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Frans Jongeneelen & Wil ten Berge
EPAA workshop, Ispra (I), 13-14 October 2011
A generic, cross-chemical predictive PBTK-model
PBTK-model in exposure/dose assessment
2
Schematic from: Georgopoulos
What were the demands for our generic PBTK-model?
Simple and easy model• To be used for occupational and/or environmental exposures
(Variability of environmental exposure is higher than biological variability)
• 1st tier estimate accuracy (one order of magnitude)• Minimum of data input• Free open source model
Choices made• QSPR prediction of partitioning• metabolism data of in vitro metabolism kinetics• MS Excel software platform• Species: man (in rest or light activity ), rat, mouse
3
Result: Overview of the PBTK-model IndusChemFate
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Exposure scenario Three routes of uptake:
Inhalation - concentrationDermal – dose rateOral - dose
Duration of exposure Personal Protective Equipment Species Physical activity level (rest/ light)
PBTK-model
Compound data Physical-chemical properties:
Density Molecular weight Vapour pressure Log(Kow) at pH 5.5 and 7.4 Water Solubility
Biochemical parameters : Metabolism (kM and Vmax) Renal tubulair resorption Enterohepatic circulation ratio
0,00E+00
5,00E-05
1,00E-04
1,50E-04
2,00E-04
2,50E-04
3,00E-04
3,50E-04
4,00E-04
4,50E-04
0,000 10,000 20,000 30,000 40,000 50,000 60,000 70,000 80,000
Hours
Pyrene and metabolites (Venous Blood)
VenBl C0 µmol/l
VenBl C1 µmol/l
VenBl C2 µmol/l
Scheme of the physiology of the PBTK-model
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Lungs
Excretion of parent compound in urine
InhalationExhalation
VENOUS
BLOOD
ARTERIAL
BLOOD
Oralintake
Dermalload
Heart
Brain
Dermis
Adipose
Muscle
Bone
Stomach + intestine
Liver
Kidney
Evaporation
Parent compound
Cyclus of 1st metabolite
To 2nd
metabolite cyclus
Bone marrow
Lungs
Excretion of 1st metabolitein urine
Exhalation
VENOUS
BLOOD
ARTERIAL
BLOOD
Heart
Brain
Dermis
Adipose
Muscle
Bone
Stomach + intestine
Liver
Kidney
Bone marrow
Routing of chemicals and metabolites in the PBTK-model
– Absorption– Inhalation– Oral uptake– Dermal uptake
– Distribution over the body– QSPR algorithm for estimate of blood:air partitioning– QSPR algorithm for estimate of tissue:blood partitioning
– Metabolism– Saturable metabolism according to Michaelis-Menten kinetics
– Excretion– Urine – Exhaled air
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Dermal absorption module of the model
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SubstanceStratum corneum
Viable epidermis
DepositionEvaporation
Absorption
To systemic circulation
Stagnant air layer
As liquid and/or solid As vapour/gas
Skin
Vapour of substance
= New model ofAIHA-EASCnamedIH SKINPERM
Distribution over compartments in the body
– Blood:air partition coefficient• QSPR Algorithm for estimation of blood:air partitioning
based on Henry coefficient and Koa
– Blood:tissue partition coefficient• QSPR Algorithm for estimation of blood:tissue
partitioning taken from De Jong et al (1997), based on lipid content and Kow
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Metabolism– Metabolism can be modeled in all tissues– Default is metabolism in liver only– Sequential metabolism
• Parent, 1st metabolite, 2nd metabolite and so on
– Stoichiometric yield in every step• Rate of loss of parent and appearance of metabolite• Difference gives stoichimetric yield of specific metabolite
– KM and Vmax from in experiments in microsomal liver fraction or hepatocytes
• Scaled to liver weight of individual/species
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Other aspects– Excretion in urine and alveolair air– With enterohepatic circulation
• fase 2 metabolites
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The PBTK-model is build as application in MS-Excel, called IndusChemFate
• The differential equations of the PBTK-model are written in speadsheet syntax (visual basic)
• The file IndusChemFate contains 4 sheets: 1. Tutorial with instructions in short2. Worksheet
– For data entry (exposure scenario, properties of chemical under study)
