Polymeric Micelle Drug Delivery Systems Based On Tumor Microenvironment For Stimulus-responsive Controlled Drug Release

Due to the special nano-structure and physico-chemistry properties,the polymer micelles own good application prospect in drug delivery and diagnostic imaging.While several challenges and bottlenecks lie in the road to good chemotherapy efficiency for micellar drug delivery systems,and the challenges are maintaining good stability in vivo,declining the drug leakage in delivery process and stimuli-responsive release under the microenvironment in tumor site.This work designed the copolymers and the self-assembled micelles with definite structure,good stability,controlled drug release and good biocompatibility,and the structure-properties relationship between the copolymers and micelle system are investigated in detail.The present research provides some guidelines to the development of the polymeric material design?property control and the clinical application.To reinforce the stability of the micelles and prevent premature drug leakage in the bloodstream,the present work designed a novel polymeric micelles with a cross-linked structure containing disulfide and hydrazone bond in the micellar shell,which was aming to controlled delivery of insoluble drug camptothecin based on the weak acid and strong redox condition in tumor site.The amphiphilic four-arm star copolymer[poly??-caprolactone?-b-poly?poly?ethylene glycol?methyl ether methacrylate-co-p-?2-methacryloxyethoxy?benzaldehyde?]₄[4-AS-PCL-P?PEGMA-co-MAEBA?]was synthesized.The subsequent self-assembled polymeric micelles consist of two blocks,a hydrophobic PCL core acting as a container for the entrapment of the hydrophobic drug CPT and a hydrophilic shell containing PEGMA and MAEBA.The PEGMA can help maintain the stability of the micelles,while the aldehyde group of MAEBA is used to introduce the crosslinking sites.After a small molecular cross-linker and catalyst were added to the micelle solution at pH 6.5,the stimuli-responsive reversible cross-linked micelles?SCMs?were formed.Several properties of the micelles,including the self-assembly behavior,size distribution,thermodynamic stability,drug loading and controlled release were investigated.In the simulative normal conditions?pH 7.4?,the SCMs kept good stability and high capacity against extensive dilution,and the disassembly or aggregation didn't occur with 1000-fold water dilution.At the same time,the cross-linked structure acted as a“barrier”against the drug diffusion,and hold up the drug release efficiently,following with a low drug release amount of 27.2%in 108 h.However,a fast release?71.5%?was observed in the simulative tumor intracellular condition?pH 5.0 with 10 mM dithiothreitol?,which was mainly due to the break-up of the disulfide and hydrazone bond,following with the de-crosslinking of the micelles.The cellular uptake and MTT assay showed that the CPT-loaded SCMs could be internalized by HepG2 tumor cells within 12 h,and the cell viability was less than 20%after 48 h incubation.To further enhance the drug release when the SCMs de-crosslinked in tumor site,the disulfide bond was introduced between the hydrophilic and hydrophobic polymer blocks.The amphiphilic copolymer poly??-caprolactone?-b-poly?poly?ethylene glycol?methylether methacrylate-co-p-?2-methacryloxyethoxy?benzaldehyde?[PCL-SS-P?PEGMA-co-MAEBA?]was synthesized and a redox-responsive degradable SCMs was prepared,which was used for controlled delivery of insoluble drug doxorubicin?DOX?.This redox-responsive degradable SCMs kept good stability and low drug release amount of 23%?72 h?at pH 7.4,while the drug release was accelerated?62%?at pH 5.0,which was account of the de-crosslinking of the SCMs.In the simulative tumor cell intracellular condition,the de-crosslinking for the acid pH and the degradation?disassembly?of the micelles occurred simultaneously,followed by the further promotion of the drug release?87%for 72 h?.The results indicated that the introduction of disulfide bond between the amphiphilic blocks in micelle structure can promote the drug release of SCMs in tumor site.Aiming to enhance the efficiency of the drug release responding to the stimulus in tumor microenvironment,the present work mixed the copolymer PCL-SS-P?PEGMA-co-MAEBA?with a pH sensitive copolymer,poly??-caprolactone?-ss-poly?2-?dimethylamino?ethyl methacrylate?[PCL-SS-PDMAEMA].Then the mixed SCMs were obtained with the crosslinking of the micelles from the copolymer mixture in aqueous solution,which contained imine bond in the cross-linked stucture,disulfide bond in the copolymer backbone and the pH sensitive block PDMAEMA in the hydrophilic shell.By use of the pH-triggered swelling and redox-triggered degradation?disassembly?of the mixed SCMs in tumor cells,the drug release was promoted to the largest extent.The mixed SCMs owned good stability and the drug release was inhibited at pH 7.4 for a limited release amount with 72 h?25.7%?,while the drug release was as high as 95.3%when it comes to the simulative tumor cell intracellular conditon?pH 5.0,10 mM glutathione?.In terms of tumor“theranostics”with higher therapy effect,the micelle system for tumor chemotherapy together with imaging was taken into consideration with the PCL-SS-PDMAEMA micelles,which were loading with gold nanoparticles?GNPs?and DOX molecules.The copolymers PCL-SS-PDMAEMA self-assembed into core-shell micellar structure with a nano size of 149 nm,containing a disulfide bond in backbone and pH sensitive block as micllar shell.GNPs with good stability and size distribution?⁹ nm?were synthesized by means of the in situ reduction of HAuCl₄ with the hydrophilic block PDMAEMA.The self-assembly process of the copolymers was investigated by the combination of the experiment and computer simulation,and the DLS,TEM,XRD and UV-vis was taken advantage to analyse the physico-chemistry properties of the micelles.At pH 7.4,the accumulative drug release of the micelles was only 29.9%within 36 h.The protonation of PDMAEMA occurred at pH 5.0,which resulted in the swelling of the micellar architecture.At the same time,the disulfide bond broke up due to 10 mM glutathione,which lead to the disassembly of the micelles.All the two processes above resulted in a thorough drug release,which was up to 96%during 36 h.The MTT assay showed a high cytotoxicity of the DOX-loaded micelles against HepG2 and MCF-7 cells,and the cell viability was 24%and 13%respectively after 48 h incubation.At last,the in vitro CT imaging assay indicated that the GNPs-loaded micelles owned good CT diagnose effect.The micelles loaded with GNPs and DOX was a potential theranostics micelle system for tumor chemotherapy and imaging.