Cyclohexane

Carcinogenicity

Description of key information

Cyclohexane is a simple aliphatic chemical and neither cyclohexane nor its metabolites carry alerts for genotoxic activity.  Cyclohexane has been shown to be non-genotoxic both in vitro and in vivo.  These observations indicate that cyclohexane is not expected to be a genotoxic carcinogen.  This conclusion is concordant with that in the RAR (2004).
There are no data from toxicokinetics, metabolism and distribution findings to indicate any target for possible carcinogenic activity.
Cyclohexane has been examined in repeat dose inhalation studies and shown low toxicity.  The liver showed cellular changes consistent with an adaptive change to cyclohexane administration, with the principal histopathological change being hypertrophy.  A study where cyclohexane was applied to mouse skin and intended as an initiation/promotion assay was of insufficient robustness and validation to allow any clear conclusions.  The data did not indicate a concern that warrants further animal experimentation.
It is concluded that there are sufficient data to indicate that cyclohexane does not pose a risk of genotoxic carcinogenicity.  There are also sufficient data to understand that the cellular changes arising from repeated administration of cyclohexane to animals are minimal and to conclude that cyclohexane is unlikely to be carcinogenic.  The conduct of animal carcinogenicity studies is considered to be unwarranted.

Key value for chemical safety assessment

Carcinogenicity: via oral route

Endpoint conclusion

Endpoint conclusion:

no study available

Carcinogenicity: via inhalation route

Endpoint conclusion

Endpoint conclusion:

no study available

Carcinogenicity: via dermal route

Endpoint conclusion

Endpoint conclusion:

no study available

Justification for classification or non-classification

It is concluded that cyclohexane is unlikely to be carcinogenic and therefore does not warrant classification under CLP.

Additional information

There are no lifetime animal studies available on cyclohexane in order to directly investigate for possible carcinogenic activity. However, there is sufficient information available to make a satisfactory assessment of the likely carcinogenic potential of the material.

Non-testing information relevant for carcinogenicity

Cyclohexane is a simple aliphatic chemical. It contains no centres of chemical functional activity and contains no alerts for possible genotoxic activity when considered against the criteria established by Ashby and Tennant (1988). The chemical would therefore not be expected to show genotoxic activity in vitro or in vivo, and therefore would also not be expected to show carcinogenic activity due to a genotoxic mechanism. 

The metabolism of cyclohexane has been examined in both animals and humans. It has been shown to be metabolised by a simple oxidative pathway to hydroxy derivatives, including cyclohexanol, 1,2-dihyroxycyclohexane and 1,4-dihydroxy cyclohexane. None of these chemicals carry any alerts for possible genotoxic activity when considered as above.

Cyclohexane is therefore a simple aliphatic chemical, which, when evaluated using accepted structure activity relationship considerations, has no alerts for genotoxicity or subsequent carcinogenic activity.

Testing data relevant for carcinogenicity

In vitro data 

Cyclohexane has been examined for mutagenicity both in vitro and in vivo in a range of recognised core assay types. In vitro, these include assays for gene mutation in both bacteria (the Salmonella mutation assay), and mammalian cells (mouse lymphoma mutation assay).  It has shown negative results for mutagenicity in vitro. Additional examination of cyclohexane has been carried out using sister chromatid exchange in mouse lymphoma cells, unscheduled DNA synthesis in human lymphocytes and a DNA cell binding assay. The data from these assays all support the above conclusion that cyclohexane is not mutagenic in vitro (RAR, 2004). 

Cyclohexane was reported to be negative in a cell transformation assay. 

There are no data from other in vitro assays indicating a likely carcinogenic action for cyclohexane.

Animal data

Cyclohexane has been examined for mutagenicity using a bone marrow cytogenetic assay in the rat and was not found to be mutagenic. This finding was supported by an assay in Drosophila. Cyclohexane has therefore been examined in the key relevant mutagenicity assays both in vitro and in vivo and found to be non-genotoxic. There is therefore good evidence for cyclohexane being non-carcinogenic in animals with respect to a genotoxic mechanism. 

Cyclohexane has been examined in 90 day repeat dose toxicity tests in both the rat and mouse, by the inhalation route, including an examination by pathology of relevant tissues for this study type. In the rat, the findings were very limited with the main observation being an increase in relative liver weight and hepatic hypertrophy accompanied by centrilobular hypertrophy. This finding was partially reversible in the one month recovery period and is considered an adaptive change of limited toxicological significance. These findings are unremarkable after a 90 day exposure period, and do not provide any indication of significant cellular change that may predispose to a carcinogenic effect on prolonged exposure. Cyclohexane is considered to be metabolised by the cytochromes P450 family of enzymes and the hypertrophy observed in the liver may be a reflection of an adaptive response to this activity. In a 14 day preliminary study a minimal increase in mitotic figures was observed in the liver, but this was not observed in the 90 day study nor was it accompanied by an increase in absolute or relative liver weight. In the mouse, very few effects were again observed with those seen being focussed on the liver and broadly similar to those of the rat. After 14 days, an increase in liver weight with accompanying centrilobular hypertrophy and increased mitotic figures was seen, and after 90 days an increase in liver weight but with no concomitant histopathological findings. Also seen after 14 days was a slight increase in lung weight, although this was not seen after 90 days.

The toxicokinetics, metabolism and distribution findings for cyclohexane in animals indicate a chemical that is cleared primarily through expired air and in the urine, and with little potential for accumulation. There is no evidence to indicate a target organ that might be susceptible to cyclohexane toxicity.

Cyclohexane has been examined in a mouse skin assay for tumour promoting activity and also for the ability to increase ornithine decarboxylase activity. For the tumour promoting assessment, animals treated with the carcinogen dimethylbenzanthracene (DMBA) were subsequently exposed to acetone (control), or TPA, or cyclohexane or TPA plus cyclohexane. The results indicated the induction of a low number of tumours in animals treated with DMBA and then cyclohexane, suggesting a possible weak tumour promoting activity, under the conditions of the experiment. However, a reduction in the number of tumours (over animals treated with DMBA and TPA alone) was observed in animals treated with DMBA and then TPA plus cyclohexane. No information is provided on the irritant nature of the treatments given, and in view of the known irritant properties of cyclohexane, this is a possible explanation for the effects seen.  Overall, the data provided are difficult to rationalise, and, with the absence of information on dermal irritation, it is not possible to draw any clear conclusion on whether cyclohexane is exerting a significant promotional activity in this mouse skin model. There was no experiment conducted to examine for tumours in animals treated with cyclohexane alone, and so no conclusions can be drawn on the carcinogenic activity of cyclohexane. The application of cyclohexane to mouse skin was found to increase the activity of the enzyme ornithine decarboxylase, an enzyme associated with cell proliferation. The relevance of these findings for any extrapolation to carcinogenic potential is limited however, since again no data were presented to indicate the dermal toxicity produced by the cyclohexane treatment regime, and there was no assessment of any cellular consequence of the treatment. The data from this study are not considered to indicate any carcinogenic activity for cyclohexane, or to be of sufficient robustness to warrant any further animal studies to try to clarify the findings.