Pharmacokinetic Modeling of Environmental Chemicals

Part 2: Applications

Harvey J. Clewell, Ph.D.Harvey J. Clewell, Ph.D.Director, Center for Human Health AssessmentDirector, Center for Human Health Assessment

The Hamner Institutes for Health SciencesThe Hamner Institutes for Health SciencesResearch Triangle Park, North CarolinaResearch Triangle Park, North Carolina

I. Application of PBPK Models in Risk Assessments Based on Animal Studies

- vinyl chloride- vinyl chloride- trichloroethylene- trichloroethylene

II. Application of PBPK Models to Understand the Health Implications of Human Biomonitoring Data

- methylmercury- perfluorooctanoic acid

TODAY’S TOPICSTODAY’S TOPICS

Part 1: RISK ASSESSMENT

“The characterization of the potential adverse effects of human exposures to environmental hazards.”

- National Academy of Sciences, 1983

Risk Assessment Questions

• Qualitative: Is the chemical potentially harmful under ANY conditions?

• Quantitative: At what human exposure concentration does the RISK become SIGNIFICANT?

“All substances are poisons; there is none which is not a poison. The right dose differentiates a poison and a remedy.”

–- Paracelsus, 1493-1541“Dancing with proper limitations is a

salutary exercise, but when violent and long continued in a crowded room it is extremely pernicious, and has hurried many young people to the grave.”

-- A. Murray, M.D., 1826

The Dose is Important

Four Components of Risk Assessment

(National Academy of Sciences, 1983)

Agent Effect??

Hazard Identification

Risk Characterization

Agent Dose??Exposure Assessment

Dose Risk??Dose Response Assessment

Key Definitions In Contemporary Human Health

Risk AssessmentDefault – A generic, conservative (safe-sided) approach, for use when chemical-specific information is lacking

Mode of Action - in a broad sense, the critical sequence of events involved in the production of a toxic effect by a chemical

Dosimetry – Estimation of the tissue exposure to the form of the chemical (e.g., a reactive metabolite) that is most directly related to the toxic effect

absorption, distribution, metabolism, excretion

local metabolism, binding

reactivity, DNA adducts, receptor activation

cytotoxicity, DNA mutation, increased cell division

toxicity, cancer

Steps in a Toxic Mode of Action

Exposure

Tissue Dose

Molecular Interactions

Early Cellular Effects

Toxic Responses

Mode of Action Considerations

• Parent Chemical (ethylene oxide)

vs. Stable Metabolite (trichloroacetic acid from trichloroethylene)

or Reactive Metabolite (methylene chloride)

• Physical effect (acute neurotoxicity of solvents)

vs. Reactivity (formaldehyde)

or Receptor Binding (dioxin)

• Direct Genotoxicity (mutations from vinyl chloride adducts)

vs. Indirect (oxidative stress)

or Nongenotoxic (arsenic inhibition of DNA repair)

Role of PBPK Modeling in Risk Assessments for

ChemicalsDefine the relationship between external concentration or dose and an internal measure of (biologically effective) exposure:

• in experimental animals

• in subjects from human studies

• in the population of concern

Application of Pharmacokinetics in Risk Assessment

Underlying Assumption: Tissue Dose Equivalence

• Effects occur as a result of tissue exposure to the toxic form of the chemical.

• Equivalent effects will be observed at equal tissue exposure/dose in experimental animals and humans.

• Appropriate measure of tissue dose depends critically on the mode of action for the effect of the chemical.

Steps for Incorporating PBPK Modeling in Human Health Risk

AssessmentIdentify toxic effects in animals or human populations

Evaluate available data on mode(s) of action, metabolism, for compound and related chemicals

Describe potential mode(s) of action

Propose relationship between response and tissue dose

Develop/adapt an appropriate PBPK model

Estimate tissue dose during toxic exposures with model

Estimate risk in humans based on assumption of similar tissue response for equivalent target tissue dose

Applications of PBPK Modeling in Human Risk Assessment by Regulatory Agencies

Methylene Chloride (EPA, OSHA, ATSDR, Health Canada)

2-Butoxy Ethanol (EPA, Health Canada)

Vinyl Chloride (EPA)

Chloroform (Health Canada)

Dioxin (EPA)

Trichloroethylene (EPA)

Perchloroethylene (EPA)

Isopropanol (EPA)

Considering Pharmacokinetic and Mechanistic Information in Cancer Risk

Assessment

Examples:

Easy: Vinyl Chloride

Hard: Trichloroethylene

Example 1: Vinyl Chloride

• Used to produce plastics; formed in groundwater from bacterial degradation of other contaminants

• Cross-species correspondence of a rare tumor type: liver angiosarcoma in mouse, rat, and human (workers).

