Results of long-term experimental studies on the carcinogenicity of methyl tert-butyl ether

A Survey of Gasoline Ameliorator, Methyl-Tert-Butyl Ether (MTBE) on Bovine Serum Albumin: A Spectroscopy and Molecular Dynamic Simulation Study

Background

Methyl-Tert-Butyl Ether (MTBE) as a gasoline modifier is frequently added to fuels and used in plenty of worldwide applications. MTBE biodegradation in groundwater occurs slowly and produces water miscibility; therefore, it causes diverse environmental and human health concerns.

Objectives

The interaction of MTBE with bovine serum albumin (BSA) as a model protein at physiological conditions is investigated to illustrate the possible interactions of MTBE with the body's proteins.

Materials and Methods

Uv-visible, fluorescence, circular dichroism (CD) spectroscopy methods, and molecular modeling were used to analyze the MTBE's effect on BSA structure and dynamics. The constant protein concentration and various MTBE contents were used for possible interactions.

Results

The protein structural analysis shows that MTBE binds to BSA via positive enthalpy and entropy via hydrophobic interactions. Molecular docking shows the participation of several amino acids in the MTBE-BSA interaction. The CD spectroscopy results show that the BSA structure was not changed in the MTBE concentrations utilized in the study. Molecular dynamics (MD) simulation results suggest that MTBE can slightly change protein structure in the last 50ns.

Conclusion

Comparing experimental and MD simulation results demonstrated that the BSA secondary structure was maintained in the low concentration of the MTBE. The entropy and enthalpy parameters asserted the hydrophobic interaction was the major force in the interaction between the BSA and MTBE.

A Comparative Cancer Risk Evaluation of MTBE and Other Compounds (Including Naturally Occurring Compounds) in Drinking Water in New Hampshire

Methyl tert-butyl ether (MTBE) was added to gasoline in New Hampshire (NH) between 1995 and 2006 to comply with the oxygenate requirements of the 1990 Amendments to the Clean Air Act. Leaking tanks and spills released MTBE into groundwater, and as a result, MTBE has been detected in drinking water in NH. We conducted a comparative cancer risk assessment and a margin-of-safety (MOS) analysis for several constituents, including MTBE, detected in NH drinking water. Using standard risk assessment methods, we calculated cancer risks from exposure to 12 detected volatile organic compounds (VOCs), including MTBE, and to four naturally occurring compounds (i.e., arsenic, radium-226, radium-228, and radon-222) detected in NH public water supplies. We evaluated exposures to a hypothetical resident ingesting the water, dermally contacting the water while showering, and inhaling compounds volatilizing from water in the home. We then compared risk estimates for MTBE to those of the other 15 compounds. From our analysis, we concluded that the high-end cancer risk from exposure to MTBE in drinking water is lower than the risks from all the other VOCs evaluated and several thousand times lower than the risks from exposure to naturally occurring constituents, including arsenic, radium, and radon. We also conducted an MOS analysis in which we compared toxicological points of departure to the NH maximum contaminant level (MCL) of 13 µg/L. All of the MOSs were greater than or equal to 160,000, indicating a large margin of safety and demonstrating the health-protectiveness of the NH MCL for MTBE.

An investigation of methyl tert‑butyl ether‑induced cytotoxicity and protein profile in Chinese hamster ovary cells

Methyl tert-butyl ether (MTBE) is widely used as an oxygenating agent in gasoline to reduce harmful emissions. However, previous studies have demonstrated that MTBE is a cytotoxic substance that has harmful effects in vivo and in vitro. Although remarkable progress has been made in elucidating the mechanisms underlying the MTBE-induced reproductive toxicological effect in different cell lines, the precise mechanisms remain far from understood. The present study aimed to evaluate whether mammalian ovary cells were sensitive to MTBE exposure in vitro by assessing cell viability, lactate dehydrogenase (LDH) leakage, malondialdehyde (MDA) content and antioxidant enzyme activities. In addition, the effect of MTBE exposure on differential protein expression profiles was examined by two-dimensional electrophoresis and matrix-assisted laser desorption/ionization-time of flight mass spectrometry. MTBE exposure induced significant effects on cell viability, LDH leakage, plasma membrane damage and the activity of antioxidant enzymes. In the proteomic analysis, 24 proteins were demonstrated to be significantly affected by MTBE exposure. Functional analysis indicated that these proteins were involved in catalytic activity, binding, structural molecule activity, metabolic processes, cellular processes and localization, highlighting the fact that the cytotoxic mechanisms resulting from MTBE exposure are complex and diverse. The altered expression levels of two representative proteins, heat shock protein family A (Hsp70) members 8 and 9, were further confirmed by western blot analysis. The results revealed that MTBE exposure affects protein expression in Chinese hamster ovary cells and that oxidative stress and altered protein levels constitute the mechanisms underlying MTBE-induced cytotoxicity. These findings provided novel insights into the biochemical mechanisms involved in MTBE-induced cytotoxicity in the reproductive system.

