Highlighting the Need for AOPs in Streamlining Hazard Assessment Methods (for cancer assessment)

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Highlighting the Need for AOPs in Streamlining
Hazard Assessment Methods (for cancer assessment)

• Catherine Willett, PhD
• Director, Regulatory Toxicology
• Humane Society of the United States

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Outline

• Rodent Cancer Bioassay cost/benefit?
• Advantages of mechanism based approaches
• AOP approaches and activities at OECD

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1. Rodent Cancer Bioassay Cost

• Minimally 400 animals, 2 4 million USD and 3
  years
• Often performed in two species (drugs, food
  additives)
• Or rat bioassay plus shorter-term transgenic
  mouse assays1
• ras H2 p 53 /- use half the number of
  animals and for 6 months
• Reduced animal use mitigated by need for creation
  and maintenance of multiple lines
• Only improves identification if both used (geno
  and non-genotoxic)
• ? no animal savings, minimal cost savings
• Thomas RS, Pluta L, Yang L, Halsey TA. Toxicol
  Sci 2007. 97(1)55-64.
• 1Alden et al. Veterinary Pathology. 2011.
  48(3)772-784.

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1. Rodent Cancer Bioassay Benefit?

• Drugs (review of 533)
• False positives 80
• False negatives 27
• Sensitivity 73
• 12 out of 44 chemicals with human concern not
  seen in either rodent species
• When tested repeatedly, most molecules.had
  conflicting test results.
• Alden et al. Veterinary Pathology. 2011.
  48(3)772-784

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1. Rodent Cancer Bioassay Benefit?

• Other chemicals (e.g. NTP studies)
• 82 of 500 studies resulted in noncommittal
  classifications
• Only 57 of chemicals tested multiple time give
  consistent results
• 22 of all chemicals tested test positive1
• Largely due to findings at MTD and irrespective
  of biological plausibility
• Nevertheless results in classification as
  possible human carcinogen
• Positive correlation?
• 9/10 known human carcinogens have tested positive
  either rats or mice in NTP studies
• All known human carcinogens have tested positive
  in some rodent assay, with the possible exception
  of arsenic
• This result says more about the persistence of
  toxicologists than about the ability of a
  standard (rodent) protocol to predict human
  carcinogenicity.1
• Wasted Money Wasted Lives. People for the
  Ethical treatment of Animals. 2006.
  http//www.mediapeta.com/peta/pdf/Wasted-money-PDF
  .pdf
• 1 Ennerver and Lave. Reg. Toxicol. Pharma. 2003.
  38 52-57

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1. Rodent Cancer Bioassay Cost / benefit

• 400 animals
• 2 4 million USD
• 3 years

False Positives 80
2
False Negatives 27
or
82 noncommittal classifications
Too high
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2. Advantages of mechanistic approaches

• Refine design and interpretation of animal
  studies
• Demonstrate human relevance (or not)
• Improve predictivity for both human health and
  other target species
• Design assessment strategies that are not
  dependent on animal testing

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2. Early adoption of mechanistic approaches

• DNA reactive vs non-DNA reactive
• Mouse transgenic lines
• 3Rs value highly debatable
• Chemical alerts for DNA reactivity
• Used to flag chemicals
• Genotoxicity battery
• Currently used to exclude chemicals
• In vitro tests have high false positive rates
• in vivo tests are insensitive
• Battery misses non-DNA reactive chemicals
• Cell transformation assays

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2. Early adoption of mechanistic approaches

• Human Relevance Frameworks
• Characterize MoA of each class of carcinogens
• Determine which rodent MoA is possible relevant
  to humans
• Built using case studies
• Timeline stolen from V. Dellarco
• EPA IPCS 1999-2001. Conceptual Framework for
  Evaluating a Mode of Action for Chemical
  Carcinogenesis.
• ILSI 2003. Framework for human relevance analysis
  of information on carcinogenic modes of action.
• ILSI 2005. Extends Framework to non-cancer
  outcomes life stage information.
• IPCS 2006 2008 Adopts Human Relevance
  Framework Boobis, et al. IPCS framework for
  analyzing the relevance of a non-cancer mode of
  action for humans. Crit Rev Toxicol.
  200838(2)87-96.

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Proposals to improve efficiency of rodent-based
assessment

• Short-term screening tests
• Step 1 90-day screen
• E.g. hepatocarcinogenicity hepatocellular
  necrosis, hypertrophy, cytomegaly, increased
  liver weight
• Step 2 mechanistic screens (most 90-days)
• Histopathology
• Serum enzymes
• Acyl Co-A oxidase
• CYP induction
• CAR, PSR, AHR binding
• ER binding (or histologic evaluation of
  endocrine-sensitive organs)
• Iron staining
• Reversibility
• Metabolic activation
• Cohen, S.M. Toxicol. Pathol. 2010.
  38(3)487-501.

