This article throws light upon the four main terms related to agonist-antagonist interactions. They are: 1. PKX 2. PAX 3. PD’x 4. PDX.
Term # 1. PKX:
This is an expression for affinity and is defined as negative logarithm of the molar concentration of a drug that elicits an effect representing x% of the maximal response the drug can produce. Usually PK50 is taken for routine calculation.
Term # 2. PAX:
It is the negative logarithm of the molar concentration of antagonist in the presence of which the potency of the agonist is reduced by X times; that is, X times more agonist is required to produce a given response in the presence of than in absence of the antagonist. Generally PA2 value is taken, but sometimes PA10 is also considered.
A method of testing competitive or non-competitive nature of an antagonist is to determine both PA2 and PA10 values for agonist-antagonist pair on the same tissue. If the difference between these two values is to be 0.95 or between 0.8-1.2 then the antagonism is likely to be competitive.
Term # 3. PD’x:
The PA2 is a measure of the affinity of a reversible competitive antagonist for a specific receptor, PD2 is a measure of the affinity of a reversible non-competitive as well as an irreversible competitive antagonist for a specific receptor.
PD’2 is defined as the negative logarithm of the molar concentration of noncompetitive antagonist which will reduce the effect of an agonist to 1/x of its maximum (50% if it is PD’2).
PD’2 value is determined as per the following equation:
Where PD’X is the negative logarithm of the molar concentration of the antagonist employed.
EAM and EABM are the maximal response in the absence and in the presence of the antagonist, respectively.
Term # 4. PDX:
It is defined as the negative logarithm of the molar concentration of the agonist which produces an effect equal to 1/x that of the maximum response. This value is directly proportional to the logarithm of the affinity of an agonist for its receptors, because the concentration of agonist required to produce half its max. effect is inversely related to the drugs affinity.