gov.uk An Agency of the Health and Safety Executive gov.uk An Agency of the Health and Safety Executive MEGen A Tool for the Rapid Generation of PBPK Models and Good Modelling Practice George Loizou CEFIC, Brussels, Belgium, 3 June 2010

An Agency of the Health and Safety Executive Perceptions of PBPK Models Data hungry Complex Resource intensive Time consuming Compound specific – (Most models take >1 day to build and debug)

PROGRAM:FOR VOC MODELING 'This is a model for m-xylene inhalation in humans. ' 'The compartments are liver,fat, muscle,skin, richly and' 'perfused tissue. Analysing STUDY 1. blood MHA levels against' 'creatinine, designated xyl1acrb.csl' 'George D Loizou - 29/7/97' ' ' INITIAL $'Section of program' 'physiological parameters' CONSTANT BW = 70.0 $'Body weight (kg)' CONSTANT QPC = 4.87 $'Alveolar ventilation rate (l/hr)' CONSTANT QCC = 4.87 $'Cardiac output (l/hr)' CONSTANT VFC = 0.19 $'Fraction fat tissue (kg/(kg/BW))' CONSTANT VLC = 0.03 $'Fraction liver tissue (kg/(kg/BW))' CONSTANT VRC = 0.06 $'Fraction richly perfused tissue (kg/(kg/BW))' CONSTANT VSC = 0.08 $'Fraction poorly perfused tissue (kg/(kg/BW))' CONSTANT VSK = 0.10 $'Fraction of skin (kg/(kg/BW))' CONSTANT QFC = 0.05 $'Fractional blood flow to fat ((l/hr)/QC)' CONSTANT QLC = 0.24 $'Fractional blood flow to liver((l/hr)/QC)' CONSTANT QRC = 0.51 $'Fractional blood flow to RPT ((l/hr)/QC)' CONSTANT QSC = $'Fractional blood flow to SPT ((l/hr)/QC)' CONSTANT QSK = $'Fractional blood flow to skin ((l/hr)/QC)' CONSTANT VLU = $'Volume of urinary output (l/hr)' CONSTANT CRE = 1.35 $'Concentration of creatinine in urine (g/l)' 'Chemical-specific parameters for xylene' CONSTANT PL = 3.02 $'Liver/blood partition coefficient' CONSTANT PF = 77.8 $'Fat/blood partition coefficient' CONSTANT PT = 3.01 $'Tissue/blood partition' CONSTANT PKA = 50.5 $'Skin/air partition coefficient' CONSTANT PK = 1.91 $'Skin/blood partition' CONSTANT PR = 4.42 $'Richly perfused/blood partition coefficient' CONSTANT PS = 2.01 $'slowly perfused/blood partition coefficient' CONSTANT PB = 26.4 $'Blood/air partition coefficient' CONSTANT MW = $'Molecular weight (g/mol)' CONSTANT VMAXC= 10.2 $'Maximum velocity of metabolism (mg/kg/hr)' CONSTANT KM =.4 $'Michaelis-Menten constant (mg/L)' CONSTANT KL = 2.0 $'First-order metabolic rate A = 1.8 $'Exposed surface area

'Parameters for simulated experiment' CONSTANT CONC = 50. $'Inhaled concentration (ppm)' CONSTANT CVAP = 50. $'Skin exposure concentration (ppm)' CONSTANT TSTOP= 45 $'Length of experiment (hr)' CONSTANT LENGTH1= 4.0 $'End first period inhalation exposure (hr)' CONSTANT LENGTH2= 8.0 $'End second period inhalation exposure (hr)' CONSTANT LENGTH3= 12.0 $'End second period inhalation exposure (hr)' CONSTANT LENGTH4= 28 $'End second period inhalation exposure (hr)' CONSTANT LENGTH5= 32 $'End second period inhalation exposure (hr)' CONSTANT LENGTH6= 36 $'End second period inhalation exposure (hr)' CONSTANT BREAK = 0.5 $'Break in exposure' CONSTANT BREAK2 = 12 $'Overnight break in exposure' TELOS = 45 $'Total exposure period (hr)' 'Calculated parameters' VTC = 0.91-VFC-VLC-VRC-VSC-VSK $'Fraction non-fat tissue (kg/(kg/BW))' VT = VTC*BW $'Volume non-fat tissue (L)' VF = VFC*BW $'Volume fat tissue (L)' VR = VRC*BW $'Volume richly perfused compartment' VL = VLC*BW $'Volume liver (L)' VS = VSC*BW $'Volume slowly perfused compartment' VK = VSK*BW $'Volume of skin compartment' QC = QCC*BW QP = QPC*BW QF = QFC*QC $'Blood flow to fat (L/hr)' QR = QRC*QC $'Blood flow to RTP (L/hr)' QL = QLC*QC $'Blood flow to liver (L/hr)' QS = QSC*QC $'Blood flow to slowly perfused tissue (L/hr)' QK = QSK*QC $'Blood flow to skin compartment (L/hr)' QT = QC-QF-QL-QR-QS-QK $'Blood flow to non-fat tissue (L/hr)' VMAX = VMAXC*BW**0.7 CIX = CONC*MW/24450 $'Exposure concentration (mg/l)' CSL = CVAP*MW/ $'Skin exposure conc. (mg/M3)' SCHEDULE CAT1.AT. T SCHEDULE CAT2.AT. T+LENGTH1 SCHEDULE CAT1.AT. T+LENGTH1+BREAK SCHEDULE CAT2.AT. T+LENGTH2 SCHEDULE CAT1.AT. T+LENGTH2+BREAK SCHEDULE CAT2.AT. T+LENGTH3 SCHEDULE CAT1.AT. T+LENGTH3+BREAK2

