2. Why should drug molecules move?
Where does transfer takes place?
What are the mechanism of drug transfer?
What are classes of drug transporters?
What are the factors which may influence drug
transfer across membranes?
What is reversible nature of the membrane and
its clinical application?
4/15/2016 2Transfer across membranes
3. Define passive and facilitated diffusion,
active transport, and permeability
Distinguish between perfusion rate limited
and permeability rate limited passage of
drugs through membranes
Describe the role of pH in the movement of
drug through membranes
Describe the consequences of the reversible
nature of movement of drugs through
membranes
4/15/2016 3Transfer across membranes
4. Pharmacokinetics is the study of the variables that
affect drug delivery to, and removal from, its site
of action.
Pharmacokinetic parameters or variables includes
absorption, distribution, metabolism and
excretion/elimination (ADME)
For a drug to exert an effect, it must reach its
intended molecular target.
Conversely, removal of drug from its intended site
of action is an important factor in terminating drug
action.
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5. Absorption, distribution, and elimination are all
processes that require movement through
membranes
This movement is known as drug transport.
The rapidity of drug transport through membranes
is determined by the anatomic and physiologic
factors that will be discussed in this presentation
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6. Drugs must traverse a number of barriers to be
absorbed, distributed, and eliminated.
Cellular membranes and spaces impede drug
transport to varying degrees, and any one of them
can rate-limit the overall process
For example, in the skin, the stratum corneum is
the major site of impedance
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7. The plasma membrane protects and isolates the
cytoplasm from the environment and in so doing, it
presents a barrier to entry of molecules into the
cell.
This barrier is selectively permeable, allowing
physiologically important molecules to enter and
exit cells, while excluding other molecules.
4/15/2016 7Transfer across membranes
8. • barrier to water and water-soluble substances
ions glucose H2O
urea
CO2
O2
N2
halothane
10. Protein embedded in the plasma membrane are
important in facilitating transport of ions and other
molecules into the cells
These proteins are ion channels,
transporters, and pumps.
Individual membrane proteins specifically bind to
certain ligands e.g. to chloride, to glucose, to
sodium and potassium, and facilitate their
movement across the plasma membrane
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16. Most drugs pass through membranes by diffusion
The natural tendency for molecules to move down a
concentration gradient.
Movement results from the kinetic energy of the
molecules.
So drugs must cross membrane barriers to reach their
intended site of action.
These barriers are generally lipid membranes.
Therefore the degree of ionization and the lipid
solubility will affect passive drug transfer
4/15/2016 16Transfer across membranes
17. Since no work is expended by the system, the
process is known as passive diffusion
Where, the driving force for drug transfer is the
concentration of the diffusing species in each of
the compartments on either side of the
membrane.
Movement will be from a region of higher solute
concentration to a region of low solute
concentration
4/15/2016 17Transfer across membranes
18. (a) lipid-soluble molecules move readily across the
membrane
(rate depends on lipid solubility)
(b) water-soluble molecules cross via channels or pores
(a) (b)
19. The net rate of penetration is
=surface area x permeability x
concentration
difference
Whereby:
conc difference = conc side 1-conc side 2
4/15/2016 19Transfer across membranes
20. The importance of the surface area of the
membrane is readily apparent.
For example doubling the surface area
doubles the probability of collision with
the membrane and thereby increasing the
penetration rate twofold.
In general increasing surface area will
increase the penetration rate.
4/15/2016 20Transfer across membranes
21. Some drugs readily pass through a membrane,
other do not
The difference in ease of penetration is
quantitatively expressed in terms of permeability
Permeability is drug's ability to dissolve
into the barrier separating the two
compartments.
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22. The major sources of variation in permeability of a
given membrane to a drug are:
◦ Molecular size and weight
◦ Lipophilicity
◦ Charge
◦ Drug Properties
◦ Membrane thickness
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23. Molecular size has little impact on diffusion of
substance in water but has a major effect on
movement through membranes
Molecular size is important, as are shape and
charge of the molecule
Molecules with small molecular size
diffuse more rapidly than those with large
size
4/15/2016 23Transfer across membranes
24. Molecular weight MW
Drugs or ions with low molecular weight
are more permeable than those with large
molecular weight
For example K+ has a lower molecular weight and
is 30x more permeable than Na+
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25. Lipophilicity, is often characterized by partition
between lipid and water
The lipid-water partition coefficient is an
index of lipid solubility.
