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Toxicological information

Basic toxicokinetics

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Administrative data

basic toxicokinetics in vivo
Type of information:
experimental study
Adequacy of study:
key study
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
other: Non-GLP, non guideline study, published in peer reviewed literature, no restrictions, fully adequate for assessment
Cross-referenceopen allclose all
Reason / purpose for cross-reference:
reference to same study
Reason / purpose for cross-reference:
reference to other study

Data source

Reference Type:
Disposition of inhaled isobutene in F344/N rats.
Henderson R, Sabourin P, Bechtold W, Steinberg B and Chang I
Bibliographic source:
Toxicol. Appl. Pharmacol. 123:50-61

Materials and methods

Objective of study:
Principles of method if other than guideline:
The purpose of this study was to determine the effect of exposure concentration on the uptake and metabolic disposition of inhaled isobutene.
GLP compliance:
not specified

Test material

Constituent 1
Reference substance name:
Constituent 2
Chemical structure
Reference substance name:
EC Number:
EC Name:
Cas Number:
Molecular formula:
Details on test material:
Isobutene (99 + %) was obtained from Aldrich Chemical Co. (Milwaukee, WI, USA). [2-14C]isobutene (radiochemical purity of 98.5%, 2.7 mCi/mmol Lot No. 2484) was obtained from DuPont NEN Research Products (Boston, MA, USA). The purity of the [14C]isobutene was determined before use by HPLC

Test animals

Fischer 344
Details on test animals or test system and environmental conditions:
Male F344/N rats (11-15 weeks of age) from the barrier-maintained colony of the Inhalation Toxicology Research Institute (ITRI) were used. The rats were housed in polycarbonate cages (two per cage) containing hardwood chip bedding and filter caps prior to the exposures. Rooms were on a 12-hr light-dark cycle with light starting at 6:00 AM. Food (Certified Lab Blox, Allied Mills, Chicago, IL, USA) and water were provided ad libitum.
Rats were preconditioned to the nose-only exposure tubes prior to start of the exposure. The rats were placed in the tubes for I hr and 3 hr, 2 days and I day prior to exposure, respectively. All rats were killed by ip injection of 250 mg sodium pentobarbital/kg body wt.

Administration / exposure

Route of administration:
inhalation: gas
other: air
Details on exposure:

- Exposure apparatus: nose only inhalation chambers

- Source and rate of air: filtered room air
Duration and frequency of treatment / exposure:
2 hours
Doses / concentrations
Doses / Concentrations:
Target concentrations were 2, 40, 400, and 4000 ppm, ( 3.1, 75, 747, and 7474 mg/m3)
No. of animals per sex per dose / concentration:
Unlabelled isobutene: 3 rats per timepoint
Labelled isobutene: 5 rats per concentration, 3 rats per timepoint
Control animals:
Positive control reference chemical:
Details on study design:
Studies with unlabeled Isobutene:

Blood levels of isobutene and volatile metabolites. Male F344/N rats (11- 15 weeks of age) were exposed for 2 hr to 40, 400, or 4000 ppm isobutene in a nose-only exposure system. Blood levels of isobutene and volatile metabolites were determined by headspace analysis at 0.25, 0.5, 1.0, 2.0,2.25,2.75, 3.75, and 6.75 hr after initiation of the exposure.

Studies with [14C]Isobutene

Total uptake, excretion, and metabolism studies.
Male F344/N rats (11-15 weeks of age) were exposed to 40,400, or 4000 ppm [14C] isobutene for 6 hr in a nose-only exposure system. Five rats were exposed in plethysmograph tubes that allowed measurement of breathing patterns and the total amount of [14C]isobutene inhaled. These animals were killed and counted for 14C at the end of the exposure to determine total retained isobutene and the fraction of inhaled isobutene that was retained. Four exposed animals were placed in glass metabolism cages for the collection of excreta (urine, faeces, exhaled air) for 3 days after the exposure. After 3 days the amount of 14C in the excreta plus that remaining in the carcass were summed to determine if a mass balance was achieved with the original total body burdens determined in the rats killed immediately after the exposure.

