Principles of Toxicokinetics/Toxicodynanics

Transcription

1 Biochemical and Molecular Toxicology ENVR 442; TOXC 442; BIOC 442 Principles of Toxicokinetics/Toxicodynanics Kim L.R. Brouwer, PharmD, PhD

2 Pharmacokinetics/ Toxicokinetics: the study of the time course of xenobiotic absorption, distribution, metabolism and excretion

3 ADME Absorption how the xenobiotic enters the body Distribution where the xenobiotic goes in the body Metabolism what the body does to the xenobiotic Elimination how the xenobiotic is removed from the body

4 Pharmacodynamics/ Toxicodynamics: the relationship between xenobiotic concentration at the site of action and the resulting effect, including the time course and intensity of therapeutic and adverse effects

5 Pharmacokinetics/ Toxicokinetics Pharmacodynamics/ Toxicodynamics Dosage Regimen/ Exposure Plasma Concentration Site of Action Effects: Pharmacologic Toxic

6 Toxicokinetic/ADME Studies Pharmacokinetics / Bioavailability Mass Balance Tissue Distribution Metabolite Profile Plasma Protein Binding Inhibition / Induction

7 Pre-Clinical Testing Clinical Testing In Vitro PK / PD PK-Guided Dose Escalation Dose/Response Efficacy Pop n PK In Large Efficacy Trials Bio- Equivalence Generics Special Populations Product line extension Animal PK / PD Toxicology Safety Assessment Dose Selection Patient Variables Phase 3 Post Marketing Phase 2 Phase 1

8 Terms used in Pharmacokinetics/Toxicokinetics: C max T max λ t 1/2 AUC AUMC Vd ss f u Cl MRT MAT F

9 C max maximum concentration of compound observed in the matrix of interest T max time of maximum concentration

10 10000 C max Concentration T max - time of maximum concentration Time

11 lambda (λ) terminal elimination rate constant (slope from a semi-log concentration vs time plot) t 1/2 half-life: the time it takes for the concentration of the compound to decrease by 50%

12 10000 t 12 2 = ln λ Concentration slope = λ Time

13 The Half-life is Compound- and Subject-Dependent Compound Name t 1/2 (h) Rosiglitazone Avandia 3-4 Lamotrigine Lamictal Buproprion Wellbutrin 21 Fluticasone Flovent n/a Paroxetine Paxil 21 Data from LexiComp

14 Half-life is used to: Half-life Determine the time to reach steady-state (5 x t 1/2 ) Determine the time to eliminate all the compound from the body (5 t 1/2 ) Calculate dosing intervals Determine how much compound accumulates in the body given a fixed dosing interval or exposure Half-life is a secondary pharmacokinetic parameter that is determined by the volume of distribution (V) and clearance (Cl) of the compound t 1 = xV Cl

15 AUC area under the concentration vs time curve ln Concentration AUC Time

16 AUC 0 = AUC tau Single Dose Multiple Dose to Steady-State ln Concentration Time Time tau

17 Steady-state At Steady-State: Rate of input = Rate of elimination

18 AUMC area under the first moment curve Time * Concentration AUMC Time

19 Mean Residence Time (MRT) The average time one molecule resides in the body MRT = AUMC/AUC Used to express overall persistence of compound in the body

20 Clearance (Cl) volume of fluid (usually blood) from which compound is removed completely per unit time Mathematically: Cl = Dose AUC Organs that may be involved in clearance:» GI tract» Liver» Kidney» Lungs» Other sites (e.g., blood, skin)

21 Clearances (Cl) are Additive

22 Pharmacokinetic Parameters of GI158104X in Dog Dose mg/kg/day Cl IV (ml/min/kg) Cl r (ml/min/kg) Cl other (ml/min/kg)

23 Linear Pharmacokinetics Linear Kinetics = Dose-Proportional kinetics AUC (or concentration) changes proportionally with dose AUC Dose

24 Avandia Package Insert

25 Stationary Pharmacokinetics Pharmacokinetic parameters are independent of time Package insert for Tegretol (carbamazepine) Because Tegretol induces its own metabolism, the half-life is also variable. Autoinduction is completed after 3-5 weeks of a fixed dosing regimen. Initial half-life values range from hrs, decreasing to hr s on repeated doses. Tegretol is metabolized in the liver.