Evaluation of toxicity of methyl tert-butyl ether (MTBE) on mouse spermatogenic cells in vitro

As a synthetic organic chemical, methyl tert-butyl ether (MTBE) is the most common fuel oxygenate. The increasing use of MTBE has raised concern over its safety. Previous studies in vivo revealed that MTBE could alter the male reproduction system. Therefore, the current experiments were designed to evaluate whether isolated mice spermatogenic cells in vitro were sensitive to exposure to MTBE at environmental levels, and to evaluate whether spermatogenic cells had undergone changes in morphologic, activity and viability parameters after exposure to MTBE. Spermatogenic cells in vitro were incubated with medium alone (control), 100 ppb, 10 ppm, 1000 ppm, 3000 ppm MTBE, respectively, for 6, 12, 18 h. MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazo liumbromide) assay, staining with fluorescein diacetate (FDA) and propidium iodide (PI), and flow cytometric analyses were used to assess MTBE toxicity on cells and DNA. The results showed that there were no significant differences between control and treatments of ≤ 1000 ppm MTBE at the same time point. Although 3000 ppm MTBE could exert toxic effects directly on spermatogenic cells, environmental levels of MTBE did not exert toxic effects on cultured spermatogenic cells.

Interaction of insulin with methyl tert-butyl ether promotes molten globule-like state and production of reactive oxygen species

Interaction of methyl tert-butyl ether (MTBE) with proteins is a new look at its potential adverse biological effects. When MTBE is released to the environment it enters the blood stream through inhalation, and could affect the properties of various proteins. Here we investigated the interaction of MTBE with insulin and its effect on insulin structural changes. Our results showed that insulin formed a molten globule (MG)-like structure in the presence of 8 μM MTBE under physiological pH. The insulin structural changes were studied using spectroscopy methods, viscosity calculation, dynamic light scattering and differential scanning calorimetry. To delineate the mechanisms involved in MTBE-protein interactions, the formation of reactive oxygen specious (ROS) and formation of protein aggregates were measured. The chemiluminscence experiments revealed an increase in ROS production in the presence of MTBE especially in the MG-like state. These results were further confirmed by the aggregation tests, which indicated more aggregation of insulin at 40 μM MTBE compared with 8 μM. Thus, the formation of initial aggregates and exposure of the hydrophobic patches upon formation of the MG-like state in the presence of MTBE drives protein oxidation and ROS generation.

No Promoting Effect of Ethyl Tertiary-butyl Ether (ETBE) on Rat Urinary Bladder Carcinogenesis Initiated with N-Butyl-N-(4-hydroxybutyl)nitrosamine

The effects of ethyl tertiary-butyl ether (ETBE) on two-stage urinary bladder carcinogenesis in male F344 rats initiated with N-butyl-N-(4-hydroxybutyl)nitrosamine (BBN) were investigated at various dose levels with regard to possible promoting activity. Groups of 30 rats were given drinking water containing 500 ppm BBN, as an initiator, for 4 weeks and starting one week thereafter received ETBE by gavage (daily, 7 days/week) at dose levels of 0 (control), 100, 300, 500 or 1000 mg/kg/day until experimental week 36. No statistically significant differences in incidences of preneoplastic lesions, papillomas, and carcinomas of the urinary bladder were evident in rats treated with 100–1000 mg/kg/day ETBE as compared with control values. Furthermore, the average numbers of preneoplastic or neoplastic lesions per unit length of basement membrane in rats given 100–1000 mg/kg/day ETBE were also comparable to control values. However, papillomatosis of the urinary bladder was found in 4 out of 30 rats (13%) in the group given 1000 mg/kg/day ETBE, and soft stones in the urinary bladder were found in 3 out of these 4 rats. The results thus demonstrated that ETBE did not exert promotional activity on urinary bladder carcinogenesis. However, papillomatosis of the urinary bladder developed in small numbers of the rats given ETBE at 1000 mg/kg/day but not in rats given 500 mg/kg/day or lower doses.

Scientific considerations for evaluating cancer bioassays conducted by the Ramazzini Institute

BACKGROUND

The Ramazzini Institute (RI) has completed nearly 400 cancer bioassays on > 200 compounds. The European Food Safety Authority (EFSA) and others have suggested that study design and protocol differences between the RI and other laboratories by may contribute to controversy regarding cancer hazard findings, principally findings on lymphoma/leukemia diagnoses.

OBJECTIVE

We aimed to evaluate RI study design, protocol differences, and accuracy of tumor diagnoses for their impact on carcinogenic hazard characterization.

METHODS

We analyzed the findings from a recent Pathology Working Group (PWG) review of RI procedures and tumor diagnoses, evaluated consistency of RI and other laboratory findings for chemicals identified by the RI as positive for lymphoma/leukemia, and examined evidence for a number of other issues raised regarding RI bioassays. The RI cancer bioassay design and protocols were evaluated in the context of relevant risk assessment guidance from international authorities.

DISCUSSION

Although the PWG identified close agreement with RI diagnoses for most tumor types, it did not find close agreement for lymphoma/leukemia of the respiratory tract or for neoplasms of the inner ear and cranium. Here we discuss a) the implications of the PWG findings, particularly lymphoma diagnostic issues; b) differences between RI studies and those from other laboratories that are relevant to evaluating RI cancer bioassays; and c) future work that may help resolve some concerns.

CONCLUSIONS

We concluded that a) issues related to respiratory tract infections have complicated diagnoses at that site (i.e., lymphoma/leukemia), as well as for neoplasms of the inner ear and cranium, and b) there is consistency and value in RI studies for identification of other chemical-related neoplasia.