In rats exposed to 40 ppm [14C]isobutene, blood samples were collected in separate rats (three per group) at 0.5, 1,2,4,6,6.5,7.5,9.5, 15, and 24 hr after initiation of the exposure. Blood and excreta samples were analyzed for metabolites..
Because results indicated nonlinear uptake and metabolism of isobutene over the exposure range of 40-4000 ppm [14C]isobutene for 6 hr, an additional group of rats was exposed to 2 ppm isobutene
for approximately 3 hr. The purpose of this exposure was to define a linear range of exposures for uptake and metabolism of isobutene.
Details on dosing and sampling:
see above

Results and discussion

Preliminary studies:

Toxicokinetic / pharmacokinetic studies

Details on absorption:
The blood concentrations of isobutene at the end of the exposures (at equilibrium) were 7.7, 90.4, and 1552 ng/mL blood for the 39, 434, and 3790 ppm exposures, respectively. The ratio of blood concentrations (in ng/mL) to exposure concentrations (in ppm) was 0.20, 0.21, and 0.41 for
the three exposure levels, indicating that the blood concentration of isobutene increased more than proportionately at the highest exposure concentration. This indicates that the capacity of the rats to metabolise isobutene was exceeded at the higher exposure concentrations resulting in higher blood levels.
Details on excretion:
The percentages of the initial body burden excreted by different routes or retained in the body did not differ statistically for the three lower exposure levels, but the highest exposure level resulted in a greater fraction of the isobutene being exhaled. This demonstrated that at 4000 ppm, the ability of the rats to metabolise isobutane is exceeded. This results in a higher percentage of isobutene being excreted in exhaled air.
Toxicokinetic parameters
Toxicokinetic parameters:
other: not determined

Metabolite characterisation studies

Metabolites identified:
Details on metabolites:
Two urinary metabolites were identified as isobutenediol and 2-hydroxyisobutyric acid. Two other urinary metabolites were tentatively identified as sulfate conjugates of isobutenediol.

Any other information on results incl. tables

Fate of [14C]Isobutene at End of Exposure (as % of Total Body Burden mean ± SE, n = 4) 


Exposure concentration of isobutene

2.12 ppm

44 ppm

417 ppm

3930 ppm


80.0 ± 0.6

88.4± 0.3

82.4± 0.9

62.4± 2.8


0.87 ± 0.29

1.33± 0.14

1.81± 0.14

0.64± 0.16

Exhaled isobutene

7.6 ± 0.2

4.6± 0.1

5.1± 0.2

13.7± 0.9


6.4 ± 0.5

1.8± 0.2

7.2± 0.4

20.7± 2.3


5.1 ± 0.2

3.9± 0.4

4.6± 0.6

2.5± 0.9

Applicant's summary and conclusion

Interpretation of results (migrated information): low bioaccumulation potential based on study results
Saturation of the metabolism of isobutene (2-methylpropene) occurred in rats. Oxidised metabolites were excreted in the urine.
Executive summary:

 Male F344/N rats were exposed to isobutene (2-methylpropene) by inhalation at concentrations from 40 to 4000 ppm (91.8 to 918 mg/m3). A time-course evaluation of blood levels of isobutene was performed and urinary metabolites were determined. Blood levels of isobutene were linearly related to exposure up to 400ppm (918 mg/m3) but were supralinear at 4000 ppm (9180 mg/m3) indicating saturation of metabolism at this higher dose level. Absorption of inhaled isobutene was about 8% up to 40ppm but decreased at higher concentrations. At 40 ppm, over 90% of absorbed isobutene was metabolised but at 4000 ppm 20% was excreted as unchanged isobutene also indicating saturation of metabolism. Two urinary metabolites were identified as isobutenediol and 2-hydroxyisobutyric acid. Two other urinary metabolites were tentatively identified as sulfate conjugates of isobutenediol.