Exo-1,7,7-trimethylbicyclo[2.2.1]hept-2-yl acrylate

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

Study period:

2018

Reliability:

1 (reliable without restriction)

Rationale for reliability incl. deficiencies:

test procedure in accordance with generally accepted scientific standards and described in sufficient detail

Objective of study:

toxicokinetics

Qualifier:

no guideline followed

Principles of method if other than guideline:

Three male rats per compound were intravenously administered IBOA at target concentration 7.5 mg/kg (36 µmol/kg), using Intralipid 20% as the dose vehicle. After dose administration, blood samples (~200 μL) were collected at 5, 10, 30, 60, and 180 minutes into individual pre-weighed glass vials containing ethyl acetate (600 μL) acidified with 10% trifluoroacetic acid (TFA). After vortexing and subsequent centrifugation, the blood extracts underwent quantitative analysis for IBOA by gas chromatography tandem mass spectrometry (GC/MS-MS).

GLP compliance:

no

Radiolabelling:

no

Species:

rat

Strain:

Fischer 344/DuCrj

Sex:

male

Details on test animals or test system and environmental conditions:

TEST ANIMALS
- Source: Charles River (Kingston, New York)
- Age at study initiation: 9-12 weeks
- Weight at study initiation: 183-196 g
- Housing: singly in glass Roth-type metabolism cages
- Diet (e.g. ad libitum): LabDiet Certified Rodent Diet #5002 (PMI Nutrition International, St. Louis, Missouri) in pelleted form, ad libitum
- Water (e.g. ad libitum): Municipal water, ad libitum
- Acclimation period: 7d

ENVIRONMENTAL CONDITIONS
- Temperature (°C): 22°C with a range of 20°C-26°C
- Humidity (%): 50% with a range of 30-70%
- Photoperiod (hrs dark / hrs light): 12-hour light/dark (on at 6:00 a.m. and off at 6:00 p.m.)

Route of administration:

intravenous

Vehicle:

other: Intralipid 20%

Details on exposure:

Appropriate amounts of the test substance were added to sterile pharmaceutical grade Intralipid 20% using aseptic technique to obtain the appropriate concentration. The amount of dose solution administered was targeted at 2.5 mL/kg bw and injected over ~1minute to correspond to an injection rate of ~0.5 mL/min. The dose was stored at ambient temperature (19-25°C) and used within 24 hours of preparation.

Duration and frequency of treatment / exposure:

single iv treatment

Dose / conc.:

7.5 mg/kg bw/day (nominal)

Remarks:

corresponds to 36 µmol/kg

No. of animals per sex per dose / concentration:

3 males

Control animals:

yes, concurrent vehicle

other: another isobornyl ester (isobornyl acetate, IBOAC) was tested for its toxikokinetic properties for comparison reasons (rad-across background)

Positive control reference chemical:

no

Details on study design:

- Dose selection rationale: previous studies of this type have used similar equimolar ratios

Details on dosing and sampling:

TOXICOKINETIC / PHARMACOKINETIC STUDY
- Tissues and body fluids sampled: blood
- Time and frequency of sampling: 5, 10, 30, 60 and 180 minutes post-dosing

Statistics:

Descriptive statistics were used, i.e., mean ± standard deviation, where applicable. All descriptive statistic calculations were conducted using Microsoft Excel (Microsoft Corporation, Redmond, Washington) spreadsheets in full precision mode (15 digits of accuracy). Pharmacokinetic parameters that were calculated for blood used the pharmacokinetic computer modeling program PK Plus (v. 9.6, Simulation Plus, Inc., Lancaster, California, United States of America).

Preliminary studies:

The solubility of both IBOAC and IBOA was tested up to 10 mg/mL in saline and Intralipid 20%. Both test materials were insoluble at this concentration in saline but were soluble in Intralipid 20%; thus they were prepared using the latter vehicle .

