Conventional drug administration technologies display poorcontrollability, and lead to high plasma concentrations andshort duration times, which frequently lead to adverse effects.Controlled release technology aims at predictable andreproducible delivery of an active substance over an extendedperiod of time, yielding optimal response and prolongedefficiency, and thus offering considerable improvement of manytreatments. A powerful approach to controlled drug delivery isthe incorporation of the drug into a biodegradable polymericmatrix, which distributes the active substance in a controlledand sustained fashion as the polymer erodes.
This thesis describes the design of novel biodegradablepolymer matrices for controlled and sustained drug delivery.New functional and biodegradable materials with variableproperties were obtained by homopolymer blending andcopolymerization of building blocks with specific, desirableproperties; poly(adipic anhydride) (PAA), poly(trimethylenecarbonate) (PTMC), poly(1,5-dioxepan-2-one) (PDXO),poly(L-lactide) (PLLA), poly(D,L-lactide) (PDLLA), andpoly(L-lactide-co-1,5-dioxepan-2-one) (P(L-LA-co-DXO).Techniques were developed for the preparation of drug-releasingmatrices in the form of films and microspheres. Variousanalysis techniques, including Differential ScanningCalorimetry,1H-Nuclear Magnetic Resonance, InfraredSpectrometry, Scanning Electron Microscopy, Size ExclusionChromatography and UV-VIS Spectroscopy, were used forcharacterization.
Microspheres encapsulating therapeutic substances wereprepared from P(L-LA-co-DXO) and blends of PDXO with PLLA orPDLLA. The properties, storage stability,degradation and drugrelease characteristics of these matrices were explored,compared and evaluated with special regard to the morphologyand its impact on thein vitrobehavior. Sustained release of drugs wasobtained. The mode of release was strongly influenced by thehydrophilicity of the drug, and by the copolymer/blendcomposition and morphology. The lactide:DXO composition wasproven to be a versatile tool to control the morphology and inturn the rates and pattern of erosion and diffusion ofencapsulated agents from the matrices.
Films were prepared from PTMC-PAA blends, in which PAA actedas a plasticizer. Loss of the fast-degrading PAA componentenhanced the porosity and hydration of the slow-degrading PTMC.A statistical full factorial model was designed to elucidatethe influence of matrix parameters and their interactions. ThePTMC-PAA ratio, the molecular weight of the PTMC, andinteractions amongst these factors significantly influenced therelease performance, mass loss and degree of plasticization andthe relationships obtained enabled the erosion and drug releasepattern to be predicted and controlled. Moisture uptake,storage stability at different relative humidities, and thesterilizability were determined to further explore theversatility of PTMC-PAA matrices. Thein vivolocal tissue response and biocompatibility ofPTMC-PAA implants was assessed in the anterior chamber ofrabbits eyes for 1 month. PTMC-rich matrices displayed goodbiocompatibility.
Key factors that regulate the biological activity of thesepolymeric vehicles were identified as drug solubility,composition, molecular weight, stereochemical configuration,and morphology. By careful design, the degradation and drugrelease characteristics, e.g. kinetics and duration, can bealtered over a broad spectrum. This study shows that structuralchanges of the polymer backbone, and the modeling ofcomposition and morphology provide powerful means of tailoringsystems for specific applications.
Keywords: controlled release, drug delivery, microspheres,polylactide, poly(1,5-dioxepan-2-one), poly(adipic anhydride),poly(trimethylene carbonate), degradation, blends