'Chemical in Skin compartment' RAK = KP*A*(CDERM-CVK2)+QK*(CA-CVK) $'(mg/hr)' AK = INTEG (RAK,0.) $'(mg/hr)' CK = AK/VK $'(mg/L)' CVK = CK/PK $'Venous blood (mg/L)' CVK2 = CK/PKA $'Skin conc (mg/L)' 'Chemical in lean tissue compartment' RAT = QT*(CA-CVT) $' (mg/hr)' AT = INTEG(RAT,0.) $' (mg)' CT = AT/VT $'(mg/L)' CVT = CT/PT $'Venous blood (mg/L)' 'Chemical in richly-perfused tissue compartment' RAR = QR*(CA-CVR) $' (mg/hr)' AR = INTEG(RAR,0.) $' (mg)' CVR = CR/PR $' (mg/L)' CR = AR/VR $' (Venous blood (mg/L)' 'Chemical in fat compartment' RAF = QF*(CA-CVF) $'(mg/hr)' AF = INTEG(RAF,0.) $'(mg)' CF = AF/VF $'(mg/L)' CVF = CF/PF $'Venous blood (mg/L)' 'Chemical in slowly perfused compartment' RAS = QS*(CA-CVS) $'(mg/hr)' AS = INTEG(RAS,0.) $'(mg)' CS = AS/VS $'(mg/L)' CVS = CS/PS $'Venous blood (mg/L)' 'Chemical in liver compartment' RAL = QL*(CA-CVL) - RAM $' (mg/hr)' AL = INTEG(RAL,0.) $' (mg)' CL = AL/VL $' (mg/L)' CVL = CL/PL $'Venous blood (mg/L)' 'Chemical metabolism' RAM = VMAX*CVL/(KM+CVL)+KL*CVL*VL $'(mg/hr)' AM = INTEG (RAM,0.) $'(mg)'

An Agency of the Health and Safety Executive Types of Model Error Simple typographical errors - Usually easy to rectify Coding errors - Mathematical errors leading to, - Physiologically unfeasible errors - Physiologically plausible errors (most dangerous kind!)

An Agency of the Health and Safety Executive Types of Model Error Violation of Mass-Balance - Most difficult to detect and rectify (because equations are coupled) Detecting errors can be notoriously difficult and laborious to detect and rectify

An Agency of the Health and Safety Executive Encouraging wide-spread take-up Make PBPK modelling more accessible to the regulatory community Rapid generation of PBPK models Shift emphasis from mathematics to biology Make model building process transparent and auditable

An Agency of the Health and Safety Executive MEGen Model Equation Generator An open-source PBPK model equation generator A web application with an intuitive interface Enforcing an efficient workflow based on the principles of good modelling practice.

An Agency of the Health and Safety Executive MEGen Comprises: Parameter database A model equation generator

An Agency of the Health and Safety Executive The PBPK Database Contains anatomical, physiological, biochemical and physicochemical data from original source Reduces the time and effort required to extract parameters required to populate a PBPK model To reduce the incidence of mistakes

An Agency of the Health and Safety Executive The Model Equation Generator To generate model code rapidly using “future-proof” generic code To eliminate typographical errors To be mathematically consistent To impose “check functions” to maintain model integrity (e.g., mass conservation and peripheral blood perfusion rates) Current ‘proof of concept’ can reproduce most standard PBPK models in the literature in about 10 minutes

An Agency of the Health and Safety Executive Slowly Perfused Rapidly Perfused Maintaining Mass-Balance

An Agency of the Health and Safety Executive Outputs MEG/Database Fully documented table of model parameters Schematic of PBPK model Code for Modelling software

An Agency of the Health and Safety Executive Maintaining Mass-Balance The sum total of tissue and organ compartment masses should be within the body mass of the organism. The total blood flow (i.e., cardiac output) in the model should be equal to the sum of the flows to the tissue compartments of the model in order to maintain the mass balance of the chemical at all times.

An Agency of the Health and Safety Executive PBPK Database MEGXML Berkerley Madonna MCSim ACSL MATLAB XHTML Components Merged Modelling software (visualisation and exercise) “Future-proof” language Model preview

An Agency of the Health and Safety Executive

Acknowledgements Sponsors CEFIC/LRI HSE Unilever DEFRA Dr. Rory Conolly (National Center for Computational Toxicology US EPA) Acting as Independent scientific evaluator for CEFIC )