Key result

Test no.:

#1

Toxicokinetic parameters:

half-life 1st: 1.2494 min-1

Remarks:

relevant for the vast majority of applied test substance

Key result

Test no.:

#1

Toxicokinetic parameters:

other: <1.0% of the corresponding administered dose remains in the blood 10 minutes post-intravenous administration

Test no.:

#1

Toxicokinetic parameters:

AUC: 383.7 µg-min/g

Metabolites identified:

not measured

The resulting blood time course showed that the substance was quickly hydrolyzed with less than 1.0% of the corresponding administered dose remaining in the blood 10 minutes post-intravenous administration and around 0.1% remaining 180 minutes post-intravenous administration. Further pharmacokinetic analysis of the blood concentrations of the substance showed that both compounds were biphasic (alpha [α] and beta [β] phases; i.e. a two-compartment model) in which there is a dominant initial distribution and hydrolysis step (i.e. t_½α) with a subsequent phase of further hydrolysis and elimination (i.e. t_½β). The initial distribution and hydrolysis step had an average calculated t_½αvalue of 1.25 min^-1and accounts for metabolic fate of the vast majority of applied test substance quantities. The secondary hydrolysis and elimination phase (t_½β) had an average value of 120 min^-. The cause of the beta-phase associated with the minor portion of the test substance's kinetics is currently unclear and may be of limited relevance to normal physiology. The average area under the curve (AUC_0-t) value was 384 µg-min/g, respectively.

Conclusions:

Overall, the analysis of the blood pharmacokinetics indicated that the parent test material is rapidly removed from blood circulation (i.e. blood compartment)in vivoin a very similar manner. Hence, these data support a pharmacokinetics-based Read-Across for IBOAC and IBOA.

Executive summary:

This non-GLP pharmacokinetic study of isobornyl acetate (IBOAC) and isobornyl acrylate (IBOA) in rats via intravenous administration evaluated the hydrolysis and pharmacokinetic characteristics as measured by the rate of parent (IBOAC or IBOA) disappearance. The purpose of this study was to evaluate whether these data can be used in a pharmacokinetics-based Read-Across approach similar to several other well-studied (meth)acrylates.

Three male rats per compound were intravenously administered either IBOAC or IBOA at target concentrations 7.0 mg/kg (36 µmol/kg) or 7.5 mg/kg (36 µmol/kg), respectively, using Intralipid 20% as the dose vehicle. After dose administration, blood samples (~200 μL) were collected at 5, 10, 30, 60, and 180 minutes into individual pre-weighed glass vials containing ethyl acetate (600 μL) acidified with 10% trifluoroacetic acid (TFA). After vortexing and subsequent centrifugation, the blood extracts underwent quantitative analysis for either IBOAC or IBOA by gas chromatography tandem mass spectrometry (GC/MS-MS).

The resulting blood time course showed that both substances were quickly hydrolyzed with less than 1.0% of the corresponding administered dose remaining in the blood 10 minutes post-intravenous administration and around 0.1% remaining 180 minutes post-intravenous administration. Further pharmacokinetic analysis of the blood concentrations of both IBOAC and IBOA showed that both compounds were biphasic (alpha [α] and beta [β] phases; i.e. a two-compartment model) in which there is a dominant initial distribution and hydrolysis step (i.e. t_½α) with a subsequent phase of further hydrolysis and elimination (i.e. t_½β). The initial distribution and hydrolysis step had average calculated t_½αvalues of 0.866 and 1.25 min^-1for IBOAC and IBOA, respectively, and accounts for metabolic fate of the vast majority of applied test substance quantities. The secondary hydrolysis and elimination phase (t_½β) had average values of 55.1 and 119.5 min^-1for IBOAC and IBOA, respectively. The cause of the beta-phase associated with the minor portion of IBOAC and IBOMA kinetics is currently unclear and may be of limited relevance to normal physiology. The average area under the curve (AUC_0-t) values for IBOAC and IBOA were 363 and 384 µg-min/g, respectively.

Reference 2

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

GLP compliance:

not specified

Radiolabelling:

yes

Remarks:

Tritium-labelled test substance

Species:

mouse

Strain:

other: dd

Sex:

male

Route of administration:

dermal

Vehicle:

water

Dose / conc.:

3.18 other: µg/mL bath water

Remarks:

almost complete body exposed

Control animals:

no

Conclusions:

Interpretation of results: no bioaccumulation potential based on study results Isobornyl acetate is rapidly absorbed through the skin and undergoes rapid metabolism.

