New Tools of Toxicology and Exposure Science: Opportunities for Informing Low-Dose Evaluations

1. New Tools of Toxicology and Exposure Science: Opportunities for Informing Low-Dose Evaluations James Bus, PhD, DABT, ATS The Dow Chemical Company “Beyond Science and Decisions: From Problem Formulation to Dose Response” Workshop I, March 16-18, 2010 Austin, Texas

2. The Paradigm Shift: Low-dose linear response default assumption for cancer and non-cancer endpoints • Did Silver Book seriously considered critical alternative, i.e., all responses exhibit practical thresholds (non-linear behaviors)? • Evidence of thresholds (non-linear behavior) for all responses: • Radiation and chemical hormesis • DNA-reactive substances – genotoxicity assays • Whole animal cancer and non-cancer bioassays

3. Assumption of Linearity: The “natural chemicals” conundrum • Tens’s of thousands of natural chemicals in everyday environment • Many present in significant doses in “healthy foods” • Exhibit full range of toxicologic properties associated with anthropogenic chemicals, including genotoxic activity • Silver Book default assumption of linear low-dose toxicity responses further magnifies natural chemical conundrum, i.e., otherwise “healthy” foods are judged to be even “unhealthier” • Future risk assessment paradigm must be able to differentiate healthy food from true chemical risks >>> What can new tools of toxicology and exposure science reveal?

4. Value of new tools of toxicology: Dose-response Naciff et.al., Tox.Sci. 2005

5. Genetic susceptibility Susceptibility: Dose response implications PON1 KO? Wild-type Response Dose

6. * * 20 MNU MMS 15 * Reticulocyte-MN (‰ ave) 10 * * 5 * 0 0 0.01 0.1 0.5 1 5 10 25 50 Dose group (mkd, 4d) Nonlinear Dose Response for Micronuclei (MN) in Reticulocytesas Measured by Flow Cytometry

7. 125 100 75 Genes Significantly Changed vs. Control (#) 50 25 0 0.5 5 50 MMS (mkd, 4d) Genes Significantly Changed in Liver vs. Control as Measured by Microarray ↝ Agilent complete rat genome (41,121) ↝ GeneSpring prefilter (21,205) ↝ ANOVA p<0.001 (126)↝ Tukey’s post-hoc p<0.05 (20 and 121)

8. Mega-Fish Study: Non-linear Response to Genotoxic Carcinogen Dibenzo[a,l] pyrene Bailey et.al., Chem.Res.Toxicol. 22: 1264-1276 (2009)

9. Value of new tools of toxicology: Modes of Action • Rapid identification of “alternative” modes of action • Liver tumors: PPARα vs CYP P450 enzyme induction • Improved animal models, e.g., humanized mice • Identify and test potential animal-model-specific responses • Test hypothesized “common” and/or “cumulative” modes of action • AhR-mediated toxicity • Assess question of “how to add” risks of complex mixtures

10. Exposure-dosimetry relationships: Implications for future risk assessment • Advancements in analytical and modeling technologies rapidly improving both animal dosimetry and human exposure evaluations (biomonitoring) • Refinements to “Margin of Exposure” approaches • “Biomonitoring Equivalents” approach (Hays et.al., Reg.Toxicol.Pharmacol 47: 96-109, 2007) • Internal dosimetry for short half-life compounds • Steady-state concentrations under conditions of toxicity test • Peak and/or steady-state AUCs in human exposures

11. Linking Animal Toxicity Tests to Human Exposure From: Hays et.al., Reg.Toxicol.Pharmacol 47: 96-109, 2007

12. Linking Animal Test Doses to Human Exposure Saghir et.al., TAAP 211: 245, 2006

13. Summary • New tools of toxicology offer unprecedented opportunities to better characterize the shape of the dose response under dose-exposure conditions relevant to real world exposure • Emerging data provides biological basis for existence of toxicological thresholds (non-linearities), even for genotoxic substances • Human exposure advances and information can be integrated into dose-response considerations of toxicity tests • Opportunities to improve Margin of Exposure risk approaches • Objective: Differentiate “healthy” from “harmful”