Abstract
Massive bone allografts commonly used for craniofacial reconstruction can exhibit challenging complications such as non-union, fracture, or rejection. Growing evidence suggests the largest barrier to successful long term allograft incorporation is delayed or absent vascularization, as bone grafts are difficult to vascularize. To this end, this work utilized two novel bone graft drug loading strategies to sustain the local release of FTY720, a selective agonist for sphingosine 1-phosphate (S1P) receptors, in rat cranial defects: loaded in a coating of poly(lactic-co-glycolic acid) (PLAGA) or directly adsorbed to graft. To explore the polymer coating delivery method, uncoated grafts, vehicle coated, low dose FTY720 in PLAGA (1:200 w:w) and high dose FTY720 in PLAGA (1:40) were implanted into critical size calvarial bone defects. The ability of local FTY720 delivery to promote angiogenesis, maximize osteoinductivity, and improve graft incorporation by recruitment of bone progenitor cells from surrounding soft tissues and microcirculation was evaluated. FTY720 bioactivity after polymer encapsulation and release was confirmed with a sphingosine kinase 2 assays. HPLC-MS quantified about 50% loaded FTY720 release of total encapsulated drug (4.5 μg) after 5 days. Following 2 weeks of defect healing, FTY720 delivery led to statistically significant increases in bone volumes compared to controls. The rate and extent of enhanced bone growth persisted through week 4, but by week 8, increases in bone formation in FTY720 groups were no longer statistically significant. However, micro-computed tomography (microCT) of contrast enhanced vascular ingrowth (MICROFIL®) and histological analysis showed enhanced integration as well as directed bone growth in both high and low dose FTY720 groups compared to controls. The cranial defect model was again used to explore direct adsorption of FTY720 to bone grafts as a therapy with empty defects, xenograft only, or FTY720 loaded xenografts. Again, FTY720 improved graft remodeling and enhanced bone healing in the void space starting after week 2 and continuing through week 12. Empty defects saw undirected bone growth which began to regress after week 10. This work capitalized on existing mechanical and biomaterial properties of devitalized bone and delivered a small molecule compound to constitutively target vascular and osseous remodeling at the graft-host bone interface. Such results support continued evaluation of drug-eluting grafts as a viable strategy to improve functional outcome and long-term success of massive bone graft implants.
