1,1-difluoroethane

Basic toxicokinetics

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: This study was selected as the key study because the information provided for the hazard endpoint is sufficient for the purpose of classification and labelling and/or risk assessment.

Data source

Reference

Reference Type:

study report

Title:

Unnamed

Year:

1995

Report date:

1995

Materials and methods

Objective of study:

metabolism

Principles of method if other than guideline:

Rats were exposed to the test material via inhalation. Partition coefficients for the test substance between air and blood, liver, fat, and muscle were measured. The data for rate of chemical loss from the chamber and the partition coefficients were used with a physiologically-based pharmacokinetic model (PBPK) to estimate the kinetic constants for metabolism in the whole rat. Urine was sampled and tested for metabolites.

GLP compliance:

not specified

Test material

Test material information

Constituent 1

Reference substance name:

1,1-difluoroethane

EC Number:

200-866-1

EC Name:

1,1-difluoroethane

Cas Number:

75-37-6

Molecular formula:

C2H4F2

IUPAC Name:

1,1-difluoroethane

Details on test material:

- Purity: 100%

Radiolabelling:

no

Test animals

Species:

rat

Strain:

other: Crl:CD®BR

Sex:

male

Details on test animals or test system and environmental conditions:

TEST ANIMALS
- Age at study initiation: Not reported
- Weight at study initiation: Approximately 250-300 g
- Fasting period before study: No
- Housing: Singly in wire-mesh cages suspended above absorbent cageboard
- Individual metabolism cages: No
- Diet (e.g. ad libitum): ad libitum, except during exposure
- Water (e.g. ad libitum): ad libitum, except during exposure
- Acclimation period: 1 week

ENVIRONMENTAL CONDITIONS
- Temperature (°C): Not reported
- Humidity (%): Not reported
- Air changes (per hr): Not reported
- Photoperiod (hrs dark / hrs light): Not reported

Administration / exposure

Route of administration:

inhalation

Vehicle:

not specified

Details on exposure:

TYPE OF INHALATION EXPOSURE: whole body

GENERATION OF TEST ATMOSPHERE / CHAMPER DESCRIPTION
- Exposure apparatus: 13 L chamber
- Method of holding animals in test chamber: Not reported
- Source and rate of air: Not reported
- Method of conditioning air: Not reported
- System of generating particulates/aerosols: The flow rate of the atmosphere through the chamber was approximately 2 L/min, with approximately 5% of the total flow being directed through gas chromatograph (GC). The concentrations of test compound in the exposure chamber were monitored every 10 min by GC. CO2 was removed from the chamber with a soda lime trap. Oxygen concentrations in the chamber were also monitored and regulated to 21±2%.
- Treatment of exhaust air: Not reported

Partition Coefficient Study: For the tissue partition coefficient study, a vial equilibration method was used by injecting the test compound into the headspace above blood or a tissue homogenate and then determining the amount of chemical in the headspace of the vial by GC after an equilibration time.

Duration and frequency of treatment / exposure:

4-5 hours

Doses / concentrations

Remarks:

Doses / Concentrations:
Metabolism (in vivo): Approximately 370, 470, 1500, and 2650 ppm
Tissue Solubility: 750 ppm (starting concentration)

No. of animals per sex per dose / concentration:

Metabolism (in vivo): 3 male rats per concentration

Control animals:

no

Details on study design:

Tissue Solubility: Tissue solubility was measured by determining tissue:air partition coefficients for blood, liver, fat, and muscle from male rats. A vial equilibration method was used by injecting the test substance into the headspace above blood or a tissue homogenate and then determining the amount of chemical in the headspace of the vial by gas chromatography after an equilibration time. The GC was equipped with a flame ionization detector. The sample volume was 1.0 mL and the oven was held at 40°C. The injector and detector temperatures were 150°C and 225°C, respectively. The partition coefficient was calculated from the difference in the measured headspace concentration compared to the concentration in a reference vial with no blood or tissue.

