Registration Dossier

Administrative data

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

Data source

Reference
Reference Type:
publication
Title:
Synergism between mercaptans and ammonia or fatty acids in the production of coma-A possible role for mercaptans in the pathogenesis of hepatic coma
Author:
Zieve L, Doizaki WM, Zieve FJ
Year:
1974
Bibliographic source:
J. Lab. Clin. Med., 83, 16-28

Materials and methods

Test guideline
Qualifier:
no guideline followed
Principles of method if other than guideline:
The coma-producing properties of methanethiol were studies in rats.
GLP compliance:
not specified
Limit test:
no

Test material

Reference
Name:
Unnamed
Type:
Constituent
Details on test material:
Source: Eastman Organic Chemicals, Rochester, N. Y.
Purity was confirmed by injecting the material received into the gas chromatograph and observing a single peak.

Administration / exposure

Route of administration:
inhalation: vapour
Vehicle:
unchanged (no vehicle)
Details on exposure:
Methyl mercaptan was administered in the gas phase. Rats were placed singly in a 27 liter desiccator, with the opening in the lid covered by a rubber septum through which the compound could be injected into the chamber. In each instance the test animal was placed inside the appropriate chamber prior to the injection of the dose of the compound.
Analytical verification of doses or concentrations:
no
Details on analytical verification of doses or concentrations:
The mercaptan content of the gas phase in the chamber was not routinely analyzed; the concentrations recorded in individual experiments were calculated from the dose injected. At gas-chamber concentrations of 1 per cent by volume, the differences between calculated and measured values were less than 5 per cent; at 0.5 per cent by volume, less than 10 per cent; at 0.2 per cent by volume, less than 15 per cent; and at 0.1 per cent by volume, less than 25 per cent. The error of measurement of these mercaptans by gas chromatography was less than 5 per cent.
No. of animals per sex per dose:
3-8

Examinations

Observations and clinical examinations performed and frequency:
For the construction of the dose-response curves, the animais were observed until they became comatose or until 15 minutes after the administration of the compound. Coma was defined as complete loss of the righting reflex. Each point on the curves was based upon at least 3 and as many as 8 rats. All animals recovered consciousness within 30 minutes.
Statistics:
For each curve a CD50. was defined as the dose causing coma in 50 per cent of the animals. It was obtained by dropping a vertical line to the x-axis from the point on the dose-response curve corresponding to a 50 per cent coma incidence.

Results and discussion

Results of examinations

Clinical signs:
effects observed, treatment-related
Mortality:
mortality observed, treatment-related
Details on results:
The CD50 for methanethiol was approximately 0.16% by volume (1% = 0.44 mmoles/l). Coma occurred in 100 per cent of rats when the dose of methanethiol given was 0.2 per cent, and the largest dose of methanethiol which produced no coma was 0.12 per cent.

The rats went through a brief excitement phase before they became groggy. With doses producing coma in 50 per cent of animals, the excitement phase lasted approximately 2 minutes, then passed quickly into a phase lasting 1 minute in which the rat became groggy and lethargic. This was followed in the next minute or two by frank coma, the rat falling to its side and rolling over on its back as the chamber was tipped up slightly. With smaller doses the excitement phase and the pre-coma phase were prolonged; with larger doses the entire sequence occurred more quickly. If the rat was removed from the chamber as soon as he became comatose, the coma did not last more than 30 minutes as a rule, and upon recovery the rat appeared and remained alert and active.

The blood level of methanethiol that best separated the group of rats that became comatose from those that did not, was approximately 0.5 nanomole per milliliter.

Applicant's summary and conclusion

Executive summary:

In 1974, Zieve et al. reported on the "coma-producing" properties of methanethiol. Male rats, weighing 285-325 g, were exposed singly for up to 15 minutes to methanethiol at concentrations of approximately 600-2,200 ppm (1,182-4,334 mg/cu m). Three to eight rats were exposed at each concentration. For static exposures, the desired thiol concentrations were achieved by injecting the required amount of thiol through a rubber septum in the lid of the chamber into which the rat had been placed. Occasional analysis of chamber air showed that actual concentrations varied by 5% at a 1%-by-volume (10,000 ppm) concentration and by less than 25% at the 0.1%-by-volume (1,000 ppm) concentration. For the determination of dose-response relationships, the animals were observed until they had completely lost the righting reflex or until they had been exposed to the thiol for 15 minutes. The concentration of thiol in the blood was determined after a 4-minute exposure.

The thiol concentration at which 50% of the rats lost their righting reflex was 1,600 ppm (3,152 mg/cu m) for methanethiol. The rats went through a brief excitement period before they became "groggy and lethargic," and then their righting reflex was lost within the next few minutes. The duration of the phase of excitement and beginning depression varied inversely with the concentration of the thiol inhaled. Methanethiol at 2,000 ppm (3,940 mg/cu m) caused all rats to lose their righting reflex, whereas 1,200 ppm (2,364 mg/cu m) was the highest concentration at which no animal lost the righting reflex. If the animals were removed from the chamber immediately after losing the righting reflex, "consciousness" was regained within 30 minutes. How consciousness was determined was not stated in the publication. The ratio of the concentration of thiol in the blood to that inhaled was determined. Rats exposed to methanethiol at an air concentration of 0.066 millimole/liter had blood levels of the substance that ranged from 0 to 0.5 millimole methanethiol/ml blood. From the data presented, no dose-response relationship could be determined.