Absorption of drug in the body

1. ABSORPTION OF DRUGS
Pharmacokinetics:
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2. • Absorption is defined as the process of movement of
unchanged drug from its site of administration to the
blood stream.
Both the rate and the extent of drug absorption are
important because they determine the duration and
intensity of drug action.
A drug that is completely but slowly absorbed may fail
to show therapeutic response as the minimum effective
concentration is never reached.
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3. On the other hand, a rapidly and completely
absorbed drug attains the therapeutic level easily to
elicit its pharmacological effect.
Regardless of the route of administration, drugs
must be in solution form to be absorbed.
Thus, solid forms (e.g. powders, capsules and tablets)
must be able to disintegrate and dissolve in body
fluids.
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4. Factors Effecting Absorption of Drugs
a. A. Physico-chemical properties of drug:
b. Important physico- chemical properties which
influence drug absorption are lipid solubility (oil:
water partition coefficient), pKa and molecular size
of the drug.
c. Lipid soluble and unionised drugs are rapidly and
efficiently absorbed into the blood circulation.
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5. a. B. Nature and type of dosage form:
b. Drugs given in aqueous solutions are more rapidly
absorbed than those given in oily, suspension or
solid form because these mix more readily with the
aqueous phase at the absorptive site.
c. As a general rule, the absorption of a drug from
various dosage forms occurs in the following order:
solutions>emulsions > suspensions > capsules >
tablets> sustained release products.
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6. a. C. Concentration and volume:
b. Drugs given in high concentration/dose or volume
are absorbed more rapidly than those given in
dilute concentration and small volume.
c.
This is because high concentration and large volume
develop high concentration gradient, an essential
requirement for passive diffusion.
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7. a. D. Blood flow to site of administration:
b. Better perfusion rate of the absorptive tissue
enhances absorption of the drug.
c. This is because blood circulation removes the
drug from the site of absorption and maintains
concentration gradient across the membrane.
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8. e. E. Area of absorbing surface:
f. Drugs are absorbed more rapidly from the site
having a large surface area than that possessing
small area.
g. For example, drugs are more rapidly
absorbed from the intestine than from the stomach.
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9. e.F. Route of administration:
f.Absorption of drugs from 1M sites is relatively rapid,
but much slower in comparison to IP injections.
Absorption of. drugs from SC sites is slower than
those from 1M sites, but is faster than the oral route.
g. Absorption of lipid soluble drugs from pulmonary
mucosa is very rapid because of its rich vascular and
vast surface area.
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10. e. G. Disease states:
f. Several disease states can influence the rate and
extent of drug absorption.
g. For example, acid- base imbalance may alter body
fluid pH, infection may alter membrane
permeability, and cardiovascular disease may
affect blood flow to absorbing sites.
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11. e. Gastrointestinal Tract
• The oral mucosa has a thin epithelium and rich
vascularity, which favors absorption; however,
contact time in mouth is usually very brief for
substantial absorption to take
place.
• According to the pH- partition hypothesis, acidic
drugs are best absorbed from stomach (acidic pH),
and basic drugs from intestine (alkaline pH) in
which conditions they are unionised to a large
extent.
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12. • However even for acidic drugs, absorption from
stomach is less because the gastric mucosa is thick
and covered with mucus, and the surface area is
small.
• Therefore, practically most acidic and basic drugs
are absorbed mainly from the intestine (intestine
has large surface area, high perfusion rate, long
residence time of drug, and pH
range of 5 to 7.5)
• Presence of food in the GI tract often dilutes the
drug and retards its absorption.
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13. • Rapid intestinal motility decreases the
drug absorption.
• Absorption of drugs can also be greatly affected by
other concurrently ingested drugs.
• Several other factors may affect the absorption of
drugs from the GI tract.
• These include molecular size and shape of
drug, type of dosage form, morphological and
functional differences of Gl tract among various
animal species, and contents, intestinal blood and
lymph flow, and pathological condition of gastric and
intestinal epithelium.
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14. • Gastrointestinal fluids usually promote
disintegration (breakdown of solid dosage forms
into small particle) and dissolution (solid to liquid
phase) of dosage forms, but may inactivate some
drugs (e.g. penicillin-G by gastric acid and
insulin by peptidases).
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15. Parenteral Sites
• Drug absorption from intramuscular and
subcutaneous sites is rapid as drugs are deposited
directly in vicinity of the capillaries.
• Lipid soluble drugs pass readily across the whole
surface of the capillary endothelium.
• Many drugs which are not absorbed orally are
able to reach systemic circulation when given
parenterally.
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16. • Absorption of drugs from 1M sites is relatively rapid
but depends on the vascularity of the injection site.
• Aqueous solutions of drugs are usually absorbed from
the IM injection site within 10-30 minutes.
• Incorporation of hyaluronidase facilitates drug
absorption from SC injection by promoting spread, while
addition of vasoconstrictors ( e.g. epinephrine) retards
drug absorption.
• Increasing blood supply to the IM or SC injection site
by application of heat, massage or exercise hastens rate of
absorption, while immobilisation of site, local cooling, or
application of a tourniquet retards drug absorption.
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17. Pulmonary Sites (Alveoli)
• Some drugs intended for systemic effects are
administered by inhalation either as gases (e.g.
volatile and gaseous anaesthetics) or aerosols.
• The volatile and gaseous anaesthetics are absorbed
practically instantaneously.
• The large surface area of alveoli, high
permeability of alveolar epithelium and rich
perfusion rate also facilitate their rapid
absorption.
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18. • In case of aerosols, the drug delivery to lungs is
largely dependent on the particle size of the
aerosolised droplets.
• Small particles reach deep into the respiratory
tract and may cause rapid absorption from alveoli
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19. Topical Sites
• Topical application of drugs to skin or mucous
membranes is mainly meant for local effects.
• However, drugs may be absorbed through the skin
or mucous membranes, or in some cases are
intended to be absorbed to produce systemic effects.
• Systemic absorption after topical application
depends primarily on lipid solubility of the drugs;
lipid insoluble drugs generally penetrate the skin or
mucous membranes poorly.
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20. • Absorption of drugs through skin is usually poor
because the keratinised epidermis behaves like a
barrier.
• However in some cases, it may be enhanced
by rubbing, use of occlusive dressing.
• Certain solvents (e.g. dimethyl sulphoxide; DMSO)
may facilitate the penetration of drugs through skin.
• Inflammation, cuts, rashes, mild bums, etc, promote
drug absorption through skin.
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21. • Absorption of most drugs through mucous
membranes is usually more rapid than the skin
because mucous membranes are thin and highly
vascular.
Some drugs are applied to the mucous membranes
to facilitate absorption and produce systemic effects.
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