Absorption distribution

1. Introduction of Biopharmaceutic and
Pharmacokinetics
Subject: Biopharmaceutic and Pharmacokinetics
Unit-I
Prepared by: Kajale F. V.
(M.Pharm Pharmacology)
Shivai Charitable trust’s College of
Pharmacy.

2.  Biopharmaceutics and Pharmacokinetics: An Introduction
• Biopharmaceutics is defined as the study of factors influencing the rate and
amount of drug that reaches the systemic circulation and the use of this
information to optimize the therapeutic efficacy of the drug products. The
process of movement of drug from its site of administration to the systemic
circulation is called as absorption.
• Biopharmaceutics examines the interrelationship of the physical and
chemical properties of the drug, the dosage form in which the drug is given,
and the route of administration on the rate and extent of systemic drug
absorption.

3. • Its four key chapters are:
• Absorption: The drug's entry into the bloodstream, the gateway to its
action.
• Distribution: The drug's travel to different tissues, seeking its designated
targets.
• Metabolism: The drug's transformation by the body, often paving the
way for elimination.
• Excretion: The removal of the drug and its metabolites, ensuring their
safe departure.

4. • Physicochemical properties of the drug:
• These intrinsic characteristics, such as solubility, permeability, and
ionization, directly influence how readily the drug crosses biological
barriers.
• Formulation characteristics:
• Factors like dissolution rate, dosage form design, and excipients significantly
impact the drug's release from its formulation and subsequent availability for
absorption.
• Physiological factors: The pH, gastrointestinal motility, and presence of
transporters within the body's absorption sites play a crucial role in
facilitating or hindering drug uptake.

6. • The cell membrane, also known as the plasma membrane or cytoplasmic
membrane, is a biological membrane that separates and protects the interior
of a cell from the outside environment (the extracellular space). It is found in
all living cells and is essential for their survival.
 Structure
• The cell membrane is made up of a lipid bilayer, which is two layers of
phospholipids arranged in a tail-to-tail fashion.
• The phospholipids have a hydrophilic (water-loving) head and a
hydrophobic (water-hating) tail. This arrangement creates a barrier that is
selectivel y permeable, meaning that only certain molecules can pass through
it.
• In addition to phospholipids, the cell membrane also contains proteins,
carbohydrates, and cholesterol. The proteins have a variety of functions,
including transport, signaling, and cell adhesion.
• The carbohydrates are involved in cell-cell recognition and communication.
Cholesterol helps to maintain the fluidity of the membrane.

7.  Passage of Drugs Across Cell Membrane
(Mechanisms of Drug Absorption)
A. Transcellular/Intracellular
Transport
1. Passive Transport
a. Passive diffusion.
b. Pore transport.
c. Ion-pair transport.
d. Facilitated- or mediated-diffusion.
2. Active Transport
a. Primary active transport.
b. Secondary active transport
i. Symport (co-transport).
ii. Antiport (counter-transport).
B. Paracellular/Intercellular
Transport
1. Permeation through tight junctions
of epithelial cells
2. Persorption
C. Vesicular or Corpuscular
Transport (Endocytosis)
1. Pinocytosis.
2. Phagocytosis.

8.  Passive diffusion
• Passive diffusion is the process by which molecules spontaneously diffuse from a
region of higher concentration to a region of lower concentration. This process is
passive because no external energy is expended.
• Passive diffusion is the major absorption process for most drugs. The driving force for
passive diffusion is higher drug concentrations, typically on the mucosal side compared
to the blood as in the case of oral drug absorption. According to Fick’s law of diffusion,
drug molecules diffuse from a region of high drug concentration to a region of low
drug concentration.
• where dQ/dt = rate of diffusion, D = diffusion coefficient, A = surface area of
membrane, K = lipid–water partition coefficient of drug in the biologic
membrane that controls drug permeation, h = membrane thickness, and CGI −
Cp = difference between the concentrations of drug in the gastrointestinal tract
and in the plasma.