Rate of Metabolism: Metabolism was measured by gas uptake methods. The rate of decline in the test substance chamber was used, along with the experimentally derived partition coefficients, to estimate the kinetic constants for metabolism in the rat according to a PBPK model. The kinetic constants were estimated by visual fitting of the model to the data. Then, with the visually fitted constants as a starting point, an optimization of the kinetic constants was performed by SimuSolv®. The optimization was performed by varying the Km and Vmax (with no constraints) and fitting the model to the concentration of the test material in the chamber. The optimization method was a Nelder-Mead search with a weighting parameter of 2.0. The search was terminated when the standard deviation of the log likelihood function was less than 0.00001.

Identification of Metabolites: Urine was collected from the bottom of the exposure chamber after the end of the exposure and stored at -20°C until analysis. Since difluoroacetic acid was postulated as a potential metabolite, the urine collected during exposure was analyzed for fluorinated compounds by 19F NMR. The probe was 5 mm, the pulse width 4 µsec, and the recycle time was 1.33 sec. Spectra were acquired at room temperature without proton decoupling. The reference spectrum was for CFCl3 at 0 ppm. The atmosphere of the inhalation chamber was monitored for the appearance of new peaks during the exposure, which would be an indication of the production of volatile metabolites.

Results and discussion

Toxicokinetic / pharmacokinetic studies

Details on distribution in tissues:

The test substance is only slightly soluble in blood and tissue. The partition coefficients were 1.69 for blood:air, 1.69 for liver:air, 5.63 for fat:air, and 1.47 for muscle:air. The low partition coefficients indicate that only a small portion of an inhaled concentration would be absorbed and that little accumulation would be expected in tissues, mostly in fat.

Toxicokinetic parameters

Toxicokinetic parameters 1

Toxicokinetic parameters:

other: Vmax = 7.8 ± 0.9 mg/hr/kg

Toxicokinetic parameters 2

Toxicokinetic parameters:

other: Km = 27.9 ± 3.7 mg/L

Metabolite characterisation studies

Metabolites identified:

yes

Details on metabolites:

Analysis of gas uptake data with a PBPK model showed that there was measurable metabolism of the test substance occurring in the rat. The best fit to the PBPK model was achieved by Michaelis-Menten kinetics. The concentration where metabolism was saturated was approximately 75000 ppm. NMR data showed only one fluoride ion and a trace of acyl fluoride in the urine of exposed rats. There were no fluoroacetate or difluoroacetate detected, which indicated that metabolism of the test substance proceeds through an oxidative route starting with the carbon-hydrogen bond on the -CHF2 carbon. The likely end-product of metabolism was acetic acid.

Applicant's summary and conclusion

Conclusions:

Interpretation of results (migrated information): other: metabolized slowly
The study and the conclusions which are drawn from it fulfil the quality criteria (validity, reliability, repeatability). The test substance is metabolized slowly in the rat, most likely through an oxidative route to acetic acid.

Executive summary:

Groups of male rats were exposed to the test substance by inhalation at starting concentrations of approximately 370, 470, 1500, and 2650 ppm in a closed, recirculating chamber. The duration of exposure was 4-5 hours. The chamber concentrations were monitored by gas chromatography every 10 minutes. Partition coefficients for the test substance between air and blood, liver, fat, and muscle were measured. The data for rate of chemical loss from the chamber and the partition coefficients were used with a physiologically-based pharmacokinetic model (PBPK) to estimate the kinetic constants for metabolism in the whole rat. Samples of urine were taken from the exposure chamber after the end of exposure an analyzed for metabolites by¹⁹F NMR.

 

The partition coefficients for the test substance were 1.69 for blood:air, 1.69 for liver:air, 5.63 for fat:air, and 1.47 for muscle:air. Analysis of the gas uptake data with a PBPK model showed that there was measurable metabolism of the test substance occurring in the rat. The estimated Vmax and Km were 7.8±0.9 mg/hr/kg and 27.9±3.7 mg/L, respectively. The best fit to the model was achieved by Michaelis-Menten kinetics. The concentration where metabolism was saturated was approximately 75000 ppm. ¹⁹F NMR showed fluoride ion and traces of an acyl fluoride in the urine. This indicates that the test substance was predominantly metabolized at the –CHF₂carbon. The likely end-product of test substance metabolism was acetic acid. The results of this study show that the test substance is metabolized slowly in the rat, most likely through an oxidative route to acetic acid.