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Adverse Outcome Pathways

An adverse outcome pathway (AOP) is a model that identifies the sequence of biochemical events required to produce a toxic effect when an organism is exposed to a substance. Construction of an AOP can:

  • Organize information about biological interactions and toxicity mechanisms into models that describe how exposure to a substance might cause illness or injury
  • Suggest cell- or biochemical-based tests for pathway elements that could be used to develop testing strategies for targeted toxicity
  • Identify steps in a toxicity mechanism that need improved characterization

NICEATM's Integrated Chemical Environment will soon be a resource for applying AOPs to assess substance toxicity, with resources to link tests and their endpoints to AOPs planned for release in spring 2019.

The Organisation for Economic Co-operation and Development (OECD) is actively supporting AOP development within its work to standardize testing methods to assess substance toxicity. OECD maintains a wiki-based interface for developing descriptions of AOPs and issues formal descriptions of well-defined AOPs.

OECD scientists joined colleagues from U.S. government agencies, industry, and academia at a 2014 workshop organized by NICEATM to discuss the status of AOP development efforts and how AOPs could be applied to regulatory testing. The workshop featured demonstrations of the OECD wiki and the Effectopedia, a data collection and collaboration tool for developing AOPs. A workshop report (Kleinstreuer et al. 2016) was published in 2016.

Elements of an Adverse Outcome Pathway

 AOPs are made up of specific elements:

  • A molecular initiating event is an interaction between the toxic substance and an organism, such as binding of the substance to a receptor or protein. This interaction begins the toxicity process.
  • Key events after the molecular initiating event characterize the progression of the toxicity. Early key events can include changes in protein production or molecular signaling that occur in individual cells. Later key events can include altered tissue or organ function. The links between key events are described by key event relationships.
  • Adverse outcomes may occur at individual or population levels. An adverse outcome for an individual organism can include disease, impaired development, or impaired reproduction. Population adverse outcomes can include changes in population structure or local extinction of a species.

Example elements of an adverse outcome pathway

Using AOPs for Research & Testing

Traditional methods used to gather toxicity information involve treating animals and observing outcomes. However, thousands of substances currently on the market lack full toxicity data. Traditional testing methods are too expensive and labor-intensive to fully determine the toxicity of these substances within a reasonable time frame.

There is increasing interest in approaches that gather toxicity data using high-throughput cell- and biochemical-based tests. Each test is designed to assess a specific activity, such as protein binding or receptor activation, for the substance being tested. Because toxic effects cannot be predicted by any one test, it is important to test a substance using a number of these high-throughput and biochemical tests, and then to evaluate the combined data to predict potential toxic effects. AOPs are key to combining the data generated by this approach.

Once an AOP is defined for an adverse outcome, researchers can identify specific cell- or biochemical-based tests that represent the molecular initiating events, key events, and key event relationships for that pathway. To support this goal, NICEATM is currently mapping high-throughput screening assays to endpoints such as acute systemic toxicity and developmental toxicity. A set of tests covering steps in the AOP can then be used to screen substances in an efficient, cost-effective manner for those most or least likely to cause an identified adverse outcome. This approach also supports animal welfare goals by minimizing or eliminating animal use. For example, an AOP defined for skin sensitization is the basis for several non-animal testing approaches developed or evaluated by NICEATM.

AOPs clarify the events and mechanisms involved in toxicity, which can help with classification and prioritization of substances for further or future testing. For example, substances lacking the properties needed to cause a molecular initiating event would be unlikely to result in an adverse outcome, so testing of those substances might not be necessary or could at least be postponed. AOPs can be linked together by common key events to form AOP networks, which are helpful in understanding more complex toxicity endpoints such as cancer or developmental defects.

The process of defining AOPs and AOP networks can also help researchers and test method developers identify areas needing improved characterization. Knowledge gaps that prevent an AOP from being fully defined indicate the need for more basic research; key events that are not represented by any suitable tests suggest future areas for test method development.