate and methanol

Endpoint summary

Link to relevant study record(s)

Reference

Reference 1

Endpoint:

basic toxicokinetics in vivo

Type of information:

experimental study

Adequacy of study:

weight of evidence

Reliability:

1 (reliable without restriction)

Rationale for reliability incl. deficiencies:

other: well documented GLP study, a reliability of 1 is assigned.

Reason / purpose for cross-reference:

reference to other study

Objective of study:

toxicokinetics

Principles of method if other than guideline:

Method: other

GLP compliance:

yes

Species:

rat

Route of administration:

inhalation

Vehicle:

not specified

Duration and frequency of treatment / exposure:

6 hours

Dose / conc.:

6.04 mg/L air

No. of animals per sex per dose / concentration:

Males: 10 Females: 10

Toxicokinetic examinations revealed that the blood levels of methyl acetate were below the detection limit of 5 ppm (v/v) immediately after the end of exposure and thereafter, indicating rapid clearence of the test compund from the blood.

Conclusions:

Toxicokinetic examinations revealed that the blood levels of methyl acetate were below the detection limit of 5 ppm immediately after the end of exposure and thereafter.
No bioaccumulation potential based on study results.

Executive summary:

The study reported methyl acetate concentration less than 5 ppm (v/v; < 4.6 mg/L) in rats immediately after 6 hours inhalation exposure (2,000 ppm) on the last day of a subacute study (6 hours daily, 5 days/week) and 30, 60, 120 min. as well as 18 hours later. At each time point two male rats and two female rats were sacrified to collect blood.

Reference 2

Endpoint:

basic toxicokinetics in vivo

Type of information:

experimental study

Adequacy of study:

weight of evidence

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

other: Well documented study, but only summary is in English. Details are in Japanese.

Objective of study:

metabolism

Principles of method if other than guideline:

Chemical analyses of metabolites in ureine.

GLP compliance:

not specified

Species:

other: human

Route of administration:

inhalation

Vehicle:

unchanged (no vehicle)

Duration and frequency of treatment / exposure:

2 h exposure time, twice a day at the interval of 2 h. Exposure was repeated for 3-4 days.

Remarks:

Doses / Concentrations:
200 ppm

No. of animals per sex per dose / concentration:

2 human beings

Details on study design:

Urine was collected at regular intervals for determination of the methanol content (either metabolite from methyl acetate or administered as such).

Metabolites identified:

yes

Details on metabolites:

methanol as a metabolite of methyl acetate

Conclusions:

No bioaccumulation potential based on study results. The methanol concentration in urine can serve as an indicator of the deegree of exposure to methyl acetate.

Executive summary:

Methyl acetate is rapidly metabolised in humans after inhalation. Methanol can serve as an indicator of metabolic degradation.

 

Repeated inhalation 2h-exposure of each of 2 test persons to about 200 ppm methyl acetate (165 290 ppm; approx. 610 mg/m3) respectively to methanol (160-225 ppm) 2 times per day (2h interval) over 3-4 days resulted in each of the first exposures per day to enhanced methanol concentrations in urine, which reached a maximum after each second exposure per day respectively within 4 h after second exposure (>10 mg methanol/L urine). At each next morning the values restored to normal (< 5 mg/L).

Reference 3

Endpoint:

basic toxicokinetics in vivo

Type of information:

experimental study

Adequacy of study:

weight of evidence

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

other: Investigation which cannot be subsumed undere a testing guideline, but which is well documented and scientifically acceptable.

Objective of study:

metabolism

Principles of method if other than guideline:

Methyl acetate and other esters were administered by inhalation. The hydrolyses rate and the hydrolyses products were determined enzymatically.

