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Toxicokinetics
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Toxicokinetics is the study of the drug movement
around the body (Absorption, Distribution,
metabolism, and Elimination)
Toxicokinetic data is best derived using radio labeled
dose of the drug. This allows for following the fate of
the drug, metabolic products, distribution in the
tissue, storage sites, as well as its elimination.
Unfortunately, these methods do not provide
knowledge about proportion of the drug left intact to
its metabolites.
TK is concerned with what the body does to the
toxicant.
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
Toxicodynamics
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Toxicodynamics is the study of toxic actions of
xenobiotic substances on living systems.
Toxicodynamics is concerned with processes
and changes that occur to the drug at the target
tissue, including metabolism and binding that
results in an adverse effect.
Simply, TD is concerned with what the
toxicant do to the body
Dosage
Exposure
Plasma Site of
action
Conc.
Toxicokinetics
Toxic
Effects
Toxicodynamics
Toxicokinetic (TK) processes
ABSORPTION
xenobiotic
EXTERNAL
MEMBRANE
BARRIERS
skin
G.I. tract
lungs
DISTRIBUTION
BLOOD PLASMA
TISSUES
depots
METABOLISM
PHASE-1
PHASE-2
EXCRETION
KIDNEYS
LIVER
lungs
saliva
sweat
breast milk
Disposition of Xenobiotics
Ingestion
Inhalation
Intravenous
Intraperitoneal
Subcutaneous
Gastrointestinal
tract
absorption
Intramuscular
Lung
Dermal
Liver
Blood and lymph
Bile
extracellular
fluid
fat
distribution
Kidney
Bladder
feces
Urine
Lung
Secretory
Structures
soft
tissue
Alveoli
Expired Air
body
organs
Secretions
bone
excretion
Toxicokinetics (ADME)
Toxicokinetics study four processes:
1.
Absorption
2.
Distribution
3.
Metabolism
4.
Excretion
Metabolism and excretion processes are
combined as a single process called
elimination
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The toxicokinetics of a chemical are determined
by measuring the concentrations of the
chemical in plasma (usually) or blood at
various times following a single dose. The
fundamental parameters that define the rates
and extents of distribution and elimination are
derived from data following an intravenous or
oral dose.
Important principles of
toxicokinetics
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1.
2.
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The effect which a drug produces is
dependent on:
The dose
The concentration in the target organ
The kinetics of a drug may differ from
therapeutic dose to its toxic dose
Toxicokinetics is important in predicting the
plasma concentration of a drug
Toxicokinetics and toxicity
Toxicity depends on:
 Duration and concentration of drug at the portal of entry
 The rate and amount (extent) of drug absorbed; toxicity
will be low at slow absorption rates. This means that a highly
toxic drug that is poorly absorbed may have same hazard as
another with low toxicity but is highly absorbed.
 The distribution of drug within the body; where most drugs
are distributed in highly perfused organs like brain, liver and
kidneys. However, in some cases, the organ in which the drug
is concentrated may not necessarily suffer the damage. An
example is organochlorine compounds concentrated in adipose
tissue while the target organ is the brain.
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The efficiency of biotransformation and nature of
metabolites; where, in some cases, a drug may be
transformed to a more toxic metabolite or a more
lipid soluble or water soluble metabolite, which
affects absorption and distribution
The ability of the drug to pass through cell
membranes and interact with cell constituents.
Example, some organochlorines affect the DNA
The amount and storage duration of the drug or
its metabolites in the tissue. These may induce
toxicity after a long time after exposure. Lead in
bones is an example
The rate and site of excretion; where the more rapid
the excretion, the less toxicity it will produce
1. Absorption
The term absorption describes the process of the transfer of the
parent chemical from the site of administration into the general
circulation, and applies whenever the chemical is administered
via an extravascular route (i.e. not by direct intravascular
injection).
