Sodium methanethiolate

Endpoint summary

Additional information

In vitro study

A Salmonella typhimurium reverse mutation assay was performed with sodium methanethiolate according to OECD Guideline 471 (Molinier, 1992). S. typhimurium strains, TA98, TA100, TA102, TA1535 and TA1537, were exposed to five concentrations of sodium methanethiolate in the presence and absence of a metabolic activation system using the plate incorporation and the preincubation methods. All doses of sodium methanethiolate, vehicle control and positive controls were plated in triplicate and incubated for 48-72 hours at 37 ºC. The positive controls gave expected responses. Sodium methanethiolate was cytotoxic at ≥1000 and ≥2500 µg/plate, in the absence and presence of a metabolic activation system, respectively. Treatment of the S. typhimurium strains with sodium methanethiolate in the presence or absence of metabolic activation did not induce any significant increases in the revertant numbers. 

The results of a cytogenetic assay with human lymphocytes (OECD Guideline 473) were equivocal with and without metabolic activation at sodium methanethiolate concentrations of 30, 60, 90, 120, 240 or 480 µg/ml (Molinier, 1995). Treatment times without metabolic activation were 20 or 44 hours. Treatment time with metabolic activation was 3 hours with a 20-or 44-hour post-treatment incubation prior to harvest. Treatment with sodium methanethiolate did not induce structural chromosome aberrations with or without metabolic activation. At the 44-hour treatment period without metabolic activation, an increase in polyploidy was observed (4% and 14.5% at 90 and 120 µg/ml, respectively compared to 0% for control). A second test for 44 hours was then conducted at 50, 100 and 150 µg/ml. The mitotic index was reduced >90% at 150 µg/ml and numerical aberrations were increased (3%) at 100 µg/ml. No effect was seen at 50 µg/ml.

In vivo study

Sodium methylmercaptide was tested in a Mammalian Erythrocyte Micronucleus Test, according to the OECD n° 474 Guideline and EC 92/69/EEC B.12 guidelines in compliance with the Principles of Good Laboratory Practice (Haddouk, 1999). A preliminary toxicity test was performed to define the dose-levels to be used for the cytogenetic study. In the main study, three groups of five male and five female Swiss Ico: OF1 (IOPS Caw) mice received two oral treatments of sodium methylmercaptide at dose-levels of 12.5, 25 or 50 mg/kg/day, at a 24-hour interval. One group of five males and five females received the vehicle (distilled water) under the same experimental conditions, and acted as control group. One group of five males and five females received the positive control test substance (cyclophosphamide) once by oral route at the dose-level of 50 mg/kg. The animals of the treated and vehicle control groups were killed 24 hours after the last treatment and the animals of the positive control group were killed 24 hours after the single treatment. Bone marrow smears were then prepared. For each animal, the number of the micronucleated polychromatic erythrocytes (MPE) was counted in 2000 polychromatic erythrocytes. The polychromatic (PE) and normochromatic (NE) erythrocyte ratio was established by scoring a total of 1000 erythrocytes (PE + NE).

The top dose-level for the cytogenetic test was selected according to the criteria specified in the international guidelines; since toxic effects were noted in the preliminary test, the top dose-level was based on the toxicity level, such that a higher dose-level was expected to induce lethality. Consequently, 50 mg/kg/day was selected as the top dose-level. The two other dose-levels were 25 and 12.5 mg/kg/day. For both males and females, the mean values of MPE as well as the PE/NE ratio in the groups treated with the test substance, were equivalent to those of the vehicle group and no significant difference was noted. The mean values of MPE as well as the PE/NE ratio for the vehicle and positive controls were consistent with the historical data. Cyclophosphamide induced a highly significant increase (p < 0.001) in the frequency of MPE, indicating the sensitivity of the test system.

Sodium methylmercaptide does not induce damage to the chromosomes or the mitotic apparatus of mice bone marrow cells after two oral administrations, with a 24-hour interval, at the dose-levels of 12.5, 25 or 50 mg/kg/day.

Short description of key information:
Sodium methylmercaptide was found to be negative in the in vitro gene mutations assays on bacteria (Ames test). Conflicting results were obtains in chromosomal aberration assays, ambiguous on humal lymphocytes and negative on Chinese hamster lung fibroblast cells. Sodium methylmercaptide was negative in the micronucleus test with mice.

Endpoint Conclusion: No adverse effect observed (negative)

Justification for classification or non-classification

According to REGULATION (EC) No 1272/2008 and DIRECTIVE 67/548/EEC criteria and the available test results in vitro and in vivo, sodium methylmercaptide should not be classified as germ cell mutagen.