Tertiary-Butanol: a toxicological review

Tert-Butanol is an important intermediate in industrial chemical synthesis, particularly of fuel oxygenates. Human exposure to tert-butanol may occur following fuel oxygenate metabolism or biodegradation. It is poorly absorbed through skin, but is rapidly absorbed upon inhalation or ingestion and distributed to tissues throughout the body. Elimination from blood is slower and the half-life increases with dose. It is largely metabolised by oxidation via 2-methyl-1,2-propanediol to 2-hydroxyisobutyrate, the dominant urinary metabolites. Conjugations also occur and acetone may be found in urine at high doses. The single-dose systemic toxicity of tert-butanol is low, but it is irritant to skin and eyes; high oral doses produce ataxia and hypoactivity and repeated exposure can induce dependence. Tert-Butanol is not definable as a genotoxin and has no effects specific for reproduction or development; developmental delay occurred only with marked maternal toxicity. Target organs for toxicity clearly identified are kidney in male rats and urinary bladder, particularly in males, of both rats and mice. Increased tumour incidences observed were renal tubule cell adenomas in male rats and thyroid follicular cell adenomas in female mice and, non-significantly, at an intermediate dose in male mice. The renal adenomas were associated with alpha(2u)-globulin nephropathy and, to a lesser extent, exacerbation of chronic progressive nephropathy. Neither of these modes of action can function in humans. The thyroid tumour response could be strain-specific. No thyroid toxicity was observed and a study of hepatic gene expression and enzyme induction and thyroid hormone status has suggested a possible mode of action.

Commentary: further comments on Mycoplasma pulmonis and lymphoma in bioassays of rats

The authors recently assessed the likelihood that lifetime cancer bioassays of aspartame, methanol, and methyl tertiary butyl ether conducted with conventional (not specific pathogen free) Sprague-Dawley rats were compromised by Mycoplasma pulmonis disease. From the tumor data and other information, the authors concluded that the rats used in these bioassays likely had M pulmonis disease and that lesions of the disease were plausibly interpreted as lymphoma. Subsequently, they analyzed the nonneoplastic lesion data from these bioassays for occurrence of inflammatory lesions and found that 2,267 of 2,960 rats (76.6%) were reported to have bronchitis, the signature lesion of M pulmonis disease, and that 633 rats (21.4%) were reported to have otitis, another common lesion of the disease. Also, documentation is now available containing serologic evidence of mycoplasma infection in the rats. In contrast, the reports of 6 National Toxicology Program bioassays based on specific pathogen-free Sprague-Dawley rats listed no instances of bronchitis or otitis. These findings provide substantial additional evidence that the bioassays in question were compromised by M pulmonis disease. Therefore, the reported induction of lymphoma in these studies should not be considered in cancer risk assessments. The authors also found that inflammatory lesions were prevalent in lymph nodes, thymus, pleura, and brain. Finally, they found that of all 328 cases of lymphoimmunoblastic lymphoma affecting the lung (the primary form of lymphoma reported), 218 (66.5%) occurred within the first 104 weeks of the studies, showing that occurrence of such lesions was not due to appearance in rats surviving beyond that interval.

MTBE: A poster child for exposure assessment as central to effective TSCA reform

Advanced exposure assessment is vital to improving the Toxic Substances Control Act.

Applications of the Poly-K Statistical Test to Life-Time Cancer Bioassay Studies

The statistical analysis of cancer bioassay data has historically depended on the pathological determination of the experimental animal's cause of death. The poly-k statistical test has provided a method of statistical analysis of animal bioassay data without the need for cause of death information. The test has been shown to have good statistical properties in the typical 2-year cancer bioassay. However, while the poly-k test has been applied to chronic lifetime animal studies, it has not been formally evaluated with respect to the operating characteristics of this statistical test when applied to such studies. Thus, our objective is to assess the performance of the poly-k test for lifetime studies and to make comparisons with other tests. We observed in one recent lifetime study of the gasoline additive MTBE that the application of the poly-k test was not statistically robust. Simulation studies were subsequently conducted for a limited number of scenarios of lifetime cancer bioassays. These simulations showed that the poly-k test is not statistically robust for testing effect of increasing dose in some lifetime cancer studies.

"ToxRTool", a new tool to assess the reliability of toxicological data

Evaluation of the reliability of toxicological data is of key importance for regulatory decision-making. In particular, the new EU Regulations concerning the registration, evaluation, authorisation and restriction of chemicals (REACH) and classification, labelling and packaging (CLP) according to the new globally harmonised system (GHS) rely on the integration of all available toxicological information. The so-called Klimisch categories, although well established and widely used, lack detailed criteria for assigning data quality to categories. A software-based tool (ToxRTool) was developed within the context of a project funded by the European Commission to provide comprehensive criteria and guidance for reliability evaluations of toxicological data. It is applicable to various types of experimental data, endpoints and studies (study reports, peer-reviewed publications) and leads to the assignment to Klimisch categories 1, 2 or 3. The tool aims to increase transparency and to harmonise approaches of reliability assessment. The tool consists of two parts, one to evaluate in vivo and one to evaluate in vitro data. The prototypes of the tool were tested in two independent inter-rater experiments. This approach allowed the analysis of the performance of the tool in practice and the identification and minimisation of sources of heterogeneity in evaluation results. The final version, ToxRTool, is publicly available for testing and use.

