Does ozone inhalation cause adverse metabolic effects in humans? A systematic review.

We utilized a practical, transparent approach for systematically reviewing a chemical-specific evidence base. This approach was used for a case study of ozone inhalation exposure and adverse metabolic effects (overweight/obesity, Type 1 diabetes [T1D], Type 2 diabetes [T2D], and metabolic syndrome). We followed the basic principles of systematic review. Studies were defined as "Suitable" or "Supplemental." The evidence for Suitable studies was characterized as strong or weak. An overall causality judgment for each outcome was then determined as either causal, suggestive, insufficient, or not likely. Fifteen epidemiologic and 33 toxicologic studies were Suitable for evidence synthesis. The strength of the human evidence was weak for all outcomes. The toxicologic evidence was weak for all outcomes except two: body weight, and impaired glucose tolerance/homeostasis and fasting/baseline hyperglycemia. The combined epidemiologic and toxicologic evidence was categorized as weak for overweight/obesity, T1D, and metabolic syndrome,. The association between ozone exposure and T2D was determined to be insufficient or suggestive. The streamlined approach described in this paper is transparent and focuses on key elements. As systematic review guidelines are becoming increasingly complex, it is worth exploring the extent to which related health outcomes should be combined or kept distinct, and the merits of focusing on critical elements to select studies suitable for causal inference. We recommend that systematic review results be used to target discussions around specific research needs for advancing causal determinations.

Improving risk assessment approaches for chemicals with both endogenous and exogenous exposures.

To conduct risk assessments of exogenous chemicals for which there are also endogenous exposures, knowledge of the chemistry and biology of both types of exposures needs to be integrated into problem formulation and carried through to risk characterization. This issue is framed in a risk assessment context, highlighting the importance of quantifying increments of dose from all sources of the same or similar chemicals interacting with biological targets; understanding the influence of endogenous chemical concentrations on disease risk; and assessing total dose to targets in evaluating risk from incremental environmental exposures. Examples of recent assessments illustrate the importance of addressing this issue. Evaluations of data on blood or organ concentrations of ammonia, methanol, formaldehyde, acetaldehyde, and three gaseous signaling molecules (hydrogen sulfide, carbon monoxide, and nitric oxide) provide examples where current data are already informing perspectives on relative exposures at the portal of entry and systemically. To facilitate quality risk assessments of exogenous chemicals with endogenous exposures, a series of specific questions are presented that need to be addressed in systematic review to enhance problem formulation, improve the development of holistic conceptual models, and to facilitate the identification of priority data needs for improving risk assessments.

Development of an adverse outcome pathway for chemically induced hepatocellular carcinoma: case study of AFB1, a human carcinogen with a mutagenic mode of action.

Adverse outcome pathways (AOPs) are frameworks starting with a molecular initiating event (MIE), followed by key events (KEs) linked by KE relationships (KERs), ultimately resulting in a specific adverse outcome. Relevant data for the pathway and each KE/KER are evaluated to assess biological plausibility, weight-of-evidence, and confidence. We aimed to describe an AOP relevant to chemicals directly inducing mutation in cancer critical gene(s), via the formation of chemical-specific pro-mutagenic DNA adduct(s), as an early critical step in tumor etiology. Such chemicals have mutagenic modes-of-action (MOA) for tumor induction. To assist with developing this AOP, Aflatoxin B1 (AFB1) was selected as a case study because it has a rich database and is considered to have a mutagenic MOA. AFB1 information was used to define specific KEs, KERs, and to inform development of a generic AOP for mutagen-induced hepatocellular carcinoma (HCC). In assessing the AFB1 information, it became clear that existing data are, in fact, not optimal and for some KEs/KERs, the definitive data are not available. In particular, while there is substantial information that AFB1 can induce mutations (based on a number of mutation assays), the definitive evidence - the ability to induce mutation in the cancer critical gene(s) in the tumor target tissue - is not available. Thus, it is necessary to consider the patterns of results in the weight-of-evidence for KEs and KERs. It was important to determine whether there was sufficient evidence that AFB1 can induce the necessary critical mutations early in the carcinogenic process, which was the case.

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.

Quantitative weight of evidence to assess confidence in potential modes of action.

