This document discusses drug interactions at plasma and tissue binding sites. It describes the mechanisms of protein drug binding, including reversible and irreversible binding via hydrogen bonds, hydrophobic bonds, ionic bonds, and Vander Waal's forces. It explains how drugs can bind to blood components like plasma proteins, albumin, alpha-1-acid glycoprotein, lipoproteins, globulins, and blood cells. It also discusses how drugs can bind to extravascular tissues in organs like the liver, kidneys, lungs, and muscles. The significance of protein and tissue binding on drug absorption, distribution, elimination, therapy and drug targeting is explained.

1. Drug Interaction At Plasma
And Tissue Binding Site
Presented by
Arabinda Changmai
M.Pharm 2nd sem
Department of pharmacy
ADTU

2. CONTEN
T
 INTRODUCTION
 MECHANISMS OF PROTEIN DRUG BINDING
 BINDING OF DRUGS TO BLOOD COMPONENTS
 PLASMS PROTEIN DRUG BINDING
 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

3. INTRODUCTION
3
A drug in a body can interact with several tissue components in which
two major categories are-
• Blood
• Extravascular tissue
The interacting molecules are generally the macromolecules such as
proteins, DNA or adipose.
The phenomenon of complex formation with protein is calls as protein
drug binding.
It is mainly two types-
• Intracellular binding
where drugs binds to cell protein
• Extracellular binding
where drugs binds to extracellular protein

4. Mechanisms of protein drugbinding:
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 drug binding though rare, arises as a result of covalent
binding and is often a reason for the carcinogenicity or tissue
toxicity of the drug.

5. TO BLOOD
COMPONENTS
Plasma
protein
Blood
cell
TO EXTRAVASCULAR
TISSUE
Liver
Kidney
Bones
etc.
BINDING OF DRUG

6. BINDING OF DRUGSTO BLOODCOMPONENTS
1. PLASMA PROTEIN-DRUG BINDING
 The main interaction of drug in the blood compartment is with the
plasma protein which are present in abundant amounts and in large
variety
 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.

7. 2. Binding of drug to human serum Albumin (HAS)
It is the most abundant plasma protein, having a molecular weight of
65,000 with a large binding capacity.
Four different sites on HSA have been identified for drug-binding.
Site 1- Warfarin and azapropazone binding site
Site 2- Diazepam binding site
Site 3- Digitoxin binding site
Site 4- Tamoxifen binding site

8. 4. 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
3.BINDING OF DRUG TO ὰ1-ACID GLYCOPROTEIN
 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.

9.  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
 Very low density lipoprotein (VLDL)
 Low density lipoprotein (LDL)
 High density lipoprotein (HDL)

10. 5.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.
• ᵝ2- globulin:- Binds to carotinoids
• γ-globulin:- Binds specifically to antigens.

11. 12
6. BINDING OF DRUGS 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
• Haemoglobin: It has a M.W. of 64,500 Dal. Drugs like phenytoin,
pentobarbital bind to haemoglobin.
• Carbonic anhydrase: Carbonic anhydrase inhibitors drugs are bind to
it like acetazolamide
• Cell membrane: Imipramine & chlorpromazine are reported to bind
with the RBC membrane.

12. 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 HSA.
• 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

13. DETERMINATION OF PROTEIN-DRUG BINDING
1. Indirect techniques
–It based on separation of bound form from the free micro-
molecule.
–Equilibrium Dialysis, Dynamic Dialysis, Ultra filtration,
Ultracentrifugation, Gel filtration are generally applied in
biological samples.
2. Direct techniques
–Do not require the separation of bound form from
micromolecules.
–UV-Spectroscopy, Fluorimetry, HPLC are used.

14. FACTOR AFFECTING PROTEIN-DRUG BINDING
1. Drug-related factors
– Physicochemical characteristics of drug
Protein binding is directly related to the lipophilicity & stereo selectivity of
drug
– Concentration of drug in the body
The extent of protein-drug binding can change with both changes in
drug as well as protein concentration.
– Affinity of a drug for a particular binding component
Drug having their own higher specific protein binding site.

15. 2. Protein/ tissue related factors
– 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.
– Concentration of protein or binding component
The amount of several proteins and tissue components available for
binding, changes during disease state
– 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.

16. 3. Drug interactions
a) Competition between drugs for the binding sites [ Displacement
interactions]
a) When two or more drugs can bind to the same site, competition
between them for interaction with the binding site results.
b) Such drug-drug interaction for the common binding site is called as
displacement interaction
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)

17. c) Allosteric changes in protein molecule
• The process involves alteration of the protein structure by the
drug or it’s metabolite thereby modifying its binding capacity
• The agent that produce such an effect is called as allosteric
effector.
4. Patient-related factors
a) Age
b) Inter-subject 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.

18. SIGNIFICANCE OF PROTEIN/TISSUE BINDING OF
DRUGS
Absorption
Systemic solubility of drugs
Distribution
Tissue binding, apparent volume of distribution and drug
storage
Elimination
Displacement interaction and toxicity
Diagnosis
Therapy and drug targeting

19. ABSORPTION
• The absorption equilibrium is attained by transfer of free drug from
the site of administration into the systemic circulation and when
the concentration in these two compartments become equal.
• However, binding of the absorbed drug to plasma proteins
decreases free drug concentration and disturbs such an equilibrium
• Thus, sink conditions and the concentration gradient are
reestablished which now act as the driving force for further
absorption.
• This is particularly useful incase of ionized drugs which are
transported with difficulty.

20. SYSTEMIC SOLUBILITY OF DRUGS
• Water insoluble drugs ,neutral endogenous macromolecules such as
Heparin, Oil soluble vitamins are circulated and distributed to tissues
by binding especially to lipoproteins which act as a vehicle for such
hydrophobic compounds.
DIAGNOSIS
The chlorine atom of chloroquine when replaced with radiolabeled I-
131 can be used to visualize melanomas of the eye since chloroquine
has a tendency to interact with the melanin of eyes

21. DISTRIBUTION
• Plasma protein binding restricts the entry of drugs that have
specific affinity for certain tissues .
• This prevents accumulation of large fraction of drugs in such
tissues and thus, reduce toxic reactions.
• Plasma protein binding thus favors uniform distribution of
drugs throughout the body by its buffer function.
• A protein bound drug in particular does not cross the BBB ,the
placental barrier and the glomerulus.

22. ELIMINATION
• Only the unbound drug is capable of being eliminated.
• This is because the drug protein complex cannot penetrate
into the liver.
• The large molecular size of the complex also prevents it from
getting filtered through the glomerulus .
• Thus, drugs which are more than 95% bound are eliminated
slowly i.e. they have long elimination half lives

23. THERAPY AND DRUG TARGETING
• The binding of drugs to lipoproteins can be used for site
specific delivery of hydrophilic moieties.
• This is particularly useful in certain cancer therapy's because
certain tumor cells have greater affinity to for LDL then
normal cells.
• Thus binding a suitable antineoplastic to it can be used as
therapeutic tool.

24. CONCLUSION
• All pharmacokinetic parameters can be influenced by protein
binding.
• Bound drug cannot penetrate through blood capillaries, so
that the bound drug pharmacologically inert.
• Plasma–protein bound drug have longer elimination half lives
compare to the free drug.
• Protein bound drug doesn’t cross BBB and placental barrier.

25. 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. Paradkar A. ,Bakliwal S. ,Biopharamaceutics and pharmacokinetics
,2nd ed. , Nirali prakashan , p. 3.12-3.15.