Methyl-tert-butyl ether (MTBE): integration of rat and mouse carcinogenicity data with mode of action and human and rodent bioassay dosimetry and toxicokinetics indicates MTBE is not a plausible human carcinogen.

Methyl-tert-butyl ether (MTBE) is a fuel oxygenate used in non-United States geographies. Multiple health reviews conclude that MTBE is not a human-relevant carcinogen, and this review provides updated mode of action (MOA), exposure, dosimetry and risk perspectives supporting those conclusions. MTBE is non-genotoxic and has large margins of exposure between blood concentrations at the overall rat 400 ppm inhalation NOAEL and blood concentrations in typical workplace or general population exposures. Non-cancer and threshold cancer hazard quotients range from a high of 0.046 for fuel-pump gasoline station attendants and are 100-1,000-fold lower for general population exposures. Cancer risks conservatively assuming genotoxicity for these same scenarios are all less than 1 × 10-6. The onset of MTBE nonlinear toxicokinetics (TK) in rats at inhalation exposures less than 3,000 ppm, a dose that is also not practically achievable in fuel-use scenarios, indicates that high-dose specific male rat kidney and testes (3,000 and 8,000 ppm) and female mouse liver tumors (8000 ppm) are not quantitatively relevant to humans. Mode of action analyses also indicate MTBE male rat kidney tumors, and lesser so female mouse liver tumors, are not qualitatively relevant to humans. Thus, an integrated analysis of the toxicology, exposure/dosimetry, TK, and MOA data indicates that MTBE presents minimal human cancer and non-cancer risks.

Bruce Nathan Ames - Paradigm shifts inside the cancer research revolution.

Dr. Bruce Ames turned 92 on December 16, 2020. He considers his most recent work linking adequate consumption of 30 known vitamins and minerals with successful aging to be his most important contribution. With the passage of time, it is not uncommon for the accomplishments of a well-known scientist to undergo a parsimonious reductionism in the public mind - Pasteur's vaccine, Mendel's peas, Pavlov's dogs, Ames' test. Those of us in the research generation subsequent to Dr. Ames' are undoubtedly affected by our own unconscious tendencies toward accepting the outstanding achievements of the past as commonplace. In doing so, seminal advances made by earlier investigators are often inadvertently subsumed into common knowledge. But having followed Ames' work since the mid-1970s, we are cognizant that the eponymous Ames Test is but a single chapter in a long and rich narrative. That narrative begins with Ames' classic studies on the histidine operon of Salmonella, for which he was elected to the National Academy of Sciences. A summary of the historical progression of the understanding of chemical carcinogenesis to which Ames and his colleagues contributed is provided. Any summary of a topic as expansive and complex as the ongoing unraveling of the mechanisms underlying chemical carcinogenesis will only touch upon some of the major conceptual advances to which Ames and his colleagues contributed. We hope that scientists of all ages familiar with Ames only through the eponymous Ames Test will further investigate the historical progression of the conceptualization of cancer caused by chemical exposure. As the field of chemical carcinogenesis gradually moves away from primary reliance on animal testing to alternative protocols under the rubric of New Approach Methodologies (NAM) an understanding of where we have been might help to guide where we should go.

A review of the genotoxicity of the industrial chemical cumene.

The purpose of this review is to evaluate the literature on the genotoxicity of cumene (CAS # 98-82-8) and to assess the role of mutagenicity, if any, in the mode of action for cumene-induced rodent tumors. The studies reviewed included microbial mutagenicity, DNA damage/ repair, cytogenetic effects, and gene mutations. In reviewing these studies, attention was paid to their conformance to applicable OECD test guidelines which are considered as internationally recognized standards for performing these assays. Cumene was not a bacterial mutagen and did not induce Hprt mutations in CHO cell cultures. In the primary rat hepatocyte cultures, cumene induced unscheduled DNA synthesis in one study but this response could not be reproduced in an independent study using a similar protocol. In a study that is not fully compliant to the current OECD guideline, no increase in chromosomal aberrations was observed in CHO cells treated with cumene. The weight of the evidence (WoE) from multiple in vivo studies indicates that cumene is not a clastogen or aneugen. The weak positive response in an in vivo comet assay in the rat liver and mouse lung tissues is of questionable significance due to several study deficiencies. The genotoxicity profile of cumene does not match that of a classic DNA-reactive molecule and the available data does not support a conclusion that cumene is an in vivo mutagen. As such, mutagenicity does not appear to be an early key event in cumene-induced rodent tumors and alternate hypothesized non-mutagenic modes-of-action are presented. Further data are necessary to rule in or rule out a particular MoA.

