International Journal of PharmaceuticsPreviously we showed that some excipients can inhibit breast cancer resistance protein (BCRP/ABCG2) in vitro and in vivo.We then evaluated the reversibility and themodeof BCRP inhibition of excipients, such as Tween 20 and Pluronic P85, by the intracellular mitoxantrone uptake study. To evaluate the reversibility of BCRP inhibitory effects, BCRP expressing cellswere preincubated with the excipients and the intracellular mitoxantrone uptake was determined after removing or not removing the excipients. To evaluate the mode of BCRP inhibitory effects, the intracellular mitoxantrone uptake at the different mitoxantrone concentrations in the medium with the excipients was determined. Both Tween 20 and Pluronic P85 increased the mitoxantrone uptake in BCRP expressing cells, but these effectswere disappeared when the excipientswere removed. Moreover, both excipients increased the uptake at lowsubstrate concentrations. However, at high substrate concentrations, Tween 20 increased the uptake to less extent compared with low substrate concentrations, whereas there was no such effect of Pluronic P85. Taken together, Pluronic P85 and Tween 20 appear to inhibit BCRP-mediated efflux of mitoxantrone reversibly and the inhibition mode of Pluronic P85 may be competitive but not that of Tween 20, which may be mixed type.