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.

Assessment of the applicability of the threshold of toxicological concern for per- and polyfluoroalkyl substances.

While toxicity information is available for selected PFAS, little or no information is available for most, thereby necessitating a resource-effective approach to screen and prioritize those needing further safety assessment. The threshold of toxicological concern (TTC) approach proposes a de minimis exposure value based on chemical structure and toxicology of similar substances. The applicability of the TTC approach to PFAS was tested by incorporating a data set of no-observed-adverse-effect level (NOAEL) values for 27 PFAS into the Munro TTC data set. All substances were assigned into Cramer Class III and the cumulative distribution of the NOAELs evaluated. The TTC value for the PFAS-enriched data set was not statistically different compared to the Munro data set. Derived human exposure level for the PFAS-enriched data set was 1.3 µg/kg/day. Structural chemical profiles showed the PFAS-enriched data set had distinct chemotypes with lack of similarity to substances in the Munro data set using Maximum Common Structures. The incorporation of these 27 PFAS did not significantly change TTC Cramer Class III distribution and expanded the chemical space, supporting the potential use of the TTC approach for PFAS chemicals.

A systematic approach to evaluate plausible modes of actions for mouse lung tumors in mice exposed to 4-methylimidozole.

The National Toxicology Program (NTP) reported that chronic dietary exposure to 4-methylimidazole (4-MeI) increased the incidence of lung adenomas/carcinomas beyond the normally high spontaneous rate in B6C3F1 mice. To examine plausible modes of action (MoAs) for mouse lung tumors (MLTs) upon exposure to high levels of 4-MeI, and their relevance in assessing human risk, a systematic approach was used to identify and evaluate mechanistic data (in vitro and in vivo) in the primary and secondary literature, along with high-throughput screening assay data. Study quality, relevance, and activity of mechanistic data identified across the evidence-base were organized according to key characteristics of carcinogens (KCCs) to identify potential key events in known or novel MLT MoAs. Integration of these evidence streams provided confirmation that 4-MeI lacks genotoxic and cytotoxic activity with some evidence to support a lack of mitogenic activity. Further evaluation of contextual and chemical-specific characteristics of 4-MeI was consequently undertaken. Due to lack of genotoxicity, along with transcriptomic and histopathological lung changes up to 28 and 90 days of exposure, the collective evidence suggests MLTs observed following exposure to high levels of 4-MeI develop at a late stage in the mouse chronic bioassay, albeit the exact MoA remains unclear.

Comparison of threshold of toxicological concern (TTC) values to oral reference dose (RfD) values.

Thousands of chemicals have limited, or no hazard data readily available to characterize human risk. The threshold of toxicological concern (TTC) constitutes a science-based tool for screening level risk-based prioritization of chemicals with low exposure. Herein we compare TTC values to more rigorously derived reference dose (RfD) values for 288 chemicals in the U.S. Environmental Protection Agency's (US EPA) Integrated Risk Information System (IRIS) database. Using the Cramer decision tree and the Kroes tiered decision tree approaches to determine TTC values, the TCC for the majority of these chemicals were determined to be lower than their corresponding RfD values. The ratio of log10(RfD/TCC) was used to measure the differences between these values and the mean ratio for the substances evaluated was ~0.74 and ~0.79 for the Cramer and Kroes approach, respectively, when considering the Cramer Classes only. These data indicate that the RfD values for Cramer Class III compounds were, on average, ~6-fold higher than their TTC value. These analyses indicate that provisional oral toxicity values might be estimated from TTCs in data-poor or emergency situations; moreover, RfD values that are well below TTC values (e.g., 2 standard deviations below the log10(Ratio)) might be overly conservative and targets for re-evaluation.

An Adverse Outcome Pathway (AOP) for forestomach tumors induced by non-genotoxic initiating events.