9. • The drug moves down the concentration gradient indicating downhill
transport
• The rate of drug transfer is directly proportional to the concentration
gradient between GI fluids and the blood compartment
• Greater the area and lesser the thickness of the membrane, faster the
diffusion; thus, more rapid is the rate o f drug absorption from the intestine
than from the stomach
• Equilibrium is attained when the concentration on either side of the
membrane becomes equal.
• Drugs which can exist in both ionized and unionized forms approach
equilibrium primarily by the transfer of the unionized species; the rate of
transfer of unionized species is 3 to 4 times the rate for ionized drugs.

10. • Greater the membrane/water partition coefficient of drug, faster the
absorption; since the membrane is lipoidal in nature, a lipophilic drug
diffuses at a faster rate by solubilizing in the lipid layer of the membrane.
• The drug diffuses rapidly when the volume of GI fluid is low; conversely,
dilution of GI fluids decreases the drug concentration in these fluids (CGIT)
and lower the concentration gradient (CGIT - C). This phenomena is,
however, made use of in treating cases of oral overdose or poisoning.
• The process is dependent, to a lesser extent, on the square root of the
molecular size of the drug – drugs having molecular weights between 100 to
400 Daltons are effectively absorbed passively. The diffusion generally
decreases with increase in the molecular weight of the compound.
However, there are exceptions—for example, cyclosporin A, a peptide of
molecular weight 1200, is absorbed orally much better than any other
peptide.

11.  Pore Transport
• It is also called as convective transport, bulk flow or filtration.
• Very small molecules (such as urea, water, and sugars) are able to cross
cell membranes rapidly, as if the membrane contained channels or pores.
• Although such pores have never been directly observed by microscopy, the
model of drug permeation through aqueous pores is used to explain renal
excretion of drugs and the uptake of drugs into the liver.
• A certain type of protein called a transport protein may form an open
channel across the lipid membrane of the cell. Small molecules including
drugs move through the channel by diffusion more rapidly than at other
parts of the membrane.(<400 Daltons)

12. Pore Transport

13.  Ion-Pair Transport
• Strong electrolyte drugs are highly ionized or charged molecules, such as
quaternary nitrogen compounds with extreme pKa values.
• Strong electrolyte drugs maintain their charge at all physiologic pH values
and penetrate membranes poorly.
• When the ionized drug is linked with an oppositely charged ion, an ion pair
is formed in which the overall charge of the pair is neutral. This neutral
drug complex diffuses more easily across the membrane.
• Example, Propranolol a basic drug that forms an ion pair with Oleic acid,
and Quinine, which forms ion pairs with Hexylsalicylate.
• The complexation of amphotericin B and DSPG (Distearoyl phosphatidyl
glycerol) in some amphotericin B/liposome products.

14. • Ion pairing may rapidly alter distribution, reduce high plasma free drug
concentration, and reduce renal toxicity.

15.  Facilitated or mediated diffusion
• Facilitated diffusion is also a carrier-mediated transport system, differing
from active transport in that the drug moves along a concentration gradient
(i.e., moves from a region of high drug concentration to a region of low drug
concentration). Therefore, this system does not require energy input.
• Various carrier-mediated systems (transporters) are present at the intestinal
brush border and basolateral membrane for the absorption of specific ions
and nutrients essential for the body. Both influx and efflux transporters are
present in the brush border and basolateral membrane that will increase
drug absorption (influx transporter) or decrease drug absorption (efflux
transporter).
• Examples of a transport system include entry of glucose into RBCs and
intestinal absorption of vitamins B1 and B2.

18.  Active Transport
• Active transport is described by the ability to transport drug against a
concentration gradient that is, from regions of low drug concentrations to
regions of high drug concentrations.
 Primary active transport
• Primary active transport, also known as direct active transport, is a vital
cellular process that directly utilizes metabolic energy to transport molecules
across a cell membrane.
• Unlike passive transport, where molecules move freely based on
concentration gradients, active transport requires energy. Also, the process
transfers only one ion or molecule and in only one direction, and hence
called as uniporter e.g. absorption of glucose.