• Carcinogenic at doses with no evidence of toxicity

• DNA-reactive, mutagenic

• Likely to be carcinogenic even at low doses

Considering Pharmacokinetic and Mechanistic Information in Cancer Risk

Assessment

Vinyl Chloride

ChloroethyleneEpoxide

Chloroacetaldehyde

P450

Epoxide Hydrolase

DNA AdductsCO2

H2O

Tissue AdductsGlutathioneConjugates

GSH

GSH

Vinyl ChlorideVinyl Chloride

ChloroethyleneEpoxide

ChloroethyleneEpoxide

ChloroacetaldehydeChloroacetaldehyde

P450

Epoxide Hydrolase

DNA AdductsDNA AdductsCO2CO2

H2O

Tissue AdductsTissue AdductsGlutathioneConjugatesGlutathioneConjugates

GSH

GSH

Metabolism of Vinyl Chloride

Dose metric: Dose metric: concentration of concentration of chloroethylene epoxidechloroethylene epoxide

QPQC

QF

QS

QR

QL

Lungs

Fat

Rapidly Perfused Tissues

Slowly Perfused Tissues

Liver

CI CX

CVF

CVR

CVS

CVL

Reactive MetabolitesCO2 Glutathione Conjugate

Tissue/DNA Adducts

CA

CA

CA

KCO2 KGSM

VMAX2KM2

VMAX1KM1

KFEE

CA

GSH

KGSM

KS KO

KZER

KA

KB

QPQC

QF

QS

QR

QL

LungsLungs

FatFat

Rapidly Perfused TissuesRapidly Perfused Tissues

Slowly Perfused TissuesSlowly Perfused Tissues

LiverLiver

CI CX

CVF

CVR

CVS

CVL

Reactive MetabolitesReactive MetabolitesCO2CO2 Glutathione ConjugateGlutathione Conjugate

Tissue/DNA AdductsTissue/DNA Adducts

CA

CA

CA

KCO2 KGSM

VMAX2KM2

VMAX1KM1

KFEE

CA

GSHGSH

KGSM

KS KO

KZER

KA

KB

PBPK Model for Vinyl Chloride (Clewell et al. 2001)

Dose metric: Dose metric: production rate of production rate of reactive metabolite reactive metabolite per gram liverper gram liver

1

10

100

1000

10000

0 1 2 3 4 5 6

Hours

Cha

mbe

r Con

cent

ratio

n (p

pm)

250 ppm550 ppm1250 ppm3200 ppm

Rats -- Pharmacokinetics

0

2000

4000

6000

8000

10000

0.1 1 10 100 1000 10000

Concentration (ppm)

Tota

l Am

ount

Met

abol

ized

(mg)

Rats -- Metabolism

0.1

1

10

0 0.1 0.2 0.3 0.4 0.5

Hours

Cha

mbe

r Con

cent

ratio

n (p

pm)

KM1=0.1

KM1=1.0

Human -- Subject A

0.1

1

10

0 0.1 0.2 0.3 0.4 0.5

Hours

Cha

mbe

r Con

cent

ratio

n (p

pm)

KM1=0.1

KM1=1.0

Human -- Subject B

Human risk estimates (per million) for lifetime exposure

to 1 ppb vinyl chloride in air based on the incidence

of liver angiosarcoma in animal bioassays

Animal Bioassay Study 95% UCL Risk / million / ppb

Males Females

Maltoni - Mouse Inhalation 1.52 3.27

Maltoni - Rat Inhalation 5.17 2.24

Feron - Rat Diet 3.05 1.10

Maltoni - Rat Gavage 8.68 15.70

Comparison of Cancer Risk Estimates for Vinyl Chloride

Basis

Old EPA -- Animal

PBPK -- Animal

PBPK -- Human (Epidemiology)

Inhalation(1 ug/m3)

84.0 x 10-6

1.1 x 10-6

0.2 - 1.7 x 10-6

Drinking Water(1 ug/L)

54.0 x 10-6

0.7 x 10-6

Example 2: Trichloroethylene

• Popular solvent for degreasing ; replaced by perchloroethylene for dry cleaning

• Lung and liver tumors in mice but not rats; kidney tumors in rats but not mice

• Equivocal human evidence (contradictory studies)

