Design,Preparation And In Vitro Performance As Drug Carriers Of Amphiphilic Star-Shaped Polymers

Polymeric micelles capable of responding to various biorelevant triggers,such as pH gradients,hyperthermia,reducing and oxidizing environments,have been widely investigated as smart carriers for controlled drug delivery.Incorporation of the reducible disulfide links into delivery systems has been repeatedly highlighted to realize elegantly the tradeoff between extracellular stability and intracellular high therapeutic efficacy.Of all the reported introducing strategies,esterification seems to be the most extensive adopted one,however,suffers from relatively low yield,especially for the esterification coupling between two macromolecules,due to the low condensation efficiency.Therefore,there is considerable scope to adopt a more efficient coupling strategy to address these issues.The preparation of various types of miktoarm star polymers with precisely controlled structures（A ₂B,ABC,AB₂C₂,etc.）has made significant progress due to the considerable advances in the synthetic strategies,including multistep protections/deprotections,orthogonality,and integration of different polymerization techniques.However,compared to the well-developed synthesis methodologies,the investigations on miktoarm star copolymers as drug delivery vehicles remain relatively unexplored,especially for the relationship of their branched structures and properties as drug delivery systems.Conjugation of various active targeting ligands to the surface of nanocarriers to realize the specific recognition by the corresponding receptors localized on the membrane of the cancer cells has provided a powerful means toward enhanced cancer therapy.Folic acid（FA）is one of the most used targeting ligands due to the overexpressed FA receptors in many cancer cell lines.However,conjugation of hydrophobicFAtothesurfaceofnanocarriersusuallyaltersthe hydrophilic/hydrophobic balance of the stabilized nanoparticles,leading to their thermodynamic instability and subsequent formation of aggregates,which apparently compromises the in vivo long circulation and minimized side effects of nanocarriers.The currently leading strategy to overcome this issue is to incorporate a protecting hydrophilic stealth that can be deshielded to expose the targeting ligand at the desired tumor site,which generally involves multistep chemical modifications,conjugations and purifications.Therefore,we aims to develop a facile and efficient stratergy to promote FA-targeted anticancer drug delivery.To address the issues mentioned above,the following three systems were designed in this thesis.The details contents are listed as follows,1.Cu（I）-catalyzed azide-alkyne cycloaddition（CuAAc）click coupling with high efficiency and high selectivity was used to incorporate disulfide links to the star-shaped amphiphilic block copolymers in the first charpter.Interestingly,in addition to the synthetic advantages,the click coupling-introduced disulfide joints further endowed the resulting star-shaped copolymer-based nanocarriers with higher therapeutic efficacy than the analogues containing esterification-generated disulfide links,in terms of better buffering capacity,greater selective drug release in reducing over nonreducing environments,and higher in vitro cytotoxicity.The disclosed effect of introducing strategy of disulfide link on the delivery efficacy of the drug carriers provides new insights into the structure–property relationship of reduction-sensitive delivery systems,as well as offers new opportunities for the future design and development of intelligent nanocarriers toward enhanced therapeutic efficacy.2.In the second charpter,four different amphiphilic miktoarm star copolymers with the respectively identical molecular weights（MWs）of hydrophilic and hydrophobic moieties but different star structures using heteroinitiators were synthesized by protection/deprotection strategies and integrated ring-opening polymerization of hydrophobicε-caprolactone（ε-CL）and atom transfer radical polymerization of hydrophilic oligo（ethylene glycol）monomethyl ether methacrylate（OEGMA）.Further screening of an optimal formulation for anticancer drug delivery by the stability of micelles,in vitro drug loading capacity,drug release properties,cellular uptake efficacy,and cytotoxicity of doxorubicin（DOX）-loaded micelles showed that PCL₃POEGMA₁ micelles possessed the lowest critical micelle concentration,the highest drug loading content,and enhanced therapeutic efficiency for DOX release of all the synthesized four star copolymer constructs.This study thus provides preliminary guidelines and rationalities for the construction of amphiphilic miktoarm star polymers toward enhanced anticancer drug delivery.3.A combination strategy of micelle complex and reducible conjugation was reported in the third charpter to promote folic acid（FA）-targeted anticancer drug delivery.FA was first conjugated to the terminus of the hydrophilic block of a reduction-sensitivemiktoarmstar-shapedamphiphiliccopolymer,PCL₃-SS-POEGMA₁ with the previously optimized star structure by click coupling via a reducible disulfide link.The resulting PCL₃-SS-POEGMA₁-SS-FA was further mixed with the parent PCL₃-SS-POEGMA₁ to afford a micelle complex with both reducibly conjugated and relatively low amount of FA targeting ligands toward excellent FA-mediated targeted drug delivery without compromised salt stability of micelles,leading to the greater in vitro cytotoxicity against HeLa cells relative to the other formulations.Therefore the combined strategy develop herein provides a simple and powerful means to promote FA-mediated anti-cancer drug delivery.