Controlled Syntheses Of Polyphosphoesters And Their Applications In Drug Delivery

Polyphosphoesters are a series of biodegradable polymers with repeating phosphoester bonds in the backbone. Owing to their good biocompatibility, and biodegradability through hydrolysis as well as enzyme catalytic degradation, polyphosphoester are raising great interest in biomedical applications.The purposes of this dissertation are to overcome the problem of uncontrollability encountered by traditional syntheses methods of polyphosphoesters, and to develop nano-drug carriers based on polyphosphoesters with controllable properties. We have studied two catalytic systems for ring-opening polymerization of cyclic phosphoester monomers, and demonstrated that the polymerization is with living characteristics and thus can be utilized to synthesize polyphosphoesters with controlled molecular weight, composition and structure. Taking the advantages of the controlled synthesis methods and the unique properties of polyphosphoester, we have constructed a few nano-carriers for drug delivery, including micellar nanoparticles and nanogels with tunable sizes and properties, which have been applied for targeted delivery of chemotherapeutic drugs, intracellular responsive drug delivery and reversal of multi-drug resistance of cancer cells.The main content and conclusions of this dissertation are summarized below:1. We have investigated the ring-opening polymerization of cyclic phosphoester monomers using aluminum isopropoxide and stannous octoate as the initiators. The polymerization kinetics studies reveal the living characteristics of polymerization. The molecular weight and molecular architecture of polymers, including block copolymers, can be well-controlled by adjusting the molar ratios of monomers and the initiator as well as the reaction time. It has also been found that, the polymerization rate of cyclic phosphoester monomer is highly depended on the structure of pendant side group and the ring substituent group when using stannous octoate as the catalyst. The polymerization method also facilitates the synthesis of functionalized polyphosphoesters with pendant functional modification (e.g. by "click" chemistry).2. Taking the advantages of the controlled synthesis methods above and the unique properties of polyphosphoester, including the biodegradability, water solubility and good biocompatibility, we have developed a series of triblock copolymers of poly(s-caprolactone) and poly(ethyl ethylene phosphate) (PCL-b-PEEP). We have investigated the self-assembly behavior of those block copolymers, and constructed micellar nanoparticles with hydrophilic poly(ethyl ethylene phosphate) (PEEP) as the shell material for drug delivery. It has been proved that micelles with PCL core and PEEP shell are cytocompatible, while the size of micelles can be finely tuned by adjusting the molecular weight and composition of the copolymers. Moreover, the critical micellization concentrations of the micelles are relatively lower compared with micelles bearing poly(ethylene glycol) (PEG) shell, indicating that micelles with PEEP shell can be more stable thermodynamically. The drug release of paclitaxel from those micelles is correlative to the polymer structures.PCL-b-PEEP copolymers obtained through ring-opening polymerization catalyzed by Al(O'Pr)3 or stannous octoate bear hydroxyl end groups. The hydroxyl groups have been further modified to achieve active targeting in drug delivery for cancer therapy. With surface conjugation of galactose to the end of PEEP chain, the micellar nanoparticles can targeted deliver paclitaxel to HepG2 cells via interaction with asialoglycoprotein receptor (ASGP-R) presented on the cell membranes, leading to higher cytotoxicity when compared with the micelles without galactose ligands.3. We have developed multi-responsive micelles as nano-drug carriers based on the thermoresponsibility of polyphosphoesters, and investigated the intracellular drug release behaviors. The responsibility of micelles composed of poly(ε-caprolactone) and thermoresponsive polyphosphoester can be widely tuned by adjusting the molecular weight and polymer composition. The multi-responsibilities (thermo-, pH-and light-responsibilities) have been simply achieved and finely controlled by subtle chain terminal modification of polyphosphoester chain. Such stimuli-responsive micelles lead to responsive drug release properties in response to multi-environmental stimuli, which are adjustable by the subtle chain terminal groups. The micelles with carboxyl groups on the surface exhibit thermo-sensitivity in responsive to pH variation, thus the endosomal/lysosomal pH (pH 5.5) triggers rapid drug release from the micelles at 37℃, owing to that pH lowers the lowest critical solution temperature (LCST) of micelles to a temperature lower than 37℃. On the contrast, at pH 7.4, the LCST of the micelles is higher than 37℃and the drug release is much slower. Thus, with the encapsulation of doxorubicin, the dual pH-/thermo responsive micelles significantly increase the cytotoxicity of doxorubicin against both MCF-7 and drug-resistant MCF-7/ADR cancer cells.4. We have designed and synthesized water soluble block copolymers composed of poly(ethylene glycol) and thermoresponsive polyphosphoesters. Based on the influence of salt on the thermoresponsibility, we have developed multi-functional nano-hydrogel with polyphosphoester core using a template-free method, and investigated the drug delivery behaviors. It is demonstrated that the composition of polymer, salt concentration and molecular weight of the polymer influence the thermoresponsibility. The polymers form self-assemblies when the environmental temperature is higher than its LCST, and the size of assemblies depends on the concentration of both polymer and salt. The self-assemblies are proven to be biocompatible and biodegradable. Photo-crosslinking of the salt-induced assemblies leads to formation of nanogels. Moreover, with the integration of lactosyl moieties onto the surface of nanogels, the nanogels can specifically bind HepG2 cells mediated by ligand-receptor interaction, thus deliver drug to cells more efficiently, resulting enhanced cytotoxicity.5. The multidrug resistance of cancer cells against chemotherapeutic drugs is a major factor in the failure of chemotherapy. To overcome the multidrug resistance, we have designed and synthesized a disulfide-linked biodegradable diblock copolymer of poly(ε-caprolactone) and poly(ethyl ethylene phosphate) (PCL-S-S-PEEP), which forms micellar nanoparticles but detaches the PEEP shell in response to the intracellular reduction conditions (e.g., GSH). In one aspect, the micelles can enter cancer cells through internalization pathways, which protecting the drug from the efflux by P-glycoprotein located on the cell membranes, and resulting in drug accumulation in MCF-7/ADR drug resistant cells. In another aspect, the intracellular reduction conditions break the disulfide linkages between hydrophobic PCL and hydrophilic PEEP chains, thus destroy the micelles and accelerate the intracellular rapid release of doxorubicin incorporated in the micelles. The shell-detachable drug-loaded micelles in response to intracellular reduction conditions significantly overcome the drug resistance of MCF-7/ADR cells, lowering the IC50 to 15% of the free doxorubicin drug.