The phenomenon of complex formation of drug with protein is called as Protein drug binding. The proteins are particularly responsible for such an interaction. A drug can interact with several tissue components.

1. Protein Drug Binding

2. CONTENTS
 INTRODUCTION
 CLASSES OF PROTEIN BINDING
 BINDING OF DRUGS TO BLOOD COMPONENTS
a)PLASMS PROTEIN DRUG BINDING
b)BINDING OF DRUG TO BLOOD CELLS
BINDING OF DRUG TO EXTRA VASCULAR TISSUE PROTEIN
FACTORS AFFECTING PROTEIN DRUG BINDING
SIGNIFICANCE OF PROTEIN/TISSUE BINDING OF DRUG
 REFRENCES
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INTRODUCTION:-

4. Protein drug binding:-
The phenomenon of complex formation of drug with
protein is called as Protein drug binding. The proteins are
particularly responsible for such an interaction. A drug can
interact with several tissue components.
 Binding of drug falls into 2 classes:
1) Blood
a) Plasma proteins.
b) Blood cells.
2) Extra vascular tissue proteins, fats, bones, etc.
 Protein binding may be divided into:
1)Intracellular binding,
2)Extracellular binding.
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5. Mechanisms of protein drug binding:
Binding of drugs to proteins is generally of reversible
&irreversible. Reversible generally involves weak chemical
bond such as:
1)Hydrogen bonds
2)Hydrophobic bonds
3)Ionic bonds
4)Vander waal’s forces.
Irreversible though rare, arises as a result of covalent
binding and is often a reason for the carcinogenicity or tissue
toxicity of the drug.
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7. A) BINDING OF DRUGS TO BLOOD COMPONENTS:-
1) Plasma Protein-drug binding.
The binding of drugs to plasma proteins is reversible.
The extent or order of binding of drug to plasma proteins
is:
Albumin ›ὰ1-Acid glycoprotein ›Lipoproteins ›Globulins.
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a)Binding of drug to human serum Albumin.
It is the most abundant plasma protein (59%), having M.W. of
65,000 with large drug binding capacity. Endogenous compounds &
Large variety of all types of drugs.
4 diff. sites on HSA for drug binding.
- Site I: warfarin & azapropazone binding site.
- Site II: diazepam binding site.
- Site III: digitoxin binding site.
- Site IV: tamoxifen binding site.

9. b) Binding of drug to ὰ1-Acid glycoprotein: (orosomucoid)
It has a M.W. 44,000 and plasma conc. range of 0.04
to 0.1 g%. It binds to no. of basic drugs like imipramine,
lidocaine, propranolol, quinidine.
c) Binding of drug to Lipoproteins:
Lipoproteins are amphiphilic in nature. It contains
combination of lipid & apoproteins. The lipophilic lipid
consist of triglycerides & cholesteryl esters and hydrophilic
apoprotein consists of free cholesterol &proteins. 9

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 The M.W. of lipoproteins from 2 lakhs to 34 lakhs
depends on their chemical composition. They are classify on
the basis of their density :
- Chylomicrons
- VLDL
- LDL
- HDL

11. d) Binding of drug to Globulins:
It mainly binds to endogenous substances. In plasma
several globulins have been identified.
• ἀ1-globulin:- (transcortin) corticosteroid binding globulin.
• ἀ2-globulin:- (ceruloplasmin) it binds vita. A, D, E, K &
cupric ions.
•
.
ᵝ1-globulin:- (transferrin) it binds to ferrous ions.
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12. 2) Binding of drug to Blood cells.
In blood 40% of blood cells of which major
component is RBC (95%). The RBC is 500 times in diameter
as the albumin. The rate & extent of entry into RBC is more
for lipophilic drugs. Thus, significant RBC-drug binding is
possible. The RBC comprises of 3 components
a)Haemoglobin: It has a M.W. of 64,500 Dal. Drugs like
phenytoin, pentobarbital bind to haemoglobin.
b)Carbonic anhydrase: Carbonic anhydrase inhibitors drugs
are bind to it like acetazolamide & chlorthalidone.
c)Cell membrane: Imipramine & chlorpromazine are reported
to bind with the RBC membrane. 12

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14. B) BINDING OF DRUGS TO EXTRAVASCULAR
TISSUES:-
 The tissue-drug binding is much more significant because
the body tissues comprise 40% of the body wt which is
100 times that of HAS.
 A tissue can act as the storage site for drugs.
 Factors that influence localization of drug in tissues are
lipophilicity & structural features of the drug, perfusion rate,
pH differences etc.
 The order of binding of drug to extravascular tissue is:
Liver ›Kidney › Lung › Muscles
 Several example of extravascular tissue-drug binding are:
Liver, Lungs, Kidneys, skin, eyes, hairs, etc.
It also seen in hairs, bones, fats & nucleic acids etc. 14

