Liwei Ma

Synthesis and Phase Transition of Poly(N-isopropylacrylamide)-Based Thermo-Sensitive Cyclic Brush Polymer

Polymers with advanced topological architectures are promising materials for wide applications due to their structure-generated unique properties different from that of the linear analogues. The elegant integration of stimuli-responsive polymers with such advanced architectures can create novel materials with virtues from both moieties, are thus a hot subject of research for both fundamental and practical investigations. To fabricate cyclic brush polymer-based intelligent materials for biomedical applications, herein, we designed and synthesized thermo-sensitive cyclic brush polymers with poly(N-isopropylacrylamide) (PNIPAAm) brushes by controlled living radical polymerization using cyclic multimacroinitiator. The thermo-induced phase transition behaviors of the resultant cyclic brush polymers with different compositions were investigated in detail by temperature-dependent optical transmittance measurements, and compared with the properties of bottlebrush and linear counterparts. Interestingly, the cloud point transition temperature (Tcp) of cyclic brush PNIPAAm could be regulated by the chain length of PNIPAAm brush. Although the bottlebrush polymers with the same composition exhibited similarly structurally dependent Tcps behaviors to the cyclic brush polymers, the cyclic brush PNIPAAm did show higher critical aggregation concentration (CAC) and enhanced stability against dilution than the bottlebrush counterpart. The readily tailorable Tcps together with the ability to form highly stable nanoparticles makes thermo-sensitive cyclic brush PNIPAAm a promising candidate for controlled drug delivery.

pH-sensitive drug release of star-shaped micelles with OEG brush corona

Star-shaped polymers are attractive as drug carriers due to the integration of relatively easy synthesis compared to the preparation of hyper-branched dendrimers and their potential ability to form unimolecular nanoparticles with enhanced stability to survive the high dilution-associated deformation and subsequently premature drug release in the body fluids and blood during circulation. Clearly, the star structure exerts a significant effect on their bioproperties and potential applications. In this study, three star-shaped amphiphilic copolymers with the same polymer compositions but different star structures were synthesized to investigate the effect of star architecture on their properties as well as potential applications as drug carriers. Three different branched alcohols, 2-(hydroxymethyl) propane-1,3-diol, pentaerythritol and dipentaerythritol with 3, 4, and 6 respective hydroxyl groups were chosen as the starting core to generate the target star-shaped amphiphilic block copolymers composed of hydrophobic poly(e-caprolactone) (PCL) and hydrophilic poly(oligoethyleneglycol methacrylate) (POEGMA) with 3, 4, and 6 corresponding star arms by a combination of ring-opening polymerization (ROP) and atom transfer radical polymerization (ATRP). Dynamic light scattering (DLS) measurements and pyrene fluorescence probe technique confirmed the capability of the resultant star-shaped amphiphilic copolymers to form unimolecular micelles with average diameters smaller than 50 nm in an aqueous phase. A comparison of the drug loading capacity revealed that the micelles of 4-star-PCL–POEGMA (4s-PCL–POEGMA) exhibited the highest drug loading content (DLC) of all the three formulations. More importantly, in vitro doxorubicin (DOX) release study showed unique pH-mediated drug release behaviors, i.e., dramatically accelerated release at pH 6.0 but a much slower profile at pH 7.4, from these generally recognized “pH-insensitive” 4-arm star-shaped micelles. An acid-triggered degradation and an acid–base titration were performed to further reveal that such interesting pH-responsive drug release behaviors were attributed primarily to the hydrophilic corona of OEG brushes. This study is believed to provide a new insight into the structure–bioproperties relationship of star-shaped polymers, thus should be useful for the future design and development of novel star-shaped polymers for controlled drug delivery.

