# Presentation: 282242

## Background and Objectives of Ongoing Studies

**Annie M. Jarabek**
**National Center for Environmental Assessment**
**U.S. Environmental Protection Agency**

- Perchlorate Stakeholders Forum
- Sponsored by the IPSC
- Henderson, NV
- 19-21 May 1998

## Outline

- Background
- Definition of the RfD
- Derivation of the RfD
- Basis of the provisional RfD
- Review of perchlorate database
- Recommended new studies
- Description of different study designs
- Objectives of each study
- Strategy for synthesis of data
- Summary

## Outline

- Background
- Definition of the RfD
- Derivation of the RfD
- Basis of the provisional RfD
- Review of perchlorate database
- Recommended new studies
- Description of different study designs
- Objectives of each study
- Strategy for synthesis of data
- Summary

## Definition

- An oral reference dose (RfD) is an estimate (with uncertainty spanning perhaps an order of magnitude) of a daily oral exposure to the human population (including sensitive subgroups) that is likely to be without appreciable risk of deleterious noncancer health effects during a lifetime.

##   Research          Assessment             Management

***  ******Research          ******Assessment******             Management***

**EPA Scientific Research/**
**Data Collection**

**Risk Assessment**

- Dose-Response
- Assessment

-  Animal Toxicology
-  Clinical Studies
-  Epidemiology
-  Cell/Tissue
-       Experiments
-  Computational
-      Methods
-  Monitoring/
-      Surveillance

***Research***
***Needs***

- Hazard
- Identification

- Exposure
- Assessment

***Risk***
***Characteri-***
***zation***

- Control
- Options

- Non-risk
- Analyses

**Risk Management**

**Collaboration**

**Collaboration**

**External Input**
**into Research  **

-  Other Federal Agencies
-  States/Local
-  Academia
-  Industry
-  Public Interest/Environmental

**Decisions**

## Minimum Data Base for Derivation of an RfD 

**Mammalian Data Base****

**A. Two Chronic Oral **

**Bioassays in Different Species**

**B. **

**One Subchronic Oral **

**Bioassay **

**Confidence**

**High***

**Low**

**Comments**

**Minimum Data Base **

**for High Confidence**

**Minimum Data Base **

**for Estimation of an **

**RfD**

**One 2-Generation **

**Reproductive Study**

**C. Two Developmental Toxicity **

**Studies in Different Species**

- *   Rationale is to address all potentially critical life stages
- ** Rationale is to use different species to evaluate variability in species
-     sensitivity unless a particular laboratory animal model is more appropriate

## A High Confidence RfD is Based on Data 

**Reproductive**

**Developmental**

**General Toxicity**

**A High Confidence RfD is Based on Data **
**that Addresses All Potentially **
**Critical Life Stages.**

## RfD Derivation

**Hazard identification and data array analysis****	**
**Designation of effect levels (NOAEL, BMD)**
**Choice of critical effect**
**Dosimetric adjustment**
**Application of uncertainty factors (UF)**
**Characterization of uncertainty (confidence levels)**

## Data Array and Oral Reference Dose (RfD) Derivation

**Data Array and Oral Reference Dose (RfD) Derivation**

## RfD

**RfD**

## RfC  =

**NOAEL*[HED] = **
**	****	****The NOAEL or equivalent effect level obtained with an ****	****	****alternate approach (*), dosimetrically-adjusted to a ****	****	****	****human equivalent dose [HED].**
**	****UF = **
**	****	****Uncertainty factor(s) applied to account for the ****	****	****	****extrapolation required from the characteristics of the ****	****	****experimental regimen to the assumed human scenario, ****	****	****and**
**	****MF = **
**	****	****Modifying factor to account for scientific uncertainties ****	****	****in the study(ies) chosen as the basis for the ****	****	****	****	****operational derivation, e.g., poor statistical power or ****	****	****exposure characterization.**

