An empirical approach to sufficient similarity: combining exposure data and mixtures toxicology data

Combined Exposures and Mixtures Research: An Enduring NIEHS Priority

BACKGROUND

The National Institute of Environmental Health Sciences (NIEHS) continues to prioritize research to better understand the health effects resulting from exposure to mixtures of chemical and nonchemical stressors. Mixtures research activities over the last decade were informed by expert input during the development and deliberations of the 2011 NIEHS Workshop "Advancing Research on Mixtures: New Perspectives and Approaches for Predicting Adverse Human Health Effects." NIEHS mixtures research efforts since then have focused on key themes including a) prioritizing mixtures for study, b) translating mixtures data from in vitro and in vivo studies, c) developing cross-disciplinary collaborations, d) informing component-based and whole-mixture assessment approaches, e) developing sufficient similarity methods to compare across complex mixtures, f) using systems-based approaches to evaluate mixtures, and g) focusing on management and integration of mixtures-related data.

OBJECTIVES

We aimed to describe NIEHS driven research on mixtures and combined exposures over the last decade and present areas for future attention.

RESULTS

Intramural and extramural mixtures research projects have incorporated a diverse array of chemicals (e.g., polycyclic aromatic hydrocarbons, botanicals, personal care products, wildfire emissions) and nonchemical stressors (e.g., socioeconomic factors, social adversity) and have focused on many diseases (e.g., breast cancer, atherosclerosis, immune disruption). We have made significant progress in certain areas, such as developing statistical methods for evaluating multiple chemical associations in epidemiology and building translational mixtures projects that include both in vitro and in vivo models.

DISCUSSION

Moving forward, additional work is needed to improve mixtures data integration, elucidate interactions between chemical and nonchemical stressors, and resolve the geospatial and temporal nature of mixture exposures. Continued mixtures research will be critical to informing cumulative impact assessments and addressing complex challenges, such as environmental justice and climate change. https://doi.org/10.1289/EHP14340.

Modeling PAH Mixture Interactions in a Human In Vitro Organotypic Respiratory Model

One of the most significant challenges in human health risk assessment is to evaluate hazards from exposure to environmental chemical mixtures. Polycyclic aromatic hydrocarbons (PAHs) are a class of ubiquitous contaminants typically found as mixtures in gaseous and particulate phases in ambient air pollution associated with petrochemicals from Superfund sites and the burning of fossil fuels. However, little is understood about how PAHs in mixtures contribute to toxicity in lung cells. To investigate mixture interactions and component additivity from environmentally relevant PAHs, two synthetic mixtures were created from PAHs identified in passive air samplers at a legacy creosote site impacted by wildfires. The primary human bronchial epithelial cells differentiated at the air-liquid interface were treated with PAH mixtures at environmentally relevant proportions and evaluated for the differential expression of transcriptional biomarkers related to xenobiotic metabolism, oxidative stress response, barrier integrity, and DNA damage response. Component additivity was evaluated across all endpoints using two independent action (IA) models with and without the scaling of components by toxic equivalence factors. Both IA models exhibited trends that were unlike the observed mixture response and generally underestimated the toxicity across dose suggesting the potential for non-additive interactions of components. Overall, this study provides an example of the usefulness of mixture toxicity assessment with the currently available methods while demonstrating the need for more complex yet interpretable mixture response evaluation methods for environmental samples.

Wrangling Whole Mixtures Risk Assessment: Recent Advances in Determining Sufficient Similarity

Human health risk assessments for complex mixtures can address real-world exposures and protect public health. While risk assessors typically prefer whole mixture approaches over component-based approaches, data from the precise exposure of interest are often unavailable and surrogate data from a sufficiently similar mixture(s) are required. This review describes recent advances in determining sufficient similarity of whole, complex mixtures spanning the comparison of chemical features, bioactivity profiles, and statistical evaluation to determine "thresholds of similarity". Case studies, including water disinfection byproducts, botanical ingredients, and wildfire emissions, are used to highlight tools and methods. Limitations to application of sufficient similarity in risk-based decision making are reviewed and recommendations presented for developing best practice guidelines.

From Cohort to Cohort: A Similar Mixture Approach (SMACH) to Evaluate Exposures to a Mixture Leading to Thyroid-Mediated Neurodevelopmental Effects Using NHANES Data

Prenatal exposure to a mixture (MIX N) of eight endocrine-disrupting chemicals has been associated with language delay in children in a Swedish pregnancy cohort. A novel approach was proposed linking this epidemiological association with experimental evidence, where the effect of MIX N on thyroid hormone signaling was assessed using the Xenopus eleuthero-embryonic thyroid assay (XETA OECD TG248). From this experimental data, a point of departure (PoD) was derived based on OECD guidance. Our aim in the current study was to use updated toxicokinetic models to compare exposures of women of reproductive age in the US population to MIX N using a Similar Mixture Approach (SMACH). Based on our findings, 66% of women of reproductive age in the US (roughly 38 million women) had exposures sufficiently similar to MIX N. For this subset, a Similar Mixture Risk Index (SMRI_HI) was calculated comparing their exposures to the PoD. Women with SMRI_HI > 1 represent 1.1 million women of reproductive age. Older women, Mexican American and other/multi race women were less likely to have high SMRI_HI values compared to Non-Hispanic White women. These findings indicate that a reference mixture of chemicals identified in a Swedish cohort-and tested in an experimental model for establishment of (PoDs)-is also of health relevance in a US population.

