Charge-Conversional & Reduction-Sensitive Polymer Micelles As Anti-Cancer Drug Carriers

Nowadays, cancer has posed a great threat to human health. There are millions of patients died of cancer every year. Among various treatments for cancers, chemotherapy continues to be the first selectivity. Due to the poor aqueous solubility, instability and undesirable side effects to normal tissues, conventional chemotherapy drugs face enormous challenge in their clinical application. The development of nanocarriers for drug delivery provides a feasible method to break this bottleneck. This strategy has many advantages including the increased solubility of poorly soluble drugs, improved thermal stability, prolonged circulation time, reduced toxicity to normal tissues and enhanced anti-tumor activity. However, there are also some drawbacks in conventional drug carriers, such as the uncontrolled and incomplete drug release, resulting in the decreased bioavailability of drugs and the development of multidrug resistance. In order to solve these problems, persist attentions have been focused on the development of stimuli-triggered drug carriers. Taking advantage of tumor microenvironment, the combination of charge-conversion and reduction-sensitive can be applied in the engineering of nanocarriers. Due to the slightly acidic microenvironment of tumor and organelles, the nanoparticles can achieve surface charge conversion, promoting the uptake of nanoparticles by tumor cell. There is a higher concentration of reductive glutathione (GSH) inside cancer cells than normal cells and the blood. When nanoparticles were internalized into cancer cells, the drug would be rapidly released from nanocarriers owing to the intracellular reductive environment, resulting potent anti-tumor activity. In this dissertation, we designed and prepared a series of charge-conversion and reduction-sensitive drug delivery systems, whose physicochemical properties and biological properties were systematically studied. This thesis can be further categorized into four parts as described below:1. In this work, an amphiphilic block copolymer PLA-SS-PHEMA was synthesized via combination of ring opening reaction (ROP) and atom transfer radical polymerization (ATRP), which could self-assemble to form spherical micelles with uniform sizes. Due to their excellent thermal stability and blood compatibility, the micelles have the potential to be used as anticancer drug carriers. Doxorubicin (DOX) was loaded into the micelles with high drug-loading efficiency and entrapment efficiency. In the presence of reductive agents such as glutathione (GSH), DOX could be rapidly released from nanocarriers due to the disassembly of micelles caused by the breakage of disulfide bonds. Once they entered cancer cells via the clathrin -mediated endocytosis, the micelles achieved intracellular reduction-responsive drug release, resulting in the excellent anti-tumor activity.2. We designed and synthesized a novel DOX-prodrug (DOX-SS-DOX) by attaching a thiol-responsive domain to the doxorubicin. By the self-assembly of DOX-prodrug and PLA-SS-PHEMA, a dual reduction responsive drug delivery system was prepared. In vitro release results indicated that the cleavage of the intervening disulfide bonds in both carrier and prodrug in response to reductive environment led to fast release of anticancer drug. In vitro cytotoxicity results demonstrated that the drug loaded micelles showed almost no cytotoxicity to normal cells, while they could effectively inhibit tumor cell proliferation. When the micelles were internalized into cancer cells via the clathrin-mediated endocytosis, DOX-prodrug would be rapidly released from nanocarriers and further be reduced to free DOX under reductive environments, which resulted in potent antitumor activity.3. In this work, we designed and synthesized an amphiphilic block copolymer PLA-SS-PAEMA/DMMA via the introduction of DMMA derived amide groups in the side chain and disulfide bonds in the main chain. Due to their amphiphilic nature, the polymer would self-assembly to form uniform spherical micelles in aqueous solution. The micelles possessed excellent thermal stability and blood compatibility, which was beneficial to circulation through human body after intravenous injection. Under normal physiological conditions, the negatively charged micelles were able to slightly adsorb bovine serum albumin (BSA). Whereas in slightly acidic environment, the surface of the micelles gradually changed from negative value to positive value due to the cleavage of amide bonds and release of the amino groups, enhancing the uptake of micelles by cancer cells. By the electrostatic interactions between polymer and anti-cancer drug, the micelles could effectively load DOX with high drug-loading efficiency and entrapment efficiency. In vitro release results indicated the reductive agents and low pH could effectively promote the release of DOX from the micelles. Cell experiments showed the synergetic effect of charge reversal under weakly acidic conditions and intracellular reduction responsive release of DOX resulted in the excellent antitumor efficacy of the micelles, which entered cancer cells via the clathrin-mediated endocytosis.4. A novel zwitterionic polymer PLA-SS-PBEMA/DMMA was synthesized in this work, which could be self-assembly to form uniform micelles composed of hydrophobic PLA core and hydrophlic PBEMA/DMMA corona. Because of their excellent thermal stability and blood compatibility, the polymer micelles had good in vivo applications prospect. The neutral charged polymeric micelles showed excellent protein-resistant under normal physiological conditions. Due to the slightly acidic tumor microenvironment, the surface of the micelles gradually changed from neutral value to positive value, promoting the uptake of micelles by cancer cells. In vitro release results suggested that DOX would be rapidly released from nanocarriers under reductive conditions. Due to the synergetic effect of charge-conversion under weakly acidic conditions and intracellular reduction-responsive release of DOX, the micelles exhibited excellent anti-tumor activity. The clathrin-mediated endocytosis had the most pronounced contribution to cell internalization of the PLA-SS-PBEMA/DMMA micelles.