Differential toxic effects of methyl tertiary butyl ether and tert-butanol on rat fibroblasts in vitro

Methyl tertiary butyl ether (MTBE) is the most widely used motor vehicle fuel oxygenate since it reduces harmful emissions due to gasoline combustion. However, the significant increase in its use in recent years has raised new questions related to its potential toxicity. In fact, although available data are somehow conflicting, there is evidence that MTBE is a toxic substance that may have harmful effects on both animals and humans and an unresolved problem is the role played by MTBE metabolites, especially tertiary butyl alcohol (TBA), in determining toxic effects due to MTBE exposure. In this study, the toxic effects of MTBE have been analyzed on a normal diploid rat fibroblast cell line (Rat-1) and compared to the effects of TBA. The results obtained suggest that both MTBE and TBA inhibit cell growth in vitro but with different mechanisms in terms of effects on the cell cycle progression and on the modulation of cell cycle regulatory proteins. In fact, MTBE caused an accumulation of cells in the S-phase of the cell cycle, whereas TBA caused an accumulation in the G0/G1-phase with different effects on the expression of cyclin D1, p27Kip1, and p53. Moreover, both MTBE and TBA were also shown to induce DNA damage, as assessed in terms of oxidative DNA damage and nuclear DNA fragmentation, that appeared to be susceptible of repair by the cell DNA-repair machinery. In conclusion, these findings suggest that both MTBE and TBA can exert, by acting through different molecular mechanisms, important biological effects on fibroblasts in vitro. Further studies are warranted to shed light on the mechanisms responsible for the observed effects and on their potential significance for the in-vivo exposure.

Physiologically based pharmacokinetic model of methyl tertiary butyl ether and tertiary butyl alcohol dosimetry in male rats based on binding to alpha2u-globulin

Current physiologically based pharmacokinetic (PBPK) models for the fuel additive methyl tertiary butyl ether (MTBE) and its metabolite tertiary butyl alcohol (TBA) have not included a mechanism for chemical binding to the male rat-specific protein alpha2u-globulin, which has been postulated to be responsible for renal effects in male rats observed in toxicity and carcinogenicity studies with MTBE. The objective of this work was to expand the previously published models for MTBE to include binding to alpha2u-globulin in the kidney of male rats. In the model, metabolism of MTBE was assumed to occur only in the liver via two saturable pathways. TBA metabolism was assumed to occur only in the liver via one saturable, low-affinity pathway and to be inducible following repeated exposures. The binding of MTBE and TBA to alpha2u-globulin was modeled as saturable and competitive and was assumed to only affect the rate of hydrolysis of alpha2u-globulin in the kidney. The developed model characterized the differences in kidney concentrations of MTBE and TBA in male versus female rats from inhalation exposures to MTBE, as well as the observed changes in blood and tissue concentrations from repeated exposure to TBA. The model-predicted binding affinity of MTBE to alpha2u-globulin was greater than TBA, and the hydrolysis rate of chemically bound alpha2u-globulin was approximately 30% of the unbound protein. This PBPK model supports the role of MTBE and TBA binding to the male rat-specific protein alpha2u-globulin as essential for predicting concentrations of these chemicals in the kidney following exposure.

Effects of subchronic methyl tert-butyl ether ether exposure on male Sprague-Dawley rats

Methyl tert-butyl ether (MTBE) is an additive used to oxygenate gasoline to improve air quality by reducing tailpipe emissions of carbon monoxide and ozone precursors. Although several toxicity studies in rats have been conducted to examine the acute, subchronic, and chronic toxicities by employing various routes of exposure to MTBE, few data were available on the effects of MTBE exposure on blood. In this study, MTBE was administered to rats at dose levels of 0, 400, 800, and 1600 mg/kg/day, respectively. After 2- or 4-weeks treatment period, rats were euthanized and blood was collected for the assay of hematological indicators and blood biochemistry indicators. Some organs, including brain, heart, liver, spleen, lung, kidneys, testes, epididymis, thymus, and prostate, were immediately removed and weighed. Possible subchronic health effects of MTBE exposure by gavage were evaluated on mortality, body weight, relative organ weight, hematology, and blood biochemistry indicators in male Sprague-Dawley rats. The results indicated that MTBE did not disrupt the growth rate of rats. Relative organ weight showed change in heart, liver, kidney, testes, thymus, and prostate. In the 2-week treatment, MTBE exerted toxicity on white blood cell count, including lymphocyte, granulocyte, and eosinophil. This finding was especially strong at 1600 mg/kg/day MTBE. In the 4-week treatment, hemoglobin at high dose MTBE significantly increased. The results of the assay for the biochemistry indicators and relative organ weight indicated that MTBE could impair liver and kidney functions and also have adverse effects on lipid metabolism and immune system. It was conducted that subchronic MTBE exposure induced the adverse effects occurring in the relative organ weight, the hematological indicators, and the biochemistry indicators under high MTBE dose.

Methyltertiary-Butyl Ether: Studies for Potential Human Health Hazards

When methyl tertiary-butyl ether (MTBE) in gasoline was first introduced to reduce vehicle exhaust emissions and comply with the Clean Air Act, in the United States, a pattern of complaints emerged characterised by seven "key symptoms." Later, carefully controlled volunteer studies did not confirm the existence of the specific key symptoms, although one study of self-reported sensitive (SRS) people did suggest that a threshold at about 11-15% MTBE in gasoline may exist for SRSs in total symptom scores. Neurobehavioral and psychophysiological studies on volunteers, including SRSs, found no adverse responses associated with MTBE at likely exposure levels. MTBE is well and rapidly absorbed following oral and inhalation exposures. Cmax values for MTBE are achieved almost immediately after oral dosing and within 2 h of continuous inhalation. It is rapidly eliminated, either by exhalation as unchanged MTBE or by urinary excretion of its less volatile metabolites. Metabolism is more rapid humans than in rats, for both MTBE and tert-butyl alcohol (TBA), its more persistent primary metabolite. The other primary metabolite, formaldehyde, is detoxified at a rate very much greater than its formation from MTBE. MTBE has no specific effects on reproduction or development, or on genetic material. Neurological effects were observed only at very high concentrations. In carcinogenicity studies of MTBE, TBA, and methanol (included as an endogenous precursor of formaldehyde, without the presence of TBA), some increases in tumor incidence have been observed, but consistency of outcome was lacking and even some degree of replication was observed in only three cases, none of which had human relevance: alpha(2u)-globulin nephropathy-related renal tubule cell adenoma in male rats; Leydig-cell adenoma in male rats, but not in mice, which provide the better model of the human disease; and B-cell-derived lymphoma/leukemia of doubtful pathogenesis that arose mainly in lungs of orally dosed female rats. In addition, hepatocellular adenomas were significantly higher in female CD-1 mice and thyroid follicular-cell adenomas were increased in female B6C3F1 mice treated with TBA, but these results lack any independent confirmation, which would have been possible from a number of other studies.