**RfC  =**

**NOAEL*[HED]**
**UF X MF**

## Factors for Uncertaintiesin Applied Extrapolations

**10****H****	****	****Human to Sensitive Human**
**10****A****	****	****Experimental Animal to Human**
**10****S****	****	****Subchronic to Chronic Duration**
**10****L****	****	****LOAEL(HEC) to NOAEL(HEC)**
**10****D****	****	****Incomplete to Complete Data Base**

**Modifying Factor**

**MF Professional Assessment of Scientific Uncertainties of the Study and Data Base not Explicitly Addressed Above.  Default for the MF is 1.0 e.g., applied for small sample size or poor exposure characterization.**

## Extrapolation Uncertainties

- Interspecies

**Rat to Human**

- Intrahuman

**Variability Across Humans**

## Schematic of UF Components Incorporated Into Exposure-Dose-Response Characterization

**Schematic of UF Components Incorporated Into Exposure-Dose-Response Characterization**

**3**

**3**

**3**

**3**

**Pharmacokinetic**
**parameters & processes**

**Pharmacodynamic**
**parameters & processes**

## Basis of the Provisional RfD

**Initial correspondence to EPA Region IX (Dec 92) from Superfund Health Risk Technical Support Center (NCEA-Cin)**
**Principal study = Stanbury & Wyngaarden (1952)**
**NOAEL = 0.14 mg/kg-day for 100% iodide release **
**UF = 1000**
- intrahuman variability (10)
- less than chronic data (10)
- database deficiencies (10)
**Drinking water criteria = 3.5 ppb based on 70 kg / 2 L water**

## Second Provisional RfD (1995)

**Revision based on PSG submission to Superfund Health Risk Technical Support Center (NCEA-Cin)**
**Same principal study and NOAEL**
**Different UF**
- intrahuman variability (10)
- less than chronic data (10)
- database deficiencies decreased (3)
**Drinking water criteria = 18 ppb based on 70 kg / 2 L water**

## Provisional RfDMarch 1997 External Peer Review

**Proposed by TERA**
**Same prinicipal study, critical effect**
**Another, different UF = 100**
- intrahuman reduced (3)
- subchronic to chronic (3)
- LOAEL to NOAEL (3)
- Database deficiencies (3)

## March 1997 External Peer Review Process

- Independent experts selected from government, industry, academia, consulting firms and environmental groups by TERA Board of Trustees
- Conflict of interest disclosed and discussed
- Review lasted 3 hours and included:
- presentation by sponsor
- discussion by review panel of database, hazard identification, dose-response derivation, and other issues
- opportunity for registered observers to comment
- polling panel for consensus
- identification of outstanding issues

## March 1997 External Peer Reviewers 

- March 1997 External Peer Reviewers

- Dr. Robert Benson, U.S. EPA, Region VIII
- Dr. John Christopher*, California EPA
- Dr. Gary Diamond, Syracuse Research Corporation
- Dr. Marvin Friedman, Cytec Industries, Inc.
- Ms. Annie Jarabek, U.S. EPA, National Center for Environmental Assessment
- Ms. Bette Meek, Health Canada
- Dr. Kenneth Poirier, Procter and Gamble Company
- Dr. Jon Reid, University of Cincinnati
- Ms. Ruthann Rudel, Silent Spring Institute
- Experts Available to Peer Review Panel
- Dr.James Fagin, University of Cincinnati Department of Endocrinology
- Dr. Charles Capen, Ohio State University Department of Veterinary Biosciences
- Dr. Daniel Caldwell, principal investigator of the Caldwell et al. (1996) study
- * Dr. Christopher was not polled for consensus

## March 1997 External Peer Review

- Inadequate data base for derivation
- Available mechanistic insights suggest special studies and synthesis strategy
- Eight (8) additional new categories of studies recommended

## Deficiencies of Clinical Data

- Adult subjects
- Typically subjects with thyroids altered by disease or other treatments
- Few pregnant subjects
- Acute or short-term exposure duration
- Limited range of doses

## Outline

- Background
- Definition of the RfD
- Derivation of the RfD
- Basis of the provisional RfD
- Review of perchlorate database
- Recommended new studies
- Description of different study designs
- Objectives of each study
- Strategy for synthesis of data
- Summary

## Established Perchlorate Toxicity

**Established Perchlorate Toxicity**

- The only systematically studied and
- established effect is the anti-thyroid effect due to competitive inhibition of iodine uptake.