Executive summary:

The dermal resorption of Isobornyl acetate was tested in male dd-mice. The Tritium-marked substance was added to a foam bath fluid (3.18 µg/ml bath water) in order to reflect realistic conditions of taking a bath. The substance was applied to a surface of 3 cm² of the shaved and depilated skin with a fitted cylinder. After 5, 10, 20, 30 and 40 minutes, 0.1 ml blood was tapped from the tail vein, respectively. The maximum concentration of 60 ng Isobornyl acetate/ml was detected after 10 min. Thereafter, a rapid degradation occurred, resulting in 15 ng/ml after 40 min.

Reference 3

Endpoint:

basic toxicokinetics in vivo

Type of information:

experimental study

Adequacy of study:

key study

Study period:

2018

Reliability:

1 (reliable without restriction)

Rationale for reliability incl. deficiencies:

test procedure in accordance with generally accepted scientific standards and described in sufficient detail

Objective of study:

toxicokinetics

Qualifier:

no guideline followed

Principles of method if other than guideline:

Three male rats per compound were intravenously administered IBOAC at target concentration 7.0 mg/kg (36 µmol/kg), using Intralipid 20% as the dose vehicle. After dose administration, blood samples (~200 μL) were collected at 5, 10, 30, 60, and 180 minutes into individual pre-weighed glass vials containing ethyl acetate (600 μL) acidified with 10% trifluoroacetic acid (TFA). After vortexing and subsequent centrifugation, the blood extracts underwent quantitative analysis for IBOAC by gas chromatography tandem mass spectrometry (GC/MS-MS).

GLP compliance:

no

Radiolabelling:

no

Species:

rat

Strain:

Fischer 344/DuCrj

Sex:

male

Details on test animals or test system and environmental conditions:

TEST ANIMALS
- Source: Charles River (Kingston, New York)
- Age at study initiation: 9-12 weeks
- Weight at study initiation: 182-190 g
- Housing: singly in glass Roth-type metabolism cages
- Diet (e.g. ad libitum): LabDiet Certified Rodent Diet #5002 (PMI Nutrition International, St. Louis, Missouri) in pelleted form, ad libitum
- Water (e.g. ad libitum): Municipal water, ad libitum
- Acclimation period: 7d

ENVIRONMENTAL CONDITIONS
- Temperature (°C): 22°C with a range of 20°C-26°C
- Humidity (%): 50% with a range of 30-70%
- Photoperiod (hrs dark / hrs light): 12-hour light/dark (on at 6:00 a.m. and off at 6:00 p.m.)

Route of administration:

intravenous

Vehicle:

other: Intralipid 20%

Details on exposure:

Appropriate amounts of the test substance were added to sterile pharmaceutical grade Intralipid 20% using aseptic technique to obtain the appropriate concentration. The amount of dose solution administered was targeted at 2.5 mL/kg bw and injected over ~1minute to correspond to an injection rate of ~0.5 mL/min. The dose was stored at ambient temperature (19-25°C) and used within 24 hours of preparation.

Duration and frequency of treatment / exposure:

single iv treatment

Dose / conc.:

7 mg/kg bw/day (nominal)

Remarks:

corresponds to 36 µmol/kg

No. of animals per sex per dose / concentration:

3 males

Control animals:

yes, concurrent vehicle

other: another isobornyl ester (isobornyl arylate, IBOA) was tested for its toxikokinetic properties for comparison reasons (rad-across background)

Positive control reference chemical:

no

Details on study design:

- Dose selection rationale: previous studies of this type have used similar equimolar ratios

Details on dosing and sampling:

TOXICOKINETIC / PHARMACOKINETIC STUDY
- Tissues and body fluids sampled: blood
- Time and frequency of sampling: 5, 10, 30, 60 and 180 minutes post-dosing

Statistics:

Descriptive statistics were used, i.e., mean ± standard deviation, where applicable. All descriptive statistic calculations were conducted using Microsoft Excel (Microsoft Corporation, Redmond, Washington) spreadsheets in full precision mode (15 digits of accuracy). Pharmacokinetic parameters that were calculated for blood used the pharmacokinetic computer modeling program PK Plus (v. 9.6, Simulation Plus, Inc., Lancaster, California, United States of America).

Preliminary studies:

The solubility of both IBOAC and IBOA was tested up to 10 mg/mL in saline and Intralipid 20%. Both test materials were insoluble at this concentration in saline but were soluble in Intralipid 20%; thus they were prepared using the latter vehicle .

