Mechanism of transport of drugs across the cell membrane

1. PHARMACOLOGY
PHARMACOKINETICS
PHARMACODYNAMICS
Dr. Anoop Kumar
Assistant Professor
Pharmacology and Toxicology
NIPER-R, Lucknow

2. PHARMACOLOGY – 2 main parts
1. As soon as a drug is administered, the body starts
working on it i.e What body does to drugs –
 called PHARMACOKINETICS ----- ❶
(Pharmacon = Drugs; Kinetics = Movement)
i.e. whatever happens TO the DRUG while
moving into, through, and out of the body
CONVERSELY … … …
2. Drugs also start working (acting) on the body …
i.e. what Drug does TO the BODY……..
…….. and cause EFFECTS on systems –
- Desirable or
- Undesirable
 called PHARMACODYNAMICS ----- ❷

3. MOVEMENT OF DRUGS IN BODY – Ph-Kinetics
 In each of these 5 processes –
1.Absorption
2.Transport & Distribution
3.Storage
4.Metabolism/Biotransformation
5.Excretion ………
drug molecules have to move ACROSS at least
2 CELL MEMBRANES, e.g. 
(1) of the tissue where drug is administered (e.g. Gut) &
(2) of blood vessel (as during the Absorption);
OR
(1) of the blood vessel &
(2) of the tissues in which Biotransformation, Excretion,
or Storage will occur.

4. GUT
BV
LIVER
ABSORPTION
EXCRETION
ACTION
Drugs must cross cell membranes of the
tissues during Ph-kinetics & Ph-dynamics
DISTRIBUTION

5. A.
HOW DO THE DRUGS CROSS THE
CELL MEMBRANES?
or
WHAT PROCESSES TRANSPORT /
Translocate the DRUGS
ACROSS CELL MEMBRANES?
or
How do the Chemical (Drug) Molecules
PERMEATE INTO THE CELL?

6. Movement of Drugs in Body: Drugs move by –
☛ Mass or Bulk Flow (as by blood), OR
☛ DIFFUSION, molecule by molecule, over a short distance
(as across cell membrane)
• Molecules flow
across the cell
membrane
• From high conc. to
lower conc. (Down
the gradient)
• Movement stops
only when mols on
two sides reach
equilibrium
DIFFUSION

7. DIFFUSION
No NET Molecular flow as the equilibrium has been reached.
Equal number of molecules exist on both sides of membrane.

8.  Drugs movement by Mass or Bulk Flow (as by blood)
may not vary much from drug to drug
 But Drug Movement by DIFFUSION process varies
with the
 Nature of Chemical Molecule, &
 Nature of Cell membrane
 Cell Membranes are Lipid-rich & they act as barriers
between various ‘aqueous’ compartments of body
 Drugs have to move thru both aqueous & lipid phases
 ‘Aqueous diffusion’ (as in GIT lumen) takes the drug to
lipid membrane (GIT epithelium) to be crossed
 ‘Lipid Diffusion’ is used for Crossing Cell Membrane
 On the other side of membrane, drug again diffuses
in ‘Aqueous’ Medium, e.g. to ECF or the blood
MOVEMENT OF DRUGS IN BODY

9. Factors determining drug transfer across cells
o Molecular Size of drug
o Area of Membrane which has to be crossed
o Vascularity of tissue whose cells are to be crosses
Size of drug molecules can vary widely
 Most are about 250-450 Da
 Some light ones are N2O (44) and Ethanol (46)
 d-Tubocurarine is quite heavy - ~700
 Peptides (e.g. ADH) are still larger ~1000-2000
 Insulin – a small Protein ~6000
 Most drugs are much smaller than Albumin (~65000);
other proteins can be still bigger
Molecular Size is very important factor because -
 Different Processes are needed to cross the
cell membrane by different types of molecules
MOVEMENT OF DRUGS IN BODY

10. TRANSPORT OF DRUGSACROSS CELL MEMBRANES
Cell Membrane / Plasma Membrane –
o Consists of a LIPID BI-LAYER
o Whose Hydrophilic Heads are oriented outwards, &
o Hydrocarbon Tails are oriented inwards to form a continuous
Lipophilic (Hydrophobic) phase
o Individual lipid molsecules can move laterally to provide
flexibility, fluidity, and high electrical resistance to membrane
which is relatively ‘impermeable to highly polar molecules’
o Protein mols embedded in the bilayer serve as -
Hydrophilic
Lipophilic
LIPIDBILAYER
☛ CHANNELS for ions
or for water-soluble
molecules, or
☛ TRANSPORTERS of
molecules, or
☛ RECEPTORS for
pharmacological
actions