GLP compliance:

not specified

Species:

other: rabbits, rats, syrian hamster

Strain:

other: rats: F344/N rabbits: New Zealand white syrian hamster: no data

Sex:

male

Details on test animals or test system and environmental conditions:

Barrier-raised maIe F344/N rats, 15-20 weeks of age, reared at the Institute, were used in these studies. Rats were housed 2 per filter-topped, polycarbonate cage, on sterilized hardwood chip bedding. The animal rooms were maintained at 25°C with 40-60% relative humidity. The rooms were maintained on a 12 h light/12 h dark cycle, beginning at 600 a.m. Species-formulated food (Allied Mills, Chicago, IL) and water were available ad libitum. All rats’ cages were changed on a weekly basis. Rabbits used in these studies were New Zealand white male rabbits (Bell Rabbits, Clovis, NM), 20 to 26 weeks old. Male Syrian hamsters (Charles River, Wilmington, MA) were 14-18 weeks old at sacrifice. After receipt of shipment, the rabbits or hamsters were quarantined (a minimum of 2 weeks) and only healthy animals were used. The rabbits were kept in wire cages and were allowed water and food (Wayne Rabbit Chow) ad libitum. They were on a 12 h light/12 h dark cycle prior to sacrifice for their tissues. Hamsters were maintained as described above for rats.

Route of administration:

inhalation

Vehicle:

unchanged (no vehicle)

Control animals:

no

Details on study design:

The nasal, lung, and liver tissues were removed. Analyses of methyl acetate was by ion chromatography and enzymatically.

Preliminary studies:

The hydrolyses rate of methyl acetate was determined to be 15 Nmol/mg S-9 protein/min.

Metabolites identified:

no

Degradation products (CAS No./EC No./EINECS Name): 64-19-7     200-580-7 acetic acid

Degradation products (CAS No./EC No./EINECS Name): 67-56-1     200-659-6 methanol

 

Esters are readily hydrolyzed by respiratory tract enzymes. Species and tissue differences were apparent. The nasal ethmoturbinates had especially high levels of esterase activity with tissue weight-normalized activities from rabbits and hamsters for most substrates exceeding all other tissues tested, including liver.

 

The hydrolysis rate of methyl acetate was 15 nmol/mg S-9 protein/min (+-  3 SE) with rat ethmoturbinates S-9 with an observed logarithm of the  hydrolysis rate of 1.17.

 

Methyl acetate is quickly hydrolyzed to methanol and acetic acid (acetate).

Conclusions:

No bioaccumulation potential based on study results. Inhaled methyl acetate is rapidly hydrolysed.

Executive summary:

Inhaled methyl acetate is rapidly hydrolysed.

Reference 4

Endpoint:

basic toxicokinetics in vivo

Type of information:

experimental study

Adequacy of study:

weight of evidence

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

other: well documented study, a reliability of 2 is assigned.

Objective of study:

absorption

Principles of method if other than guideline:

Method: other

GLP compliance:

not specified

Species:

human

Route of administration:

inhalation

Duration and frequency of treatment / exposure:

10 minutes

Dose / conc.:

0.3 mg/L air

No. of animals per sex per dose / concentration:

Males: 4

Metabolites identified:

yes

Degradation products (CAS No./EC No./EINECS Name): 64-19-7 200-580-7 acetic acid
Degradation products (CAS No./EC No./EINECS Name): 67-56-1 200-659-6 methanol

The percentage solvent in end-exhaled air and in mixed-exhaled air increased after the start of the exposure and reached a quasi-steady-state level within a few minutes. The mean respiratory uptake for the last 5 minutes of methyl acetate respiration was 60.4%. Methanol, its metabolite, was detected in exhaled air (concentration: 1.3 ppm) at the first minute reaching a quasi-steady state level of 3 ppm at the 5th minute, and suggesting removal of the solvent by metabolism in the wall tissue of the respiratory tract.

Conclusions:

No bioaccumulation potential based on study results

Reference 5

Endpoint:

basic toxicokinetics in vitro / ex vivo

Type of information:

experimental study

Adequacy of study:

weight of evidence

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

other: Investigation which cannot be subsumed undere a testing guideline, but which is well documented and scientifically fully acceptable.

Objective of study:

toxicokinetics

Principles of method if other than guideline:

Methyl acetate was incubated with blood. The concentration of methyl acetate and formed methanol was measured with GC and GC-Ms.

GLP compliance:

not specified

Route of administration:

other: Methyl acetate was added to blood samples

Vehicle:

unchanged (no vehicle)

Duration and frequency of treatment / exposure:

2 - 8 h

Metabolites identified:

yes

Details on metabolites:

Methanol

The time-course of methanol formation and quantitative relation with methyl acetate disappearance was examined by incubating methyl acetate with blood at 36°C. Two controls were run in parallel; one was incubated in the absence of methyl acetate and the other was incubated without blood.

