pa2 and pd2 values
TRANSCRIPT
PA2 and PD2 VALUES
Dr.Shamshi AzmiJRDept. of Pharmacology
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.
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.
Competitive antagonism
• Antagonist competes with the agonist for the same binding site on
receptor.
Reversible competitive antagonism• Here binding between antagonist and receptor is reversible.
• Antagonism can be overcome by increasing concentration of the
agonist and vice-versa.
• There is parallel shift of dose response curve to right with increasing concentration of antagonist but maximal response remains same.
Dose Ratio(r)
Emax
response
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.
• Higher the dose ratio more specific is the antagonist.
• This “r” can be found out from the extent of the rightward shift of
DRC.
• Dose ratio(r) = EC’50/EC50
• EC’50 means the dose of agonist at which 50% of effect is produced
in presence of antagonist.
Irreversible competitive antagonist
• Antagonist binds to receptor irreversibly by forming covalent
bond.
• As the dose of antagonist is increased, the slope as well as
maximal response of the agonist decreases.
• With sufficiently high dose of antagonist, no amount of agonist
will produce response.
response
Non competitive antagonist
• Antagonist binds to different site of the receptor such that
agonist is unable to combine with the receptor or is unable to
produce response.
• Its effect is similar to irreversible competitive antagonist, where
antagonist cannot be displaced even with higher concentration
of agonist.
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.
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)
Experimental estimation of drug antagonism
Antagonism of drug induced contractions are of two types-
• Preventive: antagonist is first added into the bath to
prevent the effect of agonist added subsequently.
• Curative: effect (here contractions) is first produced by
the agonist and then, antagonist is added to counter it.
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
Parallel shift of DRC to right
• 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.
• The extent of the rightward shift tells us by what ratio we have
to increase the dose of agonist to get the same effect.
Dose Ratio(r)
Emax
response
Double reciprocal plot of Lineweaver and Burk (Chen and Russell,1950)
• If the points lie on straight line and if the stragiht lines
determined in the presence and absence of antagonist
intersect on the line corresponding to infinite dose then
the antagonism is said to be competitive.
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
Method of determination of PA2
• Organ bath is connected with two bottles one containing only
physiological salt solution and other containing antagonist
solution.
• A number of sub maximal
contractions are first obtained
with an agonist at regular
intervals till a constant
response is produced.
• Then organ bath is filled with
solution containing antagonist
and dose of agonist is doubled
in its presence
• The response some times increases initially but gradually
diminishes and reaches steady level.
• Concentration of antagonist that produces a response slightly
greater than the original response to single dose of agonist is
first used.
• Then the bath is connected to the bottle containing the salt
solution to allow for complete recovery of its effect.
• The assay is concluded by producing series of maximal
contractions.
• Then the experiment is repeated with the concentration of
antagonist that produces slightly smaller response then the
original response to a single dose of agonist.
Plotting of pA2 values
• The contraction height of the single dose of agonist in the
absence of antagonist and contraction height of double dose
agonist following two concentrations of antagonist are
expressed as percentage of maximal contraction .
• Later, two percentage contraction are then plotted against the
two doses of antagonist on log scale.
• The concentration of antagonist that produces response equal
to single dose of agonist is found by interpolation.
Schild’s plot
• Most commonly used method for estimating pA2 value
• Let’s say: Drug A is an agonist; drug B is an competitive antagonist
• Xb :concentration of antagonist used
• Kb : equilibrium constant for the antagonist
• Addition of B will shift the DRC 0f A towards right
• The extent of shift / dose ratio(r ) =(Xb/Kb) +1
• “r”will help in determining Kb for B
Expressing this logarithmically, we get schild’s equation
• Log( r-1) = log Xb- log Kb
• Very simple & useful equation
Thus, plot of log(r-1) on the y-axis and –logXb on the x–axis gives
us SCHILD’S PLOT
pA2 value is directly read out at the point where line intersects x-axis at zero level
of y-axis
Intercept (E)- pA2 value for the antagonist
Applications of pA2 measurements
• Principle advantage: gives us simple numbers
• Helps us differentiate between competitive & non competitive
antagonism.
• Drugs that act on the same receptors can be expected to be antagonized
by the same concentration of a competitive antagonist, even though they
themselves may differ considerably in activity.
• If the given agonist-antagonist pair gives similar pAx values on different
preparations, it is suggestive that receptors are identical.
• On other hand, different pAx values are indicative of different receptors.
pA scale
High values: high specificity
Horizontal lines: non specific antagonism
Steep lines: highly specific
antagonists
pD2’ value
• Measures the affinity of a 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.
• Determined by the equation
pD2’= pDx + log[ (E1\E2) – 1]
where,
• pDx is negative molar concentration of the antagonist
employed.
• E1 and E2 are the maximal contraction heights in the absence
and presence of antagonist respectively.
• Pharmacological drug antagonism can be assessed and
measured practically by different methods.
• Most commonly used methods : pA2 value estimation and
Schild’s plot.
• Dose ratio is the most important factor which needs to be
determined in drug antagonism.
Thank you…