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Glossary of Terms: Methods of Toxicity Testing and Risk Assessment

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This Web page identifies and explains the glossay terms used for the development and evaluation of new technologies in molecular, cellular, and computational sciences to supplement or replace more traditional methods of toxicity testing and risk assessment.
Read more about additional Web-based glossary sources.

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Active Ingredient – chemical or substance component of a pesticide product that can kill, repel, attract, mitigate or control a pest or that acts as a plant growth regulator, desiccant, or nitrogen stabilizer. The remainder of a formulated pesticide product consists of one or more “inert ingredients” (such as water, solvents, emulsifiers, surfactants, clay and propellants), which are there for reasons other than pesticidal activity. By law, the active ingredient must be identified by name on the label together with its percentage by weight.

AcTOR (Aggregated Computational Toxicology Resource) – Web site developed by EPA’s Office of Research and Development that utilizes a collection of databases collated or developed by EPA. This site provides easy access into more than 200 sources of publicly available data on environmental chemicals in AcTOR. These are searchable by chemical name, structure and other identifiers. Data include chemical structure, physical-chemical values, in vitro assay data (e.g., ToxCast) and in vivo toxicology data. Chemicals include, but are not limited to, high and medium production volume industrial chemicals, pesticides (active and inert ingredients), and potential ground and drinking water contaminants. More details on AcTOR.

Antimicrobial Pesticide – substance or mixture of substances used to kill or suppress the growth of harmful microorganisms such as bacteria, viruses, or fungi on inanimate objects and surfaces. Read more about antimicrobial pesticides.

ASTER – The Assessment Tools for the Evaluation of Risk (ASTER) was developed by EPA’s Office of Research and Development, to assist EPA in hazard ranking and the development of comprehensive environmental risk assessments for aquatic organisms and wildlife. It is designed to provide high quality data for discrete chemicals in the associated databases (i.e., ECOTOX and EcoChem) and QSAR-based estimates when data are lacking. More details on ASTER.

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Bioactivity Profiling – evaluation of chemicals and their effects on organisms. This term is typically used to indicate the use of rapid high throughput screening (HTS) assays that provide a comprehensive profile of a chemical’s effects on cellular molecules and processes.

Biochemical Pesticide – chemicals that are either naturally occurring or identical to naturally occurring substances such as hormones, pheromones, and enzymes. Biochemicals function as pesticides through non-toxic, non-lethal modes of action, such as disrupting the mating pattern of insects, regulating growth, or acting as repellants. Since biochemicals tend to be environmentally compatible, they are important to Integrated Pest Management programs.

Bovine Corneal Opacity and Permeability Assay (BCOP) – a test method developed in the search for a potential replacement for the Draize rabbit eye test. It uses bovine (cow) eyes that are received shortly after the slaughter of the animal so the cells are still viable. The corneas are excised and treated with a chemical to determine its potential to damage the eye. The BCOP model predicts effects similar to many human corneal responses. The Agency is proposing a pilot interim policy to use BCOP in combination with other non-animal alternative tests such as EpiOcular ™ Model and Cytosensor to address the full range of eye irritancy (i.e., Toxicity Categories I through IV) for a limited class of antimicrobial cleaning agents.

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Computational Models – term that refers to computerized predictive tools. Sometimes referred to as “in silico” models.

Conventional Pesticide - pesticides that are chemicals or other substances developed and produced primarily or only for use as pesticides. As opposed to a biological pesticide, conventional pesticide refers to any man-made chemical that can be used to kill pests. The term is generally used in reference to active ingredients. For example, pyrethrins were developed and used almost exclusively as pesticides.

Cytosensor Microphysiometer (CM) Assay – a non-animal test method developed in search for a replacement for the Draize rabbit eye test. This assay evaluates the potential eye toxicity of a chemical by measuring the dose required to reduce the metabolic rate in treated cells in vitro. A very sensitive instrument called a microphysiometer is used to electronically measure the metabolic rate of cell populations through small changes in acidic metabolites in the medium. The rate is constant in an undamaged cell population. If the cells are injured, an altered metabolic rate is found. The CM assay would be used in combination with other non-animal alternative tests (see Bovine Corneal Opacity and Permeability Assay).

