In pharmacology, the term mechanism of action (MOA) refers to the specific biochemical interaction through which a drug substance produces its pharmacological effect. A mechanism of action usually includes mention of the specific molecular targets to which the drug binds, such as an enzyme or receptor.

1. Mechanism Of Drug Action
Present By-
Mr. Dnyaneshwar Balu Gutale
M. Pharmacy (Pharmacology)
Vidyabharti College of Pharmacy Amravati

2. Introduction
• A drug may produce its effects by a number of
mechanisms like combination with enzymes, cell
membrane or other components of a cell.
• The action either be physical or chemical and are
classified at four levels:
• Molecular- Receptor, ion channel, enzyme, carrier
molecules
• Cellular-Transduction e.g., G protein, ion channel,
enzyme
• Tissue- Contraction, secretion, metabolic activity,
proliferationSystem-CNS, CVS, etc.

3. Types of effects
Local Effect
When the drug effect occurs in immediate vicinity of application, this is
known as local effect.
Systemic Effect
When the drug effect occurs away from the site of administration, this
is known as systemic effect.
Primary effect
Primary effect is the effect for which the drug is administered or the
treatment of the disease for which the drug is given
Side effect
All other effects occurring in addition are known as secondary effects.
When the secondary effects are undesirable by the Patient, these are
called adverse effects or side effects.

4. Mechanisms
Most drugs act by altering the various body control systems, which may
be receptors, enzymes or ion channels.
These various mechanisms include:
1. Physical mechanisms
2. Chemical mechanisms
3. Drug-receptor interactions
4. Drug- enzyme interactions
5. Drug-channel interactions
6. Miscellaneous mechanisms

5. Mechanism of Drug Action – Physical Mechanisms
• When the drug does not produce any chemical reaction or change in
the cells of the body and the effect is only physical, the mechanisms
involved are called physical mechanisms.
• These includes:
• Local Application- Emollients, Counter irritants, Massagers, pastes
• Physically acting antacids- Milk, Mucin
• Bulk forming purgatives- Bran, Ispaghula, Magnessium sulphate

6. Mechanism of Drug Action – Chemical
Mechanisms
In chemical mechanisms, drugs act by producing chemical reactions in
the body.
These include:
•Chemically acting antacids- NaHCO3
• Chelating agents- Dimercaprol, penicillamine, desferrioxamine

7. Mechanism of Drug Action –Drug
Receptor Interaction
Receptor:
Macromolecules protein in nature which are target sites for
drugs.
Most drugs have to bind receptors to produce effects.
Receptors are located mostly on the cell membrane but certain
intracellular receptors are found as well.

8. • The binding of drug with receptor may by of two types:
• Reversible binding
• Irreversible binding
• In reversible binding, the bond between the drug and receptor is very
weak ionic, hydrogen or van der wall. This the effect is short lived.
• In irreversible binding, very strong covalent bonds are present, which
prolongs the effects of drug. The effect continues until the drug is
excreted or new receptor is generated.
There are three forms of binding to receptors:
• Agonists
• Antagonists
• Partial agonists

9. Ligands
• Ligands are the endogenous substances, molecules or
compounds which bind with receptors present in the body.
• E.g. acetyl choline, adrenaline, noradrenalin,
neurotransmitters like glutamate, aspartate and GABA.
• They produce various effects and interfere with the flow of
ions through channels called ligand gated channels.
• Their action may:
• Resemble with natural ligand
• Block the natural ligand

10. Agonists
• They are the drugs which when bind receptors, cause
activation of receptors.
• They have the capacity to produce chain reactions in the
receptors which ultimately bring about the effects.
• Agonists have two properties:
1. Affinity for receptor
2. Capability to produce chain reactions in the cells having
capability of intrinsic activity or efficacy

11. • When drug binds the receptor, activation occurs, and the response
gradually increases until it reaches the peak, then the response
decreases, although agonist still binds.
• This is due to desensitization of receptors, which might be overcome
by removing the agonists from the receptor.
• Example includes suxamethonium (succinyl choline), which produces
relaxation of skeletal muscles

12. • Prolong contact of tissues with the agonists results in decreased
number of receptors in the tissues called down regulation of
receptors.
• Example includes the patients suffering from bronchial asthma, in
whom by prolong usage of beta agonists down regulation occurs.
• This the effect is reduced.

13. Antagonists
• A substance which interferes with or inhibits the physiological action
of another.
• They have two properties:
• Affinity
• Do not have efficacy or intrinsic activity
• Examples include atropine, which is antagonist of acetyl choline.
Propanolol is antagonist of beta receptors.

14. • The binding of antagonist with receptor is of two types:
• Reversible binding
• Irreversible binding
• Reversible binding is also known as competitive antagonism. E.g.
atropine.
• Irreversible antagonism occurs when binding effect of antagonist is
prolonged until drug is excreted or new receptor is generated.
• Example includes phenoxy benzamine, which non- competitively
blocks action of catecholamines at beta receptors.
• Second generation H1 histamines are also non- competitive blockers.

15. Mechanism of Drug Action –Drug Enzyme
Interactions
• Drug enzyme interaction is similar to drug receptor interactions.
• The drugs resemble the natural substrates, bind enzymes and cause
change in their activity.
• This may take place by:
• Activation of enzymes
• Inhibition of enzymes

16. • This combination of drugs with the enzyme may be:
• Competitive
• Non competitive
Non competitive response is irreversible until new enzyme is generated

17. Non Competitive Inhibition
• The effects of non competitive inhibition are prolonged. These
include:
• Irreversible anticholinestrases e.g., Organophosphate compounds
• Aspirin
• MAO Inhibitors e.g., Iproniazid, Phenelzine
•
• Proton Pump Inhibitors e.g., Omeprazole, Esomeprazole

18. Mechanism of Drug Action –Drug Channel
Interactions
• In drug channel interaction, drug interfere with the flow of ions
through the channels specific for these ions.
• These include the Na+, K+,Ca++ and Cl- channels. Examples include:
• Sodium Channels:- Quindine Procainamide, Local anesthetic
• Calcium channels: Nifedipine Verapamil, Diltiazem
• Potassium Channels:- Amiodarone, sulfonylureas
• Chloride Channels:- Benzodiazepines