Ethylene Oxide Exposure in U.S. Populations Residing Near Sterilization and Other Industrial Facilities: Context Based on Endogenous and Total Equivalent Concentration Exposures.

Given ubiquitous human exposure to ethylene oxide (EO), regardless of occupation or geography, the current risk-specific concentrations (RSCs: 0.0001-0.01 ppb) from the U.S. Environmental Protection Agency (EPA) cancer risk assessment for EO are not useful metrics for managing EO exposures to the general U.S. population. The magnitude of the RSCs for EO are so low, relative to typical endogenous equivalent metabolic concentrations (1.1-5.5 ppb) that contribute ~93% of total exposure, that the RSCs provide little utility in identifying excess environmental exposures that might increase cancer risk. EO monitoring data collected in the vicinity of eight EO-emitting facilities and corresponding background locations were used to characterize potential excess exogenous concentrations. Both 50th and 90th percentile exogenous exposure concentrations were combined with the 50th percentile endogenous exposure concentration for the nonsmoking population, and then compared to percentiles of total equivalent concentration for this population. No potential total exposure concentration for these local populations exceeded the normal total equivalent concentration 95th percentile, indicating that excess facility-related exposures are unlikely to require additional management to protect public health.

Relevance of mouse lung tumors to human risk assessment.

Mouse lung is a common site for chemical tumorigenicity, but the relevance to human risk remains debated. Long-term bioassays need to be assessed for appropriateness of the dose, neither exceeding Maximum Tolerated Dose (MTD) nor Kinetically based Maximum Dose (KMD). An example of the KMD issue is 1,3-dichloropropene (1,3-D), which only produced an increased incidence of lung tumors at a dose exceeding the KMD. In addition, since mouse lung tumors are common (>1% incidence), the appropriate statistical significance is p < .01. Numerous differences exist for mouse lung and tumors compared to humans, including anatomy, respiratory rate, metabolism, tumor histogenesis, and metastatic frequency. The recent demonstration of the critical role of mouse lung specific Cyp2 F2 metabolism in mouse lung carcinogenicity including styrene or fluensulfone indicates that this tumor response is not qualitatively or quantitatively relevant to humans. For non-DNA reactive and non-mutagenic carcinogens, the mode of action involves direct mitogenicity such as for isoniazid, styrene, fluensulfone, permethrin or cytotoxicity with regeneration such as for naphthalene. However, the possibility of mixed mitogenic and cytotoxic modes of action cannot always be excluded. The numerous differences between mouse and human, combined with epidemiologic evidence of no increased cancer risk for several of these chemicals make the relevance of mouse lung tumors for human cancer risk dubious.

Use of toxicokinetic data for afidopyropen to determine the dose levels in developmental toxicity studies.