Drugs with higher lipid-water partition
coefficients will cross biologic membranes more
quickly.
Lipid soluble drug tend to penetrate lipid
membranes with ease.
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26. Charge is a another major constraint to
transmembrane passage
Thus, the larger and more polar a molecule, the
slower is its movement across membranes.
Movement is slowed even more if the
molecule is charged
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27. The general chemical properties of a drug can
greatly influence its pharmacokinetics.
For a drug to be absorbed and distributed to its
site of action or its site of elimination
it must be liberated from its formulation
It must dissolve in aqueous solutions, and at the
same time it must be able to cross several
hydrophobic barriers (e.g., plasma membrane).
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28. Solid formulations (e.g., tablets, capsules,
suppositories) must disintegrate to release the drug.
Disintegration of the dosage form may be
compromised under certain conditions (e.g., dry
mouth caused by aging, disease, or
concurrent drug treatment slows dissolution of
nitroglycerin tablets).
Drugs may be specifically formulated to allow
disintegration only in certain sections of the
gastrointestinal (GI) tract (e.g., enteric-coated tablets
disintegrate in the small intestine), for the purpose of
protecting the drug from destruction by gastric
acid of the stomach (e.g., erythromycin) or
protecting the stomach from an irritant drug
(e.g., enteric-coated aspirin).
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29. Tablets and capsules may also be formulated to
slowly release drugs (controlled-release,
extended-release, or sustained-release
formulations) and prolong their duration of action.
Sustained-release formulations are particularly
useful for drugs that have very short durations of
action
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30. Semisolid formulations include creams, ointments,
and pastes.
These formulations are generally for topical
application to the skin and require liberation and
diffusion of the drug across the skin.
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31. Liquid formulations may be suspensions or
solutions, which do not require disintegration of
the formulation and thus are generally absorbed
more readily than solid formulations.
Suspensions or solutions are also advantageous for
patients who cannot swallow tablets or capsules.
Drugs in suspension are not dissolved in the
liquid vehicle. Therefore, the drug must first dissolve
before it can be absorbed.
Drugs in solution are already dissolved.
Consequently, solutions are generally absorbed
more rapidly than suspensions.
Drug solutions may also be administered directly
into the bloodstream.
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32. The physical and chemical properties of a drug
will influence its ability to traverse biologic
membranes and to be dissolved and transported
in biologic fluids.
Smaller molecules are absorbed more
readily.
Drug shape affects affinity of the drug
for carrier molecules or other binding sites
such as plasma proteins or tissue.
Drugs of similar structure may exhibit
competition for such binding sites, which
can affect their pharmacokinetics.
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33. Another determinant of permeability is
membrane thickness, the distance a molecule
has to transverse from the site of interest (e.g an
absorption surface) to a blood capillary.
The shorter the distance, the higher is the
permeability.
This distance can vary from about 0.005-
0.01um(for cell membranes) to several
millimeters( at some skin sites)
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34. The rate of drug transport is directly
proportional to concentration
For example, rate of drug transport is
increased twofold when concentration of
drug is doubled.
Each molecule diffuse independently of the other
and system can not be saturated
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35. Note:
Both absence of competition between molecules
and
lack of saturation are characteristics of passive
diffusion
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36. There are two types of specialized transport
processes
◦ Facilitated diffusion
◦ Active transport
4/15/2016 36Transfer across membranes
37. Based on their size, charge, and low solubility in
phospholipids, many biologically important molecules
such as ions, sugars, and amino acids would be
predicted to enter into cells very slowly.
However their uptake into cells occurs at fairly rapid
rates
This can occur because ion channels or
membrane transport proteins facilitate the entry
or exit of specific molecules into and out of cells.
This is referred to as facilitated diffusion
4/15/2016 37Transfer across membranes
38. Ions gain entry into cells through ion channels.
Ion channels are selective, allowing only ions
of certain size and charge to enter.
For example
◦ calcium channels are specific for calcium and sodium
channels for sodium
◦ Ligand gated channels are regulated by
neurotransmitters
◦ Voltage gated channels are regulated by an electric
field(change in membrane potential)
4/15/2016 38Transfer across membranes
39. Transporters are transmembrane proteins that
catalyze migration of molecules into or out of cells.