The utility of rodent forestomach tumor data for hazard and risk assessment has been examined for decades because humans do not have a forestomach, and these tumors occur by varying modes of action (MOAs). We have used the MOA for ethyl acrylate (EA) to develop an Adverse Outcome Pathway (AOP) for forestomach tumors caused by non-genotoxic initiating events. These tumors occur secondary to site of contact induced epithelial cytotoxicity and regenerative repair-driven proliferation. For EA, the critical initiating event (IE) is epithelial cytotoxicity, and supporting key events (KEs) at the cellular and tissue level are increased cell proliferation (KE1) resulting in sustained hyperplasia (KE2), with the adverse outcome of forestomach papillomas and carcinomas. For EA, a pre-molecular initiating event (pre-MIE) of sustained glutathione depletion is probable. Supporting data from butylated hydroxyanisole (BHA) are also reviewed. Although there may be some variability in the pre-MIEs and IEs for BHA and EA, they share the same KEs, and evidence for BHA confers support for the AOP. Evolved Bradford Hill considerations of biological plausibility, essentiality, and empirical support were evaluated per OECD guidance. Although an MIE is not specifically described, overall confidence in the AOP is high due to well-developed and accepted evidence streams, and the AOP can be used for regulatory applications including hazard identification and risk assessment for chemicals that act by this AOP.

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 AUC0-∞ 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 AUC0-∞ 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.

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.

Reproductive and developmental toxicity evaluation of cannabidiol.

An important data gap in determining a safe level of cannabidiol (CBD) intake for consumer use is determination of CBD's potential to cause reproductive or developmental toxicity. We conducted an OECD Test Guideline 421 GLP-compliant study in rats, with extended postnatal dosing and hormone analysis, where hemp-derived CBD isolate (0, 30, 100, or 300 mg/kg-bw/d) was administered orally. Treatment-related mortality, moribundity, and decreased body weight and food consumption were observed in high-dose F0 adult animals, consistent with severe maternal toxicity. No effects were observed on testosterone concentrations, F0 reproductive performance, or reproductive organs. Hepatocellular hypertrophy in the 100- and 300 mg/kg-bw/day groups correlated with hypertrophy/hyperplasia in the thyroid gland and changes in mean thyroid hormone concentrations in F0 animals. Mean gestation length was unaffected; however, total litter loss for two females and dystocia for two additional females in the high-dose group occurred. Other developmental effects were limited to lower mean pup weights in the 300 mg/kg-bw/d group compared to those of concurrent controls. The following NOAELs were identified for CBD isolate based on this study: 100 mg/kg-bw/d for F0 systemic toxicity and female reproductive toxicity, 300 mg/kg-bw/d for F0 male reproductive toxicity, and 100 mg/kg-bw/d for F1 neonatal and F1 generation toxicity.

Oral toxicity evaluation of cannabidiol.

Use of cannabidiol (CBD) in humans has increased considerably in recent years. While currently available studies suggest that CBD is relatively safe for human consumption, data from publicly available studies on CBD conducted according to modern testing guidelines are lacking. In the current study, the potential for toxicity following repeated oral exposure to hemp-derived CBD isolate was evaluated in male and female Sprague Dawley rats. No adverse treatment-related effects were observed following administration of CBD via oral gavage for 14 and 90 days at concentrations up to 150 and 140 mg/kg-bw/d, respectively. Microscopic liver and adrenal gland changes observed in the 90-day study were determined to be resolved after a 28-day recovery period. CBD was well tolerated at these dose levels, and the results of this study are comparable to findings reported in unpublished studies conducted with other CBD isolates. The current studies were conducted as part of a broader research program to examine the safety of CBD.

Updated systematic assessment of human, animal and mechanistic evidence demonstrates lack of human carcinogenicity with consumption of aspartame.

Aspartame has been studied extensively and evaluated for its safety in foods and beverages yet concerns for its potential carcinogenicity have persisted, driven primarily by animal studies conducted at the Ramazzini Institute (RI). To address this controversy, an updated systematic review of available human, animal, and mechanistic data was conducted leveraging critical assessment tools to consider the quality and reliability of data. The evidence base includes 12 animal studies and >40 epidemiological studies reviewed by the World Health Organization which collectively demonstrate a lack of carcinogenic effect. Assessment of >1360 mechanistic endpoints, including many guideline-based genotoxicity studies, demonstrate a lack of activity associated with endpoints grouped to key characteristics of carcinogens. Other non-specific mechanistic data (e.g., mixed findings of oxidative stress across study models, tissues, and species) do not provide evidence of a biologically plausible carcinogenic pathway associated with aspartame. Taken together, available evidence supports that aspartame consumption is not carcinogenic in humans and that the inconsistent findings of the RI studies may be explained by flaws in study design and conduct (despite additional analyses to address study limitations), as acknowledged by authoritative bodies.

