Controlled delivery of angiogenic and osteogenic growth factors for bone regeneration
Patel, Zarana S.
Mikos, Antonios G.; Cox, John W.
Doctor of Philosophy
This research introduces a delivery system for the release of an angiogenic and an osteogenic growth factor to address the inability of large bone defects to sufficiently heal. These critical size defects (CSDs) affect approximately one million patients annually. Current treatments rely on the use of bone grafts or permanent orthopedic implants; alternatively, tissue engineering strategies utilize a combination of biomaterials for the delivery of osteoprogenitor cells and osteogenic growth factors. However, few studies address the importance of angiogenesis for bone regeneration, particularly in CSDs which require the presence of an underlying vasculature to recruit and support osteoprogenitor cells within the large defect. We hypothesized that controlled delivery of an angiogenic growth factor, vascular endothelial growth factor (VEGF), and an osteogenic growth factor, bone morphogenetic protein-2 (BMP-2), would demonstrate a compounded effect to induce the closure of CSDs. Both growth factors were delivered from gelatin microparticles incorporated within a porous polymer scaffold. In vitro studies showed that VEGF and BMP-2 release kinetics were affected by the extent of gelatin crosslinking, but growth factor dose was found to affect release only minimally for the doses investigated. Additionally, gelatin type was also found to affect the release profiles of BMP-2. Early delivery of VEGF and minimal burst release and sustained delivery of BMP-2 were attained in vitro and in vivo by using acidic and basic gelatin, and low and high gelatin crosslinking, respectively. A critical size rat cranial defect was also used to evaluate the regenerative potential of this dual delivery system in vivo. At 4 weeks, there was no difference in blood vessel formation between groups, but dual release groups exhibited significantly higher bone formation at 4 weeks along with increased bony bridging at 12 weeks. Bone formation at 12 weeks in the dual release and BMP-2-only groups were significantly higher than in VEGF-only groups or blank scaffolds. These results suggest a synergistic effect for dual release at early time periods and indicate faster healing times at later periods. The studies presented here demonstrate the potential of this unique dual release system for use in strategies for bone regeneration.