The evolved World Health Organization/International Programme on Chemical Safety mode of action (MOA) framework provides a structure for evaluating evidence in pathways of causally linked key events (KE) leading to adverse health effects. Although employed globally, variability in use of the MOA framework has led to different interpretations of the sufficiency of evidence in support of hypothesized MOAs. A proof of concept extension of the MOA framework is proposed for scoring confidence in the supporting data to improve scientific justification for MOA use in characterizing hazards and selecting dose-response extrapolation methods for specific chemicals. This involves selecting hypothesized MOAs, and then, for each MOA, scoring the weight of evidence (WOE) in support of causality for each KE using evolved Bradford Hill causal considerations (biological plausibility, essentiality, dose-response concordance, consistency, and analogy). This early proof of concept method is demonstrated by comparing two potential MOAs (mutagenicity and peroxisome proliferator activated receptor-alpha) for clofibrate, a rodent liver carcinogen. Quantitative confidence scoring of hypothesized MOAs is shown to be useful in characterizing the likely operative MOA. To guide method refinement and future confidence scoring for a spectrum of MOAs, areas warranting further focus and lessons learned, including the need to incorporate a narrative discussion of the weights used in the evaluation and an overall evaluation of the plausibility of the outcome, are presented.

Dose-Response for Multiple Biomarkers of Exposure and Genotoxic Effect Following Repeated Treatment of Rats with the Alkylating Agents, MMS and MNU.

The nature of the dose-response relationship for various in vivo endpoints of exposure and effect were investigated using the alkylating agents, methyl methanesulfonate (MMS) and methylnitrosourea (MNU). Six male F344 rats/group were dosed orally with 0, 0.5, 1, 5, 25 or 50mg/kg bw/day (mkd) of MMS, or 0, 0.01, 0.1, 1, 5, 10, 25 or 50 mkd of MNU, for 4 consecutive days and sacrificed 24h after the last dose. The dose-responses for multiple biomarkers of exposure and genotoxic effect were investigated. In MMS-treated rats, the hemoglobin adduct level, a systemic exposure biomarker, increased linearly with dose (r (2) = 0.9990, P < 0.05), indicating the systemic availability of MMS; however, the N7MeG DNA adduct, a target exposure biomarker, exhibited a non-linear dose-response in blood and liver tissues. Blood reticulocyte micronuclei (MN), a genotoxic effect biomarker, exhibited a clear no-observed-genotoxic-effect-level (NOGEL) of 5 mkd as a point of departure (PoD) for MMS. Two separate dose-response models, the Lutz and Lutz model and the stepwise approach using PROC REG both supported a bilinear/threshold dose-response for MN induction. Liver gene expression, a mechanistic endpoint, also exhibited a bilinear dose-response. Similarly, in MNU-treated rats, hepatic DNA adducts, gene expression changes and MN all exhibited clear PoDs, with a NOGEL of 1 mkd for MN induction, although dose-response modeling of the MNU-induced MN data showed a better statistical fit for a linear dose-response. In summary, these results provide in vivo data that support the existence of clear non-linear dose-responses for a number of biologically significant events along the pathway for genotoxicity induced by DNA-reactive agents.

Pronamide: Weight of evidence for potential estrogen, androgen or thyroid effects.

Based on the exposure potential to humans and environment, pronamide was one of 52 chemicals on the first list evaluated under US EPA's Endocrine Disruptor Screening Program (EDSP). The purpose of EDSP is to screen chemicals for their potential to interact with estrogen-, androgen-, or thyroid-signaling pathways. A battery of 11 Tier 1 assays was completed for pronamide in accordance with EDSP test guidelines. In addition, Other Scientifically Relevant Information, which included existing data from regulatory guideline studies and published literature, was used in a weight-of-evidence (WoE) evaluation of potential endocrine activity. The WoE conclusion is that pronamide does not interact directly with estrogen, androgen, or thyroid receptors or post-receptor events. Across in vivo studies, the liver is consistently and reproducibly the target organ for pronamide's effects. Pronamide activates hepatocytic nuclear receptors (including constitutive androstane receptor), induces hepatic enzymes, produces hepatocellular hypertrophy and increases liver weights. These changes are coupled with increased metabolic activity and a subsequent increased metabolism and/or clearance of both steroid and thyroid hormones. Thus, while pronamide alters some endocrine-sensitive endpoints in EDSP Tier 1 assays, effects on liver metabolism likely explain altered hormone levels and indirect endocrine changes.

Mode of action and human relevance of pronamide-induced rat thyroid tumors.

