Trimethyl orthoacetate

Endpoint summary

Link to relevant study record(s)

Reference

Endpoint:

basic toxicokinetics in vivo

Type of information:

other: expert statement

Adequacy of study:

key study

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

other: An extended assessment of the toxicokinetic behaviour of TMOA was performed, taking into account the chemical structure, the available physico-chemical-data and the available toxicity data.

Objective of study:

absorption

distribution

excretion

metabolism

Guideline:

other: ECHA Guidance R.7c

Principles of method if other than guideline:

An assessment of toxicological behaviour of TMOA is based on its physico-chemical properties and on the results of available toxicity data.

GLP compliance:

no

Type:

absorption

Results:

The absorption rate of TMOA is assumed to be 100 % via the oral, dermal and inhalation route.

Type:

distribution

Results:

The substance and the hydrolysis products are expected to be distributed widely through the body via blood circulation. Transport between cells through cell membranes might also be possible for the hydrolysis products methanol and acetic acid

Type:

metabolism

Results:

Methanol is metabolised in the liver leading to the formation of toxic formic acid.

Type:

excretion

Results:

The main excretion route for the hydrolysis products acetic acid is urinary excretion. Methanol is, besides urinary excretion, also excreted via exhalation.

Details on absorption:

Oral absorption
Based on the results of the hydrolysis study the registered substance is expected to readily hydrolyse completely in the acidic environment of the stomach into acetic acid and methanol. Therefore, the absorption rate and the toxicity profile are believed to be triggered by the hydrolysis products acetic acid and methanol. Absorption by passive diffusion is favoured for substances with moderate log P values (between -1 and 4). With a log P of -0.77 and -0.17, methanol and acetic acid are likely to follow this route of absorption. Additionally, based on their low molecular weight (< 200 g/mol) and high water solubility, both hydrolysis products may also be absorbed by passing through aqueous pores or being carried through the epithelial barrier by the bulk passage of water.
Since no signs of toxicity were observed in the acute oral toxicity study in rats (Hüls AG 1995a), this study does not give any indication regarding oral absorption.
Taken together, based on the physical-chemical properties of TMOA and its hydrolysis products acetic acid and methanol an oral absorption rate of 100 % is assumed.

Dermal absorption
Absorption in the stratum corneum is favoured for substances with a molecular weight below 100 g/mol and very unlikely for chemicals with a molecular weight above 500 g/mol. To cross the lipid-rich stratum corneum a certain degree of lipophilicity is required (Log P > 0). The registered substance has a molecular weight of 120 g/mol and a log P of 1.13. Based on the low molecular weight and the log P above 0, the registered substance might be able to be absorbed by the stratum corneum. To partition from the stratum corneum into the viable part of the epidermis, a substance must be sufficiently soluble in water (>1 mg/). TMOA has a water solubility of 10.8 g/L. Thus, penetration into the deeper, viable layers of the epidermis is likely.
In a skin irritation study according to OECD Guideline 404 TMOA was shown to have irritating effects (Hüls AG 1995c), which may additionally favour dermal absorption via uptake through a potentially compromised skin barrier. Furthermore, TMOA was tested positive in GPMT according to OECD Guideline 406 (Hüls AG 1995d). This underlines the assumption that the substance is absorbed by the viable part of the epidermis.
All in all, based on the physical-chemical properties and the toxicological profile of TMOA, a dermal absorption rate of 100 % is assumed.

Respiratory absorption
As the substance is a liquid, no inhalable particles occur. Due to the vapour pressure of 4.14 kPa at 25°C TMOA is moderately available for inhalation as a vapour. Thus, uptake of the substance into the lung cannot be excluded. Penetration to the lower respiratory tract is favoured for substances with low water solubility, which do not dissolve in the mucus lining the respiratory tract. The moderate water solubility indicates that TMOA may dissolve in the mucus to a certain degree. However, penetration of TMOA to the alveolar region of the lung cannot be ruled out. Having a log P value of 1.13, absorption directly across the respiratory tract epithelium by passive diffusion is favoured for TMOA. In addition, it is expected that TMOA will hydrolyse in the lung. The available hydrolysis study (AlessaChemie 2004) does not provide an estimate of the actual hydrolysis half-life of the substance in the respiratory tract. However, it indicates that the parent compound is only present in the lung for a limited time. The hydrolysis products acetic acid and methanol may also be absorbed by passive diffusion across the respiratory tract epithelium. Furthermore, their high water solubility also favours absorption by passing through aqueous pores or being carried through the epithelial barrier by the bulk passage of water.
Based on the available data, it can be concluded that inhalatory exposure is possible. If respiratory exposure takes place, a high systemic availability is assumed. As worst case, 100 % inhalation absorption is assumed.