Many chemicals will be metabolized or transformed during
their passage from the site of administration into the
general circulation, so that little parent chemical may
reach the general circulation, this raises the possibility of
confusion in discussing the ‘extent of absorption’ depending on
whether the data refer to the parent chemical, or to metabolites
or both (when radiolabeling is used). This confusion is
resolved by the proper use of the term bioavailability (the
fraction of the dose administered that reaches the general
circulation as the parent compound) to describe the extent of
absorption.
Mechanism of Membrane
Permeation
1.
2.
3.
4.
Passive diffusion
Active transport
Facilitated transport
Pinocytosis and phagocytosis
Drugs are absorbed by the following
processes:
Passive transport
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This can occur by simple diffusion due to concentration
gradient or
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By passage of drugs through the pores (of the kidney and
capillaries), i.e. by filtration
Passive transport is affected by:

Ability of the drug to dissolve in the lipid portion of the cell
membrane
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The size of the drug, in case it is water soluble. Aqueous
pores are about 4Ao which will allow drugs of 100-200 amu
to pass
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Presence of the drug in its nonionized form
1.
Uptake by Passive diffusion
• Uncharged molecules may diffuse along
conc. gradient until equilibrium is
reached
• No substrate specificity
• Small MW < 0.4 nm (e.g. CO, N20,
HCN) can move through cell pores
• Lipophilic chemicals may diffuse
through the lipid bilayer
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)
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
2. Special transport
Two types of special transport mechanisms can
be identified:
1.
Active diffusion:
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Independent of or against conc. gradient
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Require energy
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Substrate –specific
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Rate limited by no. of carriers
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Example: Ca-pump (Ca2+ -ATPase)
2. Facilitated diffusion: Occurs when a drug has
a specific carrier protein, and does not occur
against concentration gradient
 Carried by trans-membrane carrier along
concentration gradient
 Energy independent
 May enhance transport up to 50,000 folds
 Example: Calmodulin for facilitated transport
of Ca2+
3. Additional transport: occurs by endocytosis;
where :
 Phagocytes (cell eating) engulf the solid large
particles suspended in the intracellular fluid
 Pinocytes (cell drinking) in which very small
suspended particles or liquids are engulfed
Factors affecting gastrointestinal
absorption
1. Types of cells at the specific site:
An example is the sublingual cells which are highly
vascularized which allows for rapid absorption
2. Period of time that drugs remain at the site:
Drugs are poorly absorbed within the mouth because the
time a drug spends in the mouth is very short, while
high absorption can occur in the intestine due to the
long time a drug spends there
3. pH
This factor affects the ionizability of the drug.
The acidic nature of the fluid in the stomach
facilitates the absorption of weakly acidic
drugs, while both weakly acidic and basic
drugs are well absorbed in the small intestine
since the pH there is almost neutral
4. The concentration at the absorption site
5. Presence of food or binding substances:
These will decrease the concentration of the free
drug and thus will lower its absorption
6. Rate of gastric emptying:
As emptying rate is decreased, absorption in the
stomach will increase
7. Gastrointestinal motility:
This will decrease the amount absorbed in the stomach
while increase the amount absorbed in the intestine
8. Absorbing surface area of the intestine
9. Blood flow to the site
10. Intestinal bacteria and gastrointestinal enzyme level
11. General condition of the patient:
Comatose decrease motility thus affecting absorption
12. Drug formulation: whether it is a slow release or
other form
Factors affecting pulmonary
absorption
Solubility of the drug in the blood
2.
Particle size
Large particles are deposited in the nasal tract > 5
microns; 2-5 micron particles are deposited mainly
in the tracheabronchial region; while particles less
than 1 micron penetrate into the alveolar sacs and
absorbed into the blood
3. Water solubility
High water solubility volatile drugs are absorbed in the
nasal tract; while low water solubility drugs will
reach the bronchioles to alveoli
1.
Airway anatomy
bronchial tree
trachea
•
•
diffusion distance: ~20 mm
total gas exchange area: ~80 m2
Airway anatomy
trachea
alveoli
capillaries
bronchial tree
•
•
diffusion distance blood/air: ~20 mm
total exchange gas exchange area: ~80 m2
Factors affecting dermal absorption
1. Condition of the skin: Stratum corneum serves as the main
barrier. When abraded, increased absorption will result
2. Skin permeability coefficient
This represents the rate at which a particular drug penetrates the
skin
3. Body region
Not all regions of the body have the same skin thickness.