19. (i) Ion transporters :
• These are responsible for transporting ions in or out of cells. e.g. of ATP-driven ion pump is
proton pump which is implicated in acidification of intracellular compartments.
(a) Organic anion transporter: absorption of drugs pravastatin and atorvastatin.
(b) Organic cation transporter: absorption of drugs diphenhydramine.
(ii) ABC transporters (ATP-binding cassette) :
• These are responsible for transporting small foreign molecules (like drugs and toxins)
especially out of cells (and thus called as efflux pumps). e.g. p-glycoprotein (P-gp).
• It is responsible for pumping hydrophobic drugs especially anticancer drugs out of cells.
Presence of large quantity of this protein thus makes the cells resistant to a variety of drugs
used in cancer chemotherapy, a phenomenon called as multi-drug resistance.
• ABC transporters present in brain capillaries pump a wide range of drugs out of brain.

21.  Secondary active transport
• Secondary active transport involves a transporter protein that couples the
movement of an ion (usually Na⁺ or H⁺) down its electrochemical
gradient to the uphill movement of another molecule or ion against a
concentration or electrochemical gradient.
• the energy stored in the electrochemical gradient of one ion is used to
transport another solute against its gradient.
• Cotransport (Symport):
• In cotransport, the driving ion (e.g., Na⁺) and the driven molecule/ion are
transported in the same direction. Cotransporters (or symporters) facilitate
this process. e.g. peptide transporter called as H+ coupled peptide
transporter (PEPT1) which is implicated in the intestinal absorption of
peptide-like drugs such as lactam antibiotics.

22. • Exchange (Antiport):
• In exchange, the driving ion and the driven molecule/ion move in
opposite directions. Exchangers (or antiporters) are responsible for this
type of transport.
• The drug is transported from a region of lower to one of higher
concentration i.e. against the concentration gradient (in the case of ions,
against an electrochemical gradient) or uphill transport, without any
regard for equilibrium. The process is faster than passive diffusion.
• As the process requires outflow of energy, it can be inhibited by
metabolic poisons that interfere with energy production like fluorides,
cyanide and dinitrophenol and lack of oxygen, etc.

23.  Permeation through tight junctions of epithelial cells
• Tight junctions play a crucial role in regulating the permeability of ions,
nutrients, and water across epithelial cell layers. These specialized junctions
create a selectively permeable barrier that supports the absorption of
nutrients and the secretion of waste, while simultaneously preventing the
intrusion of luminal content.
• Structure: Tight junctions, also known as zonula occludens, form belt-like
structures between adjacent epithelial cells. These junctions are essential
for cell adhesion and paracellular barrier functions.
• Proteins Involved: Integral membrane proteins called claudins constitute
the basic framework of tight junction strands. Claudins are part of a family of
at least 24 members in mice and humans.

24. • Other proteins like occludin, tricellulin, JAMs (junctional adhesion
molecules), and CAR (coxsackie and adenovirus receptor) also contribute
to tight junction formation.
• Organization: Membrane-anchored scaffolding proteins, such as ZO-1/2,
help establish the high-level organization of tight junction strands.

25.  Persorption
• Persorption is a mechanism of drug absorption that involves the passage of a
drug molecule through temporary openings formed by the shedding of two
neighboring epithelial cells into the lumen of the intestine.
• This process is relatively uncommon compared to other absorption
mechanisms like passive diffusion and active transport, but it can play a role
in the absorption of certain drugs, particularly macromolecules and
particulate matter.(Peptides, proteins)

26.  Pinocytosis
Endocytosis and exocytosis are the processes of moving specific
macromolecules into and out of a cell, respectively.
During Pinocytosis, Phagocytosis, or Transcytosis, the cell membrane
invaginates to surround the material and then engulfs the material,
incorporating it inside the cell Subsequently, the cell membrane containing the
material forms a vesicle or vacuole within the cell.(Sabin polio vaccine and
various large proteins.)
Transcytosis is the process by which various macromolecules are transported
across the interior of a cell. In transcytosis, the vesicle fuses with the plasma
membrane to release the encapsulated material to another side of the cell.
Vesicles are employed to intake the macromolecules on one side of the cell,
draw them across the cell, and eject them on the other side.

