Construction And Characterization Of Smart Drug Delivery Systems For Cancer Treatment

Currently, chemotherapy offers an important strategy among various cancer treatments and is an indispensable choice for most cancer cases. However, most chemotherapeutic drugs suffer from several limitations including poor solubility and stability, undesirable toxicity, nonspecific selectivity, low accumulation in tumors, relatively short half-life, and adverse side effects for healthy tissues, which prevent their widespread clinical application. Hence, it has been a hot research topic to construct highly efficient delivery systems of chemotherapy drugs with low toxicity for cancer therapy. Up to now, numerous strategies have been put forward to realize this goal. Among them, environment-responsive drug delivery systems adaptable to microenvironmental stimuli associated with tumor tissues and cells have gained significant attention because they have not only demonstrated good water solubility and enhanced stability, but also targeted the tumor site and released the loaded drug in a controlled manner. In this dissertation, we chose chemotherapeutic agents used in clinic or promising anticancer drugs as model drugs and constructed a serious of stimuli-responsive drug delivery systems. They can be divided into two categories:(1) smart small molecular prodrugs by introduction of stimuli-responsive functional groups into the drugs;(2) smart polymeric nano-drug delivery systems using stimuli-responsive polymers as carriers. The main contents are described as below: 1. Synthesis of tumor microenvironment-responsive pixantrone-based prodrug and its anticancer activity evaluationGenerally, small molecular chemotherapeutic drugs have no specific selectivity between tumor and normal tissues and could induce adverse side effects. Therefore, how to improve the selectivity of drugs in cancer treatment is a great challenge. In this work, pH-responsive pixantrone-based prodrug containing acid-sensitive amide linkage has been synthesized through the amidation reaction between 2,3-dimethylmaleic anhydride and pixantrone. The in vitro cell experiments were conducted to investigate the effects of pixantrone-based prodrug under a simulated physiological condition(pH ≈ 7.4) and in a simulated tumor environment(pH ≈ 6.5). It has been demonstrated that pixantrone-based prodrug was negatively charged, thus leading to reduced cellular internalization. Moreover, pixantrone-based prodrug exhibited significant lower cytotoxicity when compared with pixantrone. In contrast, the amide bond between an amino and anhydride was cleavable under slightly acidic conditions such as pH 6.5 and pixantrone-based prodrug reversed its charge from negative to positive, which facilitated cell internalization. Correspondingly, this prodrug has showed enhanced cytotoxicity in tumor cells. Compared with pixantrone maleate, this pixantrone-based prodrug displayed prolonged in vivo circulation time, thus facilitating the accumulation of drugs at tumor sites. This pH-responsive pixantrone-based prodrug system with improved selectivity against tumor cells provides a new strategy for selective cancer therapy. 2. Preparation and anticancer activity evaluation of dual-responsive nanoprodrugAlthough small molecular prodrugs obtained from chemical modification of chemotherapeutic drugs show promising anticancer activity, they still face several problems such as low accumulation in tumors, short circulation time, rapid blood/renal clearance, poor in vivo stability and so on. To address these limitations, the dual pH-/redox-responsive nanoprodrug system has been successfully prepared. Firstly, the amphiphilic prodrug was synthesized by conjugating biocompatible lipoic acid to the hydrophobic anticancer agent cinnamaldehyde through an acylhydrazone bond. Then the nanoparticles were prepared from cinnamaldehyde-lipoic acid derivatives and readily polymerized using a catalytic amount of dithiothreito. This nanoprodrug system is beneficial for prolonging the in vivo circulation time and then accumulating at the tumor sites through the enhanced permeation and retention(EPR) effect. Meanwhile, it is stable against dilution and can delivery cinnamaldehyde efficiently without leakage. In the acidic and reductive endosomal environment, this nanoprodrug system disassembled rapidly and then released the encapsulated drugs due to the acid-sensitive acylhydrazone bond and redox-sensitive disulfide bond, furthermore enhancing the inhibition of tumor cell proliferation efficently. In addition, owing to the existence of the hydrophobic segments in the nanprodrug system, hydrophobic anticancer drug doxorubicin(Dox) could be encapsulated into it. In vitro cell evaluation demonstrated that Dox-loaded nanoprodrugs could enhance apoptosis of Hela cells significantly, which shows tremendous potential in combination therapy. 3. Preparation of pixantrone/poly(γ-glutamic acid) nanoparticles through supramolecular co-assembly for oral chemotherapyUsually, polymeric nanoparticles as drug carriers can largely improve drug solubility in water, prolong the in vivo circulation time and enhance the targeting ability to the tumor tissues, thereby decreasing the side effects and leading to better therapeutic effect. Among them, oral delivery systems based on polymeric nanoparticles have attracted intensive attention because of their convenience, safety and cost-effectiveness in comparison to intravenous administration systems. However, the practical applications of these nanoparticles have been greatly hindered due to the complicated preparation process and the usage of harmful organic solvents in their preparation. To overcome these problems, a facile and green approach is proposed to construct pixantrone/poly(γ-glutamic acid) nanoparticles(PIX/γ-PGA NPs) as an oral drug delivery system through the electrostatic interactions between the polyelectrolyte γ-PGA and the anticancer drug pixantrone dimaleate(PDM). The size, drug loading content and efficiency, and the surface charge of nanoparticles could be controlled by adjusting the solution volume ratio of PDM to γ-PGA. Due to the electrostatic interaction, the PIX/γ-PGA NPs showed pH-triggered release property. They keep stable in the acidic environment and then dissociate rapidly under the alkaline condition. Furthermore, these nanoparticles could be effectively internalized by Lovo cancer cells and exhibited improved inhibition in comparison with free pixantrone. PIX/g-PGA NPs with a pH-triggered drug release property are a promising oral drug delivery system for cancer therapy. This practical approach may be extended to polymeric nanoparticle systems of other cationic drugs, thus showing great significance in application. 4. Chitosan-based nanocarriers with pH and light dual-response for anticancer drug deliveryDespite their great potential for drug delivery, polymeric nanocarriers still face some inevitable problems. On the one hand, polymeric nanoparticles formed by supramolecular self-assembly of amphiphilic copolymers are always too fragile to protect the encapsulated drugs while circulating in blood due to the dynamic nature, which may cause serious side effects. On the other hand, after these nanoparticles accumulate preferentially in tumor sites via the EPR effect, they may not easily be internalized by the tumor cells and release the anticancer drug intracellularly, which would reduce the therapeutic efficacy. Therefore, it is of great significance to design intelligent polymeric nanocarriers that are stable under physiological conditions but can fall apart in a controlled fashion upon stimulation to release the payloads. In this study, chitosan-based cross-linked nanocarriers with pH and light dual response were successfully prepared. The results demonstrated that cross-linked polymeric micelles are stable enough in a neutral environment(pH ≈ 7.4), whereas rapid disassembly was observed under the acidic environment with UV light irradiation because of the cleavage of cross-linked imine bonds and the disaggregation of hydrophobic light-sensitive o-nitrobenzyl succinate. The hydrophobic anticancer drug camptothecin(CPT) was selected as a model drug and loaded into crosslinked polymeric micelles. In vitro evaluation showed that these carriers improved the stability of the encapsulated CPT and released them rapidly at low pH with the light irradiation inside the tumor cells, thereby enhancing the inhibition of tumor cell proliferation.