# MoE Approach: An Alternate Approach 

# to Evaluate Bioactivity and Exposure

S. Marty (S. Chaudhuri, M. Bartels) mmarty@dow.com The Dow Chemical Company December 3, 2014 FIFRA SAP Meeting: “Integrated Bioactivity and Exposure Ranking: A Computational Approach for the Prioritization and Screening of Chemicals in the Endocrine Disruptor Screening Program”

## Margin of Exposure Approach to ToxCast Data
• Conceptually similar, but approach differs from US EPA IBER 

### scoring

– AUC approach not available

– ExpoCast data not used 

### • Alternate approach: Uses exposure to interpret ToxCast data • Takes advantage of existing data on exposures for pesticides

– Focus on General Public 

### • Use IVIVE extrapolations to determine human internal dosimetry • If sufficient MoE between the lowest AC50 

### concentration in 

### ToxCast and internal dosimetry based on estimated human exposures, then compound is of ‘Low Concern” • Results agree with the EPA’s conclusions with IBER approach DOW RESTRICTED

## Approach for MoE Calculations
• Downloaded “active results” from ToxCast (September 2014) 

### • Separated data: endocrine- related and non -endocrine assays • Used lowest AC 50 

### values (AUC approach not available) • Future determinations could use AC 5

### , point of departure, etc. • Cell cytotoxicity was considered in paired assays • EPA risk assessment values used to estimate exposures • Use IVIVE extrapolations (TK) utilizing measured external exposure to predict human blood concentrations (internal dose) • MoE was based on the highest exposed group • TK parameters were adjusted for age - related physiology • MoE : Compared the lowest AC 50 

### concentration in ToxCast assays with internal blood concentrations estimated from human exposures 3

# IBER Estimations

3. Refined TK 1. Ag Chem Exposure Data 2. Lowest AC 50 Value

Margin of ExposureTest Chemical ExpoCast Estimated

Exposure – mg/kg/day Highest Dietary Exposure (Chronic) – mg/kg/day Source of Data Benfluralin 8.44E -8 to 9.35E - 6

(Total 95%ile: 6.386E - 6 ) 1.4E -5 (20-50+ yr olds) 7.0E - 6 (1 - 2 yr olds) Chronic Dietary

Risk Assessment, RED, EPA, 2004 

## 1. Ag Chemical Exposure Data: Dietary

Exposures Ag Chemical Exposures are: 

### • Well characterized; acquired as part of registration process

- ExpoCast data may span orders of magnitude • Evaluated by age -group



### - Can focus on age group with highest predicted exposure

## 2. Lowest AC50 

## Values – Endocrine and Non endocrine • AC 50 

## values from ToxCast Phase 2

– AC 50 

## has greater stability

– AC 5 

## or POD could be used for future calculations Test Chemical ToxCast AUC ToxCast Phase 2 Lowest AC50 Values

ER Agonist Endocrine Non -endocrine Benfluralin 0.0413 9 μM (OT_ERa_EREGFP_0120) 11.7 μM (ATG_NRF2_ARE_CIS)

## 3. EPA: Toxicokinetics to Estimate External Dose • HTTK based on Wetmore et al. , 2010: 1) Whole body is at the same concentration 2) Environmental exposure is constant and uniform 3) Assumes 100% absorption 

### 4) Plasma concentration is at steady state ( Css ) 5) Includes elimination due to liver metabolism • Substrate depletion by primary hepatocytes 

### 6) Includes elimination via passive glomerular filtration • Unbound chemical determined by rapid equilibrium dialysis • Adjusted for plasma protein binding 

### 7) Css predicted for steady state dose of 1 mg/kg BW/day • Linear dose adjustment based on Css value using a PK model • Monte Carlo analysis to simulate human variability in TK in adultsGastroPlus (v. 8.5): Oral absorption model attached to PBPK Model Refined predictions: plasma concentrations and fraction absorbed from GI 1) Highest exposure (mg/kg/day) was split into 3 daily exposures

– Mimic 3 meals per day

– Administered for a period of 28 days 2) Integrated metabolic clearance, plasma protein binding, and renal clearance (Wetmore et al., 2010) 3) Used variables in generic PBPK model of fed humans

– Age -appropriate physiology 4) Metabolic clearance rate at 1 or 10 μM were used depending on AC 50 (> 5 μM used 10 μM rate) 5) PBPK model run under fed conditions 6) Permeability was based on physiologically relevant QSAR predictions of GI permeation in human 7) Oral absorption uses the Advanced Compartmental Absorption and Transit (ACAT) model to predict passive absorption across the gut, account for soluble and insoluble portions of administered dose

– Benfluralin absorption = 31.5% 

## 3. Alternative Approach: Toxicokinetics to Estimate

Internal Dose (μM)Test Compound IBER

Scores Age

Ranges MOE by

Wetmore Approach MOE by

adjusting for fraction absorbed (Wetmore Approach) MOE by Gastro

Plus PBPK (Using Wetmore in vitro assays)
Benfluralin 79,231

(ER) 20-50+ yrs old 52,957 (E) 168,386 (E) 24,938,182 (E)

68,845 (NE) 218,902 (NE) 32,419,636 (NE) 1,000,000 (AR) 1-2 yrs

old 105,915 (E) 336,772 (E) 2,414,016,000 (E) 137,689 (NE) 437,803 (NE) 3,138,220,800 (NE) 

## MOE Values for Benfluralin

## Options to Consider for Bioactivity/Exposure

Profiling
• Use the most refined exposure data • For TK, use PBPK models that account for:

– Percent absorption

– Absorption rate

– Age- related physiology 

### • Determine suitable MoE to differentiate chemicals that require additional follow- up work • Alternate approach yielded the same conclusion as the 

### EPA’s IBER approach – Low Priority for ER and AR screening.

# Thank You!