27. An example of exocytosis is the transport of a protein such as insulin from
insulin-producing cells of the pancreas into the extracellular space.

28.  Factors Influencing Drug Absorption Though GIT
A. Pharmaceutical Factors:
I. Physicochemical Properties of Drug Substances
1. Drug Solubility and Dissolution Rate
2. Particle Size and Effective Surface Area
3. Polymorphism and Amorphism
4. Pseudo Polymorphism (Hydrates/Solvates)
5. Salt Form of The Drug
6. Lipophilicity of The Drug
7. pka of The Drug And Gastrointestinal PH
8. Drug Stability
9. Stereochemical Nature of The Drug

29. Pharmaceutical Ingredients
(Pharmaco-technical Factors)
1. Disintegration time
2. Dissolution time
3. Manufacturing variables
4. Pharmaceutical ingredients
5. Nature and type of dosage form
6. Product age and storage
conditions
B. PATIENT RELATED FACTORS:
1. Age
2. Gastric emptying time
5. Disease states
6. Blood flow through the GIT
7. Gastrointestinal contents
a. Other drugs
b. Food
c. Fluids
d. Other normal GI contents
8. Presystemic metabolism by:
a. Luminal enzymes
b. Gut wall enzymes
c. Bacterial enzymes

30. • Drug Solubility and Dissolution Rate
• Drug solubility refers to the maximum amount of a drug that can dissolve
in a specific solvent, typically water, at a given temperature and pressure.
• It is expressed as milligrams per milliliter (mg/mL) or grams per liter
(g/L).
• Highly soluble drugs dissolve readily, while poorly soluble drugs have
limited solubility.
• Dissolution rate refers to the speed at which a solid drug dissolves in a
solvent. It is measured as the percentage of drug dissolved over time.
• Drug solubility and dissolution rate are critical factors governing drug
absorption and effectiveness.
• Understanding these properties is essential for developing safe and
effective medications.

31. o Biopharmaceutics Classification System (BCS)
• The Biopharmaceutics Classification System (BCS) is a framework used to
differentiate drugs based on their solubility and permeability, which
significantly impact their oral absorption. Let’s delve into the details:
1. Class I (High Permeability, High Solubility):
• Examples: Metoprolol and Paracetamol.
• These compounds are well absorbed and typically have a higher absorption
rate than excretion.
2. Class II (High Permeability, Low Solubility):
• Examples: Glibenclamide, Bicalutamide, Ezetimibe, and Aceclofenac.
• The bioavailability of these products is limited by their solvation rate.
There’s a correlation between in vivo bioavailability and in vitro solvation.

32. 3. Class III (Low Permeability, High Solubility):
• Example: Cimetidine.
• Absorption is restricted by the permeation rate, but the drug solvates rapidly.
If the formulation doesn’t alter permeability or gastrointestinal duration time,
Class I criteria can be applied.
4. Class IV (Low Permeability, Low Solubility):
• Example: Bifonazole
• These compounds have poor bioavailability, often not being well absorbed
across the intestinal mucosa, leading to high variability.

33. o Theories of Drug Dissolution
• 1. Diffusion Layer Model/Film Theory
• Diffusion layer model, also known as the film theory, which provides a
physical explanation for the dissolution process. In this model, the limiting
step is the diffusion of molecules through a stagnant film of liquid (referred
to as a hydrodynamic boundary layer) around the solid surface.
• Two Steps:
• Solution of the Solid: Initially, the solid substance dissolves to form a
stagnant film or diffusive layer. This layer becomes saturated with the
drug.
• Diffusion of Soluble Solute: Next, the soluble solute (drug) diffuses from
this stagnant layer into the bulk of the solution. This diffusion process is
the rate-determining step in drug dissolution.

34. • Assumptions:
1. A diffusion layer (or stagnant liquid film layer) of thickness h surrounds
the surface of the dissolving particle.
2. The concentration of the drug within this layer is saturated due to the
initial dissolution of the solid.
• Significance:
1. The diffusion layer model helps us understand how the drug molecules
move from the solid surface into the surrounding solution during
dissolution.