Details on distribution in tissues:

Since TMOA readily hydrolyses into acetic acid and methanol, the distribution and accumulative potential of acetic acid and methanol can follow more or less independent ways. Due to their low molecular weight and high water solubility, acetic acid and methanol are expected to be widely distributed in the body via blood circulation. As small water soluble substances, acetic acid and methanol might be able to diffuse through aqueous channels and pores of cell membranes into cells.
Based on the low log P (< 3) accumulation in fatty tissue or stratum corneum is expected neither for the parent compound TMOA, nor for the hydrolysis products acetic acid and methanol.

Details on excretion:

A major excretion route of methanol from the circulatory system is exhaled air (30 to 60 % of absorbed methanol). In addition, methanol is metabolised in the liver. Here, methanol is converted to formaldehyde via alcohol dehydrogenase (ADH). This is readily oxidised to formic acid (formate) via aldehyde dehydrogenase (ALDH). Formic acid is further metabolised to carbon dioxide via tetrahydrofolate (FH4)-dependent C1 metabolism. However, the metabolism of formic acid is rather slow. Therefore, formic acid accumulates in the blood leading to acidosis. The neurotoxic effects occurring after methanol intoxication are attributed to this acidosis caused by the metabolite formic acid (Fuhrmann 2006).
Acetic acid is usually fully ionised to acetate at physiological pHs. It can be bound to Coenzyme-A forming acetyl-CoA. In this form it enters the citric acid cycle.
Besides the expiration of methanol itself and its metabolite CO2, urinary excretion is another route of elimination of methanol and formic acid. For acetic acid, urinary excretion is expected to be the main excretion route.

Metabolites identified:

yes

Details on metabolites:

formic acid

Executive summary:

The absorption rate of TMOA is assumed to be 100 % via the oral, dermal and inhalation route. The substance and the hydrolysis products are expected to be distributed widely through the body via blood circulation. Transport between cells through cell membranes might also be possible for the hydrolysis products methanol and acetic acid. Methanol is metabolised in the liver leading to the formation of toxic formic acid. The main excretion route for the hydrolysis products acetic acid is urinary excretion. Methanol is, besides urinary excretion, also excreted via exhalation.

Description of key information

The absorption rate of TMOA is assumed to be 100 % via the oral, dermal and inhalation route. The substance and the hydrolysis products are expected to be distributed widely through the body via blood circulation. Transport between cells through cell membranes might also be possible for the hydrolysis products methanol and acetic acid. Methanol is metabolised in the liver leading to the formation of toxic formic acid. The main excretion route for the hydrolysis products acetic acid is urinary excretion. Methanol is, besides urinary excretion, also excreted via exhalation.

Key value for chemical safety assessment

Bioaccumulation potential:

no bioaccumulation potential

Absorption rate - oral (%):

100

Absorption rate - dermal (%):

100

Absorption rate - inhalation (%):

100

Additional information

Toxicokinetic Statement for trimethyl orthoacetate (CAS 1445-45-0)

General

There are no ADME studies available for trimethyl orthoacetate (TMOA) (CAS 1445-45-0). The toxicokinetic profile was not determined by actual absorption, distribution, metabolism or excretion measurements. Rather, toxicokinetics of TMOA was assessed based on the physical-chemical properties in combination with results of toxicity studies.

Substance identity

Table 1: Physical-chemical properties of TMOA and the derivatives acetic acid and methanol

	TMOA (CAS 1445-45-0)	Acetic acid (CAS 64-19-7)	Methanol (CAS 67-56-1)
Structural formula	C5H12O3	C2H4O2	CH4O
Molecular Weight [g/mol]	120	60	32
Physical state	liquid	liquid	liquid
Water Solubility (20°C)	1.08E4 mg/L (QSAR WSKOW v1.42, EPIWIN 2010)	6.03E5 mg/L (handbook data)	1E6 mg/L (handbook data)
Log P	1.13 (QSAR, EPIWIN 2010)	-0.17 (handbook data)	-0.77 (handbook data)
Vapour Pressure (25°C)	4140 Pa (QSAR, EPIWIN 2010)	20.8 Pa (handbook data)	169 Pa (handbook data)
Boiling point	108 – 109°C (experimental study)	118°C (handbook data)	64.7°C (handbook data)

