Development and Characterization of Biodegradable Elastomers for Localized Angiogenic Growth Factor Delivery
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Therapeutic angiogenesis is a promising technique to treat ischemia by creating new blood vessels. The aim of this thesis was to develop and characterize biodegradable elastomers for localized delivery of growth factors and to investigate the ability of released growth factors to induce angiogenesis. An osmotic delivery mechanism using photo-cross-linked elastomers based on trimethylene carbonate (TMC) was used to deliver vascular endothelial growth factor (VEGF165) and hepatocyte growth factor (HGF) alone or in combination at two different doses. It was hypothesized that elastomers made of TMC can provide an effective osmotic release using trehalose as a main osmotigen and that the use of TMC would eliminate the microenvironmental pH drop implicated in denaturing acid sensitive growth factors. To obtain an insight into the degrading zone in which growth factors will be released, the in vivo degradation mechanism and tissue response were investigated. The in vivo degradation of D,L-lactide/ε-caprolactone (DLLACL) elastomers that degrade by hydrolysis was investigated for comparison. Cross-link-density played a significant role in the degradation pattern of DLLACL elastomers. TMC and TMCCL elastomers degraded by surface erosion and oxidation played a significant role in their in vivo degradation. To obtain an efficient release, the mechanical properties of TMC elastomers were tailored by copolymerizing TMC with CL and DLLA and/or by controlling the cross-link density. The delivery device was able to provide a sustained release of growth factors for longer than two weeks with no initial burst. Cell based bioactivity assays indicated that released growth factors were highly bioactive over the entire release period. Microenvironmental pH studies using FITC-BSA indicated no significant drop in pH in TMC elastomers that contained small amounts of DLLA. Using 125I-VEGF165, it was found that the osmotic delivery can provide a direct in vivo-in vitro release correlation. Released growth factors were able to induce angiogenesis in rats when tested by subcutaneous implantation. Angiogenesis was dose dependent for both VEGF165 and HGF. Combined release of VEGF and HGF achieved the best results. The formed blood vessels were stable during the active release period, and they were normal looking and connected to the surrounding vasculature.