Page 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

Page 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

Page 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?

Page 4

®

Value of new tools of toxicology: Dose-response

Naciff et.al., Tox.Sci. 2005

Author/Date

®

Susceptibility: Dose response implications

Dose

Response

Wild-type

Genetic susceptibility

PON1 KO?

Author/Date

®

Nonlinear Dose Response for Micronuclei (MN) in Reticulocytesas Measured by Flow Cytometry

0 0.01 0.1 0.5 1 5 10 25 500

5

10

15

20

MMS

MNU

Dose group (mkd, 4d)

Ret

icu

locy

te-M

N (

‰ a

ve)

*

**

*

**

Author/Date

®

Number of Genes Significantly Changed

25 mkd 50 mkd

1.5 fold 8 112

2 fold 3 79

3 fold 2 38

4 fold 2 31

5 fold 0 20

Genes Significantly Changed in Liver vs. Control as Measured by Microarray

0

25

50

75

100

125

0.5 5 50

MMS (mkd, 4d)

Ge

ne

s S

ign

ific

an

tly

Ch

an

ge

d v

s. C

on

tro

l (#

)

↝ 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)

Page 8

®

Mega-Fish Study: Non-linear Response to Genotoxic Carcinogen Dibenzo[a,l] pyrene

Bailey et.al., Chem.Res.Toxicol. 22: 1264-1276 (2009)

Page 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

Page 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

Page 11

®

Linking Animal Toxicity Tests to Human Exposure

From: Hays et.al., Reg.Toxicol.Pharmacol 47: 96-109, 2007

Page 12

®

Linking Animal Test Doses to Human Exposure

Saghir et.al., TAAP Saghir et.al., TAAP 211: 245, 2006211: 245, 2006

Page 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”