Wildfire Variable Toxicity: Identifying Biomass Smoke Exposure Groupings through Transcriptomic Similarity Scoring

The prevalence of wildfires continues to grow globally with exposures resulting in increased disease risk. Characterizing these health risks remains difficult due to the wide landscape of exposures that can result from different burn conditions and fuel types. This study tested the hypothesis that biomass smoke exposures from variable fuels and combustion conditions group together based on similar transcriptional response profiles, informing which wildfire-relevant exposures may be considered as a group for health risk evaluations. Mice (female CD-1) were exposed via oropharyngeal aspiration to equal mass biomass smoke condensates produced from flaming or smoldering burns of eucalyptus, peat, pine, pine needles, or red oak species. Lung transcriptomic signatures were used to calculate transcriptomic similarity scores across exposures, which informed exposure groupings. Exposures from flaming peat, flaming eucalyptus, and smoldering eucalyptus induced the greatest responses, with flaming peat grouping with the pro-inflammatory agent lipopolysaccharide. Smoldering red oak and smoldering peat induced the least transcriptomic response. Groupings paralleled pulmonary toxicity markers, though they were better substantiated by higher data dimensionality and resolution provided through -omic-based evaluation. Interestingly, groupings based on smoke chemistry signatures differed from transcriptomic/toxicity-based groupings. Wildfire-relevant exposure groupings yield insights into risk assessment strategies to ultimately protect public health.

Leveraging Multiple Data Streams for Prioritization of Mixtures for Hazard Characterization

There is a growing need to establish alternative approaches for mixture safety assessment of polycyclic aromatic hydrocarbons (PAHs). Due to limitations with current component-based approaches, and the lack of established methods for using whole mixtures, a promising alternative is to use sufficiently similar mixtures; although, an established framework is lacking. In this study, several approaches are explored to form sufficiently similar mixtures. Multiple data streams including environmental concentrations and empirically and predicted toxicity data for cancer and non-cancer endpoints were used to prioritize chemical components for mixture formations. Air samplers were analyzed for unsubstituted and alkylated PAHs. A synthetic mixture of identified PAHs was created (Creosote-Fire Mix). Existing toxicity values and chemical concentrations were incorporated to identify hazardous components in the Creosote-Fire Mix. Sufficiently similar mixtures of the Creosote-Fire Mix were formed based on (1) relative abundance; (2) toxicity values; and (3) a combination approach incorporating toxicity and abundance. Hazard characterization of these mixtures was performed using high-throughput screening in primary normal human bronchial epithelium (NHBE) and zebrafish. Differences in chemical composition and potency were observed between mixture formation approaches. The toxicity-based approach (Tox Mix) was the most potent mixture in both models. The combination approach (Weighted-Tox Mix) was determined to be the ideal approach due its ability to prioritize chemicals with high exposure and hazard potential.

From cohorts to molecules: Adverse impacts of endocrine disrupting mixtures

Convergent evidence associates exposure to endocrine disrupting chemicals (EDCs) with major human diseases, even at regulation-compliant concentrations. This might be because humans are exposed to EDC mixtures, whereas chemical regulation is based on a risk assessment of individual compounds. Here, we developed a mixture-centered risk assessment strategy that integrates epidemiological and experimental evidence. We identified that exposure to an EDC mixture in early pregnancy is associated with language delay in offspring. At human-relevant concentrations, this mixture disrupted hormone-regulated and disease-relevant regulatory networks in human brain organoids and in the model organisms Xenopus leavis and Danio rerio, as well as behavioral responses. Reinterrogating epidemiological data, we found that up to 54% of the children had prenatal exposures above experimentally derived levels of concern, reaching, for the upper decile compared with the lowest decile of exposure, a 3.3 times higher risk of language delay.

