Absorption is the first stage of pharmacokinetics where a drug enters systemic circulation after administration. It involves drug transport across biological membranes via passive diffusion, facilitated diffusion, active transport, or endocytosis. Factors affecting drug absorption include route of administration, gastric emptying time, presence of food, gastrointestinal diseases, and pharmaceutical factors like dosage form, particle size, and salt form. Bioavailability is the fraction of administered drug that reaches systemic circulation and is a measure of both the rate and extent of drug absorption. It can be quantitatively evaluated by comparing the area under the plasma concentration-time curve after oral versus intravenous administration.

1. ABSORPTION
Dr chintan Doshi
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2. PHARMACOKINETICS
• Simply means, the science which deals with
 “ What the Body does to the drug ”
• Includes 4 stages –
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PHARMACOKINETICS
ABSORPTION DISTRIBUTION METABOLISM EXCRETION

3. 3
Drug
administration
Crosses
biological membrane
Transport
Absorption
Enters
Systemic
Circulation
Reaches
body tissues
Distribution

4. ABSORPTION
30-Dec-13
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5. BIOAVAILABILITY
• DEFINITION :-
“ The Rate at which & the Extent to which the
Active Concentration of drug is available at the desired
site of action. ”
• Concept came into existence when an unusual incidence
occurred in Australia in 1968.
• Phenytoin toxicity in Epileptic patients.
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6. BIOEQUIVALENCE
 Definition :
• If two or more dosage forms of the same drug reach the
blood circulation at the same relative rate & same
relative extent, they are called Bioequivalent
preparations of the generic drug.
• Clinically important for the drugs having steep dose-
response relationship,
• e.g.
 Zero order kinetics ( phenytoin, warfarin )
 Narrow margin of safety ( Theophylline, Tetracycline )
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7. Measurement of Bioavailability
• Oral administration of 2 brand products of same drug
⇓
Plasma concentration – Time curve is obtained
⇓
3 characteristics noted & compared :
1) Cmax
2) Tmax
3) AUC
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Rate of Absorption
Extent of Absorption

8. 8

9. Quantitative Evaluation of Bioavailability
Drug Bioavailability ( % ) = AUC ( Oral ) x 100
AUC ( I.V. )
AUC (oral) = AUC obtained after oral administration of
single dose
AUC (IV) = AUC obtained after IV administration of same
drug in the same dose.
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10. Drug transport process
DRUGS ARE TRANSPORTED ACROSS
THE MENBRANE
PASSIVE DIFFUSION
AND FILTRATION
SPECIALIZED
TRANSPORT
CARRIER
MEDIATED
FACILITATED
DIFFUSION
ACTIVE
TRANSPORT
PRIMARY SECONDORY
PINOCYTOSIS
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11. Diffusion
Higher
conc.
Lower
conc.
M
E
M
B
R
A
N
E
Lipid soluble
Lipid
insoluble
DIFFUSION
FILTRATION
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12. Carrier mediated transport
• Facilitated
M
E
M
B
R
A
N
E
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13. • Active transport
AA
M
E
M
B
R
A
N
E
ATP ADP
e.g.- levodopa
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14. Active transport
PRIMARY
◦ Energy obtained directly
from ATP
◦ Eg. P glycoprotein
SECONDARY
◦ Energy obtained from
movment of other solute
(Na+)
◦ Eg- Na/K/2CL symporter
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15. • Primary Active transport
AA
M
E
M
B
R
A
N
E
ATP ADP
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16. • Secondary Active Transport
AA
M
E
M
B
R
A
N
E
Na+ K +
ATPase
ATP ADP
Na+ K +
ATPase
H+ H+
GLUCOSE
Na+
K+
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17. Pinocytosis / Cell Drinking
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• Uptake of fluid solutes

18. 18
FACTORS AFFECTING
DRUG ABSORPTION
&
BIOAVAILABILITY

19. A) Pharmacological Factors:
(1) Route of Administration :
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20. Absorption via G.I.T.
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Absorption
Site
pH
Drugs
absorbed
Example
Stomach Acidic
Lipid soluble
Non-ionized
Acidic/Neutral
Aspirin
Mouth
Lipid soluble
Non-ionized
Basic/Neutral
Isosorbide
Small
intestine Morphine
Colon Diazepam
Rectal Ergotamine

21. Absorption via Parenteral Sites :
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Intravenous injection
⇓
Direct in bloodstream
⇓
Completely absorbed,
Rapidly distributed
Intramuscular
&
Subcutaneous injection
⇓
Passive diffusion
Absorption : i.m. > s.c.

22. Absorption via Lungs :
• Rapid absorption of Lipid-soluble drugs
• From pulmonary epithelium & mucous
membrane of trachea & lungs
• Ex. - General anaesthetics ( vapourized form )
- Salbutamol ( aqueous spray )
- Disodium cromoglycate ( suspended
microfined particles)
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23. Absorption via Topical sites :
• Dermis – permeable for lipid-soluble drugs
◦ Transdermal patches for – GTN, Scopolamine.
◦ Applied on mucous membranes – Oxytocin,
Vasopressin
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24. (2) Age :
 Infants :
◦ High gastric pH
◦ Less intestinal surface & blood flow
 Elderly :
◦ Achlorhydria
◦ Less intestinal blood flow
◦ Altered gastric emptying
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25. ◦ Any factor which ↑: ↑ absorption of drug
◦ Permits drug to reach large intestinal surface
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(3) Gastric Emptying Time