Epidemiology, toxicokinetics, and health effects of methyl tert-butyl ether (MTBE)

This paper reviews the published information assessing the kinetics and potential for adverse health effects related to exposure to the fuel oxygenate, methyl tert-butyl ether (MTBE). Data were obtained from previously published reports, using human data where possible. If human data were not available, animal studies were cited. The kinetic profile of MTBE in humans is similar for ingestion and inhalation. The concentrations of MTBE to which the general public is expected to be exposed are orders of magnitude below concentrations that have caused adverse health effects in animals. Controlled human studies have not replicated early epidemiology studies that suggested, but did not confirm, a possible association between MTBE exposure and nonspecific health complaints.

Methyl tert-butyl ether (MTBE) induced Ca(2+)-dependent cytotoxicity in isolated rabbit tracheal epithelial cells

As a volatile synthetic organic chemical, methyl tert-butyl ether (MTBE) was the most common gasoline additive. The increasing use of MTBE raised concern over its health safety. Inhalation was the principle route of exposure for the general population. This study used a model of rabbit tracheal epithelial cells (RTEs) in primary culture to investigate the cytotoxic effects induced by MTBE and the potential mechanism. RTEs were incubated with medium alone (control), 0.5, 50, 5000ppm MTBE respectively. MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazo liumbromide) assay, staining with fluorescein diacetate, propidium iodide and lactate dehydrogenase leakage ratio were used to assess MTBE cytotoxicity on cells. We also observed a significant elevation in cytosolic Ca2+ by fluorescence probe Fluo-3AM at 3, 6 and 12h following exposure to MTBE. Loss of mitochondrial membrane potential (MMP) was detected following 12 and 24h treatment of NP and assessment by rhodamine 123 (Rh123) staining. Activity changes of the Ca(2+)-ATPase, Ca(2+)-Mg(2+)-ATPase following MTBE treatment displayed a similar trend, suggesting an initial elevation before 6h and subsequent dramatic decrease at 12h. Our results demonstrated that induction of cell injury, associated with mitochondrial dysfunction, and alterations in cytosolic Ca2+ in RTEs represent key mechanisms by which MTBE exerts its cytotoxic effects.

Effects of MTBE on the reported incidence of Leydig cell tumors in Sprague-Dawley rats: range of possible Poly-3 results

An increased Leydig cell tumor (LCT) incidence has been reported in a study of Sprague-Dawley (SD) rats exposed via gavage to 1000 (but not 250)mg/kgday MTBE; it is unclear, however, if this finding was indeed dose-related or due to the statistical analyses not having adequately accounted for the increased survival rate in the high-dose animals and/or for multiple statistical comparisons. To address this question, we conducted Hoel-Walburg and Poly-3 analyses, using p-values of 0.01 for pair-wise comparisons and 0.005 for trend tests of common tumors. We found that MTBE does not cause a statistically significant increase in LCTs in SD rats when survival is appropriately taken into account. In addition, the original study reported some overall survival data, but did not specify which rats had LCTs. This led us to conduct separate Poly-3 analyses for the most extreme scenarios of survival age and tumor incidence to provide an illustrative example of approaches for analyzing the impact of survival rates on tumor findings in the absence of animal-specific survival data. We found this method to provide results similar to analyses using the actual data, suggesting that it can be used when full survival data are not available.

Benzene-induced cancers: abridged history and occupational health impact

Benzene-induced cancer in humans was first reported in the late 1920s. Carcinogenesis findings in animals were not reported conclusively until 1979. Industry exploited this "discrepancy" to discredit the use of animal bioassays as surrogates for human exposure experience. The cardinal reason for the delay between first recognizing leukemia in humans and sought-after neoplasia in animals centers on poor design and conduct of experimental studies. The first evidence of carcinogenicity in animals manifested as malignant tumors of the zymbal glands (sebaceous glands in the ear canal) of rats, and industry attempted to discount this as being irrelevant to humans, as this organ is vestigial and not present per se in humans. Nonetheless, shortly thereafter benzene was shown to be carcinogenic to multiple organ sites in both sexes of multiple strains and multiple species of laboratory animals exposed via various routes. This paper presents a condensed history of the benzene bioassay story with mention of benzene-associated human cancers.