35. dC/dt = k (Cs-Cb)
• dC/dt = dissolution rate of the drug,
• k = dissolution rate constant,
• Cs = concentration of drug in the stagnant layer (also called as the
saturation or maximum drug solubility),
• Cb = concentration of drug in the bulk of the solution at time t.

36. 2. Danckwert’s Model (Penetration or Surface Renewal Theory)
• Danckwert did not approve of the existence of a stagnant layer and
suggested that turbulence in the dissolution medium exists at the
solid/liquid interface.
• solute containing packets are continuously replaced with new packets of
fresh solvent due to which the drug concentration at the solid/liquid
interface never reaches Cs and has a lower limiting value of Ci.
• Since the solvent packets are exposed to new solid surface each time, the
theory is called as surface renewal theory.
dC/dt = dm/dt = A (Cs-Cb) ꝩ D
m = mass of solid dissolved,
ꝩ = rate of surface renewal (or the interfacial tension).

37. 3. Interfacial Barrier Model (Double Barrier or Limited Solvation Theory)
• The diffusion layer model and the Danckwert’s model were based on two
assumptions:
1. The rate-determining step that controls dissolution is the mass transport.
2. Solid-solution equilibrium is achieved at the solid/liquid interface.
• According to the interfacial barrier model, an intermediate concentration can
exist at the interface as a result of solvation mechanism and is a function of
solubility rather than diffusion. When considering the dissolution of a crystal,
each face of the crystal will have a different interfacial barrier.
• G = Ki (Cs-Cb)
• G = dissolution rate per unit area
• Ki = effective interfacial transport constant.

38. • Particle Size and Effective Surface Area of the Drug
• The surface area increases with decreasing particle size, a decrease in particle
size, which can be accomplished by micronisation, will result in higher
dissolution rates.
• Greater the effective surface area, more intimate the contact between the
solid surface and the aqueous solvent and faster the dissolution. But it is only
when micronisation reduces the size of particles below 0.1 microns that there
is an increase in the intrinsic solubility and dissolution rate of the drug.
• E.g micronisation of poorly aqueous soluble drugs like griseofulvin,
chloramphenicol and several salts of tetracycline results in superior
dissolution rates in comparison to the simple milled form of these drugs.
• E.g. Polysorbate 80 increases the bioavailability of phenacetin by
promoting its wettability.

39. • Polymorphism and Amorphism
• When a substance exists in more than one crystalline form, the different
forms are designated as polymorphs and the phenomenon as polymorphism.
1. Enantiotropic polymorph is the one which can be reversibly changed into
another form by altering the temperature or pressure e.g. sulphur,
2. Monotropic polymorph is the one which is unstable at all temperatures and
pressures e.g. glyceryl stearates.
• e.g. The polymorphic form III of riboflavin is 20 times more water soluble
than the form I.
• Hydrates/Solvates (Pseudopolymorphism)
• When the solvent in association with the drug is water, the solvate is known
as a hydrate. Hydrates are most common solvate forms of drugs.

40. • The anhydrous form of a drug has greater aqueous solubility than the
hydrates.
• This is because the hydrates are already in interaction with water and
therefore have less energy for crystal break-up in comparison to the
anhydrates (thermodynamically higher energy state) for further interaction
with water.
• E.g. The anhydrous form of theophylline and ampicillin have higher
aqueous solubilities, dissolve at a faster rate and show better bioavailability
in comparison to their monohydrate and trihydrate forms respectively.
• Salt Form of the Drug
• One of the easiest approaches to enhance the solubility and dissolution rate of
such drugs is to convert them into their salt forms.
• Generally, with weakly acidic drugs, a strong base salt is prepared such as the