Individual transporters are able to bind to and
interact with only certain molecules.
They are specific to certain molecules
Their specificity is similar to an enzyme’s
specificity to its substrate.
4/15/2016 39Transfer across membranes
40. Catalyzed transport via transporters( and ion
channels) requires a concentration gradient of
solute or substrate being transported.
Molecules binds to their specific transport protein
with a certain affinity, represented as Km
Km is equal to the concentration of solute that
yields half maximal velocity of transport.
4/15/2016 40Transfer across membranes
41. This maximal velocity (Vmax) of transport will be
achieved when all available transporter protein are
bound by their specific solute
After that point, the addition of more solute will not
result in an increased rate of uptake into the cells.
Therefore, membrane transport via
transporters and via ion channels is
saturable process
4/15/2016 41Transfer across membranes
43. rate of
diffusion
Concn of substance
simple diffusion
Simple vs. Facilitated
Tm facilitated diffusion
Vmax
rate of diffusion ∝ (Co-Ci)
44. Example of facilitated diffusion is glucose transport.
GLUT family members transport glucose from area
of higher concentration to an area of lower
concentration.
In order to be transported, the sugar molecules binds
to the GLUT protein and the membrane direction of
the sugar-GLUT complex is reversed so that the sugar
is released to the other side of the membrane
After the sugar has dissociated, the GLUT flips
orientation and is readied for another cycle of
transport.
4/15/2016 44Transfer across membranes
48. Active transport occurs when molecules or ions
are moved against their concentration gradients
across the membrane.
Energy is required to move these solutes into and
out of cells
The energy used is derived from adenosine
triphosphate(ATP)
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49. Two active transport mechanism
◦ Primary active transport
◦ Secondary active transport
In Primary active transport , membrane proteins that
bind to and transport the molecules or ions against their
gradients must possess the enzymatic activity to
hydrolyze ATP to harness its energy
Known as ATPases, these transporters are ATP powered
pumps that directly hydrolyze ATP to ADP and inorganic
phosphate.
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50. Class Substrate(s) transported
P Ions (H+,Na+,K+,Ca2+)
F H+ only
V H+ only
ABC Ions, drugs, xenobiotics
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51. Secondary active transport is the process by
which ion gradients generated by ATP powered
pumps are used to power transport of other
molecules and ions against their own
concentration gradients.
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52. The ion gradients established by primary active
transport can drive the transport of ions and small
molecules against their concentration gradients in
secondary active transport.
We have 2 secondary active transporters
◦ Symporters
◦ antiporters
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53. Are secondary active transporters that move
substrate in the same direction ( to the inside or to
the outside) across the membrane
Example : glucose symporter SGLT 1
4/15/2016 53Transfer across membranes
54. Secondary Active Transport
1. Co-transport (co-porters): substance is
transported in the same direction as the “driver” ion (Na+
)
Examples:
inside
outside
Na+
AA Na+ gluc 2 HCO3
-Na+
- co-transport and counter-transport -
55. Are secondary active transporters that move one
substrate into and another substrate out of the cell
across the membranes.
Examples :sodium-calcium exchanger
4/15/2016 55Transfer across membranes
56. 2. Counter-transport (anti-porters): substance is
transported in the opposite direction as the “driver” ion (Na+
)
Examples:
Na+
Ca2+
Na+
H+
Cl-
/H+
Na+
/HCO3
-
outside
inside
57. Active transport is mediated by a very large
family of transporters collectively referred to as
ATP binding cassette transporters (or ABC
transporters).
These transporters rely on adenosine triphosphate
(ATP) as a source of energy to transport drug
molecules across biologic membranes.
There are several important features of this
mechanism, including saturability, structural
selectivity, and ATP dependence.
4/15/2016 57Transfer across membranes
58. In contrast to passive diffusion, these carriers
often exhibit a concentration beyond which no
further increase in transport occurs
(Saturability)
These transporters exhibit a varying degree of
structural selectivity for drugs and endogenous
molecules (Structural Selectivity).
4/15/2016 58Transfer across membranes
59. Structurally similar molecules will compete for
binding at these transporters.
This is an important mechanism of drug
interaction.
