112. Alternative Bone-grafting Nanomaterials: A Nanoparticle-based Treatment and Assessment Tool for Autologous Bone Grafting

PURPOSE: Autologous bone grafting in the immature craniofacial skeleton continues to be a challenge across a spectrum of disease processes. Limitations include a dearth of donor sites, donor site morbidity, potential resorption, and an immature diploic space in young children. Our laboratory’s recent work has demonstrated the feasibility of turning bone tissue into nanoparticles for tissue engineering applications in the craniofacial skeleton. The ICG/BPs, bone-based nanoparticles (BPs) with encapsulated indocyanine green dye (ICG), can be used as a therapeutic bone-grafting material with in-situ, real time monitoring using non-ionizing Near-Infrared fluorescence imaging (NIR). In this study, we sought to determine if this multi-functional treatment and assessment tool could serve as a viable substitute for repairing non-healing calvarial bone defects in a small animal model. We hypothesized that ICG/BPs allow for sustained calvarial bone healing that can be spatially and temporally controlled and inversely correlated with real-time fluorescence using NIR imaging. METHODS: ICG/BPs were synthesized using previously established methods, and to enhance therapeutic efficacy, bone morphogenetic protein 2 (BMP2) peptide was additionally encapsulated, creating ICG/BMP2/BPs. The regenerative efficacy and NIR monitoring capabilities were tested in vivo using 8-week-old rats with bilateral non-healing calvarial defects. The defects were divided into two groups: 1) one side untreated, the other treated with ICG/BPs, and 2) one side untreated, the other treated with ICG/BMP2/BPs. NIR imaging was conducted at various intervals over 10 weeks to assess material degradation, and post-euthanasia, skulls were analyzed for osteogenic capacity and bone growth. RESULTS: Over the course of 10 weeks, we found a robust NIR imaging capability of both the ICG/BPs and ICG/BMP2/BPs which enabled us to accurately measure and quantify the material degradation within the defect. Using this quantification method, we determined a difference in the degradation kinetics of each material, with the ICG/BPs degrading at a more rapid pace than the ICG/BMP2/BPs, having lost all fluorescence by days 35 and 56, respectively. Ex vivo analysis of the bone tissues using gross observations and computed tomography (CT) indicated the ICG/BPs were effective at stimulating bone growth when compared to a non-treated defect. However, the addition of BMP2 appeared to improve the therapeutic efficacy of the BPs, with the ICG/BMP2/BP group showing near-complete bone growth. CONCLUSION: Bone-based nanoparticles applied to a non-healing calvarial defect model were associated with bone regeneration that can be tracked in real time using non-ionizing fluorescence-based imaging. The nanoparticle functionality includes 1) robust imaging capabilities for in vivo monitoring that inversely correlates with bone healing, 2) spatial and temporal control of tissue regeneration, and 3) effective bone-stimulating capabilities. NIR imaging revealed varying release kinetics between regular BPs and BMP2/BPs, possibly contributing to the quality and quantity of bone tissue formed after 10 weeks. Future directions will include a thorough investigation into the kinetics of each material over time and how the degradation of the BPs, with subsequent ICG and BMP2 release, can be translated into a fully reliable treatment and assessment option for calvarial defect repair.