41. • Drug pKa and Lipophilicity and pH
• The GIT is a simple lipoidal barrier to the transport of drug.
• Larger the fraction of unionised drug, faster the absorption.
• Greater the lipophilicity (Ko/w) of the unionised drug, better the absorption.
• Drug pKa and Gastrointestinal pH
• The lower the pKa of an acidic drug, stronger the acid i.e. greater the
proportion of ionised form at a particular pH. Higher the pKa of a basic drug,
stronger the base.
• E.g Pentobarbital 8.1 pH Unionised at all pH values; absorbed along the
entire length of GIT
• E.g. Aspirin 3.5 pH Unionised in gastric pH and ionised in intestinal; better
absorbed from stomach

42. • Lipophilicity and Drug Absorption
• a perfect hydrophilic-lipophilic balance (HLB) should be there in the
structure of the drug for optimum bioavailability
• The lipid solubility of a drug is measured by a parameter called as log P
where P is oil/water partition coefficient (Ko/w or simply P) value of the
drug.
• Drug Stability
• A drug for oral use may destabilize either during its shelf-life or in the GIT.
degradation of the drug into inactive form, and interaction with one or more
different component(s) either of the dosage form or those present in the GIT
to form a complex that is poorly soluble or is unabsorbable.
• Stereochemical Nature of Drug
• Enantiomers have identical physical and chemical properties despite

43. • Dosage Form (Pharmaco-technical) Factors
• Disintegration Time
• Disintegration time (DT) is of particular importance in case of solid dosage
forms like tablets and capsules. DT of a tablet is directly related to the
amount of binder present and the compression force (hardness) of a tablet. A
harder tablet with large amount of binder has a long DT.
• e.g. Microcrystalline cellulose.
o Pharmaceutical Ingredients/Excipients (Formulation factors)
• A drug is rarely administered in its original form. Almost always, a
convenient dosage form to be administered by a suitable route is prepared.
Such a formulation contains a number of excipients (non-drug components of
a formulation).
• Vehicle or solvent system is the major component of liquid orals and

44.  Absorption of Drug From Non Per Oral Extra-vascular Routes
• Buccal/Sublingual Administration
• Sublingual route: The drug is placed under the tongue and allowed to
dissolve.
• Buccal route: The medicament is placed between the cheek and the gum.
1. Rapid absorption and higher blood levels due to high vascularisation of the
region and therefore particularly useful for administration of antianginal
drugs.
2. No first-pass hepatic metabolism.
3. No degradation of drugs such as that encountered in the GIT
4. Presence of saliva facilitates both drug dissolution and its subsequent
permeation by keeping the oral mucosa moist.

45. • Rectal Administration
• The rectal route of drug administration is still an important route for children
and old patients. Drugs administered by this route include aspirin,
paracetamol, theophylline, barbiturates, etc.
• Topical Administration
• Intramuscular Administration
• Subcutaneous Administration
• Pulmonary Administration
• Intranasal Administration
• Intraocular Administration
• Vaginal Administration

46.  Distribution of Drugs
• Distribution which includes reversible transfer of a drug between
compartments.
• Almost all drugs having molecular weight less than 500 to 600 Daltons
easily cross the capillary membrane to diffuse into the extracellular
interstitial fluids.
• Steps in Drug Distribution
1. Permeation of free or unbound drug present in the blood through the
capillary wall (occurs rapidly) and entry into the interstitial/extracellular
fluid (ECF).
2. Permeation of drug present in the ECF through the membrane of tissue cells
and into the intracellular fluid.
(a) Rate of perfusion to the extracellular tissue
(b) Membrane permeability of the drug

47. • Physiological Barriers to Distribution of Drugs
1. Simple capillary endothelial barrier
2. Simple cell membrane barrier
3. Blood-brain barrier
4. Blood-CSF barrier
5. Blood- placental barrier
6. Blood-testis barrier. (sertoli-sertoli cell)
• The brain capillaries consist of endothelial cells which are joined to one
another by continuous tight intercellular junctions comprising what is called
as the blood-brain barrier.
• Perfusion rate is defined as the volume of blood that flows per unit time per
unit volume of the tissue.

