Species extrapolation with PBPK models

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Presentation transcript:

Species extrapolation with PBPK models Kannan Krishnan Université de Montréal, Canada

Outline Introduction Rat-Human extrapolation Rat-Fish extrapolation Single chemicals Mixtures QSARs Rat-Fish extrapolation PBPK modeling Conclusions

PBPK Models and Interspecies Extrapolation Adipose tissue Richly perfused tissues Liver Poorly perfused Lungs Inhaled Exhaled C RAT PBPK Program T HUMAN C RAT or HUMAN T SIMULATIONS INPUT PARAMETERS STRUCTURE

Functional Representation Amount Blood flow to tissue, volume, partition coefficient

Functional Representation Metabolic Clearance Vmax = maximum velocity of metabolism Km = Michaelis-Menten constant C = concentration of chemical Metabolizing enzyme, its levels, tissue volume

Interspecies extrapolation of PK of chemicals Species SPECIFIC: Species SPECIFIC: INVARIANT: Blood:air PC Flows Volumes [P-450] Vmaxc Km Tissue:air PC Blood:air PC Flows Volumes [P-450]

Mixture PBPK Models and Interspecies Extrapolation Arterial blood Fat Lungs Poorly perfused tissues Richly perfused Liver metabolism Venous blood A B C D E A B C D E C RAT Computer Program T HUMAN C RAT or HUMAN T SIMULATIONS INPUT PARAMETERS STRUCTURE

QSARs for PBPK Parameters Fragment constant approach Ppbpk =  nf·Cf Multilinear regression (SPSS®) 46 VOCs, Fragments: CH3, CH2, CH, C, C=C, H, Cl, Br, F, B-ring, 2 E1 substrates Cross-validation, external validation

Structure Input @Chemical Exposure Condition Yellow Indicates User Input

QSAR/PBPK model – Ethyl benzene

QSAR/PBPK model:Dichloromethane

Conceptual Representation Inspired chemical Expired chemical Venous Blood Arterial Blood Rest of Body Kidneys Muscle Liver Gills

Partition Coefficients Tissue:water PCs Determinants of bioconcentration factors May vary between species (rat vs fish), if the composition of tissues change from one species to another Tissue components: lipids, water, proteins Lipids (neutral, phospho) + water

Tissue composition based Computation of PCs Po:e = n-octanol:env partition coefficient Pt:e = tissue:env partition coefficient Fnt = volume fraction of neutral lipids in tissue Fpt = volume fraction of phospholipids in tissue Fwt = volume fraction of water in tissue

Interspecies differences in mechanistic determinants (PCs) Tissue and species Neutral lipid eqvt Water eqvt Muscle Rat 0.0117 0.7471 Human 0.0378 0.7573 Catfish 0.0041 0.7960 FHM 0.0194 0.8116 Trout 0.0244 0.7746

Interspecies extrapolation of tissue:air partition coefficients (Humans) Y = 0.992X R2 = 0.969 Y = 0.973X R2 = 0.933 Y = 0.886X

Interspecies extrapolation of tissue:air partition coefficients (Fish)

Application (chloroethane) CL H H Log Poa = 0.373 +0.433 +0.785 = 1.6 Log Pwa = -0.031 -0.225 +0.471 = 0.215 Rat Pla 0.0425 *101.6 + 0.7176 *100.215 = 2.87 Trout Pla 0.0261 *101.6 + 0.7649 *100.215 = 2.29

Magnitude of Interspecies Differences in Tissue:Water PCs Po :w = n-octanol:water partition coefficient Pt :a = tissue:water partition coefficient Fnt = volume fraction of neutral lipids in tissue Fpt = volume fraction of phospholipids in tissue Fwt = volume fraction of water in tissue A & B = two differents species

Interspecies extrapolation of metabolism constants Vmax in one species Allometrically scale to another species Assume Km to be species invariant Worked well for CYP2E1 subtrates

Structure-Metabolic Constants Relationship Modeling Structural feature Log Vmaxc Log Km AC .734 .382 CL .612 .569 BR .810 .296 H (on C=C) .453 .584 CH3 .795 7.08E-2 CH2 .269 -.320 CH -.211 -.845 C -1.451 -1.544 C=C -. 353 -2.07 R2 0.947 0.752 PRESS/SSY 0.10 0.89

Application (chloroethane) CL H H Log Km = 0.071 - 0.32 + 0.569 = 0.26 vs 0.19 µM Log Vmaxc = 0.795 + 0.269 + 0.612 = 1.676 vs 1.79 Human (BW=70 kg) = 937 µmol/hr 101.68 * BW0.7 Rat (BW=0.25 kg) = 18.1 µmol/hr

Interspecies extrapolation of metabolism constants Turnover rate for one species CYP concentration + tissue volume Interspecies extrapolation of Vmax… Current approach: Classification of substrates (molecular volume, log P) Isozyme-specific substrates ( in vitro QSARs) Species extrapolation based on protein [C] and tissue volume

Interspecies extrapolation of PK of organic chemicals Species SPECIFIC: Species SPECIFIC: INVARIANT: Fluid PCs Flows Volumes Enzyme [C] Vmaxc Km Blood:air PC Flows Volumes Enzyme [C]

QSAR-PBPK Modeling Physiology Parameters Equations Rat Simulations

QSPR-PBPK Modeling: Interspecies extrapolation of tissue concentrations 5 10 15 20 25 30 35 Time (hr) Tissue concentration (mg/L) Catfish FHM Trout Rat Human

QSARs – An alternative paradigm DOSE EFFECT QSAR Tissue dose or Blood Conc. QSAR PK PD Relative contribution of the TK and TD processes Extrapolations based on TK determinants

Conclusions Interspecies differences in metabolic clearance and volume of distribution can be examined using mechanism-based QSARs PBPK modeling uniquely allows the integration of such QSARs to simulate interspecies differences in PK profiles QSAR-PBPK models facilitate internal dose based risk assessment in multiple species(lethal and non-lethal effects)

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