pA ₂ value

In pharmacology, the p A ₂ value denotes the negative decadic logarithm of the concentration of an inhibiting active ingredient ( antagonist ) at which the concentration of an activating active ingredient ( agonist ) would have to be doubled in order to observe the same effect as in the absence of an antagonist:

${\displaystyle \operatorname {p} A_{2}=-\log \,[Antagonist]+\log \,(r-1)}$, in which

${\displaystyle r={\frac {EC_{50}(+Antagonist)}{EC_{50}(-Antagonist)}}}$

The p A ₂ value was introduced by Heinz Otto Schild . It can be determined with the help of the Schild plot and is a measure of the affinity of an antagonist to the receptor. The p A ₂ value corresponds approximately to the dissociation constant p K _{D of} an antagonist.

swell

1. ^ Shield HO (1947). PA, a new scale for the measurement of drug antagonism. Br. J. Pharmacol. 2: 189-206.