Abiotic degradation

Hydrolysis as a function of pH (EC-Guideline C.7, GLP) (AlessaChemie 2004)

The hydrolytic degradation of TMOA can be described as a pseudo-first order kinetic and the half-life of TMOA was determined to be < 2.4 h at pH 4, 7 and 9. Under neutral (pH 7) and acidic (pH 4) conditions the half life was < 1 h (table 1). Methanol (CAS 67-56-1) and methyl acetate (CAS 79-20-9) were identified as degradation products. The mass balance of TMOA (C₅H₁₂O₃), methanol (CH₄O) and methyl acetate (C₃H₆O₂) after relatively short reaction times are in accordance with the following reaction step:

C₅H₁₂O₃+ 1 H₂0 -> C₃H₆O₂+ 2 CH₄O.

The relative increase of methanol in comparison to the diminished amount of TMOA above the ratio of 2:1 at larger reaction times indicates the further hydrolysis of methyl acetate to acetic acid as final reaction product.

All in all, it can be concluded that the substance hydrolyses very rapidly in water into methanol and acetic acid.

Table 2: Hydrolysis as a function of pH value tested according to EC-Guideline C.7 with TMOA

Test	Temperature [°C]	pH 9		pH 7		pH 4
preliminary test	50	Time (h)	Hydrolysis (%)	Time (h)	Hydrolysis (%)	Time (h)	Hydrolysis (%)
		0	< 1	0	31.4	0	100
		1	-	1	100	1	-
		2.4	80.5	2.4	100	2.4	100
		4	92.9	4	-	4	-
further necessary	testing (yes/no)	no		no		no

Toxicological profile

Acute Oral Toxicity in the Rat – Limit test (OECD Guideline 401, GLP) (Hüls AG 1995a)

Following single application of TMOA at a dose of 2000 mg/kg bw to five male and five female rats the test material did not induce mortality. The bodyweight change was not affected by the treatment and the gross examination did not reveal any pathological changes. Hence, the test material was considered to be non-toxic under the conditions of the test.

Acute Dermal Toxicity in the Rat – limit test (OECD Guideline 402, GLP) (Hüls AG 1995b)

The test substance TMOA was applied topically (semiocclusive) to five male and five female rats at a dose of 2000 mg/kg bw. The exposure time was 24 hours. Following single application of TMOA, the animals did not show any general clinical signs of toxicity during the observation period of 14 days. All animals survived the experiment. The bodyweight change wasnot affected by the treatment and the gross examination did not reveal any pathological changes. Hence, the test material was considered to be non-toxic under the conditions of the test.

Skin irritation (OECD Guideline 404, GLP) (Hüls AG 1995c)

The average irritation indices of TMOA 24, 48 and 72 hours after the end of exposure were 1.89 and 2.0 for erythema and oedema formation, respectively. After 13 days, all three rabbits were free of signs of irritation. Therefore, the test material is considered to be not irritating according to the Draize classification scheme. No corrosive effects were noted.

Skin sensitisation (OECD Guideline 406, GLP) (Hüls AG 1995d)

Skin sensitisation potential of the test item TMOA was studied in the guinea pig maximisation test (GPMT). TMOA yielded positive responses in 12/20 animals and it therefore considered as skin sensitiser.

Oral absorption

Based on the results of the hydrolysis study the registered substance is expected to readily hydrolyse completely in the acidic environment of the stomach into acetic acid and methanol. Therefore, the absorption rate and the toxicity profile are triggered by the hydrolysis products acetic acid and methanol. Absorption by passive diffusion is favoured for substances with moderate log P values (between -1 and 4). With a log P of -0.77 and -0.17, methanol and acetic acid are likely to follow this route of absorption. Additionally, based on their low molecular weight (< 200 g/mol) and high water solubility, both hydrolysis products may also be absorbed by passing through aqueous pores or being carried through the epithelial barrier by the bulk passage of water.

Since no signs of toxicity were observed in the acute oral toxicity study in rats (OECD guideline 401) with TMOA (Hüls AG 1995a), this study does not give any indication regarding oral absorption.

Taken together, an oral absorption rate of 100 % is assumed for TMOA.

