Assessment of Low-Dose Dose-Response Relationships (Non-linear or Linear) for Genotoxicity, Focused on Induction of Mutations

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Assessment of Low-Dose Dose-Response
Relationships (Non-linear or Linear) for
Genotoxicity, Focused on Induction of Mutations
Clastogenic Effects

• ARA Case Study 26
• Team
• L.H. Pottenger, M.M. Moore, E. Zeiger, T. Zhou

Disclaimer this does not represent the views or
policy of US FDA
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Case study 26-G/M

• WHAT Dose-response for genotoxic effects from
  DNA-reactive chemicals
• WHY Driver for risk assessment
• HOW Review published data vs. guidance on
  recommended data/approaches
• Conclusions Issues

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WHAT WHYLow-Dose Dose-Response for Genotoxic
Effects

• Definitions
• Low-Dose Range of human relevance (not MTD)
• Mutation Heritable change in genetic information
  (gene mutation chromosomal event such as
  reciprocal translocation)
• Included clastogenicity (micronucleus) data (not
  heritable)
• Non-linear/threshold Threshold is a subset of
  non-linear
• Endogenous/Background for adducts genotoxic
  effects
• Genotoxic effects are a driver for decisions on
  cancer risk assessment dose-response models
  (mutagenic MOA)
• Dose-response of key events is critical input
  into MOA analysis (mutation is early key event in
  mutagenic MOA)
• Shape of dose-response for key events should be
  empirically defined with chemical-specific data,
  when available, and supported with
  hypothesis-driven theory

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HOW Our Process

• Discussed possible criteria
• Based on
• Pottenger Gollapudi, 2010
• Swenberg et al., 2008
• Several datasets preferred (different labs is
  best)
• Adequate doses, replicates controls
  reproducible results
• Preferably with dosimetry data
• Apply dose-response modelling techniques
• Statistical parameters to determine best fit
• Selected assigned candidate chemicals
• EO, EMS/ENU, MMS/MNU, Acrylamide/Glycidamide
• Shared relevant publications
• Developed chemical-specific appendices
• Agreed general conclusions
• Identified issues

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Conceptual Key Events Frameworks
Mutation
Cancer
Pottenger Gollapudi, 2010
Swenberg et al., 2008
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Conclusions

• Apparent linearity for biomarkers of exposure
  (hemoglobin DNA adducts).
• EMS(/ENU) Hb DNA adducts
• MMS(/MNU) DNA adducts based on 13C-MMS
• EO inadequate data not clear if low-dose linear
  or not (14C-EO AMS data)
• Clear, statistically supported evidence for
  low-dose non-linear/threshold dose-response for
  induction of mutations and clastogenicity for at
  least some DNA-reactive chemicals.
• EMS in vivo in vitro statistical best fit for
  non-linear/threshold dose-response model
• MMS/MNU in vitro statistical best fit for
  non-linear/threshold dose-response model

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EMS in vivo dosimetry

Gocke and Muller, 2009
Apparent linearity for in vivo induction of
protein (hemoglobin) and DNA (N7-ethylG)
adducts by EMS
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EMS in vivo clastogenicity

Gocke and Muller, 2009
Clear, statistically supported evidence for best
fit of low-dose data on in vivo induction of
clastogenicity by EMS to non-linear/threshold
dose-response model.
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EMS in vivo clastogenicity dosimetry

Gocke and Muller, 2009
Clear, evidence for low-dose non-linear/threshold
dose-response for in vivo induction of
clastogenicity by EMS, even at doses with
increasing internal dose (Hb adducts).
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EMS in vivo mutation

Gocke and Muller, 2009
Clear, statistically supported evidence for
low-dose non-linear/threshold dose-response for
in vivo induction of mutation by EMS.
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Issues

• Current effort was a limited analysis of
  available data (with time expertise
  constraints)
• Develop hypothesis (theoretical underpinnings) to
  explain a non-linear/threshold dose-response for
  genotoxicity
• Only a few examples evaluated statistically
  supported best fit of dose-response models.
• Additional evaluation (i.e., statistical
  approaches to dose-modelling) should be
  considered
• Additional examples/datasets are always useful
• Eventually develop categories based on MOA/key
  events hypothesis-driven theoretical
  understanding of MOA

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• Questions?

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Endogenous/Background DNA adducts
Endogenous DNA Lesions / Cell
Abasic Sites 30,000
N7-HEG 3,000
8-oxodG 2,400
7-(2-Oxoethyl)G 1,500
Formaldehyde (N6-HOCH2-dA) 960
AcrdG 120
M1dG 60
N2,3-eG 36
1N2-edG 30
1N6-edA 12
TOTAL 38,118
J. Swenberg, personal communication
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EMS in vivo mutation clastogenicity

Gocke and Muller, 2009
Clear, statistically supported evidence for
low-dose non-linear/threshold dose-response for
induction of mutation and clastogenicity by EMS.
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MMS and MNU in vitro mutation dosimetry
Pottenger et al., 2009
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MMS in vitro mutation dosimetry
Swenberg et al., 2008 Doak et al., 2007