## Susceptibility

- The potential for increased susceptibility is due to factors that influence:

**(1)****	****Exposure ****e.g., activity patterns and location**
**(2) ****	****Deposition / uptake and the internal target tissue dose ****	****(i.e., pharmacokinetic parameters) and toxicant-target ****	****interactions,**** e.g., metabolism rates or pathways**
**(3)****	****Tissue sensitivity (pharmacodynamics) - conditions ****	****	****which alter or enhance target tissue response, ****e.g., ****	****	****age, nutritional status, or disease states**

## Potential Perchlorate Toxicity

**Potential Perchlorate Toxicity**

- Anti-thyroid effect in pregnant women
- might cause adverse effect in developing fetus.

## Additional Suggested Target Tissues

- Reproductive function
- Immune function
- aplastic anemia
- leukopenia

## Mechanisms of Anti-Thyroid Mediated Neoplasia in Rodents

- DNA Directed:
- X - rays
- 131 I
- Genotoxic chemicals
- Indirect
- Partial thyroidectomy
- Transplantation of TSH-secreting pituitary tumors
- Iodide deficiency
- Chemicals inhibiting iodide uptake
- Chemicals inhibiting thyroid peroxidase
- Chemicals inhibiting TH
- Chemicals inhibiting conversion of T3 & T4
- Chemical inhibiting hepatic thyroid hormone metabolism and excretion

## Mode of Action Provides Important Insight to Characterization of Toxicity

- A chemical’s influence on the molecular, cellular, and physiological functions in producing tumors
- Prolonged depression of TH causes a feedback that leads to upregulation of TSH which leads to thyroid gland hyperplasia
- Genotoxic?

## Proliferative Lesions Thyroid Follicular Cells in Rodents

**Morphologic Continuum**

**Normal**

**Hyperplasia**

**Adenoma**

**Carcinoma**

**Significance in Risk Assessment**

## Existing Data Summary

- Target tissue appears to be the thyroid but available testing not comprehensive across endpoints
- Anti-thyroid effects would differ among adult versus developing fetus, children
- Anti-thyroid effects associated with benign neoplasia development in rats; a nonlinear process
- Genotoxicity not characterized
- Relevancy to human risk of rat tumors not established; presumed protective

## Outline

- Background
- Definition of the RfD
- Derivation of the RfD
- Basis of the provisional RfD
- Review of perchlorate database
- Recommended new studies
- Description of different study designs
- Objectives of each study
- Strategy for synthesis of data
- Summary

## Recommended Studies

- 90-Day subchronic bioassay
- Developmental neurotoxicity study
- Genotoxicity assays
- Mechanistic studies
- ADME -	Absorption, Distribution,            		Metabolism and Elimination
- Developmental study
- 2-Generation reproductive toxicity study
- Immunotoxicity

## 90-Day Subchronic Bioassay

- Tests for additional target tissues
- Minimum database for RfD derivation
- Added additional tests for:
- reproductive parameters
- mutagenic effects in bone marrow
- thyroid hormone levels
- recovery
- Objective is to ascertain if anti-thyroid effect is critical and its dose-response

## Developmental Neurotoxicity Study in Rats

- Examines potentially critical effect and population:  evaluates nervous system (structure and function) of fetal, newborn, and young animals
- Added thyroid histopathology and thyroid hormone level determinations to characterize anti-thyroid effect in offspring

## Genotoxicity Assays

- Tests for toxicity to DNA
- Provides mode-of-action information to evaluate potential for carcinogenicity
- May impact consideration of uncertainty factor for less than chronic data

