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# Modeling Human Variation in Response at Specific Ages Paul Hinderliter, PhD, DABT Battelle Pacific Northwest Division paul.hinderliter@pnl.gov EPA Scientific Advisory Panel, Washington, DC February 15 - 18, 20112 

# Information in this presentation addresses:
• Chapters in the report 4. Creation of Variation PBPK/PD model


# • Charge Questions 1. Physiologically Based Pharmacokinetic/Pharmacodynamic (PBPK/PD) Modeling 3. Model Calibration and Evaluation with Direct Dosing Human Studies 5. Sensitivity Analyses, Variability, and Uncertainty EPA Scientific Advisory Panel, Washington, DC, February 15 - 18, 20113 

# Variation modeling steps
• Perform a local sensitivity analysis to identify parameters in the LifeStage model that drive variation in response • Develop distributions of interindividual variation in values for the identified sensitive parameters • Define how correlations between input values are addressed • Compare prediction of variability to data from human volunteer studies.
3 EPA Scientific Advisory Panel, Washington, DC, February 15 - 18, 20114 

# Model Sensitivity Analysis

• The sensitivity coefficient of a parameter is defined by the change in peak RBC or brain AChE inhibition divided by the change in the parameter value. • In local sensitivity analysis a small change (1%) was made in the parameter value and the impact on peak RBC and brain AChE inhibition was calculated. • A sensitivity coefficient of 1 indicates that the 1% change in parameter produces a 1% change in either the peak RBC or brain AChE inhibition. • Values greater than 1 indicate a greater than 1% change in one of the two target effects. • Values close to zero indicate that neither brain nor RBC AChE levels were affected by changes in the parameter. EPA Scientific Advisory Panel, Washington, DC, February 15 - 18, 20115 

# Model Sensitivity Analysis Parameter Normalized Sensitivity Coefficient AChE Basis Tissue Volume Rapidly perfused tissues 0.09 Brain Slowly perfused tissues 0.20 Brain Liver 0.74 Brain Blood 1.77 RBC Fat 0.19 Brain Tissue Specific Blood Flows Rapidly perfused tissues <0.01 RBC Slowly perfused tissues <0.01 RBC Liver 0.30 RBC Fat <0.01 RBC Liver 0.47 RBC • Note: All of the physiological parameters above are calculated for each individual using the LifeStage model. EPA Scientific Advisory Panel, Washington, DC, February 15 - 18, 20116 

# Model Sensitivity Analysis Parameter Normalized Sensitivity Coefficient AChE Basis Liver Metabolism Cytochrome CYP that converts CPF into: Oxon (activation) 0.98 RBC TCPy (detoxification) 0.92 RBC Brain Metabolism - Cytochrome CYP that converts CPF into: Oxon (activation) 0.32 Brain TCPy (detoxification) 0.10 Brain PON1 Blood 0.26 RBC Liver 0.47 RBC 

### • Very few of the model parameters impacted the estimates of peak RBC or brain AChE inhibition. • Only 8 parameters had sensitivity coefficients greater than 0.1. It is recognized that some parameters that vary widely across a population with coefficients smaller than 0.1 might also contribute to variation in response. • However , using a cutoff criterion of 0.1 should capture the major drivers of variation in response.7 Variability Model Variability Model Data from: Lafortuna , 2005, Butte, 2000; or Valentin , 2002 

# LifeStage simulation of fat volumes

EPA Scientific Advisory Panel, Washington, DC, February 15 - 18, 20118 • Adapted from work rates from ( Jonsson et al. 2001) • Decrease in relative perfusion of major organs • Liver • Richly perfused (Red Marrow, GI Organs, Pancreas, Thyroid, Spleen, Kidney) • Increased perfusion • Skin and Muscle

– Movement and cooling • Fat

– energy stores 

### Model Compartment Change in Perfusion Change in work level Perfusion (L/ hr ) at Rest (0W) Perfusion (L/ hr ) at Work (50W) Fat 0.00017 4.8 6.3 Liver - 0.0019 25.7 22.7 Brain 0 38.2 38.2 Slowly perfused 0.075 15.9 239.7 Richly perfused - 0.0019 46.5 44.7 Total Cardiac Output 131 352



# Impact of work on Organ - Specific Perfusion Rates9 

# Z - score Tutorial

• A method of describing variation in terms of a distribution’s mean and standard deviation. • Z - scores of - 2, - 1, 0, 1, 2 are values that fall at two standard deviations below the mean, one standard deviation below the mean, the mean value, one standard deviation above the mean and two standard deviations above the mean. • The table at the right gives the Z - scores that correspond to various percentiles for normal distributions.

• Plotting the values of a distribution against their Z - score produces a straight line if the data are normally distributed. • Plotting the logs of data against Z - scores produces a straight line when the data are log - normally distributed. 

### Percentiles Z - score 1 - 2.3 5 - 1.6 10 - 1.3 25 - 0.7 50 0 75 0.7 90 1.3 95 1.6 99 2.310 • Distribution of **in vitro** enzyme liver activities for CYP (CPF to Oxon and TCPy ) and PON1 (Oxon to TCPy ). • Data points represent measured **in vitro** values scaled to the standard adult (30 samples). • Lines represent the fitted lognormal distribution used in the Variation model. • Note that the distribution is allowed to extend beyond the limits of the measured data. 