11. MULTIPLE MEMBRANE
SPANNING PROTEINS
MEMBRANE
CONSTITUENTS
CELL MEMBRANE

12. MEMBRANE
LIPIDS
PHOSPHOLIPIDS(Phophatidylcholine)

13. TRANSPORT OF DRUGSACROSS CELL MEMBRANES
1 2 3
Molecules can be Transported by following PROCESSES –
1. Passive DIFFUSION of Lipid soluble mols (Lipid Diffusion)
2. Passive DIFFUSION of water-soluble molecules
3. CARRIER MEDIATED TRANSPORT by Carrier proteins
a) ACTIVE TRANSPORT, &
b) FACILITATED DIFFUSION
4. PINOCYTOSIS (Endocytosis / Exocytosis)
5. Passive Filtration (as in Renal Glomeruli)
6. Passage via Gap Junctions

14. TRANSPORTACROSSCELLMEMBRANES–SIMPLE PASSIVE DIFFUSION
1. Simple Passive DIFFUSION (Lipid): FEATURES-
Outside
Inside
A. Lipid-solubility
Non-polar unionized molecules (electrons
uniformly distributed) dissolve freely in lipids of
membranes
Membranes are liquid at body temperature
so molecules readily diffuse across cell
Number of the molecules crossing membrane
(per unit area per unit time) depends on -
1. Permeability Coefficient, and
2. Conc. Gradient across membrane
1. Permeability Coefficient depends on
a) Solubility in lipid membrane (Partition
Coefficient between membrane phase
& aqueous environment), &
b) the Diffusivity (a measure of mobility
of molecules within the lipid, expressed
as a Diffusion Coefficient)

15. 1. Simple Passive DIFFUSION (Lipid): Features-
Partition Coefficient:
• Drug will first accumulate in the membrane until the ratio of its
concentration in the membrane and its concentration in the
extracellular fluid, equals its Partition Coefficient.
• A concentration gradient is thereby established between the
Membrane and the Intracellular space; this gradient is the
driving force for the Passive Transfer of Drug into the cell.
• A drug with a very high lipid–water partition coefficient, will
generate a HIGHER gradient, which favours more RAPID
diffusion across the membrane and into the cell.
TRANSPORTACROSSCELLMEMBRANES–SIMPLE PASSIVE DIFFUSION (contd)-

16. Simple Passive DIFFUSION (Lipid) - contd:
Outside
Inside
Diffusion Coefficient (DC) is inversely related to
square root of the Mol. Wt. of the diffusing drug
As most drugs have small Mol. Wt. (200-1000)
 DC of most drugs would not vary much
So, most drugs would diffuse readily by passive
lipid diffusion
BUT LARGER MOLECULES DO NOT MOVE
ACROSS easily, or do so MORE SLOWLY
In effect, therefore, Lipid Solubility is the key
factor for drugs’ ability to cross membranes
Pharmacokinetic properties of a drug
(asRate of absorption from the gut
Penetration into the brain / other tissues
Extent of renal elimination …….., etc.,)
involve molecular travel across membrane
So, all these properties can be predicted
from drug's lipid-solubility behaviour
TRANSPORTACROSSCELLMEMBRANES–SIMPLE PASSIVE DIFFUSION (contd)-

17. Lipid Bilayer
SMALL,
UNCHARGED
LARGE,
UNCHARGED
SMALL
CHARGED
IONS
H2O, Urea,
CO2, O2, N2
Glucose
Sucrose
H+, Na+, K+,
Ca2+, Cl-,
HCO3
-
DENIED!
DENIED!
Swoosh!
Hydrophobic
Tails
Hydrophilic
Heads
Molecular travel across Membranes
(as in Absorption)

18. Simple Passive DIFFUSION (Lipid) features - contd:
B. Down the Gradient:
• Drug molecules Passively flow “Down-the-Gradient”
across the cell membrane (from HIGH to the LOW conc.)
• Higher the lipid solubility (oil:water partition coefficient) 
higher the levels reached in membrane  higher the
gradient  higher the flow (as seen earlier)
• As the drug crosses membrane, the gradient falls and the
transfer of drug slows down until equilibrium is achieved.
TRANSPORTACROSSCELLMEMBRANES–SIMPLE PASSIVE DIFFUSION (contd)-

19. Simple Passive DIFFUSION (Lipid) features - contd:
Outside
Inside
C. No Carrier & Energy involved
• The process being Passive, does not –
• need any help from any active entity
• require or spend energy
D. Not Saturable
Passive diffusion process can handle any
amount of molecules (dose), and can not
be saturated (not limited by supply of
carrier or energy)
MOST DRUGS use Passive Diffusion for
Crossing Cell Membranes
(i.e. Most Drugs are absorbed by Passive
Diffusion thru Lipids)
TRANSPORTACROSSCELLMEMBRANES–SIMPLE PASSIVE DIFFUSION (contd)-

