Temporal delivery of multiple growth factors from polymer scaffolds to enhance neovascularization

Therapeutic angiogenesis holds great promise as a therapy to treat ischemic diseases and reduce damage to affected tissues. The complex nature of the angiogenic cascade requires the coordination of numerous cell types and various factors, several challenges with current growth factor delivery approaches have been identified. They include limited growth factor exposure time to the host tissue, systemic presence of the factor, and lack of vessel maturation due to single factor administration. To date, neovascularization approaches in many pre-clinical and clinical trials have focused on the initial stages of blood vessel formation. Although blood vessel formation is crucial to battle ischemia due to arterial occlusion, maintaining the new vascular network is vital to the establishment of a meaningful and lasting therapy.; In this thesis, an inductive tissue engineering approach was employed utilizing a polymeric, macroporous scaffold to deliver vascular endothelial growth factor (VEGF), platelet-derived growth factor-BB (PDGF-BB), and the angiopoietins-1 and -2 (Ang1, Ang2) in a temporal manner to promote new blood vessel formation and enhance maturation. The ability of a poly(lactide-co-glycolide) (PLG) system to locally release bioactive factors with distinct kinetics, in vitro and in vivo, was dependent on the protein incorporation method and polymeric properties. Direct protein incorporation into the PLG matrix positioned the factor predominantly adjacent to the pore, allowing it to be released quickly. In contrast, pre-encapsulating the protein prior to matrix fabrication deeply embeds the protein in the polymer resulting in a slower release rate. Using this polymeric system in a subcutaneous mouse model, the dual rapid release of VEGF and Ang2 provided an additive benefit in blood vessel formation. Rapid delivery of VEGF + Ang2, followed by a delayed release of PDGF and Ang1, individually or concurrently, promoted vessel maturation without significantly reducing the density of the newly formed vascular network. In contrast, releasing PDGF + Ang1 at the same time as VEGF + Ang2, reduced vessel density. Both VEGF and VEGF + Ang2 greatly limited necrosis and restored partial blood perfusion in hind limbs of SCID mice subjected to severe ischemia. These studies expose the importance of temporal interactions between these angiogenic molecules on vessel formation and maturation. The results may allow one to control the development of a functional and stable vascular bed in ischemic tissues and improve therapeutic neovascularization.