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Degradates – the product of the environmental transformations of a pesticide. Degradates can have similar properties to the original pesticide.

Draize Rabbit Eye Test – an acute toxicity test originally designed in 1944 by Food and Drug Administration (FDA) toxicologist John H. Draize. This test is used to determine the degree to which a chemical such as a pesticide, drug, or cosmetic, damages or irritates the eye. In this test a small amount of the substance is applied directly in the eye of a rabbit, and the rabbit is monitored for signs of redness, swelling, discharge, ulceration, hemorrhaging, cloudiness, or blindness in the tested eye. EPA currently requires all pesticides to be tested in the Draize test for eye irritation and uses the results to classify the products into one of four toxicity categories on the basis of that information.

Distributed Structure-Searchable Toxicity (DSSTox) – a project of EPA's Office of Research and Development that is helping to build a public data foundation for improved structure-activity and predictive toxicology capabilities. The DSSTox website provides a public forum for publishing downloadable, structure-searchable, standardized chemical structure files associated with toxicity data. More details about DSSTox.

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ECOSAR – a computerized predictive system developed by EPA’s Office of Chemical Safety and Pollution Prevention that estimates the acute and chronic toxicity of an industrial compound or pesticide to aquatic organisms such as fish, aquatic invertebrates, algae in the absence of measured data. Free download of ECOSAR.

ECOTOX – a comprehensive web-based database developed and maintained by EPA. It contains effects of single chemicals on aquatic and terrestrial organism. The data primarily come from studies published in the open literature. More details on ECOTOX.

EpiOcular ™ Model – a non-animal test method developed in the search for a replacement for the Draize rabbit eye test. This test is an in vitro model of the human corneal epithelium composed of normal human-derived epidermal keratinocytes and is used to evaluate the eye irritation potential of chemicals, particularly surfactants. The EpiOcular ™ Model would be used in combination with other non-animal alternative tests (see Bovine Corneal Opacity and Permeability Assay).

EPISuite – a computerized predictive system developed by EPA’s Office of Chemical Safety and Pollution Prevention that contains models for estimating physical-chemical properties and environmental fate endpoints in the absence of measured data. Free download of EPISuite.

Extended One-Generation Reproductive Study – a redesign of the reproduction toxicity study commonly used to evaluate potential adverse effects of chemical exposure on reproduction and development. This study design constitutes a significant departure from the current guideline studies in that the animals born to treated animals (the F1 generation) would be subjected to a far more comprehensive evaluation than what is currently done (including assessments of the developing immune and nervous systems, when integrated approaches to testing and assessment deem them necessary) and the second generation after the treatment stage (the F2 generation) would not be routinely produced. By selectively producing that second generation and combining elements of the developmental neurotoxicity and immunotoxicity studies, the Extended One Generation Reproductive Toxicity Study will use fewer animals while providing a more in-depth assessment of the reproductive system and the developing organism.

Expert System – a computer program that mimics the judgment of experts by following sets of chemical-specific knowledge rules derived from measured toxicity data and knowledge of how chemicals cause toxicity in animals and humans. In computational toxicology, expert systems can be combined with QSAR models or qualitative structure-activity relationships and used to make predictions of toxicity. Expert systems ideally provide transparency with regard to the data (see knowledge base) underlying the rules and their mechanistic rationale.

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FDA QSAR Models – a series of computer models constructed by FDA and distributed by Leadscope, Inc., to analyze datasets of chemical structures and related biological or toxicity data. Models are being used for data analysis in the following areas: high-throughput screening, lead optimization, compound acquisition, structure-based data mining and in silico toxicology. EPA’s Office of Pesticide Programs is working with FDA to explore the use of these computer program and to build toxicity databases that include pharmaceutical and pesticide study data to support better hazard predictions across different chemical classes and modes of action. Read more about FDA QSAR models.