Conclusions:

No bioaccumulation potential based on study results. Methyl acetate does not accumulate in human blood under normal conditions.

Executive summary:

Almost 60% of methyl acetate added was converted to methanol in 2 h, and methyl acetate was exhausted in 8 h.

Reference 6

Endpoint:

basic toxicokinetics in vivo

Type of information:

experimental study

Adequacy of study:

weight of evidence

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

other: Peer reviewed data by Employers's Liability Insurance association

Reason / purpose for cross-reference:

reference to other study

Objective of study:

metabolism

Qualifier:

no guideline available

Principles of method if other than guideline:

Inhalation expsoure of tracheotomized rabbits to methyl acetate. Determination of methyl acetate in blood as well as in inhaled and exhaled air.

GLP compliance:

not specified

Radiolabelling:

no

Species:

rabbit

Strain:

not specified

Sex:

not specified

Vehicle:

unchanged (no vehicle)

Details on exposure:

Testing chamber with 645 l volume.

Duration and frequency of treatment / exposure:

Duration of exposure up to 3 h.

Details on dosing and sampling:

After start of inhalation blood samples were periodically taken and analysed for thier content of methyl acetate. Methyl acetate was determined in inhaled and exhaled air in short intervals.

Metabolites identified:

no

Methyl acetate was found in exhaled air shortly after start of inhalation exposure. Steady state concentration established within several minutes. Concentration of methyl acetate in exhaled air was 20 - 50 % of the concentration is inhaled air (up to 20 mg/L). Methyl acetate could not be detected in blood which indicates rapid hydrolyis in the body.

Conclusions:

No bioaccumulation potential based on study results
Methyl acetate is rapidly metabolised after inhalative exposure in rabbits.

Executive summary:

Shortly after start of inhalation exposure of tracheotomized rabbits to methyl acetate in a 645 L chamber for up to 3 hours methyl acetate was found in the exhaled air in a concentration of up to 30-50% of the inhaled quantity (up to 20 mg/L). After termination of exposure no methyl acetate was detected in the exhaled air. Although there was retention of methyl acetate during inhalation, methyl acetate could not be found in the blood indicating a rapid hydrolysis.

Reference 7

Endpoint:

basic toxicokinetics in vivo

Type of information:

experimental study

Adequacy of study:

weight of evidence

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

study well documented, meets generally accepted scientific principles, acceptable for assessment

Reason / purpose for cross-reference:

reference to same study

Objective of study:

toxicokinetics

Principles of method if other than guideline:

The study was performed to examine the absorption of methanol after oral administration and the rate and the extent of methanol absorption during inhalation exposure in rats and mice.

GLP compliance:

not specified

Radiolabelling:

no

Species:

other: rats and mice

Strain:

other: Sprague-Dawley and CD-1

Sex:

female

Route of administration:

other: oral:gavage and inhalation: vapour

Vehicle:

other: water and unchanged

Dose / conc.:

26.6 mg/L air

Dose / conc.:

13.3 mg/L air

Dose / conc.:

6.7 mg/L air

Dose / conc.:

1.3 mg/L air

Dose / conc.:

2 500 mg/kg bw/day

Dose / conc.:

100 mg/kg bw/day

Type:

clearance

Results:

for 2500 mg/kg bw: 60.7 ± 1.4 mg/hour/kg

Type:

clearance

Results:

for 100 mg/kg bw: 117.0 ± 3 mg/hour/kg

Type:

absorption

Results:

100% for 100 and 2500 mg/kg bw within minutes

Type:

absorption

Results:

Mean fractional respiratory absorption (13.3 and 26.3 mg/L): 70%

Type:

absorption

Results:

Mean fractional respiratory absorption (1.3 and 6.7 mg/L): 85%

Key result

Test no.:

#4

Toxicokinetic parameters:

Cmax: 1000 mg/L at 6.7 mg/L methanol (8 h exposure)

Key result

Test no.:

#3

Toxicokinetic parameters:

Cmax: 3500 mg/L at 6.7 mg/L methanol (8h exposure)

Key result

Test no.:

