Is the multidrug transporter a flippase?

The phenomenon of multidrug resistance is correlated with the presence of a membrane protein, P-glycoprotein, which pumps a wide variety of drugs out of cells thus reducing their toxicity. However, the mechanism of this pumping action remains unclear. In this article, we suggest that several properties of the multidrug transporter may be explained if it acts as a 'flippase' to transport drugs from the inner leaflet of the lipid bilayer to the outer or to the external medium.