Ethyl tertiary-butyl ether: a toxicological review

A number of oxygenated compounds (oxygenates) are available for use in gasoline to reduce vehicle exhaust emissions, reduce the aromatic compound content, and avoid the use of organo-lead compounds, while maintaining high octane numbers. Ethyl tertiary-butyl ether (ETBE) is one such compound. The current use of ETBE in gasoline or petrol is modest but increasing, with consequently similar trends in the potential for human exposure. Inhalation is the most likely mode of exposure, with about 30% of inhaled ETBE being retained by the lungs and distributed around the body. Following cessation of exposure, the blood concentration of ETBE falls rapidly, largely as a result of its metabolism to tertiary-butyl alcohol (TBA) and acetaldehyde. TBA may be further metabolized, first to 2-methyl-1,2-propanediol and then to 2-hydroxyisobutyrate, the two dominant metabolites found in urine of volunteers and rats. The rapid oxidation of acetaldehyde suggests that its blood concentration is unlikely to rise above normal as a result of human exposure to sources of ETBE. Single-dose toxicity tests show that ETBE has low toxicity and is essentially nonirritant to eyes and skin; it did not cause sensitization in a maximization test in guinea pigs. Neurological effects have been observed only at very high exposure concentrations. There is evidence for an effect of ETBE on the kidney of rats. Increases in kidney weight were seen in both sexes, but protein droplet accumulation (with alpha(2u)-globulin involvement) and sustained increases in cell proliferation occurred only in males. In liver, centrilobular necrosis was induced in mice, but not rats, after exposure by inhalation, although this lesion was reported in some rats exposed to very high oral doses of ETBE. The proportion of liver cells engaged in S-phase DNA synthesis was increased in mice of both sexes exposed by inhalation. ETBE has no specific effects on reproduction, development, or genetic material. Carcinogenicity studies have been conducted with ETBE, TBA, and ethanol (included in this review as an endogenous precursor of acetaldehyde in the absence of TBA). A single experiment with ETBE in rats and several experiments with ethanol in rats and mice were not considered adequate for an evaluation of ETBE carcinogenicity. In male rats only, TBA induced alpha(2u)-globulin nephropathy-related renal tubule adenomas. These are generally considered to have no human relevance. In addition, increases in thyroid follicular cell adenoma incidence were associated with TBA treatment in female mice. This result lacks independent confirmation and is not supported by experiments in which similar or higher internal doses of TBA were delivered.

Methyl tert-butyl ether (MTBE)-induced cytotoxicity and oxidative stress in isolated rat spermatogenic cells

Methyl tert-butyl ether (MTBE) is a class of synthetic organic chemical. In the USA, MTBE pollution is regarded as a serious environmental problem. The objective of the present study was to investigate the cytotoxic effects and oxidative stress induced by MTBE in isolated rat spermatogenic cells. In cytotoxic experiments, spermatogenic cells isolated from the testes of adult Sprague-Dawley rats by a mechanical procedure without the use of trypsin were incubated with medium alone (control), 0.5, 5, 50 mm MTBE, respectively, for 6, 12 and 18 h. MTT assay, staining with fluorescein diacetate (FDA) and propidium iodide (PI) and flow cytometric analyses were used. In oxidative stress experiments, the spermatogenic cells were incubated with medium alone (control) and with 0.5, 50 microm, 5 mm MTBE. For 1, 2, 6, 12, 18 h incubation, ROS production was tested using a 2',7'-dichlorofluorescein diacetate (DCHF-DA) probe; for 1, 3, 6, 12, 18 h incubation, cytosolic superoxide dismutase (SOD) and extracellular SOD (SOD(EX)) activity was assessed; and for 18 h incubation, lipid peroxidation was assessed. The results showed that MTBE at high doses significantly decreased the spermatogenic cell viability and increased plasma membrane damage and the ratio of necrotic cells compared with the control. Assessment of the MTBE-induced oxidative stress revealed that MTBE increased the production of reactive oxygen species (ROS) and enhanced lipid peroxidation. In addition, although SOD(EX) activity increased at a high dose level, cytosolic SOD activity decreased. These results suggest that an increase of MTBE-induced ROS production and an enhancement of membrane lipid peroxidation may play an important role in its cytotoxicity in isolated rat spermatogenic cells.

Identification and elimination of polysiloxane curing agent interference encountered in the quantification of low-picogram per milliliter methyl tert-butyl ether in blood by solid-phase microextraction headspace analysis

Widespread use of the gasoline additive methyl tert-butyl ether (MTBE) and the subsequent human exposure that follows have led to the need to quantify MTBE in a variety of complex biological matrixes. In this work, we demonstrate our latest MTBE quantification assay for whole blood and uncover previously unidentified contamination sources that prevented routine quantification in the low picogram per milliliter (parts per trillion, ppt) range despite a sensitive and selective analytical approach. The most significant and unexpected sources of contamination were found in reagents and laboratory materials most relevant to sample preparation and quantification. In particular, significant levels of MTBE were identified in sample vial septa that use poly(dimethylsiloxane) (PDMS)-based polymers synthesized with peroxide curing agents having tert-butyl side groups. We propose that MTBE is one of the byproducts of these curing agents, which cross-link PDMS via the methyl side groups. Residual MTBE levels of approximately 20 microg/septa are seen in septa whose formulations use these curing agents. Fortunately, these levels can be significantly reduced (i.e., <0.2 ng/septa) by additional processing. Performance achieved with this sample preparation approach is demonstrated using a mass spectrometry-based method to quantify blood MTBE levels in the low-ppt range.