#2

Toxicokinetic parameters:

half-life 1st: 7.6 min (2500 mg/kg bw)

Key result

Test no.:

#1

Toxicokinetic parameters:

half-life 1st: 1.5 min (100 mg/kg bw)

Details on metabolites:

After oral administration of 100 and 2500 mg methanol/kg to female rats, gastrointestinal absorption was 100% within minutes (abs. half-life 1.5 min and 7.6 min, respectively).
The maximum elimination rate is about twice as high in mice as in rat: 117.0 ± 3 and 60.7 ± 1.4 mg/hour/kg.
After inhalation at exposure concentrations from 1.3 to 6.7 mg/L (corresponding to 1000 to 5000 ppm), the mean fractional respiratory absorption of methanol in rats and mice was found to about 85%. At exposure concentrations from 13.3 to 26.6 mg/L (corresponding to 10000 - 20000 ppm), the mean fractional respiratory absorption of methanol tended to be lower (approximately 70%) in rats, but not in mice.
Blood levels in rats were about 1000 mg/L (observed: 1047 ± 298 mg/L; predicted: 1018 mg/L) at 6.7 mg/L methanol (corresponding to 5000 ppm; 8 hour exposure). Blood levels in mice were about 3500 mg/L (observed: 3580 ± 599 mg/L; predicted: 4188 mg/L) at 6.7 mg/L methanol (corresponding to 5000 ppm; 8 hour exposure).

Reference 8

Endpoint:

basic toxicokinetics in vivo

Type of information:

experimental study

Adequacy of study:

weight of evidence

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

study well documented, meets generally accepted scientific principles, acceptable for assessment

Reason / purpose for cross-reference:

reference to same study

Objective of study:

toxicokinetics

Qualifier:

equivalent or similar to guideline

Guideline:

OECD Guideline 417 (Toxicokinetics)

Deviations:

yes

Remarks:

- only females were studied; determination of the amount of test substance only in urine and blood; no analysis of metabolites

GLP compliance:

not specified

Radiolabelling:

no

Species:

mouse

Strain:

CD-1

Sex:

female

Route of administration:

inhalation: vapour

Vehicle:

unchanged (no vehicle)

Duration and frequency of treatment / exposure:

8 hours for one day

Dose / conc.:

13 mg/L air

Dose / conc.:

6.7 mg/L air

Dose / conc.:

3.3 mg/L air

No. of animals per sex per dose / concentration:

Mice were exposed individually and in groups of 8 to 9 animals per dose.

Control animals:

yes

Details on metabolites:

Mice exposed to methanol in air at concentrations of 3.3 - 13 mg/L (corresponding to 2500 to 10000 ppm) absorbed the substance rapidly. Blood methanol concentrations were approximately 500 - 3000 mg/L at 1 hour and 500 - 4000 mg/L at 6 hours exposure.
Total 8 hours ventilation decreased slightly with increasing exposure concentration. The calculated values of the fraction of inhaled methanol absorbed was approx. 85% in mice. No statistically significant differences between exposed groups were observed for the fractional absorption.
Measured ventilation, fractional absorption, and systemic kinetic parameters were combinded in a semiphysiologic pharmacokinetic model that yielded accurate predictions of blood methanol concentrations during and after an 8 hours exposure. Model predictions for the mouse were compared to previously developed inhalation toxicokinetic model for the rat.
The comparison demonstrated that at similar methanol vapour concentrations, mice evidenced a two- to threefold higher blood methanol cencentration than rats, despite the fact that V(max) for methanol elimination in the mouse is twofold higher than that in the rat. The fractional absorption was higher in mice (approx. 85%) than in rats (56 - 87%) (p. 251; Perkins et al., 1995).
At the teratogenic exposure concentration in mice of 6.7 mg/L (corresponding to 5000 ppm), the following blood levels would be achieved mouse: 2313 ± 338 mg/L (exposed as group; Perkins et al., 1995); rat: 1047 ± 298 mg/L (Pollack and Brouwer, 1996).

