Amphiphilic linear-dendritic block copolymers for drug delivery
Author(s)Nguyen, Phuong, Ph. D. Massachusetts Institute of Technology
Massachusetts Institute of Technology. Dept. of Chemical Engineering.
Paula T. Hammond and Robert Langer.
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Polymeric drug delivery systems have been widely used in the pharmaceutical industry. Such systems can solubilize and sequester hydrophobic drugs from degradation, thereby increasing circulation half-life and efficacy. However, there are still challenges in the design of drug delivery vehicles to achieve efficient drug delivery in a site-specific manner. In this thesis, an amphiphilic linear-dendritic block copolymer was designed, synthesized, and applied as a new polymeric drug delivery platform. First, to develop the drug delivery vehicle, an ABA dendritic-linear-dendritic block copolymer consisting of poly(amidoamine) (PAMAM) and poly(propylene oxide) (PPO) was synthesized. In order to determine the viability of the linear-dendritic block copolymer as a drug delivery vehicle, the solution-phase self-assembly behavior and the self-assembled structures were characterized experimentally and through molecular dynamics simulations. The triblock self-assembles in aqueous media to form stable micelles with low CMC values. Dynamic light scattering results and TEM indicate the formation of particles ranging from 9 to 18 nm in diameter, with smaller diameters exhibited at higher generations. Static light scattering also confirmed the trend where the aggregation number decreased with higher generations. The experimental characterization results indicated that the physical characteristics of the PPO-PAMAM micelles were desirable and within the design specifications necessary for drug delivery. The experimental results were utilized to set up simulations where further knowledge of the microstructure of the micelles formed could be gained. It was found that the block copolymers simulated formed micelles in the same size range that was seen experimentally. However, the simulations indicated that the micelles displayed greater asphericity than dendrimers.(cont) Backfolding of the terminal amine ends was encountered, which would have implications for the configuration and spacing of any additional targeting ligand attached to the dendritic ends. Further analysis revealed that with increasing generation, the porosity of the micelles increased, which could affect the diffusion rate of drugs released out of the system. Another important finding detailed the preferential localization of a model hydrophobid drug, triclosan, in an equilibrated micelle structure. Additional experiments were performed to assess the feasibility of the nanoparticles for drug delivery applications. Drug loading studies were performed with a model hydrophobic drug, triclosan, resulting in high loading efficiencies. In comparison, linear block copolymers were half as efficient in loading triclosan. It was determined that the dendritic block synergistically increased the drug loading due to either acting as an additional block capable of encapsulating drug or sterically favoring the seclusion of the drug in the core. The linear-dendritic block copolymer synthesized was found to be a promising candidate for drug delivery due to its relative stability in aqueous solution and its drug encapsulation and release properties. Overall, the linear-dendritic block copolymer displayed physical characteristics and self-assembly behavior that satisfied the design criteria for a viable drug delivery vehicle. As a further step, the potential benefits of the novel linear-dendritic architecture were explored in two different drug delivery applications. First, PPO-PAMAM was explored as a circulating nanoparticle with the capability of multivalently targeting to specific cells, due to the presence of the dense functional groups on the dendritic block forming the corona of the micelles. PPO-PAMAM was functionalized with galactose and targeted to hepatocellular carcinoma cells. It was found that the polymer was not cytotoxic and could bind to the asialoglycoprotein receptor.(cont) The galactose-functionalized micelles were loaded with a chemotherapeutic, doxorubicin, and delivered to the carcinoma cells more efficiently than non-functionalized micelles and bare doxorubicin. The results from in vitro testing showed that PPO-PAMAM micelles with targeting capability are promising circulating drug delivery vehicles. In order to ensure success of subsequent testing in vivo of the targeted linear-dendritic block copolymer system, some improvements to the system were explored. First, PPO-PAMAM micelles were stabilized by physical entrapment of the hydrophobic core. An emulsion polymerization of hydrophobic methacrylate monomers created an interpenetrating polymer keeping the micelles intact at concentrations below the CMC and in a solubilizing solvent, methanol. This improvement would ensure that once injected into the bloodstream, the micelles would not destabilize and release high concentrations of drug. Another improvement that was explored was the synthesis of a new linear-dendritic block copolymer composed of a hydrophobic poly(amino acid) and a polyester dendron. Additionally, poly(ethyleneglycol) (PEG) groups were attached to the outer surface of the polyester dendron. The new system synthesized has a low CMC and is thermodynamically slow to break apart in the bloodstream. Furthermore, the micelles formed would be able to circulate for longer times with PEG aiding in evading the reticuloendothelial system. The second drug delivery application explored, which advantageously utilized the dendritic blocks on the outer surface of the block copolymer micelles was as a localized drug delivery coating created by the layer-by-layer (LbL) assembly approach. The electrostatic LbL assembly approach offers large potential in the area of drug delivery from thin films and surfaces; however, because the processing technique is aqueous-based, there have been few strategies proposed to incorporate hydrophobic molecules into these films.(cont) Here we created an LbL film that is capable of incorporating hydrophobic drug at high loadings via encapsulation with linear-dendritic block copolymer micelles and demonstrate for the first time release times of a hydrophobic antibacterial agent over a period of several weeks--a significant improvement over reports of other micelle-encapsulated thin films with release times of several minutes. The PAMAM block, which is polycationic, enabled LbL deposition with negatively charged poly(acrylic acid) (PAA). The stable PPO-PAMAM micelles incorporated into the LbL films encapsulated a hydrophobic bactericide, triclosan. Film thickness and UV-vis measurements confirm the formation of the LbL film and incorporation of triclosan into the film. Fluorescence measurements of PPO-PAMAM/PAA films with pyrene indicated the presence of hydrophobic domains in the film. GISAXS revealed regular spacing of approximately 10.5 nm in the direction parallel to the film substrate, which is approximately the same size as the PPO-PAMAM micelles in aqueous solution. Volume fraction measurements based on elemental analysis and TGA confirm the GISAXS data. An in vitro release study revealed long release times of triclosan on the order of weeks, and a Kirby Bauer test was performed on Staphylococcus Aureus demonstrating that the drug released was still active to inhibit the growth of bacteria. Linear-dendritic block copolymer micelles were successfully used in two different drug delivery applications where the dendritic block could be fully utilized. It is hoped that with the research and results presented in this thesis further development of this drug delivery platform can result in a product successfully treating a serious disease.
Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Chemical Engineering, 2007.Includes bibliographical references.
DepartmentMassachusetts Institute of Technology. Dept. of Chemical Engineering.; Massachusetts Institute of Technology. Department of Chemical Engineering
Massachusetts Institute of Technology