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.
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