Pronamide, a selective, systemic, pre- and post-emergence herbicide, caused an increased incidence of thyroid follicular cell adenomas in a rat carcinogenicity study. Thyroid tumors, as well as liver and pituitary changes, were limited only to the high-dose group. The evidence for and against specific potential modes of action (MoAs) for rat thyroid follicular cell adenomas and their relevance to humans is discussed. Pronamide is not mutagenic and therefore, direct DNA reactivity is not relevant as a MoA. The hypothesized MoA for this effect is altered homeostasis of the hypothalamic-pituitary-thyroid (HPT) axis mediated by the induction of hepatic enzymes, including uridine diphosphate glucuronosyltransferase (UGT). Evaluation of data from a series of regulatory guideline and MoA studies aimed at identifying the causative and associated key events supported a UGT-mediated MoA in the development of thyroid follicular tumors. This MoA for pronamide-induced thyroid tumors in rats, which involves increased thyroid hormone metabolism/clearance, altered thyroid hormone homeostasis and HPT stimulation is not considered relevant to humans based on quantitative species differences, making rats markedly more sensitive than humans to thyroid perturbations.

Pronamide: Human relevance of liver-mediated rat leydig cell tumors.

Dietary exposure to pronamide resulted in higher incidences of Leydig cell tumors (LCT) at 1000ppm in a 2-year cancer bioassay, but there were no testes effects at 40 or 200ppm, and no testes effects at 12-months at any concentration. A 90-day mode-of-action (MoA) study was conducted at concentrations of 0, 200, 1000 and 2000ppm. Standard parameters and stereological and proliferation analyses of LCs, targeted testis and liver gene expression, in vitro metabolism of testosterone by liver microsomes, and quantification of serum hormones and testosterone metabolites were evaluated. Increased testosterone metabolism due to increases in hepatic microsomal activity, alterations in serum hormone levels, and other data suggest that LCTs were mediated through a perturbation of the HPG-axis. Data suggest that this occurs after a threshold of exposure is reached, indicating a nonlinear/threshold dose-response. Pronamide-induced rat LCTs mediated by alterations to the HPG-axis have low relevance to humans due to quantitative differences in sensitivity between rats and humans to LCTs. Pronamide displayed no genotoxicity or direct endocrine effects. A margin of exposure approach for risk assessment and derivation of the chronic reference dose based on a point of departure of 200ppm is most appropriate and protective of human health.

An organizational approach for the assessment of DNA adduct data in risk assessment: case studies for aflatoxin B1, tamoxifen and vinyl chloride.

The framework analysis previously presented for using DNA adduct information in the risk assessment of chemical carcinogens was applied in a series of case studies which place the adduct information into context with the key events in carcinogenesis to determine whether they could be used to support a mutagenic mode of action (MOA) for the examined chemicals. Three data-rich chemicals, aflatoxin B1 (AFB1), tamoxifen (Tam) and vinyl chloride (VCl) were selected for this exercise. These chemicals were selected because they are known human carcinogens and have different characteristics: AFB1 forms a unique adduct and human exposure is through contaminated foods; Tam is a pharmaceutical given to women so that the dose and duration of exposure are known, forms unique adducts in rodents, and has both estrogenic and genotoxic properties; and VCl, to which there is industrial exposure, forms a number of adducts that are identical to endogenous adducts found in unexposed people. All three chemicals produce liver tumors in rats. AFB1 and VCl also produce liver tumors in humans, but Tam induces human uterine tumors, only. To support a mutagenic MOA, the chemical-induced adducts must be characterized, shown to be pro-mutagenic, be present in the tumor target tissue, and produce mutations of the class found in the tumor. The adducts formed by AFB1 and VCl support a mutagenic MOA for their carcinogenicity. However, the data available for Tam shows a mutagenic MOA for liver tumors in rats, but its carcinogenicity in humans is most likely via a different MOA.

A toxicological review of the propylene glycols.