48.  Factors Affecting Distribution of Drugs
1. Tissue permeability of the drug
• a. Physicochemical properties
• b. Physiological barriers to diffusion
of drugs
• 2. Organ/tissue size and perfusion
rate
• 3. Binding of drugs to tissue
components
• a. Binding of drugs to blood
components
• b. Binding of drugs to extravascular
tissue proteins
• 4. Miscellaneous factors:
• a. Age
• b. Pregnancy
• c. Obesity
• d. Diet
• e. Disease states
• f. Drug interactions.

49. • VOLUME OF DISTRIBUTION
• It is defined as the hypothetical volume of body fluid into which a drug is
dissolved or distributed.
• A drug in circulation distributes to various organs and tissues.
• Different organs and tissues contain varying concentrations of drug which can
be determined by the volume of tissues in which the drug is present.
• different tissues have different concentrations of drug. However, there exists a
constant relationship between the concentration of drug in plasma, C, and the
amount of drug in the body, X.
X= Vd × C
• where Vd = apparent volume of distribution.

51. • Plasma and tissue protein binding of drugs
• Protein binding is a crucial process where drugs attach to proteins within the
blood, impacting their distribution and effectiveness.
 Bound Drugs:
• Definition: Bound drugs are those that attach to proteins within the blood
plasma.
• Form: They exist as a complex with plasma proteins (e.g., albumin,
alpha-1 acid glycoprotein).
• Pharmacological Inactivity: Bound drugs are pharmacologically
inactive because they cannot exert their effects.
• Distribution: They remain in the bloodstream and do not readily cross
cell membranes.

52. • Metabolism and Excretion: Bound drugs are not metabolized or
excreted; they act as reservoirs.
• Equilibrium: Binding is reversible, creating an equilibrium between
bound and unbound states.
 Unbound Drugs
• Definition: Unbound drugs are the free, unconjugated form of the drug.
• Pharmacological Activity: They are pharmacologically active and
responsible for therapeutic effects.
• Cell Membrane Permeability: Unbound drugs can diffuse across cell
membranes.
• Metabolism and Clearance: They undergo metabolism and may be
excreted.
• Clinical Significance: Unbound drug levels determine efficacy and
potential side effects.

53. • Mechanisms of Protein-Drug Binding
1. Intracellular binding
• where the drug is bound to a cell protein which may be the drug receptor if
so, binding produces a pharmacological response. These receptors with
which drug interact to show response are called as primary receptors.
2. Extracellular binding
• where the drug binds to an extracellular protein but the binding does not
produce a pharmacological response. These receptors are called secondary
or silent receptors.
• Reversible binding
1. Hydrogen bonds 2. Hydrophobic bonds 3. Ionic bonds, or 4. van der Waal’s
forces.

54. • Irreversible binding
• Irreversible binding involves permanent attachment of the drug to the
receptor.
1. Covalent binding
2. Tight binding where the dissociation rate is effectively zero
 BINDING OF DRUGS TO BLOOD COMPONENTS
 Plasma Protein-Drug Binding
 Binding of Drugs to Human Serum Albumin(HAS 65,000,)
 Binding of Drugs to 1-Acid Glycoprotein ( 1-AGP or AAG)(44,000)
 Binding of Drugs to Lipoproteins
1. Chylomicrons (least dense and largest in size).
2. Very low density lipoproteins (VLDL).

55. 3. Low-density lipoproteins (LDL)
4. High-density lipoproteins (HDL)
 Binding of Drugs to Globulins
• Alfa 1 Globulin (transcortin)
• Alfa 2 Globulin (ceruloplasmin)
• Beta 1 Globulin (transferrin)
• Beta 2 Globulin
• Gamma Globulin
 Binding of Drugs to Blood Cells
• Haemoglobin
• Carbonic anhydrase
• Cell membrane

56.  TISSUE BINDING OF DRUGS (Tissue localization of drugs)
Liver > Kidney > Lung > Muscles
• Examples of Extravascular Tissue-Drug Binding:
1. Liver: Epoxides of halogenated hydrocarbons and paracetamol
irreversibly bind to liver tissues, leading to hepatotoxicity.
2. Lungs: Basic drugs like imipramine, chlorpromazine, and antihistamines
accumulate in lung tissues.
3. Kidneys: Metallothionin, a protein in kidneys, binds to heavy metals
(lead, mercury, cadmium), resulting in renal accumulation and toxicity.
4. Skin: Chloroquine and phenothiazines interact with melanin,
accumulating in the skin.
5. Eyes: Retinal pigments containing melanin bind drugs like chloroquine
and phenothiazines, contributing to retinopathy.

