Design,Synthesis,Self-assembly Behavior,and Biomedical Applications Of Novel Thermo-sensitive Double Hydrophilic Block Copolymers

Micelles formed by the self-assembly of amphiphilic block copolymers that contain both hydrophilic and hydrophbic segements have attracted considerable attention in the fields of nanomedicine as drug delivery systems due to their unique core-shell structures.Dialysis is generally recognized as the most frequently used method for the preparation of self-assembled micells from amphiphilic block copolymers.It requires necessarily the use of certain organic solvent for dissolving the polymers,and subsequent solvent exchange between water and the oragnic solvent using a dialysis membrane,therefore the dialysis strategy involving both organic solvent and time-consuming solvent exchange process hamper obviouly the facile fabrication and clinical translation of amphiphilic block copolymers-based drug deivery systems.Double hydrophilic block copolymers(DHBCs),unlike amphiphilic block copolymers,consist of two different hydrophilic segments,it is therefore readily and fully soluble in an aqueous phase.If one of the segments is temperature-sensitive,the self-assembly of double hydrophilic block copolymers can be accomplished by simply changing the ambient temperature in solution.This strategy is thus regarded as a much more facile and “green” strategy in contrast to the dialysis method discussed above.In the first part of the master thesis,we investigated the effect of polymer topology on its self-assembly behavior by designing and synthesizing a series of toothbrush-like double hydrophilic block copolymers,and reported for the first time an interesting dual-transition from unimers to vesicles,and finally to micelles based on the toothbrush-like DHBCs.In the second part,we used DHBCs for the fabrication of two types of polymeric micelle prodrugs with drug tethered to the hydrophilic micellar shell or buried within the hydrophobic micellar core,respectively,and further compared the effect of drug-conjugated sites on the properties and delivery efficacies of the two formulations in terms of the dimension of the self-assemblied micelles,cellular uptake efficacy,in vitro drug release behaviors,and in vitro cytotoxicity.The details are listed as follows,1.Sequential reversible addition-fragmentation transfer(RAFT)polymerizations were conducted to prepare the target toothbrush-like DHBC,P(NIPAAm)-b-P(OEGMA300).The thermo-induced association behaviours of P(NIPAAm)-b-P(OEGMA300)were investigated by UV-vis and DLS.TEM and SEM were performed to identify directly the morphology of the self-assemblies.P(NIPAAm)-b-P(HPMA)and P(NIPAAm)-b-P(OEGMA480)were also synthesized as control groups.The results revealed that it is the entanglement of pendent OEG brushes with PNIPAAm block that results in an interesting thermo-regulated dual transitions from unimers to vesicles,and finally to micelles in an aqueous solution upon temperature elevation.The fully collapse of far-end free PNIPAAm chain units dominates the starting unimer-to-vesicle transition,and the following vesicle-to-micelle transition is primarily dependent on the complete shrinkage of the close-end confined PNIPAAm chain units.This dual associations is also applicable to OEGMA480 monomer-based DHBC,P(NIPAAm)-b-P(OEGMA480).2.Two different DHBCs-based polymeric prodrugs,P(HPMA)-b-P(NIPAAm-st-(EGMA-DOX))and P(HPMA-st-(EGMA-DOX))-b-P(NIPAAm),with drug tethered to the P(HPMA)shell or conjugated within the hydrophobic P(NIPAAm)core at the phosiological tempareture of 37 oC above the LCST of PNIPAAm block via a pH-liable hydrazone bond were designed and developed,respectively,by consecutive RAFT polymerizations and post-polymerization decoration.The very similar thermo-sensitivity and mean particle size measured by DLS for both polymeric prodrugs probably demonstrate the insignificant effect of drug location on the properties of their self-assembled nanostructures.To validate the acidic pH-triggered drug release,in vitro DOX release behaviors were investigated at three different conditions,i.e.,pH 7.4,6.8 and 5.0,the typical extracellular pH,tumor microenvironment pH,and endosomal/lysosomal pH respectively,at 37 °C.And we conclude that drug tethered to the hydrophilic shell of micelles promotes significantly drug release from micelle prodrugs without compromising micelle stability for blood circulation.Finally,the cytotoxicity of micelle prodrugs was assessed by MTT cell viability assay,and the results revealed that drug tethered to the hydrophilic shell of micelles showed higher cytotoxicty.