The toxicological profiles of monopropylene glycol (MPG), dipropylene glycol (DPG), tripropylene glycol (TPG) and polypropylene glycols (PPG; including tetra-rich oligomers) are collectively reviewed, and assessed considering regulatory toxicology endpoints. The review confirms a rich data set for these compounds, covering all of the major toxicological endpoints of interest. The metabolism of these compounds share common pathways, and a consistent profile of toxicity is observed. The common metabolism provides scientific justification for adopting a read-across approach to describing expected hazard potential from data gaps that may exist for specific oligomers. None of the glycols reviewed presented evidence of carcinogenic, mutagenic or reproductive/developmental toxicity potential to humans. The pathologies reported in some animal studies either occurred at doses that exceeded experimental guidelines, or involved mechanisms that are likely irrelevant to human physiology and therefore are not pertinent to the exposures experienced by consumers or workers. At very high chronic doses, MPG causes a transient, slight decrease in hemoglobin in dogs and at somewhat lower doses causes Heinz bodies to form in cats in the absence of any clinical signs of anemia. Some evidence for rare, idiosyncratic skin reactions exists for MPG. However, the larger data set indicates that these compounds have low sensitization potential in animal studies, and therefore are unlikely to represent human allergens. The existing safety evaluations of the FDA, USEPA, NTP and ATSDR for these compounds are consistent and point to the conclusion that the propylene glycols present a very low risk to human health.

Advancing human health risk assessment: integrating recent advisory committee recommendations.

Over the last dozen years, many national and international expert groups have considered specific improvements to risk assessment. Many of their stated recommendations are mutually supportive, but others appear conflicting, at least in an initial assessment. This review identifies areas of consensus and difference and recommends a practical, biology-centric course forward, which includes: (1) incorporating a clear problem formulation at the outset of the assessment with a level of complexity that is appropriate for informing the relevant risk management decision; (2) using toxicokinetics and toxicodynamic information to develop Chemical Specific Adjustment Factors (CSAF); (3) using mode of action (MOA) information and an understanding of the relevant biology as the key, central organizing principle for the risk assessment; (4) integrating MOA information into dose-response assessments using existing guidelines for non-cancer and cancer assessments; (5) using a tiered, iterative approach developed by the World Health Organization/International Programme on Chemical Safety (WHO/IPCS) as a scientifically robust, fit-for-purpose approach for risk assessment of combined exposures (chemical mixtures); and (6) applying all of this knowledge to enable interpretation of human biomonitoring data in a risk context. While scientifically based defaults will remain important and useful when data on CSAF or MOA to refine an assessment are absent or insufficient, assessments should always strive to use these data. The use of available 21st century knowledge of biological processes, clinical findings, chemical interactions, and dose-response at the molecular, cellular, organ and organism levels will minimize the need for extrapolation and reliance on default approaches.

A survey of frameworks for best practices in weight-of-evidence analyses.

The National Academy of Sciences (NAS) Review of the Environmental Protection Agency's Draft IRIS Assessment of Formaldehyde proposed a "roadmap" for reform and improvement of the Agency's risk assessment process. Specifically, it called for development of a transparent and defensible methodology for weight-of-evidence (WoE) assessments. To facilitate development of an improved process, we developed a white paper that reviewed approximately 50 existing WoE frameworks, seeking insights from their variations and nominating best practices for WoE analyses of causation of chemical risks. Four phases of WoE analysis were identified and evaluated in each framework: (1) defining the causal question and developing criteria for study selection, (2) developing and applying criteria for review of individual studies, (3) evaluating and integrating evidence and (4) drawing conclusions based on inferences. We circulated the draft white paper to stakeholders and then held a facilitated, multi-disciplinary invited stakeholder workshop to broaden and deepen the discussion on methods, rationales, utility and limitations among the surveyed WoE frameworks. The workshop developed recommendations for improving the conduct of WoE evaluations. Based on the analysis of the 50 frameworks and discussions at the workshop, best practices in conducting WoE analyses were identified for each of the four phases. Many of these best practices noted from the analysis and workshop could be implemented immediately, while others may require additional refinement as part of the ongoing discussions for improving the scientific basis of chemical risk assessments.

Workshop report: identifying key issues underpinning the selection of linear or non-linear dose-response extrapolation for human health risk assessment of systemic toxicants.