RNA-sequencing (transcriptomic) data collected in liver and lung of male and female B6C3F1 mice exposed to various dose levels of 4-methylimidazole for 2, 5, or 28 days.

The National Toxicology Program (NTP) reported that chronic exposure to varying dietary concentrations of 4-methylimidazole (4-MeI) increased lung tumors in female and male mice [1]. In this study, mice (male and female B6C3F1 mice) were either administered 4-MeI by oral gavage (0, 50, 100, 200, or 300 mg/kg/day) for 2 days or exposed for 5 and 28 days to 4-MeI in the diet (0, 150, 300, 1250, or 2500 ppm) and whole transcriptome (RNA-Sequencing) data from 4-MeI-exposed B6C3F1 mice to determine whether changes occurred in the target (lung) and nontarget (liver) tissues. This analysis was conducted to provide information with which to evaluate biological processes affected by exposure to 4-MeI, with a focus on identifying key events that could be used to propose a plausible mode of action (MoA) for mouse lung tumors [2].

Kinetics of selected di-n-butyl phthalate metabolites and fetal testosterone following repeated and single administration in pregnant rats.

Human exposure to phthalic acid diesters occurs through a variety of pathways as a result of their widespread use in consumer products and plastics. Repeated doses of di-n-butyl phthalate (DBP) from gestation day (GD) 12 to 19 disrupt testosterone synthesis and male sexual development in the fetal rat. Currently little is known about the disposition of DBP metabolites, such as monobutyl phthalate (MBP) and its glucuronide conjugate (MBP-G), during gestation after repeated exposure to DBP. In order to gain a better understanding of the effect of repeated dosing on maternal and fetal metabolism and distribution, pregnant Sprague-Dawley rats were given a single dose of 500 mg/kg DBP on GD 19 or daily doses of 50, 100, and 500 mg/(kg day) from GD 12 to 19 via corn oil gavage. Dose-response evaluation revealed a non-linear increase in maternal and fetal plasma concentrations of MBP. Maternal and fetal MBP levels were slightly lower in animals after 8 days of dosing at 500 mg/(kg day). Fetal plasma MBP levels closely followed maternal plasma, while the appearance and elimination of MBP-G in fetal plasma were significantly delayed. MBP-G accumulated over time in the amniotic fluid. Inhibition of testosterone was rapid in fetal testes when exposed to DBP (500 mg/(kg day)) on GD 19. Within 24h, the level of inhibition in the fetus was similar between animals exposed to a single or multiple daily doses of 500 mg/(kg day). Examination of testosterone time-course data indicates a rapid recovery to normal levels within 24h post-dosing at DBP doses of 50 and 100 mg/(kg day), with a rebound to higher than normal concentrations at later time-points. MBP kinetics in fetal testes allows direct comparison of active metabolite concentrations and testosterone response in the fetal testes.

A Framework for Systematic Evaluation and Quantitative Integration of Mechanistic Data in Assessments of Potential Human Carcinogens.

Recently, the key characteristics of carcinogens (KCC) have been proposed as an organizational approach for the evaluation of mechanistic data related to carcinogenicity. Our objective was to develop a framework to systematically and quantitatively integrate KCC data using elements that are important to risk assessment. Methods for developing the framework included: defining objectives, identifying and accommodating key considerations for components, input, and output of the framework, and operational development via iterative testing by a multidisciplinary team. The proposed framework involves 3 steps: (1) a structured, yet flexible, appraisal of individual studies and endpoints, (2) a structured and transparent evaluation of the body of evidence for each key characteristic, and (3) an evaluation of all of the KCC-relevant data relative to tumors and/or cancer types. In step 1, data are assessed and scored for reliability, strength, and activity. In step 2, a mathematical algorithm is used to integrate (and weight) the quality, relevance, and activity for each of the KCCs. These scores facilitate subsequent evaluations related to the overall body of evidence in step 3 in which KCCs can be linked, assessing potential adverse outcome pathways or networks, and finally, considered in the context of observed carcinogenic responses in animals and/or humans. The output is an overall conclusion regarding KCC activity as it relates to carcinogenic responses. The proposed framework provides a flexible solution to quantitatively integrate KCC data in a systematic and transparent manner that provides weighting of data most well-suited for the assessment of potential human carcinogenicity.

