pIC50 and pAC50 are used to represent an Inhibition and Activation measure in log units for titration assays. RxPlora uses the term pIC50 even if it is an activation assay.

pIC50 = -log(IC50). IC50 represents the compound/substance concentration required for 50% inhibition. Thus the larger the pIC50 the more potent the compound. A pIC50 of 4 indicates an IC50 of 10^4. A commonly used cut-off for defining potent compounds is a pIC50 >= 6 or a compound with submicromolar potency. We calculate pIC50 as follows. Assuming that you ran a concentration curve study where X is the logarithm of concentration and Y is the response; Then,

   Y=Bottom + (Top-Bottom)/(1+10^((pIC50-X)*HillSlope))

Y starts at Bottom and goes to Top with a sigmoid shape. This is identical to the “four parameter logistic equation.”

To learn more on the variable slope sigmoidal titration equation, visit the GraphPad website.