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TOXICOKINETICSTOXICOKINETICS

Dr: Wael Hamdy Mansy

Department of PharmacologyKing Saud University

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Toxicokinetics - the study of the time course of toxicant absorption, distribution, metabolism, and excretion

DosageExposure

ToxicEffects

PlasmaConc.

Site ofaction

Toxicokinetics Toxicodynamics

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Toxicokinetic (TK) processes

xenobiotic

ABSORPTION DISTRIBUTION METABOLISM EXCRETION

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

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Disposition of Xenobiotics

Blood and lymph

Liver

Intravenous Intraperitoneal

Subcutaneous

Intramuscular

Dermal

extracellular fluid

fat

Secretory Structures

Bile

Kidney Lung

Bladder Alveoli

Urine Expired Air Secretions

body organs

softtissue bone

Gastrointestinal tract

Lung

feces

InhalationIngestion

absorption

distribution

excretion

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Structural model of cell membrane

HYDRO PHILE

HYDRO PHILE

HYDRO PHILE

HYDRO PHILE

HYDRO PHILE

LIPOPHILE

LIPOPHILE

INTERIOR

EXTERIOR

PASSIVE DIFFUSION

FACILITATED DIFFUSION OR

ACTIVE TRANSPORT

Phospholipid Bilayer

polar heads

non-polar tails

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.

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Mechanism of Membrane Permeation

1. Passive diffusion2. Active transport 3. Facilitated transport4. Pinocytosis

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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 endotheliumsize, shape and charge of

chemical

PASSAGE ACROSS MEMBRANES

Passive

Facilitated

Active

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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 100-70,000 to pass

through)

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• 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

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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 (Ca2+ -ATPase).

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Uptake by Pinocytosis

For large molecules ( ca 1 um)Outside: in-folding of cell membraneInside: release of moleculesExample:

Airborne toxicants across alveoli cellsCarrageenan across intestine

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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.

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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.

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Factors Affecting GI Absorption

Disintegration of dosage form and dissolution of particlesChemical stability of chemical in gastric and intestinal juices and enzymesRate of gastric emptying Motility and mixing in GI tractPresence and type of food

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Skin Absorption

Must cross several cell layers (stratum corneum, epidermis, dermis) to reach blood vessels.Factors important here are:

lipid solubilityhydration of skinsite (e.g. sole of feet vs.

scrotum)

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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

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BioavailabilityDefinition: 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

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FIRST PASS EFFECT

Intestinal vs. gastric absorption

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Extent of Absorption or Bioavailability

Dose

Destroyed in gut

Notabsorbed

Destroyed by gut wall

Destroyedby liver

tosystemiccirculation

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0

10

20

30

40

50

60

70

0 2 4 6 8 10

Plasma concentration

Time (hours)

i.v. route

oral route

Bioavailability (F)

(AUC)o

(AUC)iv

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PrinciplePrinciple

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.

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DistributionDistribution 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)

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•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

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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

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Distribution

Initial and later phases:initial determined by blood flowlater determined by tissue

affinityExamples of tissues that store chemicals:

fat for highly lipid soluble compounds

bone for lead

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100-fold increase in free pharmacologically active concentration at site of action.

NON-TOXIC TOXIC

Alter plasma binding of chemicals

1000 molecules

% bound

molecules free

99.9 90.0

100 1

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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

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Volume of Distribution (Vd)

Volume into which a drug appears to distribute with a concentration equal to its plasma concentration

Amount of drug in bodyConcentration in Plasma

Vd =

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Time

Ln of Blood (or

Plasma) Conc.

Co V = Dose / Co

Vd can be calculated after an IV dose of a substance that exhibits "one-compartment model" characteristics.

