To test the ability of nanoparticle (NP) formulations to overcome P-gp-mediated multidrug resistance (MDR), several different doxorubicin (Dox) and paclitaxel (PX)-loaded solid lipid NPs were prepared. Dox NPs showed 6-8-fold lower IC50 values in Pgp overexpressing human cancer cells than those of free Dox. The IC50 value of PX NPs was over 9-fold lower than that of Taxol® in P-gp-overexpressing cells. A series of in-vitro cell assays were used including quantitative studies on uptake and efflux, inhibition of calcein acetoxymethylester (Calcein AM) efflux, alteration of ATP levels, membrane integrity, mitochondrial membrane potential, apoptosis and cytotoxicity. Enhanced uptake and prolonged retention of Dox were observed with NP-based formulations in P-gp-overexpressing cells. Calcein AM and ATP assays confirmed that blank NPs inhibited P-gp and transiently depleted ATP. Intravenous injection of pegylated PX BTM NPs showed marked anticancer efficacy in nude mice bearing resistant NCI/ADR-RES tumors versus all control groups. NPs may be used to both target drug and biological mechanisms to overcome MDR via P-gp inhibition and ATP depletion
