The overall goal of this project is to investigate the effects of polymer microsphere size and size distribution, and the shell thickness of core-shell microparticles, on macromolecule encapsulation and release. Proteins and other macromolecules form a rapidly growing class of therapeutics. Because of short in vivo half-life and poor oral bioavailability, frequently repeated injections are usually required. Alternative formulations and delivery systems for proteins present a variety of new challenges. Controlled-release drug delivery systems may provide better control of drug concentrations, reduced side effects and improved compliance. However, design of controlled-release devices, such as biodegradable polymer microspheres, is difficult because of incomplete understanding of the mechanisms that control release and that affect protein stability. We have reported a unique precision particle fabrication (PPF) technique that produces monodisperse microspheres, with precise control of particle size, and core-shell particles with control of shell thickness. Previously, we have shown that by controlling the particle size and shell thickness, we could control small-molecule release rates, and we have discovered several unexpected mechanisms by which particle size can affect release. PPF may provide a similar opportunity to control macromolecule release. Further, we hypothesize that the core-shell morphology, in particular those with aqueous cores, may provide enhanced stability of encapsulated proteins. In this project, we will test these hypotheses by investigating in vitro release of four model compounds from uniform PLGA microspheres (Aim 1) and core-shell particles of various morphologies (Aim 2). To investigate release mechanisms, experiments are also designed to follow changes in drug distribution and particle morphology during degradation. Preliminary results suggest that microspheres undergo autocatalytic degradation due to accumulation of acidic polymer degradation products to an extent depending on diameter. Thus, we will use ratiometric confocal fluorescence microscopy to measure pH microenvironment inside degrading particles with varying diameter and shell thickness (Aim 3). Finally, we will compare stability of proteins encapsulated in solid microspheres versus microcapsules, as the latter will exhibit a smaller aqueous-organic interface and may maintain more neutral intraparticle pH (Aim 4). To now, the effects of microparticle size on the factors controlling release rates have been obscured by typically broad particle size distributions. Through precision particle fabrication, this project will result in novel fundamental insights into mechanisms controlling release as well as provide a method for enhanced control of release rates.
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