Afidopyropen is an insecticide that acts as a TRPV channel modulator in chordotonal organs of target insects and has been assessed for a wide range of toxicity endpoints including developmental toxicity in rats and rabbits. The GLP developmental toxicity study in rabbits did not produce evidence of maternal or fetal toxicity at the highest dose tested (32 mg/kg/day) but pharmacokinetics (PK) in pregnant rabbits in this study exhibited onset of PK nonlinearity from 5 mg/kg/day on, as measured by plasma Cmax and AUC. The NOAEL (32 mg/kg/day) is 9000X higher than maximum expected human dietary exposures to afidopyropen; the dose range where nonlinear PK were observed (5-15 mg/kg/day) is 1400-4200X higher. As nonlinearity occurred between 5 and 15 mg/kg/day, 32 mg/kg/day is concluded to be a sufficiently high dose (kinetically derived maximum dose) for a prenatal developmental toxicity study. As recognized by regulatory dose-selection guidance, onset of saturated PK is evidence of excessive biological stress to test animals rendering any effects at such doses of questionable relevance for human risk assessment. These data demonstrate that consideration of PK is critical for improving the dose-selection in developmental toxicity studies to enhance human relevance of animal toxicity studies.

Trichloroethylene in drinking water throughout gestation did not produce congenital heart defects in Sprague Dawley rats.

BACKGROUND: Trichloroethylene (TCE) was negative for developmental toxicity after inhalation and oral gavage exposure of pregnant rats but fetal cardiac defects were reported following drinking water exposure throughout gestation. Because of the deficiencies in this latter study, we performed another drinking water study to evaluate whether TCE causes heart defects. METHODS: Groups of 25 mated Sprague Dawley rats consumed water containing 0, 0.25, 1.5, 500, or 1,000 ppm TCE from gestational day 1-21. TCE concentrations were measured at daily formulation, when placed into water bottles each day and when water bottles were removed from cages. Four additional mated rats per group were used for plasma measurements. At termination, fetal hearts were carefully dissected fresh and examined. RESULTS: All TCE concentrations were >90% of target when initially placed in water bottles and when bottles were placed on cages. All dams survived with no clinical signs. Rats in the two higher dose groups consumed less water/day than other groups but showed no changes in maternal or fetal weights. The only fetal cardiac observation was small (<1 mm) membranous ventricular septal defect occurring in all treated and water control groups; incidences were within the range of published findings for naive animals. TCE was not detected in maternal blood, but systemic exposure was confirmed by detecting its primary oxidative metabolite, trichloroacetic acid, although only at levels above the quantitation limit in the two higher dose groups. CONCLUSIONS: Ingesting TCE in drinking water ≤1,000 ppm throughout gestation does not cause cardiac defects in rat offspring.

Considerations for refining the risk assessment process for formaldehyde: Results from an interdisciplinary workshop.

Anticipating the need to evaluate and integrate scientific evidence to inform new risk assessments or to update existing risk assessments, the Formaldehyde Panel of the American Chemistry Council (ACC), in collaboration with the University of North Carolina, convened a workshop: "Understanding Potential Human Health Cancer Risk - From Data Integration to Risk Evaluation" in October 2017. Twenty-four (24) invited-experts participated with expertise in epidemiology, toxicology, science integration and risk evaluation. Including members of the organizing committee, there were 29 participants. The meeting included eleven presentations encompassing an introduction and three sessions: (1) "integrating the formaldehyde science on nasal/nasopharyngeal carcinogenicity and potential for causality"; (2) "integrating the formaldehyde science on lymphohematopoietic cancer and potential for causality; and, (3) "formaldehyde research-data suitable for risk assessment". Here we describe key points from the presentations on epidemiology, toxicology and mechanistic studies that should inform decisions about the potential carcinogenicity of formaldehyde in humans and the discussions about approaches for structuring an integrated, comprehensive risk assessment for formaldehyde. We also note challenges expected when attempting to reconcile divergent results observed from research conducted within and across different scientific disciplines - especially toxicology and epidemiology - and in integrating diverse, multi-disciplinary mechanistic evidence.

Analytical methods impact estimates of trichloroethylene's glutathione conjugation and risk assessment.

