| In the past decades, polymeric nanoparticles have been emerged as a promising platform for targeted drug delivery. The idea drug-loaded nanoparticles are stable in long circulation, while can release drugs quickly in tumor cells. Polyurethanes(PUs) are one of the most important versatile polymers widely used for biomedical applications. Due to their excellent physical properties and biocompatibility, polyurethanes have been traditionally applied as biostable and inert materials in heart valves, catheters, vascular grafts and prostheses. In the past few years, biodegradable PUs have been attracted considerable attention in the applications of drug delivery systems and tissue engineering due to their facile synthesis, ease incorporation of biodegradable segments, such as ester and carbonate bonds. With the purpose of further improving drug release behavior and anticancer efficiency in vitro or/and in vivo, recently stimuli-responsive biodegradable PUs containing labile bonds, such as acid-labile bonds and disulfide bonds, etc., have been designed and investigated. As reported by some literatures, reduction-sensitive polyurethanes containing disulfide bonds in the backbones were prepared by using bis(2-hydroxyethyl) disulfide diol which may result in toxicity in drug delivery systems. Therefore, the aim of this thesis was to develop biocompatible and biodegradable stimuli-responsive polyurethane-based nanocarriers, which are rather stable under extracellular conditions while exhibited fast response to intracellular stimuli and released drugs right into cytosol. To this end, we have developed α-amino acid-based diol and poly(disulfide urethane)s, which could self-assemble into reduction-sensitive biodegradable micelles and applied for efficient intracellular drug release. Further modification through surface-ligand decoration and incorporation of imidazole groups for endosome escaping greatly improved the release behavior and drug efficiency.In chapter 1, a literature of overview is presented to give a brief introduction about stimuli responsive nanocarriers, polyurethanes and polyurethane based nanosytems.In chapter 2, we reported a novel reductively-degradable α-amino acid-based poly(disulfide urethane)s via polyaddition reaction of disulfide-containing bis(ethyl L-serinate amide) diol(SS-BSER) with L-lysine ethyl ester diisocyanate(LDI). After conjugation with poly(ethylene glycol)(PEG), triblock copolymer PEG-AAPU(SS)-PEG was obtained, which could self-assemble into micelles(denoted as AAPU(SS) micelles) with mean hydrodynamic diameter 155 nm. AAPU(SS) micelles showed practically non-toxic(cell viabilities ≥ 90 %) up to a tested concentration of 1.0 mg/m L both in RAW 264.7 and Doxorubicin(DOX)-Resistant MCF-7 cells(MCF-7/ADR cells) by MTT assays. In vitro release studies revealed that minimal drug release(29%) was observed within 12 h for AAPU(SS) micelles in PB(10 m M, p H 7.4) buffer. In contrast, these core-responsive micelles released 84% of DOX within 12 h in the presence of 10 m M glutathione(GSH). Confocal laser scanning microscopy(CLSM) observation showed that AAPU(SS) micelles effectively transported DOX into the perinuclear and nuclear regions in the MCF-7/ADR cells. Notably, MTT assays indicated that DOX-loaded AAPU(SS) micelles had a good antitumor activity toward RAW 264.7 and MCF-7/ADR cells. Interestingly, when DOX dosage was 40 μg/m L, the cell viability of MCF-7/ADR cells incubated with DOX-loaded AAPU(SS) micelles and free DOX·HCl for 48 h, were 22.3 % and 57.9 %, respectively. These novel reductively-degradable α-amino acid-based poly(disulfide urethane)s provided a promising platform for controlled drug delivery systems.In chapter 3, novel c(RGDf K)-decorated micelles based on two triblock copolymers, PEG-AAPU(SS)-PEG and c(RGDf K) conjugated PEG-AAPU(SS)-PEG were developed for improving cellular uptake of αvβ3-overexpressing U87 MG cells. MTT assays showed that DOX-loaded c(RGDf K)-decorated micelles containing 30 wt.% of c(RGDf K)-PEG-AAPU(SS)-PEG-c(RGDf K)(30T-AAPU(SS)) exhibited apparent targetability and significantly enhanced antitumor efficacy toward U87 MG cells with a particularly low half maximal inhibitory concentration(IC50) of 4.8 μg DOX equiv/m L, which was comparable to free DOX and approximately 3.0 times lower than that for nontargeting micelles under the same conditions. Flow cytometry analysis and CLSM observation showed cellular DOX fluorescence in U87 MG cells treated with DOX-loaded 30T-AAPU(SS) micelles was much greater than that with nontargeting micelles, signifying the importance of combining reduction-response release and active targeting. Inhibition experiment results revealed DOX-loaded targeted micelles could be specifically taken up by U87 MG cells via receptor mediated endocytosis. These c(RGDf K)-decorated reductively biodegradable poly(disulfide urethane) micelles are highly promising for the efficient intracellular delivery of various lipophilic anticancer drugs to achieve improved cancer therapy.In chapter 4, p H and reduction-responsive biodegradable micelles were developed from the histamine modified triblock copolymers(PEG-AAPU(SS)(-g-his)-PEG) for fast endosomal escaping and triggered intracellular release of DOX. Triblock copolymer PEG-AAPU(SS)(-g-his)-PEG were prepared by aminolysis reaction with ethyl group of AAPU(SS) and amines of histamine. In vitro release studies revealed that accelerated drug release(approximately to 100 %) was observed within 24 h for DOX-loaded 20H-AAPU(SS) micelles in p H 5.0 sodium acetate buffe containing 10 m M GSH. MTT results revealed that DOX-loaded 20H-AAPU(SS) micelles had improved cell cytotoxicity to RAW 264.7 and MCF-7/ADR cells(IC50 descreased about 5 and 2.7 times, respectively) comparing to NH-AAPU(SS) micelles. Interestingly, confocal laser scanning microscopic observation revealed rapid endosomal escape of the micelles for the excellent antitumor efficancy in vitro.In chapter 5, a comprehensive summary and a future perspective of this thesis work were provided. |
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