– For numerical output
3. Database of phys-chemical and biochemical properties of various chemicals
4. Graphical output sheet11
Simulation example 1 Average and individual variation in workers
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Paper: Inhaled dosimetry and biomonitoring of coal liquefaction workers (Quinlan et al, 1995)
• At coal liquefaction plants PAH occur as pollutants• 5 workers were followed during 4 shift on and 96h off work• Average pyrene in breathing zone was 1.3 µg/m3 pyrene.• 1-hydroxypyrene in urine was measured during 8 days = 192 h
Simulation example 1
Metabolism of pyrene
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Figure taken from: Luukkanen et al, 2001
Simulation example 1
Kinetics of in vitro metabolism of pyrene in human hepatic fractions
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Step Tissue Parameter and value ref
Pyrene to 1-OH-pyrene
Hepatic 9000*g fraction of 12 individuals
Vmax = 180 µmol/hr/kg tissue
KM = 4.4 µM
Jongeneelen (1987)
1-OH-Pyrene to
1-OH-pyrene- glucuronide
Hepatic microsomal fraction of 3 individuals
Vmax = 6,900 µmol/hr/kg tissue
KM = 7.7 µM
Luukkanen et al (2001)
Simulation example 1
Modeling: Data to be entered
1) Enter phys-chemical properties and biochemical parameters of parent compound and metabolites under study
2) Enter exposure scenario1) Inhalation: concentration and duration2) Dermal: dose rate and duration3) Oral: bolus dose
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Simulation example 1Entering properties & biochemdata of pyrene and metabolites
Pyrene
1-OH-Pyrene
1-OH-Pyrene-glucuronide
Parent Compound PyreneCAS 129-00-0Density (mg/cm3 or grams/litre) 1270Molecular weight 202,26Vapour Pressure (Pa) 0,0106Log(Kow) at skin pH 5.5 4,88Log(Kow) at blood pH 7.4 4,88Water solubility (mg/litre) 0,135Resorption tubuli (y/n/?) yEnterohepatic removal (relative to liver venous blood) 0Vmax Liver (parent[total] µmol/kg tissue/hr) 360Km Liver (parent[total] µmol/litre) 4,5Vmax Liver (parent[specif] µmol/kg tissue/hr) 180Km Liver (parent[specif] µmol/litre) 4,51st metabolite HydroxypyreneCAS 5315-79-7Density (mg/cm3 or grams/litre) 1000Molecular weight 218,28Vapour Pressure (Pa) 0,000022Log(Kow) at skin pH 5.5 Log(Kow) at blood pH 7.4 4,45Water solubility (mg/litre) 4Resorption tubuli (y/n/?) yEnterohepatic removal (relative to liver venous blood) 0Vmax Liver (1st metab[total] µmol/kg tissue/hr) 6900Km Liver (1st metab[total] µmol/litre) 7,7Vmax Liver (1st metab[specif] µmol/kg tissue/hr) 6900Km Liver (1st metab[specif] µmol/litre) 7,72nd metabolite Hydroxypyrene GlucuronideCAS 154717-05-2Density (mg/cm3 or grams/litre) 1000Molecular weight 394Vapour Pressure (Pa) 3,2E-17Log(Kow) at skin pH 5.5 Log(Kow) at blood pH 7.4 -2,12Water solubility (mg/litre) 40000Resorption tubuli (y/n/?) nEnterohepatic removal (relative to liver venous blood) 0,3Vmax Liver (2nd metab[total] µmol/kg tissue/hr)Km Liver (2nd metab[total] µmol/litre)Vmax Liver (2nd metab[specif] µmol/kg tissue/hr)Km Liver (2nd metab[specif] µmol/litre)
Simulation example 1
Entering exposure scenario of the coal liquefaction workers
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Airborne exposurescenario
Dermal exposurescenario
Oral intakescenario
Parameters Airborne ExposureµConcentration parent compound (mg/m3) 0,0013Start of airborne exposure (hours) 0Duration of airborne exposure (hours) 12Respiratory protection factor ( => 1) 1Dermal protection factor (air tight clothing => 1) 1
Parameters Dermal exposure to parent compoundSkin deposition pure substance (mg/cm2/hour) 0Start of skin exposure (hours) 0Duration of skin exposure (hours) 8Skin temperature (centigrade) 25Affected skin area (cm2) 5000
Parameters of oral absorptionBolusdose to stomach of parent compound (mg/kg bwt) 0Time of application (time in hours) 0Absorption rate into intestinal tissue (1/hour) 3
Simulation example 1
Run program - Results as table with levels and amounts in fluids and tissues
Simulation example 1
Run program- Results as graphs
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0,00E+00