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60. Characteristics in common with passive facilitated
diffusion are saturability, specificity, and
competitive inhibition.
Active transport is distinguished from passive
facilitated diffusion by the net movement of
substance against a concentration gradient, which
can be large.
The maintenance of this gradient requires
metabolic energy
Therefore active transport can be impeded by
metabolic inhibitors.
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62. • occurs down a concn.
gradient
• no mediator or involves
a “channel” or “carrier”
• no additional energy
• occurs against a concn.
gradient
• involves a “carrier”
• requires ENERGY
Figure 4-2; Guyton & Hall
63. Most drugs must be transferred from their site of
administration to the blood stream in order to
reach their target tissues within the body, except
for intravenously administered drugs.
But drugs administered other ways may be only
partially absorbed.
Oral administration is the common route
and it requires the drug to penetrate the intestinal
mucosa in order to access the blood stream.
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64. Drugs designed to reach the CNS must cross the
Blood brain barrier.
Transport proteins facilitate the movement of
drugs across biological membrane.
And active transport processes appear to be most
commonly used by some drugs.
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65. Many drug transporters function as primary and
secondary active transporters
Based on sequence similarities, drugs
transporters have been classified as
◦ Solute carriers(SLCs) and
◦ ATP binding cassette (ABC) transporters
The structures, functions, and tissue distributions
of drug transporters within each group can vary
widely
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66. The solute carriers are classified into four major
families
◦ Peptide transporters(PEPT)
◦ Organic anionic transporting polypeptides(OATP)
◦ Organic cation transporters and
◦ H+/ organic cation antiporters
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67. Protons (H+) and peptides are cotransported by
PEPT proteins that transport small peptides
Responsible for transporting drugs such as B-
lactam antibiotics( including penicillin) and
ACE inhibitors across the intestinal epithelium
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69. These transporters catalyze the movement of
amphipathic organic compounds such as bile
salts, steroids, and thyroid hormones.
One member of family,OATP1B1, is responsible
for hepatic uptake of pravastatin and ACE
inhibitor, enalapril.
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71. This family makes up a large group of drug
transporters
Various members of this family are expressed in
liver, kidney, skeletal muscles and the intestinal
brush border.
Renal secretion of drugs and toxins is
mediated in part by organic cation
transporters.
The drug metformin is taken up by a transporter
in this family.
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72. Organic cations are reabsorbed by the
proton/organic cation antiporter in brush border
membranes.
An oppositely directed proton gradient is the
driving force of the transport.
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74. These ATP binding proteins that use primary active
transport are responsible for exporting ions, drugs,
and xenobiotics from cells.
Substrates of ABC transporters including anticancer
agents, antiviral agents, calcium channel
blockers and immunosuppressive agents
Multi drug resistance(MDR) can develop when
cells expel the therapeutic agents designed to inhibit
or kill them.
Inhibitors of ABC transporters are a way to prevent
MDR
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75. ABCB1 or P-glycoprotein(P-gp) is an ABC
transporter often implicated in MDR
Responsible for extruding chemotherapeutic
agents from cancer cells.
ABCB1 is also expressed in normal tissues, for
example, in the intestinal epithelial brush border ,
ABCB1 is responsible for efflux of xenobiotics
before they reach the circulation.
4/15/2016 75Transfer across membranes
80. Blood, perfusing tissue, delivers and remove
substances.
Viewing any tissue as whole, the movement of
drug through membranes cannot be divorced from
perfusion considerations
When membranes offer virtually no barrier, the
slowest or rate limiting step is perfusion, and not
permeability.
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82. The passage of ethanol and many small drugs
across the small intestine is similarly perfusion
rate limited.
As membrane resistance to drug increases, the
rate limitation moves away from one of perfusion
to one of permeability.
The problem now lies in penetrating the
membrane, not in delivering drug to, or removing it
from, the tissue.
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83. This increase in resistance may arise for the same
drug crossing membranes of increasing thickness
i.e the multiple cell layers of the epidermis
are less permeable to a drug than the
single cell layer of the capillary
epithelium.
For the same membrane, resistance increases
with increasing size and polarity of the molecule
4/15/2016 83Transfer across membranes
84. Most drugs are weak acids or weak bases
and exist in solution as an equilibrium between
un-ionized and ionized forms.