Reference 9

Endpoint:

basic toxicokinetics in vivo

Type of information:

experimental study

Adequacy of study:

weight of evidence

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

study well documented, meets generally accepted scientific principles, acceptable for assessment

Reason / purpose for cross-reference:

reference to same study

Objective of study:

toxicokinetics

Qualifier:

equivalent or similar to guideline

Guideline:

OECD Guideline 417 (Toxicokinetics)

Deviations:

yes

Remarks:

- only females were studied; determination of the amount of test substance only in urine and blood; no analysis of metabolites

GLP compliance:

not specified

Radiolabelling:

no

Species:

rat

Strain:

Sprague-Dawley

Sex:

female

Route of administration:

inhalation: vapour

Vehicle:

unchanged (no vehicle)

Duration and frequency of treatment / exposure:

8 hours for one day

Dose / conc.:

26.6 mg/L air

Dose / conc.:

20 mg/L air

Dose / conc.:

13 mg/L air

Dose / conc.:

6.7 mg/L air

Dose / conc.:

1.3 mg/L air

No. of animals per sex per dose / concentration:

3 - 4

Control animals:

yes

Details on metabolites:

Blood methanol concentrations were approximately 100 - 4000 mg/L after 8 hours exposure to 1.3 to 26.6 mg/L methanol vapour.
The influence of inhaled methanol exposure on total 8 hours ventilation remained fairly constant during exposure for each rat. The mass of methanol extracted from the air and the mass in the blood pluss mass eliminated were on balance approximately equal during the course of the experiment regardless of concentration. The total amount of methanol taken up by the animal increased as exposure concentration increased, the relationship, however, was not proportional, which suggests that the fraction absorbed differed across exposure concentrations. The fraction absorbed decreased as exposure concentration increased: fractional absorption of inhaled methanol (1.3 to 26.6 mg/L) was 87% to 60%.

Comparison of rat and mouse: The same environmental methanol vapour concentration lead to very different blood methanol levels and these were higher in mice although the maximum elimination rate was about twice as high in as in rats (Pollack and Brouwer 1996).
At an concentration in mice of 6.7 mg/L (5000 ppm, which was teratogenic in mice), the following blood levels were estimated: mouse 2313 ± 338 mg/L (exposed as group; Perkins et al., 1995); rat 1047 ± 298 mg/L (Pollack and Brouwer, 1996).
In conclusion, the same environmental methanol vapour concentration leads to very different blood methanol level. Therefore, risk assessment for methanol should be based on blood concentrations following inhalation exposure, not on exposure concentrations itself. The differences between the teratogenic potential of methanol in rats versus mice, with mice being approximately two-fold more sensitive than rats may be due to the differences in blood concentrations.

Description of key information

The reaction mass of methyl acetate and methanol is assessed on the basis of the individual constituents methyl acetate and methanol using a read-across approach from the supporting substances (structural analogue or surrogate).

 

Methyl acetate

Several experimental studies on toxicokinetics are available for methyl acetate. Furthermore, the toxicokinetic behavior of methyl acetate was assessed from substance-specific physicochemical properties and the available toxicological studies of the substance. Both its physicochemical properties and results from in vivo studies show that methyl acetate is bioavailable by all routes (100% absorption rate for oral and inhalative uptake). In vitro and in vivo studies show that methyl acetate is hydrolysed into acetic acid and methanol by esterases at the intestinal barrier and/or blood fastly. Therefore, further toxicokinetic assessment is exclusively based on the metabolites.

It is known from literature that acetic acid is almost completely absorbed from the gastrointestinal (GI) tract and is also absorbed via the lungs following inhalation. It is a weak acid and hence at the pH of body fluids will be found in both the acid and acetate forms. Acetic acid is rapidly metabolised in plasma and most tissues, with a half-life of the order of a few minutes. Acetate is readily converted to acetyl-CoA, which enters the citric acid cycle, being converted eventually to carbon dioxide. Due to conversion of acetic acid to carbon dioxide only a small amount of acetic acid is excreted unchanged as acetate in the urine.

The second metabolite, methanol, is assessed separately as constituent of the target substance and is therefore addressed in the following.