Forehead versus palm
4. Lipid solubility
The more lipid soluble the drug is the more it will be absorbed
5. Skin hydration
Rate of Absorption
The rate of absorption may be of toxicological importance
because it is a major determinant of the peak plasma
concentration and, therefore, the likelihood of acute toxic
effects. Transfer of chemicals from the gut lumen, lungs, or
skin into the general circulation involves movement across cell
membranes, and simple passive diffusion of the unionized
molecule down a concentration gradient is the most
important mechanism. Lipid-soluble molecules tend to cross
cell membranes easily and are absorbed more rapidly than
water-soluble ones. The gut wall and lungs provide a large and
permeable surface area and allow rapid absorption; in contrast
the skin is relatively impermeable and even highly lipidsoluble chemicals can enter only slowly
The lipid solubility and rate of absorption depend on the extent of
ionization of the chemical. Compounds are most absorbed
from regions of the gastrointestinal tract at which they are least
ionized. Weak bases are not absorbed from the stomach, but
are absorbed from the duodenum which has a higher pH,
whereas weak acids are absorbed from the stomach. The rate
of absorption can be affected by the vehicle in which the
compound is given, because rapid absorption requires the
establishment of a molecular solution of the chemical in the
gut lumen. Extremely lipid soluble compounds, such as
dioxins, may be only partially absorbed, because they do not
form a molecular solution in the aqueous phase of the
intestinal contents.
Extent of Absorption
The extent of absorption is important in determining the total
body exposure or internal dose, and therefore is an important
variable during chronic toxicity studies and/or chronic human
exposure. The extent of absorption depends on the extent to
which the chemical is transferred from the site of
administration into the local tissue, and the extent to which
it is metabolized or broken down by local tissues prior to
reaching the general circulation. An additional variable
affecting the extent of absorption is the rate of removal from
the site of administration by other processes compared with
the rate of absorption
Chemicals given via the gastrointestinal tract may be
subject to a wide range of pH values and
metabolizing enzymes in the gut lumen, gut wall, and
liver before they reach the general circulation. The
initial loss of chemical prior to it ever entering the
blood is termed first-pass metabolism or presystemic metabolism; it may in some cases remove
up to 100% of the administered dose so that none of
the parent chemical reaches the general circulation.
The intestinal lumen contains a range of hydrolytic
enzymes involved in the digestion of nutrients. The
gut wall can perform similar hydrolytic reactions and
contains enzymes that can oxidize many drugs
FIRST PASS
EFFECT
Intestinal vs.
gastric
absorption
Absorption and Bioavailability
Irrespective of the reason that is responsible for the incomplete
absorption of the chemical as the parent compound, it is
essential that there is a parameter which defines the extent of
transfer of the intact chemical from the site of administration
into the general circulation. This parameter is the
bioavailability, which is simply the fraction of the dose
administered that reaches the general circulation as the parent
compound. (The term bioavailability is perhaps the most
misused of all kinetic parameters and is sometimes used
incorrectly in a general sense as the amount of drug available
specifically to the site of toxicity).
Extent of Absorption or Bioavailability
Destroyed
in gut
Not
absorbed
Destroyed
by gut wall
Destroyed
by liver
Dose
to
systemic
circulation
Bioavailability
Definition: the fraction of the administered
dose reaching the systemic circulation and is
thus a measure of first pass elimination
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
Liver vein
Systemic
circulation
Liver
Liver artery
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
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.
