Responsive Gold Nanoparticles As Drug Carrier For Overcoming Drug Resistance In Cancer Cells

Resistance of tumor cells, including multidrug resistance (MDR) to chemotherapeutic drugs is a major impediment to the success of cancer chemotherapy. As one of the most important mechanisms involved in many MDR cells, P-glycoprotein (P-gp), is an important transporter. It is capable of effluxing a broad range of structurally and functionally distinct anticancer agents, which is often overexpressed in the plasma membrane of many MDR cells. Therefore, inhibition, or bypass of P-gp-mediated drug efflux or P-gp expression has become an important strategy in overcoming MDR.Nanoparticles can enter cells by an endocytosis pathway and the exocytosis of nanoparticles may be independent from the P-gp pathway. Thus, it can be expected that an ideal nanoparticular delivery system which can significantly overcome MDR would be dependent on the high efficiency of cellular entry of the nanoparticles as well as on the subsequent rapid release of the cytotoxic drug intracellularly. This dissertation mainly focuses on the development of responsive gold nanomaterials as the drug carrier for intracellular drug delivery to overcome multi-drug resistance in cancer cells. This dissertation can be categorized into four main parts as described below:1. To prove the concept that rapid drug release from the nanoparticles in the cells would potentially render the reversal of drug resistance of cancer cells, we have developed a drug delivery system that tethers doxorubicin onto the surface of gold nanoparticles with a poly(ethylene glycol) spacer via an acid-labile linkage (DOX-Hyd@AuNPs). We have investigated the pH-responsive release of doxorubicin from doxorubicin-tethered gold nanoparticles (AuNPs). The fluorescence emission intensity of the processor LA-PEG-Hyd-DOX significantly decreases after the reaction with AuNPs, demonstrating the presence of nanosurface energy transfer (NSET) between the doxorubicinyl groups and AuNPs. However, the quenched fluorescence of doxorubicin recovers rapidly once doxorubicin is released from the nanoparticles due to the cessation of NSET. We find that the release of doxorubicin from DOX-Hyd@AuNPs is accelerated at lower pH values, indicating that DOX-Hyd@AuNPs may be an ideal nanoparticular delivery system for overcoming the drug resistance in cancer cells. 2. Based on the observations as mentioned above, we have further demonstrated that multidrug resistance in cancer cells can be significantly overcome by a combination of highly efficient cellular entry and a responsive intracellular release of doxorubicin from the gold nanoparticles in acidic organelles. DOX-Hyd@AuNPs has achieved enhanced drug accumulation and retention in multidrug resistant MCF-7/ADR cancer cells when it is compared with free doxorubicin. It can release doxorubicin in response to the pH of acidic organelles following endocytosis, which has been shown by the recovered fluorescence of doxorubicin from quenching due to NSET between the doxorubicinyl groups and the gold nanoparticles. DOX-Hyd@AuNPs therefore can significantly enhance the cytotoxicity of doxorubicin and induce elevated apoptosis of MCF-7/ADR cancer cells. With a combined therapeutic potential and ability to probe drug release, DOX-Hyd@AuNPs represent a model with dual roles in overcoming MDR in cancer cells and probing the intracellular release of drug from its delivery system.3. We have explored the ablation of cancer stem cells (CSCs) in vitro by the combination of photothermal therapy and chemotherapy using a multifunctional doxorubicin tethered gold nanoshell DOX-Hyd@HAuNS. We find that DOX-Hyd@HAuNS treatment to mammospheres of MDA-MB-231cells can enhance the DOX accumulation in the cells and facilitate intercellular DOX release to overcome the drug resistance. Under the NIR laser irradiation, it has been observed that DOX-Hyd@HAuNS treatment to CSCs significantly reduces the formation of mammospheres in comparison with free DOX or barely HAuNS treatment at the same dose(s), due to the photothermal effect of gold nanoshell under the NIR laser irradiation. This study provides evidence that combination of photothermal therapy with chemotherapy using the multifunctional gold nanoshell carrier may synergesically inhibit the "sternness" of cancer stem cells.4. Biodegradable polymer vesicle for drug delivery is reported. Poly(s-caprolactone)-block-poly(ethyl ethylene phosphate) with well-defined structure (PCL150-b-PEEP30) has been prepared by ring-opening polymerization. It forms vesicles in aqueous solution using the thin-film hydration method and further exclusion of the as-formed vesicles results in vesicles at nano-size, demonstrated by confocal laser scanning microscope (CLSM) and transmission electron microscopy observations. Doxorubicin (DOX) has been loaded into the vesicles with a loading content of4.38%using an acid gradient method. The release of DOX from the vesicles is accelerated in the presence of an enzyme phosphodiesterase I that is known to catalyze the degradation of polyphosphoester, achieving83.8%release of total loaded DOX in140h. The DOX-loaded vesicles can be successfully internalized by A549cells, and it results in enhanced inhibition to A549cell proliferation, likely owning to the sustained intracellular release of DOX as observed by CLSM. With these properties, the vesicles based on the block copolymer of PCL and PEEP are attractive as drug carriers for pharmaceutical application.