development of a physiologically based pharmacokinetic and pharmacodynamic model to quantitate...
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Development of a Physiologically Based Pharmacokinetic and Pharmacodynamic Model to Quantitate Biomarkers of
Exposure to Organophosphorus Insecticides
Charles Timchalk, Ahmed Kousba and Torka S. PoetBattelle, Pacific Northwest Division, Richland WA
QQ PON1 Polymorphism
1.E-08
1.E-07
1.E-06
1.E-05
1.E-04
1.E-03
1.E-02
1.E-01
Brai
n O
xon
(AUC
)
5 mg/kg
0.5 mg/kg
0.05 mg/kg
0.005 mg/kg
Lowest PON1 Activity
#7 Variability
0
20
40
60
80
100
0 200 400 600 800
Time (hrs)
% o
f Con
trol
Dermal 5 mg/kgOral 0.5 mg/kg
#5 Rat & Human Validation Studies #6 Saliva Biomonitoring
Blood TCP
0
1
10
100
0 3 6 9 12Time (hrs)
umol
/L
50 mg/kg
10 mg/kg
1 mg/kg
Saliva TCP
0.01
0.10
1.00
0 3 6 9 12
Time (hrs)
umol
/L
50 mg/kg
1 mg/kg
Plasma ChE Inhibition
0255075
100
0 3 6 9 12
Time (hrs)
% o
f Con
trol
50 mg/kg10 mg/kg1 mg/kg
TCP Kinetics
ChE InhibitionSaliva ChE Inhibition
0255075
100
0 3 6 9 12
Time (hrs)
% o
f Con
trol
50 mg/kg10 mg/kg1 mg/kg
Free Esterase Oxon-Esterase Aged Complex
Synthesis of New Esterase
“Free”CPF-Oxon +
Degradation of Esterase Released TCP Metabolite
mole hr-1
Regeneration
hr -1
B-Esterase (B-EST) Inhibition(shaded compartments)
CPF-Oxon
Km1, Vmax1
Compartment Model Ke B-EST (AChE, BuChE, CaE)
A-EST* Km3,4, Vmax3,4
BloodV
enou
s B
lood
Arte
rial B
lood
Fat
Slowly Perfused
Rapid Perfused
Diaphragm
Brain
Liver
CPF
Oxon
Qc
Ven
ous
Blo
odSlowly Perfused
Rapid Perfused
Brain
Liver
Ca
Arte
rial B
lood
Qs
Qr
Qd
Qb Cvb
Ql
Cv
Cvs
Cvr
Cvd
Cvl
Stomach
KaS HydrolysisKm2, Vmax2
Diaphragm
FatQf Cvf CvoCao
Qc
Qfo Cvfo
Qso Cvso
Qro Cvro
Qdo Cvdo
Qbo Cvbo
Qlo Cvlo
Intestine
KsI
KaI
TCP
SkinQsk Cvsk
GavageExposure
DermalExposure
Kp
SA
* Liver and blood only.
Dietary Exposure
Kzero
hr -1
InhibitionuM hr-1 -1
hr -1
Fa Fa
##4 PBPK/PD Model4 PBPK/PD Model
#8 Age-Dependent Response
0
20
40
60
80
100
0 10 20 30
Time (hrs)
% o
f Con
trol Adult Rat
PND17 Neonate Rat~4x
Brain AChE Inhibition, Oral 15 mg/kgBrain AChE Inhibition, Oral 15 mg/kg
Background:This project entails development and validation of a physiologically based
pharmacokinetic and pharmacodynamic (PBPK/PD) model (see #1) for the organophosphorus insecticide chlorpyrifos (CPF) to quantitate dosimetry and acetylcholinesterase (AChE) inhibition in young rats and children.
Chlorpyrifos is metabolized to an active oxon metabolite which is a potent inhibitor of AChE. Toxicity is due to the inhibition of AChE resulting in a broad range of neurotoxic effects (see #2).
It is hypothesized that an age-dependent decrement in chlorpyrifos metabolism correlates with increased sensitivity of young animals and potentially children. A balance between the contribution of various parameters like metabolism can increase or decrease the potential toxicity (see #3).