The glutathione (GSH) conjugates, S-(1,2-dichlorovinyl)-glutathione (DCVG) and S-(1,2-dichlorovinyl)-L-cysteine (DCVC), have been implicated in kidney toxicity and kidney cancer from trichloroethylene (TCE) exposure. Considerable differences in blood and tissue levels of DCVG and DCVC have been reported, depending on whether HPLC/UV (High Performance Liquid Chromatography-Ultraviolet) or HPLC/MS (HPLC-Mass Spectrometry) was used. A side-by-side comparison of analytical results with HPLC/UV and HPLC/MS/MS (High Performance Liquid Chromatography-Tandem Mass Spectrometry) detection was undertaken to quantitatively compare estimates for DCVG and DCVG using rat and human tissues. For the HPLC method, DCVG and DCVC were initially derivatized with fluorodinitrobenzene (DNP). The results from the HPLC/UV method showed that derivatized-DCVC eluted at the solvent front and could not be quantified. Derivatized-DCVG, however, was quantified but significant interference was observed in all four control tissues (rat blood, liver, kidney; and human blood), resulting in average spike recoveries of 222-22,990%. In contrast, direct analysis of spiked tissues by HPLC/MS/MS resulted in recoveries of 82-127% and 89-117% for DCVG and DCVC, respectively. These differences in analytical results were further confirmed in tissues from TCE-treated rats, e.g., DCVG levels in rat liver were 18,000 times higher by HPLC/UV as compared to HPLC/MS/MS. Fraction collection of the derivatized-DCVG peak (obtained with the HPLC-UV method), followed by peak identification via an HPLC/UV/Q-TOF/MS/MS method, identified DNP-derivatized endogenous glutamate as the primary interfering substance that contributed to and exaggerated recoveries of DCVG. Thus, estimates of DCVG based on the HPLC/UV methods are not reliable; they will over-estimate the formation of the GSH conjugates of TCE and will artifactually exaggerate the potential cancer risk in humans from TCE exposure. Therefore, it is recommended that any characterization of cancer risks from TCE exposure attributable to the GSH conjugates of TCE rely on results obtained with the more specific and reliable HPLC/MS/MS method.

Strain-related differences in mouse lung gene expression over a two-year period of inhalation exposure to styrene: Relevance to human risk assessment.

Both CD-1 and C57BL/6 wildtype (C57BL/6-WT) mice show equivalent short-term lung toxicity from exposures to styrene, while long-term tumor responses are greater in CD-1 mice. We analyzed lung gene expression from styrene exposures lasting from 1-day to 2-years in male mice from these two strains, including a Cyp2f2(-/-) knockout (C57BL/6-KO) and a Cyp2F1/2A13/2B6 transgenic mouse (C57BL/6-TG). With short term exposures (1-day to 1-week), CD-1 and C57BL/6-WT mice had thousands of differentially expressed genes (DEGs), consistent with changes in pathways for cell proliferation, cellular lipid metabolism, DNA-replication and inflammation. C57BL/6-WT mice responded within a single day; CD-1 mice required several days of exposure. The numbers of exposure related DEGs were greatly reduced at longer times (4-weeks to 2-years) with enrichment only for biological oxidations in C57BL/6-WT and metabolism of lipids and lipoproteins in CD-1. Gene expression results indicate a non-genotoxic, mouse specific mode of action for short-term styrene responses related to activation of nuclear receptor signaling and cell proliferation. Greater tumor susceptibility in CD-1 mice correlated with the presence of the Pas1 loci, differential Cytochrome P450 gene expression, down-regulation of Nr4a, and greater inflammatory pathway activation. Very few exposure-related responses occurred at any time in C57BL/6-KO or -TG mice indicating that neither the short term nor long term responses of styrene in mice are relevant endpoints for assessing human risks.

Based on an analysis of mode of action, styrene-induced mouse lung tumors are not a human cancer concern.