 

Methanol

Methanol is readily absorbed after inhalation (60% absorption rate), ingestion (100% absorption rate) and dermal contact and distributed rapidly to organs and tissues. Metabolisation to formaldehyde by alcohol dehydrogenase (humans and monkeys) or by the catalase-peroxidase pathway (rodents) takes place in the liver. It is then further metabolised to formic acid or formate and finally to carbon dioxide. The rate of formate oxidation depends on the availability of folate, which varies amongst species and is considered a key determinant in species differences in sensitivity to acute methanol toxicity. Methanol is metabolized almost completely (98%), with more than 90% of the administered dose exhaled as carbon dioxide, as shown in animals and from experience with humans. Only small amounts of unmetabolised methanol are excreted renally or pulmonary.

 

Conclusion

The target substance, the reaction mass of methyl acetate and methanol, is composed of the constituents methanol (approx. 80%) and methyl acetate (approx. 20%). Taking into account that methyl acetate is rapidly metabolised into acetic acid and methanol, the source substances for assessing the toxicokinetics are methanol and acetic acid, which are degraded in the metabolic pathways described above.

Key value for chemical safety assessment

Bioaccumulation potential:

no bioaccumulation potential

Additional information

Methyl acetate

Several experimental studies on toxicokinetics are available for methyl acetate. Furthermore, the toxicokinetik behaviour of methyl acetate was assessed from substance-specific physico-chemical properties and the available toxicological studies of the substance. Therefore, in accordance with Annex VIII, Column 1, Item 8.8 of Regulation (EC) 1907/2006 and with Guidance on information requirements and chemical safety assessment Chapter R.7c: Endpoint specific guidance (ECHA, 2012), assessment of the toxicokinetic behavior of the substance methyl acetate was conducted based on the relevant available information.

 

Due to high-vapour pressure of methyl acetate (220 hPa at 20°C), water solubility (295 g/L) and partition coefficient (log Pow) of 0.18 indicate bioavailability of the substance at all routes. Methyl acetate is absorbed via the lungs in animals and humans, absorption via the oral route is demonstrated. After absorption the substance undergoes hydrolysis to methanol and acetic acid.

 

Toxicokinetics in vitro/ex vivo, RL2

There are several studies available that show the hydrolysis of methyl acetate. Ghittori et al. (1984) reported on hydrolysis of methyl acetate in blood at 37ºC (concentration: 2.18 μg/L; 7 hours later: 1.96 μg/L). The products were identified as acetic acid and methanol.

 

Toxicokinetics in vitro/ex vivo, RL2

Mizunuma et al. (1992) detected hydrolysis of methyl acetate to methanol in vitro when methyl acetate was incubated with human blood (27.9 μg/mL) for 2 to 8 hours at 36°C. The velocity of the reaction was so fast that 60% of methyl acetate was converted to methanol in 2 hours, almost all of methyl acetate disappeared in 8 hours (detection limit 0.1 μg/ml). Concentrations were analysed by head-space gas chromatography, the methanol formation was further confirmed by means of gas chromatography-mass spectrometry. The capacity to hydrolyse methyl acetate was evenly distributed in cellular and noncellular fractions of blood.

 

Toxicokinetics in vivo, RL2

After incubation of methyl acetate with carboxylic esterases from nasal mucosa of male F344 rats (S9-homogenate) a rate of hydrolysis of 15 + 3 nmol/mg S9-protein/min was determined (Dahl et al., 1987). Cleavage of methyl acetate into methanol and acetic acid in the gastrointestinal tract takes place by esterases of the gastric mucosa. It is furthermore hydrolysed by esterases of the blood. It is roughly estimated that the half-life of methyl acetate amounts to 2 – 3 h (rat blood) and ca. 2 h (human blood).

 

Toxicokinetics in vivo, RL1

HMR (1999) reported methyl acetate concentration less than 5 ppm (v/v; < 4.6 mg/L) in rats immediately after 6 hours inhalation exposure (2,000 ppm) on the last day of a subacute study (6 hours daily, 5 days/week) and 30, 60, 120 min. as well as 18 hours later. At each time point two male rats and two female rats were sacrified to collect blood.

 

Toxicokinetics in vivo, RL1

Shortly after start of inhalation exposure of tracheotomized rabbits to methyl acetate in a 645 L chamber for up to 3 hours methyl acetate was found in the exhaled air in a concentration of up to 30-50% of the inhaled quantity (up to 20 mg/L). After termination of exposure no methyl acetate was detected in the exhaled air. Although there was retention of methyl acetate during inhalation, methyl acetate could not be found in the blood indicating a rapid hydrolysis (BG Chemie, 1995).

