(532d) Gene Delivery from Polymer Scaffolds for Angiogenesis

Recent advances in clinical islet transplantation have increased the rates of insulin independence, indicating that cell replacement therapy is a viable treatment for type I diabetes. However, significant challenges remain that include minimizing the number of islets required, improving islet survival, and identifying the optimal transplant site. The efficacy of islet transplantation is limited, in part, by the detachment of islets from their native extracellular matrix and the loss of vascular connections which occur during the isolation process. This vulnerability of newly transplanted islets may be alleviated by the creation of a supportive environment. Our approach employs porous polymer scaffolds capable of sustained gene delivery to provide a support for islet attachment and manipulate signals present within the local environment. Scaffold-mediated gene delivery can provide extended production of angiogenic proteins to induce the formation of a dense vascular network that may enhance islet survival and function. We are investigating the relationship between scaffold properties, the extent of transgene expression in vivo, and vascular ingrowth. In particular, we focus on the immediate and early post-transplantation period, which is critical for stimulating rapid vascular ingrowth to prevent islet cell apoptosis. Highly porous poly(lactide-co-glycolide) (PLG) scaffolds are fabricated using a gas foaming/particulate leaching process that allows for the incorporation of plasmid DNA. Scaffolds loaded with plasmid DNA encoding for firefly luciferase were implanted in mice and transgene expression was evaluated using bioluminescence imaging and immunohistochemistry. Prolonged transgene expression for over one month was demonstrated at two transplant sites: the subcutaneous space and epididymal fat. A comparison of scaffolds fabricated using different molecular weights (MW) of PLG resulted in high levels of transgene expression for at least 2 weeks for both polymer formulations, indicating that MW can be varied without negatively impacting transgene expression. Additionally, we have shown that transplantation of islets on DNA-releasing scaffolds does not affect the extent of transgene expression obtained. The formation of new blood vessels within scaffolds delivering angiogenic factors is evaluated using microcomputed tomography, which provides 3-D visualization and quantification of the vascular networks. We have demonstrated that increasing the local concentration of vascular endothelial growth factor (VEGF) leads to a dramatic increase in blood vessel density by 3 weeks relative to control scaffolds. The use of polymer scaffolds capable of sustained gene delivery to create an environment that supports islet cell survival and function may prove to be a powerful approach for improving the efficacy of islet transplantation.