1. Bioassay of
Antagonist
Presenter-
Dr Arun Singh
Senior Resident
Department of Pharmacology
SMS Medical College, Jaipur

2. Overview
Introduction
Types of Antagonist
Schild Plot Method, pA2 and pA10 values
Parallel Shift
Lineweaver-Burk Plot

3. Introduction
• The term antagonist refers to any drug which can completely or
partially block a response of an agonist.
• So, an antagonist is the opposite of an agonist which stimulates an
action and these two are considered as a prime agent in
pharmacology.
• Antagonist activity may be reversible or irreversible depending on
antagonist–receptor complex which in turn depends on the nature of
antagonist receptor binding.

4. • When assessing an antagonist, the following points should kept in mind such
as,
a. To check whether the antagonism is surmountable by increasing the
concentration of agonist or not
b. To check whether the antagonism is reversible or not (does agonist regain
response, after washing of antagonist)
Note: This is due to identification of the type of antagonism. Such as, if an
antagonist is surmountable or reversible, it is likely to be competitive.

5. • Experimentally antagonist is divided into the two groups:
1. Preventive
When the antagonist used before addition of agonist, it is called the
preventive antagonist
2. Curative
In this process of antagonism, first agonist and then antagonist is added In
pharmacology, several different types of antagonist are described such as
Competitive antagonist, Non-competitive antagonist, Physiological
(functional) antagonist, Pharmacokinetic antagonist and Chemical
antagonist.

6. (I) Competitive antagonist
• Antagonists bind to the receptor of agonist but show no efficacy, e.g.
propranolol, naloxone, etc.)

7. Fig. 2.22: (A)
Cumulative plot
showing competitive
antagonism (parallel
shift) in the
presence of
antagonism

8. • Fig. 2.22: (C) Competitive antagonism
response plot, showing a partial response
block of an agonist which is regained by the
repeated tissue washing

9. (II) Non-competitive antagonist
• Antagonists bind to the receptor at
sites which is not related to the
agonist binding site for example
Ca2+ blockers.

10. • Fig. 2.22: (B) Cumulative plot of non-
competitive antagonism in the presence of
antagonist

11. (III) Physiological (functional) antagonist
• Antagonist has the opposite biological
action of the agonist, by action on a
different receptor, e.g.: salbutamol a
β2-adrenoceptor agonist given in acute
asthmatic attack antagonizes the
bronchoconstrictor action.

12. (IV) Pharmacokinetic antagonist
• It reduces the free blood concentration of drug at its
target site either by reducing drug absorption or
accelerating renal or hepatic elimination.

13. (V) Chemical antagonist
• Particular drug may antagonize the action of a second
drug by binding and inactivating the second drug.
• For example protamine binds to heparin and makes it
unavailable for interactions with proteins involved in
the formation of blood clots.

14. But, in the practical
pharmacology, the
identification of
antagonist is limited to
the competitive or
noncompetitive or
physiological antagonists
only.
For analyzing the
antagonist, a dose-
response curve of agonist
is made in the absence and
presence of a fixed
concentration of
antagonist, which shifts
the DRC to the right
(parallel shift), with the
same maximum response
and the same shape; for
the competitive
antagonist.

15. Antagonist has no effect of its own, so, it is determined
experimentally by the intensity to block the agonist activity.
The process of dose determination is same as it is with the agonist.
First, the agonist response is obtained thereafter percentage
inhibition is determined by using the different doses of antagonist.
Sometimes, percentage inhibition is converted into the probit scale
against the log dose to make graph more linear.

17. • As a general rule antagonist takes a
long-time to block an action of
agonist.
• In the experimental purpose, the use
of antagonist concentration is around
10 times less than the agonist
concentration, i.e. if agonist used in
the concentration of 1 X 10-9 M then
the concentration of antagonist used
in the same experiment will be 1 X
10–10 M.
• The exposure time of antagonist to
tissue should be around 15min.

18. But in the routine experiment it is
also seen that 3-5 min contact
time of antagonist with tissue can
also elicited the result.
Hence, various methods are
employed for the identification of
competitive antagonist, 1. Schild
plot method and pA2, pA10
values 2. Parallel shift of DRC 3.
Double reciprocal plot
Lineweaver and Burk

19. Schild Plot
Method and
pA2, pA10
Values
• Generally, potencies of competitive
antagonist are expressed as pA2 values or
pAx values.
• Hence the term, pA2 is defined as the
negative logarithm to base 10 of the
antagonist concentration in molar units
corresponding to a dose-ratio of 2, i.e. the
concentration that produces a 2-fold shift in
the response of agonist concentration-
response curve.

20. • Kd (the dissociation constant of the
antagonist for the receptor) is the other
form of expressing the potencies of
competitive antagonists.
• Procedures (both design and model) for
estimating Kd and pA2 have also been
developed from the results of an antagonist
inhibition curve in the presence of a fixed
concentration of the agonist.

21. • Suppose, the antagonist is of competitive type, then the dose ratio will be
expressed as,
Dose ratio (d) = 1+ (antagonist)/(Kd)
d-1 = (antagonist)/(Kd)
log (d-1)= log [(antagonist)/(Kd)]
log (d-1)= log(antagonist)- log(Kd)
Note: If the antagonist is competitive, then slope will be 1.0 and the X-
intercept and Y-intercept will both be equal to the Kd of the antagonist.

23. The other formula to identify the competitive inhibition is by the difference of
pA2 and pA10 (pA2 – pA10).
If the difference (pA2 – pA10) is 0.95 or in the ranges of 0.8 - 1.20, the
inhibition is competitive. (pA2 and pA10, where 2 and 10 is the dose ratio)
The limitation of the above mentioned methods is that no one applies to
evaluate antagonist potencies for compounds that are not pure antagonist,
i.e. partial and inverse agonists and antagonists lacking intrinsic activity.

24. • So, in these cases Waud Model is applied for the estimation of pA2, Kd, and
IC50.
• Waud Method requires fewer experimental units compared to the Schild
method to plot a graph.
• Once the experiments are completed a series of dose ratios (DR) are
calculated for observed responses.
• For example, the ratio of the dose of agonist (A’) to produce a specific
effect (e.g. half maximal effect) in the presence of the antagonist (B) to the
dose required in the absence of the antagonist (A) is calculated.

25. Parallel Shift Plot
• It is the simplest form of experimental approach of identifying an
antagonist in which the shift of DRC is parallel towards right after
addition of competitive antagonist is seen.

27. Lineweaver-Burk Plot or Woolf-LineweaverBurk Plot/graph
• This method was developed by Hans Lineweaver and Dean Burk in 1934 for
the assessment of enzyme kinetics.
• Lineweaver-Burk plot is inverse of response plotted against the inverse of
dose concentration of agonist, which was invented to analyze how fast a
drug can produce its response, against a present antagonist.
• This method applied to distinguish between competitive, noncompetitive
and uncompetitive antagonists.

29. Findings of Different Antagonist -
• Competitive inhibition: Have the same y-intercept as uninhibited
(since Vmax is unaffected by competitive inhibitors the inverse of
Vmax also doesn’t change) but there are different slopes and x-
intercepts between the two data sets.
• Noncompetitive inhibition: Produces plots with the same x-intercept
as uninhibited (Km is unaffected) but different slopes and yintercepts.
• Uncompetitive inhibition: Causes different intercepts on both the y-
and x-axes but the same slope.