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High-Throughput Systems (HTS) – in vitro biochemical or cellular assays that can be run efficiently on a large number of compounds to determine their activity on different biological targets such as ion channels, receptors, enzymes, proteins, and signaling pathways.

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Inert Pesticide Ingredient – substances that are not an active ingredient Exit EPA disclaimer and are included in a pesticide formulation Exit EPA disclaimer for reasons other than pesticidal activity. Read more about inerts.

In silico – is a general term used to mean "performed on computer or via computer simulation."

In vitro – experiments or tests done under controlled experimental conditions outside of the body, such as in a test tube or laboratory dish. These tests tend to focus on organs, tissues, cells, cellular components, proteins, and/or biomolecules.

In vivo – tests or evaluations performed within an intact, living organism such as a laboratory animal or humans.

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Knowledge Bases – repositories of data on toxicity or biological activity ideally combined with chemical structure information to better leverage existing resources and to facilitate predicting effects, or for building models to predict effects of chemicals with little or no data.

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Metabolites - breakdown products, intermediates, and other biochemical substances produced by biological processes.

MetaPath – a Metabolism Pathway database under development by EPA’s Office of Research and Development to store summary information on rat metabolism of pesticides, plant and livestock residues, and environmental degradates that is obtained from the reviews of regulatory studies. MetaPath will provide risk assessors ready access to metabolite, residue and degradate information. Facilitating searches by chemical structure will increase efficiency of information retrieval in the pesticide program and allow critical analysis and interpretation of data in a manner currently not possible.

Metabolic Simulator – a system that is being developed by EPA’s Office of Research and Development to predict chemical metabolism; it which will be based on a metabolism pathway database called MetaPath.

Mode of Action – understanding how chemicals perturb normal biological function; the key steps in the toxic response after chemical interaction at the target site that is responsible for the physiological outcome or pathology of the chemical.

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Omics – refers to a broad field of study in biology, ending in the suffix ''-omics'' such as genomics, proteomics, transcriptomics.

OncoLogic – computer software program developed by EPA’s Office of Chemical Safety and Pollution Prevention designed to predict the potential cancer-causing effects of a chemical by applying structure activity relationship (SAR) analysis and expert knowledge of how chemicals cause cancer in animals and humans. Free download of OncoLogic.

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PBT Profiler – web-based screening-level tool used in the absence of data to help identify chemicals that may potentially persist in the environment, bioaccumulate, and be toxic to aquatic life. Access PBT Profiler.

Physiologically Based Pharmacokinetic (PBPK) Model – a mathematical model that estimates the dose to a target tissue or organ by taking into account the rate of absorption into the body, distribution among target organs and tissues, metabolism, and excretion.

Priority Setting – An activity that ranks chemicals on their hazard and/or exposure potential based on specified criteria in a defined regulatory context. Priority setting takes full advantage of existing knowledge about potential chemical exposure and toxicity and computer-aided predictions such as QSAR to determine which chemicals (or group of chemicals), and for which end points, are of greater concern in gathering additional data (typically in vivo) to better evaluate potential adverse effects on humans and wildlife.

Proteomics – study of the function of all expressed proteins.

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Quantitative Structure-Activity Relationship (QSAR) – a mathematical relationship between a quantifiable aspect of chemical structure and a chemical property or reactivity or a well defined biological activity, such as toxicity. Using a sample set of chemicals, a relationship is established between one or many physical-chemical properties a chemical possesses due to its structure and a chemical property or biological activity of concern. This mathematical expression is then used to predict the chemical property or biological response expected from other chemicals with similar structures. It is based on the presumption that similar molecules or chemical structures have similar properties or biological activities or toxicity potential. Read about the principles that OECD has developed to facilitate the appropriate use of QSAR modeling for regulatory purposes.