Key result

Test no.:

#1

Toxicokinetic parameters:

half-life 1st: 0.8661 min-1

Remarks:

relevant for the vast majority of applied test substance

Key result

Test no.:

#1

Toxicokinetic parameters:

other: <1.0% of the corresponding administered dose remains in the blood 10 minutes post-intravenous administration

Test no.:

#1

Toxicokinetic parameters:

AUC: 363.3 µg-min/g

Metabolites identified:

not measured

The resulting blood time course showed that the substance was quickly hydrolyzed with less than 1.0% of the corresponding administered dose remaining in the blood 10 minutes post-intravenous administration and around 0.1% remaining 180 minutes post-intravenous administration. Further pharmacokinetic analysis of the blood concentrations of the substance showed that both compounds were biphasic (alpha [α] and beta [β] phases; i.e. a two-compartment model) in which there is a dominant initial distribution and hydrolysis step (i.e. t_½α) with a subsequent phase of further hydrolysis and elimination (i.e. t_½β). The initial distribution and hydrolysis step had an average calculated t_½αvalue of 0.87 min^-1and accounts for metabolic fate of the vast majority of applied test substance quantities. The secondary hydrolysis and elimination phase (t_½β) had an average value of 55 min^-. The cause of the beta-phase associated with the minor portion of the test substance's kinetics is currently unclear and may be of limited relevance to normal physiology. The average area under the curve (AUC_0-t) value was 363 µg-min/g, respectively.

Conclusions:

Overall, the analysis of the blood pharmacokinetics indicated that the parent test material is rapidly removed from blood circulation (i.e. blood compartment)in vivoin a very similar manner. Hence, these data support a pharmacokinetics-based Read-Across for IBOAC and IBOA.

Executive summary:

This non-GLP pharmacokinetic study of isobornyl acetate (IBOAC) and isobornyl methacrylate (IBOA) in rats via intravenous administration evaluated the hydrolysis and pharmacokinetic characteristics as measured by the rate of parent (IBOAC or IBOA) disappearance. The purpose of this study was to evaluate whether these data can be used in a pharmacokinetics-based Read-Across approach similar to several other well-studied (meth)acrylates.

Three male rats per compound were intravenously administered either IBOAC or IBOA at target concentrations 7.0 mg/kg (36 µmol/kg) or 7.5 mg/kg (36 µmol/kg), respectively, using Intralipid 20% as the dose vehicle. After dose administration, blood samples (~200 μL) were collected at 5, 10, 30, 60, and 180 minutes into individual pre-weighed glass vials containing ethyl acetate (600 μL) acidified with 10% trifluoroacetic acid (TFA). After vortexing and subsequent centrifugation, the blood extracts underwent quantitative analysis for either IBOAC or IBOA by gas chromatography tandem mass spectrometry (GC/MS-MS).

The resulting blood time course showed that both substances were quickly hydrolyzed with less than 1.0% of the corresponding administered dose remaining in the blood 10 minutes post-intravenous administration and around 0.1% remaining 180 minutes post-intravenous administration. Further pharmacokinetic analysis of the blood concentrations of both IBOAC and IBOA showed that both compounds were biphasic (alpha [α] and beta [β] phases; i.e. a two-compartment model) in which there is a dominant initial distribution and hydrolysis step (i.e. t_½α) with a subsequent phase of further hydrolysis and elimination (i.e. t_½β). The initial distribution and hydrolysis step had average calculated t_½αvalues of 0.8661 and 1.2494 min^-1for IBOAC and IBOA, respectively, and accounts for metabolic fate of the vast majority of applied test substance quantities. The secondary hydrolysis and elimination phase (t_½β) had average values of 55.0728 and 119.4828 min^-1for IBOAC and IBOA, respectively. The cause of the beta-phase associated with the minor portion of IBOAC and IBOMA kinetics is currently unclear and may be of limited relevance to normal physiology. The average area under the curve (AUC_0-t) values for IBOAC and IBOA were 363.300 and 383.700 µg-min/g, respectively.

NOTE: Any of data in this dataset are disseminated by the European Union on a right-to-know basis and this is not a publication in the same sense as a book or an article in a journal. The right of ownership in any part of this information is reserved by the data owner(s). The use of this information for any other, e.g. commercial purpose is strictly reserved to the data owners and those persons or legal entities having paid the respective access fee for the intended purpose.