Based on 13 chronic studies, styrene exposure causes lung tumors in mice, but no tumor increases in other organs in mice or rats. Extensive research into the mode of action demonstrates the key events and human relevance. Key events are: metabolism of styrene by CYP2F2 in mouse lung club cells to ring-oxidized metabolites; changes in gene expression for metabolism of lipids and lipoproteins, cell cycle and mitotic M-M/G1 phases; cytotoxicity and mitogenesis in club cells; and progression to preneoplastic/neoplastic lesions in lung. Although styrene-7,8-oxide (SO) is a common genotoxic styrene metabolite in in vitro studies, the data clearly demonstrate that SO is not the proximate toxicant and that styrene does not induce a genotoxic mode of action. Based on complete attenuation of styrene short-term and chronic toxicity in CYP2F2 knockout mice and similar attenuation in CYP2F1 (humanized) transgenic mice, limited metabolism of styrene in human lung by CYP2F1, 2 + orders of magnitude lower SO levels in human lung compared to mouse lung, and lack of styrene-related increase in lung cancer in humans, styrene does not present a risk of cancer to humans.

Editor's Highlight: Complete Attenuation of Mouse Lung Cell Proliferation and Tumorigenicity in CYP2F2 Knockout and CYP2F1 Humanized Mice Exposed to Inhaled Styrene for up to 2 Years Supports a Lack of Human Relevance.

Styrene is a mouse-specific lung carcinogen, and short-term mode of action studies have demonstrated that cytotoxicity and/or cell proliferation, and genomic changes are dependent on CYP2F2 metabolism. The current study examined histopathology, cell proliferation, and genomic changes in CD-1, C57BL/6 (WT), CYP2F2(-/-) (KO), and CYP2F2(-/-) (CYP2F1, 2B6, 2A13-transgene) (TG; humanized) mice following exposure for up to 104 weeks to 0- or 120-ppm styrene vapor. Five mice per treatment group were sacrificed at 1, 26, 52, and 78 weeks. Additional 50 mice per treatment group were followed until death or 104 weeks of exposure. Cytotoxicity was present in the terminal bronchioles of some CD-1 and WT mice exposed to styrene, but not in KO or TG mice. Hyperplasia in the terminal bronchioles was present in CD-1 and WT mice exposed to styrene, but not in KO or TG mice. Increased cell proliferation, measured by KI-67 staining, occurred in CD-1 and WT mice exposed to styrene for 1 week, but not after 26, 52, or 78 weeks, nor in KO or TG mice. Styrene increased the incidence of bronchioloalveolar adenomas and carcinomas in CD-1 mice. No increase in lung tumors was found in WT despite clear evidence of lung toxicity, or, KO or TG mice. The absence of preneoplastic lesions and tumorigenicity in KO and TG mice indicates that mouse-specific CYP2F2 metabolism is responsible for both the short-term and chronic toxicity and tumorigenicity of styrene, and activation of styrene by CYP2F2 is a rodent MOA that is neither quantitatively or qualitatively relevant to humans.

How well can carcinogenicity be predicted by high throughput "characteristics of carcinogens" mechanistic data?

IARC has begun using ToxCast/Tox21 data in efforts to represent key characteristics of carcinogens to organize and weigh mechanistic evidence in cancer hazard determinations and this implicit inference approach also is being considered by USEPA. To determine how well ToxCast/Tox21 data can explicitly predict cancer hazard, this approach was evaluated with statistical analyses and machine learning prediction algorithms. Substances USEPA previously classified as having cancer hazard potential were designated as positives and substances not posing a carcinogenic hazard were designated as negatives. Then ToxCast/Tox21 data were analyzed both with and without adjusting for the cytotoxicity burst effect commonly observed in such assays. Using the same assignments as IARC of ToxCast/Tox21 assays to the seven key characteristics of carcinogens, the ability to predict cancer hazard for each key characteristic, alone or in combination, was found to be no better than chance. Hence, we have little scientific confidence in IARC's inference models derived from current ToxCast/Tox21 assays for key characteristics to predict cancer. This finding supports the need for a more rigorous mode-of-action pathway-based framework to organize, evaluate, and integrate mechanistic evidence with animal toxicity, epidemiological investigations, and knowledge of exposure and dosimetry to evaluate potential carcinogenic hazards and risks to humans.

Assessing molecular initiating events (MIEs), key events (KEs) and modulating factors (MFs) for styrene responses in mouse lungs using whole genome gene expression profiling following 1-day and multi-week exposures.