Nondetectable or minimal detectable residue levels of N-(n-butyl) thiophosphoric triamide in bovine tissues and milk from a 28-d NBPT dosing study.

N-(n-butyl) thiophosphoric triamide (NBPT) (Figure 1) is an active ingredient in nitrogen stabilizer (urease inhibitor), which temporarily inhibits the action of the urease enzyme to improve the efficiency of urea-containing fertilizers. Given the potential for NBPT residues to be present in milk and tissues of dairy cattle, due diligence is needed to demonstrate the safety of NBPT in urea-based fertilizers used on forages and crops intended for consumption by Holstein dairy cows. This study used controlled dosing of NBPT in capsule form to dairy cattle for 28 d, followed by a 14-d depuration phase to assess the potential for residues to exist in milk and tissues of dairy cattle at exaggerated use levels. Fourteen lactating cows were selected for the dosing and depuration phases of the study, based on health, body weight (BW), and milk production. There were four treatment groups: 0 mg NBPT/kg BW (Control) (n = 2 cows), 1 mg NBPT/kg BW (1×) (n = 3 cows), 3 mg NBPT/kg BW (3×) (n = 3 cows), and 10 mg NBPT/kg/BW (10×) (n = 6 cows); levels were based on maximum tolerable amount of urea that a cow can ingest on a daily basis (1×) and the maximum concentration of NBPT commercially used when treating urea (0.1 wt% NBPT in urea). At the end of the 28-d dosing phase, cows were randomly selected for the 14-d depuration phase of the study (one control and three 10× cows). The results showed no NBPT residue is detectable at all dose levels, except that a residue level was above the lower limit of quantitation in a single milk and subcutaneous fat sample in the highest (10×) treatment group, which represents the level of NBPT that would be theoretically present in 10× the lethal dosing of daily consumable urea to a cow. Overall, the study demonstrated that it is unlikely for NBPT residues to be present in cattle milk or edible tissues or to cause negative effects on animal health under good agricultural practice.

14C-labeled pulegone and metabolites binding to alpha2u-globulin in kidneys of male F-344 rats.

Pulegone is a major constituent of pennyroyal oil and a minor component of peppermint oil. Pulegone is biotransformed to menthofuran and menthones (diastereomeric menthone and isomenthone) in pennyroyal and peppermint as well as in rodents. Pulegone and menthofuran are hepatotoxic to rodents, and menthones are less toxic. The metabolism and disposition of pulegone and menthofuran were previously studied in rodents, and higher concentrations of pulegone- and menthofuran-derived radioactivity were observed in male than female rat kidney. One explanation is the association of pulegone and metabolites with a male rat-specific protein, alpha2u-globulin. To test this hypothesis, male and female rats were dosed orally with 14C-labeled pulegone (80 mg/kg, 120 microCi/kg) or menthofuran (60 mg/kg, 120 microCi/kg) or menthones (80 mg/kg, 120 microCi/kg) in corn oil, and the kidney cytosol was prepared 24 h after dosing. An equilibrium dialysis experiment showed that in all three studies the radioactivity was associated with kidney cytosol proteins of male but not female rats. The chemicals present in the male rat kidney cytosol after dialysis were extracted with dichloromethane and characterized by high-performance liquid chromatography (HPLC) and gas chromatography/mass spectrometry (GC-MS). All parent compounds were detected, and the metabolites characterized included piperitone from pulegone or menthones treatment, menthones and possibly 8-hydroxymenthones from pulegone treatment, and mintlactones (diastereomeric mintlactone and isomintlactone) and 7a-hydroxymintlactone from menthofuran treatment. Analysis of the male rat kidney cytosol by a gel filtration column demonstrated that the retention was due to reversible binding of these chemicals with the male rat-specific protein alpha2u-globulin. However, binding of pulegone and/or metabolites to alpha2u-globulin did not produce accumulation of this protein in the kidney.