26 Volume of Distribution at Steady-State (Vd ss ) a parameter that relates plasma concentration to total mass of compound in the body Mathematically: Dose AUMC Vdss = * AUC 2

27 The Volume of Distribution is Compound- and Subject-Dependent Compound Name V (L) Rosiglitazone Avandia 18 Lamotrigine Lamictal 70 Buproprion Wellbutrin 140 Fluticasone Flovent 280 Paroxetine Paxil 560 Data from LexiComp

28 Fraction Unbound (f u ) fraction of drug that is not bound to plasma proteins the unbound concentration is in equilibrium between the tissues and blood Blood Bound Drug Unbound Drug Bound Drug Tissue Unbound Drug

29 Protein-Binding Changes May be Clinically Relevant for the Following Types of Compounds and Routes of Administration IV Administration High Extraction Ratio Low Extraction Ratio Hepatic Clearance Yes No Nonhepatic Clearance Yes No Oral Administration Hepatic Clearance No No Nonhepatic Clearance Yes No No drugs from a list of 456 met these criteria Benet and Hoener, Clin Pharmacol Ther 71:115, 2002

30 Organ Extraction (E) C in Q Clearing Organ Q C out Drug Elimination E = C in - C out C in

31 When Is Protein Binding Important? In vitro ADME/preclinical toxicology studies Scaling pharmacokinetic/pharmacodynamic parameters from animal models to humans Calculating human doses from in vitro measures of target concentrations Therapeutic drug monitoring of blood/plasma concentrations for drugs with a narrow therapeutic index

32 Bioavailability (F) fraction of the administered dose that reaches the systemic circulation intact 0 < F < 1 Mathematically: F AUC = PO * AUC IV Dose Dose IV PO

33 Relative Bioavailability fraction of the dose of a test product that reaches the systemic circulation relative to a reference product Mathematically: F AUC = test * AUC ref Dose ref Dose test

34 Bioavailability is an important determinant of pharmacologic response and therapeutic effectiveness

35 Estimation of Oral Bioavailability (F) After an IV and Oral Dose of Compound X Dose mg/kg/day AUC IV 1 (µg*hr/ml) AUC PO 1 (µg*hr/ml) F mean, n=4

36 Factors Responsible for Poor or Variable Oral Bioavailability Physicochemical Factors Physiologic Factors

37 Influence of Formulation Factors on the Concentration-Time Profile Formulations with different release rates (absorption kinetics) Rosiglitazone Plasma Concentration, ng/ml 200 E C Formulation #1 G D Formulation #2 G H H E F 150 D C EF GH H E Formulation #3 F Formulation #4 H F D G E G Formulation #5 G H H Formulation #6 D CE F F F C F HG G 100 E D D H FE G E G CE C H F H D D C E F CF 50 G G D D E H G H F D C H G EE CE 0 CC DD DDD C CCC D F GE F G H FF GH H CE Time, h

38 Influence of Formulation on the Bioavailability of a Compound Concentration (ng/ml) Serum Concentration vs Time Profile Time (hours) Form. A Form. B Bioequivalence studies are performed to select the formulation which gives the greatest oral exposure to the drug

39 Special Studies: Effects of Food on Exposure AUC (inf) (ng*hr/ml) Often the fed/fasted state of the animal/human can influence the absorption of drugs Food can enhance or inhibit absorption 0 Fed Fasted Food can impact clinical dosing

40 Physiologic Factors Responsible for Poor or Variable Oral Bioavailability Cell Membranes ph, Volume and Content of GI Fluid GI Motility Vascularity and Blood Flow Enzymatic Degradation / Metabolism Disease States

41 The Gastrointestinal Tract

42 Factors Affecting Systemic Availability Gut Lumen Gut Wall Portal Vein Protein Binding RBC Partitioning Blood/Plasma Stability Systemic Circulation Solubility Chemical Stability Permeability Mucosal Metabolism Transport/Efflux Feces Metabolism Liver Metabolism Biliary Excretion

43 Presystemic Elimination: (First-Pass Effect) Loss or metabolism of drug between the administration and sampling site Example: Grapefruit Juice Decreases Felodipine Bioavailability

44 In Vivo Pharmacokinetic and Bioavailability Studies Resulting Information: pharmacokinetic parameters (t 1/2, AUC, Vd ss, Cl, F) in the species used for the multiple dose toxicology studies rate and extent of drug absorption primary routes of elimination Utility: identifies bioavailability problems parameters are used in scaling to humans to predict the initial human dose

45 Additional Studies for Compounds That Exhibit Poor Bioavailability In Vivo Studies to Assess Sites of Presystemic Elimination:

46 Assessing Sites of Presystemic Elimination Dose Dose Stomach Portal Vein Dose Duodenum Jejunum & Ileum Liver Systemic Circulation

47 Additional Studies for Compounds That Exhibit Poor Bioavailability In Vivo Portal Vein Administration Studies: AUC F PV PV= * AUCIV Dose Dose If F PV approximates 100%, presystemic elimination occurs prior to the portal vein (gut lumen or gut wall) If F PV approximates F PO, presystemic elimination occurs primarily in the liver IV PV

48 Assessing Sites of Presystemic Elimination Dose Dose Dose Stomach Portal Vein Dose Duodenum Systemic Circulation Jejunum & Ileum Liver

49 Case Study: Assessing Sites of Presystemic Elimination Route of Administration AUC (µg*hr/ml) Bioavailability (%) Oral Intraduodenal Portal Vein Intravenous

50 In Situ and In Vitro Models to Assess Sites of Presystemic Elimination Models to Assess Presystemic Elimination Prior to the Portal Vein Isolated Perfused Intestinal Segments Everted Gut Sacs Intestinal Rings Cell Monolayers Models to Assess Hepatic Presystemic Elimination Isolated Perfused Liver Liver Slices Hepatocytes (freshly isolated/cultured)

