Protein Binding

Protein Binding

Learning
Objectives

At the end of this lecture, the student will be able to:

• Identify the drug binding components

• List the vascular and extravascular components to which
the drug can bind

• Differentiate between plasma protein binding and tissue
binding

• Discuss the factors affecting protein binding

• Analyse the influence of protein binding on the
pharmacokinetics of the drug

• Significance of drug displacement interactions

• Discuss the kinetics of protein binding

• Analyse the significance of drug displacement interactions

• Discuss the kinetics of protein binding

Protein
Binding of Drugs

• A drug in the body can interact with several tissue
components of which the two major categories are blood and extravascular tissue

• The interacting molecules are generally the macromolecules
such as proteins, DNA

Importance
of protein binding:

• The bound drug is both pharmacokinetically as well as
pharmacodynamically inert because a protein bound drug is neither metabolized
nor pharmacologically active.

• A bound drug is also restricted since it remains confined
to a particular tissue and because of its enormous size cannot undergo
transport and thus its half-life is increased

Types of Binding

• Reversible

• Irreversible

Binding of
drug to globulin

α1 globulin Bind
to a number of steroidal drug cortisone , prednisolone $ thyroxin  , cynocobalamine

α2 globulin
(ceruloplasmin ) bind to Vit. A D E K

γ- globulin Bind
to antigen

β1-globulin
(transferrin ) bind to ferrous ion

β2-globulin bind
to carotenoid

Binding of
drug to blood cells

Haemoglobin Bind
to phenytoin, pentobarbital, phenothiazine

Carbonic anhydrase
Acetazolamide, chlorthalidone

Cell membrane
Imepramine, chlorpramazine bind to RBCs cell membrane

Tissue
binding of drug

Majority of drug bind to extravascular tissue

The order of binding – liver > kidney > lung >
muscle

Liver – epoxide of number of halogenated hydrocarbon,
paracetamol

Lung – basic drug imipramine, chlorpromazine, antihistamines,

Kidney – metallothionin bind to heavy metal, lead, Hg, Cd,

Skin – chloroquine & phenothiazine

Eye – chloroquine & phenothiazine

Hairs- arsenicals, chloroquine bind to hair shaft.

Bone – tetracycline

Fats – thiopental, pesticide- DDT

Comparison
between Plasma Protein Drug Binding and Tissue Drug Binding

Plasma protein- drug binding	Tissue – drug binding
Binding involves weak bonds and thus reversible	Binding generally involves strong and covalent bonds and thus irreversible
Drugs that bind to plasma proteins have small apparent volume of distribution	Drugs that bind to extravascular tissues have large apparent volume of distribution
Half-life of plasma protein bound drug is relatively short	Half-life of extravascular tissue bound drug is relatively long
Does not result in toxicity	Tissue toxicity is common
Displacement from binding sites is possible by other drugs	Displacement by other drugs generally does not occur
Competition between drugs for binding to plasma protein can occur	Tissue-drug binding is generally non- competitive

Factors
affecting drug protein binding

1.  Factor relating to the drug

a) Physicochemical characteristic of drug

b) Concentration of drug in the body

c) Affinity of drug for a particular component

2. Factor relating to
the protein and other binding component

a) Physicochemical characteristic of the protein or binding
component

b) Concentration of protein or binding component

c) Num. of binding site on the binding site

3. Drug interaction

4. Patient related factor

Drug
related factors

Concentration of Drug
in the Body

• The extent of drug – protein binding can change with both
change in drug and protein concentration

• The conc. of drug that binding HSA does not have much of
an influence as the therapeutic concentration of any drug is insufficient to
saturate it

Ex: Therapeutic concentration of lidocaine can saturate AAG
with which it binding as the conc. of AAG is much less in comparison to that of
HSA in blood

Drug Protein / Tissue
Affinity

• Lidocaine have greater affinity for AAG than HSA

• Digoxin have greater affinity for protein of cardiac
muscle than skeleton muscles or plasma

Protein or
tissue related factor

Physicochemical property of protein / binding component

– Lipoprotein or adipose tissue tend to bind lipophilic drug
by dissolving them to lipid core

– The physiological pH determine the presence of anionic or
cationic group on the albumin molecule to bind a variety of drug

Concentration of
protein / binding component

• Mostly all drug bind to albumin as it presents a higher
concentration than other protein

Number of binding
sites on the protein

Albumin has a large number of binding site as compare to
other protein and is a high capacity binding component

• Several drug capable to binding at more than one binding
site

Examples: flucoxacillin, flurbiprofen, ketoprofen, tamoxifen
and dicoumarol bind to both primary and secondary site of albumin

• Indomethacin binds to three different sites

• AAG is a protein with limited binding capacity b/c of it
low – conc. and molecular size.

