Multidrug resistance (MDR) is a major obstacle limiting chemotherapeutic efficacy. The purpose of these studies was to investigate the potential application of injectable paclitaxel (PX) and doxorubicin (Dox)-loaded nanoparticles (NPs) engineered from oil-in-water microemulsion precursors for overcoming P-glycoprotein (P-gp)- mediated drug resistance in solid tumors. An in-vitro study was performed to test whether the oil (stearyl alcohol and cetyl alcohol) used to make lipid nanoparticles could be metabolized. The results showed that the concentrations of the fatty alcohols within nanoparticles, which were quantitatively determined over time by gas chromatography, decreased to only 10-20% of the initial concentration after 15-24 h of incubation with horse liver dehydrogenase (HLADH) and NAD+ at 37ºC. Moreover, the surfactant Brij 78 (polyoxyethylene 20-sterayl ether) in the nanoparticles influenced the activity of the enzyme. Novel Cremophor EL-free paclitaxel-loaded nanoparticles were developed using experimental design combining Taguchi array and sequential simplex optimization. The resulting PX G78 and PX BTM NPs were stable at 4ºC over five months and in PBS at 37ºC over 102 h. Release of PX from PX NPs was slow and sustained without initial burst release. Interestingly, PX BTM NPs could be lyophilized without cryoprotectants and without changing any physiochemical properties and bioactivities. Cytotoxicity studies in breast cancer MDA-MB-231 cells showed that PX NPs have similar anti-cancer activities compared to Taxol. Optimized Dox-loaded NPs were prepared using an ion-pair agent, sodium tetradecyl sulfate (STS), to mask Dox charge and to enhance its entrapment in NPs. In-vitro cytotoxicity studies were carried out in both sensitive and resistant human cancer cells treated with PX and Dox-loaded NPs. All of drug-loaded NPs decreased IC50 values by 6-13-fold in resistant cells compared to free drugs. A series of in-vitro assays were used to understand the underlying mechanisms. The results, in part, showed that the NPs inhibited P-gp and transiently depleted ATP, leading to enhanced uptake and prolonged retention of the drugs in P-gp-overexpressing cancer cells. Finally, in-vivo anti-cancer efficacy studies were performed using pegylated PX BTM NPs after intravenous (i.v.) injection and showed marked anti-cancer efficacy in nude mice bearing resistant NCI/ADR-RES tumors versus all control groups. These results suggest that NPs may be used to both target drug and biological mechanisms to overcome MDR.

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