Agonists and Antagonist.pdf

Receptor Drug Intearction
(Agonist and Antagonist)
Prof. Shaikh Abusufiyan
Assistant Professor,
AIKTC-School of Pharmacy,
New Panvel-410206
Pharma Learning Forever

At the end of this e-learning session you are able to…
A. Discuss interaction of agonist
and antagonist with receptors

Receptor:
l It is defined as a macromolecule or binding
site located on the surface or inside the
effector cell
serves to recognize the signal
molecule/drug and initiate the response to it
but itself has no other function.

The following terms are used in describing
drug-receptor interaction:
Agonist:
l An agent which activates R to produce an
effect similar to that of the physiological
signal molecule.
lnverse agonist:
l An agent which activates a receptor to
produce an effect in the opposite direction
to that of the agonist.

Antagonist:
l An agent which prevents the action of an
agonist on a R or the subsequent
response, but does not have any effect of
its own.
Partial agonist:
l An agent which activates a R to produce
submaximal effect but antagonizes the
action of a full agonist.

Ligand (Latin: ligare- to bind):
l Any molecule which attaches selectively to
particular R or sites.
l The term only indicates affinity or binding
l Agonists and competitive antagonists are
both ligands of the same R.

Agonists
l Have both affinity and maximal intrinsic
activity (IA = 1) e.g. adrenaline, histamine,
morphine.
Competitive antagonists
l Have affinity but no intrinsic activity (IA =
0), e.g. atropine, chlorpheniramine,
naloxone.

l Partial agonists have affinity and
submaximal intrinsic activity (IA between 0
and 1) e.g. dichloroisoproterenol (on Beta
adrenergic R).
l lnverse agonists have affinity but intrinsic
activity with a minus sign (IA between 0
and - 1) e.g. on benzodiazepine R.

l It has also been demonstrated that many
full agonists can produce maximal
response even while occupying <7 % of
the available R.

Q&A
Q.1 What is differnce between agonist and antagonist?
Q.2 If, IA = 0. Is it agonist or antagonists?
Q.3 An agent which activates a receptor to produce
submaximal effect is called as _______________.

The two-state Receptor model OR Theory
l The Receptor is believed to exist in two
interchangeable states: Ra (active) and Rl
(inactive).
l In the case of majority of Receptor, the Rl
state is favoured at equilibrium
l No or very weak signal is generated in the
absence of the agonist:- the R exhibits no
constitutive activation.

l If an agonist has only slightly greater
affinity for Ra than for Ri
the equilibrium is only modestly shifted
towards Ra
Submaximal response is produced and the
drug is called a partial agonist (C).

l The inverse agonist (D) has high affinity for
the Ri state
opposite response

l The competitive antagonist (B) binds to
Ra and Rl with equal affinity - the
equilibrium is not altered
no response is generated

ANTAGONISM
l When one drug decreases or abolishes the
action of another they are said to be
antagonistic
l Usually in an antagonistic pair one drug is
inactive as such but decreases the effect
of the other.

l Receptor antagonism is of 2 types:
- competitive
- or non-competitive.

A. Competitive antagonism (equilibrium
type)
l The antagonist is chemically similar to the
agonist
l Competes with it and binds to the same site

l Because the antagonist has affinity but no
intrinsic activity
No response is produced and the log DRC of the
agonistis shifted to the right.
l Since antagonist binding is reversible and
depends on the relative concentration
Higher concentration of the agonist progressively
overcomes the blocking effect of antagonist.

Non-competitive antagonism:
l The antagonist is chemically unrelated to
the agonist
l Binds to a different allosteric site altering
the R in such a way that it is
-unable to combine with the agonist.
-or unable to transduce the response

l Because the agonist and the antagonist are
combining with different sites
High agonist concentration is unable to
reverse the block completely.
l Increasing concentrations of the antagonist
progressively flatten the agonist DRC.

l Noncompetitive antagonists have been
produced experimentally, but are not in
clinical use.

Features of competitive and noncompetitive
antagonism are compared below:
q Competative (Equilibrium type)
q Antagonist Bind with the same
receptor as agonist
q Antagonist chemically resemble
with the agonist
q Parallel rightward shift of agonist
DRC
q Maximum response can be
attained by increasing dose of
agonist
q Intensity of action is depends on
the concentration of both agonist
and antagonist
q Eg. Acetylcholine and atropine
q Non-competative
q Bind to another site of the
receptors
q Does not resemble
q Flattening of agonist DRC
q Maximum response is suppressed
q Maximum response is only depend
on the concentration of
antagonist
q Eg. Diazepam and Bicucullin

Q&A
Q.1 True or false. Higher concentration of the agonist
progressively overcomes the blocking effect of non-cometative
antagonist.
Q.2 Which type of antagonist resemble agonist?
Q.3 Identify type of antagonism where Maximum response is
only depend on the concentration of antagonist

Reference:
K D Tripathi. Essentials of Medical Pharmacology. Seventh Edition. Jaypee
Publication. Page no:11-15.

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Copyright @ Presentation
The said presentation is copyright under Copyright @shaikhabusufiyan2022
The presentation is for education purpose only, don’t use the same for any
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Agonists and Antagonist.pdf

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