The document discusses various mechanisms of drug absorption including transcellular transport processes like passive diffusion, carrier-mediated transport, and active transport. It describes passive diffusion in detail, noting it is the principal mechanism of drug absorption and follows Fick's first law of diffusion. Carrier-mediated transport includes facilitated diffusion which operates down a concentration gradient faster than passive diffusion, and active transport which requires ATP and includes primary active transport and secondary active transport like symport and antiport. The document also briefly discusses paracellular transport between epithelial cells and vesicular transport methods.

1. Dr S RAMKANTH, M. Pharm., Ph.D.,
Professor & Head, Department of Pharmaceutics
KARPAGAM COLLEGE OF PHARMACY, COIMBATROE – 32, TAMIL NADU
email id: ramkanthsg@gmail.com Cell: +919618312122
Mechanism of Drug Absorption
UNIT 1

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Mechanism of Absorption
1. Transcellular / intracellular transport
A. Passive Transport Processes
I. Passive diffusion
II. Pore transport
III. Ion- pair transport
IV. Facilitated diffusion
B. Active transport processes
I. Primary
II. Secondary
a. Symport (Co-transport)
b. Antiport (Counter transport)
2. Paracellular / Intercellular Transport
A. Permeation through tight junctions of
epithelial cells
B. Persorption
3. Vesicular or Corpuscular Transport (Endocytosis)
A. Pinocytosis
B. Phagocytosis
There three broad categories are

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Passive Diffusion
 Also known as Non ionic Diffusion
 90% of drug
 Principle of Concentration gradient / Electrochemical Gradient
Diffusion

4. Passive Diffusion (Cont.)
 Passive diffusion is best expressed by Fick’s first law of diffusion which states
that the drug molecules diffuse from a region of higher concentration to one of
lower concentration until equilibrium is attained and that the rate of diffusion is
directly proportional to the concentration gradient across the membrane.
𝒅𝑸
𝒅𝒕
=
𝑫𝑨𝑲 𝒎/ 𝒘
𝒉
(𝑪𝑮𝑰𝑻 − 𝑪)
dQ/dt = rate of drug diffusion
D = diffusion coefficient of the drug
A = surface area of the absorbing membrane
Km/w = partition coefficient of the drug between
membrane and the aqueous phase
(CGIT – C) = difference in the concentration of drug in GI
fluid & the plasma
h = thickness of the membrane (length)
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Passive Diffusion (Cont.)
Based on the equation, certain characteristics of passive diffusions are
 Concentration Gradient – Down Hill Process
 Rate of drug transfer is proportional to concentration gradient between the
GI fluid & Blood
 Greater the surface area and less thickness of the membrane – Better
absorption Equilibrium will be attained upon equal concentration on both sides
 Drug with Ionized / Unionized drug both approaches where Unionized drug
transfer better
 Greater the membrane or water partition co-efficient, faster the absorption
 Dilution of GI fluid is used in treatment of over dose or poisoning

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Passive Diffusion (Cont.)
 Passive diffusion is energy dependent and non saturable
 It depends on square root of molecular weight of the drug
 100 to 400 Da
 Diffusion decreases with increase in molecular weight
 Maintenance of Sink Condition
 DAKm/w/h may be constant in few condition hence its replace by P in the
previous equation 𝒅𝑸
𝒅𝒕
= 𝑷 𝑪𝑮𝑰𝑻

7. Pore Transport
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 It is also called as convective transport, bulk flow or filtration
 The driving force is hydrostatic pressure or the osmotic differences across the
membrane
 The process is important in the absorption of low molecular weight (less than
100), generally water-soluble drugs through narrow, aqueous-filled channels
ex: urea, water and sugars Chain-like or linear compounds of molecular weight up to 400 Daltons can be
absorbed by filtration. For example, the straight-chain alkanes.
 Drug permeation through water-filled channels is importance in renal
excretion, removal of drug from the cerebrospinal fluid and entry of drugs into
the liver

8. Ion-Pair Transport
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 Absorption of drugs like quaternary ammonium compounds (Examples are
benzalkonium chloride, benzethonium chloride) and sulphonic acids (sulfonic
acid), which ionise under all pH conditions, is ion-pair transport
 Despite their low o/w partition coefficient values, such agents penetrate the
membrane by forming reversible neutral complexes with endogenous ions of
the GIT like mucin.
 Such neutral complexes have both the required lipophilicity as well as
aqueous solubility for passive diffusion. Such a phenomenon is called as ion-
pair transport

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Ion-Pair Transport

10. Carrier-Mediated Transport
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 Some polar drugs cross the membrane more readily than can be predicted from
their concentration gradient and partition coefficient values. Like
monosaccharides, amino acids and vitamins will be poorly absorbed
 The mechanism is involved is carrier that binds reversibly or non-covalently
with the solute molecules to be transported.
 This carrier-solute complex traverses across the membrane to the other side
where it dissociates and discharges the solute molecule.
 The carrier then returns to its original site to complete the cycle by accepting
a fresh molecule of solute.
 Carriers in membranes are proteins (transport proteins) and may be an
enzyme or some other component of the membrane

11. Carrier-Mediated Transport - Important Characteristics
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Structure Specific – False Nutrients
Competitive binding
Capacity Limited
Mixed Order Kinetics
Bioavailability of drug absoption decreased with increase in
dose
Absorption window

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Carrier Mediated Transport
 Two types of Carrier Mediated Transport are – Facilitated Diffusion & Active
Transport Facilitated Diffusion
 It is a carrier-mediated transport system that operates down the
concentration gradient (downhill transport) but at a much a faster rate than
can be accounted by simple passive diffusion.
 The driving force is concentration gradient (hence a passive process).
 Since no energy expenditure is involved, the process is not inhibited by
metabolic poisons that interfere with energy production.

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Active Transport Processes
This transport mechanism requires energy in the form ATP. Active transport
mechanisms are further subdivided into -
 Primary active transport – In this process, there is direct ATP requirement.
Moreover, the process transfers only one ion or molecule and in only one
direction, and hence called as uniporter e.g. absorption of glucose.
 Secondary active transport – In these processes, there is no direct requirement
of ATP i.e. it takes advantage of previously existing concentration gradient. The
energy required in transporting an ion aids transport of another ion or molecule
(co-transport or coupled transport) either in the same direction or in the
opposite direction.

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Active Transport
 Accordingly this process is further subdivided into
 Symport (co-transport) – involves movement of both molecules
in the same direction e.g. Na+-glucose symporter
 Antiport (counter-transport) – involves movement of molecules
in the opposite direction e.g. H+ ions using the Na+ gradient in
the kidneys.

15. symportantiport
ATP ATP
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Active Transport

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https://www.youtube.com/watch?v=dUMNd6Eunbw

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Paracellular/Intercellular Transport
 It is defined as the transport of drugs through the junctions between
the GI epithelial cells. This pathway is of minor importance in drug
absorption. The two paracellular transport mechanisms involved in drug
absorption are – Permeation through tight junctions of epithelial cells – this process
basically occurs through openings which are little bigger than the
aqueous pores. Compounds such as insulin and cardiac glycosides are
taken up this mechanism Persorption – is permeation of drug through temporary openings
formed by shedding of two neighbouring epithelial cells into the lumen

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https://www.youtube.com/watch?v=eWkFJx_tz2w

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