2. Overall Objectives Demonstrate the existence of new modalities of toxic tissue injury with increasing dose using a series of representative case examples. Examine the impact of dose-dependent transitions on the risk assessment process. Provide a forum for multi-sector discussion of data needs, experimental design, and principles for incorporating dose-dependent transitions into risk assessment decisions.

3. Challenge The shape of the dose-response curve may be significantly affected by the existence of multiple mechanisms of toxicity. For example, critical, limiting steps in any given mechanistic pathway may become overwhelmed with increasing exposures, signaling the emergence of new modalities of toxic tissue injury at higher doses.

4. Challenge (cont.) Chemical-specific case studies show that, as the dose of an agent increases, dose- dependent transitions such as receptor interactions, altered homeostasis, and saturation of pharmacokinetic and repair mechanisms can and do occur.

5. Challenge (cont.) Determining which mechanisms are operative throughout the dose-response curve and examining the impact of these dose- dependent transitions in mechanisms of toxicity will have significant implications for interpretation of data sets for risk assessment.

8. Basic Tenet The course from the point of exposure to expression of a biological response consists of a series of interrelated yet independent processes, each with its own set of finite kinetic characteristics. Saturation of any one of these active processes alters the course of the toxic response, which may be reflected in a deviation from a log-linear relationship as one explores the full dose-response curve. Deviations from linear dose-response relationships confound the extrapolation of experimental laboratory data to accurately predict human health outcomes

9. Finite capacity Absorption –Passive (dose-linear) –Active –Facilitated Distribution –Serum binding –Tissue transporters Uptake [glycos] Export [ABC transporters] Tissue storage –Specific binding proteins [fabp, GST, MT] –Non-specific storage depots [lipid] Excretion –Filter –Secrete [organic anions] Metabolic transformation –Activation [P450, MFO] –Detoxification [conjugations, esterase] –Cofactor depletion [conjugate base]

10. Finite capacity Dynamic - –Receptor Finite number association/dissociation turnover/reactivation –[OP’s, PPAR, ANS] Target - –Defense [oxidative stress] –Repair [DNA] –Replacement [cell necrosis- stimulated proliferation]

11. Examples/Case Studies Metabolic activation/detoxification [Acetaminophen] [Ethylene Glycol]

12. Acetaminophen Metabolic Disposition

13. Acetaminophen protein adducts/ GSH depletion - Time Course (Mitchell et al.)

14. Acetaminophen protein adducts/ GSH depletion - Dose-dependence (Mitchell et al.)

15. (Hinson et al.) APAP Binding = f[GSH]

16. Acetaminophen Metabolic Disposition

17. (Mitchell et al.) P450-dependent Metabolic Disposition of Acetaminophen

18. Acetaminophen

19. Ethylene Glycol (EG) (GA) Rate-limit

21. (Marshall, 1982) EG GA Oxal

22. Ethylene Glycol (EG) (GA) Low dose High dose

23. Examples/Case Studies: Altered Repair/Replacement [Propylene oxide]

24. Propylene Oxide O CH 3

26. Conclusions/concerns dose % response 1.Documentation of dose-dependent transitions a.Dictated by saturation of specific kinetic steps i.Necessity to identify and characterize mechanisms Saturation of detox, defense, repair Saturation of activation

27. Conclusions/concerns dose % response 2.Key to applying to safety assessment is where transitions occur with respect to expt’l dose and human dose a.Explore mechanisms at doses in range of the transitions Typical lab animal doses Typical human doses

28. Conclusions/concerns dose % response 3.Dose extrapolations assume that in the absence of evidence to the contrary, similar transitions occur across species, gender, age (targets, metabolic profile, disposition, receptors, defense, repair, cell cycle kinetics)

29. Conclusions/concerns dose % response 4.Now that we recognize the existence of dose-dependent transitions in drug-induced toxicities, how do we go about applying the concept to reducing the uncertainty of safety assessment estimates?

30. Proposed Definitions  “Transition” - a change in the relationship of the response rate as a function of dose, which may be indicated as a change in the slope of the dose- response curve and reflects a change in key underlying kinetic and/or dynamic factors that influence the mechanism responsible for the observed toxicity.  A transition usually occurs over a range of doses

31. Importance/Relevance  Transitions in the dose-response curve occur experimentally for a number of differentially-acting chemicals and should be factored into the risk assessment process to reduce uncertainty –Risk assessments should be based on the ‘best science’ – consideration of dose-dependent transitions in the mechanism of toxicity is an example of integrating the ‘best science’.

32. Origins  Transitions may reflect either kinetic or dynamic determinants –Importance of both PBPK and biologically-based modeling –Identification of key determinant factor influencing that transition  Identification of adaptive/compensatory responses in the respective species  Essential elements (O 2, Fe, Cu, Mn, Vit A)

33. Importance/Relevance  Identification of the transition ‘phase’ in the dose- response relationship is critical to the effective extrapolation between species, gender, age, etc. –Extrapolation beyond the tested dose-range –Estimating margins with respect to exposure

34. Dose-Selection  Dose selection should emphasize the transition region of the dose-response curve.  Current testing strategies likely will not capture transitions in the dose-response relationship.  Key concern is where within the dose-response relationship the transition occurs with regard to other points, such as NOAEL

35. Mechanism-based Biomarkers  Characterization of the mechanism of toxicity in animals reveals useful biomarkers of response, which are essential to anticipating points of departure in the dose- response relationship for humans. –Focus on endpoints that are linked to observed adverse effect and reliable in humans (bridging biomarkers). –Opportunity to consider human data.

36. Mechanism-based Biomarkers  Assume that more molecular end-points yield a d/r curve to the left (more sensitive) of the actual whole animal toxic end-point  A better understanding of molecular mechanisms will allow the integration of new approaches such as genomics, etc. in the R/A and R/C processes

37. Interspecies Concordance  If a dose-dependent transition is established for experimental species, the default assumption is that a similar transition occurs in humans.  Unless there is evidence to the contrary, it is assumed that the same mechanisms of toxicity are operative in humans as in experimental species.

38. Implementation  Applying the concept of dose-dependent transitions in R/A requires a much better understanding of exposure – can’t be exclusively hazard-driven.  Must provide incentives to generate the data

39. Achieving Acceptance  Acceptance is a far greater hurdle than conducting the scientific studies; –there is a critical need for communication to convince the risk managers of the value for change.

40. Conclusion  Risk assessments should be based on the ‘best science’ – consideration of dose- dependent transitions in the mechanism of toxicity is an example of integrating the ‘best science’.