Styrene increased lung tumors in mice at chronic inhalation exposures of 20ppm and greater. MIEs, KEs and MFs were examined using gene expression in three strains of male mice (the parental C57BL/6 strain, a CYP2F2(-/-) knock out and a CYP2F2(-/-) transgenic containing human CYP2F1, 2A13 and 2B6). Exposures were for 1-day and 1, 4 and 26weeks. After 1-day exposures at 1, 5, 10, 20, 40 and 120ppm significant increases in differentially expressed genes (DEGs) occurred only in parental strain lungs where there was already an increase in DEGs at 5ppm and then many thousands of DEGs by 120ppm. Enrichment for 1-day and 1-week exposures included cell cycle, mitotic M-M/G1 phases, DNA-synthesis and metabolism of lipids and lipoproteins pathways. The numbers of DEGs decreased steadily over time with no DEGs meeting both statistical significance and fold-change criteria at 26weeks. At 4 and 26weeks, some key transcription factors (TFs) - Nr1d1, Nr1d2, Dbp, Tef, Hlf, Per3, Per2 and Bhlhe40 - were upregulated (|FC|>1.5), while others - Npas, Arntl, Nfil3, Nr4a1, Nr4a2, and Nr4a3 - were down-regulated. At all times, consistent changes in gene expression only occurred in the parental strain. Our results support a MIE for styrene of direct mitogenicity from mouse-specific CYP2F2-mediated metabolites activating Nr4a signaling. Longer-term MFs include down-regulation of Nr4a genes and shifts in both circadian clock TFs and other TFs, linking circadian clock to cellular metabolism. We found no gene expression changes indicative of cytotoxicity or activation of p53-mediated DNA-damage pathways.

Combining transcriptomics and PBPK modeling indicates a primary role of hypoxia and altered circadian signaling in dichloromethane carcinogenicity in mouse lung and liver.

Dichloromethane (DCM) is a lung and liver carcinogen in mice at inhalation exposures≥2000ppm. The modes of action (MOA) of these responses have been attributed to formation of genotoxic, reactive metabolite(s). Here, we examined gene expression in lung and liver from female B6C3F1 mice exposed to 0, 100, 500, 2000, 3000 and 4000ppm DCM for 90days. We also simulated dose measures - rates of DCM oxidation to carbon monoxide (CO) in lung and liver and expected blood carboxyhemoglobin (HbCO) time courses with a PBPK model inclusive of both conjugation and oxidation pathways. Expression of large numbers of genes was altered at 100ppm with maximal changes in the numbers occurring by 500 or 2000ppm. Most changes in genes common to the two tissues were related to cellular metabolism and circadian clock. At the lower concentrations, the changes in metabolism-related genes were discordant - up in liver and down in lung. These processes included organelle biogenesis, TCA cycle, and respiratory electron transport. Changes in circadian cycle genes - primarily transcription factors - showed strong concentration-related response at higher concentrations (Arntl, Npas2, and Clock were down-regulated; Cry2, Wee1, Bhlhe40, Per3, Nr1d1, Nr1d2 and Dbp) were up-regulated with similar directionality in both tissues. Overall, persistently elevated HbCO from DCM oxidation appears to cause extended periods of hypoxia, leading to altered circadian coupling to cellular metabolism. The dose response for altered circadian processes correlates with the cancer outcome. We found no evidence of changes in genes indicative of responses to cytotoxic, DNA-reactive metabolites.

IARC use of oxidative stress as key mode of action characteristic for facilitating cancer classification: Glyphosate case example illustrating a lack of robustness in interpretative implementation.