Fabrication of supramolecular star-shaped amphiphilic copolymers for ROS-triggered drug release

Star-shaped copolymers with branched structures can form unimolecular micelles with better stability than the micelles self-assembled from conventional linear copolymers. However, the synthesis of star-shaped copolymers with precisely controlled degree of branching (DB) suffers from complicated sequential polymerizations and multi-step purification procedures, as well as repeated optimizations of polymer compositions. The use of a supramolecular host-guest pair as the block junction would significantly simplify the preparation. Moreover, the star-shaped copolymer-based unimolecular micelle provides an elegant solution to the tradeoff between extracellular stability and intracellular high therapeutic efficacy if the association/dissociation of the supramolecular host-guest joint can be triggered by the biologically relevant stimuli. For this purpose, in this study, a panel of supramolecular star-shaped amphiphilic block copolymers with 9, 12, and 18 arms were designed and fabricated by host-guest complexations between the ring-opening polymerization (ROP)-synthesized star-shaped poly(ε-caprolactone) (PCL) with 3, 4, and 6 arms end-capped with ferrocene (Fc) (PCL-Fc) and the atom transfer radical polymerization (ATRP)-produced 3-arm poly(oligo ethylene glycol) methacrylates (POEGMA) with different degrees of polymerization (DPs) of 24, 30, 47 initiated by β-cyclodextrin (β-CD) (3Br-β-CD-POEGMA). The effect of DB and polymer composition on the self-assembled properties of the five star-shaped copolymers was investigated by dynamic light scattering (DLS), transmission electron microscopy (TEM), and fluorescence spectrometery. Interestingly, the micelles self-assembled from 12-arm star-shaped copolymers exhibited greater stability than the 9- and 18-arm formulations. The potential of the resulting supramolecular star-shaped amphiphilic copolymers as drug carriers was evaluated by an in vitro drug release study, which confirmed the ROS-triggered accelerated drug release from the doxorubicin (DOX)-loaded supramolecular star-shaped micelles due to the oxidation-induced dissociation of β-CD/Fc pair and the consequent loss of the colloidal stability of the star-shaped micelles. Studies of the delivery efficacy by an in vitro cytotoxicity study further indicated that higher DBs and longer hydrophilic arm compromised the therapeutic efficacy of the DOX-loaded supramolecular star-shaped micelles, resulting in significantly reduced cytotoxicity, as measured by increased IC50 value. Overall, our results revealed that the screening of hydrophilic block by DB and MW for an optimized star-shaped copolymer should balance the stability versus therapeutic efficacy tradeoff for a comprehensive consideration. Therefore, the 12-arm star-shaped copolymer with POEGMA30 is the best formulation tested.

Fabrication of Hyperbranched Block-Statistical Copolymer-Based Prodrug with Dual Sensitivities for Controlled Release

Dendrimer with hyperbranched structure and multivalent surface is regarded as one of the most promising candidates close to the ideal drug delivery systems, but the clinical translation and scale-up production of dendrimer has been hampered significantly by the synthetic difficulties. Therefore, there is considerable scope for the development of novel hyperbranched polymer that can not only address the drawbacks of dendrimer but maintain its advantages. The reversible addition-fragmentation chain transfer self-condensing vinyl polymerization (RAFT-SCVP) technique has enabled facile preparation of segmented hyperbranched polymer (SHP) by using chain transfer monomer (CTM)-based double-head agent during the past decade. Meanwhile, the design and development of block-statistical copolymers has been proven in our recent studies to be a simple yet effective way to address the extracellular stability vs intracellular high delivery efficacy dilemma. To integrate the advantages of both hyperbranched and block-statistical structures, we herein reported the fabrication of hyperbranched block-statistical copolymer-based prodrug with pH and reduction dual sensitivities using RAFT-SCVP and post-polymerization click coupling. The external homo oligo(ethylene glycol methyl ether methacrylate) (OEGMA) block provides sufficient extracellularly colloidal stability for the nanocarriers by steric hindrance, and the interior OEGMA units incorporated by the statistical copolymerization promote intracellular drug release by facilitating the permeation of GSH and H+ for the cleavage of the reduction-responsive disulfide bond and pH-liable carbonate link as well as weakening the hydrophobic encapsulation of drug molecules. The delivery efficacy of the target hyperbranched block-statistical copolymer-based prodrug was evaluated in terms of in vitro drug release and cytotoxicity studies, which confirms both acidic pH and reduction-triggered drug release for inhibiting proliferation of HeLa cells. Interestingly, the simultaneous application of both acidic pH and GSH triggers promoted significantly the cleavage and release of CPT compared to the exertion of single trigger. This study thus developed a facile approach toward hyperbranched polymer-based prodrugs with high therapeutic efficacy for anticancer drug delivery.