51 Toxicokinetics Dose Selection Dose Proportionality Multiple Dose Pharmacokinetics Induction/Inhibition Exposure Verification

52 Toxicokinetics: Dose Selection Determination of the optimum dose primarily focused on the relationship between dose and exposure» linearity» saturation

53 C max and AUC Following Escalating Doses of Compound A Dose mg/kg/day C max 1 (ug/ml) AUC 1 (ug*hr/ml) mean, n=4

54 Dose Selection AUC 6000 (ng*h/ml) Dose (mg/kg/day) Rat Dog

55 Saturation of Absorption Bioavailability (%) Mouse Dog Dose (mg/kg)

56 Toxicokinetics: Dose Proportionality A = AUC increases proportional to dose. B = Less than proportional increase in exposure: Absorptionlimited exposure. C = Greater than proportional increase in exposure: Capacitylimited elimination AUC hr*µg/m C A B Dose (mg/kg) Interpretation of results demonstrating lower toxicity at higher doses depends on whether compound exhibits dose-proportional kinetics.

57 Toxicokinetics: Multiple Dose Pharmacokinetics Determine how drug distribution is altered after multiple dosing Examine changes in clearance, half-life, accumulation, linearity Assure dose related continuous exposure of the animals to the test compound Evaluate: Toxicokinetic changes with dose and time Effect of advancing age of the animals Decreases in lean body mass, total body water, cardiac output, renal, hepatic and GI tract blood flow

58 Terminal t 1/2 (mean and SD) on Days 1 and 30 During a Rat 1-Month Oral Multiple Dosing Study M ales Females Half-life (hr) Half-life (hr) Dose Day 1 Day 30 Dose Day 1 Day 30 (mg/kg/day) (mg/kg/day)

59 Effect of Age on the Toxicokinetics of N-Acetylprocainamide HCl After IV Administration of 100 mg/kg in Rats Age Parameters 13 Weeks Mean (n=7) CV% 26 Weeks Mean (n=11) CV% 52 Weeks Mean (n=7) CV% Half-life (hr) C (0) (ug/ml) AUC (ug*hr/ml) Cl (L/hr/kg) Vd (L/kg) From: S. Garattini, Drug Metabolism Reviews 13, (3), 1982

60 Time (Exposure) Dependence Inhibition Normal 20 Induction Time 100 Inhibition Normal Induction 10 Induction and Inhibition of metabolism as examples of elimination changes Induction or Inhibition may occur without affecting the overall Time disposition of the test compound

61 Exposure Verification - Example Continuous IV infusion for 14 days in rats and dogs Infusion solution at ph = 5 Blood samples taken at various times to document exposure Pilot (24 hr) study in both species

62 Exposure Verification Concentration (ng/ml) Males Females Expected Time (h)

63 Special Toxicokinetic Issues Toxicity / Potency Evaluations Metabolic Sites Correlation of Residual Drug with Toxicity Species Selection Gender Effects Placental Transfer Milk Transfer

64 Coverage for Human Metabolites Species Selection confirmation that the species chosen for oncogenicity studies supports the human metabolism is there coverage in the animal species for all metabolites generated in human systems From: Ferrero, J.L. and Brendel, K. Advances in Pharmacology V43, pp , 1997.

65 Use of Whole Body Autoradiography (WBA) to Assess Effect of GF on Antiviral Distribution in Rats Antiviral Alone Ratio Brain CSF Antiviral + GF Blood Brain CSF 0.50 Ratio CSF/Blood 0.00 Brain/Blood Antiviral Alone Antiviral + GF Brain/Blood CSF/Blood

66 Toxicokinetic Modeling Classic Compartmental Modeling 1-Compartment Model Multi-compartment Model Physiologic Compartmental Modeling Perfusion-Limited Compartments Diffusion-Limited Compartments Specialized Compartments

67 Scheme Describing Disposition of Prodrug and Metabolite in Isolated Perfused Liver and Sandwich- Cultured Hepatocytes Yan et al. J Pharmacol Exp Ther, 337: , 2011

68 Intrahepatic Binding Markedly Influences Disposition of Active Metabolite Hepatic accumulation of active metabolite was extensive (>95% of total formed) Hepatic unbound fraction (f u,l ) of furamidine was only 0.3% explaining, at least in part, low perfusate (systemic) exposure of furamidine. Yan et al. J Pharmacol Exp Ther, 337: , 2011

69 Schematic of Physiologically-Based Pharmacokinetic Model 1. Parameters» Molecular Weight» Log P» Partition Coefficients» Protein Binding» Metabolic (Vmax & Km)» Clearance 2. Structure Organs & tissues involved Organ Weights Organ Flows Perfusion/Diffusion and/or Transport 3. Equations Mass Balance Clearance from tissues

70 Schematic Structure of Physiologically-Based Toxicokinetic Model for Inhaled Propylene Oxide Csanády G A, Filser J G Toxicol. Sci. 2006;95:37-62