McKim Workshop on Strategic Approaches for Reducing Data Redundancy in Cancer Assessment Duluth, MN, USA 19 May, 2010

Goal & ProblemCategoriesMechanism, Modes, & PathwaysThe Example of Skin SensitizationAdverse Outcome PathwaysCritical Events & Integrated Testing StrategySummary

To place every discreet organic chemical in to a category for every hazard endpoint of interest.Easy for data-rich acute effects.Hard for data-sparse chronic effects.With this in mind the concept of outcome pathways is proposed.

There is discontinuity between chemical and biological spaces (substances, which are similar in molecule structure, are often dissimilar in terms of their toxciity including the ability to elicit a particular hazard endpoint, as well as potency within that endpoint).We often have difficulty in forming toxicologically meaningful groups, especially for elaborate hazard endpoints such as cancer.

Provide a means of evaluating all members for common toxicological behavior or consistent trends among data for an endpoint (measured data on a few category members can be used to estimate the missing values for one or more untested member).Identification of a consistent pattern of toxic effects within a category increases the confidence in the reliability of the results for all.This is predicated on a priori binning the chemical in the correct category.Category formation is a key topic of predictive toxicology.

The goal is to develop a TMC, which enables a transparent, defensible assessment through mechanistic comparisons without further testing.This shifts the emphasis to intrinsic chemical activity and critical biological events and away from statistical parameters, especially a fixation on fit and predictivity.Data for different in vivo endpoints differs so several ways will be needed to form TMCs.

Are formed as a result of a common 1) chemical reactivity mechanism, 2) biological mechanism, 3) mode of toxic action (based on receptor, enzyme or basic cellular processes), or 4) molecular similarityWhen one moves from a common chemical reactivity-based category to a receptor or common cellular process-based category and even more so to molecular similarity-based category confidence in whether the chemical in question truly belongs to the category diminishes

Toxicologically related to DNA- and protein-binding.Directly applicable to a limited number of hazard endpoints where the Molecular Initiating Event (MIE) is the rate limiting factor in the in vivo effect.Important but not the total answer to forming TMCs.

In mammalian pharmacology and toxicology literature the MechTA denotes the sequence of events leading from the absorption of an effective dose of a chemical to the specific biological response in the target . Understanding a chemicals MechTA requires understanding the causality and temporal relationships between the steps to a particular toxic endpoint, as well as the steps that lead to an effective dose at the biological target(s).

Meeting this definition of a MechTA requires an exceptionally large amount of high quality data, which only can be attained for a very limited number of compounds.This is currently out of reach for the vast majority of industrial organic compounds.One cannot impose the MechTA criteria to forming TMCs and expect to make progress in the near term.

Foundation can be traced to the studies of McKim et al. (1987) and their fish acute toxicity syndromes, which are represented by selected biochemical and/or physiological effects of exposure selected as key responses measured in vivo from exposure to model chemicals.Require less data than MechTA approach.Successful in forming TMCs for acute aquatic toxicity.

Designed to describe knowledge concerning the linkages between chemical structure of the target compound and the in vivo outcome of regulatory interest.The term adverse outcome pathway has been selected so not to cause confusion with the term Toxicity Pathway (United States National Research Council in their document entitled Toxicity Testing in the Twenty-first Century: A Vision and a Strategy, 2007).

Facilitates the use of in silico, in chemico, and in vitro (cellular, molecular, and biochemical) endpoints in forecasting in vivo effects.Assimilates MIEs with measurements of key biological process.

Karlberg et al. Chem. Res. Toxicol. 2008, 21, 53-69.1. Haptenation; 2. Epidermal inflammation & LC activation; 3. LC migration; 4. DC: T cell interaction; 5. T cell proliferation; 6. Increase in hapten-specific T cells; 7. Hapten re-exposure; 8. Acute inflammation; 9. T cell-mediated inflammation

Events, which are:Hypothesized in the pathway.Essential to the induction of the adverse outcome.Measurable

A data generating and data gathering exercise.Largely focused on in silico, in chemico, and in vitro endpoints.Selected ITS endpoints must have biological relevance to the hazard endpoint in question, which is most transparent when linked to an AOP.

Toxicants electrophile or chemicals converted to a reactive metabolite.Molecular site(s) of action are nucleophilic sites (cysteine and lysine) in proteins.MIE is covalent (irreversible) perturbation of dermal proteins.Biochemical paths are incompletely known, but includes stimulation of selected cellular responses (e.g., antioxidant-response element).

EVENTMEASUREMENTProtein binding in silico predictions from structureChemical reactivity w/ SH + for TH1, - for TH2Chemical reactivity w/ NH2 - for TH1, + for TH2Keratinocyte stimulation of + for TH1, - for TH2Keap1-Nrf2-ARE cellular Pathway

Dendritic cell expression of IL4 - for TH1, + for TH2

Dendritic cell expression of IL8 + for TH1, - for TH2

The sequence of events from chemical structure through the MIE event to the in vivo outcome.Designed to avoid mixing data from multiple mechanisms, which can cause the same in vivo outcome.An organizing principle for hazard assessment, especially for elaborate endpoints.

Describes a technique for grouping chemicals based on both up-stream chemical and down-stream biological processes. Shifts emphasis from just intrinsic chemical activity to chemical activity plus the cascade of events that occur across the different levels of biological organization.

Allows a shift from animal testing to hypothesis testing.Provides a basis for chemical extrapolation.Provides for comparisons across level of biological organization.Provides for consideration of life form & life stage at exposure.Provides a basis for species extrapolation.

Chemical Reactivity ProfilesReceptor, DNA,ProteinInteractionsMechanistic ProfilingBiological ResponsesCurrent OECD ToolboxToxicantMacro-Molecular InteractionsMolecular Initiating Event

Lethality

Sensitization

Birth Defect

Reproductive Impairment

CancerAlteredFunction

Altered DevelopmentGene Activation

Protein Production

Signal AlterationChemical Reactivity ProfilesReceptor, DNA,ProteinInteractionsStructure

ExtinctionCellularOrganMechanistic ProfilingIn VivoTestingBiological ResponsesITS and Adverse Outcome PathwayToxicantOrganismMacro-Molecular InteractionsMolecular Initiating EventPopulationCellular & In Vitro Testing

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