• Tumors generally associated with toxicity

• Little evidence of direct interaction with DNA

• Unlikely to be carcinogenic at low doses

Considering Pharmacokinetic and Mechanistic Information in Cancer Risk

Assessment

PBPK Model for TCE (Clewell and Andersen, 2004)QPCI CX

VMTB, KMTB

KAD KAS

KTSDKTD PDose

CVG

QG

QTBCVTB

CA

QC

CV

QC

QFCVF

QRCVR

QSCVS

KF VM, KM

QLCVL

Alveolar Blood

Alveolar Air

Tracheo-Bronchial Tissue

Lung Toxicity

Fat Tissue

Rapidly Perfused Tissue

Slowly Perfused Tissue

Stomach LumenGut Lumen

Gut Tissue

Liver TissueKidney Toxicity Liver Effects

Comparison of Linear Cancer Risk Estimates (per million) for Vinyl

Chloride and TCEBasis

Vinyl Chloride:

Old EPA

PBPK -- Animal

PBPK -- Human

TCE:

Old EPA

PBPK -- Animal

Inhalation(1 ug/m3)

84.0

1.1

0.2 - 1.7

1.3

3.5

Drinking Water(1 ug/L)

54

0.7

0.32

1.2

So… low-dose risk estimates using PBPK modeling would seem to suggest that TCE is a more potent carcinogen than vinyl chloride!

(What’s wrong with this picture?)

TCE Risk Assessment Factors

Exposure TCE

TCADCACHL

DCVC

mitogenicity

toxicity

DNA interaction

liver tumors

lung tumors

kidney tumors

Pharmacokinetics/Metabolism

Mechanism

ResponsePBPK modeling can only go so far…Also need an understanding of the toxic mechanism to interpret low-dose risks

• Issue: – Detection of chemicals in human blood (“chemical

trespass”)– Uncertain relationship to doses in animal toxicity studies

• Goal: – Reconstruct exposures– Compare to regulatory guidelines

(MCL, RfD, etc)

• Tools:– Pharmacokinetic (PBPK) models– Monte Carlo analysis of exposure variability and sampling

uncertainty

• Products:– Margins of safety – Objective interpretation of biomonitoring data

Part 2: Use of PBPK Modeling to Interpret Human Biomonitoring Data

Relationship of Human Biomonitoring Data to Animal Toxicity Data

Chemical concentrations in human blood from biomonitoring studies

Human exposures(Chemical concentrations in

environment)

Chemical concentrations in animal blood in

toxicity studies

Animal exposures(Administered doses in

toxicity studies)

Pharmacokinetic modeling

Pharmacokinetic Modeling

Traditional risk assessment

Margin of safety

Forw

ard dosim

etryR

everse dosim

etry

• Accidental poisoning episode– Iraq – 1972

• Seed grain, treated with methylmercury fungicide, inadvertently used to prepare bread

• Exposures continued over 1- to 3-month period

• Symptoms (late walking, late talking, neurological performance) observed in children of asymptomatic mothers exposed during pregnancy

Reconstructing Exposure with a PBPK Model: An Example with Methylmercury

P B P K M o d e l fo r M e H g E x p o s u r e

k vk rb c Q c

Q k

k u

R e d B lo o d C e lls

P la sm a

K id n e y

U rin e

D iv

Q rR ic h ly P e r fu se d

Q s

Q b r

Q p l

k h

k b r

k fe

Q l

S lo w ly P e rfu se d

H a ir

B ra in B lo o d

B ra in

P la c e n ta

F e tu s

L iv e r

Q g Q g

k r

k b G u tIn o rg a n ic M e rc u ryk i

D o ra l

k o

k fk d

In te s t in e

k fF e c e sIn o rg a n ic M e rc u ry

k l

Q fF a t

PBPK Model for Gestational Exposure to Methylmercury

Clewell et al. 1999, Shipp et al. 2000

Fetal Compartment

Qpl

kfeQfe

krbcf

Qfbr

Qfb

Placenta

Fetal Plasma

Fetal RBC's

Fetal Brain

Fetal Body

Effect of Changes in Fetal and Maternal Physiology on Dosimetry

Non-human primates exposed to a constant daily dose of methylmercury during gestation

0

0 . 5

1

1 . 5

2

2 . 5

0 5 0 1 0 0 1 5 0 2 0 0 2 5 0 3 0 0 3 5 0

D a y s

Me

Hg

in

Blo

od

(p

pm

)

F e t a l B l o o d

M a t e r n a l B l o o d

5 0 µ g M e H g / k g / d a y

Exposure Reconstruction With a PBPK ModelIraqi woman exposed during pregnancy

to grain contaminated with methylmercury

0

5 0

1 0 0

1 5 0

2 0 0

2 5 0

3 0 0

3 5 0

4 0 0

4 5 0

0 2 0 0 4 0 0 6 0 0 8 0 0

D a y s

Me

Hg

in

Ha

ir (

pp

m)

