chapter 7
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1
CHAPTER 7
ABSORPTION KINETICS
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ABSORPTIONGIT
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ABSORPTION FROM GIT
Oral Dosage Forms
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Advantages of Oral Drugs
Convenient, portable, no pain Easy to take Cheap, no need for sterilization Compact, multi-dose bottles Automated machines producing
tablets in large quantities Variety- fast release, enteric coated,
capsules, slow release, …..
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ABSORPTION
Definition: is the net transfer of drug from the site of absorption into the circulating fluids of the body.
For Oral Absorption1- Cross the epithelium of the
GIT and entering the blood via capillaries
2- Passing through the hepato-portal system intact into the systemic circulation
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ABSORPTION
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Biological Membranes
No matter by which route a drug is administered it must pass through several to many biological membranes during the process of absorption, distribution, biotransformation and elimination.
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Cell Membrane Structure
It is a bimolecular layer of lipid material entrained between two parallel monomolecular layers of proteins.
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Cell Membrane Structure
The cell membrane appears to be perforated by water-filled pores of various sizes, varying from about 4 to 10 A
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Drug Transport Transport is the movement of drug from one place to another within the body. Most drugs pass through membranes by diffusion. The process is passive because no external energy is expended.
TRANSCELLULARPARACELLULAR
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PASSIVE DIFFUSION
The passage of drug
molecules occurring from
the side of high drug
concentration to low drug
concentration
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Fick’s law of diffusion
Q: is the net quantity of drug transferred across the membrane, t: is the time
Ch: is the conc on one side (GIT) and Cl: that on the other side (plasma)
x: is the thickness of the membrane
A: is surface area of membrane and D: is the diffusion coefficient related to permeability
k: is the partition coefficient of the drug
x
CCDAk
dt
dQ lh )(
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SMALL INTESTINE VILLI
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PERMEABILITYThe permeability of a membrane to a drug depends
on physico-chemical properties of drugs: Lipophilicity: membranes are highly permeable to lipid
soluble drugs
Molecular size: important in paracellular route and in
drugs bound to plasma protein. Macromolecules such
as proteins do not traverse cell membrane or do so
very poorly
Charge: cell membranes are more permeable to
unionized forms of drugs because of more lipid
solubility
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PERMEABILITY
a
sa C
CpkpH log
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Carrier-Mediated Transport
Active Transport The drug is transported against a concentration gradient .This
system is an ENERGY consuming system.
Example: Glucose and Amino acids transport.
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Passive Facilitated Diffusion
A drug carrier is
Required but no ENERGY
is necessary. e.g. vitamin
B12 transport. Drug
moves along conc
gradient (from high to
low), downhill but faster
CARRIER
DRUG
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DRUG TRANSPORT
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Characteristics of GIT
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Effect of Food on Drug AbsorptionPropranolol
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Effect of Diseases on Drug AbsorptionDiseases that cause changes in: Intestinal blood flow GI motility Stomach emptying time Gastric and intestinal pH Permeability of the gut wall Bile and digestive enzyme secretion Alteration of normal GI flora
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Simulation of Drug Absorption by Dissolution MethodsDissolution tests in vitro measure the rate and extent of dissolution of the drug from a dosage form in an aqueous medium
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ABSORPTION KINETICSPlasma Concentration-Time Curve
Absorption
Phase
Elimination
Phase
Time
Cp
Cmax
Tmax
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First-Order Absorption
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Absorption
Zero-Order Absorption: is seen with controlled
release dosage forms as well as with poorly soluble
drugs. The rate of input is constant.
First-Order Absorption: is seen with the majority of
extravascular administration (oral, IM, SC, rectal,
ect..) Most PK models assume first-order absorption
unless otherwise stated.
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One Compartment Model for First-Order Absorption and First-Order Elimination
Gastrointestinal, Percutaneous, Subcutaneous,
Intramuscular, Ocular, Nasal, Pulmonary, Sublingual,…
Drug in dosage form
Drug particlesIn body fluid
Drug in solution
CentralCompartment
(Plasma)
Release
Dissolution
ka kel
EliminationAbsorption
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COMPARTMENTAL MODEL
One compartment model with Extravascular
administration
Route of Administration: Oral, IM, SC, Rectal, ect…
CentralCompartment
ka kelDrug inGIT
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First-Order Absorption ModelRate of change = rate of input – rate of output
Integrated Equation:
BelGIaB DkDFk
dt
dD
Beltk
aB DkeDFk
dt
dDa
0
)()(
0 tktk
elad
ap
ael eekkV
DFkC
)( tktkp
ael eeAC
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The Residual MethodThe rising phase is not log-linear because absorption
and elimination are occurring simultaneously
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The Residual Method
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The Residual Method
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The Residual Method
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Cmax and tmax
The time needed to reach Cmax is tmax
At the Cmax the rate of drug absorbed is equal to the rate of drug eliminated
ela
ela
kk
kkt
)ln(
max
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Lag TimeThe time delay prior to the commencement of
first-order drug absorption is known as lag time
Time
Cp
Lag time
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FLIP-FLOP of ka and kel
In a few cases, the kel obtained from oral
absorption data does not agree with that
obtained after i.v. bolus injection. For
example, the kel calculated after i.v. bolus
injection of a drug was 1.72 hr -1, whereas
the kel calculated after oral administration
was 0.7 hr -1. When ka was obtained by the
method of residuals, the rather surprising
result was that the ka was 1.72 hr -1
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FLIP-FLOP of ka and kel
Drugs observed to have flip-flop characteristics are drugs with fast elimination (kel > ka)
The chance for flip-flop of ka and kel is greater for drugs that have a kel > 0.69 hr-1
The flip-flop problem also often arises when evaluating controlled-release products
The only way to be certain of the estimates is to compare the kel calculated after oral administration with the kel from intravenous data.
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FLIP-FLOP of ka and kel
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Effect of size of the dose of a drug on the peak concentration and time of peak concentrationThe time of peak conc is the same for all doses
A >B >C
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Effect of altering ka on Cmax and TmaxThe faster the absorption the higher is the Cmax and the
shorter is the Tmax
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The faster the elimination the lower is the Cmax and the
shorter is the Tmax
Effect of altering kel on Cmax and Tmax
Cp
ka= 0.5 hr-1
kel= 0.2 hr-1
ka= 0.5 hr-1
kel= 20 hr-1
Time
ka= 0.5 hr-1
kel= 0.02 hr-1
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Equations
)( tktkp
ael eeAC
)(0
elad
a
kkV
DFkA
ela
ela
kk
kkt
)/ln(max
elkt
693.02/1
Cl
DoseFAUC
.
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