Toxicity and carcinogenicity of t-butyl alcohol in rats and mice following chronic exposure in drinking water

Methyl-tertiary-butyl-ether (MTBE) misclassified

Methyl-tertiary-butyl-ether (MTBE) was introduced into motor fuels in 1992 to reduce carbon monoxide automotive emissions in areas where the National Ambient Air Quality Standards for CO were exceeded. At a meeting of the National Toxicology Program's Board of Scientific Counselors (2-3 December 1998), data were presented showing that exposure to MTBE caused increased incidence of liver tumors, renal adenomas, carcinomas and interstitial cell adenomas of the testes in male, and lymphomas and leukemia in female CD1 mice [National Toxicology Program, 1998]. Despite this evidence, the NTP Board defeated a motion to list MTBE as "Reasonably anticipated to be a human carcinogen" by a vote of 6 to 5. This decision directly contravenes rules and procedures previously established by NTP for assessing carcinogenicity of chemical compounds. Good public health policy dictates that the NTP Board conduct another review of MTBE with proper consideration of the criteria that have been established for listing agents as carcinogens. Millions of Americans who are exposed daily to this chemical deserve an unbiased evaluation of carcinogenic agents being introduced into the environment.

Evaluation of the in vivo interaction of methyl tert-butyl ether with alpha2u-globulin in male F-344 rats

Methyl tert-butyl ether (MTBE), a fuel additive blended into unleaded gasoline, decreases emissions of selected air pollutants. Exposure to MTBE causes a low incidence of renal tumors in male, but not female, F-344 rats. A number of chemicals that cause male rat-specific renal tumors also cause a syndrome unique to male rats referred to as alpha2u-globulin (alpha2u) nephropathy (alpha2u-N). Previous investigations have demonstrated that MTBE exposure induces a mild accumulation of alpha2u in male F-344 rats. The objective of the present study was to determine if MTBE, or a metabolite of MTBE, interacts with alpha2u in male rats administered MTBE orally. Eleven-week-old male and female F-344 rats were administered 750 mg [14C]MTBE/kg body wt or an equivalent volume of 10% emulphor orally for 4 consecutive days. Although [14C]MTBE-treated male rats exhibited a statistically significant increase in renal alpha2u concentration, the total radioactivity recovered was similar in kidney samples from [14C]MTBE-treated male and female rats. Further analysis of kidney cytosol prepared from [14C]MTBE-treated rats revealed that a slightly greater percentage of radioactivity coeluted on a G-25 gel filtration column with the total protein fraction from male rats than from female rats. Gel filtration (Sephadex G-75 column) and anion exchange chromatography, however, did not demonstrate any coelution of MTBE-derived radioactivity with the low-molecular-weight protein fraction or alpha2u fraction, respectively, in kidney cytosol prepared from [14C]MTBE-treated male or female rats. Further experimentation using a sealed vial equilibration system demonstrated that d-limonene oxide, a chemical with a high affinity for alpha2u, displaced MTBE in male, but not female, rat kidney samples following administration of MTBE. These findings provide indirect evidence that MTBE interacts with a male-specific protein such as alpha2u in male F-344 rats. Since the pathogenesis of alpha2u-N is dependent on the formation of a reversibly bound chemical-alpha2u complex, demonstration of an in vivo interaction of MTBE or one of its metabolites with alpha2u supports the alpha2u mechanism as a cause of MTBE-induced protein droplet nephropathy in male rats.

Potential health effects of gasoline and its constituents: A review of current literature (1990-1997) on toxicological data

We reviewed toxicological studies, both experimental and epidemiological, that appeared in international literature in the period 1990-1997 and included both leaded and unleaded gasolines as well as their components and additives. The aim of this overview was to select, arrange, and present references of scientific papers published during the period under consideration and to summarize the data in order to give a comprehensive picture of the results of toxicological studies performed in laboratory animals (including carcinogenic, teratogenic, or embryotoxic activity), mutagenicity and genotoxic aspects in mammalian and bacterial systems, and epidemiological results obtained in humans in relation to gasoline exposure. This paper draws attention to the inherent difficulties in assessing with precision any potential adverse effects on health, that is, the risk of possible damage to man and his environment from gasoline. The difficulty of risk assessment still exists despite the fact that the studies examined are definitely more technically valid than those of earlier years. The uncertainty in overall risk determination from gasoline exposure also derives from the conflicting results of different studies, from the lack of a correct scientific approach in some studies, from the variable characteristics of the different gasoline mixtures, and from the difficulties of correctly handling potentially confounding variables related to lifestyle (e.g., cigarette smoking, drug use) or to preexisting pathological conditions. In this respect, this paper highlights the need for accurately assessing the conclusive explanations reported in scientific papers so as to avoid the spread of inaccurate or misleading information on gasoline toxicity in nonscientific papers and in mass-media messages.

Methyl tert butyl ether systemic toxicity

In male F344 rats exposed in a chronic inhalation study to methyl tertiary butyl ether (MTBE) a treatment related increase in severity of chronic nephropathy and mortality and an increase in hyaline droplets in the kidney were noted. Liver weights were increased in both rats and mice but no histological lesions other than hypertrophy are seen. Transient CNS effects but no indications of permanent nervous system effects were noted. MTBE is not a reproductive or developmental hazard. MTBE is rapidly absorbed. MTBE with some metabolite, tertiary butyl alcohol (TBA) and a little CO2 are excreted in the air. The urinary excretion products in animals are TBA metabolites, while in humans the urinary excretion products are MTBE and TBA. A comparison of the systematic responses of the possible metabolites TBA and formaldehyde indicate that they are not responsible for toxicity associated with MTBE, except that TBA may be partially responsible for the kidney effects reported. Animals and humans are similar in the uptake and excretion though with some differences in metabolism of MTBE. This supports the use of the animal data as a surrogate for humans.

Carcinogenicity studies on MTBE: critical review and interpretation

Chronic inhalation of toxic concentrations of MTBE caused renal tubular cell neoplasms in male Fischer 344 rats and hepatocellular adenomas in female CD-1 mice. In Sprague-Dawley rats the oral administration of MTBE was associated with increased incidences of Leydig cell tumors and of lymphomas and leukemias (combined) in males and females, respectively. Neither lymphomas nor leukemias were individually increased in treated females. leydig cell tumors are common in rats and do not predict human responses to drugs and chemicals. Neither MTBE nor its metabolite, t-butyl alcohol, possess mutagenic potential and a second metabolite, formaldehyde, is mutagenic in vitro but in vivo results are equivocal. MTBE-induced neoplasms are most likely produced through a nongenetic mechanism which requires chronic exposure to toxic doses. Because of the intense odor (and taste) of MTBE, humans will not tolerate either air or water concentrations sufficient to produce the cytotoxic precursors required to promote cellular proliferation.

Pharmacokinetics of tertiary butyl alcohol in male and female Fischer 344 rats

Tertiary butyl alcohol (TBA) is a small aliphatic alcohol with multiple industrial and scientific uses. A comprehensive pharmacokinetic profile for TBA has not been determined in rats. The purpose of this study was to fully characterize the pharmacokinetics of TBA in male and female F-344 rats following intravenous administration of 37.5, 75, 150 and 300 mg/kg TBA. TBA was observed to undergo a rapid distribution phase followed by a slower elimination phase. The steady-state volume of distribution for TBA was roughly 4.5 times greater than total body water, and the clearance was lower than the estimated glomerular filtration rate. The elimination of TBA appears to saturate at higher doses, as evidenced by a disproportional increase in area under the concentration-time curve and decreased rate of clearance.