Calculation of Bioavailability
The fraction absorbed as the intact compound or bioavailability (F) is
determined by comparison with intravenous (i.v.) dosing (where F = 1 by
definition). The bioavailability can be determined from the area under the
plasma concentration–time curve (AUC) of the parent compound , or the
percentage dose excreted in urine as the parent compound, i.e. for an oral
dose:
2. Distribution
Distribution is the reversible transfer of the
chemical between the general circulation and
the tissues. Irreversible processes such as
excretion, metabolism, or covalent binding are
part of elimination and do not contribute to
distribution parameters. The important
distribution parameters relate to the rate and
extent of distribution.
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
Rate of Distribution
The rate at which a chemical may enter or leave a tissue may be
limited by two factors:
(i) the ability of the compound to cross cell membranes and
(ii) the blood flow to the tissues in which the chemical
accumulates.
The rate of distribution of highly water-soluble compounds may
be slow due to their slow transfer from plasma into body
tissues such as liver and muscle; water-soluble compounds do
not accumulate in adipose tissue. In contrast, very lipid-soluble
chemicals may rapidly cross cell membranes but the rate of
distribution may be slow because they accumulate in adipose
tissue, and their overall distribution rate may be limited by
blood flow to adipose tissue
The rate of distribution is indicated by the distribution
rate constant, which is determined from the decrease
in plasma concentrations in early time points after an
intravenous dose. The rate constants refer to a mean
rate of removal from the circulation and may not
correlate with uptake into a specific tissue. Once an
equilibrium has been reached between the general
circulation and a tissue, any process which lowers the
blood (plasma) concentration will cause a parallel
decrease in the tissue concentration.
Factors affecting distribution
Blood flow
Drugs are readily distributed to highly perfused tissue
like brain, liver, and kidneys
2. Permeability limitations
Many drugs do not readily enter the brain due to the
blood brain barrier
3. Protein binding
Acidic drugs are bound to the most abundant plasma
protein (albumin); while basic drugs bind to a-1acid glycoprotein.
1.
4. Effect of pH
The pH of the blood or tissue affect the
ionization of the drug and thus its distribution
5. Age
In old people, Protein binding and body water
will decrease, thus increasing the concentration
of the drug per unit time
6. Existence of storage sites:
These include: Adipose tissue, plasma proteins,
liver, kidneys, and bone
Extent of Distribution
The extent of tissue distribution of a chemical depends
on the relative affinity of the blood or plasma
compared with the tissues. Highly water-soluble
compounds that are unable to cross cell membranes
readily are largely restricted to extracellular fluid
(about 13 L per 70 kg body weight). Water-soluble
compounds capable of crossing cell membranes (e.g.
caffeine, ethanol) are largely present in total body
water (about 41 L per 70 kg body weight).
Lipid-soluble compounds frequently show
extensive uptake into tissues and may be
present in the lipids of cell membranes and
adipose tissue.
. A factor which may further complicate the
plasma/tissue partitioning is that some
chemicals bind reversibly to circulating
proteins such as albumin (for acid molecules)
and acid glycoprotein (for basic molecules).
The extent and pattern of tissue distribution can
be investigated by direct measurement of
tissue concentrations in animals. Tissue
concentrations cannot be measured in human
studies and, therefore, the extent of distribution
in humans has to be determined based solely
on the concentrations remaining in plasma or
blood after distribution is complete.
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).
Volume of Distribution (Vd)
Volume into which a drug appears to
distribute with a concentration equal to its
plasma concentration after distribution is
complete
Vd = Amount of drug in body
Concentration in Plasma
when a chemical shows a more extensive reversible
uptake into one or more tissues the plasma
concentration will be lowered and the value Vd will
increase. For highly lipid-soluble chemicals, such as
organochlorine pesticides, which accumulate in
adipose tissue, the plasma concentration may be so
low that the value of Vd may be many liters for each
kilogram of body weight. This is not a real volume of
plasma and therefore Vd is called the apparent volume
of distribution.
It is an important parameter because extensive
reversible distribution into tissues, which will
give a high value of Vd , is associated with a
low elimination rate and a long half-life . It
must be emphasized that the apparent volume
of distribution simply reflects the extent to
which the chemical has moved out of the site
of measurement (the general circulation) into
tissues, and it does not reflect uptake into any
specific tissue(s).