The report of an Expert Panel convened by the National Research Council (NRC) of the National Academy of Sciences (NAS), entitled Science and Decisions: Advancing Risk Assessment (National Research Council, 2009a), includes a recommendation to use, as a default approach, low-dose linear extrapolation for systemic toxicity. This recommendation represents a significant departure from long-standing risk assessment practices for non-cancer toxicity, where the most appropriate No Observed Adverse Effect Level (NOAEL) or Benchmark Dose (BMD) of the critical effect in the key study is selected, and then a "safe exposure" level is derived by applying uncertainty factors to account for dataset completeness, potential greater sensitivity of humans when compared with experimental animals, and for potential variability of sensitivity in humans. A workshop was held to "frame" issues raised by the NAS report that needed further study. Workshop objectives included the following: (1) identify the issues raised by the 2009 NRC report and discuss the extent to which existing science may (or may not) align with the NAS analyses and recommendations, and (2) identify/develop possible actions to assist in advancing deeper and broader considerations of some of the critical issues presented by the 2009 NAS Panel. Experts invited to this "Framing" Workshop encompassed a full spectrum of toxicology and risk assessment disciplines; in particular, expertise in molecular interactions and dose-response of biological systems were well represented. The recommendations developed at this Framing Workshop provide specific ideas on possible further steps to facilitate deeper and broader consideration of the issues underpinning dose-response extrapolation in the risk assessment of systemic toxicants.

Derivation of inhalation toxicity reference values for propylene oxide using mode of action analysis: example of a threshold carcinogen.

Propylene oxide (PO) is an important industrial chemical used primarily in the synthesis of other compounds. Inhalation carcinogenesis studies in rodents, with no-observed-adverse-effect levels (NOAELs) of 100 and 200 ppm, have revealed that chronic, high exposure to PO can induce tumors at the site of contact. Despite these characteristics, there is no evidence that typical environmental or occupational exposures to PO constitute a health risk for humans. The nongenotoxic effects of PO (glutathione depletion and cell proliferation) that augment its DNA-reactive and non-DNA-reactive genotoxicity are expected to be similar in humans and rodents. Available evidence on mode-of-action suggests that cancer induction by PO at the site of contact in rodents is characterized by a practical threshold. Human toxicity reference values for potential carcinogenic effects of PO were derived based on nasal tumors identified in rodent studies and specified uncertainty factors. The 95% lower confidence limit on the dose producing a 10% increase in additional tumor risk (LED10) was calculated using the rat and mouse data sets. The human reference values derived from the rat and mouse LED10 values were 0.7 and 0.5 ppm PO, respectively. A similar noncancer reference value, 0.4 ppm, was derived on the basis of non-neoplastic nasal effects in rats.

Creating context for the use of DNA adduct data in cancer risk assessment: I. Data organization.

The assessment of human cancer risk from chemical exposure requires the integration of diverse types of data. Such data involve effects at the cell and tissue levels. This report focuses on the specific utility of one type of data, namely DNA adducts. Emphasis is placed on the appreciation that such DNA adduct data cannot be used in isolation in the risk assessment process but must be used in an integrated fashion with other information. As emerging technologies provide even more sensitive quantitative measurements of DNA adducts, integration that establishes links between DNA adducts and accepted outcome measures becomes critical for risk assessment. The present report proposes an organizational approach for the assessment of DNA adduct data (e.g., type of adduct, frequency, persistence, type of repair process) in concert with other relevant data, such as dosimetry, toxicity, mutagenicity, genotoxicity, and tumor incidence, to inform characterization of the mode of action. DNA adducts are considered biomarkers of exposure, whereas gene mutations and chromosomal alterations are often biomarkers of early biological effects and also can be bioindicators of the carcinogenic process.

A case for a new paradigm in genetic toxicology testing.

The field of genetic toxicology is in need of a transformational change in experimental approaches and data interpretation so that genotoxicity data can better inform risk assessment. The historical approach of the one-hit theory for DNA-reactive chemicals and the view of genotoxicity as an inherent property of a chemical are being challenged, based on a better understanding of the complexity of molecular mechanisms of mutation. A seemingly simple, but hitherto rarely practiced, approach that could help catapult the field forward is the application of the fundamental tenet of toxicology, a better understanding of the dose-response. There is a growing body of evidence to support the existence of thresholds/NOAELs for genotoxic effects, even from DNA-reactive chemicals. It is also suggested that a better understanding of the internal and/or effective dose to the critical target, for both in vitro and in vivo experiments, can significantly help to improve characterization of the shape of the dose-response curve and serve to support cross-species extrapolation. These experimental design and data interpretation approaches will render genetic toxicology data more useful to inform the mode-of-action-based risk assessment process, and provide the paradigm shift necessary to help bring the field into the 21st century.

Dose-response and operational thresholds/NOAELs for in vitro mutagenic effects from DNA-reactive mutagens, MMS and MNU.

