Transcript Document

TOXICOKINETICS
Absorption
Non-compartment
Zero
First
Order of reaction
Quantitative model
Compartment
Physiological-based TK
Clearance
Half-life
Volume of distribution
Bioavailability
Active
Passive
Ingestion
Inhalation
Skin penetration
Parenteral
Parameters
Distribution
Circulation
Adipose tissue
Highly perfused ogan
Blood-brain barrier
Metabolism
Phase 1
Phase 2
Toxicokinetics
Membrane transportation
Excretion
Kidney
Lung
Feces
Saliva
Lactation
Sweating
Wongwiwat Tassaneeyakul
Department of Toxicology
Khon Kaen University
1
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?
Dosage
Exposure
Plasma Site of
action
Conc.
Toxicokinetics
Toxic
Effects
Toxicodynamics
2
Toxicokinetic (TK) processes
ABSORPTION
xenobiotic
EXTERNAL
MEMBRANE
BARRIERS
skin
G.I. tract
lungs
DISTRIBUTION
BLOOD PLASMA
TISSUES
pools
depots
sinks
METABOLISM
PHASE-1
Oxidation
PHASE-2
conjugation
EXCRETION
KIDNEYS
LIVER
lungs
saliva
sweat
breast milk
3
Disposition of Xenobiotics
Ingestion
Inhalati on
Intravenous
Intraperitoneal
Subcut aneous
Gast rointest inal
tract
absorption
Intramuscular
Lung
Dermal
Liv er
Blood and ly mph
Bile
ext racellular
f luid
f at
distribution
Kidney
Bladder
f eces
Urine
Lung
Secret ory
Structures
sof t
tissue
Alv eoli
Expired Air
body
organs
Secret ions
bone
excretion
4
Structural model of cell membrane
The ‘lipid sieve’
model explain how
lipophilic small
cpds can permeate
through the
membrane by
passive diffusion
HYDRO
PHILE
EXTERIOR
HYDRO
PHILE
HYDRO
PHILE
polar heads
HYDRO
PHILE
non-polar tails
LIPOPHILE
Phospholipid
Bilayer
hydrophilic cpds
cannot permeate
unless there is a
specific membrane
transport channel
or pump.
INTERIOR
FACILITATED DIFFUSION
OR
ACTIVE TRANSPORT
HYDRO
PHILE
PASSIVE
DIFFUSION
LIPOPHILE
5
6
Mechanism of Membrane
Permeation
1.
2.
3.
4.
Passive diffusion
Active transport
Facilitated transport
Pinocytosis
7
Transfer of Chemicals across Membranes
PASSAGE ACROSS
MEMBRANES
Passive
Facilitated
Active
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
8
Uptake by Passive diffusion
• Uncharged molecules may diffuse along
conc. gradient until equilibrium is
reached
• No substrate specific
• Small MW < 0.4 nm (e.g. CO, N20,
HCN) can move through cell pores
• Lipophilic chemicals may diffuse
through the lipid bilayer
9
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)
10
Weak Acids and Weak Bases
HA <==> H+ + A[ UI ]
[I]
pKa = pH + log(HA/A-)
pH = 2
B + H+ <==> BH+
[ UI ]
pKa = pH+ log(BH+/B)
pKa = 4.5 (a weak acid)
0.1 = [ I ]
[ I ] = 9990
100 = [ UI ]
[ UI ] = 100
100.1
[I]
pH = 7.4
= total drug = 10090
11
Flicks’s 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
12
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++
13
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)
14
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
15
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: INTRAVENOUS> INHALATION >ORAL
> DERMAL EXPOSURE.
16
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.
