TOXICOKINETICS

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TOXICOKINETICS
Wongwiwat Tassaneeyakul Department of
Toxicology Khon Kaen University
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Toxicokinetics - the study of the time course of
toxicant absorption, distribution, metabolism,
and excretion How can we predict variability
among individuals? How can we extrapolate from
animal models to humans?
Plasma Conc.
Site of action
Dosage Exposure
Toxic Effects
Toxicokinetics
Toxicodynamics
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Toxicokinetic (TK) processes
ABSORPTION
DISTRIBUTION
METABOLISM
EXCRETION
EXTERNAL
BLOOD PLASMA
PHASE-1
KIDNEYS
MEMBRANE
Oxidation
LIVER
BARRIERS
xenobiotic
lungs
TISSUES
saliva
skin
PHASE-2
pools
sweat
G.I. tract
conjugation
depots
breast milk
lungs
sinks
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Disposition of Xenobiotics
absorption
distribution
excretion
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Structural model of cell membrane
The lipid sieve model explain how lipophilic
small cpds can permeate through the membrane by
passive diffusion hydrophilic cpds cannot
permeate unless there is a specific membrane
transport channel or pump.
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Mechanism of Membrane Permeation

• Passive diffusion
• Active transport
• Facilitated transport
• 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 endothelium
• size, shape and charge of chemical

PASSAGE ACROSS MEMBRANES
Active
Passive
Facilitated
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Uptake by Passive diffusion

• Uncharged molecules may diffuse along conc.
  gradient until equilibrium is reached
• No substrate specific
• Small MW lt 0.4 nm (e.g. CO, N20, HCN) can move
  through cell pores
• Lipophilic chemicals may diffuse through the
  lipid bilayer

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Uptake by Passive diffusion

• First order rate 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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Weak Acids and Weak Bases
HA ltgt H A- B H ltgt BH
UI I
UI I
pKa pH log(HA/A-) pKa
pH log(BH/B)
pKa 4.5 (a weak acid)
pH 2
pH 7.4
0.1 I
I 9990
100 UI
UI 100
100.1 total drug 10090
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Flickss law and Diffusion
dD/dt KA (Co - Ci) / t
Where dD/dt rate of mass transfer across the
membrane K constant (coefficient of
permeability) A Cross sectional area of
membrane exposed to the compound C0
Concentration of the toxicant outside the
membrane Ci Concentration of the toxicant
inside the membrane t Thickness of the membrane
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Facilitated Transport

• 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

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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. OC)
• Ca-pump (Ca2 -ATPase)

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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
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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. Rate the following
processes in order of fastest to slowest
INTRAVENOUSgt INHALATION gtORAL gt 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
  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

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Lungs Absorption

• For gases, vapors and volatile liquids, aerosols
  and particles
• In general large surface area, thin barrier,
  high blood flow rapid absorption
• Bloodair partition coefficient
• influence of respiratory rate and blood flow
• Bloodtissue partition coefficient

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Lungs Absorption
REMOVAL OF PARTICLES
Absorption of Aerosols and Particles 1-
Particle Size 2- Water solubility of the
chemical present in the aerosol or particle
Lymph
Physical
Phagocytosis
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Airway anatomy
bronchial tree
trachea

• diffusion distance 20 mm
• total exchange gas exchange area 80 m2

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Airway anatomy
alveoli
trachea
capillaries
bronchial tree

• diffusion distance blood/air 20 mm
• total exchange gas exchange area 80 m2

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Absorption Area in the Respiratory System
Nasopharynge 5-30 µm
Trachea Bronchi Bronchioles 1-5 µm
Alveolar Region 1 µm
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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)

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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 other
  factors
• Intravenous

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Bioavailability
Definition the fraction of the administered
dose reaching 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 First Pass Effect
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Systemic circulation
Liver vein
Liver
Liver artery
Vena portae and tributaries
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FIRST PASS EFFECT Intestinal vs. gastric
absorption
Wilkinson, NEJM 2005
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Extent of Absorption or Bioavailability
Destroyed in gut
Not absorbed
Destroyed by gut wall
Destroyed by liver
Dose
to systemic circulation
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Bioavailability (F)
Plasma concentration
(AUC)o (AUC)iv
i.v. route
oral route
Time (hours)
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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.
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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)

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Distribution into body compartments

• Plasma 3.5 liters. (heparin, plasma expanders)
• Extracellular fluid 14 liters.
• (tubocurarine, charged polar compounds)
• Total body water 40 liters. (ethanol)
• Transcellular small. CSF, eye, fetus (must
  pass tight junctions)

