Butane, 2-ethoxy-2-methyl-

Endpoint summary

Description of key information

The oral NOAEL of TAEE is 100 mg/kg bw/day based on the increased ALT activity in females and significant increased relative liver weights accompanied by diffuse enlargement of the hepatocytes without the normal zonal pattern in males at mid- and high-dose levels, and increased adrenal weights in both sexes at the high dose level (observed in a subacute toxicity study).
For the inhalation route, no TAEE specific data are available. Therefore, data of the structural analogues TAME (2-methoxy-2-methylbutane) and ETBE (2-ethoxy-2-methylpropane) were considered. The (lower) inhalation NOAEC reported for TAME, 250 ppm (1060 mg/m3), based on organ (liver, adrenals and kidneys) weight increases (rats and mice) and histopathological liver effects (mice) will be used for the derivation of the long-term inhalation DNEL for systemic effects for TAEE. The introduction of an additional safety factor in order to take into account the uncertainty for using the data on TAME to derive a DNEL for TAEE is considered to be unnecessary based on the available data. Only correction for molecular weight (102 g/mol for TAME vs. 116 g/mol for TAEE) will be applied.

Key value for chemical safety assessment

Repeated dose toxicity: via oral route - systemic effects

Endpoint conclusion

Dose descriptor:

NOAEL

100 mg/kg bw/day

Study duration:

subacute

Species:

rat

Repeated dose toxicity: inhalation - systemic effects

Endpoint conclusion

Dose descriptor:

NOAEC

1 060 mg/m³

Study duration:

subchronic

Species:

rat

Additional information

Oral route

The sub-acute oral toxicity of TAEE was studied in a GLP-compliant OECD 422 guideline study in which male and female rats received 0, 100, 375 or 750 mg/kg bw/day of the test substance by oral gavage in corn oil (Covance Laboratories Ltd, 2009d). Males were dosed for up to 8 wk (56 d), females for approximately 6 wk (41-46 d). Clinical signs were generally unremarkable, however males given 375 and 750 mg/kg bw/day, and some females given 750 mg/kg bw/day, were slightly ataxic and less active relative to controls. There were no toxicologically significant alterations in body weight, weight gain, food intake, haematology or clinical chemistry parameters. Necropsy findings were observed in the adrenals, liver and kidneys.

Adrenals

Adrenal weights were statistically significantly increased at the highest dose tested in both sexes. Microscopic findings in the adrenals of animals given 750 mg/kg bw/day were comparable with controls, and there was no correlation with the macroscopic findings of large adrenals or with the increase in adrenal weights seen at necropsy.

Kidneys

Dose-related increases in male adjusted relative kidney were observed. Macroscopically, pale kidneys were seen in treated males. Microscopically, a dose related increase in the severity of hyaline droplets was apparent in treated males compared with controls, which correlated with the findings of pale kidney recorded macroscopically. Hyaline droplets were characterised by eosinophilic cytoplasmic inclusions in the proximal tubular cells of the kidney. Males from the high dose groups also showed an increase in incidence of minor focal nephropathy compared with controls. Focal nephropathy was characterised by foci of basophilic tubules, with occasional casts and inflammatory cell infiltration. The levels of focal nephropathy in males given 100 or 375 mg/kg bw/day were considered to be within the normal background range. The kidneys of treated females were comparable with controls. The observed kidney effects in males at all dose levels resemble the male rat-specific syndrome of α2u-globulin nephropathy as a primary mode of action for kidney toxicity. This pathologic process has no human counterpart and is therefore not relevant for human risk assessment.

Liver

Dose-related increases in male adjusted relative liver weights were reported at all dose levels. The livers of treated females were comparable with controls. Microscopically, hepatocyte hypertrophy was present in all males and correlated with the finding of large liver recorded macroscopically. The changes were characterized in high dose males by a diffuse enlargement of the hepatocytes, with an overall increase in the size of the liver lobules and without the normal zonal pattern. At the lower doses the enlarged hepatocytes were more obvious in the centrilobular areas of the liver lobule. Alterations in clinical chemistry parameters were unremarkable in males, but females showed a significant trend for increased ALT activity (+82%, +89% for intermediate and high dose groups, respectively).

Although the increases in relative liver weights with minor histopathological changes have been observed at all dose levels, the effects observed at 100 mg/kg bw/day are considered to be of minor importance, taking into account the magnitude of the relative weight increase (<20%) and the slight nature of the histopathological effects at this dose. These effects are presumably mainly related to metabolic adaptation. According to ECETOC Technical Report No. 85 (2002), increased liver weight either in the absence of adverse histological changes, or accompanied by zonal (usually centrilobular) hypertrophy of the hepatocytes is generally regarded as an adaptive effect associated with enzyme induction or smooth endoplasmic reticulum proliferation. In such cases, the magnitude of the increase in the liver weight should be taken into consideration when deciding on the adversity of these effects.