Increased accumulation of drug on the side of a
membrane where pH favors greater ionization of
drug has led to the pH partition hypothesis
4/15/2016 84Transfer across membranes
85. According to this hypothesis, only un-ionized
nonpolar drug penetrate the membrane,
and at equilibrium, the concentration of the un-
ionized species are equal on both sides.
The un-ionized form is assumed to be
sufficiently lipophilic to transverse
membrane. If it is not, theory predicts that there
is no transfer, irrespective of pH.
The fraction of the un-ionized is controlled
by both the pH of the medium and pKa of
the drug.
4/15/2016 85Transfer across membranes
86. According to the Henderson- Hasselbach :
for acids
For bases
4/15/2016 86Transfer across membranes
87. As log10(1)=0
The pKa of a compound is the pH at
which the un-ionized and ionized
concentrations are equal.
The pKa is a characteristic of the drug
Consider for example the anticoagulant warfarin.
Warfarin is an acid with pKa 4.8 i.e. equimolar
concentrations of un-ionized and ionized drug
exist in solution at pH 4.8.
Stated differently 50% of the drug is un-ionized
at this pH
4/15/2016 87Transfer across membranes
88. Very weak acids such as phenytoin and many
barbiturates, whose pKa values are greater than
7.5 are essentially un-ionized at all pH values
For these acids drug transport should be rapid
and independent of pH, provided the un-ionized
form is permeable.
4/15/2016 88Transfer across membranes
89. For acids with pKa values between 3 and 7.5,
these compounds a change in rate of transport
with pH is expected
For acids with pKa values less than 2.5 the
fraction of un-ionized is so low that transport
across the membrane e.g gut may be slow even
under the most acidic conditions
4/15/2016 89Transfer across membranes
90. A similar analysis indicates that a base must be
very weak, pKa less than 5, for transport to be
independent of pH.
For example caffeine (pKa 0.8) is rapidly
transported and shows no pH dependent
absorption.
Only with stronger bases, those with pKa values
between 5 and 11, is pH dependent transport
expected.
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91. At the usually low pH of the gastric fluid, these
bases exist almost exclusively in the ionized form,
and for these gastric transport should be slow
Passage of these bases should be more rapid
from a less acidic environment.
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92. However the likely influence of pH on the rate
process depends on where the rate limitation lies.
Only if the limitation is in permeability is an effect
of pH on the rate expected
If the limitation is in perfusion, the problem is not
one of movement of drug through membranes
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93. However with all the explanation given, it
must be noted that , the ionized form do
penetrate membranes but at a slower rate
than un-ionized form.
For example: a variety of quaternary ammonium
compounds e.g.propantheline bromide which are
always ionized elicit systemic effects when given
orally.
4/15/2016 93Transfer across membranes
94. Figure 2-2 Application of pH trapping to renal drug
elimination. 4/15/2016 94Transfer across membranes
95. Many drugs bind to plasma protein and tissue
components
Such a binding is reversible and usually so rapid
that an equilibrium is established within
milliseconds
Only unbound drug is thought to be
generally capable of passing through
membranes
The protein bound form being too large to
do so.
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96. It is important to remember that drug
transport is generally bidirectional.
An important application can be made of a
transport in the opposite direction.
For example, repeated oral administration of
charcoal can hasten removal from the body of
drugs such as digoxin, phenylbutazone,
phenobarbital, and digitoxin in cases of drug
overdose.
Because of extensive adsorption of drug to
charcoal.
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97. The concepts of this presentation on passage of
the drugs across membranes are important to an
understanding of movement of drugs into, within,
and out of the body.
Provide a good foundation before starting learning
about the PK parameters-absorption, distribution,
and elimination.
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98. Why should drug molecules move?
Where does transfer takes place?
What are the mechanism of drug transfer?
What are classes of drug transporters?
What are the factors which may influence drug
transfer across membranes?
What is reversible nature of the membrane and
its clinical application?
4/15/2016 98Transfer across membranes
99. Clinical pharmacokinetics and pharmacodynamics
: Concepts and application by Malcolm
Rowland and Thomas N Tozer;4th
Edition,2011
Lippincott’s illustrated review: Cell and
Molecular Biology by Nalini Chandar & Susan
Viselli, Unit III pg 132-154.
4/15/2016 99Transfer across membranes
