Physiologically based pharmacokinetic model for ethyl tertiary-butyl ether and tertiary-butyl alcohol in rats: Contribution of binding to α2u-globulin in male rats and high-exposure nonlinear kinetics to toxicity and cancer outcomes

Assessment of the genotoxicity of tert-butyl alcohol in an in vivo thyroid comet assay

Chronic exposure to high (20,000 ppm) concentrations of tert-butyl alcohol (TBA) in drinking water, equivalent to ~2100 mg/kg bodyweight per day, is associated with slight increases in the incidence of thyroid follicular cell adenomas and carcinomas in mice, with no other indications of carcinogenicity. In a recent toxicological review of TBA, the U.S. EPA determined that the genotoxic potential of TBA was inconclusive, largely based on non-standard studies such as in vitro comet assays. As such, the potential role of genotoxicity in the mode of action of thyroid tumors and therefore human relevance was considered uncertain. To address the potential role of genotoxicity in TBA-associated thyroid tumor formation, CD-1 mice were exposed up to a maximum tolerated dose of 1500 mg/kg-day via oral gavage for two consecutive days and DNA damage was assessed with the comet assay in the thyroid. Blood TBA levels were analyzed by headspace GC-MS to confirm systemic tissue exposure. At study termination, no significant increases (DNA breakage) or decreases (DNA crosslinks) in %DNA tail were observed in TBA exposed mice. In contrast, oral gavage of the positive control ethyl methanesulfonate significantly increased %DNA tail in the thyroid. These findings are consistent with most genotoxicity studies on TBA and provide mechanistic support for non-linear, threshold toxicity criteria for TBA. While the mode of action for the thyroid tumors remains unclear, linear low dose extrapolation methods for TBA appear more a matter of policy than science.

Investigation of the potential mutagenicity of ethyl tertiary-butyl ether in the tumor target tissue using transgenic Big Blue Fischer 344 rats following whole body inhalation exposure

Ethyl tertiary-butyl ether (ETBE) is a fuel oxygenate used for the efficiency of motor vehicle fuels and their octane ratings. ETBE has been reported to induce liver adenomas in male rats in a 2-year bioassay at the highest inhalation concentration tested of 5000 ppm. To investigate the potential mutagenicity of ETBE in the liver, male Big Blue Fischer 344 rats were exposed for 28 consecutive days (6 h/day) to 0, 500, 1500, and 5000 ppm ETBE. The treated rats were sacrificed 3 days post-exposure and the frequencies of cII mutants were evaluated in the liver and bone marrow tissues. The mutant frequency (MF) of the liver in the negative control group was 36.3 × 10^-6 and this value was not significantly different in ETBE-exposed animals (39.4, 37.3, and 45.9 × 10^-6 in 500, 1500, and 5000 ppm groups, respectively). In the bone marrow, the mean MF in the negative control was 32.9 × 10^-6 which was not different from the means of the exposed groups (33.8, 22.6, and 32.0 × 10^-6 for groups exposed to 500, 1500 and 5000 ppm, respectively). These data, along with consistent negative response reported in the literature for other apical genotoxicity endpoints informs that mutagenicity is not likely the initial key event in the mode of action for ETBE-induced hepatocarcinogenesis in the rat.

Adverse outcome pathway (AOP): α2u-globulin nephropathy and kidney tumors in male rats

The National Research Council's vision of using adverse outcome pathways (AOPs) as a framework to assist with toxicity assessment for regulatory requirements of chemical assessment has continued to gain traction since its release in 2007. The need to expand the AOP knowledge base has gained urgency, with the U.S. Environmental Protection Agency's directive to eliminate reliance on animal toxicity testing by 2035. To meet these needs, our goal was to elucidate the AOP for male-rat-specific kidney cancer. Male-rat-specific kidney tumors occur through the ability of structurally diverse substances to induce α2u-globulin nephropathy (α2u-N), a well-studied mode of action (MoA) not relevant in humans that results in kidney tumor formation in male rats. An accepted AOP may help facilitate the differentiation from other kidney tumors MoAs. Following identification and review of relevant in vitro and in vivo literature, both the MIE and subsequent KEs were identified. Based on the weight of evidence from the various resources, the confidence in this AOP is high. Uses of this AOP include hazard identification, development of in vitro assays to determine if the MoA is through α2u-N and not relevant to humans resulting in decreased use of animals, and regulatory applications.