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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 flow
• later determined by tissue affinity
• Examples of tissues that store chemicals
• fat for highly lipid soluble compounds
• bone for lead

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Alter plasma binding of chemicals
1000 molecules
90.0
99.9
bound
100
1
molecules free
100-fold increase in free pharmacologically
active concentration at site of
action. NON-TOXIC
TOXIC
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volume of distribution
Chemicals appear to distribute in the body as if
it were a single compartment. The magnitude of
the chemicals distribution is given by the
apparent volume of distribution (Vd).
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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 body
Vd
Concentration in Plasma
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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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Examples of apparent Vds for some drugs
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Competition-displacement between xenobiotics
high bioavailability
low bioavailability
tolbutamide warfarin (antocoagulant)
tolbutamide (hypoglycemic drug)
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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

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Free-plasma and erythrocyte-bound
xenobioticsexample lead binding to ALAD protein
plasma Pb
Blood Pb
erythrocyte Pb
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Free-plasma and erythrocyte-bound
xenobioticsexample lead binding to ALAD protein
CNS (brain)
spongy bone
kidney
higher neurotoxicity
avg plasma Pb
avg erythrocyte Pb
average blood Pb
ALAD-1 polymorphism
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Normal blood capillaries
most capillaries are fenestrated small gaps in
capillary wallnot tightly sealed allows
paracellular permeation of small plasma solutes
hydrophiles can pass thru capillary wall into
tissue ECF must be smaller than 100 A lipophiles
cannot easily permeate capillary wall by
paracellular permeation mostly bound to plasma
proteins permeate capillary wall by passive
diffusion in free plasma phase
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Brain capillaries blood-brain barrier (BBB)

• brain capillaries are unfenestrated -- no gaps
• cell membrane of capillary endothelium cells
  sealed shut
• tight intercellular junctions constitute the
  blood brain barrier (BBB)
• paracellular permeation of plasma solutes is
  impossible
• hydrophiles dissolved in blood typically cannot
  pass through the BBB into brain
• lipophiles can easily permeate the BBB by
  transcellular permeation (passive diffusion)

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Capillary structure
General circulation
Central nervous system blood brain barrier
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Elimination

• Includes all mechanisms for removing xenobiotics
  from the body
• Kel is the elimination rate constant
• One compartment model
• Slope -kel/2.3
• Two Compartment model
• ? distribution Constant
• slope ß/-2.3 and ?is the elimination rate
  constant
• Is calculated after pseudoequilibrium has been
  established

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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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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

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Nephron Structure
(A.C. Guyton, Textbook of Medical Physiology,
Philadelphia, W.B. Saunders Co. 1991
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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
• Biliary Secretion active transport, 4 categories

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The enterohepatic shunt/circulation
Drug
Liver
Bile formation
Bile
duct
Biotransformation glucuronide produced
Hydrolysis by beta glucuronidase
gall bladder
Portal circulation
Gut
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EXCRETION BY OTHER ROUTES

• LUNG - For gases and volatile liquids by
  diffusion.
• Excretion rate depends on partial pressure of
  gas and bloodair partition coefficient.
• MOTHERS 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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Variations in Rates of Absorption and Elimination
on Plasma Concentration of an Orally Administered
Chemical
Plasma concentration
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Example of one or two compartment model
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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

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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.

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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
DRUNKENNESS
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10000
Zero Order Elimination
1000
Plasma Concentration
100
10
1
0
1
2
3
4
5
6
Time
logCt logCo - Kel . t
2.303
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First Order Elimination
dC/dt k
-Kel.t
Ct Co e
lnCt lnCo Kel .t
Plasma concentration
logCt logCo - Kel . t
2.303
y b a.x
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Plasma Concentration Profile after a Single I.V.
Injection
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lnCt lnCo Kel.t
Vd Dose/C0
When t 0, C C0, i.e., the concentration at
time zero when distribution is complete and
elimination has not started yet. Use this value
and the dose to calculate Vd.
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lnCt lnCo Kel.t
t1/2 0.693/Kel
When Ct ½ C0, then Kel.t 0.693. This is the
time for the plasma concentration to reach half
the original, i.e., the half-life of elimination.

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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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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 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 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

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Single dose
Toxic level
Cumulation
plasma conc
Time
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The time to reach steady state is 4 t1/2s
Concentration due to repeated doses
Concentration due to a single dose
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Toxicokinetic parameters

• Vol 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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Variability in Toxicokinetics
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50
40
Concentration (mg/L)
Plasma Drug
30
20
10
0
0
5
10
15
Daily Dose (mg/kg)
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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.