0

1

2

3

4

5

6

Me

Hg

in

Blo

od

(p

pm

)M a t e r n a l h a i r

M a t e r n a l b l o o d

I n f a n t b l o o d

M a t e r n a lE x p o s u r e

P r e g n a n c y

4 2 µ g / k g / d a y1 0 8 d a y s Estimated

exposure: 42 ug/kg/day

EPA Reference Dose: 0.1 ug/kg/day

Exposure Reconstruction for perfluoro-octanoic acid

Perfluoro-octanoic acid (PFOA) is used in the production of “non-stick” surface coatings; it is also a by-product of the production of water- and grease-repellent finshes

PFOA is highly persistent compound that has been found in human blood and in the environment, raising public concerns regarding the possible effects of exposure

In this study, a pharmacokinetic model of PFOA was used to estimate exposures in a population exposed to high concentrations of PFOA in drinking water and in a group of workers exposed to PFOA in the workplace

Schematic for a physiologically-motivated renal resorption pharmacokinetic model for

PFOA

k21dose

Central Compartment(Volume of distribution; Free f raction in serum; Serum conc)

Tissue Compartment(Amount in tissue)

Filtrate Compartment(Volume of renal fi ltrate; Renal fi ltration rate;

Saturable resorption)

k12

Qfil Tm, Kt

Elimination

k21dose

Central Compartment(Volume of distribution; Free f raction in serum; Serum conc)

Tissue Compartment(Amount in tissue)

Filtrate Compartment(Volume of renal fi ltrate; Renal fi ltration rate;

Saturable resorption)

k12

Qfil Tm, Kt

Elimination

Predicted time course of PFOA in plasma at different exposure levels

Occupational exposure ng/kg/day: 150 90* 46

Environmental exposure

* Estimated safe exposure based on effects in animal studies

Se

rum

PF

OA

Co

nc

en

tra

tio

n (

ng

/mL

)

Blood levels in general population: 5 ng/mL)

(Clewell et al., 2004)

Transplacental exposure to dioxin in maternal blood

Dilution of infant dioxin concentration by rapid growth

Different fractional volume of fat between male and female effects dioxin concentration

Application of PBPK Modeling to Predict the EffectOf Age-Dependent PK on Dioxin Blood Levels

Predicted blood levels assuming a constant daily exposure throughout life

Summary: Use of PBPK Modeling in Risk Assessments for Environmental

Chemicals• Pharmacokinetics can be used to improve the accuracy of extrapolations across species, and to estimate exposures associated with human biomonitoring results

BUT:

• Mechanistic data is essential for the selection of the appropriate dose metric to use in pharmacokinetic modeling as well as for the selection of the appropriate approach for characterizing the dose-response below the range of experimental observation of toxic effects

Physiological Pharmacokinetic Modeling Applications

References

Andersen, M.E., Clewell, H.J. III, Gargas, M.I., Smith, F.A., and Reitz, R.H. (1987). Physiologically-based pharmacokinetics and the risk assessment process for methylene chloride. Toxicol. Appl. Pharmacol. 87, 185

Clewell, H.J., III and Andersen, M.E. 2004. Applying mode-of-action and pharmacokinetic considerations in contemporary cancer risk assessments: An example with trichloroethylene. Crit Rev Toxicol 34(5):385-445.

Clewell, H.J., Gearhart, J.M., Gentry, P.R., Covington, T.R., VanLandingham, C.B., Crump, K.S., and Shipp, A.M. 1999. Evaluation of the uncertainty in an oral Reference Dose for methylmercury due to interindividual variability in pharmacokinetics. Risk Anal 19:547-558.

Clewell, H.J., Gentry, P.R., Covington, T.R., Sarangapani, R., and Teeguarden, J.G. 2004. Evaluation of the potential impact of age- and gender-specific pharmacokinetic differences on tissue dosimetry. Toxicol. Sci. 79:381-393.

Clewell, H.J., Gentry, P.R., Gearhart, J.M., Allen, B.C., Andersen, M.E., 2001. Comparison of cancer risk estimates for vinyl chloride using animal and human data with a PBPK model. Sci. Total Environ. 274 (1-3), 37–66.

Shipp, A.M., Gentry, P.R., Lawrence, G., VanLandingham, C., Covington, C., Clewell, H.J., Gribben, K., and Crump, K. 2000. Determination of a site-specific reference dose for methylmercury for fish-eating populations. Toxicol Indust Health 16(9-10):335-438.

Tan, Y.-M., Liao, Kai H., Conolly, R.B., Blount, B.C., Mason, A.M., and Clewell, H.J. 2006. Use of a physiologically based pharmacokinetic model to identify exposures consistent with human biomonitoring data for chloroform. J. Toxicol. Environ. Health, Part A, 69:1727-1756.