Predicting nonlinear relationships between external and internal concentrations with physiologically based pharmacokinetic modeling

Although external concentrations are more readily quantified and often used as the metric for regulating and mitigating exposures to environmental chemicals, the toxicological response to an environmental chemical is more directly related to its internal concentrations than the external concentration. The processes of absorption, distribution, metabolism, and excretion (ADME) determine the quantitative relationship between the external and internal concentrations, and these processes are often susceptible to saturation at high concentrations, which can lead to nonlinear changes in internal concentrations that deviate from proportionality. Using generic physiologically-based pharmacokinetic (PBPK) models, we explored how saturable absorption or clearance influence the shape of the internal to external concentration (IEC) relationship. We used the models for hypothetical chemicals to show how differences in kinetic parameters can impact the shape of an IEC relationship; and models for styrene and caffeine to explore how exposure route, frequency, and duration impact the IEC relationships in rat and human exposures. We also analyzed available plasma concentration data for 2,4-dichlorophenoxyacetic acid to demonstrate how a PBPK modeling approach can be an alternative to common statistical methods for analyzing dose proportionality. A PBPK modeling approach can be a valuable tool used in the early stages of a chemical safety assessment program to optimize the design of longer-term animal toxicity studies or to interpret study results.

Heading Towards a Possible Rebirth of the Induced Renal Cell Carcinoma Models?

Introduction: Animal models are interesting tools to improve our knowledge of the pathophysiological processes underlying kidney cancer development. Recent advances have been made in the understanding of the genetic founding events underlying clear cell renal carcinoma. The aim of this paper was to review and discuss the characteristics of all the induced animal models of renal carcinogenesis that have been described in the scientific literature to date and to see if and how they could regain some use in the light of the latest discoveries. Methods: The authors reviewed all the papers available in PubMed regarding induced animal models of renal carcinogenesis. From this perspective, the keywords “induced”, “animal model”, and “renal cancer” were used in PubMed’s search engine. Another search was done using the keywords “induced”, “animal model”, and “kidney cancer”. PRISMA recommendations were used to develop the literature review. Results: Seventy-eight studies were included in this review. Results were presented depending on the mechanisms used to induce carcinogenesis in each model: induction by carcinogens, hormones, viral induction, or induction by other agents. Discussion focused on the possibility to rethink these different induced animal models and use them to answer new research questions. Conclusion: Many induced animal models have been developed in the past to study renal cancer. While these models seemed unable to yield new knowledge, the latest advances in the understanding of the genetics behind renal carcinogenesis could well bring the models back to the forefront.

SIRT1 exhibits antioxidative effects in HT22 cells induced by tert-butyl alcohol

Tertiary butyl alcohol (TBA) is a principal metabolite of methyl tertiary-butyl ether (MTBE), a common pollutant worldwide in the ground or underground water, which is found to produce nervous system damage. Nevertheless, few data regarding the effects of TBA has been reported. Studies indicated that oxidative stress plays a pivotal role in MTBE neurotoxic mechanism. Sirtuin 1 (SIRT1) has been reported to exert a neuroprotective effect on various neurologic diseases via resistance to oxidative stress by deacetylating its substrates. In this study, we examined levels of oxidative stress after exposure to TBA for 6 h in HT22 cells and HT22 cells with SIRT1 silencing (transfected with SIRT1 siRNA) or high expression (preconditioned with agonists SRT1720). We found that TBA activated oxidative stress by increasing generation of intracellular reactive oxygen species (ROS), malondialdehyde (MDA) and Oxidized glutathione (GSSG), and decreasing contents of superoxide dismutase (SOD) and glutathione reductase (GSH). In additional, levels of TBA-induced oxidative stress were aggravated when SIRT1 silenced but alleviated when SIRT1 enhanced. Our study indicated that SIRT1 mitigated oxidative stress induced by TBA.

Physiologically based pharmacokinetic model for ethyl tertiary-butyl ether and tertiary-butyl alcohol in rats: Contribution of binding to α2u-globulin in male rats and high-exposure nonlinear kinetics to toxicity and cancer outcomes

In cancer bioassays, inhalation, but not drinking water exposure to ethyl tertiary-butyl ether (ETBE), caused liver tumors in male rats, while tertiary-butyl alcohol (TBA), an ETBE metabolite, caused kidney tumors in male rats following exposure via drinking water. To understand the contribution of ETBE and TBA kinetics under varying exposure scenarios to these tumor responses, a physiologically based pharmacokinetic model was developed based on a previously published model for methyl tertiary-butyl ether, a structurally similar chemical, and verified against the literature and study report data. The model included ETBE and TBA binding to the male rat-specific protein α2u-globulin, which plays a role in the ETBE and TBA kidney response observed in male rats. Metabolism of ETBE and TBA was described as a single, saturable pathway in the liver. The model predicted similar kidney AUC_0-∞ for TBA for various exposure scenarios from ETBE and TBA cancer bioassays, supporting a male-rat-specific mode of action for TBA-induced kidney tumors. The model also predicted nonlinear kinetics at ETBE inhalation exposure concentrations above ~2000 ppm, based on blood AUC_0-∞ for ETBE and TBA. The shift from linear to nonlinear kinetics at exposure concentrations below the concentration associated with liver tumors in rats (5000 ppm) suggests the mode of action for liver tumors operates under nonlinear kinetics following chronic exposure and is not relevant for assessing human risk. Copyright © 2016 The Authors Journal of Applied Toxicology Published by John Wiley & Sons Ltd.