Reference 4

Endpoint:

basic toxicokinetics in vivo

Type of information:

experimental study

Adequacy of study:

key study

Study period:

1988

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

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

Principles of method if other than guideline:

Radiolabelled n-butyl acrylate was administered to rats. Radioactivity was determined in exhaled carbon dioxide, urine, feces and different tissue samples. Identification of metabolites was performed in urine, bile and kidney extracts.

GLP compliance:

not specified

Specific details on test material used for the study:

- Physical state: liquid

Radiolabelling:

yes

Species:

rat

Strain:

Fischer 344

Sex:

male

Details on test animals or test system and environmental conditions:

TEST ANIMALS
- Source:
- Age at study initiation: 8 - 10 weeks
- Weight at study initiation: 180 - 220 g
- Fasting period before study:
- Housing:
- Individual metabolism cages: yes/no
- Diet: NIH 31 rat chow
- Water: ad libitum

Route of administration:

oral: gavage

Vehicle:

corn oil

Duration and frequency of treatment / exposure:

single dose

Dose / conc.:

4 mg/kg bw/day

Dose / conc.:

40 mg/kg bw/day

Dose / conc.:

400 mg/kg bw/day

No. of animals per sex per dose / concentration:

3

Control animals:

no

Details on study design:

Radio-labelled butyl acrylate was diluted with unlabeled butyl acrylate to achieve 20 µCi/kg bw for 40 and 400-mg/kg bw doses. Undiluted radio-labelled BA was administered at 7µCi/kg bw for 4-mg/kg bw dose. At sacrifice, animals were dissected and tissues were removed, and weighed. Tissues were stored at -20°C. Excretion of radioactive metabolites was determined by collection of urine and feces.
Collection of carbon dioxide and other exhalation products was performed by placing the animals in glass metabolism cages after administration. The percentage of the total dose of carbon-14 labelled butyl acrylate eliminated as carbon-14 carbon dioxide and other volatile substances was determined by counting triplicate 1-ml aliquots of each trapping solution.

Details on dosing and sampling:

Urine, bile and 0 - 15 min kidney extract samples
- Method type(s) for identification: HPLC, UV, NMR, FAB-MS

Details on absorption:

Butyl acrylate was rapidly absorbed and metabolised after oral administration. The acrylate moiety was metabolised primarily to carbon dioxide, accounting for elimination of approximately 75 % of the administered radiolabelled material.

Details on distribution in tissues:

The distribution of butyl acrylate derived radioactivity in the major tissues examined was mostly independent on the dose applied. An exception was the adipose tissue.
Most of the radioactivity was found in the adipose tissue.

Details on excretion:

Exhaled carbon dioxide was up to 75 % of the administered radiolabelled substance. Excretions in urine and feces were about 10 % and 2 % of the dose, respectively.

Metabolites identified:

yes

Details on metabolites:

N-acetyl-S-(2-carboxyethyl)cysteine and N-acetyl-S-(2-carboxyethyl)cysteine-S-oxide were the major metabolites identified in urine.

Distribution and excretion of butyl acrylate derived radioactivity 24 h after oral administration in rats

Fraction / Tissue	Percentage of total dose
	Dose: 4 mg/kg	Dose: 40 mg/kg	Dose: 400 mg/kg
Blood	1.9 ± 0.2*	2.0 ± 0.1	2.0 ± 0.1
Liver	1.9 ± 0.2	2.6 ± 0.4	2.3 ± 0.1
Kidney	0.3 ± 0.0	0.3 ± 0.0	0.4 ± 0.0
Skin	3.4 ± 0.3	2.9 ± 0.3	3.2 ± 0.5
Adipose	1.6 ± 0.3	8.6 ± 4.7	5.7 ± 0.9
Muscle	5.9± 0.3	5.4 ± 1.0	5.7 ± 0.3
Urine	12.6 ± 2.3	7.7 ± 1.0	7.6 ± 0.5
Feces	2.2 ± 0.7	2.1 ± 0.3	2.1 ± 0.6
Carbon dioxide	74.2 ± 7.6	65.5 ± 4.6	78.0 ± 2.7
Volatiles	1.6 ± 1.3	0.8 ± 0.5	1.7 ± 1.5

*Mean ± Standard Deviation from three animals

Conclusions:

no bioaccumulation potential based on study results
Butyl acrylate was rapidly absorbed and metabolised in rats after oral administration. It was mainly degraded to carbon dioxide. About 10 % of the substance was excreted in urine, 2 % in feces. In case of saturation of metabolic pathways, there is evidence for acrylic acid occurrence in urine indicating that ester hydrolysis is not the limiting step.

