1 TOXICOKINETICS Dr: Wael Hamdy Mansy Department of Pharmacology King Saud University

2 Toxicokinetics - the study of the time course of toxicant absorption, distribution, metabolism, and excretion Dosage Exposure Toxic Effects Plasma Conc. Site of action ToxicokineticsToxicodynamics

3 Toxicokinetic (TK) processes xenobiotic ABSORPTIONDISTRIBUTIONMETABOLISMEXCRETION EXTERNAL MEMBRANE BARRIERS skin G.I. tract lungs pools depots sinks BLOOD PLASMA TISSUES PHASE-1 Oxidation PHASE-2 conjugation KIDNEYS LIVER lungs saliva sweat breast milk

4 Disposition of Xenobiotics absorption distribution excretion

5 Structural model of cell membrane The lipid bilayer model explain how lipophilic xenobiotics can permeate through the membrane by passive diffusion hydrophilic xenobiotics can’t permeate unless there is a specific membrane transport channel or pump.

6 Mechanism of Membrane Permeation 1.Passive diffusion 2.Active transport 3.Facilitated transport 4.Pinocytosis

7 Transfer of Chemicals across Membranes Passive transport determined by: - Permeability of surface - Concentration gradient - Surface area Permeability depends on: For cell membranes: - Lipid solubility - pH of medium - pK of chemical For endothelium size, shape and charge of chemical PASSAGE ACROSS MEMBRANES Passive Facilitated Active

8 Uptake by Passive diffusion Passive diffusion, depends on Concentration gradient Surface area (alveoli  25 x body surface) Thickness Lipid solubility & ionization Molecular size (membrane pore size = 4-40 A, allowing MW of ,000 to pass through)

9 Carried by trans-membrane carrier along concentration gradient Energy independent May enhance transport up to 50,000 folds Example: Calmodulin for facilitated transport of Ca ++ Facilitated Transport

10 Active Transport Independent of or against conc. gradient Require energy Substrate –specific Rate limited by no. of carriers Example: P-glycoprotein pump for xenobiotics (e.g. oleoresin capsicum or OC gas ) and Ca-pump (Ca 2+ - ATPase).

11 Uptake by Pinocytosis For large molecules ( ca 1 um) Outside: in-folding of cell membrane Inside: release of molecules Example: Airborne toxicants across alveoli cells Carrageenan across intestine

12 Rate of Absorption The rate of absorption determines the time of onset and the degree of acute toxicity. This is largely because time to peak (Tmax) and maximum concentration (Cmax) after each exposure depend on the rate of absorption. Quiz: Rate the following processes in order of fastest to slowest: INTRAVENOUS> INHALATION >ORAL > DERMAL EXPOSURE.

13 Factors Affecting Absorption Determinants of Passive Transfer (lipid solubility, pH, pK, area, concentration gradient). Blood flow Dissolution in the aqueous medium surrounding the absorbing surface.

14 Factors Affecting GI Absorption Disintegration of dosage form and dissolution of particles Chemical stability of chemical in gastric and intestinal juices and enzymes Rate of gastric emptying Motility and mixing in GI tract Presence and type of food

15 Skin Absorption Must cross several cell layers (stratum corneum, epidermis, dermis) to reach blood vessels. Factors important here are: lipid solubility hydration of skin site (e.g. sole of feet vs. scrotum)

16 Other Routes of Exposure Intraperitoneal large surface area, vascularized, first pass effect. Intramuscular, subcutaneous, intradermal: absorption through endothelial pores into the circulation; blood flow is most important Intravenous

17 Bioavailability Definition: the fraction of the administered dose reaching unchanged to the systemic circulation for i.v.: 100% for non i.v.: ranges from 0 to 100% e.g. lidocaine bioavailability 35% due to destruction in gastric acid and liver metabolism

18 FIRST PASS EFFECT Intestinal vs. gastric absorption

19 Extent of Absorption or Bioavailability Dose Destroyed in gut Not absorbed Destroyed by gut wall Destroyed by liver to systemic circulation

20 Plasma concentration Time (hours) i.v. route oral route Bioavailability (F) (AUC) o (AUC) iv

21 Principle For xenobiotics taken by routes other than the iv, the extent of absorption and the bioavailability must be understood in order to determine whether a certain exposure dose will induce toxic effects or not. It will also explain why the same dose may cause toxicity by one route but not the other.

22 Distribution Distribution is second phase of TK process defines where in the body a xenobiotic will go after absorption Perfusion-limited tissue distribution perfusion rate defines rate of blood flow to organs highly perfused tissues (often more vulnerable) liver, kidneys, lung, brain poorly perfused tissues (often less vulnerable) skin, fat, connective tissues, bone, muscle (variable)

23 Plasma 3.5 liters. (heparin, plasma expanders) Extracellular fluid 11 liters. (tubocurarine, charged polar compounds) Intracellular water 28 liters. Total body water 42 liters. (ethanol) Transcellular small. CSF, eye, fetus (must pass tight junctions) Distribution into body compartments

24 Distribution Rapid process relative to absorption and elimination Extent depends on - blood flow - size, M.W. of molecule - lipid solubility and ionization - plasma protein binding - tissue binding

25 Distribution Initial and later phases: initial determined by blood flow later determined by tissue affinity Examples of tissues that store chemicals: fat for highly lipid soluble compounds bone for lead

fold increase in free pharmacologically active concentration at site of action. NON-TOXIC TOXIC Alter plasma binding of chemicals 1000 molecules % bound molecules free

