Registration Dossier

Administrative data

Endpoint:
in vivo mammalian somatic cell study: cytogenicity / erythrocyte micronucleus
Remarks:
Type of genotoxicity: chromosome aberration
Type of information:
experimental study
Adequacy of study:
key study
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
other: Study design comparable to OECD Guideline 474 (exposure for 3 months; see also section 7.5.3)

Data source

Reference
Reference Type:
publication
Title:
Unnamed
Year:
2007

Materials and methods

Test guideline
Qualifier:
equivalent or similar to
Guideline:
OECD Guideline 474 (Mammalian Erythrocyte Micronucleus Test)
Deviations:
not applicable
Principles of method if other than guideline:
At the end of a 3-month toxicity study with alpha-methylstyrene (see section 7.5.3; male and female B6C3F1 mice were exposed via inhalation to concentrations of 0, 75, 150, 300, 600 or 1000 ppm on 6 hours plus T90 (12 minutes) per day on 5 days/week over 14 weeks), peripheral blood samples were obtained from male and female mice. Smears were immediately prepared and fixed in absolute methanol. The methanol-fixed slides were stained with acridine orange and coded. Slides were scanned to determine the frequency of micronuclei in 1000 normochromatic erythrocytes (NCEs) in each of up to 10 animals per exposure group and in 1000 polychromatic erythrocytes (PCEs) in up to 10 chamber control and 1000 ppm mice. PCEs in a population of 1000 erythrocytes was determined as a measure of bone marrow toxicity. The results were tabulated as the mean of the pooled results from all animals within a treatment group, plus or minus the standard error of the mean.
GLP compliance:
not specified
Type of assay:
micronucleus assay

Test material

Reference
Name:
Unnamed
Type:
Constituent
Details on test material:
Lot/batch No.: BNW 13871-4
Purity: 99.5 %

Test animals

Species:
mouse
Strain:
B6C3F1
Sex:
male/female
Details on test animals and environmental conditions:
see section 7.5.3

Administration / exposure

Route of administration:
inhalation
Vehicle:
clean air
Details on exposure:
see section 7.5.3
Duration of treatment / exposure:
14 weeks
Frequency of treatment:
6 hours plus T90 (12 minutes) per day on 5 days/week
Post exposure period:
No
Doses / concentrations
Remarks:
Doses / Concentrations:
0, 75, 150, 300, 600 or 1000 ppm
Basis:
nominal conc.
No. of animals per sex per dose:
10 m / 10 f
Control animals:
yes, concurrent vehicle

Examinations

Details of tissue and slide preparation:
Smears were immediately prepared and fixed in absolute methanol. The methanol-fixed slides were stained with acridine orange and coded. Slides were scanned to determine the frequency of micronuclei in 1000 normochromatic erythrocytes (NCEs) in each of up to 10 animals per exposure group and in 1000 polychromatic erythrocytes (PCEs) in up to 10 chamber control and 1000 ppm mice. PCEs in a population of 1000 erythrocytes was determined as a measure of bone marrow toxicity. The results were tabulated as the mean of the pooled results from all animals within a treatment group, plus or minus the standard error of the mean.
Statistics:
The frequency of micronucleated cells among PCEs or NCEs was analyzed by a statistical software package that tested for increasing trend over exposure groups using a one-tailed Cochran-Armitage trend test, followed by pairwise comparisons between each exposure group and the control group. In the presence of excess binomial variation, as detected by a binomial dispersion test, the binomial variance of the Cochran-Armitage test was adjusted upward in proportion to the excess variation. In the micronucleus test, an individual trial was considered positive if the trend test P value was <= 0.025 or if the P value for any single exposed group was <= 0.025/N where N equals the number of exposed groups. A final call of positive for micronucleus induction is preferably based on reproducibly positive trials. Ultimately, the final call was determined by the scientific staff after considering the results of statistical analyses, reproducibility of any effects observed, and the magnitudes of those effects.

Results and discussion

Test resultsopen allclose all
Sex:
male
Genotoxicity:
negative
Sex:
female
Genotoxicity:
positive

Applicant's summary and conclusion

Conclusions:
No significant increases in the frequencies of micronucleated erythrocytes were seen in blood samples of male mice obtained at the conclusion of the 3-month study. However, in female mice from the 3-month study, a significant increase in micronucleated erythrocytes was observed in the 1000 ppm group. In view of the nature of the observed increases in micronucleated cells in these female mice and the clear absence of clastogenic effect observed in mammalian cells in vitro, the significance of this weak positive result is uncertain (high dose only, without trend).
Executive summary:

In this study (design comparable to OECD Guideline 474), groups of 10 male and 10 female B6C3F1 mice were exposed by whole-body inhalation to alpha-methylstyrene at concentrations of 0, 75, 150, 300, 600 or 1000 ppm for 6 hrs per day and 5 days per week for 14 weeks.

No significant increases in the frequencies of micronucleated erythrocytes were seen in blood samples of male mice obtained at the conclusion of the 3-month study.

However, in female mice from the 3 -month study, a significant increase in micronucleated NCEs was observed at the highest exposure concentration of 1000 ppm, resulting in a negative call for male mice and a positive call in this assay for female mice. Reticulocytes (polychromatic immature erythrocytes; PCEs) were also scored for frequency of micronucleated cells in male and female mice. No increase in micronucleated PCEs was observed in either sex at the highest exposure concentration of 1000 ppm, indicating that the damage observed in the mature erythrocyte population in 1000 ppm females was reflective of long-term accumulation of damage and was not detectable immediately after exposure by analyzing recently-formed (within 48 hours) reticulocytes.

Toxic effects (see also section 7.5.3):

Two females in the 1000 ppm group died before exposure on day 3. Final mean body weights of 75, 300, and 1000 ppm females were significantly less than those of the chamber controls; final mean body weight gains of mice exposed to >= 300 ppm were also significantly less.

Ataxia was observed at 1000 ppm. The absolute liver weights of >= 600 ppm females and the relative liver weights of >= 300 females were significantly increased. The oestrous cycle lengths of >= 600 ppm females were significantly longer than that of the chamber controls. Minimal to mild centrilobular hypertrophy was present in the livers of females at >= 600 ppm. The incidences of exposure-related nasal lesions, including atrophy and hyperplasia of Bowman’s glands and atrophy and metaplasia of the olfactory epithelium, were significantly increased in all exposed groups. The incidences of hyaline degeneration, characterized by the accumulation of eosinophilic globules in the cytoplasm of the respiratory epithelium, were significantly increased in females at >= 150 ppm.