 

In conclusion, the NOAEL for repeated dose toxicity has been set at 100 mg/kg bw/day, based on the increased ALT activity in females and significant increased relative liver weights accompanied by diffuse enlargement of the hepatocytes without the normal zonal pattern in males at mid- and high-dose levels, and increased adrenal weights in both sexes at the high dose level.

For the structural analogue TAME, a subacute oral study is available. Five male and five female Sprague-Dawley rats were administered 0, 125, 500 or 1000 mg/kg bw TAME in corn oil by gavage (Daughtrey et al., 1995). The dose was given once daily, 7 days a week for a period of 29 days.The critical effects observed were an increased adrenal (absolute and relative) and kidney (relative) weight at 500 and 1000 mg/kg bw in males. The organ weight increases in the kidney and adrenals were not accompanied by any histopathological changes. The liver weights were not statistically significantly affected. The NOAEL was set at 125 mg/kg bw/day.

Inhalation route

No inhalation repeated dose toxicity data are available on TAEE. However, Article 13 of the REACH legislation states that, in case no appropriate animal studies are available for assessment, information should be generated whenever possible by means other than vertebrate animal tests, i.e., applying alternative methods such as in vitro tests, QSARs, grouping and read-across. For the structural analogues of the substance, TAME (2-methoxy-2-methylbutane) and ETBE (2-ethoxy-2-methylpropane), data are available regarding inhalation repeated dose toxicity (analogue approach). As TAEE and TAME cause the same type of effects after repeated oral exposure and as toxicokinetic data on TAME and ETBE show that the route of administration appeared not to have an influence on the metabolic process, read-across to the inhalation data of TAME and ETBE is allowed for TAEE.

 

TAME:

For inhalation repeated dose toxicity, two 90 days studies are available, one with F344 rats and one with CD1 mice (Huntingdon Life Sciences, 1997). In addition, a subacute toxicity study with SD rats is available (IIT Research Institute, 1992). The 90 days studies are considered key (longer exposure duration, lower NOAEC). In these studies the exposure concentrations were 250, 1500 and 2500 / 3500 ppm (1060, 6360, 10600 / 14840 mg/m³). An overall NOAEC of 250 ppm (1060 mg/m³) was established based on organ (adrenals, kidneys and liver) weight increases in male and female rats and based on an increase in the labeling index of hepatocytes and microscopic evidence of centrilobular, hepatocellular hypertrophy in male and female mice.

ETBE:

One 28-day inhalation study with SD rats (IIT Research Institute, 1991b), a 90-day study with F344 rats and a 90-day study with CD1 mice (CIIT, 1996a and 1996b) were available for assessment. The 90 days studies are considered key (longer exposure duration, lower NOAEC). In these studies the exposure concentrations were 500, 1750 and 5000 ppm (2100, 7350 and 21000 mg/m³). The overall NOAEC for rats and mice is 500 ppm (2100 mg/m3) based on liver effects (mainly female mice, 1750 and 5000 ppm (7350 and 21000 mg/m³)).

 

Considering all information reported above, and using a precautionary principle, the (lower) inhalation NOAEC reported for TAME (1060 mg/m³) based on organ (liver, adrenals and kidneys) weight increases in rats will be used for the derivation of the long-term inhalation DNEL for systemic effects for TAEE. It should also be noted that in the available subacute oral studies with TAEE and TAME, the observed NOAELs for systemic toxicity are of the same order of magnitude (100 and 125 mg/kg bw/day, respectively), suggesting similar toxicodynamics for both substances. Therefore the introduction of an additional safety factor in order to take into account the uncertainty for using the data on TAME to derive a DNEL for TAEE is considered to be unnecessary in this case, and only correction for molecular weight (102 g/mol for TAMEvs.116 g/mol for TAEE) will be applied.

Repeated dose toxicity: via oral route - systemic effects (target organ) digestive: liver; urogenital: kidneys

Repeated dose toxicity: inhalation - systemic effects (target organ) digestive: liver; glandular: adrenal gland; neurologic: central nervous system; urogenital: kidneys

Justification for classification or non-classification

In accordance with Directive 67/548/EEC and EU Classification, Labelling and Packaging of Substances and Mixtures (CLP) Regulation (EC) No. 1272/2008, classification is not necessary for repeated dose toxicity.