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Additional information

In vitro data

The mutagenic potential of SMBT was evaluated in a bacterial mutagenicity test (Monsanto Co. 1976). Although the study is reliable the test design of the study does not comply with the current guideline with regard to the kind of tester strains used. Here, the tester strains Salmonella typhimurium TA 98, TA 100, TA 1535, TA 1537 and TA 1538 were used. Treatment by the plate incorporation method was done in presence or absence of metabolic activation (S9-mix). A concentration range of 0.001 µl/plate to 5 µl/plate was evaluated. Toxicity of the test substance was noted at 5 µl/plate in all tester strains evaluated with and without metabolic activation. No mutagenic response was noted in any of the tester strains evaluated with and without metabolic activation. A slight but not dose dependent increase in revertans was noted in tester strain TA1538 in the lower dose groups (0.001 and 0.01 µl/plate) with metabolic activation. No increase was noted in a repeated experiment in the same dose range; thus the relevance of these finding was found questionable. The author concluded that the test substance was negative under the experimental conditions used with and without metabolic activation.

The negative finding was confirmed by another bacterial mutation assay (Goodyear 1979). The Salmonella typhimurium tester strains TA 1535, TA98, TA1537 and TA100 were tested with test substance concentrations ranging from 0.1 to 100 µg/plate with and without metabolic activation. No mutagenic effects were observed in any of the tester strains evaluated with and without metabolic activation.

The mutagenic potential of SMBT was evaluated in another bacterial mutation assay using tester strains TA100, TA1535, TA97 and TA98 with and without metabolic activation (NTP 1984). A slight increase in revertants was noted in tester strain TA98 with metabolic activation. No mutagenic activity was noted in any of the other tester strains evaluated with and without metabolic activation. The authors concluded that the test substance was weak positive in the assay with metabolic activation.

There are only limited bacterial mutation assay data, no mammalian cell mutation assay data and no mammalian cell cytogenetic data available for SMBT. A read-across approach is performed with data from MBT (benzothiazole-2-thiol) (see discussion endpoint summary toxicokinetics).

Read across MBT

No mutagenic activity of MBT was noted in several baceterial mutation assays (e.g. CMA 1984). The mutagenic potential of MBT was evaluated in a HGPRT assay with CHO cells (CMA 1984). A preliminary cytotoxicity assay was done with test substance concentrations of 0.03, 0.1, 0.33, 1.0, 3.3, 10.0, 33.33, 100.0, 333.3 and 1000 µg/ml with and without metabolic activation (S9-mix). Without metabolic activation cytotoxicity was indicated at 100, 333.3 and 1000 µg/ml; at 33.33µg/ml a survival of 58% was indicated. With metabolic activation cytotoxicity was noted at 1000 µg/ml; relative survival at 333.33µg/ml was 18 %. Based on the findings from the cytotoxicity assay a concentration range of 10 to 300µg/ml (with metabolic activation) and 1 to 50µg/ml (without metabolic activation) was evaluated in the mutation assay. No increase in the mutation frequency was observed. The authors concluded that the test substance MBT was negative under the experimental conditions used.

The negative finding in the HGPRT assay was confirmed in another mammalian cell mutation assay. No significant increase in mutation frequency was noted in a mouse lymphoma assay (WTR 1997).

In a chromosome aberration assay with CHL cells an induction of polyploidy cells and endoreduplications were noted after treatment with 0.2 µg/ml MBT. The frequency of polyploidy cells and endoreduplications was 3.6% and 6.2 % without metabolic activation and 2 % and 0.4 % with metabolic activation, respectively. No endoreduplications were noted in the solvent control (Matsuoka 2005). Furthermore, in a limited chromosome aberration assay with CHO cells an increase in aberrant cells were noted (NTP 1988). However, the findings are questionable because of significant chemically induced cell cycle delay, which presumably indicated high toxicity. In addition, in a limited sister chromatid exchange (SCE) assay a relative increase in SCE’s was noted in presence of metabolic activation (NTP 1988). However, the relevance of this finding is questionable because of presumed high toxicity indicated by a significant chemically induced cell cycle delay and the lacking of a dose-response relationship.

