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Toxicological information

Basic toxicokinetics

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Administrative data

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:
other: Investigators report, produced as a part of Health Effects Institue Research Report 102; well-conducted and documented study, adequate for assessment.

Data source

Reference
Reference Type:
publication
Title:
Unnamed
Year:
2001

Materials and methods

Objective of study:
metabolism
Principles of method if other than guideline:
4 hours exposure of rats to 2 concentrations of MTBE vapour, followed by quantification of MTBE and its metabolites in blood and urine.
GLP compliance:
not specified

Test material

Reference
Name:
Unnamed
Type:
Constituent
Details on test material:
- Name of test material (as cited in study report): MTBE
- Analytical purity: > 99.8%
- Supplier: Aldrich Chemical Company (Deisenhofen, Germany).
Radiolabelling:
no

Test animals

Species:
rat
Strain:
Fischer 344
Sex:
male/female
Details on test animals and environmental conditions:
TEST ANIMALS
- Source: Harlan Winkelmann, Borchen, Germany
- Age at study initiation: 12 weeks
- Weight at study initiation: 210-240 g (males); 190-220 g (females)
- Fasting period before study:
- Housing: during the exposure: separate Macrolon cages; after the exposure metabolic cages
- Diet: ad libitum
- Water: ad libitum
- Acclimation period: 3 days


ENVIRONMENTAL CONDITIONS
- Temperature (°C): 21
- Photoperiod (hrs dark / hrs light): 12/12

Administration / exposure

Route of administration:
inhalation: vapour
Vehicle:
unchanged (no vehicle)
Details on exposure:
TYPE OF INHALATION EXPOSURE: whole body

GENERATION OF TEST ATMOSPHERE / CHAMPER DESCRIPTION
- Exposure apparatus: dynamic exposure chamber, total volume 8 m3
- Method of holding animals in test chamber: caged
- Rate of air: 28 m3/hour
- Temperature: 22 °C
- humidity: 50-60%
Duration and frequency of treatment / exposure:
4 hours
Doses / concentrations
Remarks:
Doses / Concentrations:
4 and 40 ppm
No. of animals per sex per dose:
5/sex/dose
Control animals:
no
Details on dosing and sampling:
PHARMACOKINETIC STUDY (Absorption, distribution, excretion)
- Tissues and body fluids sampled : urine, blood
- Time and frequency of sampling: urine was collected at 4 °C for 72 h at 6-h intervals. Blood samples were taken at the end of the exposure.
- Other:


METABOLITE CHARACTERISATION STUDIES
- Tissues and body fluids sampled: urine, blood
- Method type(s) for identification: GC-MS
Statistics:
Statistical analysis were performed using a Student t test. To determine possible sex differences, all data from the male and female animals were compared using a t test. P values of < 0.05 were considered significant. Half-times were calculated using exponential regression in Excel. The curve-fitting function of the program was used, and curves were stripped based on correlation coefficients. All correlation values (r2) > 0.95 were considered for separation.

Results and discussion

Toxicokinetic / pharmacokinetic studies

Details on absorption:
The concentrations of MTBE in rat blood immediately after the end of exposure to 4 and 40 ppm MTBE were 2.3 ± 1.0 and 5.9 ± 1.8 μM, respectively.
Besides MTBE, tert-butyl alcohol (TBA) was also detected in blood of the rats after the end of exposure at concentrations 2.9 ± 0.5 and 36.7 ± 10.8 μM, respectively.
TBA was also detected in low concentrations in blood samples taken from control rats; however, blood samples taken from rats after exposure to 4 ppm showed statistically significant increases in TBA concentrations at the end of exposure.
Details on distribution in tissues:
No data.
Details on excretion:
MTBE was cleared from blood with a half-time of 30 minutes. No sex differences in the apparent elimination half-times were found.
In rat urine, the ether concentrations were already below the limit of detection in the first available samples (6 h after the end of exposure). The rate of excretion of MTBE metabolites in rat urine was slower compared to exhaled breath. All MTBE metabolites were eliminated with apparent half-times of elimination of less than 5 h.
Toxicokinetic parameters
Test no.:
#1
Toxicokinetic parameters:
half-life 1st: 0.5 ± 0.2 h

Metabolite characterisation studies

Metabolites identified:
yes
Details on metabolites:
TBA, 2-methyl-1,2-propanediol, and 2-hydroxyisobutyrate are MTBE metabolites excreted in urine. Based on the confirmed structures of the metabolites, MTBE biotransformation proceeds by oxidation of the methyl group in MTBE to give an intermediate hemiacetal, which decomposes with release of formaldehyde to TBA.

Any other information on results incl. tables

Formation of the other MTBE metabolites involves further biotransformation of TBA formed in the first step of metabolic pathway. Conjugation of TBA with activated glucuronic acid results in excretion of the glucuronide conjugate. The two other metabolites (2-methyl-1,2-propanediol and 2-hydroxyisobutyrate) present in urine of TBA-treated animals and also in urine of rats exposed to MTBE suggest further oxidative metabolism of the intermediate metabolite TBA. The likely pathway for formation of these metabolites involves oxidation of TBA by CYP to give 2-methyl-1,2-propanediol. TBA is not a substrate for alcohol dehydrogenase, but it is oxidized by rat liver microsomes to formaldehyde and acetone under conditions consistent with an involvement of CYP (Cederbaum and Cohen 1980, Cederbaum et al 1983). CYPmediated oxidation of a C–H bond in one of the methyl groups of TBA results in excretion of the diol metabolite. Further oxidation of 2-methyl-1,2-propanediol results in 2-hydroxyisobutyrate, which is excreted as a major metabolite of TBA, as well as of MTBE.

Applicant's summary and conclusion