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Proposals to improve efficiency of rodent-based
assessment

• Testing strategy to rule out carcinogens
• Negative genotoxicity in standard battery
• Negative hormonal perturbation in chronic studies
• effects on endocrine-related tissues, hormone
  levels
• Lack of histopathologic risk factors in 6 month
  rat study
• neoplasia in any tissue
• Negative in 6 month transgenic mouse
• Both genotox and non-genotox models required?
• Positives go into a standard rat bioassay
• Sistare et al. Toxicol. Path. 2011.39 716-744.

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2. Molecular approaches

• Gene expression profiling following 13 week
  exposure
• Predict lung tumors in B6C3F1 female mice
• Overall accuracy 77.5 in predicting mouse lung
  tumors
• 25 chemicals sufficient to develop predictive
  model
• (could create predictive models for 8 organs
  using 200 chemicals)
• Gene profiles placed into biologic process
  categories1
• Used to calculate BMD values for liver or lung
• Some transcriptional and tumor incident BMD
  values correlate well
• ? transcriptional BMD values could potentially be
  used as points of departure for cancer as well as
  non-cancer risk assessment
• Thomas, R.S. et al. Toxicol. Sci. 2009. 112(2)
  311-321.
• 1 Thomas, R.S. et al. Toxicol. Sci. 2011. 120
  (1) 194-205.
• Could the predictive capacity of omics be
  improved by AOPs?

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3. Adverse Outcome Pathway Approaches

• Adverse Outcome Pathway a chemical and
  biological description of what occurs when a
  substance interacts with a living organism and
  results in an adverse reaction a biological map
  from the initiating event through the resulting
  adverse outcome that describes both mechanism and
  mode of action.

From Ankley et al. Environ.Toxicol.Chem. 2010.
29 (3) 730741.
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3. Adverse Outcome Pathway Approaches

• AOPs can be useful for
• Near-term
• Developing chemical categories and structure
  activity relationships
• Increasing certainty of interpretation of both
  existing and new information
• Developing integrated testing strategies that
  maximize useful information gained from minimal
  testing
• Longer-term
• Identifying key events for which non-animal tests
  can be developed, thereby facilitating
  mechanism-based, non-animal chemical assessment
• Creating predictive toxicological assessments
  with low uncertainty and high human relevance

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3. AOP activities at OECD expert consultation
on the ER decision framework in Feb 2009
ER-mediated Reproductive Impairment
In vivo
POPULATION
CELLULAR Response
TISSUE/ORGAN
MOLECULAR Target
INDIVIDUAL
Skewed Sex Ratios Yr Class
Liver Altered proteins Gonad Ova-testis Sex-reve
rsed Fecundity
Sex reversal Altered behavior Repro.
Liver Cells Altered Protein Expression Vitellog
enin
Receptor Binding ER Binding
Chemicals
Toxicity Pathway
Adverse Outcome Pathway
P. Schmieder, McKim conference 2008.
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3. AOP activities at OECD

• 2010 Workshop on using mechanistic information in
  forming chemical categories near-term
  recommendations
• 1) Develop AOPs for well-established effects
  (e.g., skin sensitization) as well as several
  longer term health and ecotoxicological
  endpoints.
• 3) Establish, populate and maintain an
  accessible, electronic repository e.g.
  Effectopedia.
• 4) Develop a strategic plan including
• an information template for developing and
  assessing AOPs
• guiding principles for assessing completeness and
  acceptance of an AOP
• a format for attaining mutual acceptance of an
  AOP.
• 5) Harmonize terminology associated with AOPs.
• 6) Integrate AOPs in the OECD QSAR Toolbox.

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3. AOP activities at OECD Sensitization draft AOP
OECD 2011. (Draft) The Adverse Outcome Pathway
for Skin Sensitisation Initiated by Covalent
Binding to Proteins
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3. AOP activities at OECD

• Agreed at last Joint Meeting in June 2011 that
  AOPs were to become the cornerstone for all
  future projects in the test guidelines programme.
  
• Under development
• A process for development and maintenance of AOPs
• A Guidance Document for Developing and Assessing
  Completeness of AOPs

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Thank you.
Catherine Willett, PhD Director, Regulatory
Toxicology, Risk Assessment and
Alternatives kwillett_at_humanesociety.orgt
01.240.599.6785   The Humane Society of the
United States2100 L Street NW    Washington, DC
20037humanesociety.org/animalsinlaboratories