Self-assembly Nano-sized Micelles Prepared From Bioresorbable Polylactide-poly(Ethylene Glycol) Block Copolymers As Controlled Drug Release System

In the past two decades, nano-sized micelles derived from amphiphilic block copolymers of polylactide-poly(ethylene glycol) (PLA-PEG) have been widely investigated as novel controlled drug release system. These micelles possess a core-shell structure:the hydrophobic PLA segments aggregate to form an inner core able to incorporate hydrophobic drugs; the outer shell consists of hydrophilic PEG blocks able to maintain the stability of micelles in water. However, the most common method to prepare micelles in literature is solvent evaporation or dialysis, involving the use of organic solvents such as dichloromethane, acetonitrile and acetone, which might cause side effects to human body.In this work, a novel micelle preparation method called direct dissolution has been suggested. This method presents great potential for applications in the drug delivery field due to its advantages including absence of organic solvents and easy formulation. In addition, L/D mixed micelles were obtained by dissolving equal molar PLLA-PEG and PDLA-PEG, in order to investigate the influence of stereocomplexation effect on micelle properties. The main contents are shown as following:(1) A series of PLA-PEG diblock and triblock copolymers were obtained by ring-opening polymerization of L-or D-lactide in the presence of mono-or dihydroxyl PEG using nontoxic zinc lactate as catalyst. The molecular weight and distribution, composition, crystallization behavior and thermal properties of these copolymers were characterized. Bioresorbable micelles were prepared by direct dissolution method. Surface tension measurements were used to determine the critical micellar concentration (CMC) of the micelles. The results show that L/D mixed micelles are more stable than single ones due to strong stereocomplexation effect between PLLA and PDLA blocks. The properties of the polymeric micelles strongly depend on the chain structure and composition of the copolymers. CMC values in the presence of salt and at 37℃suggested that the micelles could exhibit good stability in vivo. Thermodynamic parameters calculated from the dependence of CMC on temperature indicate that the micellization process is spontaneous and driven by entropy gain. Dynamic light scattering (DLS) and transmission electron microscopy (TEM) were used to observe the size and morphology of the micelles.(2) The aggregation behavior of PLA-PEG block copolymer micelles in aqueous medium was investigated in detail. The results show that the average hydrodynamic radius (Rh) of L/D mixed micelles is lower than that of PLLA-PEG single ones. It is also confirmed that the micelle size increases with increasing temperature and hydrophobic block length, but decreases after salt addition. Aqueous gel permeation chromatography (GPC) was used for the first time to evaluate the molecular weight (Mw) and aggregation number (Nagg) of the micelles, in comparison with data obtained from static light scattering (SLS). It appears that mixed micelles present lower Nagg than single copolymer ones. Nagg decreases with increasing hydrophobic block length and upon addition of NaCl, but increases with elevating temperature. The gyration (Rg) and Rg/Rh ratio data show that both increase with increasing temperature. As temperature increases from 15℃to 35℃, the second Virial coefficient (A2) of PLA-PEG copolymers turns from negative to positive, which means that water changes from poor solvent to good solvent. The average density p of PLA-PEG micelles decreases with increasing temperature, confirming that micelles exhibit a looser structure at higher temperature due to water swelling effect.(3) A series of PLA-PEG block copolymer micelles were prepared by two different methods:direct dissolution and dialysis. The hydrolytic degradation properties of these micelles were investigated. During the investigated degradation time period up to 8 weeks, the size of micelles by dialysis remains stable, while that by direct dissolution increases after 3-5 weeks, followed by a collapse of the structure. This is because the former micelles possess a more compact structure due to the different formation mechanisms. Micelles with longer hydrophobic PLA blocks present a more compact structure, leading to less size change during the same degradation time. The structure of L/D mixed micelles can be preserved for longer time than that of single ones. With degradation proceeding, the average molecular weight of copolymer decreases and the weight distribution becomes wider, especially for micelles by dialysis and L/D mixed micelles which possess a more compact structure. This can be attributed to the concentrated carboxyl endgroups of short LA oligomers which may accelerate the hydrolysis of remaining ester bonds. The PEG content in the copolymer chains increases during degradation, leading to a decrease of glass transition temperature (Tg) and crystallization temperature (Tc) of the copolymers. However, the remaining LA oligomers have much effect on the crystallinity of PEG blocks, thus resulting in the decrease of melting transition temperature (Tm) and melting enthalpy (△Hm). (4) Drug-loaded PLA-PEG micelles were prepared by direct dissolution method, using a hydrophobic anticancer drug, paclitaxel, as model drug. In vitro and in vivo properties were studied in comparison with micelles by dialysis method. The direct dissolution method yields comparable drug encapsulation efficiency (EE) and loading content (LC) as the dialysis method. The drug encapsulation ability is higher for L/D mixed copolymer micelles than single micelles due to stereocomplexation. Diameters of drug-loaded micelles are larger than blank ones as determined by DLS. In vitro drug release properties of the micelles strongly depend on the micelle fabrication method and copolymer composition. Faster release was obtained for micelles derived from direct dissolution method. The in vivo experiments show that paclitaxel is widely distributed and kept at high concentration levels in various tissues after administration of drug-loaded micelles. Compared with current clinical formulation and micelles by dialysis, direct dissolution micelles with paclitaxel exhibit the highest antitumor ability.