A simple method for predicting serum protein binding of compounds from IC50 shift analysis for in vitro assays

The shift in apparent IC50 that attends addition of serum proteins to in vitro cellular, enzymatic, and receptor binding assays can be used to determine the dissociation constant for compound–serum protein complexes. We show here that a simple linear relationship exists between the apparent IC50 in the presence of serum protein and the inverse of the apparent Kd for the compound–serum protein complex. Using a series of cell-active kinase inhibitors we demonstrate that the Kd value derived in this way can be used to predict the extent of protein binding in serum for various compounds. This method should provide a simple means of assessing the relative serum protein binding propensity of compounds early in the compound optimization phase of drug discovery campaigns.