bp/pk 520

1. PROTEIN BINDING
UNIT- 4 BIOPHARMACEUTICS AND PHARMACOKINETICS (PE- 520)
PRIYANSHA SINGH (M.S. PHARM- PHARMACOLOGY & TOXICOLOGY, NIPER GUWAHATI)

2. BASIC DIAGRAM OF PROTEIN BINDING

3. WHAT IS PROTEIN BINDING
 A drug in the body can interact with several tissue components of which the two major categories are
blood and extravascular tissues. The interacting molecules are generally the macromolecules such as
proteins, DNA or adipose.
 The proteins are particularly responsible for such an interaction. The phenomena of complex
formation with proteins is called as protein binding of drugs.
 The importance of such a binding derives from the fact that the bound drug is both
pharmacokinetically as well as pharmacodynamically inert i.e. a protein bound drug is neither
metabolized nor excreted nor is it pharmacologically active.
 A bound drug is also restricted since it remains confined to a particular tissue for which it has
greater affinity. Moreover, such a bound drug, because of its enormous size, cannot undergo
membrane transport and thus its half-life is increased.

4. MECHANISM BEHIND PROTEIN BINDING
 Binding of drugs generally involves weak chemical bonds such as hydrogen bonds, hydrophobic
bonds, ionic bonds or van der Waal 's forces and, therefore, is a reversible process.
 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; for example, covalent binding of chloroform and
paracetamol metabolites to liver results in hepatotoxicity.
 Binding of drugs falls into 2 classes:
1. Binding of drugs to blood components like-a) Plasma proteins & b) Blood cells
2. Binding of drugs to extravascular tissue proteins, fats, bones, etc.

5. BINDING OF DRUGS TO BLOOD COMPONENTS
Pharmacological class-
1. Imipramine- TCA
Antidepressant
2. Lidocaine- Local Anesthetic
(surface linked/soluble and
amide linked),
Antiarrhythmic (class 1B)
3. Quinidine- Class 1A
antiarrhythmic
4. Chlorpromazine-
phenothiazine antipsychotic
5. Cyanocobalamine- Vitamin
B12
6. Cupric ion= Cu+ ions

6. BINDING OF DRUGS TO BLOOD COMPONENTS
 The main interaction of drug, in the blood compartment is with the plasma proteins 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 drugs to various plasma proteins is:
albumin > alpha 1-acid glycoprotein > lipoproteins > globulins.
 A short concept- all proteins have some pockets/ specific sites in which only a specific
drug binds depending upon the molecular weight, lipophilicity and concentration of the
protein present.
 Now we will have a look at binding of drugs with
1. Human Serum Albumin (HSA)
2. Alpha 1- acid glycoprotein
3. Lipoproteins
4. Globulins
5. RBC components

7. A. BINDING OF DRUGS TO HUMAN SERUM ALBUMIN
 It is one of the most abundant plasma protein (upto 60%), having M.W. of 65k with large binding capacity.
 Both endogenous compounds such as fatty acids, bilirubin and tryptophan as well as drugs bind to HSA. A large variety of
drugs ranging from weak acids, neutral compounds to weak bases bind to HSA.
 A drug can bind to more than one site in which case the main binding site is called as the primary site and the other as the
secondary site; for example, site I is the primary site for dicoumarol and site II is then the secondary site
 Groups of drugs that bind to the same site, compete with each other for binding, but drugs that bind to one site do not
competitively inhibit binding of drugs to other sites. However, they may either promote or retard binding of a drug to another
site by energetic coupling mechanisms.
Pharmacological class
1. Warfarin- oral anticoagulant (coumarin derivatives)
2. Azapropazone- NSAID
3. Diazepam- Benzodiazepines (intermediate acting)
4. Digitoxin- Cardiac glycoside
5. Tamoxifen- SERM used against breast cancer

8. B. BINDING OF DRUGS WITH ALPHA 1 GLYCOPROTEIN
(Beta blocker)

9. C. BINDING TO LIPOPROTEINS
Why is HDL good
Pharmacological class
1. Diclofenac- NSAID (phenylacetic
acids- non selective COX inhibitor)
2. Cyclosporin A- immunosuppressant

10. D. BINDING TO GLOBULIN

11. E. BINDING OF DRUGS WITH RBC AND ITS COMPONENTS
Pharmacological class-
1. Phenytoin- antiepileptic
(hydantoin)
2. Pentobarbital-
Barbiturate (short acting)
3. Acetazolamide- Diuretics
(carbonic anhydrase
inhibitor)
4. Chlorthalidone- thiazide
diuretic

12. F. BINDING OF DRUGS TO TISSUE PROTEINS
 The body tissues, apart from HSA, comprise 40% of the body weight which is l00 times that of HSA.
Hence, tissue-drug binding is much more significant than thought to be.
 A drug can bind to one or more of the several tissue components. Tissue-drug binding is important in
distribution from two viewpoints: firstly, it increases the apparent volume of distribution of drugs in
contrast to plasma protein binding which decreases it; this is because the parameter is related to the
ratio of amount of drug in the body to the plasma concentration of free drug and the latter is decreased
under conditions of extensive tissue binding of drugs.
 Secondly, tissue-drug binding results in localization of a drug at a specific ·site in the body (with a
subsequent increase in biological half-life). This is more so because a number of drugs bind irreversibly
with the tissues (contrast to plasma protein-drug binding); for example, oxidation products of
paracetamol, phenacetin, chloroform, carbon tetrachloride and bromobenzene bind covalently to
peptic tissues.