Executive summary:

Radiolabelled butyl acrylate was administered orally to rats in doses of 4, 40 and 400 mg/kg bw.

The substance was rapidly absorbed and metabolised. The main fraction was exhaled as carbon dioxide (about 75%). About 10 % was found in urine, 2 % in feces. Most of the substance was hydrolysed to acrylic acid, which was further metabolised to compounds available for oxidation. A smaller portion formed glutathione conjugates. In the high dose group, there is evidence for the presence of acrylic acid in the urine indicating that the first metabolic step, the carboxyl esterase catalysed ester hydrolysis, is not the limiting step in case of saturation of metabolic pathways.

Description of key information

Experimentally proven rapid hydrolysis of IBOA to the acryic metabolite acylic acid/AA and alcohol metabolite isoborneol/ IBO so that systemic effects of IBOA can be asessessed with the help of these metabolites and the respective metabolite donor substances (butyl acrylate/BA and isobornyl acetate/ IBOAC). No bioaccumulation potential of IBOA.

AA is further metabolised in a secondary pathway for propionic acid metabolism, finally down to to endogenic acetate/ acetyl-CoA and respirable CO2. IBO is further metabolised to Camphor/CAM which is further oxidized, conjugated and eliminated in the urine.

Key value for chemical safety assessment

Bioaccumulation potential:

no bioaccumulation potential

Additional information

Ester hydrolysis by carboxylesterases has been established as the primary step in the metabolism of esters of simple carboxyl acids like acryl acid. 

For IBOA, this hydrolytic step leads to the rapid formation of AA and IBO. Thus, systemic effects of IBOA can also be assessed by read acoss to these metabolites and their respective donor substances (i.e. substances, which were rapidly metabolised to these metabolites) with a high level of confidence and according to ECHA's RAAF guidance (2017). This read across is justified in detail in the attached read across justification.

Primary metabolism of Isobornyl Acrylate

A comparative, non-GLP toxicokinetic study of both IBOAC (as metabolite donor substance for IBO) and IBOA in rats via intravenous administration is available which evaluated the hydrolysis rate of both IBOAC and IBOA in vivo (DOW, 2019).

Three male rats per compound were intravenously administered either IBOAC or IBOA at 7.0 mg/kg (36 µmol/kg) or 7.5 mg/kg (36 µmol/kg), respectively, using Intralipid 20% as the dose vehicle. After dose administration, blood samples (~200 μl) were collected at 5, 10, 30, 60, and 180 minutes into individual pre-weighed glass vials containing ethyl acetate (600 μL) acidified with 10% trifluoroacetic acid. After vortexing and subsequent centrifugation, the blood extracts underwent quantitative analysis for either IBOAC or IBOA by gas chromatography tandem mass spectrometry (GC/MS-MS).

 

The resulting blood time course showed that both substances were quickly hydrolyzed with less than 1.0% of the corresponding administered dose remaining in the blood 10 minutes post-intravenous administration and around 0.1% remaining 180 minutes post-intravenous administration. Further pharmacokinetic analysis of the blood concentrations of both IBOAC and IBOA showed that both compounds were biphasic (alpha [α] and beta [β] phases; i.e. a two-compartment model) in which there is a dominant initial distribution and hydrolysis step (i.e. t_½α) with a subsequent phase of further hydrolysis and elimination (i.e. t_½β). The initial distribution and hydrolysis step had average calculated t_½αvalues of 0.866 and 1.25 min^-1for IBOAC and IBOA, respectively, and accounts for metabolic fate of the vast majority of applied test substance quantities. The secondary hydrolysis and elimination phase (t_½β) had average values of 55.1 and 119 min^-1for IBOAC and IBOA, respectively. The cause of the beta-phase associated with the minor portion of IBOAC and IBOMA kinetics is currently unclear and may be of limited relevance to normal physiology. The average area under the curve (AUC_0-t) values for IBOAC and IBOA were 363 and 384 µg-min/g, respectively.