# In vitro enzyme liver activities11 • Total Vmax of human enzymatic PON1 metabolism of CPF - oxon to TCPy in the plasma over various ages. • Symbols represent **in vivo** rates extrapolated from the **in vitro** data. • Error bars show the 95% CI of Vmax values used in the model. 

# Total human PON1 metabolism

0.01 0.1 1 10 100 1 10 100 1000 10000 

### QR RR QQ



## Age (yr)Vmax (mmol/hr)



### 0.01 0.1 1 10 100 1 10 100 1000 10000 

### QR RR QQ



## Age (yr)Vmax (mmol/hr)12 -3 -2 -1 0 1 2 3 10 -2 10 -1 10 0 10 1 10 2 10 3 10 4 10 5 10 6 10 7 Liver PON Blood PON Liver CPF to TCP Liver CPF to oxon Brain CPF to TCP Brain CPF to oxon Z-ScoreVmax (**μ**mol/hrl)

-3 -2 -1 0 1 2 3 10 -2 10 -1 10 0 10 1 10 2 10 3 10 4 10 5 10 6 10 7 Liver PON Blood PON Liver CPF to TCP Liver CPF to oxon Brain CPF to TCP Brain CPF to oxon Z-ScoreVmax (**μ**mol/hrl) The range of the variation in metabolism can be inferred by comparing the ranges of the curves for the various enzymes. 

# Variation in vivo metabolic rates Children

Adults

EPA Scientific Advisory Panel, Washington, DC, February 15 - 18, 201113 0.01

0.1

1 10Blood TCP (nmol/ml) 0.5 mg/kg 1 mg/kg 2 mg/kg

• AChE inhibition and TCPy in blood in volunteers ( Kisicki **et al.** , 1999 ) following a single oral dose of CPF. • Triangles and error bars reflect actual data. • Black line is mean predictions and gray area presents the 5th and 95 percentiles of predicted range of response for individuals. 

# AChE inhibition and TCPy in blood in volunteers

0 24 48 72 96 120 Hours 0 24 48 72 96 120 Hours 0 24 48 72 96 120 50 60 70 80 90 100 110

HoursRBC AChE (% basal) 0.5 mg/kg 1 mg/kg 2 mg/kg 0 24 48 72 96 120 Hours 0 24 48 72 96 120 Hours 0 24 48 72 96 120 Hours14 Physical Activity (PA)

PA+ P+ CYP + Liver and Plasma PON1 PA + Physiology (P) Liver PON1 Liver and Plasma PON1 PA+ P+ CYP Enzymes 

## Relative variation in RBC AChE Inhibition from a single dose of 0.5 mg/kg (adult)

* Ratio of RBC AChE inhibition to the median individual15 

## Comparison of RBC AChE i nhibition from a single dose of 0.5 mg/kg across age Physical Activity (PA)

PA+ P+ CYP + Liver and Plasma PON1 PA + Physiology (P) Liver PON1 Liver and Plasma PON1 PA+ P+ CYP Enzymes

Adults (30 years) Children (3 years) Infants (6 months)16 

## Comparison of RBC AChE inhibition from a single dose of 0.1 - 1.0 mg/kg (adult) Physical Activity (PA)

PA+ P+ CYP + Liver and Plasma PON1 PA + Physiology (P) Liver PON1 Liver and Plasma PON1 PA+ P+ CYP Enzymes

0.5 mg/kg 1.0 mg/kg 0.1 mg/kg17



## Comparison of RBC and Brain AChE inhibition from a single dose of 0.5 mg/kg (adult) Physical Activity (PA)

PA+ P+ CYP + Liver and Plasma PON1 PA + Physiology (P) Liver PON1 Liver and Plasma PON1 PA+ P+ CYP Enzymes

RBC Brain

EPA Scientific Advisory Panel, Washington, DC, February 15 - 18, 201118 

# Summary of Age and Dose Dependencies

EPA Scientific Advisory Panel, Washington, DC, February 15 - 18, 201119 Peak blood concentrations of CPF and CPF - oxon following a single oral dose 

# Dose response relationships Chlorpyrifos Chlorpyrifos Oxon20 • Peak blood RBC AChE inhibition following single oral doses. • The lines represent the typical 30 year - old and 6 month - old from the LifeStage model. 

# Dose response of RBC AChE

0.01 0.1 1 10 0.01

0.1 1 10 100 1000

Dose (mg/kg) 0.01 0.1 1 10 0.001

0.01

0.1 1 10 100
30 yr old 6 mo old Dose (mg/kg) 

## % Basal % Control21 • Peak blood RBC AChE inhibition following single oral doses. • The lines represent the typical 30 year - old and 6 month - old from the LifeStage model. 

# Dose response of Brain AChE % Basal % Control

0.01 0.1 1 10 0.1 1 10 100 1000

Dose (mg/kg) 0.01 0.1 1 10 0.001

0.01

0.1 1 10 100

Dose (mg/kg)22 

# Summary
• Identified most sensitive model parameters and added estimate of interindividual variation in the LifeStage model • Simulations of human volunteer studies compare favorably • The range of various dose metrics can be calculated as a function of dose

EPA Scientific Advisory Panel, Washington, DC, February 15 - 18, 2011