2-ethoxy-2-methylpropane

Specific investigations: other studies

Additional information

Cell proliferation in rat forestomach

A non-significant trend for increased BrdU labelling in forestomach was apparent in male rats after gavage administration of ETBE at 1000 mg/kg bw/day for 1 week, but not after treatment with 300 or 100 mg/kg bw/day (DIMS Institute of Medical Science Inc., 2008c). A statistically significant increase in BrdU incorporation was also seen in the positive controls (butylated hydroxyanisole-treated). No microscopic lesions were present in ETBE-treated animals following conventional histopathological assessment, whereas focal squamous cell hyperplasia (and a single incidence of ulceration) was present in all positive control animals. Following 4 weeks treatment with ETBE, labelling indices were unremarkable in all groups with a single non-dose related occurrence of focal squamous cell hyperplasia noted at 300 mg ETBE/kg bw/day. The results indicate that gavage administration of ETBE is associated with a transient increase in cell proliferation in rat forestomach with an overall sub-acute NOAEL of 300 mg/kg bw/day. However this organ has no morphological equivalent in humans, and the results will therefore not be taken into account for the purposes of deriving a DNEL.

Rat serum thyroid hormone levels

Levels of triiodothyronine, thyroxine and thryoid-stimulating hormone were measured in rat serum following oral (gavage) administration of ETBE at doses levels of 0, 100, 300 or 1000 mg/kg bw/day for 2 or 4 weeks (DIMS Institute of Medical Science Inc., 2008d); sodium phenobarbital (administered via feed) served as a positive control. Treatment levels were based on those used in an initiation-promotion study where hyperplastic and neoplastic lesions of the thyroid were observed in carcinogen-initiated rats given 300 and 1000 mg ETBE /kg bw /day (DIMS Institute of Medical Science Inc., 2008a; see Carcinogenicity section). Decreases in serum T3, free T3, T4, and free T4 were observed in rats given 1000 mg/kg bw/day ETBE, however levels in the positive control group were unchanged (reason unclear). Absolute and relative thyroid weights were decreased by around 20% after 4 wk treatment with 1000 mg/kg bw/day ETBE but not after 2 weeks treatment or at lower exposures. Absolute and relative liver weights were significantly increased at 300 and 1000 mg/kg bw/day following 2 weeks and 4 weeks treatment with ETBE. These changes in serum thyroid hormone levels are considered linked to enhanced metabolism by microsomal enzymes present in the liver, enzymes whose activity is increased by ETBE. While the incidence of rodent thyroid tumours is modulated via a negative feed-back through the pituitary-thyroid axis (Capen et al., 2002), it is also noted that the incidence of C-cell adenomas of the thyroid was decreased in a dose-related manner in an oral carcinogenicity study on ETBE (Japan Bioassay Research Center, 2010a; see Carcinogenicity section) suggesting that these alterations in thyroid hormone status are of doubtful toxicological relevance, and therefore not relevant for the purposes of DNEL setting.

In a Study to elucidate possible mode of action (MOA) and human relevance of hepatotumorigenicity in rats for ethyl tertiary butyl ether (ETBE). Male F344 rats were administered ethyl tert-butyl ether (ETBE) at doses of 0, 150 and 1000 mg/kg bodyweight twice a day by gavage for 1 and 2 weeks. For comparison, non-genotoxic carcinogen phenobarbital (PB) was applied at a dose of 500 ppm in diet. Significant increase of P450 total content and hydroxyl radical levels were elicited by both low and high doses of ETBE and PB treatments at weeks 1 and 2, and 8-OHdG formation at week 2, accompanied accumulation of the P450 enzymes CYP2B1/2B2, CYP3A1/3A2 and CYP2C6, and downregulation of DNA oxoguanine glycosylase 1, induction of apoptosis and cell cycle arrest in hepatocytes, respectively. Up-regulation of CYP2E1 and CYP1A1 at weeks 1 and 2, and peroxisome proliferation at week 2 were found in high dose ETBE group. Proteome analysis predicted activation of upstream regulators of gene expression altered by ETBE including constitutive androstane receptor (CAR), pregnane-X-receptor (PXR) and peroxisome proliferator-activated receptors (PPARs). These results indicate that the Mode of Action (MOA) of ETBE hepato-tumorigenicity in rats may be related to induction of oxidative stress, 8-OHdG formation, subsequent cell cycle arrest, and apoptosis, suggesting regenerative cell proliferation after week 2, predominantly via activation of CAR and PXR nuclear receptors by a mechanism similar to that of PB, and differentially by activation of PPARs. The MOA for ETBE hepato-tumorigenicity in rats is unlikely to be relevant to humans.

Capen, C, DeLellis, RA, Yarrington, JT (2002). Endocrine system. In: Handbook of Toxicological Pathology, Haschek, WM, Rousseaux, CG and Wallig, MA (eds), Vol 2, Academic Press, San Diego, CA.