3. Introduction
In 1947, Sir Heinz Otto Schild devised a scale ,
known as pA scale, to express drug antagonism.
Developed methods for assessing & measuring drug
antagonism, like pA2 measure and Schild’s Plot.
Introduced the use of term Dose Ratio.
4. Drug antagonism
•An antagonist is the drug which completely or
partially blocks the effect of agonist in its presence.
•Antagonist have only affinity but no intrinsic activity
or efficacy, i.e. efficacy is zero & affinity is one.
5.
6.
7. •There is parallel shift of dose response curve to right with
increasing concentration of antagonist but maximal
response remains same.
Dose
Ratio(r)
Emax
r
e
s
p
o
n
s
e
8. • A characteristic difference between competitive and
noncompetitive antagonists is that competitive antagonists
reduce agonist potency , whereas noncompetitive
antagonists
reduce agonist efficacy.
9.
10. Dose Ratio
•The dose ratio(r) is the factor by which the concentration
of the agonist has to be multiplied to produce a given
response in presence of antagonist.
•This “r” can be found out from the extent of the
rightward shift of DRC.
11. •Dose Ratio(r) =
𝐸𝐶 50 𝑎𝑓𝑡𝑒𝑟 𝑎𝑛𝑡𝑎𝑔𝑜𝑛𝑖𝑠𝑡
𝐸𝐶 50 𝑏𝑒𝑓𝑜𝑟𝑒 𝑎𝑛𝑡𝑎𝑔𝑜𝑛𝑖𝑠𝑡
•EC50 concentration of a drug required to
produce a response that is 50% of the maximum
response achievable.
•Higher the Dose Ratio, more specific is the antagonist.
12. pAx Value
pA scale proposed by Schild to express drug antagonism.
pAx: x denotes the number by which the agonist dose has
to be increased to get the effect of single dose in the
presence of antagonist.
x can be 2 or 10( pA2 and pA10).
13. pA2 value
pA2 is the measure of the affinity of a reversible
competitive antagonist for a specific receptor.
Defined as the negative log of molar concentration of
the antagonist which will reduce the effect of double
dose of the agonist drug to that of a single dose in the
absence of antagonist.
14. Example
•Drug with concentration A 0.4 ml response
recorded 16 mm
•Drug with concentration 2A 0.8 ml response
recorded 28 mm
•As per definition, pA2
Negative log of molar concentration of antagonist
required to reduce response of 2A A.
15. Methods used for determination of
types of drug antagonism
Parallel shift of DRC to the right
Double reciprocal plot of Lineweaver and
Burk
Difference between pA2 and pA10 values
Schild’s plot
16. Parallel Shift Plot
•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.
•If the DRC of agonist shifts to the right without any
depression of maximal response following an
antagonist, it is likely that the antagonist is of a
competitive nature.
•However, it needs further confirmation.
17. Parallel shift of DRC to right
• The extent of the rightward shift tells us by what ratio
we have to increase the dose of agonist to get the
same effect.
18. 2) Lineweaver-Burk Plot or Woolf-
Lineweaver-Burk Plot/graph
•This method was developed by Hans Lineweaver and
Dean Burk in 1934 for the assessment of enzyme
kinetics.
•Inverse response is 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 is applied to distinguish between
competitive and noncompetitive antagonists.
19. Schild plot
• Most commonly used method for estimating pA2 values for
pharmacological competitive antagonists is to plot (DR-1)
against negative log molar concentration of the antagonist.
• DR agonist dose ratio
• When the schild plot gives a statistically acceptable straight
line with a slope, then the antagonism is competitive.
20. Steps for schild plot
pA2 value is directly read out at the point where the line intersects
the abscissa at the zero level of the ordinate.
Plot log (DR-1) as ordinate against either log molar concentration
or against negative log molar concentration of antagonist.
Determine the dose ratio from these curves for each
concentration of antagonist.
Plot log dose response curve in the absence and presence of
antagonist.
21. The pA2 value is read off the graph at the intercept of the line with x when
log (DR – 1) = 0.
The pA2 value does not have any units.
Schild plot
22. Difference between pA2 & pA10
values
• By this method competitive and non competitive nature of
antagonist can be determined.
• Both pA2 and pA10 values for agonist- antagonist pair is
determined on the same tissue.
• If the difference between them is approximately 0.95 (0.8-
1.2): Competitive antagonism
23. Calculate pA2 value of Prazosin using
Noradrenaline as agonist in rat
anococcygeus muscle preparation
Record the Graded responses of
noradrenaline in a geometric progression on
the rat anococcygeus muscle preparation till
getting the maximal effect.