QSAR-Based Expert System for Predicting Estrogenic Activity – set of expert rules that were developed by EPA’s Office of Research and Development to predict the potential of a chemical to bind to estrogen receptors, which could initiate a pathway ending in an adverse effect on reproduction. The rules were developed by testing a few key chemicals within chemicals classes (i.e., food use inert ingredients and antimicrobial pesticide actives) for which little or no effects data are available, to decide which chemicals are more likely to have estrogenic effects and to test those first. By testing key chemicals of each type, an understanding is gained of the underlying relationship between a chemical's structure and the ability of the chemical to bind to estrogen receptor proteins. This knowledge and the rules developed are used to predict which related but untested chemicals are likely to behave the same as tested ones. The approach allows focusing of resources on the few chemicals with the greatest likelihood to produce adverse outcomes instead of randomly selecting which chemicals to test first for estrogenic effects. This QSAR approach is helping prioritize chemicals for the Endocrine Disruptor Screening Program Tier 1 screening.

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Read Across from Analogs/Categories – “Read across” is a technique of filling data gaps. To “read across” is to apply data from a tested chemical for a particular property or effect (cancer, reproductive toxicity, etc.) to a similar untested chemical. The read across technique is often applied within groups of similar chemicals assembled for assessment using either analog approach (grouping based on a very limited number of chemicals) or category approach (grouping based on a larger number of chemicals). In an analog/category approach, not every chemical needs to be tested for every endpoint. Read more about "Read Across from Analogs/Categories." Exit EPA disclaimer

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Screening – an activity using less expensive tests typically of shorter duration to provide preliminary information about chemical toxicity potential. Results from screening assays are typically used to set priorities for those chemicals (or groups of chemicals) that need further evaluation. These tests may consist of in vivo or in vitro assays. Screening can also be done by the use of QSAR modeling.

Systems Biology – holistic approach to the study of biology with the objective of simultaneously monitoring all biological processes operating as an integrated system.

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ToxCast™ – an EPA Office of Research and Development research effort designed to develop a cost-effective approach for rapidly prioritizing in vivo toxicity testing of large numbers of chemicals. Using data from state-of-the-art high-throughput system (HTS) bioassays, ToxCast™ is building computational models to forecast the potential human toxicity of chemicals. Read more about ToxCast.

ToxRefDB – a comprehensive relational database of mammalian toxicity studies developed by EPA’s Office of Research and Development in collaboration with EPA’s Office of Pesticide Programs (OPP). The ToxRefDB will be used by EPA for ToxCast and other research projects. ToxRefDB is being used by OPP to analyze what we have learned about the contribution of required regulatory animal toxicity studies (e.g., as toxicity data submitted to the Agency for pesticide registration) to support improved study designs as well as interpretation of animal toxicity data. Read more about ToxRefDB.

Toxicity Pathways – The 2007 NRC report on Toxicity Testing in the 21st Century envisioned that new technologies will help us to better understand how chemicals perturb normal biological function, and thus identify toxicity pathways. Potential toxic effects of chemicals would be predicted based upon in vitro bioactivity profiles derived from a chemical’s effects on cellular molecules and processes. The interpretation of chemically-induced perturbations in toxicity pathways will depend upon linking in vitro effects with adverse outcomes in vivo, and on computer modeling that extrapolates to predicted responses in whole tissues, organisms and populations based on realistic human or environmental exposures.

Toxicogenomics –study of the roles that genes play in the biological responses to environmental toxicants and stressors by the collection, interpretation, and storage of information about gene and protein activity.

Transcriptomics – study of gene expression at the RNA level.

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Virtual Embryo (v-Embryo™) – research program in EPA’s Office of Research and Development developing a sophisticated computer model of a mammalian embryo. This model can be used to better understand the prenatal risks posed by environmental chemicals and to ultimately predict a chemical’s potential developmental toxicity in silico. v-Embryo™ will provide a public portal, knowledgebase, and computational infrastructure for modeling genetic-cellular networks and developmental toxicity pathways.

Virtual Liver (v-Liver™) – research program in EPA’s Office of Research and Development developing a large-scale biologically-based computer model of the organ. The long-term objective of v-Liver™ is to simulate the effect of chemicals accurately, efficiently and more humanely.

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Additional Web-based Glossary Sources

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