The uptake, distribution, metabolism, and excretion of methyl tertiary-butyl ether inhaled alone and in combination with gasoline vapor

The purpose of these studies was to evaluate the tissue uptake, distribution, metabolism, and excretion of methyl tertiary-butyl ether (MTBE) in rats and to determine the effects of coinhalation of the volatile fraction of unleaded gasoline on these parameters. Male F344 rats were exposed nose-only once for 4 h to 4, 40, or 400 ppm 14C-MTBE and to 20 and 200 ppm of the light fraction of unleaded gasoline (LFG) containing 4 and 40 ppm 14C-MTBE, respectively. To evaluate the effects of repeated inhalation of LFG on the fate of inhaled MTBE, rats were exposed for 7 consecutive days to 20 and 200 ppm LFG followed on d 8 by exposure to LFG containing 14C-MTBE. Three subgroups of rats were included for evaluation of respiratory parameters, rates and routes of excretion, and tissue distribution and elimination. MTBE and its chief metabolite, tertiary-butyl alcohol, were quantitated in blood and kidney (immediately after exposure), and the major urinary metabolites, 2-hydroxyisobutyric acid and 2-methyl-1,2- propanediol, were identified and quantified in urine. Inhalation of MTBE alone or as a component of LFG had no concentration-dependent effect on respiratory minute volume. The initial body burdens (IBBs) of MTBE equivalents achieved after 4 h of exposure to MTBE did not increase linearly with exposure concentration. MTBE equivalents rapidly distributed to all tissues examined, with the largest percentages distributed to liver. Between 40 and 400 ppm, there was a significant reduction in percentage of the IBB present in the major organs examined, both immediately and 72 h after exposure. At 400 ppm, the elimination rates of MTBE equivalents from tissues changed significantly. Furthermore, at 400 ppm there was a significant decrease in the elimination half-time of volatile organic compounds (VOCs) in breath and a significant increase in the percentage of the IBB of MTBE equivalents eliminated as VOCs in breath. LFG coexposure significantly decreased the percentage of the MTBE equivalent IBBs in tissues and increased rates of elimination of MTBE equivalents. The study results indicate that the uptake and fate of inhaled MTBE are altered upon increasing exposure levels from 4 to 400 ppm, suggesting that toxic effects observed previously upon repeated inhalation of concentrations of 400 ppm or greater may not necessarily be linearly extrapolated to effects that might occur at lower concentrations. Furthermore, coexposure to LFG, whether acute or repeated, decreases tissue burdens of MTBE equivalents and enhances the elimination rate of MTBE and its metabolites, thereby potentially reducing the toxic effects of the MTBE compared to when it is inhaled alone.

Absence of carcinogenic activity in Fischer rats and B6C3F1 mice following 103-week inhalation exposures to toluene

Toluene, methylbenzene, is used to back-blend gasoline, as a chemical intermediate, and as a solvent; more than 7 million tonnes are produced each year in the United States. Following 14-15-week toxicity studies to estimate appropriate exposure concentrations for the carcinogenesis bioassays, toxicology and carcinogenesis studies of toluene (>99% pure) were conducted by whole-body inhalation exposures of F344/N rats and B6C3F1 mice of each sex for 15 months or two years. Toluene levels were 0 (chamber controls), 600, and 1,200 ppm for rats and 0, 120, 600, and 1200 ppm for mice. Exposures were 6.5 hr/day 5 days/wk. Genetic toxicology studies using Salmonella typhimurium, mouse L5178Y lymphoma cells, and Chinese hamster ovary cells were negative. No chemically related neoplasm was found in male rats, and one nasal, two kidney, and two forestomach neoplasms observed in female rats were considered not to be associated with the toluene exposure. For mice, no biologically important increase was observed for any nonneoplastic or neoplastic lesion. Studies by others had reported carcinogenicity of toluene, especially for total malignant tumors.

Results of long-term carcinogenicity bioassays on tert-amyl-methyl-ether (TAME) and di-isopropyl-ether (DIPE) in rats

Tert-amyl-methyl ether (TAME) was administered by gavage in extra virgin olive oil solution at concentrations of 750, 250, or 0 mg/kg bw to groups of 100 male and 100 female Sprague-Dawley rats 8 weeks old at the start of the experiment. Di-isopropyl ether (DIPE) was administered in the same manner at the doses of 1000, 250, or 0 mg/kg body weight to groups of 100 male and 100 female Sprague-Dawley rats. TAME and DIPE were each delivered in 1-mL solution 4 days a week for 78 weeks. Control animals received 1 mL of extra virgin olive oil without TAME or DIPE. At the end of the treatment period, all animals were kept under observation until spontaneous death. Under these test conditions, TAME and DIPE were found to be potential carcinogenic agents for various organs and tissues.

MTBE and aromatic hydrocarbons in North Carolina stormwater runoff

A total of 249 stormwater samples were collected from 46 different sampling locations in North Carolina over an approximate 1-year period and analyzed to identify land use types where fuel oxygenates and aromatic hydrocarbons may be present in higher concentrations and at greater frequency. Samples were analyzed by gas chromatography-mass spectrometry in ion selective mode to achieve a quantitation limit of 0.05 microg/l. m-,p-Xylene and toluene were detected in over half of all samples analyzed, followed by MTBE: o-xylene: 1,3,5-trimethylbenzene: ethylbenzene; and 1,2,4-trimethylbenzene. Benzene, DIPE, TAME and 1,2,3-trimethylbenzene were detected in < 10% of the samples analyzed. Median contaminant concentrations (when detected) varied from 0.07 microg/l for ethylbenzene to 0.11 microg/l for toluene. All of the locations with significantly higher contaminant concentrations were associated with direct runoff from a gas station or discharge of contaminated groundwater from a former leaking underground storage tank. For all of the aromatic hydrocarbons, the maximum observed contaminant concentrations were over an order of magnitude lower than current drinking water standards.