## Mechanistic Studies

- Aid to quantitative interspecies extrapolation - basis to extend PBPK model
- Additional developmental studies to evaluate thyroid TH in fetal and post-natal periods
- Determine relative sensitivity of fetal/postnatal thyroid versus adult
- Determine relative sensitivity of rat versus human

## ADME study

- Literature review of perchlorate discharge test
- Protocols proposed to evaluate perchlorate kinetics, iodine inhibition kinetics and thyroid hormone homeostasis
- Basis for development of physiologically-based pharmacokinetic (PBPK) model

## PBPK Model Schematic

## Developmental Studyin Rabbits

- Endpoint required for greater confidence in database, may reduce UF for data deficiencies if not critical effect
- Definitive test for toxicity during organ development (birth defects)
- Added additional groups, hormone analysis and thyroid histopathology to evaluate second species

## 2-Generation Reproductive Toxicity Study

- Evaluates fertility of adults and viability of (toxicity in) offspring in rats
- Tests for reproductive parameters over two generations
- Added analysis of thyroid hormones and thyroid histopathology at various time points
- Endpoint required for greater confidence in database, may reduce UF for database deficiencies if not critical effect

## Immunotoxicity Study

- Evaluates immune system structure and function
- Motivated by case reports of aplastic anemia and leukopenia
- May reduce UF for database deficiencies if not critical effect

## Recommended Studies Summary

**Study****	****	****	****Description****	****	****Use in assessment**

- 1.  Developmental	Evaluates nervous 	Potentially critical effect;
-      neurotoxicity + TH	system in fetal		comparison of developmental
- 			and postnatal rats		versus adult effects on TH
- 2.  90-Day subchronic	Tests for other target 	Minimum database for RfD
-      bioassay + TH	 +	tissues;	evaluates	dose-response for TH in young
-      repro + genotox +	effect on TH in young 	adult rats; additional info on     	recovery		adult rats	  	other; may allow decrease in 						UF for database deficiencies
- 3.  Genotoxicity assays	Tests for toxicity to DNA	Mode of action information 						for thyroid neoplasia; may 							reduce UF for database
- 						deficiencies
- 4.  Mechanistic studies	Evaluate mechanism 	Interspecies extrapolation;
- 			of  TH response		determine susceptible
- 			and sensitivity in rats 	subpopulation
- 			and humans

## Recommended Studies Summary

**Study****	****	****	****Description****	****	****Use in assessment**

- 5.  ADME studies		Characterizes absorption, 	Interspecies extrapolation
- 			distribution, metabolism
- 			and elimination in rats
- 			and humans; iodine
- 			inhibition and perchlorate
- 			kinetices, hormone
- 			homeostasis
- 6.  Developmental study 	Evaluates birth defects	Potentially critical effect; data in
-      + TH			in rabbits		second species for TH effects;  						may reduce UF for database 						deficiencies
- 7.  2 - Generation		Evaluates fertility of  	Potentially critical effect;  may
-      reproductive toxicity	adult rats and toxicity	reduce UF for database
-      + TH			in offspring over two	deficiencies
- 			generations
- 8.  Immunotoxicity	Evaluates immune	May reduced UF for database
- 			system structure and	deficiencies if not critical effect
- 			function

## Outline

- Background
- Definition of the RfD
- Derivation of the RfD
- Basis of the provisional RfD
- Review of perchlorate database
- Recommended new studies
- Description of different study designs
- Objectives of each study
- Strategy for synthesis of data
- Summary

## Revised RfD

- Data across comprehensive array of endpoints to establish target tissue
- Mechanistically-motivated special studies to characterize critical dose-response relationships
- Future refinements as required by new data

## Slide 46

## Annie M. Jarabek

- National Center for Environmental Assessment
- (MD-52)
- U.S. EPA
- Research Triangle Park, NC  27711
- Phone:	919.541.4847
- FAX:		919.541.1818
- E-mail:  jarabek.annie@epa.gov