The International Agency for Research on Cancer (IARC) has formulated 10 key characteristics of human carcinogens to incorporate mechanistic data into cancer hazard classifications. The analysis used glyphosate as a case example to examine the robustness of IARC's determination of oxidative stress as "strong" evidence supporting a plausible cancer mechanism in humans. The IARC analysis primarily relied on 14 human/mammalian studies; 19 non-mammalian studies were uninformative of human cancer given the broad spectrum of test species and extensive use of formulations and aquatic testing. The mammalian studies had substantial experimental limitations for informing cancer mechanism including use of: single doses and time points; cytotoxic/toxic test doses; tissues not identified as potential cancer targets; glyphosate formulations or mixtures; technically limited oxidative stress biomarkers. The doses were many orders of magnitude higher than human exposures determined in human biomonitoring studies. The glyphosate case example reveals that the IARC evaluation fell substantially short of "strong" supporting evidence of oxidative stress as a plausible human cancer mechanism, and suggests that other IARC monographs relying on the 10 key characteristics approach should be similarly examined for a lack of robust data integration fundamental to reasonable mode of action evaluations.

Analysis of Moms Across America report suggesting bioaccumulation of glyphosate in U.S. mother's breast milk: Implausibility based on inconsistency with available body of glyphosate animal toxicokinetic, human biomonitoring, and physico-chemical data.

The non-peer-reviewed biomonitoring report published online by Moms Across America (MAA; Honeycutt and Rowlands, 2014) does not support the conclusion that glyphosate concentrations detected in a limited number of urine samples from women, men and children, or breast milk from nursing mothers, pose a health risk to the public, including nursing children. Systemically absorbed doses of glyphosate estimated from the MAA urine biomonitoring data and from other published biomonitoring studies indicate that daily glyphosate doses are substantially below health protective reference standards (ADIs; RfDs) established by regulatory agencies. The MAA report also suggested that detection of relatively high glyphosate concentrations in breast milk in 3 of 10 sampled women raised a concern for bioaccumulation in breast milk. However, the breast milk concentrations reported by MAA are highly implausible when considered in context to low daily systemic doses of glyphosate estimated from human urine biomonitoring data, and also are inconsistent with animal toxicokinetic data demonstrating no evidence of retention in tissues or milk after single- or multiple-dose glyphosate treatment. In addition, toxicokinetic studies in lactating goats have shown that glyphosate does not partition into milk at concentrations greater than blood, and that only a very small percentage of the total administered dose (<0.03%) is ultimately excreted into milk. The toxicokinetic studies also indicate that human glyphosate exposures estimated from urine biomonitoring fall thousands-of-fold short of external doses capable of producing blood concentrations sufficient to result in the breast milk concentrations described in the MAA report. Finally, in contrast to highly lipophilic compounds with bioaccumulation potential in breast milk, the physico-chemical properties of glyphosate indicate that it is highly hydrophilic (ionized) at physiological pH and unlikely to preferentially distribute into breast milk.

Risk assessments for chronic exposure of children and prospective parents to ethylbenzene (CAS No. 100-41-4).

Potential chronic health risks for children and prospective parents exposed to ethylbenzene were evaluated in response to the Voluntary Children's Chemical Evaluation Program. Ethylbenzene exposure was found to be predominately via inhalation with recent data demonstrating continuing decreases in releases and both outdoor and indoor concentrations over the past several decades. The proportion of ethylbenzene in ambient air that is attributable to the ethylbenzene/styrene chain of commerce appears to be relatively very small, less than 0.1% based on recent relative emission estimates. Toxicity reference values were derived from the available data, with physiologically based pharmacokinetic models and benchmark dose methods used to assess dose-response relationships. An inhalation non-cancer reference concentration or RfC of 0.3 parts per million (ppm) was derived based on ototoxicity. Similarly, an oral non-cancer reference dose or RfD of 0.5 mg/kg body weight/day was derived based on liver effects. For the cancer assessment, emphasis was placed upon mode of action information. Three of four rodent tumor types were determined not to be relevant to human health. A cancer reference value of 0.48 ppm was derived based on mouse lung tumors. The risk characterization for ethylbenzene indicated that even the most highly exposed children and prospective parents are not at risk for non-cancer or cancer effects of ethylbenzene.