(169e) Delivery of SDF-1α Lowers the Effective BMP-2 Dose for in Vivo Mineralization
The use of bone morphogenic proteins (BMPs) shows promise in therapies for improving bone regeneration; however, high supraphysiological concentrations required for a desired osteoinductive effect, costs, and patient variability have prevented the full advantages of BMP-based therapeutics from being realized. Thus, one strategy is to deliver synergistic molecules with BMP to enhance efficacy and lower doses. In this work, a matrix metalloprotease (MMP)-sensitive hyaluronic acid (HA)-based hydrogel was used for the delivery of both stromal cell-derived factor-1 alpha (SDF-1α) and BMP-2 towards improving BMP-induced osteogenesis. SDF-1α has been shown to play an important role in stem cell trafficking and HA hydrogels are known to increase extracellular matrix production. Additionally, our lab has previously shown that HA is capable of inducing cell chemotaxis through the CD44 receptor.
Maleimide-modified HA (MaHA) was cross-linked using a difunctional MMP-sensitive peptide according to an addition reaction, allowing for protease-mediated hydrogel degradation and growth factor release. The compressive and rheological properties of MaHA hydrogels were characterized as a function of hydrogel polymer content (1-5 wt%) and functionalization (10-40%f), where mechanical properties and gelation time were strongly dependent on cross-linking density. To determine growth factor release, MaHA hydrogels loaded with either 100 ng/scaffold of SDF-1α or BMP-2 were degraded in 10, 2, or 1 U/ml type II collagenase (non-specific MMP degradation) and compared to degradation in buffer alone. In general, BMP-2 and SDF-1α release corresponded closely with hydrogel degradation profiles, where limited release occurred through diffusion in the absence of MMPs. In the presence of MMPs, hydrogel degradation and growth factor release was controlled as a function of both collagenase concentration and hydrogel formulation, where faster growth factor release occurred at higher collagenase concentrations and for hydrogels with lower cross-link densities. In vitro SDF-1α bioactivity was determined by quantifying cellular invasion or outgrowth from a hydrogel-encapsulated cell pellet into the surrounding hydrogel over time. After four days, significant invasion from the cell pellet was observed for hydrogels loaded with 250 ng of SDF-1α and limited invasion was seen without any SDF-1α loaded into the hydrogels. Lastly, a critical-sized cranial defect was used to determine the effect of growth factor delivery on bone formation in vivo. The animal group with combined SDF-1α and BMP-2 delivery showed significantly higher bone formation when compared to groups loaded with the same BMP-2 or SDF-1α concentration delivered separately or the empty defect group. Furthermore, histology indicated increased in vivo cellular invasion for hydrogels loaded with SDF-1α.