Fabrication of Thermosensitive Cyclic Brush Copolymer with Enhanced Therapeutic Efficacy for Anticancer Drug Delivery

Adaptation of cyclic brush polymer for drug delivery applications remains largely unexplored. Herein, cyclic brush copolymer of poly(2-hydroxyethyl methacrylate-g-poly(N-isopropylacrylamide-st-N-hydroxyethylacrylamide)) (cb-P(HEMA-g-P(NIPAAm-st-HEAAm))), comprising a cyclic core of PHEMA and thermosensitive brushes of statistical copolymer of P(NIPAAm-st-HEAAm), is designed and synthesized successfully via a graft-from approach using atom transfer free radical polymerization from a cyclic multimacroinitiator. The composition of the brush is optimized to endow the resulting cyclic brush copolymer with a lower critical solution temperature (LCST) slightly above the physiological temperature, but lower than the localized temperature of tumor tissue, which is suitable for the hyperthermia-triggered anticancer drug delivery. Critical aggregation concentration determination reveals better stability for the unimolecular nanoparticle formed by the cyclic brush copolymer than that formed by the bottlebrush analogue. The dramatically increased size with elevated temperatures from below to above the LCST confirms hyperthermia-induced aggregation for both formulations. Such structural destabilization promotes significantly the drug release at 40 °C. Most importantly, the drug-loaded cyclic brush copolymer shows enhanced in vitro cytotoxicity against HeLa cells than the bottlebrush counterpart. The better stability and higher therapeutic efficacy demonstrates that the thermosensitive cyclic brush copolymer is a better formulation than bottle brush copolymer for controlled drug release applications.

Dual transitions of toothbrush-like double hydrophilic block copolymers

An interesting thermo-regulated dual transition from unimers to vesicles, and finally to micelles, was reported for the first time, based on a toothbrush-like double hydrophilic block copolymer (DHBC), poly(N-isopropylacrylamide)-b-poly(oligo(ethylene glycol) monomethyl ether methacrylate) (P(NIPAAm)-b-P(OEGMA)), which was feasibly attributed to the presence of pendent oligo(ethylene glycol) (OEG) brushes and their entanglement with the stimuli-responsive block.

A comparison study to investigate the effect of the drug-loading site on its delivery efficacy using double hydrophilic block copolymer-based prodrugs

Polymeric delivery vehicles can improve the safety and efficacy of chemotherapy drugs by facilitating preferential tumor delivery. Double hydrophilic block copolymer (DHBC)-based prodrugs are considered as ideal candidates for drug delivery due to the elegant integration of benefits from both structures including polymeric prodrugs superior protection and minimal premature drug release using covalent links and a DHBC-based green self-assembly strategy by a simple stimulus in a pure aqueous phase without the use of any organic solvent. Clearly, the location of drug molecules in the polymeric prodrugs has exerted a significant effect on their therapeutic efficiency. However, there has been no published data so far, to our knowledge, reporting the effect of drug-conjugated sites on its therapeutic efficacy, as well as some basic guidelines that can be followed to direct the future design of polymeric prodrugs. To this end, herein a thermo-sensitive DHBC, poly(N-(2-hydroxypropyl) methacrylamide)-b-poly(N-isopropyl acrylamide) (P(HPMA)-b-P(NIPAAm)), was designed and synthesized by successive reversible addition and fragmentation chain transfer (RAFT) polymerizations, and was chosen as a platform to clarify this issue. An anti-cancer drug, doxorubicin (DOX) was conjugated to the hydrophilic PHPMA block and the temperature-responsive P(NIPAAm) block, respectively, through a pH-liable hydrazone bond to fabricate two different types of polymeric prodrugs with the drug tethered to the micellar hydrophilic PHPMA shell or encapsulated within the hydrophobic P(NIPAAm) core upon temperature elevation above its lower critical solution temperature (LCST). A detailed comparison study was carried out to investigate which structure exhibits better properties and higher therapeutic efficacy in terms of micellar size, stability, cellular uptake, drug loading capacity, drug release behaviors and cell viability. The results showed the self-assembly of both DHBC-based prodrugs into well-dispersed spherical micelles with similar average hydrodynamic diameters (Dh) around 150 nm in phosphate buffer (PBS, pH 7.4) at 37 °C, but a higher drug loading content (DLC), and enhanced pH-mediated drug release, i.e., much accelerated drug release at pH 5.0, while slower at pH 7.4, as well as enhanced cytotoxicity when the drug was conjugated to the hydrophilic shell of the micelles. The guidelines obtained in this study are thus believed to direct the future design and development of polymeric prodrugs for efficient cancer therapy.