PBPK/PD models (see #4) allow for the integration of all the key parameters associated with toxicity and can be used to quantitate both the dosimetry and biological response (AChE inhibition), over a range of doses, exposure conditions (single vs. repeated), and exposure routes (oral, dermal or inhalation). This tool can be used by risk assessors to make a more biologically based assessment for risk associated with exposure to organophosphorus insecticides.
Dose BioavailabilityMetabolic ActivationMetabolic Detoxification A-Esterase CaE
Increased Toxicity Decreased Toxicity
DoseBioavailabilityMetabolic ActivationMetabolic Detoxification A-Esterase CaE
##33 OP Pharmacokinetic BalanceOP Pharmacokinetic Balance
Exposure Tissue Dose Oxon
Tissue Interaction
Tissue Interaction
AChEInhibition Neurotoxicity
Pharmacokinetics
Pharmacodynamics
##11
N
Cl Cl
ClOP
SC2H5O
C2H5O
N
Cl
Cl
Cl
HO
ChlorpyrifosS
P-450
N
Cl Cl
ClOP
OC2H5O
C2H5O Oxon
OHP
SC2H5O
C2H5ODiethylthiophosphate
3,5,6-trichloro-2-pyridinol (TCP)
OHP
OC2H5O
C2H5ODiethylphosphate
N
Cl
Cl
Cl
Conjugates- O
Sulfate or glucuronides of TCP
P-450
ToxicityAChE Inhibition
##2 Metabolism2 Metabolism
Results:
The PBPK/PD model has been validated using dosimetry and dynamic response (cholinesterase (ChE) inhibition) data obtained from animal and human studies. Figure #5 illustrates the plasma ChE response (data points) and PBPK/PD model fit (lines) for human volunteers exposed to chlorpyrifos both dermally and orally. The model does an excellent job of fitting a range of experimental data.
To evaluate the potential utility of saliva for biomonitoring, studies were undertaken to measure the amount of metabolite (TCP) present in saliva and the degree of salivary ChE inhibition following a oral exposure to chlorpyrifos (see Figure #6). These results suggest that saliva may be useful for biomonitoring for organophosphorus insecticides.
To assess the impact of variability associated with detoxification by human metabolism in adults a Monte Carlo analysis was conducted over a broad ranges of doses (see Figure #7). A metabolic polymorphism has the greatest impact on dosimetry (brain oxon AUC) at doses that overwhelm other detoxification pathways.
The PBPK/PD model was modified to allometrically scale (based on body weight) the age-dependent development of metabolizing enzymes and ChE enzyme activity, and simulations were compared against experimental data. These simulations (see Figures #8 and #9) are consistent with differences in the acute toxicity response between neonatal and adult rats (4-fold difference in sensitivity). However, the model also suggest that metabolism in neonates my be adequate at environmentally relevant exposure concentrations.
Groups Brain CPF-Oxon AUC (moles L-1 hr-1)
0.5 mg/kg
1.0 mg/kg
5 mg/kg
10 mg/kg
50 mg/kg
Adult Rat 0.02 0.03 0.13 0.31 1.64
Neonatal Rat (PND4) 0.02 (1.0)
0.06 (2.0)
0.36 (2.8)
0.77 (2.5)
6.51** (4.0)
Values in parenthesis are the ratio of brain CPF-oxon AUC for neonatal: adult rats.**Lethal dose level.
##9 Dose Comparison Adult vs. Neonatal Rat9 Dose Comparison Adult vs. Neonatal Rat
Conclusions & Impact:This EPA-STAR project has resulted in the development and validation of an integrated PBPK/PD
model for organophosphorus insecticides that can be used to quantitate age-dependent dosimetry and dynamic response following exposure to chlorpyrifos. This model can be used to address risk assessment issues specifically dealing with children’s susceptibility and cumulative risk.
The model framework can be readily extended to other important organophosphorus and carbamate insecticides.
The quantitation of key metabolites and ChE activity in saliva following in vivo exposure represents an important opportunity for development of non-invasive biomonitoring technology for the rapid detection of organophosphorus insecticide exposure. This approach can be readily adapted to other important pesticides and potentially used as a tool for the rapid assessment of exposure to chemical warfare nerve agents.