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Atelier PK sur articles 5 thèmes Absorption / biodisponibilité Métabolisme hépatique et effet de premier passage Les modèles in vitro d’étude de l’absorption : pertinence par rapport aux modèles in vivo Métabolisme intestinal et effet de premier passage Présentation de la stratégie d’extrapolation décrite dans l’article 5, en insistant sur les fondements théoriques et les limites de cette approche. 2 Article 5: In vitro/in vivo extrapolation of metabolic clearance 3 Why precociously predict in vivo metabolic (hepatic) clearance ? To evaluate oral bioavailability Can I administer my drug by the oral route ? 4 Can a new drug be developed for oral route ? Gut Lumen Portal vein Gut Wall 1 : fabs 2 : Fgut Liver 3 : FH Foral f abs F G F H 5 Can a new drug be developed for oral route ? Components of oral bioavailability Hepatic first-pass effect Foral f abs F G F H Cl H Foral, max 1 QH 6 Bioavailability and interindividual variability CV (%) 100 75 50 25 0 0 25 50 75 100 125 150 F% Hellriegel et al, 1996 Clin. Pharmacol. Ther 7 Bioavailability and interindividual variability Overexposure (adverse effects) Mean Exposure AUC or concentrations I.V. Threshold For toxicity p.o. Threshold for efficacy p.o. Dose 1 Underexposure (therapeutic failure, resistance) 5 Dose x 5 8 Two solutions for early evaluation of hepatic clearance (CLH) during development Interspecies extrapolation (from preclinical species to human) In vitro to in vivo extrapolation 9 Availability of in vitro systems Purified enzymes Subcellular fractions Hepatocytes S9, microsomes Suspensions, primary cultures Liver slices 10 Strategy for in vitro/in vivo extrapolation metabolism CLint, in vitro Microsomes Hepatocytes Scaling factors Clearance model CLint, in vivo ° In vitro fu, QH CLH Cl H Foral, max 1 QH 11 Strategy for in vitro/in vivo extrapolation metabolism CLint, in vitro Microsomes Hepatocytes Scaling factors Clearance model CLint, in vivo ° In vitro fu, QH CLH Cl H Foral, max 1 QH 12 In vitro metabolism Drug (concentration C0) E E E Free drug (free concentration) No limited diffusion to enzymes (E) 13 In vitro intrinsic clearance Quantification of metabolism by CLint Rate of metabolism CLint C Rate : M.T-1 , C: M.V-1 => CLint expressed in V.T-1 (flow units) “Intrinsic clearance (CLint) is a pure measure of enzyme activity towards a drug and is not influenced by other physiological determinants such as hepatic blood flow or drug binding within the blood matrix” 14 Michaelis-Menten kinetics Initial rate V Vmax Vmax/2 V= KM Vmax . C KM + C concentration Vmax : maximum rate of metabolism (related to enzyme quantity) KM : Michaelis constant (related to affinity between enzyme and analyte) Michaelis-Menten kinetics Rate CLint C Initial rate Vmax Initial rate C KM C Intrinsic clearance conc 16 Michaelis-Menten kinetics Vmax Rate C KM C V max CLint KM C When C << KM First-order / linear kinetics Vmax Rate C KM Clearance is constant V max CLint KM 17 Michaelis-Menten kinetics When C << KM : Initial rate Vmax Rate C KM Intrinsic clearance Graphic : slope of tangent conc The highest intrinsic clearance is obtained for C << KM 18 Strategy for in vitro/in vivo extrapolation metabolism CLint, in vitro Microsomes Hepatocytes Scaling factors Clearance model CLint, in vivo ° In vitro fu, QH CLH Cl H Foral, max 1 QH Scaling factors (SF) Microsomes CLint is expressed in µL/min/mg microsomal protein Hepatocytes CLint is expressed in µL/min/106 hepatic cells 20 Scaling factors (SF) From test tube to liver : quantitative relationship CLint,in vivo SF CLint,in vitro Ex: (mL/min) (µL/min/106 hepatic cells) 21 Scaling factors : rat liver Relevance of these scaling factors ? 22 CLint,in vivo CLint,in vitro SF log(CL int,in vivo) log(CL int,in vitro) log(SF) SF = 1.5 x 109 cells SF = 500 mg microsomal proteins Underestimation of in vivo clearance hepatocytes microsomes 23 Scaling factors (SF) Issue of experimental conditions (not taken into account) Documented species Human, rat Veterinary species ? Other species : to establish scaling factors Experimental determination Allometric scaling 24 Strategy for in vitro/in vivo extrapolation metabolism CLint, in vitro Microsomes Hepatocytes Clearance model Scaling factors CLint, in vivo ° In vitro fu, QH CLH Cl H Foral, max 1 QH In vitro vs. vivo situation ORGAN (ex : liver) cell drug cell Cu input CL organ Cu output cell Organ blood flow Q drug Metabolism Rate metabolism QE C site of metabolism (E=extraction coefficient) 26 Models of hepatic clearance Assumptions : no active transport only free drug crosses plasma membranes good mixing of hepatic arterial blood and hepatic portal blood homogenous distribution of enzymes within the liver f (QH ; fu ; CLint) ° CLH = =E Example: well-stirred model CL Q f u CLint H H f u CLint Q (= venous equilibration model) H 27 Models of hepatic clearance f (QH ; fu ; CLint) Model Complexity Well-stirred model ° CLH = Sinusoidal perfusion model Dispersion model Differences between models Low EH: minimal differences between models When EH ≥ 0.7: obvious differences between models, which become considerable when EH ≥ 0.9 28 29 Validation of in vitro/in vivo extrapolation In vitro metabolism Scaling factors Clearance model In vivo PK Vmax CLint, in vitro CLint, in vivo KM CLint, in vivo CLH CLtot 30 Validation of in vitro/in vivo extrapolation In vivo pharmacokinetic studies Intravenous administration Plasma concentration - time profile Urinary excretion of unchanged drug (Xu) CLTOT CLH CLR ε CLH Dose X u AUC plasma 31 Validation of in vitro/in vivo extrapolation In vivo pharmacokinetic studies In vivo intrinsic clearance (homogeneous model) 1 Q H D IV X u CL int,in vivo f u AUC IV Q H D IV X u 32 Validation of in vitro/in vivo extrapolation Clint,in vivo (mL/min/g liver) lidocaïne Correct prediction warfarin Important underestimation Clint,in vitro (mL/min/g liver) Iwatsubo et al. Pharmacol Ther, 73, 147-171, 1997 Reasons for discrepancies between Clint,in vitro and Clint,in vivo Extra-hepatic metabolism Drug transport through membranes Slow equilibrium between blood and hepatocytes Presence of active transport Interindividual variability Intrinsic : genetic polymorphism / P450 identification Extrinsic : liver sample handling / scaling factors Validation of in vitro/in vivo extrapolation EH : classification of compounds LOW INTERMEDIATE HIGH Hepatic extraction ratios high LOW low INTERMEDIATE HIGH ORAL BIOAVAILABILITY Clint,in vitro (mL/min/106 cells) Lavé et al. Clin Pharmacokinet, 36, 1999 EARLY PHARMACOKINETIC SCREENING Validation of in vitro/in vivo extrapolation Houston Biochem Pharmacol, 47, 1994