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Administrative data

Endpoint:
neurotoxicity: chronic inhalation
Type of information:
experimental study
Adequacy of study:
weight of evidence
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
study well documented, meets generally accepted scientific principles, acceptable for assessment

Data source

Reference
Reference Type:
publication
Title:
Unnamed
Year:
1996

Materials and methods

Principles of method if other than guideline:
The present study of oxide neurotoxicity was one component of a more comprehensive evaluation of the chronic inhalation toxicity of EtO and PO.
GLP compliance:
not specified
Limit test:
no

Test material

Constituent 1
Chemical structure
Reference substance name:
Ethylene oxide
EC Number:
200-849-9
EC Name:
Ethylene oxide
Cas Number:
75-21-8
Molecular formula:
C2H4O
IUPAC Name:
oxirane
Specific details on test material used for the study:
- Physical appearance: gas
- Purity: 99.7%

Test animals

Species:
monkey
Strain:
Macaca fascicularis
Sex:
male
Details on test animals or test system and environmental conditions:
Mean body weight: 5.26 kg for the control group, 5.39 kg for 50 ppm, 5.21 kg for for 100 ppm.
Animals were housed in stainless steel primate cages and provided with Purina monkey chow once a day and water ad libitum.

Administration / exposure

Route of administration:
inhalation
Duration of treatment / exposure:
24 months
Frequency of treatment:
7 h/day, 5 d/weeks
Doses / concentrationsopen allclose all
Dose / conc.:
50 ppm (nominal)
Dose / conc.:
100 ppm (nominal)
No. of animals per sex per dose:
12
Control animals:
yes, sham-exposed

Examinations

Observations and clinical examinations performed and frequency:
Body weight measurement, MCV testing, electroencephalograms, neuropathology

Results and discussion

Results of examinations

Details on results:
Body Weight: Following the initiation of exposures, the mean weights of all five groups declined below base line. With the exception of the EtO 100 group, mean weights recovered to baseline levels within 5-13 weeks. Results in the 50 ppm EtO group did not differ statistically from
those found in the control group, with each group showing a 12.0% increase in mean body weight over the two years of exposure. The 100 ppm EtO group, however, did not recover but remained at about 95% of baseline weight throughout the exposure period. By week 25, the EtO group weight was significantly lower than controls and remained lower throughout the phase 1. This decreased mean weight relative to controls continued in the animals that were retained during the subsequent 7-year-observation period (phase 2).
MCV: There was no significant difference between oxide exposure groups and controls. For two animals in the 100 ppm EtO group, however, a large reduction in sciatic MCV was found after 12 months of exposure. These two animals had normal MCVs of 59.2 and 73.7 m/s at the onset of the study, but these MCVs declined below 40 m/s by the end of 12 months. These MCVs remained stable at this level for the remaining 12 months of exposure. The multivariate analyses of MCVs in EtO-exposed animals were: sciatic-tibial nerve and ulnar nerve. MCV data obtained from ten monkeys in phase 2 (at the end of study year) were not statistically significant for either the sciatic nerve or the ulnar nerve.
Peak-to-peak amplitude in response to nerve stimulation was also unaffected by the exposures and did not differ among exposure conditions in phases 1 and 2 of the study. No univariate analyses were performed for individual sessions because the multivariate analyses were not significant.
Electroencephalograms: EtO exposure resulted in no detectable between-group effects on EEG; however, RMS power declined in all groups over the first 12 months of exposure. The percentages of delta, theta, alpha and beta EEG activities did not vary significantly from session to session or between groups as assessed by a multivariate analysis. A similar lack of effect was found for RMS power. An analysis of occipital lead EEGs also found no group differences.
Neuropathology (Phase 1): As the total ionic strength of the fixative was twice that of the intracellular sodium ion concentration, the perfusate was hypertonic, resulting in perfusion fixation artifacts. The artifacts mimicked demyelination in peripheral nerve sections and therefore precluded interpretation of that tissue. However, spinal cord and brain tissues were not compromised. Evaluations of spinal cord, cerebral white matter, cortex, nuclei, and tracts were conducted using a single-blind procedure by neuropathologists at the Midwest Research Institute, Kansas City, Missouri. Lesions of indicating axonal dystrophy, as defined by Pentschew and Schwartz (1962) and Cowen and Olmstead (1963), were present in the area of the medulla oblongata, restricted to the nucleus gracilis of the oxide-exposed animals, but not the controls. Axonal bodies in the nudeus gracilis were counted on each side. Severity of the lesion was graded according to Sung (1964), based on the work of Fujisawa (1967) as follows: (1) negative/trace (0-1 body in one nuclear area only); (2) slight (1-5 bodies in each nuclear area); (3) moderate (6-10 bodies), or (4) severe (>11 bodies in each nuclear area). Using this grading scheme, results for each of the two animals for the various exposure and control conditions were: 50 ppm EtO, slight, severe; 100 ppm EtO, negative, slight. One control animal was rated negative/trace, and the other control animal had no axonal bodies. In addition, demyelination in the extreme distal portion of the fasciculus gracilis was found in two EtO monkeys, one from each exposure group. Statistical analyses of these data were not conducted because of the small sample sizes.
Neuropathology (Phase 2): The foIlow-up neuropathology assessments were conducted by Experimental Pathology Laboratories, Incorporated, Hemdon, Virginia, seyen years after the exposure terminated. Microscopic grading of axonal bodies in the nucleus gracilis were the same as those employed by the Midwest Research Institute. The results were: 50 ppm EtO, slight, moderate; 100 ppm EtO, slight, moderate; controls, slight, moderate.

Effect levels

Dose descriptor:
NOAEC
Effect level:
50 ppm
Based on:
test mat.

Target system / organ toxicity

Critical effects observed:
no

Applicant's summary and conclusion

Conclusions:
Multiple axonal bodies were found in the nucleus gracilis in seven of eight oxide-exposed animals, and demyelination was found in two monkeys exposed to ethylene oxide. In contrast, a single axonal body was found in one of the two sham-control monkeys. However, the lack of a dose-response relationship suggests that this effect may not be related to oxide exposure. In a follow-up study, nerve conduction velocity and neuropathology were assessed in the remaining monkeys seven years after exposure terminated, but again, treatment-related effects could not be detected.

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