Vd = Dose / Initial Conc

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Drug L/Kg L/70 kg

Sulfisoxazole 0.16 11.2

Phenytoin 0.63 44.1

Phenobarbital 0.55 38.5

Diazepam 2.4 168

Digoxin 7 490

Examples of apparent Vd’s for some drugs

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drug 2 ( )

Albumin

Albumin

Extracellular Fluid

Blood Plasma

drug 1 ( )

Displacement of one highly bound drug by another

active molecules free in solution

AlbuminAlbumin

capillary wall

inactive molecules bound to albumin

greater affinity for plasma albumin binding sites

moderate affinity for plasma albumin binding sites

tolbutamide(hypoglycemic drug)

tolbutamide+

warfarin(anticoagulant)

highbioavailability

lowbioavailability

Competition-displacement between xenobiotics

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Distribution

Blood Brain Barrier – characteristics:1. No pores in endothelial membrane2. Transporter in endothelial cells3. Glial cells surround endothelial cells4. Less protein concentration in interstitial fluidPassage across Placenta

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Clearance (CL)

Defined rate xenobiotic eliminated from the body Can be defined for various

organs in the body Sum of all routes of

elimination

CLtotal = CLliver + CLkidney + CLintestine

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KID N EYf iltra tion

secretion(reab sorp tion )

LIVERm etabolism

excretion

LU N GSexhalation

OTH ER Sm other's m ilk

sw eat, sa liva etc .

Elim inationof chem icals from the body

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Elimination by the Kidney

Excretion - major1) glomerular filtrationglomerular structure, size constraints, protein binding 2) tubular reabsorption/secretion- acidification/alkalinization,- active transport,

competitive/saturable organic acids/bases,

-protein bindingMetabolism - minor

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Elimination by the Liver

Metabolism - major

1) Phase I and II reactions2) 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

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The enterohepatic shunt/circulation

Portal circulation

Liver

gall bladder

Gut

Bile

duct

Drug

Biotransformation;glucuronide produced

Bile formation

Hydrolysis bybeta glucuronidase

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EXCRETION BY OTHER ROUTESLUNG - 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

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Quantitative Aspects of Toxicokinetics

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Dose

Pla

sma

Co

nce

ntr

atio

n

0 1 2 3 4 5 6 7 8 90

2

4

6

8

10

12

TOXIC RANGE

THERAPEUTIC RANGE

SUB-THERAPEUTIC

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0

2

4

6

8

10

12

14

0 5 10 15 20

TIME (hours)

Pla

sma

con

cen

trat

ion

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

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Example of one or two compartment model

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Two Compartment Model

Assumes xenobiotic enters the first compartmentAssumes that xenobiotic is distributed to the second compartment and a pseudoequilibrium is establishedElimination is from the first compartment

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EliminationZero 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.

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Zero Order Elimination Pharmaco-Toxicokinetics of Ethanol

Mild intoxication at 1 mg/ml in plasmaHow 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

DONEDRUNKENNES

S

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Time

Pla

sma

Co

nce

ntr

atio

n

0 1 2 3 4 5 61

10

100

1000

10000

Zero Order Elimination

logCt = logCo - Kel . t 2.303

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Time

Plasma Concentration

0 1 2 3 4 5 61

10

100

1000

10000

C0

Distribution equilibrium

Elimination only

Distribution and Elimination

Plasma Concentration Profile after a Single I.V. Injection

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Principle

Elimination of chemicals from the body usually follows first order

kinetics with a characteristic half-life (t1/2) and fractional rate

constant (Kel).

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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.

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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 = 0.693 x Vd/CL

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PrinciplePrinciple

The half-life of elimination of a chemical (and its residence in the body) depends on itsclearance and its volume of distribution

t1/2 is proportional to Vdt1/2 is inversely proportional to CL

t1/2 = 0.693 x Vd/CL

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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 outputrate 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

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0

1

2

3

4

5

6

7

0 5 10 15 20 25 30

Time

pla

sma

con

c

Cumulation

Toxic level

Single dose

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Concentration due to a single dose

Concentration due to repeated doses

The time to reach steady state is ~4 t1/2’s

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Toxicokinetic parametersVol of distribution V = DOSE / Co

Plasma clearance CL = Kel .Vd

plasma half-life (t1/2)

t1/2 = 0.693 / Kel

or directly from graph

Bioavailability F = (AUC)x /

(AUC)iv

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Daily Dose (mg/kg)

Pla

sma

Dru

gC

on

cen

trat

ion

(m

g/L

)

0 5 10 150

10

20

30

40

50

60

Variability in Toxicokinetics

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CONCLUSIONCONCLUSION

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.