Cyclohexanone oxime

Endpoint summary

Additional information

In vitro

CHO was tested for mutagenicity in Salmonella typhimurium strains TA97, TA98, TA100 and TA1535 with or without metabolic activation (preincubation method). A positive response was only obtained in TA1535 with metabolic activation by S9 mix from hamster liver in 2 independent trials, the other tests (including TA1535 with metabolic activation by S9 rat liver fraction) were negative (Burka, 1996, RL2).

Prival (2001) repeated the test (RL2) in strains TA100 and TA1535, only under metabolic activation by hamster S9 liver mix, to reassess the results by Burka (1996). As in the previous study, a positive response was obtained in TA1535 with metabolic activation, but not in TA100. According to Prival (2001), this result is surprising, as strain TA100 was derived from TA1535 and is generally considered to be more sensitive than TA1535 for the same type of mutation.

Rogers-Back et al. (1988) performed an Ames-test with Salmonella strains TA98, TA100, TA1535, TA1537, TA1538 and could not detect mutagenic responses without or with metabolic activation by rat and hamster S9 mix (RL2). The difference to Burka (1996) and Prival (2001) may be due to the different method (plate incorporation vs. preincubation protocol).

Supplemental information comes from an abstract publication (RL4) by Araki (2008), who performed another Ames-test with Salmonella strains TA98, TA100, TA2637 and E.coli WP2 uvrA/pKM101 as preincubation test, with and without metabolic activation (no information on species of which S9 liver mix was derived). This study extended the range of tester strains to E.coli WP2 uvrA/pKM101. The results were consistently negative.

 

CHO was tested for the induction of chromosome aberrations in cultured Chinese hamster ovary cells in vitro with and without metabolic activation by rat liver S9 fraction. Significant responses were observed in the two trials without metabolic activation, but only in one dose group in each of the two trials (low- or mid-dose, respectively), the effects were not dose related and the dose-trends were negative. The authors considered the observed effects to be equivocal (without metabolic activation) and negative (with metabolic activation), respectively, but according to the lack of a dose-response relationship, the study results are interpreted as negative (Burka, 1996, RL2).

A L5178Y TK+/TK- mouse lymphoma assay in vitro revealed a dose-related increase of mutant frequencies in the test without metabolic activation, but not under metabolic activation. Cytotoxicity was observable especially in the trial without metabolic activation, but also with metabolic activation (Rogers-Back et al., 1988, RL2)

 

In vivo

An in vivo micronucleus test was included in the subchronic study by this author, where mice were exposed 3 months via drinking water in concentrations up to 10000 mg/L. No induction of micronuclei was observed in peripheral erythrocytes. A negative response was also observed in a separate micronucleus test in mice after intraperitoneal application of doses up to 1000 mg/kg bw/day on 3 consecutive days (Burka, 1996, RL2).

 

The test item was assessed in the in vivo UDS assay for its potential to induce DNA repair (UDS) in the hepatocytes of rats 4 and 16 hours after oral single application of 375 and 750 mg/kg bw. The highest dose was estimated in three pre-experiments to be the maximum applicable dose. The viability of the hepatocytes was not substantially affected by the in vivo treatment with the test item. None of the tested dose levels revealed UDS induction in the hepatocytes of the treated animals as compared to the corresponding vehicle controls.

Appropriate reference mutagens were used as positive controls. They induced UDS, thus indicating the validity of the test (Harlan RCC, 2013, RL 1).

A less reliable study (RL3) revealed no mutagenic response in a Drosophila melanogaster feeding test. n the (SLRL) test in Male Drosophila melanogaster were exposed to 8.8 mM in nutrient solution and mated with untreated females for two 3-day brood periods and a 4-day brood period. This treatment did not induce sex-linked recessive lethal (SLRL) mutations (Vogel and Chandler, 1974, RL3).

Justification for selection of genetic toxicity endpoint
3 key studies with negative in vivo results: Burka et al., 1996a, b; Harlan, 2013

Short description of key information:
In vitro
Cyclohexanone oxime (CHO) was mutagenic in Salmonella typh. strain TA1535 in reliable bacterial mutagenicity tests. It did not induce chromosome aberrations in Chinese hamster ovary cells with and without metabolic activation (RL2 study). CHO was mutagenic in a reliable (RL2) mouse lymphoma TK+/- assay without metabolic activation and negative with metabolic activation.

In vivo
No micronucleus formation was observed in peripheral blood cells of mice, exposed in drinking water at concentrations up to 10000 mg/L for 90 days. A negative response was also observed in bone marrow of mice after 3 intraperitoneal applications (both RL2 studies).
No UDS was induced in vivo in a guideline study (RL1) in rat liver hepatocytes after oral exposure up to 750 mg/kg bw.
A less reliable study (RL3) revealed no mutagenic response in a Drosophila melanogaster feeding test.

Endpoint Conclusion: No adverse effect observed (negative)

Justification for classification or non-classification

There are positive in vitro results in S. typhimurium strain TA1535 with metabolic activation by hamster S9 mix, but not in the other tester strains or E.coli. A positive result was also observed in a mouse lymphoma assay without metabolic activation.

The results of reliable and less reliable in vivo studies were consistently negative.

 

Based on the negative in vivo findings the test substance has not to be classified for genotoxic effects according to Regulation (EC) No 1272/2008.