ABSORPTION OF DRUGS
Download
Report
Transcript ABSORPTION OF DRUGS
DR.SOBAN SADIQ
Pharmacokinetics
Absorption
Distribution
Metabolism(Biotransformation)
Excretion
Routes of drug administration
Enteral(Through Alimentary tract)
Oral
Sublingual or buccal
Rectal
Parenteral(Through Injection)
Intravenous
Intramuscular
Subcutaneous
Intradermal
Intraperitoneal
Intrapleural
Intracardiac
Intra-arterial
Intrathecal
Intra-articular
Absorption is the transfer of a drug from its site of
administration to the blood stream.
Factors affecting drug absorption:
1-Lipid-water partition co-efficient: Non electrolyte
drug depends upon lipid solubility.
More lipid soluble and less water soluble that is it has
high lipid-water partition co-efficient, it will absorbed
rapidly
2- Drug solubility: drugs given in aqueous solutions
are more rapidly soluble than when given in oily
solution, suspension or solid form.
3- Dosage form: Tablets and capsules, rate of
disintegration and dissolution is limiting factor in their
absorption. After dissolution, smaller the particle size,
more efficient will be absorption
4- Circulation at the site: Increased blood flow
increase absorption
How blood flow increase?
How blood flow decrease?
5- Area of absorbing surface : Absorbed more from
large surface areas for example intestinal mucosa
6- Effect of pH: Most drugs are either weak acids or
weak bases. Weak electrolytes, in addition to lipid
solubility, depends upon its degree of ionization which
is influenced by pH of the area.
Weak acids become less ionized(charged) in an acidic
medium and weak bases become less ionized in an
alkaline medium
Unionized drug is lipid soluble and diffusible
Acidic drug will absorb more in stomach or intestine?
So,
Lipid soluble(hydrophobic), uncharged, unionized
will cross the membrane rapidly than lipid
insoluble(hydrophilic or water soluble),charged
and ionized.
Basic drug will absorb more from intestine because it
becomes unionized in basic medium. In acidic
medium basic drug will become more ionized and
thus no absorption will takes place.
Functional integrity of the GIT: Increased peristaltic
activity as in diarrhea reduces drug absorption
Increased gastric emptying time, absorption will be
more.
Bioavailability
The fraction of unchanged drug reaching the systemic
circulation following administration by any route”
or
The percentage of administered drug that reaches the
systemic circulation in a chemically unchanged form”
Thus by definition a drug that is administered by
intravenous route has 100% bioavailability
Main organ of metabolism is liver
Specific drugs are metabolized in gut wall, skin,
lungs
Factors affecting bioavailability
1-First-pass hepatic metabolism:
when a drug is absorbed across GIT, it enters the
portal circulation before entering the systemic
circulation.
If the drug is rapidly metabolized by the liver ,the
amount of unchanged drug that gains access to the
systemic circulation is decreased
2-Absorption
Solubility of the drug: hydrophobic drug will absorb
more so bioavailability will be more
Chemical instability: some drugs are unstable in pH
of the gastric contents. Others are destroyed in GIT by
degradative enzymes e.g. insulin so bioavailability?
Particle size: smaller the particle size more
absorption will be there. so bioavailability?
Plasma half life
The time required for the concentration of drug in the
plasma to decrease to one half of its initial value.
for example if the initial conc. of drug is 100mg and if
the half life is 1 hr, only 50mg will remain in the
plasma at the end of 1 hr.
Time :
Cp (mg/dl):
0
100
1hr 2hr
50 25
3hr 4hr
12.5 6.25
So from this table we can deduce that the half-life of this
drug is 1 hour.
Importance
It denotes how quicky a drug is removed from the
plasma by biotransformation or excretion
Since drug require a minimum conc. in the plasma to
produce pharmacological action, a drug which is
eliminated quickly requires more frequent dosing than
a drug with a long half life.
It thus indicates the duration of action of drug and
therfore it determines the frequency of administration
of dose of the drug for therapeutic effectiveness.
Complete drug elimination occur in 4-5 half lives.
After that drug will reach steady state concentration in
the plasma.
(drug administered=drug eliminated)
1-50 %
2-75%(50+25)
3-87.5%(50+25+12.5)
4-93.75%(50+25+12.5+6.25)
1-Alkalinization of urine hastens the excretion of :
A-Weakly basic drugs
B-Weakly acidic drugs
C-Strong electrolytes
D-Nonpolar drugs
2-Diffusion of drugs across cell membrane :
A-Is dependent upon metabolic activity of the cell
B-Is competitively inhibited by chemically related
drugs
C-Is affected by extent of ionization of drug
molecules
D-Exhibits saturation kinetics
3-The most important factor governing absorption of a
drug from intact skin is :
A-Molecular weight of the drug
B-Site of application
C-Lipid solubility of the drug
D- Nature of the base used in formulation
4-What kind of substances can’t permeate membranes
by passive diffusion?
A- Lipid-soluble
B- Non-ionized substances
C- Hydrophobic substances
D- Hydrophilic substances
Which route of administration of drug will provide
highest bioavailability:
A-Subcutaneous
B-Oral
C-Sublingual
D-Rectal
Which route of drug administration is most likely lead
to the first-pass effect?
a) Sublingual
b) Oral
c) Intravenous
d) Intramuscular
Which of the following factor does not influence the
oral bioavailability of the drugs
A-metabolism by gut wall enzymes
B-Decomposition by hydrolytic gut enzymes
C-Chelation with existing food in stomach
D-Plasma half life