Development of functional polymeirc micelles responding to intracellular microenvironments for cancer chemotherapy

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Polymeric micelles, the self-assemblies of block copolymers, are a promising nanocarrier system for drug delivery. Indeed, several formulations have been studied in clinical trials. Polymeric micelles are characterized by a size of several tens nm and unique core-shell structure, where the drug-loaded core is surrounded by biocompatible poly(ethylene glycol) (PEG) chains. These characteristics of polymeric micelles allow their longevity in the bloodstream and effective tumor accumulation after their systemic administration. In this project, we have prepared pH-sensitive polymeric micelles, in which adriamycin (ADR) is attached to the side chain of the core-forming segment of the block copolymers via an acid-labile hydrazone bond. The system showed appreciable drug release under intracellular low pH conditions, while exhibiting no drug release under physiological pH conditions. We applied such pH-sensitive polymeric micelles for the treatment of pancreatic cancers, which are known to be intractable due to hypovascularity and thick fibrosis. As a result, pH-sensitive polymeric micelles showed significant antitumor efficacy against subcutaneous pancreatic cancers in mice. Meanwhile, the pH-sensitive micelles were modified with a folate molecule to construct the polymeric micelles with the tumor-targetability. A large number of cancer cells are known to overexpress folate-binding proteins (FBP). The folate-conjugated micelles were more efficiently taken up by the FBP-overexpressing cancer cells, achieving remarkable antitumor effect at much lower ADR concentrations than the non-targeted micelles. These results indicate that the use of the folate-conjugated micelles may lower the effective doses over free ADR, improving the safety of the clinical chemotherapy. Thus, functional pH-sensitive polymeric micelles developed in this study are expected to improve the safety and efficacy of the treatments against intractable malignant tumors.