Reduction Sensitive Shell Shaddable Biodegradable Polymeric Nanocarriers For Precise Control Of Intracelular Drug Release And Anticancer Activity

Traditional polyester based micelle and polymersome nanocarriers often exhibit slowdrug release profiles in vivo, while stimuli sensitive micelles and polymersomes couldrelease drugs or proteins very quickly. However, there is lack of fine control in drug releasefrom the biodegradable micelles and polymersomes so far. In this thesis, we designedbiodegradable micelles and polymersomes based on a mixture reducible polyethyleneglycol-SS-polycaprolactone （PEG-SS-PCL） and nonreducible PEG-PCL, which exhibitedtunable DOX or protein release profiles, elegantly manipulated by varying the compositionof the two diblock copolymers.In chapter2of the thesis, effects of disulfide contents on reduction-sensitivity, triggereddrug release as well as anti-tumor activity of shell-sheddable micelles self-assembled fromwere systematically investigated. Interestingly, in contrast to rapid aggregation ofPEG-SS-PCL micelles, mixed micelles containing10⁹0wt.%PEG-PCL displayed littlesize change in response to10mM dithiothreitol （DTT）. The in vitro release studies showedthat under intracellular-mimicking reductive environments, DOX release rate increased withincreasing PEG-SS-PCL contents in the micelles, in which about29.4,42.7,77.9and86.9%DOX was released within12h from micelles containing30,50,70and90wt.%PEG-SS-PCL, respectively. In contrast, DOX release was minimal （<20%） undernon-reductive physiological conditions. Notably, flow cytometry displayed clear correlationbetween cellular DOX levels and PEG-SS-PCL contents in DOX-loaded micelles. Moreover,confocal laser scanning microscopy （CLSM） observations indicated progressively strongerDOX fluorescence in RAW264.7cells following12h treatment with DOX-loaded micellescontaining increasing PEG-SS-PCL contents. In addition, MTT assays in RAW264.7cellsshowed that the cytotoxicity of DOX-loaded micelles was augmented proportionally toPEG-SS-PCL contents, signifying the role of reduction-triggered “active” drug release incells.In chapter3of the thesis, we continued the study with the polymersomes whichcomposed of PEG-SS-PCL/PEG-PCL block copolymers with high PCL molecular weight of 18kDa. We investigated the encapsulation and release of hydrophilic macromacromoleculelike proteins and dextran of the reducible polymersomes. The reducible polymersomes couldshed off PEG shell partially to form polymersomes with porous membranes underbioreducible mimicking environments, and by controlling PEG-SS-PCL content the size andthe number of the pores formed and thus the protein release could be manipulated. Theprotein loaded polymersomes with size of143-153nm （PDI0.1-0.2） could be prepared viadirect hydration method. It is found that reduction triggered protein and dextran release wasintimately dependent on the PEG-SS-PCL content in the polymersomes, possibly due to thepore formation resulting from PEGshedding under reducible conditions. By studying the invitro release of proteins and dextran of different molecular weight and size, we analyzed thepermeability of the polymersomes in the presence of10mM DTT. After fit the release datawe got the permeability of the different polymersomes for different macromolecules inbetween0.7-12×10^-13m/s. Moreover, high PEG-SS-PCL in the polymersomes or smallmacromolecules led to high permeability. Furthermore, the pore size formed in10mM DTTin the polymersome membranes was at lease13nm. These results have shown that the invitro drug or protein release, intracellular drug release and therefore anti-tumor activity ofnano drugs can be precisely controlled by extentof reduction-triggered shedding offhydrophilic shells.