2,00E-10
4,00E-10
6,00E-10
8,00E-10
1,00E-09
1,20E-09
1,40E-09
1,60E-09
1,80E-09
0,000 50,000 100,000 150,000 200,000 250,000
Hours
Pyrene and metabolites (Alveolar Air)
AlvAir C0 µmol/l AlvAir C1 µmol/l AlvAir C2 µmol/l
0,00E+00
5,00E-05
1,00E-04
1,50E-04
2,00E-04
2,50E-04
3,00E-04
3,50E-04
4,00E-04
4,50E-04
0,000 50,000 100,000 150,000 200,000 250,000
Hours
Pyrene and metabolites (Venous Blood)
VenBl C0 µmol/l VenBl C1 µmol/l VenBl C2 µmol/l
0,00E+00
5,00E-03
1,00E-02
1,50E-02
2,00E-02
2,50E-02
3,00E-02
3,50E-02
4,00E-02
4,50E-02
5,00E-02
0,000 50,000 100,000 150,000 200,000 250,000
Hours
Pyrene and metabolites (Urine)
UrinConc C0 µmol/l UrinConc C1 µmol/l UrinConc C2 µmol/l
Figure 3: Urine
Figure 2: Blood
Figure 1: Alveolair air
Simulation Example 1
Result: predicted concentration total 1-OHP in urine of the coal liquefaction workers (in umol/L)
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0,00
0,01
0,02
0,03
0,04
0,05
0,06
0,07
0,081 12 23 34 45 56 67 78 89 100
111
122
133
144
155
166
177
188
199
21Note: Dermal exposure was not measured and set at zero in simulation
Simulation example 1 Comparison measured 1-OHP in urine versus predicted
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Simulation Example 1
Result: Estimated fate of mass of pyrene + metabolites
0,00
0,05
0,10
0,15
0,20
0,25
0,30Mass of pyrene + metabolites (in µmol)
After 43 h After 102 h After 198 h
Simulation example 1
Conclusion
The average level of hydroxypyrene can be predicted when the exposure scenario is known
A rough predictive model does when we realise that differences of external exposure between workers are large
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Simulation example 2: dermal uptake after desinfection of hands with ethanol
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Nr. Compound Type of study
Exposure route
Exposure scenario Reference
1 Pyrene Observa-tional study of workers
Inhalation + dermal
4*12 on work, 96 hr off work
Quinlan, 1995
2 Ethanol Volunteer study
Application on skin, dermal
10 times disinfection of hands and arms. Rubbing during 80 min.
Kramer, 2007
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Dermal + inhalation of evaporated ethanol!
Solid line: dermal only exposure scenario
Simulation example 2Ethanol in blood after disinfecting of hands and arms (Kramer et al, 2007)
Simulation example 2
Conclusion
Dermal uptake of ethanol can be predicted
Evaporation of ethanol seems to be a significant extra route of uptake of ethanol after handrubbing
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What are the differences between this PBTK-model and other PBTK-models?
• GENERIC MODEL– Distribution of the chemical/metabolite in the body is
estimated by algorithms, thus model can easily can be used for many different volatile and semi-volatile chemicals
• RUNS IN WIDELY AVAILABLE SOFTWARE– The software application is running in MS EXCEL, a
software platform that is widely available
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Suggested application of this PBTK-model IndusChemFate
Chemical health risk assessors• Volatile and semi-volatile compounds• A priori (= 1st tier) estimation of concentration in body tisues
and/or body fluids at specific exposure scenarios• Screening of absorpion and fate of data-poor substances in
the human body• In vivo extrapolation of in vitro ADME data• Comparisons of toxicokinetics in different species (in-vivo to
in vivo extrapolation)• Assessment of contribution of dermal uptake to body burden
Student and non-experts• Educational tool to understand toxicokinetics of chemicals in
human body in relation to phys-chem properties
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Where to get more info?• Download EXCEL-application file and user manual:
– Website CEFIC LRI, webpage IndusChemFatehttp://www.cefic-lri.org/lri-toolbox/induschemfate
• Read Paper (in Annals Occupational Hygiene 2011) • See at our website: www.industox.nl• Ask us to do a live-demonstration
Acknowledgement
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Funding from CEFIC-LRI
Thank you
Any questions?