A Novel Approach to Chemical Mixture Risk Assessment-Linking Data from Population-Based Epidemiology and Experimental Animal Tests

Humans are continuously exposed to chemicals with suspected or proven endocrine disrupting chemicals (EDCs). Risk management of EDCs presents a major unmet challenge because the available data for adverse health effects are generated by examining one compound at a time, whereas real-life exposures are to mixtures of chemicals. In this work, we integrate epidemiological and experimental evidence toward a whole mixture strategy for risk assessment. To illustrate, we conduct the following four steps in a case study: (1) identification of single EDCs ("bad actors")-measured in prenatal blood/urine in the SELMA study-that are associated with a shorter anogenital distance (AGD) in baby boys; (2) definition and construction of a "typical" mixture consisting of the "bad actors" identified in Step 1; (3) experimentally testing this mixture in an in vivo animal model to estimate a dose-response relationship and determine a point of departure (i.e., reference dose [RfD]) associated with an adverse health outcome; and (4) use a statistical measure of "sufficient similarity" to compare the experimental RfD (from Step 3) to the exposure measured in the human population and generate a "similar mixture risk indicator" (SMRI). The objective of this exercise is to generate a proof of concept for the systematic integration of epidemiological and experimental evidence with mixture risk assessment strategies. Using a whole mixture approach, we could find a higher rate of pregnant women under risk (13%) when comparing with the data from more traditional models of additivity (3%), or a compound-by-compound strategy (1.6%).

Integrating chemical similarity and bioequivalence: A pilot study on quality consistency evaluation of dispensing granule and traditional decoction of Scutellariae Radix by a totality-of-the-evidence approach

There is an increasing focus on the quality consistency evaluation of dispensing granule in traditional Chinese medicines (TCMs). According to the guideline from Chinese Pharmacopoeia Commission, the substantial equivalence of dispensing granule and traditional decoction should be determined, and the chromatographic fingerprint has been recommended as a comprehensive qualitative approach to assess the quality consistency between dispensing granule and traditional decoction. However, a high-degree chemical similarity does not equal a bioequivalence. Attempting to realize the quality evaluation by integrating chemical consistency and bioequivalence, we herein proposed a totality-of-the-evidence approach based on clustering analysis and equivalence evaluation taking the dispensing granule and traditional decoction of Scutellariae Radix (SR) as a typical case. Chemical fingerprints were developed by high performance liquid chromatography coupled with photodiode array detector and quadrupole time-of-flight mass spectrometry (HPLC-PDA/QTOF-MS). Subsequently, a feature selection strategy, integrated linear and nonlinear correlation analysis, was carried out to assess the correlation between chemical profiles and biological activities. Finally, quality consistency between the dispensing granule and the traditional decoction was determined by bioactive marker-guided hierarchical clustering analysis (HCA), k-means clustering method and bioequivalence evaluation. The available evidence suggested that not all the dispensing granule of SR were sufficiently similar to the traditional decoction. This study provides an applicable methodology for quality consistency evaluation of dispensing granule and traditional decoction in TCMs.

How similar is similar enough? A sufficient similarity case study with Ginkgo biloba extract

Botanical dietary supplements are complex mixtures that can be highly variable in composition and quality, making safety evaluation difficult. A key challenge is determining how diverse products in the marketplace relate to chemically and toxicologically characterized reference samples (i.e., how similar must a product be in order to be well-represented by the tested reference sample?). Ginkgo biloba extract (GBE) was used as a case study to develop and evaluate approaches for determining sufficient similarity. Multiple GBE extracts were evaluated for chemical and biological-response similarity. Chemical similarity was assessed using untargeted and targeted chemistry approaches. Biological similarity was evaluated using in vitro liver models and short-term rodent studies. Statistical and data visualization methods were then used to make decisions about the similarity of products to the reference sample. A majority of the 26 GBE samples tested (62%) were consistently determined to be sufficiently similar to the reference sample, while 27% were different from the reference GBE, and 12% were either similar or different depending on the method used. This case study demonstrated that approaches to evaluate sufficient similarity allow for critical evaluation of complex mixtures so that safety data from the tested reference can be applied to untested materials.

The safety of green tea and green tea extract consumption in adults - Results of a systematic review

A systematic review of published toxicology and human intervention studies was performed to characterize potential hazards associated with consumption of green tea and its preparations. A review of toxicological evidence from laboratory studies revealed the liver as the target organ and hepatotoxicity as the critical effect, which was strongly associated with certain dosing conditions (e.g. bolus dose via gavage, fasting), and positively correlated with total catechin and epigallocatechingallate (EGCG) content. A review of adverse event (AE) data from 159 human intervention studies yielded findings consistent with toxicological evidence in that a limited range of concentrated, catechin-rich green tea preparations resulted in hepatic AEs in a dose-dependent manner when ingested in large bolus doses, but not when consumed as brewed tea or extracts in beverages or as part of food. Toxico- and pharmacokinetic evidence further suggests internal dose of catechins is a key determinant in the occurrence and severity of hepatotoxicity. A safe intake level of 338 mg EGCG/day for adults was derived from toxicological and human safety data for tea preparations ingested as a solid bolus dose. An Observed Safe Level (OSL) of 704 mg EGCG/day might be considered for tea preparations in beverage form based on human AE data.

