Synthesis, Self-assembly And Drug Release Of PMAA Biodegradable PMAA-b-PLA-b-PMAA Multi-block Copolymers

Amphiphilic block copolymers have attracted tremendous attention in bioscience, materials science and pharmaceutical science because of their excellent properties. The synthesis and micellization of pH-sensitive amphiphilic block polymers have become one of the hot topics in the research of copolymer micelle-based drug nanocarriers. Albeit amphiphilic block copolymers have exhibited great potentials in the field of solubilizing poorly water-soluble drugs, sustained and controlled drug delivery and targeted drug delivery, there exist some problems demanding prompt solutions, such as the stability in blood circulation, biodegradabiliry and pH-responsive targeted drug release, which still cannot meet the higher requirements for biomedical applications. The structural forms of the amphiphilic block copolymers have impact on the self-assembly and their properties as drug carriers, non-linear copolymers such as star block copolymers, hyperbranched copolymers and H-type copolymers has therefore attracted great interest. In particular, the H-type amphiphilic block copolymers are expected to achieve unique physicochemical properties of the self-assembly micelles in that they have a space symmetrical structure and can combine two different blocks by connecting two symmetrical side chains with the main chain ends. Based on the above-mentioned description, pH-sensitive biodegradable polymethacrylic acid-block-polylactic acid-block-polymethacrylic acid (PMAA2-b-PLA-b-PMAA2) H-type multiblock copolymers were synthesized in this work by using PLA with good biocompatibility and biodegradability as hydrophobic blocks and PMAA containing carboxyl groups as stimuli-responsive blocks. The self-assembly behavior in aqueous solution, the loading and release behavior of anticancer drugs and biomedical applications of the block copolymers have been studied as well.The specific studies were summarized as follows.1. Preparation and characterization of H-type multi-block copolymer PMAA2-b-PLA-b-PMAA2A series of PtBMA2-b-PLA-b-PtBMA2 amphiphilic multi-block copolymers with different block molar ratios and relative molecular mass were synthesized through atom transfer radical polymerization (ATRP) using Cl2-PLA-Cl2 as a macroinitiator and tertiary-butyl methacrylate (tBMA) with various molar ratios of [tBMA] to [Cl2-PLA-Cl2] as a monomer. Then, the pH-sensitive PMAA2-b-PLA-b-PMAA2 multi-block copolymer was obtained by the acidolysis reaction of PtBMA2-b-PLA-b-PtBMA2. The chemical structure, constitution and relative molecular mass of the synthesized block copolymers were characterized by 1H NMR spectrum, infrared spectrum (FT-IR) and gel permeation chromatography (GPC).2. Micellization behavior and measurements of physicochemical propertiesThe self-assembly micellization behavior of the synthesized copolymers in aqueous solution were investigated by transmission electron microscopy (TEM), dynamic light scattering (DLS) and fluorescence spectroscopy. The experimental results showed that the spherical micelles with regular core-shell structure were formed by the self-assembly of the synthesized multi-block copolymer in aqueous solution or in various physiological environments. The hydrodynamic diameters were all below 200 nm, from 139.5 to 196.5 nm. It is very meaningful for the copolymer micelles with small size to serve for drug controlled release carrier. The CAC values characterized by fluorescence spectroscopy were very low from 19.7 to 32.5 mg L-1, and increased with extending the PMAA block length. The low CAC values revealed that the synthesized multi-block copolymer micelles were very stable even in diluted PBS solutions. Zeta potentials measurements revealed that the copolymer micelles were negatively charged due to ionized carboxyl groups in various PBS solutions. The change in light transmittance with pH values was investigated by ultraviolet-visible spectrophotometer (UV-vis), and the results indicated that the transmittance decreased with the reducing the pH values. H-type multi-block copolymer micelles were pH-sensitive and the pH phase transition was located at about 5.77. These characteristic parameters were affected by the composition and the length of the chains.3. Research on drug loading and drug release behaviorHydrophobic anti-cancer drugs,10-hydroxy camptothecin (HCPT) and paclitaxel (PTX), were chosen as model drugs to research the drug loading and releasing. The results indicated that the drug encapsulation efficiency depends on not only the length of PMAA chains and copolymer constitutions, but also the drug structure. The drug encapsulation efficiency for HCPT and PTX can reach up to 60.2 and 78.2%, respectively, which was higher than that of other linear amphiphilic block copolymers. The drug release behavior of PTX showed that it was pH-sensitive and the drug release rate was faster in PBS solution of pH 5.6-7.4 than in that of pH 1.4, which is very important for the copolymer micelle based pharmaceutical preparation to be used for treatment of small intestinal cancers.The in vitro biodegradation evaluation revealed that the degradation rate of the micelles changed with changing the copolymer constitution and environment conditions. The degradation rate in pH 7.4 was far higher than that in pH 1.4. This trend was well in agreement with the pH-sensitive PTX release. Moreover, the drug-loaded micelles had good stability and their drug leakage was very low after even 35 days’ storage. All these results revealed that the degradable pH-sensitive PMAA2-b-PLA-b-PMAA2 H-type multi-block copolymer micelles synthesized in this work can be used as hydrophobic anti-cancer drug targeted release carriers to treat small intestinal cancer.4. Studies on biological properties of the micelles-based drug formulationMTT assays were used to evaluate the in vitro cytotoxicity of pH-sensitive multi-block copolymers in various environments with different pH values. The assessment results demonstrated that the blank copolymers were almost non-cytotoxic against L929 cells, suggesting that the H-type multi-block copolymer micelles were nontoxic. On the contrary, the cytotoxicity of the PTX-loaded copolymer micelles was affected by pH, and was high at pH of 7.4,but was low at pH of 1.4. The results suggested that the synthesized copolymer micelles drug formulation could efficiently kill the tumor cells in small intestinal, but did no harm to the normal cells in stomach, exhibiting obvious targeted therapeutic effect.