17
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
18
Lungs Absorption
For gases, vapors and volatile liquids,
aerosols and particles
In general: large surface area, thin barrier,
high blood flow
rapid absorption
Blood:air partition coefficient –
influence of respiratory rate and blood flow
Blood:tissue partition coefficient
19
Lungs Absorption
REMOVAL OF
PARTICLES
Physical
Lymph
Phagocytosis
Absorption of Aerosols and
Particles:
1- Particle Size
2- Water solubility of the
chemical present in the
aerosol or particle
20
Airway anatomy
bronchial tree
trachea
•
•
diffusion distance: ~20 mm
total exchange gas exchange area: ~80 m2
21
Airway anatomy
trachea
alveoli
capillaries
bronchial tree
•
•
diffusion distance blood/air: ~20 mm
total exchange gas exchange area: ~80 m2
22
Absorption Area in the Respiratory System
Nasopharynge
5-30 µm
Trachea
Bronchi
Bronchioles
1-5 µm
Alveolar Region
1 µm
23
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)
24
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
25
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
26
Systemic
circulation
Liver vein
Liver
Vena portae and tributaries
Liver artery
27
FIRST PASS
EFFECT
Intestinal vs.
gastric
absorption
Wilkinson, NEJM 2005
28
Extent of Absorption or Bioavailability
Destroyed
in gut
Not
absorbed
Destroyed
by gut wall
Destroyed
by liver
Dose
to
systemic
circulation
29
Plasma concentration
70
60
Bioavailability (F)
(AUC)o
(AUC)iv
i.v. route
50
40
oral route
30
20
Time (hours)
10
0
0
2
4
6
8
10
30
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.
31
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)
32
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)
33
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
34
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
35
Alter plasma binding of chemicals
1000 molecules
99.9
% bound
90.0
1
molecules free
100
100-fold increase in free pharmacologically
active concentration at site of action.
NON-TOXIC
TOXIC
36
volume of distribution
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).
37
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
38
39
Co
V = Dose / Co
Ln of
Blood (or
Plasma)
Conc.
Time
Vd can be calculated after an IV dose of a substance that
exhibits "one-compartment model" characteristics.
Vd = Dose / Initial Conc
40
Examples of apparent Vd’s for some drugs
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
7
490
Digoxin
41
Competition-displacement between xenobiotics
Extr ace llular
Fluid
low
bioavailability
high
bioavailability
capillary wall
Blood
Plas m a
inact ive molecules
bound to albumin
act ive molecules
free in solut ion
Albumin
Albumin
Albumin
Albumin
tolbutamide
(hypoglycemic drug)
dr ug 1 ( )
moderat e af f inity
f or plasma albumin
binding sit es
dr ug 2 ( )
great er af f init y f or
plasma albumin
binding sit es
tolbutamide
+
warfarin
(antocoagulant)
42
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
43
Free-plasma and erythrocyte-bound xenobiotics
example: lead binding to ALAD protein
plasma Pb++
Blood
Pb++
erythrocyte Pb++
Allos te r ic inhibition of ALA de hydr atas e e nzym e by Le ad
s ubs tr ate binding
Pb++
Zn++
SH SH SH SH
s ite A
S S SH SH
SH SH SH
s ite B
Pb ++
ALA de hydr atas e
SH
S S SH SH
s ite A
s ite B
ALA de hydr atas e
reversib le
inhib i ti on
native enzyme
homotropic allosteric complex
irreversib l e inhib i tion
Pb++
SH SH S
s ite A
S
SH
SH SH SH
s ite B
44
Free-plasma and erythrocyte-bound xenobiotics
example: lead binding to ALAD protein
CNS (brain)
spongy
bone
kidney
higher
neurotoxicity
spongy
bone
CNS (brain)
lower
neurotoxicity
higher
renal toxicity
avg plasma Pb++
lower plasma Pb++
avg erythrocyte Pb++
higher
erythrocyte Pb++
Allosteric inhibition of ALA dehydratase enzyme by Lead
Allosteric inhibition of ALA dehydratase enzyme by Lead
substrate binding
average
blood