 

Methyl acetate is rapidly metabolised in humans after inhalation. Methanol can serve as an indicator of metabolic degradation.

 

Toxicokinetics in vivo, RL2

Repeated inhalation 2h-exposure of each of 2 test persons to about 200 ppm methyl acetate (165 290 ppm; approx. 610 mg/m3) respectively to methanol (160-225 ppm) 2 times per day (2h interval) over 3-4 days resulted in each of the first exposures per day to enhanced methanol concentrations in urine, which reached a maximum after each second exposure per day respectively within 4 h after second exposure (>10 mg methanol/L urine). At each next morning the values restored to normal (< 5 mg/l) (Tada et al., 1974).

 

Toxicokinetics in vivo, RL2

Kumagai et al (1999) reported that the percentage of methyl acetate in end-exhaled air and in mixed-exhaled air of human beings increased after the start of the exposure and reached a quasi-steady-state level within a few minutes. The mean respiratory uptake for the last 5 minutes of methyl acetate respiration was 60.4%. Methanol, its metabolite, was detected in exhaled air (concentration: 1.3 ppm) at the first minute reaching a quasi-steady state level of 3 ppm at the 5th minute, and suggesting removal of the solvent by metabolism in the wall tissue of the respiratory tract.

 

The main metabolite is methanol which itself is metabolised to formic acid. Methanol is rapidly and completely absorbed from the gastrointestinal tract following oral ingestion, reaching peak serum levels within 30-60 minutes, depending on the presence or absence of food in the stomach. Methanol distributes readily and uniformly to organs and tissues in direct proportion to their water content, and has a volume of distribution of 0.6-0.7 L/kg b.w. (IPCS/WHO, 1997). Methanol is rapidly and extensively metabolised in the liver, first to formaldehyde, by alcohol dehydrogenase in humans, then to formic acid or formate (depending on pH), primarily by formaldehyde dehydrogenase and finally to carbon dioxide, catalysed by formyl-tetrahydrofolic acid (THF) synthetase. Formic acid combines with THF to give 10-formyl-THF which is then converted into carbon dioxide by formyl-THF dehydrogenase. (IPCS/WHO, 1997; Cruzan, 2009). The rate of formate oxidation depends on the availability of folate, which varies amongst species. This is a key determinant in species differences in sensitivity to acute methanol toxicity. In general folate levels are lower in primates than in rodents (Johlin et al., 1987). The elimination half-life of methanol in humans is 2.5-3.0 h, although at high doses saturation of elimination results in more prolonged half-lives (Jones, 1987). In contrast, the elimination of formaldehyde is extremely rapid, with a half-life of approximately 1.5 min, in Cynomolgus monkeys (McMartin et al., 1979). Therefore interspecies differences in the metabolism were considered mainly of concern at dose levels leading to acute toxicity. Thus rat is a useful model to indicate subacute/subchronic toxic effects below sublethal dosages.

 

The second hydrolysis product acetic acid is almost completely absorbed from the gastrointestinal (GI) tract and is also absorbed via the lungs following inhalation. It is a weak acid and hence at the pH of body fluids will be found in both the acid and acetate forms. Acetic acid is rapidly metabolised in plasma and most tissues, with a half-life of the order of a few minutes (3-5 min, depending on dose) (Freundt, 1973, as cited by ECHA, 2012).

 

Acetate is readily converted to acetyl-CoA, which enters the citric acid cycle, being converted eventually to carbon dioxide (Smith et al., 2007).

 

Therefore, in blood and urine only methanol and acetic acid were found, not methyl acetate. Almost 97 % of an oral dose of methanol is eliminated as CO2 (IPCS/WHO, 1997; Cruzan, 2009). A small amount of methanol (~2 %) is excreted unchanged in urine and expired air. Due to convertion of acetic acid to carbon dioxide only a small amount (~0.6 %) of acetic acid is excreted unchanged, in the urine as acetate (Smith et al., 2007).