Kinetics of genistein and its conjugated metabolites in pregnant Sprague-Dawley rats following single and repeated genistein administration.

Diets high in soy-based products are well known for their estrogenic activity. Genistein, the predominant phytoestrogen present in soy, is known to interact with estrogen receptors (ER) alpha and beta and elicits reproductive effects in developing rodents. In the rat, genistein is metabolized predominantly to glucuronide and sulfate conjugates, neither of which is capable of activating ER. Therefore, it is critical to understand the delivery of free and conjugated genistein across the placenta to the fetus following maternal genistein exposure such that the potential fetal exposure to free genistein can be assessed. Genistein (4 or 40 mg/kg) was administered to pregnant Sprague-Dawley rats by oral gavage daily from gestation day (GD) 5 through 19 or on GD 19 alone. Maternal and GD 19 fetal tissues were collected 0.5, 1, 2, 4, 6, 8, 12, and 24 h following administration of the final dose on GD 19. Concentrations of genistein, genistein glucuronide, and genistein sulfate were quantitated by LC-MS/MS. In maternal plasma, genistein glucuronide was the predominant metabolite. In the fetal plasma, genistein glucuronide and genistein sulfate were the primary metabolites. Genistein levels in maternal and fetal plasma were much lower than its conjugates. The concentration of genistein in placental tissue was higher than either conjugate. Fetal concentrations of unconjugated genistein following administration of 40 mg/kg were above the EC50 for ERbeta activation. Repeated administration of 40 mg/kg genistein resulted in minor changes in genistein kinetics in the pregnant rat compared to single administration of the same dose. These data suggest that conjugated forms of genistein are not transported across the placenta. High placental concentrations of genistein indicate the placenta is a potential target organ for genistein action during gestation.

Pharmacokinetics of monobutylphthalate, the active metabolite of di-n-butylphthalate, in pregnant rats.

Di-n-butylphthalate (DBP) is a phthalic acid ester used as a plasticizer and solvent. DBP is a developmental toxicant in rats and mice, with adverse effects arising from the monoester metabolite monobutyl phthalate (MBP). The objective of this study was to evaluate the pharmacokinetics of MBP and monobutyl phthalate glucuronide (MBP-G) in pregnant rats following intravenous (i.v.) dosing with MBP. Pregnant dams were i.v. dosed with aqueous MBP (10, 30, or 50mg MBP/kg body weight) on gestation day (GD) 19. The pharmacokinetics of MBP and MBP-G were rapid: MBP was metabolized to MBP-G within 5 min, and MBP and MBP-G disappeared from maternal and fetal plasma within 24h of dosing. Results were consistent with two previous studies that utilized oral doses of DBP, suggesting that chemical (DBP versus MBP), vehicle (oil versus aqueous), dose level, and route (oral versus i.v.) have minimal effects on the maternal pharmacokinetics of MBP and MBP-G. This study provides direct pharmacokinetic analysis for MBP and MBP-G in pregnant rats during fetal male reproductive development, and indicates that future pharmacokinetic or toxicology studies can reliably utilize oral dosing with DBP.

Male mice deficient in microsomal epoxide hydrolase are not susceptible to benzene-induced toxicity.

Enzymes involved in benzene metabolism are likely genetic determinants of benzene-induced toxicity. Polymorphisms in human microsomal epoxide hydrolase (mEH) are associated with an increased risk of developing leukemia, specifically those associated with benzene. This study was designed to investigate the importance of mEH in benzene-induced toxicity. Male and female mEH-deficient (mEH-/-) mice and background mice (129/Sv) were exposed to inhaled benzene (0, 10, 50, or 100 ppm) 5 days/week, 6 h/day, for a two-week duration. Total white blood cell counts and bone marrow cell counts were used to assess hematotoxicity and myelotoxicity. Micronucleated peripheral blood cells were counted to assess genotoxicity, and the p21 mRNA level in bone marrow cells was used as a determinant of the p53-regulated DNA damage response. Male mEH-/- mice did not have any significant hematotoxicity or myelotoxicity at the highest benzene exposure compared to the male 129/Sv mice. Significant hematotoxicity or myelotoxicity did not occur in the female mEH-/- or 129/Sv mice. Male mEH-/- mice were also unresponsive to benzene-induced genotoxicity compared to a significant induction in the male 129/Sv mice. The female mEH-/- and 129/Sv mice were virtually unresponsive to benzene-induced genotoxicity. While p21 mRNA expression was highly induced in male 129/Sv mice after exposure to 100-ppm benzene, no significant alteration was observed in male mEH-/- mice. Likewise, p21 mRNA expression in female mEH-/- mice was not significantly induced upon benzene exposure whereas a significant induction was observed in female 129/Sv mice. Thus mEH appears to be critical in benzene-induced toxicity in male, but not female, mice.

