Thesis (Sc.D.)--Harvard--Massachusetts Institute of Technology Division of Health Sciences and Technology, 2000. / Includes bibliographical references. / In the past decade, biodegradable polymers have become the materials of choice for a variety of biomaterials applications. In particular, poly(lactic-co-glycolic acid) (PLGA) microspheres have been extensively studied for controlled-release protein delivery. However, significant issues arise in formulating such delivery systems since few proteins have been successfully encapsulated and released from these microspheres. Here, methods are developed to determine the causes of protein instability and solutions are provided for overcoming these problems. A commonly used technique for protein encapsulation in PLGA microspheres is the double-emulsion method. The harsh processing associated with this method can cause denaturation of the encapsulated protein. Herein, we have used Fourier transform infrared (FTIR) spectroscopy to determine the secondary structures of two model proteins, bovine serum albumin (BSA) and chicken egg-white lysozyme, within PLGA microspheres. This is a novel technique for in situ evaluation of proteins within microspheres and potentially a powerful and quick method for assessment of formulations. Results for both proteins indicate changes in structure upon entrapment within the microspheres. However, addition of the stabilizing excipient trehalose prevents the denaturing effects incurred during processing. In addition, BSA released from microspheres is improved by incorporation of trehalose. With microspheres made by double emulsion, there is often a large, initial burst of drug release upon injection. This results in inefficient use of therapy. To prevent this loss, a modified spontaneous emulsification method was explored. / (cont.) With this procedure both protein and polymer are soluble in a single solvent system thus avoiding creation of a water/solvent interface. The process was optimized for microsphere size and protein loading. Addition of a charged surfactant served to improve protein solubility and thus increase protein loading. In vitro and in vivo release kinetics showed a minimal burst, lower than that found with double emulsion microspheres, followed by sustained release. Upon injection of the microspheres in vivo, the PLGA microspheres begin to degrade. Degradation of the polymer generates acidic monomers, and acidification of the inner polymer environment is a central issue in the development of these devices for drug delivery. To quantitatively determine the intraparticle acidity, pH-sensitive fluorescent dyes were entrapped within the microspheres and imaged with confocal fluorescence microscopy. The technique allows visualization of the spatial and temporal distribution of pH within the degrading microspheres. The data indicate the formation of a very acidic environment within the particles with the minimum pH as low as 1.5. The images show a pH gradient, with the most acidic environment at the center of the spheres and higher pH near the edges, which is characteristic of diffusion-controlled release of the acidic degradation products. Strategies to avoid the accumulation of acidic monomers involve decreasing the diffusion distance of the degradation products by either decreasing the overall diameter of the microspheres or creating porous particles. / by Karen Fu. / Sc.D.
| Identifer | oai:union.ndltd.org:MIT/oai:dspace.mit.edu:1721.1/8422 |
| Date | January 2000 |
| Creators | Fu, Karen, 1967- |
| Contributors | Robert S. Langer and Alexander M. Klibanov., Harvard University--MIT Division of Health Sciences and Technology., Harvard University--MIT Division of Health Sciences and Technology. |
| Publisher | Massachusetts Institute of Technology |
| Source Sets | M.I.T. Theses and Dissertation |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | 155 leaves, 10795961 bytes, 10795720 bytes, application/pdf, application/pdf, application/pdf |
| Rights | M.I.T. theses are protected by copyright. They may be viewed from this source for any purpose, but reproduction or distribution in any format is prohibited without written permission. See provided URL for inquiries about permission., http://dspace.mit.edu/handle/1721.1/7582 |
Page generated in 0.0017 seconds