• The AAG has only one binding site for lidocaine , in
presence of HSA two binding site have been reported due to direct interaction
b/w them

Significance
of protein binding of drug

• Absorption

• Systemic solubility of drug

• Distribution

• Tissue binding, apparent volume of distribution and drug
storage

• Elimination

• Displacement interaction and toxicity

• Diagnosis

• Therapy and drug targeting

Absorption

• The binding of absorbed drug to plasma proteins decrease
free drug conc

• Thus sink condition and conc. gradient are established
acting as the driving force for further absorption.

Systemic
solubility of drug

• Water insoluble drugs, neutral endogenous macromolecules,
like heparin, steroids, and oil soluble vitamin are circulated and distributed
to tissue by binding especially to lipoprotein

• LP act as a vehicle for the circulation of such
hydrophobic drug compounds

Distribution

• The plasma protein-drug binding favours uniform
distribution of drug throughout the body by its buffer function

• A protein bound drug in particular does not cross the BBB,
placental barrier and the glomerulus

From the above
equation, it is clear that greater the unbound or free concentration of drug in
plasma, larger its Vd

Elimination

• The drug – protein complex cannot penetrate into the
liverthe chief metabolizing organ

• The larger molecular size also prevents it from getting
filtered through the glomerulus

• Only the unbound or free drug is capable of being
eliminated

• Drugs which are more than 95% bound are eliminated slowly,
i.e. they have long elimination half-live

Drug	Percentage binding	Elimination half- life
Tetracycline	65%	8.5h
Doxycycline	93%	15.1

• Exception: Penicillin is extensively bound but has short
elimination half live

• Reason: Rapid equilibrium is achieved between the free and
bound drug

Displacement
interaction and toxicity

Displacement
interaction and toxicity

1. Displacement of bilirubin from albumin by NSAID’S

2. Displacement of digoxin from its tissue (cardiac muscle)
binding site by quinidine

Digoxin has a high volume of distribution since it is
extensively bound to extravascular tissue

Quinidine can displace digoxin from its binding site,
resulting in high unbound drug concentration in tissues.

Volume of distribution

A. Drugs with large
volume of distribution like digoxin

• Even a substantial increase in the degree of displacement
of drug in plasma may not effect large increase in free drug concentration

Reasons:

i. Only a small fraction of such drug is present in plasma,
most of it is localized in extravascular tissues

ii. Following displacement, the free drug redistributes in
extravascular tissues

B. Drugs with small
volume of distribution

• Example- Warfarin, displacement can result in large
increase in free drug concentration in plasma

Diagnosis

• The chlorine atom of chlroroquine when replaced with
radiolabelled I-131 can be used to visualize melanomas of the eye since
chloroquine has a tendency to interact with the melanin of eyes

• The thyroid gland has a great affinity for iodine
containing compounds; hence any disorder of the same can be detected by tagging
such a compound with a radioisotope of iodine

Therapy and Drug
Targeting

• The binding of drugs to lipoproteins can be used for
site-specific delivery of drugs

• This is particularly useful in cancer therapies since
certain tumor cells have greater affinity for LDL than normal tissues.

• Thus binding of a suitable antineoplastic to it can be
used as a therapeutic tool

• Oestradiol binds selectively and strongly to prostrate and
thus prostate cancer can be treated by attaching nitrogen mustard to oestradiol
for targeting of prostate glands

Kinetics of
protein drug binding

…………………………………

Summary

• Drug binding components – blood components and
extravascular componets

• Blood – plasma proteins, cell components

• Tissue binding : liver > kidney > lung > muscle

• Plasma binding- reviersible, competitive, no toxicity

• Tissue binding – irreversible, non-competitive, toxicity

• Factors affecting protein binding Drug related, protein
related, interaction, disease related

• Protein binding has effect on absorption, distribution,
metabolism and drug excretion

• Displacement of drugs with large Vd  no significant
effect of displacement

• Displacement of drugs with small Vd  significant increase
of free plasma drug concentration

• Protein binding has applications in diagnosis and drug
therapy

• Measurement of protein binding – Direct plot, Double
reciprocal plot, Scatchard plot, Hitchcock plot

• Protein binding has effect on absorption, distribution,
metabolism and drug excretion

• Displacement of drugs with large Vd à no significant effect
of displacement

• Displacement of drugs with small Vd à significant increase
of free plasma drug concentration

• Protein binding has applications in diagnosis and drug therapy

• Measurement of protein binding – Direct plot, Double
reciprocal plot, Scatchard plot, Hitchcock plot

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