20. 2. CARRIER-MEDIATED TRANSPORT:
• Many cells possess specialized carrier
proteins for the transport specially of the
physiological mols e.g. glucose, steroids,
aminoacids, etc. (and many drugs as well)
• Carrier Proteins bind with the mols at one end
of membrane, & then change conformation to
release the mols at other end of membrane.
• In some cases, such systems operate purely passively,
without any energy source or consumption
 facilitate transmembrane equilibration of
the transported mols in the direction of its
concentration gradient
 called FACILITATED DIFFUSION (FD)
• Ex: Transport of glucose across Sk. M. by Insulin-sensitive
GLUT4 transporter, Aminoacids (L-dopa) into brain
• FD is Specific, Saturable, Faster than Simple diffusion &
Not Energy-dependant
TRANSPORTACROSSCELLMEMBRANES–CARRIER MEDIATED

21. 2A. Carrier-mediated (Facilitated) Diffusion:
• But in Facilitated
Diffusion, the rate of
diffusion approaches a
maximum (ceiling),
called Vmax, beyond
which it can not
increase even if conc.
of the diffusing
substance increases
• Because of the limited
capacity of the Carrier
protein
• In the Simple Diffusion, molecular flow increases in direct
proportion to the concentration of the diffusing substance
TRANSPORTACROSSCELLMEMBRANES–FACILITATED DIFFUSION

22. The molecule to be transported enters the pore & binds to the
Carrier Protein  Next, in split-second, the carrier protein
undergoes a conformational /chemical change.
So that the pore now opens to
the opposite side of membrane
to release the molecule.
Max rate of such molecular
transport here can be equal,
but not more, than the rate of
conformational change by
carrier protein.
This mechanism allows the
transported molecule to
“diffuse”—in either direction
through membrane.
TRANSPORTACROSSCELLMEMBRANES–FACILITATED DIFFUSION (contd)-

23. 2B: Carrier mediated ACTIVE TRANSPORT
• When Carrier proteins Translocate drug mols against (or
Up) the gradient, it is called ACTIVE TRANSPORT
• The carrier protein imparts energy to the transported
substance to move against the electrochemical gradient.
• Active Transport : features -
-Used for Lipid-insoluble mols, ions
-Large molecules transported
-Energy-Dependant
-can be Inhibited by reducing ATP (energy) levels
-Specific for particular chemicals
-Saturable at high ligand conc.
-Transports Against, or Up-, the Conc. Gradient
-Liable to Competitive Inhibition by molecules sharing
same carrier (co-transported mols)
-Can carry drug into, or out of, the cell
TRANSPORTACROSSCELLMEMBRANES–ACTIVE TRANSPORT

24. 2B: Carrier mediated ACTIVE TRANSPORT (contd) -
Can be of 2 types –
(a) Primary Active Transport - the energy is derived directly
from breakdown of ATP / other high-energy –PO4 compound.
• Used mainly for movement of IONS – Na+, K+, Ca++, H+, Cl-
etc.  Na+/K+ (ATPase) Pump, Ca++ Pump, H+ Pump
(Parietal cells in Stomach, Renal Distal Tubule), etc.
Primary Active Transport
Na+/K+ Pump
TRANSPORTACROSSCELLMEMBRANES–ACTIVE TRANSPORT (contd)-

25. 2B: Carrier mediated ACTIVE TRANSPORT (contd) -
(b) Secondary Active Transport - the energy is Secondarily
derived from stores in the form of Ionic Gradient between two
sides of cell membrane, created originally by primary active
transport like Na+/K+ Pump.
• Another carrier protein in membrane serves as attachment
points and it couples Na+ with mols to be pulled along with it.
• The energy gradient of Na+ ion causes both the Na+ ion & the
other substance to be co-transported to the interior of the
cell e.g. Na+-Glucose & Na+-Aminoacids co-transport
Secondary Active Transport
Na+-Glucose Cotransport
TRANSPORTACROSSCELLMEMBRANES–ACTIVE TRANSPORT (contd)-

26. Carrier mediated TRANSPORT - contd:
ACTIVE TRANSPORT –contd
• Active transport is important for transport
across the –
*Blood-Brain Barrier
*Gastrointestinal tract epithelium
*Renal tubule membrane
*Biliary tract
*Placenta.
• Drug Ex: -Penicillin by renal tubules,
-Melphalan by leucocytes,
-Iodine by Thyroid cells
• Two Superfamilies of Transporters are now known -
-SLC (Solute carrier) superfamily - has 43 families
-ABC superfamily - has 7 gr of families (designated
by letters A to H e.g. ABCA, ABCB ..) details in GG
TRANSPORTACROSSCELLMEMBRANES–ACTIVE TRANSPORT (contd)-