Select 2 doses of noradrenaline from the
linear range of the DRC i.e A and 2A and
eliciting submaximal responses R1 and R2
respectively.
Consider the response(R2) due to double the
dose of noradrenaline(2A) as 100%.Determine
the corresponding percentage of
response(R1)for the single dose of
noradrenaline.
24. Add prazosin, 10 times lesser concentration
than 2A dose of noradrenaline (B1),then
record the response of 2A in presence of
prazosin and percentage inhibition of
response is determined.
Repeat earlier step to record the
responses of 2A in the presence of
increased molar concentrations of
prazosin(B2,B3,B4 etc).
Determine the corresponding
percentage inhibition of response of
2A in the presence of different
concentration of Prazosin.
Plot a graph representing negative log of
molar concentration of prazosin in X-axis and
% response to the double dose of
noradrenaline(2A) in absence and presence of
prazosin on Y-axis.
25.
26.
27.
28. Applications of pA2
1. Quantifying Antagonist Potency:
• The pA2 value provides a measure of how potent a
competitive antagonist is in blocking the effects of an agonist.
• Higher pA2 values indicate greater potency in inhibiting the
agonist's effects.
2. Comparing Antagonists:
• pA2 values allow for a direct comparison of different
antagonists targeting the same receptor.
• Antagonists with higher pA2 values are more effective at
blocking the receptor.
29. 3. Understanding Mechanisms of Antagonism:
• By comparing pA2 values of various antagonists, insights
into the mechanisms of action and binding affinities of
these compounds at specific receptors can be gained.
4. Predicting Clinical Effects:
• The pA2 value can aid in predicting the clinical efficacy of
competitive antagonists.
• Higher pA2 values suggest that lower doses of the
antagonist might be required for therapeutic effects.
5. Selecting Antagonists for Specific Situations:
• When designing treatment regimens, selecting an
antagonist with an appropriate pA2 value can be crucial.
• For example, if rapid and reversible blockade is desired, an
antagonist with a lower pA2 value might be chosen.
30. 6. Studying Receptor Pharmacology:
• The pA2 value is widely used in receptor pharmacology
research to investigate the interactions between agonists
and competitive antagonists, providing insights into receptor
kinetics and binding properties.
7. Drug Development:
• Determining the pA2 value during the preclinical stage of
drug development helps in selecting lead compounds with
optimal antagonistic properties for further development.
8. Understanding Dose-Response Relationships:
The pA2 value contributes to understanding dose response
relationships by quantifying the concentration at which
antagonists effectively inhibit the agonist response.
31. pA2 scale
High values: high
specificity
Horizontal lines:
non specific
antagonism
Steep lines: highly
specific antagonists
pA2 values of four antagonists against histamine and acetylcholine as
measured on guinea pig ileum.
Points on the two scales referring to the same antagonist are joined.
32. pD2
• The pD2 value represents the negative logarithm of the
concentration of a drug required to produce 50% of the
maximum response (EC50).
• pD2=−log(EC50)
• pD2 value allows for a standardized comparison of the
potency of different drugs or the same drug under varying
conditions.
• By converting the EC50 values into logarithmic scale, it
becomes easier to compare the potencies of drugs with
different EC50 values.
33.
34. Applications of pD2:
Drug Development:
•Researchers use pD2 values to compare the potencies of
different compounds during drug development.
•This helps in selecting the most potent compounds for further
study.
Therapeutic Dosing:
• Understanding the pD2 value aids in determining appropriate
dosages of drugs for therapeutic use.
• Drugs with lower pD2 values require lower concentrations to
achieve the desired effect.
Drug Interaction Studies:
• pD2 values can be used to assess potential drug interactions.
•If two drugs have similar pD2 values, they might compete for the
same receptor or enzyme, potentially leading to interactions.
35. pD2’
• Measures the affinity of a reversible non competitive as well
as irreversible competitive antagonist for a specific receptor.
• Defined as negative logarithm of the molar concentration of
non competitive antagonist which will reduce the effect of
an agonist to one-half (50%) its maximum.
36. •Determined by the equation
pD2’= pDx + log[ (E AMEABM) – 1]
where,
•pDx is negative molar concentration of the antagonist
employed.
•EAM and EABM are the maximal contraction heights in the
absence and presence of antagonist respectively.
37. • measure of the affinity of a
reversible competitive antagonist.
pA2
• measure of the potency of agonist.
pD2
• Measures the affinity of a reversible
non competitive as well as
irreversible competitive antagonist.
pD2’
Defined as the negative log of molar concentration of the antagonist which will reduce the effect of double dose of the agonist drug to that of a single dose.