Dermal absorption

Absorption in the stratum corneum is favoured for substances with a molecular weight below 100 g/mol and very unlikely for chemicals with a molecular weight above 500 g/mol. To cross the lipid-rich stratum corneum a certain degree of lipophilicity is required (Log P > 0). The registered substance has a molecular weight of 120 g/mol and a log P of 1.13. Based on the low molecular weight and the log P above 0, the registered substance might be able to be absorbed by the stratum corneum. To partition from the stratum corneum into the viable part of the epidermis, a substance must be sufficiently soluble in water (>1 mg/). TMOA has a water solubility of 1.08E4 mg/L. Thus, penetration into the deeper, viable layers of the epidermis is likely.

In a skin irritation study according to OECD Guideline 404 TMOA was shown to have irritating effects (Hüls AG 1995c), which may additionally favour dermal absorption via uptake through a potentially compromised skin barrier. Furthermore, TMOA was tested positive in GPMT according to OECD Guideline 406 (Hüls AG 1995d). This underlines the assumption that the substance is absorbed by the viable part of the epidermis.

All in all, based on the physical-chemical properties and the toxicological profile of TMOA, a dermal absorption rate of 100 % is assumed.

Respiratory absorption

As the substance is a liquid, no inhalable particles occur. Due to the vapour pressure of 4.14 kPa at 25°C TMOA is moderately available for inhalation as a vapour. Thus, uptake of the substance into the lung cannot be fully excluded. Penetration to the lower respiratory tract is favoured for substances with low water solubility, which do not dissolve in the mucus lining the respiratory tract. The moderate water solubility indicates that TMOA may dissolve in the mucus to a certain degree. However, penetration of TMOA to the alveolar region of the lung cannot be ruled out. Having a log P value of 1.13, absorption directly across the respiratory tract epithelium by passive diffusion is favoured for TMOA. In addition, it is expected that TMOA will hydrolyse in the lung. The available hydrolysis study (AlessaChemie 2004) does not provide an estimate of the actual hydrolysis half-life of the substance in the respiratory tract. However, it indicates that the parent compound is only present in the lung for a limited time. The hydrolysis products acetic acid and methanol may also be absorbed by passive diffusion across the respiratory tract epithelium. Furthermore, their high water solubility also favours absorption by passing through aqueous pores or being carried through the epithelial barrier by the bulk passage of water.

Based on the available data, it can be concluded that inhalatory exposure is limited but possible. If respiratory exposure takes place, systemic availability can be assumed. As worst case, 100 % inhalation absorption is assumed.

Distribution and Accumulation

Since TMOA readily hydrolyses into acetic acid and methanol, the distribution and accumulative potential of acetic acid and methanol can follow more or less independent ways. Due to their low molecular weight and high water solubility, acetic acid and methanol are expected to be widely distributed in the body via blood circulation. As small water soluble substances, acetic acid and methanol might be able to diffuse through aqueous channels and pores of cell membranes into cells.

Based on the low log P (< 3) accumulation in fatty tissue or stratum corneum is expected neither for the parent compound TMOA, nor for the hydrolysis products acetic acid and methanol.

Metabolism and Elimination

A major excretion route of methanol from the circulatory system is exhaled air (30 to 60 % of absorbed methanol). In addition, methanol is metabolised in the liver. Here, methanol is converted to formaldehyde via alcohol dehydrogenase (ADH). This is readily oxidised to formic acid (formate) via aldehyde dehydrogenase (ALDH). Formic acid is further metabolised to carbon dioxide via tetrahydrofolate (FH4)-dependent C1 metabolism. However, the metabolism of formic acid is rather slow. Therefore, formic acid accumulate in the blood leading to acidosis. The neurotoxic effects occurring after methanol intoxication are attributed to this acidosis caused by the metabolite formic acid (Fuhrmann 2006).

Acetic acid is usually fully ionised to acetate at physiological pHs. It can be bound to Coenzyme-A forming acetyl-CoA. In this form it enters the citric acid cycle.

Besides the expiration of methanol itself and its metabolite CO₂, urinary excretion is another route of elimination of methanol and formic acid. For acetic acid, urinary excretion is expected to be the main excretion route.

References

ECHA (2017): Guidance on Information Requirements and Chemical Safety Assessment; Chapter R.7c: Endpoint specific guidance, version 3, ISBN 978-92-9495-838-9

Fuhrmann GF (2006): Toxikologie für Naturwissenschaftler, B.G. Teubner Verlag, ISBN 3-8351-0024-6