 

 

Methanol

Methanol is readily absorbed after inhalation, ingestion and dermal contact and distributed rapidly throughout the body. The clearance from the body is mainly due to metabolism (up to 98%), with more than 90% of the administered dose exhaled as carbon dioxide. Renal and pulmonary excretion rates contribute to only about 2 – 3%. The metabolism and toxicokinetics of methanol varies by species and dose. In humans, the half-life time is approximately 2.5 – 3 hours at doses lower than 100 mg/kg bw. At higher doses, the half life can be 24 hours or more (IPCS/WHO, 1977; Kavet and Nauss, 1990).

 

The mammalian metabolism of methanol occurs mainly in the liver, where methanol is initially converted to formaldehyde, which is in turn converted to formate. Formate is converted to carbon dioxide and water. In humans and monkeys, the oxidation to formaldehyde is mediated by alcohol dehydrogenases and basically limited to the capacity of those enzymes. In rodents, the oxidation to formaldehyde predominantly employes the catalase-peroxidase pathway which is of less capacity than the ADH-pathway in humans but on the other hand produces oxygen radicals which may be involved into the developmental effects in rodents which - in contrast to humans - tolerate high methanol levels without signs of CNS or retinal toxicity. The last oxidation step, conversion of formate to carbon dioxide employes formyl-tetrahydrofolate synthetase a co-enzyme, is of comparably low capacity in primates which leads to a low clearance of formate, possibly also from sensitive target tissues (such as CNS and the retina) (DFG 1999; IPCS/WHO, 1997; Dorman et al., 1994; Medinsky et al., 1997, Medinsky and Dorman, 1995; Mc Martin et al., 1977).

 

In humans, when exposed to methanol via inhalation up to an air concentration 65 mg/m3, no increase of blood methanol is expected. Up to 260 mg/m3 (single or repeated exposure) the methanol blood level is likely to increase only 2- to 4- fold above the endogenous methanol concentration in humans, but still remains significantly below 10 mg/L (Lee et al., 1992; NTP, 2003). Up to air concentrations of 1600 mg/m3 the blood methanol levels increase to a similar extent in rats, monkeys, and humans. However, above this concentration rats show a steep exponential increase which apparently reflects the saturation of the catalase-dependent pathway. A smaller exponential increase was observed in monkeys, whereas in humans there appears to be a linear relationship between air concentrations and blood methanol levels.

 

Baseline levels of formate in blood are about 3 to 19 mg/L (0.07 – 0.4 mM) in humans. Toxic blood formate concentrations are reported to be 220 mg/L and higher (> 5 mM formate). Inhalation of about 1200 mg methanol/m3 for 2.5 hours contributed only insignificantly to the internal formate pool in monkeys (in the μM-range). This also hold true for folate-deficient conditions. After repeated inhalation of 2600 mg/m3 for 6 hours/day, 5 days/week, for 1 or 2 weeks, monkeys showed no discernible increase in formate concentrations in blood (estimated body burden 200 to 300 mg/kg bw/d). Formate accumulation, however, has been observed in primates upon bolus administration of more than 500 mg Methanol/kg bw (Horton et al., 1992; Medinsky and Dorman, 1995). The critical methanol dose that saturates the folate pathway in humans is estimated to be ≥ 200 mg/kg bw. Based on data produced in monkeys, metabolic saturation in humans is also less likely to happen upon inhalation where the dose is distributed over several hours (DFG 1999; IPCS/WHO, 1997; Burbacher et al., 1999).

 

There is a strong link between saturation (zero-order) kinetics and the onset of acute toxic effects. Exposure levels in humans above 5000 ppm (750 mg/kg bw in the course of 8 hrs) are prone to a zero-order kinetic and a strong accumulation of methanol in the blood. Transient blindness has been reported for exposure levels between 1000 and 5000 ppm. (This saturation point could be reached after oral uptake at lower dose levels.) 10.000 ppm are still tolerated in rodents but would be highly detrimental in humans.

 

Conclusion: Methyl acetate is hydrolysed rapidly and completely by esterases to methanol and acetic acid in the gut and/or blood. Methanol is mainly metabolized to CO2 and eliminated through the air and acetic acid is incorporated in the endogenous metabolism. Low amounts of both substances are excreted unchanged in the urine or by air.