Lessons learned, challenges, and opportunities: the U.S. Endocrine Disruptor Screening Program.

In 1996, the U.S. Congress passed the Food Quality Protection Act and amended the Safe Drinking Water Act (SDWA) requiring the U.S. Environmental Protection Agency (EPA) to implement a screening program to investigate the potential of pesticide chemicals and drinking water contaminants to adversely affect endocrine pathways. Consequently, the EPA launched the Endocrine Disruptor Screening Program (EDSP) to develop and validate estrogen, androgen, and thyroid (EAT) pathway screening assays and to produce standardized and harmonized test guidelines for regulatory application. In 2009, the EPA issued the first set of test orders for EDSP screening and a total of 50 pesticide actives and 2 inert ingredients have been evaluated using the battery of EDSP Tier 1 screening assays (i.e., five in vitro assays and six in vivo assays). To provide a framework for retrospective analysis of the data generated and to collect the insight of multiple stakeholders involved in the testing, more than 240 scientists from government, industry, academia, and non-profit organizations recently participated in a workshop titled "Lessons Learned, Challenges, and Opportunities: The U.S. Endocrine Disruptor Screening Program." The workshop focused on the science and experience to date and was organized into three focal sessions: (a) Performance of the EDSP Tier 1 Screening Assays for Estrogen, Androgen, and Thyroid Pathways; (b) Practical Applications of Tier 1 Data; and (c) Indications and Opportunities for Future Endocrine Testing. A number of key learnings and recommendations related to future EDSP evaluations emanated from the collective sessions.

Physiologically based pharmacokinetic rat model for methyl tertiary-butyl ether; comparison of selected dose metrics following various MTBE exposure scenarios used for toxicity and carcinogenicity evaluation.

There are a number of cancer and toxicity studies that have been carried out to assess hazard from methyl tertiary-butyl ether (MTBE) exposure via inhalation and oral administration. MTBE has been detected in surface as well as ground water supplies which emphasized the need to assess the risk from exposure via drinking water contamination. This model can now be used to evaluate route-to-route extrapolation issues concerning MTBE exposures but also as a means of comparing potential dose metrics that may provide insight to differences in biological responses observed in rats following different routes of MTBE exposure. Recently an updated rat physiologically based pharmacokinetic (PBPK) model was published that relied on a description of MTBE and its metabolite tertiary-butyl alcohol (TBA) binding to alpha 2u-globulin, a male rat-specific protein. This model was used to predict concentrations of MTBE and TBA in the kidney, a target tissue in the male rat. The objective of this study was to use this model to evaluate the dosimetry of MTBE and TBA in rats following different exposure scenarios, used to evaluate the toxicity and carcinogenicity of MTBE, and compare various dose metrics under these different conditions. Model simulations suggested that although inhalation and drinking water exposures show a similar pattern of MTBE and TBA exposure in the blood and kidney (i.e. concentration-time profiles), the total blood and kidney levels following exposure of MTBE to 7.5mg/ml MTBE in the drinking water for 90 days is in the same range as administration of an oral dose of 1000 mg/kg MTBE. Evaluation of the dose metrics also supports that a high oral bolus dose (i.e. 1000 mg/kg MTBE) results in a greater percentage of the dose exhaled as MTBE with a lower percent metabolized to TBA as compared to dose of MTBE that is delivered over a longer period of time as in the case of drinking water.