13. F. BINDING OF DRUGS TO TISSUE PROTEINS
 Factors influencing localization of drugs in tissues include lipophilicity and structural features
of the drug, perfusion rate, pH differences, etc. extensive tissue-drug binding suggests that a
tissue can act as the storage site for drugs. Drug that binds to both tissue and plasma
components result in competition between drug binding sites.
 For majority of drugs that bind to extra vascular tissues, the order of binding is
 liver > kidney > lung > muscle.
 Several examples of extravascular tissue-drug binding are:
Paracetamol- acetaminophen NSAID

14. F. BINDING OF DRUGS TO TISSUE PROTEINS
Pharmacological classification
1. Metallothionin- cysteine-
rich, low molecular weight
proteins.
2. Chloroquine- antimalarial
and amebicides
3. Phenothiazine-
antipsychotic drugs
Tetracycline side effect-
fanconi syndrome,
discoloration of bone and
teeth
Aminoglycosides side effects-
ototoxicity, nephrotoxicity

16. COMPARISION BETWEEN PLASMA PROTEIN AND TISSUE PROTEIN
DRUG BINDING

17. FACTORS AFFECTING PROTEIN BINDING
 Factors relating to the drug:-
a. Physicochemical characteristics of the drug
b. Concentration of drug in the body
c. Affinity of a drug for a particular binding component
 Factors relating to the protein and other
binding components:-
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
 Drug interactions
a. Competition between drugs for the binding site
(displacement interactions)
b. Competition between drugs and normal body
constituents
c. Allosteric changes in protein molecule
 Patient related factors
a. Age
b. Inter-subject variations
c. Disease states

18. FACTORS RELATING TO THE DRUG:-

19. INFLUENCE OF PHYSICOCHEMICAL PROPERTIES OF DRUGS ON
PROTEIN BINDING

20. FACTORS RELATING TO THE PROTEIN AND OTHER BINDING
COMPONENTS:-

21. DRUG INTERACTIONS

22. AGE DIFFERENCES FROM NEONATES TO BABY

23. PATIENT RELATED FACTORS

24. KINETICS OF PROTEIN DRUG BINDING
Are drugs bound to
the protein in a
reversible or
irreversible or both
manners- majorly
drugs are reversibly
binding with drugs
which is why it shows
reversible kinetics
The drugs which show
irreversible binding are
totally out of the
question of being in
equilibrium with free
drug. It is an
irreversible equation
then.

25. KINETICS OF PROTEIN DRUG BINDING

26. KINETICS OF PROTEIN DRUG BINDING

27. KINETICS OF PROTEIN DRUG BINDING
How do we come to know by
looking at the equation 4.17 that r
has to be taken on x axis and D on y
axis?
R is a number basically
determining the ratio of no. of
moles of bound drug to the total
protein present. Therefore R is
supposed to be on Y axis

28. KINETICS OF PROTEIN DRUG BINDING

30. KINETICS OF PROTEIN DRUG BINDING

31. IMPLICATIONS OF PROTEIN BINDING ON PHARMACOKINETIC
PROPERTIES
Plasma and tissue protein binding of drugs is a major factor that affects both pharmacokinetics and
pharmacodynamics of the drug.
1. VOLUME OF DISTRIBUTION (Vd)- more the protein binding, lesser the volume of distribution and vice
versa
2. CLEARANCE- clearance again depends upon volume of distribution therefor more the protein binding,
lesser the clearance of the drug and vice versa
3. HALF LIFE- more the drug is bound to protein and especially to plasma protein, more will be the
residence time and hence the half life of the drug increases and vice versa.
4. BIOAVAILABILITY- more the drug is bound to plasma protein lesser it is pharmacologically active and hence
the bioavailability.
5. CONCENTRATION- alteration in the concentration of drug which is administered will directly affect the
protein binding of that drug. More protein binding with drug, lesser the amount of pharmacologically active
concentration of the drug.
6. DURATION OF DRUG- greater the amount of protein binding of a particular drug, longer is the half life and
hence longer is the duration of action (as well as its toxicity)

32. SIGNIFICANCE OF PROTEIN BINDING

33. SIGNIFICANCE OF PROTEIN BINDING