1.
Receptor Drug Intearction
(Agonist and Antagonist)
Prof. Shaikh Abusufiyan
Assistant Professor,
AIKTC-School of Pharmacy,
New Panvel-410206
Pharma Learning Forever

2.
At the end of this e-learning session you are able to…
A. Discuss interaction of agonist
and antagonist with receptors

3.
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.

4.
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.

5.
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.

6.
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.

7.
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.

8.
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.

9.
l It has also been demonstrated that many
full agonists can produce maximal
response even while occupying <7 % of
the available R.

10.
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 _______________.

11.
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.

12.
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).

13.
l The inverse agonist (D) has high affinity for
the Ri state
opposite response

14.
l The competitive antagonist (B) binds to
Ra and Rl with equal affinity - the
equilibrium is not altered
no response is generated

15.
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.

16.
l Receptor antagonism is of 2 types:
- competitive
- or non-competitive.

17.
A. Competitive antagonism (equilibrium
type)
l The antagonist is chemically similar to the
agonist
l Competes with it and binds to the same site

18.
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.

19.
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

20.
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.

21.
l Noncompetitive antagonists have been
produced experimentally, but are not in
clinical use.

22.
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

23.
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

24.
Reference:
K D Tripathi. Essentials of Medical Pharmacology. Seventh Edition. Jaypee
Publication. Page no:11-15.

25.
Disclaimer (Images)
The images used in this presentation are found from different sources all
over the Internet, and are assumed to be in public domain and are displayed
under the fair use principle for education purpose.
Copyright @ Presentation
The said presentation is copyright under Copyright @shaikhabusufiyan2022
The presentation is for education purpose only, don’t use the same for any
legal perspective.

26.
Copyright @shaikhabusufiyan2022