Promotion of micelle stability via a cyclic hydrophilic moiety

A novel strategy to promote micelle stability was reported by altering the topological structure of polymer species. Specifically, a cyclic hydrophilic moiety offers greater stability for the self-assembled micelles than a linear analogue. This study thus provides an alternative to enhance micelle stability for drug delivery.

Fabrication of Reduction‐Sensitive Amphiphilic Cyclic Brush Copolymer for Controlled Drug Release

The cyclic brush polymers, due to the unique topological structure, have shown in the previous studies higher delivery efficacy than the bottlebrush analogues as carriers for drug and gene transfer. However, to the best of knowledge, the preparation of reduction-sensitive cyclic brush polymers for drug delivery applications remains unexplored. For this purpose, a reduction-sensitive amphiphilic cyclic brush copolymer, poly(2-hydroxyethyl methacrylate-g-poly(ε-caprolactone)-disulfide link-poly(oligoethyleneglycol methacrylate)) (P(HEMA-g-PCL-SS-POEGMA)) with reducible block junctions bridging the hydrophobic PCL middle layer and the hydrophilic POEGMA outer corona is designed and synthesized successfully in this study via a grafting from approach using sequential ring-opening polymerization (ROP) and atom transfer free radical polymerization (ATRP) from a cyclic multimacroinitiator PHEMA. The resulting self-assembled unimolecular core-shell-corona (CSC) micelles show sufficient salt stability and efficient destabilization in the intracellular reducing environment for a promoted drug release toward a greater therapeutic efficacy relative to the reduction-insensitive analogues. The overall results demonstrate the reducible cyclic brush copolymers developed herein provides an elegant solution to the tradeoff between extracellular stability and intracellular high therapeutic efficacy toward efficient anticancer drug delivery.

Facile Fabrication of 10-Hydroxycamptothecin-Backboned Amphiphilic Polyprodrug with Precisely Tailored Drug Loading Content for Controlled Release

Polymeric prodrugs with precisely controlled drug loading content (DLC) and rapid intracellular destabilization generally require complicated chemistry that hinders large-scale manufacture. For this purpose, we reported in this study a facile construction of reduction-sensitive amphiphilic polyprodrugs with an anticancer drug, 10-hydroxycamptothecin (HCPT), and a hydrophilic poly(ethylene oxide) (PEG) moiety as the alternating building blocks of the multiblock copolymer using Cu(I)-catalyzed azide-alkyne cycloaddition (CuAAc) click coupling between azide-SS-HCPT-SS-azide and alkyne-PEG-alkyne. Adoption of PEGs with two different molecular weights (MWs) of 400 and 1450 Da (PEG400 and PEG1450) afforded two polyprodrugs with different DLCs. Both formulations can self-assemble into spherical micelles with hydrodynamic diameter smaller than 200 nm, and exhibit glutathione (GSH)-triggered degradation for promoted drug release. A further comparison study revealed that the PEG1450-based polyprodrug is a better formulation than the analogue constructed from PEG400 in terms of in vitro drug release behaviors, and cytotoxicity. This work thus provides a facile yet efficient strategy toward polymeric prodrugs with precisely controlled DLC and reduction-triggered degradation for enhanced anticancer drug delivery.