57. 6. Hairs: Arsenicals, chloroquine, and phenothiazines deposit in hair
shafts.
7. Bones: Tetracycline binds to bones and teeth, causing permanent brown-
yellow discoloration of teeth.
8. Fats: Lipophilic drugs such as thiopental and the pesticide DDT
accumulate in adipose tissues
9. Nucleic Acids: Molecular components of cells such as DNA interact
strongly with drugs like chloroquine and quinacrine resulting in
distortion of its double helical structure.
• DETERMINATION OF PROTEIN-DRUG BINDING
1. Indirect techniques are those based on the separation of bound form
from the free micromolecule. (blood, serum, plasma)
2. Direct techniques are those that do not require the separation of bound
form of drug from the free micromolecule.(UV spectroscopy, fluorimetry,
and ion-selective electrodes)

58. • FACTORS AFFECTING PROTEIN-DRUG BINDING
1. Drug related factors
• a. Physicochemical characteristics of the drug
• b. Concentration of drug in the body
• c. Affinity of a drug for a particular binding component
2. Protein/tissue related factors
• a. Physicochemical characteristics of the protein or binding agent
• b. Concentration of protein or binding component
• c. Number of binding sites on the binding agent
3. Drug interactions
• a. Competition between drugs for the binding site (displacement
interactions)
• b. Competition between the drug and normal body constituents
• c. Allosteric changes in protein molecule

59. 4. Patient related factors
a. Age
b. Inter subject variations
c. Disease states
• Clinical significance of Protein Binding (Importance)
1. Drug Availability:
• Only the unbound or free fraction of the drug exhibits pharmacological
activity and reaches its target site to exert its therapeutic effect.
2. Drug Distribution:
• Highly protein-bound drugs tend to have a larger volume of distribution,
meaning they are more dispersed throughout body fluids and tissues. This can
impact the time it takes for the drug to reach its target site and its overall
efficacy.

60. 3. Drug Interactions:
• When co-administered, multiple drugs may compete for the same binding sites
on proteins. This competition can displace one drug from its binding, leading
to an increase in the free drug concentration and potentially exaggerated
effects or toxicity.
4. Special Populations:
• Certain factors, such as disease states, pregnancy, and kidney or liver
dysfunction, can alter protein binding. These changes can affect the free drug
concentration and necessitate adjustments in drug dosing for these
populations.
5. Tissue-Specific Affinity:
• Plasma protein binding restricts the entry of drugs that have specific
affinity for certain tissues. This prevents excessive accumulation of the drug
in those tissues, reducing the risk of subsequent toxic reactions.

61.  KINETICS OF PROTEIN-DRUG BINDING
• If P represents proteins and D the drug, then applying law of mass action to
reversible protein-drug binding, we can write:
P + D ======== PD
1. Direct Plot
2. Scatchard Plot
3. Klotz Plot/Lineweaver-Burke Plot (Double Reciprocal Plot)
4. Hitchcock Plot

62.  References
1. Applied biopharmaceutics and pharmacokinetics, Leon Shargel and
Andrew B.C.YU 4th edition,Prentice-Hall Inernational edition.USA
2. Biopharmaceutics and Pharmacokinetics-A Treatise, By D. M.
Brahmankar and Sunil B.Jaiswal,Vallabh Prakashan Pitampura, Delhi.
3. Remington’s Pharmaceutical Sciences, By Mack Publishing Company,
Pennsylvnia
4. Biopharmaceutics and Clinical Pharmacokinetics-An introduction 4th
edition Revised and expanded by Rebort F Notari Marcel Dekker Inn,
New York and Basel, 1987.