26. (3) Gastric Emptying Time
• Volume of Ingested Material • Bulky material tends to empty
more slowly than liquids
• Type of Meal • carbohydrates > proteins > fats
• Body Position • Lying on the left side decreases
emptying rate,
• Right side promotes it
• Drugs
• Anticholinergics
• Narcotic analgesics
• Ethanol
• Reduction in rate of emptying
• Emotional state • Anxiety promotes,
• depression retards it
• Disease states • Hypothyroidism retards it,
• Hyperthyroidism promotes it.
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27. (4) Presence of food
FOOD INCREASES THE
ABSORPTION OF ..
Chloroquine
Griseofulvin
FOOD DECREASES THE
ABSORPTION OF ..
Ampicillin
Aspirin
Tetracycline
To be
taken
before
meals
To be
taken
after
meals
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28. Contd.
◦ Tetracycline absorption ↓ with milk
◦ Vit. C ↑absorption of iron
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29. 5)Gastrointestinial diseases
◦ Coeliac disease: Malabsorption of fat
◦ Amoxicillin show decrease absorption
◦ Gastroenteritis: oral absorption ↓
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30. 6)Drug intractions
◦ Enzyme inducer: phenobarbitone ↓ bioavalibility of other
drug
◦ Liquid paraffin: ↓ bioavaibility of VIT. A, D,E,K
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31. 7)First pass metabolism
◦ When given orally reaches direct liver for
metabolism before reaching systemic circulation
◦ Decrease bioavaibility
◦ Ex.:
◦ Levodopa
◦ Propranolol
◦ Niroglycerin
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32. 8)Pharmacogenetic factor
Slow acetalators:
◦ ↑ bioavaibility of isoniazid→ toxicity of isoniazid
◦ Seen in Americans
Fast acetalators:
o↓ bioavaibility
oSeen in Chinese, Eskimos
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33. 9)Miscelleneous factors
◦ Routes of administration
◦ Area of absorbing surface
◦ State of circulation
◦ In shock tissue perfusion ↓ and ↓ bioavalibility
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34. Pharmaceutical factors
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35. 35
Oral Administration of Drug
To fine particles
Tab or Cap
Drug in solution
Powders &
Suspensions
Solutions
Available for absorption in GIT

36. Disintegration time :
Rapid disintegration
⇓
Less disintegration time
⇓
Rapid absorption
◦ Ex.-
• Coated tablets – more disintegration time
slow absorption
• Fine dispersible tablets – less disintegration time
fast absorption
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37. 1)Particle size
◦ Microfine tablet: ↑bioavaibility
Ex:
◦ Aspirin
◦ Sprinolactone
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38. 2)Salt form
◦ Acidic drug precipitated from salt
◦ Faster dissolution and ↑bioavailability
◦ Ex.: phenytoin salts
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39. 3)Degree of ionization
◦ Non ionized lipid soluble drugs better absorbed
◦ Highly ionized drug reduce bioavailability
◦ Streptomycin
◦ Acetylcholine
◦ Neostigmine
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40. 4)Crystal form
◦ Amorphous chloramphenicol has faster dissolution and
increase bioavalibility
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41. 5)Water of hydration
◦ Anhydrous form of ampicillin has faster dissolution and
increase bioavailability
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42. Methods to Delay Absorption
1. Using Appropriate Dosage Form :
• Slow & Sustained absorption of drugs –
- Retard tablets ( pot. Chloride retard tablets )
- Depot injections ( Fluphenazine depot injectons )
- Subcutaneous implants ( testosterone pellets )
2. Changing Physical characteristics of drug :
• e.g. Procaine PnG
- Slightly water soluble
- When given i.v. ⇨ slowly absorbed & action is
prolonged
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44. 44

45. 45

46. Contd…
3. Adding a Vasoconstrictor Drug :
- Noradrenaline + Local Anaesthetic ( Xylocaine )
⇓
- Prolongs effect of local anaesthetic effect
- Reduces absorption into Systemic circulation &
systemic toxicity
4. Applying a Tourniquet :
- Tourniquet application ⇨ injection of L.A. below it
⇓
Delays systemic absorption
Prolongs action of L.A.
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47. Methods to Facilitate Absorption
• Addition of enzyme Hyaluronidase (breaks down
intercellular matrix)
⇓
Speeds absorption
• Increasing the local blood flow to the tissue
⇓
Increased absorption
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48. T h a n k
y o u…
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49. All of the following statements
regarding bioavailability
of a drug are true except:
◦ (a) It is a fraction of administered drug that reaches the
systemic circulation in unchanged form
◦ (b) Bioavailability of an orally administered drug can be
calculated by comparing the Area Under Curve after oral and
intravenous administration
◦ (c) Low oral availability always and necessarily means poor
absorption
◦ (d) Bioavailability can be determined from plasma
concentration or urinary excretion data.
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50. ◦ Drug transport mechanisms include:
◦ (a) Active transport
◦ (b) Passive transport
◦ (c) Diffusion
◦ (d) All
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51. All of the following are advantages of transdermal drug
delivery systems EXCEPT:
◦ (a) They produce high peak plasma concentration of the drug
◦ (b) They produce smooth and nonfluctuating plasma
concentration of the drug
◦ (c) They minimize interindividual variations in the achieved
plasma drug concentration
◦ (d) They avoid hepatic first pass metabolism of the drug
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52. ◦ Major mechanism of transport of drugs across
biological membranes is by:
◦ (a) Passive diffusion
◦ (b) Facilitated diffusion
◦ (c) Active transport
◦ (d) Endocytosis
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53. ◦ Pharmacodynamics includes:
◦ (a) Drug elimination
◦ (b) Drug excretion
◦ (c) Drug absorption
◦ (d) Mechanism of action
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