15. DETERMINATION OF PROTEIN-DRUG BINDING:
1)Indirect techniques:
Equilibrium Dialysis, Dynamic Dialysis, Ultra
filtration, Ultracentrifugation, Gel filtration
1)Direct techniques:
UV-Spectroscopy, Fluorimetry, HPLC.
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16. FACTOR AFFECTING PROTEIN-DRUG BINDING:
1.Drug-related factors:
a) Physicochemical characteristics of drug :-
Protein binding is directly related to the lipophilicity &
stereoselectivity of drug.
b) Concentration of drug in the body:-
The extent of protein-drug binding can change with
both changes in drug as well as protein concentration.
c) Affinity of a drug for a particular binding component:-
Drug having their own higher specific protein binding
site. 16

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2. Protein/ tissue related factors:
a) Physicochemical characteristics of protein or binding
agent:-
Lipoproteins & adipose tissue tend to bind lipophilic
drug by dissolving them in their lipid core. The
physiological pH determines the presence of active anionic
& cationic groups of drug to bind on the albumin.

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b) Concentration of protein or binding component:-
The amount of several proteins and tissue components
available for binding, changes during disease state.
c) Number of binding sites on the binding agent:-
Albumin has a large no. of binding sites as compared
to other proteins and is a high capacity binding component.
AAG having low conc. & low molecular size therefore it
has limited binding capacity.

19. 3. Drug interactions:
a) Competition between drugs for the binding sites
[ Displacement interactions]:-
A drug-drug interaction for the common binding site
is called as displacement interaction. D.I. can result in
unexpected rise in free conc. of the displaced drug which
may enhance clinical response or toxicity. Even a drug
metabolite can affect D.I.
If the drug is easily metabolisable or excretable, it’s
displacement results in significant reduction in elimination
half-life. 19

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b) Competition between drug & normal body constituents:-
The free fatty acids are known to interact with a no.
of drugs that binds primarily to HAS. the free fatty acid
level increase in,
• Physiological (fasting)
• Pathological (diabetes, myocardial infraction)
• Pharmacologically induced (after heparin or caffeine
administration)

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The fatty acids, which modify the albumin binding.
Ex.-benzodiazepines & propranolol (decreased) and
warfarin (increased).
Acidic drugs displace the bilirubin from it’s albumin
binding site. In neonates this condition produce
kernicterus.
c) Allosteric changes in protein molecule:-
Defn
- The process involves alteration of the protein
structure by the drug or it’s metabolite thereby modifying
its binding capacity. Ex.- aspirin increase the
phenylbutazone & decrease the flufenamic acid binding to
albumin.

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4. Patient-related factors:
a) Age:-
b) Intersubject variation:-
These differences have been attributed to genetic and
environmental factors.
c) Disease states:-
Almost every serious chronic illness is characterized
by decrease albumin content. Ex.-

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24. SIGNIFICANCE OF PROTEIN/TISSUE BINDING OF
DRUGS:
1)Absorption
2)Systemic solubility of drugs
3)Distribution
4)Tissue binding, apparent volume of distribution and drug
storage
5)Elimination
6)Displacement interaction and toxicity
7)Diagnosis
8)Therapy and drug targeting 24

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REFERENCES
1. Brahmankar D.M. ,Jaiswal S.B. ,Biopharamaceutics and
pharmacokinetics ;A Treatise ,2nd
ed. ,Vallabh Prakashan ,p. 116-136.
2. Tipnis H.P. ,Bajaj A. ,Principle and application of Biopharamaceutics
and pharmacokinetics ,1st
ed. ,Carrier Publication ,p. 73-84.
3. Shargel L. ,Wa-Pong S. ,Andrew B.C. Yu. ,Applied Biopharamaceutics
and pharmacokinetics ,5th
ed. ,Mc Graw Hill company ,p. 267-298.
4. Tipnis H.P. ,Nagarsenkar M.S. ,Introduction to Biopharamaceutics and
pharmacokinetics ,1st
ed. ,Nirali Prakashan ,p. 30-35.
5. Paradkar A. ,Bakliwal S. ,Biopharamaceutics and pharmacokinetics ,2nd
ed. , Nirali prakashan , p. 3.12-3.15.
6. Madan P.L. ,Biopharamaceutics and pharmacokinetics ,1st
ed. ,Jaypee
brothers Medical publisher Ltd. ,p. 82-85.
7. Tripati K.D. ,Essential of Medical pharmacology ,6th
ed. , Jaypee
brothers Medical publisher Ltd. ,p. 20-23.
8. Barar F.S.K , ,Essential of pharmacotherapeutices ,5th
ed. ,S.Chand and
Company Ltd. ,p. 43-48.

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