2-methylbutene

Carcinogenicity

Description of key information

There are no lifetime animal studies available on isoamylene (2-methylbutene) in order to directly investigate for possible carcinogenic activity. However, there is some information available to make a satisfactory assessment of the likely carcinogenic potential of the material and to propose classification. 
The applicant does not propose conducting a study since the requirement clause is an ‘and’ clause and is waived on the basis that  isoamylene (2-methylbutene) does not have a widespread dispersive use nor is there evidence of  frequent or long-term human exposure.  In addition classification as a Carc Cat 3 (R40) under DSD and Carc Cat 2 (H351) under CLP is proposed. 
 

Justification for classification or non-classification

In the absence of carcinogenicity data (animal or human) and considering the proposed classification of 2-methyl-2-butene as a Category 3 mutagen, together with the data from the repeat dose animal studies, it is concluded that the substance might appropriately be regarded as a possible carcinogen under current technical guidance (R 7.7.13.2). 

 

For this reason it is proposed that isoamylene (2M2B) warrants classification as Carc Cat 3 (R40) and the equivalent CLP, Carc Cat 2 H351 (‘Suspected of causing cancer’).

Additional information

There are no lifetime animal studies available on isoamylene (2-methylbutene) that can provide direct evidence for possible carcinogenic activity.However, information is available that can be used to make a satisfactory assessment of the likely carcinogenic potential of isoamylene (2-methylbutene) and to propose classification.

It is of note that REACH Annex X states that a carcinogenicity study may be proposed by the registrant or may be required by the Agency in accordance with Articles 40 or 41 if the substance has a widespread dispersive use or there is evidence of frequent or long-term human exposure; and the substance is classified as mutagen category 3 or there is evidence from the repeated dose study(ies) that the substance is able to induce hyperplasia and/or pre-neoplastic lesions

Testing data relevant for carcinogenicity

In vitro data

There are data from core in vitro assays covering both gene mutation and cytogenetic endpoints (bacterial mutation assay and mammalian cell cytogenetic assay) indicating a lack of genotoxic activity for isoamylene (2-methylbutene).

In vivo data

Cytogenetic studies (bone marrow micronucleus assay) in the mouse and rat report statistically significant increases in the incidence of micronucleated polychromatic erythrocytes (MPEs) in both species, with the magnitude of the increase being greater in the mouse than in the rat.A recent study examining the size of the micronuclei induced has concluded that the micronuclei are likely to represent chromosomal damage, rather than isolated whole chromosomes.Isoamylene can therefore be considered to be genotoxic in animals.The results from these studies meet the criteria for classification of 2-methyl-2-butene as a Category 3 mutagen.The results from these cytogenetic studies also indicate that a carcinogenic response in animals is possible from long term administration.

In a repeat dose rat study (OECD 422), after 4 weeks of exposure at doses up to 7000 ppm, pathological changes were noted amongst high dose females in the liver, evidenced as increased organ weights and minimal centrilobular hepatocyte hypertrophy. There was a decreased incidence of extramedullary haemopoiesis of the spleen of high dose (7000 ppm) animals, an increase in goblet cell hyperplasia in the nasal passages of high dose males; and amongst high and intermediate (2000 ppm) dose males, a slight increase in severity of mycardial inflammatory heart lesions and cortical/medullary tubular basophila in the kidneys.

The conclusion from the 28 day study was that slight effects on general systemic toxicity due to 2-methyl-2-butene were apparent in animals receiving 7000 ppm, (half the lower explosivity limit) and, to a lesser extent, at 2000 ppm. The no-effect level for general systemic toxicity was considered to be 580 ppm (1665 mg/m3).

The changes seen following repeated administration of 2-methyl-2-butene indicate a slight level of toxicity to certain tissues, including a level of hyperplasia, albeit this was only seen in the nasal epithelium following inhalation exposure and is not considered being necessarily indicative of pre-neoplasia [a marked increase in mucus-secreting cells in the respiratory epithelium may represent a physiological response to an irritant].The changes seen are slight and not indicative of major organ toxicity at doses up to 7000ppm.

Nevertheless, taken together with the positive in vivo genotoxicity data, there is reason to consider that chronic exposure to 2-methyl-2-butene may potentially result in a carcinogenic response.

Conclusion

The applicant does not propose conducting a study since the requirement clause is an ‘and’ clause and is waived on the basis that isoamylene (2-methylbutene) does not have a widespread dispersive use nor is there evidence of frequent or long-term human exposure. In addition classification as a Carc Cat 3 (R40) under DSD and Carc Cat 2 (H351) under CLP is proposed.