Pb++
substrate binding
Pb++
Zn++
SH SH SH SH
site A
S S SH SH
SH SH SH
site B
SH
S S SH SH
site A
Pb++
ALA dehydratase
site B
ALA dehydratase
elevated
blood
Pb++
SH SH SH SH
site A
-
S S SH SH
homotropic allosteric complex
SH SH S S
site A
SH
SH SH SH
site B
site A
Pb++
ALA dehydratase
SH
S S SH SH
-
site B
ALA dehydratase
reversible
inhibition
irreversible inhibition
Pb++
Pb++
Zn++
reversible
inhibition
native enzyme
kidney
SH SH SH
site B
ALA dehydratase
ALAD-1 polymorphism
heterotropic allosteric complex
denatured
native enzyme
homotropic allosteric complex
irreversible inhibition
Pb++
SH SH S S
site A
SH
SH SH SH
site B
ALA dehydratase
ALAD-2 polymorphism
heterotropic allosteric complex
denatured
45
Normal blood capillaries
most capillaries are fenestrated
small gaps in capillary wall
not 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
46
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)
47
Capillary structure
General circulation
Central nervous system:
‘blood brain barrier’
Endothelial cell
Tight junction
Basal membrane
(porous)
Astrocyte
48
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
49
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

50
Elimination
of chemicals from the body
KIDNEY
LIVER
filtration
secretion
metabolism
excretion
(reabsorption)
LUNGS
OTHERS
exhalation
mother's milk
sweat, saliva etc.
51
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
52
Nephron Structure
(A.C. Guyton, Textbook of Medical Physiology, Philadelphia, W.B. Saunders Co.; 1991
53
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
54
The enterohepatic shunt/
circulation
Drug
Liver
Bile
Bile formation
duct
Biotransformation;
Hydrolysis by
glucuronide
beta glucuronidase
gall bladder produced
Portal circulation
Gut
55
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
56
Quantitative Aspects of
Toxicokinetics
57
Plasma Concentration
12
TOXIC RANGE
10
8
THERAPEUTIC RANGE
6
4
2
SUB-THERAPEUTIC
0
0
1
2
3
4
5
6
7
8
9
Dose
58
Plasma concentration
Variations in Rates of Absorption and Elimination on
Plasma Concentration of an Orally Administered
Chemical
14
12
10
8
6
4
2
0
0
5
10
15
20
TIME (hours)
59
Example of one or two
compartment model
60
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
61
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.
62
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.
DRUNKENNES
RARELY DONE
S
63
Plasma Concentration
10000
Zero Order Elimination
1000
100
10
1
0
1
logCt = logCo - Kel . t
2.303
2
3
4
5
6
Time
64
Plasma concentration
First Order Elimination
14
dC/dt = k
12
Ct = Co e-Kel.t
lnCt = lnCo – Kel .t
10
8
logCt = logCo - Kel . t
2.303
6
4
y
2
=
b – a.x
0
0
5
10
15
20
TIME (hours)
65
66
Plasma Concentration Profile after a
Single I.V. Injection
Plasma Concentration
Distribution and Elimination
10000
C0
Elimination only
1000
100
10
Distribution equilibrium
1
0
1
2
3
4
5
6
Time
67
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.
68
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.69
Principle
Elimination of chemicals from the
body usually follows first order
kinetics with a characteristic halflife (t1/2) and fractional rate
constant (Kel).
70
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.
71
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
72
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
73
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
74
Single dose
7
6
Toxic level
plasma conc
5
4
Cumulation
3
2
1
0
0
5
10
15
20
25
30
Time
75
76
The time to reach steady
state is ~4 t1/2’s
Concentration due to
repeated doses
Concentration due to a single dose
77
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
78
Variability in Toxicokinetics
Plasma Drug
Concentration (mg/L)
60
50
40
30
20
10
0
0
5
10
15
Daily Dose (mg/kg)
79
CONCLUSIO
N
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.
80