Implementation of the basic hazard index screening for health risks associated with simultaneous exposure to multiple chemicals using a standardized target organ and systems framework

Environmental health risk assessments often involve assessing the potential health effects of exposure to multiple chemicals at once (i.e., complex mixtures). Because the possible number of chemical combinations is very large, few controlled in vivo toxicological studies with chemical mixtures are relevant or practical. In lieu of specific mixture toxicity data, the segregated hazard index (HI) approach has been used to determine whether simultaneous exposures may warrant further investigation due to their combined adverse effects. Each chemical is assigned to one or more target organs based on critical effects; HIs for each target organ are generated by summing the individual hazard quotients for each of the chemicals assigned to that organ or organ system. To conduct this phased risk assessment approach in a consistent manner, a comprehensive, systematized list of toxicity targets for implementing this approach is needed. We present a comprehensive and standardized list of toxicity target organs and systems (TTOS), with example data sets, for consistent implementation of the segregated HI method. This method is designed to facilitate the standardization of the widespread use of the basic segregated HI approach. The basic hazard index mixtures screening (BHIMS) tool allows for rapid identification of exposure concerns that may warrant further and more sophisticated assessment. Integr Environ Assess Manag 2017;13:852-860. Published 2017. This article is a US Government work and is in the public domain in the USA.

From the Cover: Vulnerability of C6 Astrocytoma Cells After Single-Compound and Joint Exposure to Type I and Type II Pyrethroid Insecticides

A primary mode-of-action of all pyrethroid insecticides (PYRs) is the disruption of the voltage-gated sodium channel electrophysiology in neurons of target pests and nontarget species. The neurological actions of PYRs on non-neuronal cells of the nervous system remain poorly investigated. In the present work, we used C6 astrocytoma cells to study PYR actions (0.1-50 μM) under the hypothesis that glial cells may be targeted by and vulnerable to PYRs. To this end, we characterized the effects of bifenthrin (BF), tefluthrin (TF), α-cypermethrin (α-CYP), and deltamethrin (DM) on the integrity of nuclear, mitochondrial, and lysosomal compartments. In general, 24- to 48-h exposures produced concentration-related impairment of cell viability. In single-compound, 24-h exposure experiments, effective concentration (EC)₁₅s 3-(4,5-dimethyl-thiazol-2-yl)-2,5-diphenyl-tetrazolium bromide (MTT assay) were computed as follows (in μM): BF, 16.1; TF, 37.3; α-CYP, 7.8; DM, 5.0. We found concentration-related damage in several C6-cell subcellular compartments (mitochondria, nuclei, and lysosomes) at ≥ 10^-1 μM levels. Last, we examined a mixture of all PYRs (ie, Σ individual EC₁₅) using MTT assays and subcellular analyses. Our findings indicate that C6 cells are responsive to nM levels of PYRs, suggesting that astroglial susceptibility may contribute to the low-dose neurological effects caused by these insecticides. This research further suggests that C6 cells may provide relevant information as a screening platform for pesticide mixtures targeting nervous system cells by expected and unexpected toxicogenic pathways potentially contributing to clinical neurotoxicity.

A tiered framework for risk-relevant characterization and ranking of chemical exposures: applications to the National Children's Study (NCS)

A challenge for large-scale environmental health investigations such as the National Children's Study (NCS), is characterizing exposures to multiple, co-occurring chemical agents with varying spatiotemporal concentrations and consequences modulated by biochemical, physiological, behavioral, socioeconomic, and environmental factors. Such investigations can benefit from systematic retrieval, analysis, and integration of diverse extant information on both contaminant patterns and exposure-relevant factors. This requires development, evaluation, and deployment of informatics methods that support flexible access and analysis of multiattribute data across multiple spatiotemporal scales. A new "Tiered Exposure Ranking" (TiER) framework, developed to support various aspects of risk-relevant exposure characterization, is described here, with examples demonstrating its application to the NCS. TiER utilizes advances in informatics computational methods, extant database content and availability, and integrative environmental/exposure/biological modeling to support both "discovery-driven" and "hypothesis-driven" analyses. "Tier 1" applications focus on "exposomic" pattern recognition for extracting information from multidimensional data sets, whereas second and higher tier applications utilize mechanistic models to develop risk-relevant exposure metrics for populations and individuals. In this article, "tier 1" applications of TiER explore identification of potentially causative associations among risk factors, for prioritizing further studies, by considering publicly available demographic/socioeconomic, behavioral, and environmental data in relation to two health endpoints (preterm birth and low birth weight). A "tier 2" application develops estimates of pollutant mixture inhalation exposure indices for NCS counties, formulated to support risk characterization for these endpoints. Applications of TiER demonstrate the feasibility of developing risk-relevant exposure characterizations for pollutants using extant environmental and demographic/socioeconomic data.