The dose-response relationships for in vitro mutagenicity induced by methylmethanesulfonate (MMS) or methylnitrosourea (MNU) in L5178Y mouse lymphoma (ML) cells were examined. DNA adducts (N7-methylguanine, N7MeG and O(6)-methylguanine, O(6)MeG) were quantified as biomarkers of exposure. Both endpoints were assessed using 5replicates/dose (4-h treatment) with MMS or MNU (0.0069-50muM), or vehicle (1% DMSO). Mutant frequency (MF) (thymidine kinase (TK) locus) was determined using the soft agar cloning methodology and a 2-day expression period; in addition, microwell and Sequester-Express-Select (SES) methods were used for MMS. Isolated DNA was acid-hydrolyzed, and adducts quantified by LC/ESI-MS/MS, using authentic and internal standards. MF dose-responses were analyzed using several statistical approaches, all of which confirmed that a threshold dose-response model provided the best fit. NOAELs for MF were 10muM MMS and 0.69muM MNU, based on ANOVA and Dunnett's test (p<0.05). N7MeG adducts were present in all cell samples, including solvent-control cells, and were increased over control levels in cells treated with >/=10muM MMS or 3.45muM MNU. O(6)MeG levels were only quantifiable at >/=10muM MNU; O(6)MeG was not quantifiable in control or MMS-treated cells at current detection limits. Thus, (1) cells treated with

ECETOC workshop on the biological significance of DNA adducts: summary of follow-up from an expert panel meeting.

This workshop on the biological significance of DNA adducts included presentations of research results in the following areas: endogenous versus exogenous adduct levels; in vitro dose-response data on adducts and mutagenesis from alkylating agents; methyltransferases and alkyl transferase-like proteins in repair of O(6)-alkylguanine adducts; mathematical modeling of threshold dose-response in mutagenesis and carcinogenesis; and the use of genomics to characterize the relationships between adducts, gene expression, and downstream adverse effects. Presentations by regulatory scientists and other authorities addressed the role of adduct and mutation data in risk characterization. Consensus statements were developed and included the following: DNA adducts should be considered as biomarkers of exposure, which may play a key role in establishing a mode of action (MOA) for cancer. Adducts themselves should not be considered as equivalent to mutations or later stage events in carcinogenesis. Although it was not possible at this time to agree on a general level of adducts below which there is no adverse biological effect, there are examples of genotoxic mutagens/carcinogens for which thresholds have been demonstrated. Evidence regarding thresholds for mutations should be considered on a case-by-case basis, in light of available MOA and mechanistic data, to build a knowledge base. Participants agreed that guidance on a recommended format for data presentation (especially agreement on units and appropriate statistical analyses) would be beneficial. Finally, for initial cases, provision of a mechanistic explanation to support a hypothesis of a threshold for mutations was essential for the eventual use of this information in risk assessment.

Critical review of styrene genotoxicity focused on the mutagenicity/clastogenicity literature and using current organization of economic cooperation and development guidance.

Styrene is an important high production volume chemical used to manufacture polymeric products. In 2018, International Agency for Research on Cancer classified styrene as probably carcinogenic to humans; National Toxicology Program lists styrene as reasonably anticipated to be a human carcinogen. The genotoxicity literature for styrene and its primary metabolite, styrene 7,8-oxide (SO), begins in the 1970s. Organization of Economic Cooperation and Development (OECD) recently updated most genotoxicity test guidelines, making substantial new recommendations for assay conduct and data evaluation for the standard mutagenicity/clastogenicity assays. Thus, a critical review of the in vitro and in vivo rodent mutagenicity/clastogenicity studies for styrene and SO, based on the latest OECD recommendations, is timely. This critical review considered whether a study was optimally designed, conducted, and interpreted and provides a critical assessment of the evidence for the mutagenicity/clastogenicity of styrene/SO. Information on the ability of styrene/SO to induce other types of genotoxicity endpoints is summarized but not critically reviewed. We conclude that when styrene is metabolized to SO, it can form DNA adducts, and positive in vitro mutagenicity/clastogenicity results can be obtained. SO is mutagenic in bacteria and the in vitro mouse lymphoma gene mutation assay. No rodent in vivo mutation studies were identified. SO is clastogenic in cultured mammalian cells. Although the in vitro assays gave positive responses, styrene/SO is not clastogenic/aneugenic in vivo in rodents. In addition to providing updated information for styrene, this review demonstrates the application of the new OECD guidelines for chemicals with large genetic toxicology databases where published results may or may not be reliable. Environ. Mol. Mutagen. 2019. © 2019 Wiley Periodicals, Inc.