Toxicology and human health effects following exposure to oxygenated or reformulated gasoline

In order to replace antiknock leaded derivatives in gasoline, legislations were enacted in the United States and other countries to find safer additives and to reduce CO, O3, and volatile organic compounds (VOCs) in non-attainment areas. Oxygenates commonly used include various alcohols and aliphatic ethers. Methyl tert-butyl ether (MTBE) is the most widely used and studied ether oxygenate and is added to gasoline at concentrations up to 15% by volume. Inhalation of fumes while fueling automobiles is the main source of human exposure to MTBE. Humans are also exposed when drinking water contaminated with MTBE. Epidemiological, clinical, animal, metabolic and kinetic studies have been carried out to address human health risks resulting from exposure to MTBE. MTBE is an animal carcinogen, but its human carcinogenic potential remains unclear. Because MTBE functions as a non-traditional genotoxicant, several mechanisms were suggested to explain its mode of action, such as, functioning as a cytotoxic as opposed to a mitogenic agent; involvement of hormonal mechanisms; or operating as a promoter instead of being a complete carcinogen. Some studies suggested that carcinogenicity of MTBE might be due to its two main metabolites, formaldehyde or tributanol. A role for DNA repair in MTBE carcinogenesis was recently unveiled, which explains some, but not all effects. The totality of the evidence shows that, for the majority of the non-occupationally exposed human population, MTBE is unlikely to produce lasting adverse health effects, and may in some cases improve health by reducing the composition of emitted harmful VOCs and other substances. A small segment of the population (e.g. asthmatic children, the elderly, and those with immunodeficiency) may be at increased risk for toxicity. However, no studies have been conducted to investigate this hypothesis. Concern over ground and surface water contamination caused by persistent MTBE has lead the Environmental Protection Agency (EPA) to proposed reducing or eliminating its use as a gasoline additive. The major potential alternatives to MTBE are other forms of ethers such as ethyl tert-butyl ether (ETBE) or tert-amyl methyl ether (TAME), and alcohols such as ethanol. More definitive studies are needed to understand the mechanism(s) by which aliphatic ethers may pose health and environmental impacts. The switch from MTBE to ethanol is not without problems. Ethanol costs more to produce, poses challenges to the gasoline distribution system, extends the spread of hydrocarbons through ground water in gasoline plumes, and in the short-term is unlikely to be available in sufficient quantity. Moreover, its metabolite acetaldehyde is a possible carcinogen that undergoes a photochemical reaction in the atmosphere to produce the respiratory irritant peroxylacetate nitrate (PAN). Congress is addressing whether the Clean Air Act Amendments (CAA) provisions concerning reformulated gasoline (RFG) should be modified to allow refineries to discontinue or lessen the use of oxygenates.

Toxicity of methyl-tert-butyl ether to freshwater organisms

Increased input of the fuel oxygenate methyl-tert-butyl ether (MTBE) into aquatic systems has led to concerns about its effect(s) on aquatic life. As part of a study conducted by University of California scientists for the State of California, the Aquatic Toxicology Laboratory, UC Davis, reviewed existing literature on toxicity of MTBE to freshwater organisms, and new information was generated on chronic, developmental toxicity in fish, and potential toxicity of MTBE to California resident species. Depending on time of exposure and endpoint measured, MTBE is toxic to various aquatic organisms at concentrations of 57-> 1000 mg/l (invertebrates), and 388-2600 mg/l (vertebrates). Developmental effects in medaka (Oryzias latipes) were not observed at concentrations up to 480 mg/l, and all fish hatched and performed feeding and swimming in a normal manner. Bacterial assays proved most sensitive with toxicity to Salmonella typhimurium measured at 7.4 mg/l within 48 h. In microalgae, decreased growth was observed at 2400 and 4800 mg/l within 5 days. MTBE does not appear to bioaccumulate in fish and is rapidly excreted or metabolized. Collectively, the available data suggests that at environmental MTBE exposure levels found in surface waters (< 0.1 mg/l) this compound is likely not acutely toxic to aquatic life. However, more information is needed on chronic and sublethal effects before we can eliminate the possibility of risk to aquatic communities at currently detected concentrations.

Long-term chemical carcinogenesis bioassays predict human cancer hazards. Issues, controversies, and uncertainties

Long-term carcinogenesis bioassays are the most valued and predictive means for identifying potential carcinogenic hazards of various agents to humans. Agents may be chemicals, chemical mixtures, multiple chemicals, combinations of chemicals, residues and contaminants, commercial products and formulations, and various exposure circumstances. Life-styles, dietary factors, and occupational exposure circumstances are very difficult, but not totally impossible, to evaluate experimentally. Historically, the first chemical bioassay took place in the early part of this century: Yamagiwa and Ichikawa in 1915, showed that coal tar applied experimentally to rabbit ears caused skin carcinomas. Since then, nearly 1500-2000 bioassays of one sort or another have been carried out. Importantly, however, some of these bioassays must be considered inadequate for judging the absence of carcinogenicity, since there were various limitations on the way they were performed: too few animals, too short a duration, too low exposure concentrations, too limited pathology, as examples. Thus, each bioassay must be critically evaluated, especially those reported to be negative, because "false negatives" are certainly more hazardous to human health than are "false positives". Likewise, one must be careful not to discount bioassay results simply because a target organ in rodents may not have a direct counterpart in humans (e.g., Zymbal glands), or because an organ site in rodents may not be a major site of cancers in humans (e.g., mouse liver). The design and conduct of a bioassay is not simple, however, and one must be fully aware of possible pitfalls as well as viable and often necessary alternatives. Similarly, evaluating results and interpreting findings must be approached with the utmost objectivity and consistency. These and other select issues, controversies, and uncertainties possibly encountered in long-term bioassays are covered in this paper. One fact remains abundantly clear: for every known human carcinogen that has been tested adequately in laboratory animals, the findings of carcinogenicity are concordant.