27 Chemicals appear to distribute in the body as if it were a single compartment. The magnitude of the chemical’s distribution is given by the apparent volume of distribution (Vd). volume of distribution

28 Volume of Distribution (Vd) Volume into which a drug appears to distribute with a concentration equal to its plasma concentration Amount of drug in body Concentration in Plasma Vd =

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30 Vd can be calculated after an IV dose of a substance that exhibits "one-compartment model" characteristics. Vd = Dose / Initial Conc

31 DrugL/KgL/70 kg Sulfisoxazole Phenytoin Phenobarbital Diazepam Digoxin7490 Examples of apparent Vd’s for some drugs

32 tolbutamide (hypoglycemic drug) tolbutamide + warfarin (anticoagulant) high bioavailability low bioavailability Competition-displacement between xenobiotics

33 Distribution Blood Brain Barrier – characteristics: 1. No pores in endothelial membrane 2. Transporter in endothelial cells 3. Glial cells surround endothelial cells 4. Less protein concentration in interstitial fluid Passage across Placenta

34 Clearance (CL) Defined rate xenobiotic eliminated from the body  Can be defined for various organs in the body  Sum of all routes of elimination  CL total = CL liver + CL kidney + CL intestine

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36 Elimination by the Kidney Excretion - major 1) glomerular filtration glomerular structure, size constraints, protein binding 2) tubular reabsorption/secretion - acidification/alkalinization, - active transport, competitive/saturable organic acids/bases, -protein binding Metabolism - minor

37 Elimination by the Liver Metabolism - major 1) Phase I and II reactions 2) Function: change a lipid soluble to more water soluble molecule to excrete in kidney 3) Possibility of active metabolites with same or different properties as parent molecule

38 The enterohepatic shunt/ circulation Portal circulation Liver gall bladder Gut Bile duct Drug Biotransformation; glucuronide produced Bile formation Hydrolysis by beta glucuronidase

39 EXCRETION BY OTHER ROUTES LUNG - For gases and volatile liquids by diffusion. Excretion rate depends on partial pressure of gas and blood:air partition coefficient. MOTHER’S MILK a) By simple diffusion mostly. Milk has high lipid content and is more acidic than plasma (traps alkaline fat soluble substances). b) Important for 2 reasons: transfer to baby, transfer from animals to humans. OTHER SECRETIONS – sweat, saliva, etc.. minor contribution

40 Quantitative Aspects of Toxicokinetics

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42 Plasma concentration Variations in Rates of Absorption and Elimination on Plasma Concentration of an Orally Administered Chemical

43 Example of one or two compartment model

44 Two Compartment Model Assumes xenobiotic enters the first compartment Assumes that xenobiotic is distributed to the second compartment and a pseudoequilibrium is established Elimination is from the first compartment

45 Elimination Zero order: constant rate of elimination irrespective of plasma concentration. First order: rate of elimination proportional to plasma concentration. Constant Fraction of drug eliminated per unit time. Rate of elimination = constant (CL) x Conc.

46 Zero Order Elimination Pharmaco- Toxicokinetics of Ethanol Mild intoxication at 1 mg/ml in plasma How much should be taken in to reach it? 42 g or 56 ml of pure ethanol (Vd x Conc.) Or 120 ml of a strong alcoholic drink like whiskey Ethanol has a constant rate of elimination of 10 ml/hour To maintain mild intoxication, at what rate must ethanol be taken now? at 10 ml/h of pure ethanol, or 20 ml/h of drink. RARELY DONE DRUNKENNES S

47 Time Plasma Concentration Zero Order Elimination logCt = logCo - Kel. t 2.303

48 Plasma Concentration Profile after a Single I.V. Injection

49 Principle Elimination of chemicals from the body usually follows first order kinetics with a characteristic half- life (t1/2) and fractional rate constant (Kel).

50 First Order Elimination Clearance (CL): volume of plasma cleared of chemical per unit time. Clearance = Rate of elimination/plasma conc. Half-life of elimination (t 1/2 ): time for plasma conc. to decrease by half. Useful in estimating: - time to reach steady state conc. - time for plasma conc. to fall after exposure stopped.

51 Rate of elimination = Kel x Amount in body = CL x Plasma Conc. Therefore, Kel x Amount = CL x Plasma Conc. Kel = CL/Vd 0.693/t1/2 = CL/Vd t1/2 = x Vd/CL

52 Principle The half-life of elimination of a chemical (and its residence in the body) depends on its clearance and its volume of distribution t1/2 is proportional to Vd t1/2 is inversely proportional to CL t1/2 = x Vd/CL

53 Multiple dosing On continuous steady administration of a chemical, plasma concentration will rise fast at first then more slowly and reach a plateau, where: rate of input = rate of output rate of administration = rate of elimination ie. steady state is reached. Therefore, at steady state: Dose (Rate of Administration) = CL x plasma conc. or steady state conc. = Dose/clearance

54 Time plasma conc Cumulation Toxic level Single dose

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56 Concentration due to a single dose Concentration due to repeated doses The time to reach steady state is ~4 t1/2’s

57 Toxicokinetic parameters Vol of distributionV = DOSE / Co Plasma clearance CL = Kel.Vd plasma half-life (t 1/2 ) t 1/2 = / Kel or directly from graph Bioavailability F =(AUC)x / (AUC)iv

58 Daily Dose (mg/kg) Plasma Drug Concentration (mg/L) Variability in Toxicokinetics

59 CONCLUSION The absorption, distribution and elimination of a chemical are qualitatively similar in all individuals. However, for several reasons, the quantitative aspects may differ considerably. Each person must be considered individually and treated accordingly.