Tunable delivery of bioactive peptides from hydroxyapatite biomaterials and allograft bone using variable-length polyglutamate domains.

Hydroxyapatite (HA) biomaterials and allograft bone are common alternatives to autogenous grafts; however, these materials lack the strong osteoinductive potential of autologous bone. Previous studies have established that polyglutamate domains, which bind selectively to HA, can be engineered onto bioactive peptides as a mechanism for coupling osteoinductive signals onto HA and allograft. In the current investigation, we adapted the polyglutamate approach to tailor delivery of a model collagen-derived peptide, Asp-Gly-Glu-Ala (DGEA), by manipulating the number of glutamates in the HA binding domain. Specifically, DGEA was modified with diglutamate (E2-DGEA), tetraglutamate (E4-DGEA), or heptaglutamate (E7-DGEA), and it was found that initial peptide binding to HA and allograft was significantly enhanced as the number of glutamates increased. We also determined that the rate of release of polyglutamate-DGEA from substrates over a 5-day interval increased proportionally as the number of glutamate residues was decreased. Additionally, we tuned the peptide release rate by creating mixtures of E2-DGEA, E4-DGEA, and E7-DGEA, and observed that release kinetics of the mixtures were distinct from pure solutions of each respective peptide. These collective results suggest that variable-length polyglutamate domains provide an effective mechanism for controlled delivery of osteoregenerative peptides on HA-containing bone graft materials.