In vivo data

There are no data available for in vivo genotoxicity of SMBT. In vivo genotoxicity data from MBT was used in a read across approach (see discussion endpoint summary toxicokinetics).

Read across with MBT

The genotoxic potential of MBT was evaluated in an in vivo micronucleus assay with CD-1 mice (4 males and 4 females per group) (CMA 1984). Single dose group animals received 300 mg/kg of MBT and multiple dose group animals received MBT in a split dose regimen with two doses of 300 mg/kg each, separated by 24 hours. The positive control article triethylenemelamine was administered intraperitonealy to a separate group of mice (4 males and 4 females) at a dose of 0.5 mg/kg. Thirty hours after treatment the positive control animals were sacrificed. The negative control animals were given two doses of corn oil separated by 24 hours and sacrificed 48 hours after the first dose. Single Dose Group I and Single Dose Group II were sacrificed at 30 and 48 hours, respectively after a single injection. Multiple Dose Group I and Multiple Dose Group II were given two doses of the test article separated by 24 hours and sacrificed at 48 and 72 hours, respectively after the initial injection. Systemic availability of the test substance was indicated by occurrence of clinical signs after application. The following signs were observed at the first dose: prostration, hypoactivity, hypernea, ptosis, tremors upon stimulation and an occasional animal exhibited a loss of righting. Observations at 4 and 24 hours following the first dose included ptosis with no other visible signs in all treated animals. No mortality occurred in the study. The results for test article MBT were negative in the micronucleus test at a dose level of 300 mg/kg in the single dose groups and with a second dose of 300 mg/kg in the multiple dose group administered in a split dose regimen. The test material did not produce a statistically significant increase in the number of micronuclei per 1000 polychromatic erythrocytes in the treated versus the control group. In addition to these criteria, all animals administered MBT were within the normal historical range of spontaneous micronuclei incidence.

The test substance MBT was evaluated in a dominant lethal test in Sprague-Dawley rats (CMA 1989). Male rats were treated with 0, 2500, 8750, or 15000 ppm MBT in the diet. Following a 13 week treatment each male was housed with two virgin female rats per week for two weeks. All females with evidence of mating from each group were sacrificed on gestation day 13 for determination of dominant lethal effects. MBT produced dose-related toxicity in male rats treated with 8750 and 15000 ppm in the diet. Toxicity was limited to reduced body weights and food consumption. When the rats were serially mated over two weeks after 13 weeks of treatment, no dominant lethal effect was observed.


Short description of key information:
The test substance SMBT was investigated for its potency to induce gene mutation in bacteria. SMBT was negative in two bacterial gene mutation studies (Monsanto Co 1976, Goodyear 1979); whereas a slight increase in revertants was noted in another bacterial mutation assay in presence of metabolic activation (NTP 1984). However, MBT was not mutagen in several bacterial mutation assays (e.g. CMA 1984).
There are no mammalian cell mutation test data available for SMBT. A read across approach is done with data from MBT. No mutagenic potential of MBT was noted in a HGPRT assay with CHO cells (CMA 1984). This negative finding is confirmed in a mouse lymphoma assay (WTR 1997). However, an increase in polyploidy cells and endoreduplications was noted in an in vitro chromosome aberration assay in CHL cells (Matsuoka 2005). Moreover an increase in chromosome aberrations and SCE’s was noted in CHO cells (NTP 1988). However the biologically relevance of these findings is questionable.
No genotoxic effects of MBT were noted in an in vivo micronucleus assay with CD-1 mice (CMA 1984). In addition, in a dominant lethal test with Sprague-Dawley rats no dominant lethal effects of the test substance were noted (CMA 1989).
In summary, no genotoxic potential of SMBT was noted in two bacterial mutation assays, whereas a rather weak response was noted in another bacterial test. In a read across approach with MBT study data no mutagenicity was noted in bacterial or mammalian cell test systems. Some evidence of clastogenic effects in vitro were noted, however the relevance of these findings is questionable. No genotoxicity in vivo was indicated for MBT. Overall, taking into account the limited study data of SMBT and the supporting data of MBT no mutagenic activity is anticipated.

Endpoint Conclusion:

Justification for classification or non-classification

No classification is required according to the classification criteria 67/548/EWG and regulation no. 1272/2008 (GHS).