Do differences in cell lines and methods used for calculation of IC50 values influence categorisation of drugs as P-glycoprotein substrates and inhibitors?

In vitro bidirectional assays are employed to determine whether a drug is a substrate and/or inhibitor of P-glycoprotein (P-gp) transport. Differences between cell lines and calculation methods can lead to variations in the determination of efflux ratios (ER) and IC50 values used to classify a drug as a P-gp substrate and inhibitor, respectively.Information was collected from the literature on ER and IC50 values with digoxin as the probe substrate using different cell lines and inhibition calculation methods. Predictive performance was evaluated by comparing [Igut]/IC50 ratios versus reported in vivo results.For known P-gp substrates, 50% of the drugs had their highest ER value in MDCK-MDR1 cells while 81% had their lowest ER value in Caco-2 cells. For 30 drugs with inhibition data, lower mean IC50 values were often observed with the Caco-2 cells and calculations based on ER. Based on the cut-off criteria of [Igut]/IC50 ≥ 10, there were no significant differences in positive or negative predictive values based on either cell line or calculation method for the drugs.Within this limited dataset, differences between cell lines or IC50 calculation methods do not seem to impact the prediction of in vivo P-gp inhibitor classification.