The P-gp-Glo Assay System with P-gp (Promega Italia, Milan, Italy) kit was used to determine the P-gp ATPase activity in the presence of the selected compounds. This assay uses isolate membranes containing human recombinant P-gp and is suitable to screen compounds for P-gp activation or inhibition in a high-throughput mode . Half-maximal inhibitory concentration (IC50) was determined by using increasing concentrations of the compounds (0.78-50 µM) in the presence of verapamil (50 µM), used as an ATPase stimulator. The reduction in ATPase activity compared to verapamil was measured by recording the firefly luciferase-generated luminescent signal, using a Victor Nivo multimode plate reader. Experiments were performed in triplicate.