Overall, the analysis of the blood pharmacokinetics indicated that the parent test material is rapidly removed from blood circulation (i.e. blood compartment)in vivoin a very similar manner. Hence, these data support a pharmacokinetics-based Read-Across for IBOAC and IBOA.

 

Subsequent metabolism of Acrylic Acid and Isoborneol within the body

Acrylic Acid (AA)

The available data are consistent with the incorporation of AA into a secondary pathway for propionic acid metabolism in which 3 -hydroxypropionate is an intermediate. In this pathway, AA is first converted to acrylyl-CoA which is subsequently oxidized to 3 -hydroxypropionate. 3 -Hydroxypropionate is, in turn, metabolized to acetate/ acetyl-CoA and CO2 via malonic semialdehyde. The resultant acetate is then incorporated into intermediary metabolism. This pathway has been reported to be a major pathway for the metabolism of propionic acid in various insect and plant species, but is a secondary pathway in mammals.

From a larger set of data, following publications were selected for demonstration reasons.

Black and coworkers (1995) performed metabolic fate studies in rats and mice after oral (and dermal) application of radiolabeled AA.

Here, C3H mice and Fischer 344 rats, respectively, were treated by gavage (40 or 150 mg/kg bw) with [14C]-AA. Mice rapidly absorbed and metabolised orally administered acrylic acid, with about 80% of the dose exhaled as 14CO2 within 24 h. Excretion in urine and faeces accounted for approximately 3% and 1% of the dose, respectively. The disposition of orally administered AA in rats was comparable to the results obtained from mice with slightly higher portions of exhaled AA High-performance liquid chromatography (HPLC) analysis of rat urine and rat and mouse tissues indicated that absorbed AA was rapidly metabolized by the ß-oxidation pathway of propionate catabolism. No unchanged AA was detected 1 h after oral administration; however, several metabolites that were more polar than AA were measured, including 3-hydroxypropionate. Neither AA nor its metabolites were detected at later times after oral administration.

In rats of the Sprague-Dawley strain, these results were confirmed by De Bethizy and coworkers (1987). The HPLC profile of metabolites observed in the urine of rats in this study indicated two major metabolites. One of the major metabolites co-eluted with 3-hydroxypropionic acid. Radioactivity could not be detected at the retention times corresponding to that of 2,3-epoxypropionic acid or N-acetyl-S-(2-carboxy-2-hydroxyethyl)cysteine leading to the conclusion that AA is not epoxidized to 2,3-epoxypropionic acid in vivo. This result was supported by an in vitro study. Hepatic microsomes were prepared using conventional methods from rats and incubations were started by the addition of 10 µL of [14C]-acrylic acid. No epoxidized metabolites could be detected and the parent compound was recovered from the incubation mixture unchanged (DeBethizy et al., 1987).

The metabolite donor substance for AA, BA was also studied for its metabolic fate. In accordance to the data of AA shown above, BA was rapidly absorbed and metabolised. The main fraction was exhaled as carbon dioxide (about 75%). About 10 % was found in urine, 2 % in feces. Most of the substance was hydrolysed to acrylic acid, which was further metabolised to compounds available for oxidation. A smaller portion formed glutathione conjugates. In the high dose group, there is evidence for the presence of acrylic acid in the urine indicating that the first metabolic step, the carboxyl esterase catalysed ester hydrolysis, is not the limiting step in case of saturation of metabolic pathways (Sanders et al., 1988).

 

Isoborneol (IBO)

In a pharmacokinetic study with 6 Wistar rats, orally administered IBO in a dose of 90 mg/kg bw was absorbed rapidly (t_max0.27 h after dosing; t_1/21.33 h). In accordance to the results of other mentioned studies in the respective publication, CAM was identified as main metabolite and showed very similar kinetic properties (t_max0.28 h after dosing; t_1/21.01 h; Sun et al. 2014).

Orientating information on the further metabolism of CAM as metabolite of IBO is available: CAM is rapidly oxidized and then conjugated in the liver to the glucuronide form. Campherol conjugated to glucuronic acid is then eliminated mainly in the urine as an inactive compound (U.S. EPA, 2006).

.