2-methylbutene

Endpoint summary

Additional information

Non-human information

In vitro data

The key studies are considered to be a bacterial mutation assay and a mammalian cell cytogenetic assay (SRC, 1981).  These are two recognised core assay types for investigating mutation in vitro and were conducted with technically pure 2M2B.

2-Methyl-2-butene (2M2B) was tested using sealed containers for a pre-incubation phase in an Ames assay in S. typhimurium strains TA1535, TA1537, TA1538, TA100, and TA98 and E. coli strains WP2 and WP2uvrA) in both the presence and absence of rat liver S9.  A range of doses up to 4000 µg/plate was used.  2M2B was negative in this assay.  In the mammalian cell cytogenetic assay, 2M2B was examined in cultured rat liver cells (RL4) at a range of doses up to 50 µg/mL in the absence of auxiliary metabolic activation. Cells were arrested in metaphase after 24 hours exposure and evaluated for chromosomal damage. 2M2B was not clastogenic in this assay. 

A negative result was also obtained for the endpoint of gene conversion in S. cerevisiae (SRC, 1981).

In vivo data

The key studies are considered to be cytogenetic studies (bone marrow micronucleus) in the mouse (EBSI, 1991a, BASF 2009) and rat (EBSI, 1991b). This is a recognised core assay type for investigating mutation in vivo.

Male mice or rats were exposed by the inhalation route to doses of 2M2B of 1005, 3207 or 9956 ppm for 6 hours per day for 2 days (EBSI, 1991a,b).  Statistically significant increases in the incidence of micronucleated polychromatic erythrocytes (MPEs) over controls were observed at the two highest dose levels in both the mouse and the rat.  The dose levels resulted in clinical signs of laboured breathing and decreased activity in a number of the animals exposed. The magnitude of the increase in MPEs was greater in the mouse, with the maximum fold-increase over controls being to 10.6x in the mouse but only 2.2x in the rat.  A recent mouse bone marrow micronucleus assay in which two strains of mouse were exposed by the inhalation route to doses of 2M2B up to 11608 ppm for 6 hours per day for 2 days confirmed these findings with increases in the incidence of MPEs to 10x control or greater in both strains (BASF, 2009).  An assessment of the size of the micronuclei concluded that they were mainly small in size, and thus indicated that the micronuclei were most likely chromosomal fragments rather than whole chromosomes. Appropriate positive controls of an aneugen and a clastogen were used for reference with the sizing.  This would be consistent with a clastogenic effect of 2M2B in the mouse rather than an aneugenic effect.

There are also further reports of micronucleus studies in mice and rats, with results generally consistent with the studies discussed above (EBSI 1990, 1991c-f).

Human information

There is no information indicating any adverse effects of isoamylene.

Short description of key information:
Isoamylene has been examined for mutagenicity both in vitro and in vivo in a range of recognised core assay types.  It has shown negative results for mutagenicity in vitro but positive results in a number of studies in vivo in the bone marrow micronucleus assay. It is concluded that the available data indicates that isoamylene is genotoxic in vivo.

Endpoint Conclusion: Adverse effect observed (positive)

Justification for classification or non-classification

Isoamylene warrants classification as Muta Cat 3, R68 under DSD. Under CLP this corresponds with Category 2 (H341): Suspected of causing genetic defects (positive in mammalian somatic cell mutagenicity tests in vivo).