27. ACTIVE TRANSPORT

28. PASSIVE
DIFFUSION
FACILITATED
DIFFUSION
ACTIVE
TRANSPORT
Lipid soluble mols Lipid insoluble mols Lipid insoluble mols
Lower Mol wt
(200-1000)
Large mol wt Larger mol wt
Nonspecific Specific Specific
Down the gradient Down the gradient Up the gradient
No carrier involved Carrier mediated Carrier mediated
No energy used No energy used /
needed
Energy dependant
Not Saturable Saturable at high
conc.
Saturable at high
conc.
Most drugs use this
process
Physiological / related
molecules – Glucose
(also Tetracyclines)
Physiological / related
mols – Steroids,
Aminoacids
Not affected by
other molecules
Co-transported mols
can compete & inhibit
Co-transported mols
can compete & inhibit

29. PASSIVE
DIFFUSION
FACILITATED
DIFFUSION
ACTIVE
TRANSPORT
Down the gradient Down the gradient Up the gradient
No carrier involved Carrier mediated Carrier mediated
No energy used No energy used /
needed
Energy dependant

30. 3. Passive Diffusion of water-soluble mols
• Passive diffusion of water-soluble molecules
depends on the mol-size
• Aqueous channels of cell membrane measure only
8-10 Å wide, which allow mols of 150-200 Da to go
• These channels belong to a larger family of specific
proteins Aquaporins, each consisting of 6 trans-membrane
domains surrounding a central pore.
• Most allow ONLY WATER to pass thru, but Aquaporin-3
also permit small water-soluble mols e.g. Urea & Glycerol
to traverse
• Few drugs also transported thru Aquaporins –
Examples (with mol. wt.):
*Lithium (~70) * Furosemide (~100)
*Ephedrine (165) * Caffeine (194)
*Vitamin C (176) *H2O2 (34)
*Vit B3 (Nicotinamide) (122)
TRANSPORTORTRANSLOCATIONPROCESSES

31. PASSIVE TRANSPORT THRU PROTEIN CHANNELS

32.  Very large or impermeant mols (>1000 Da) can enter cells
only by ENDOCYTOSIS – an ACTIVE process 2 types -
(1) Adsorptive or Phagocytic uptake of particles that
have been bound to the membrane surface, and
(2) Pinocytotic uptake, in which the particle enters the
cell as part of the fluid phase.
 The substance is engulfed by cell membrane & carried into
cell by pinching off the newly formed vesicle, as in amoeba.
 The substance is then released inside the cytosol by
breakdown of the vesicle by lysosomal enzymes.
 Ex: *Transport of vitamin B12-intrinsic factor across GIT,
*Fe –Transferrin Transport into Hb-synthesizing RBCs
*Insulin crosses BBB by Pinocytosis
 BUT not too important for Drug Translocation.
 EXOCYTOSIS is reverse of Endocytosis.
Ex: Release of neurotransmitters stored in membrane-
bound vesicles in response to nerve-action-potential
4. PINOCYTOSIS (ENDOCYTOSIS & EXOCYTOSIS)
TRANSPORTORTRANSLOCATIONPROCESSES

33. AMOEBA

34. ENDOCYTOSIS
EXOCYTOSIS
Cytoplasm

35.  Water can also cross cell membranes under hydrostatic or
osmotic pressure
 Paracellular travel of water can occur thru inter-cellular gaps
in vascular endothelium & water can carry endogenous &
drug molecules with it
 Most important example of simple filtration is Glomerular
Filtration in kidney, where capillaries have big fenestrations
(70-90 nM) that allow almost every content (except proteins)
– Ultrafiltrate – into the glomerular fluid
 Only ‘free unbound’ molecules are transported by filtration
 Conversely, capillaries in brain, have very tight inter-cellular
junctions where aqueous drug transfer can not occur (Blood
Brain Barrier)
 Capillaries in other organs may fall in between these
extremes
TRANSPORT OR TRANSLOCATION PROCESSES
5. PASSIVE FILTRATION

36.  Gap junctions exist between Epithelial / Endothelial /
Mesothelial cells of the same tissue e.g. smooth muscles,
heart, GIT, etc
 Mols <500 Da can move thru the cylindrical proteins called
Connexins (like the Aquaporins), and aqueous channels
run thru them for cell-to-cell connection
 A Connexon is a hemi-channel formed by six connexin
molecules
 One connexon docks with another connexon on an
adjacent cell, thereby forming a common channel through
which substances with molecular masses of up to around
1 kDa can pass
 Not only ions such as Ca2+ pass thru, but also a number of
organic substances such as ATP (mostly physiological)
TRANSPORT OR TRANSLOCATION PROCESSES
6. PASSAGE THRU GAP JUNCTIONS

37. PASSAGE THRU GAP JUNCTIONS - CONTD
TRANSPORT OR TRANSLOCATION PROCESSES