2014   Wellington Hsu, MD  (Northwestern University) Development of a BMP-2-binding Demineralized Bone Nanomatrix (DBN) for spinal fusion 

Northwestern Medicine

Department of Orthopaedic Surgery

Laboratory for Regenerative Technologies

Drs. Erin and Wellington Hsu

https://www.orthopaedics.northwestern.edu/research/lab.html

@TheHsuLab (Twitter)

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Proposed Study (2014 grant application):

Despite recent technological advances, there is currently no universally accepted bone graft substitute for spinal arthrodesis. Complications associated with rhBMP-2 use—ectopic bone formation, vertebral body resorption, and new cancers—have raised serious concerns, leading some surgeons and hospitals to avoid utilizing INFUSE™ entirely. Consequently, for patients at risk for a nonunion, a safe, efficacious biologic has not been established. In preliminary work, we determined that a biodegradable nanogel composed of BMP-2-binding nanofibers fuses the rat spine at a rate of 41% withoutexogenous BMP-2. Although we were the first research group to demonstrate that fusion can be achieved using a purely synthetic scaffold, we postulate that with the strategy proposed here, a 100% fusion rate may be achievable. Demineralized bone matrices are used widely, but fillers such as glycerol—which are incorporated into DBMs to improve handling characteristics—dilute the osteoinductive capacity and subsequently, the efficacy of such products. In this study, we will utilize our BMP-2 direct binding nanogel (BMP-2PA) in conjunction with a DBM powder, to create a demineralized bone nanomatrix (DBN).  This combination will harness the mechanical strength of the DBM as well as its additional source of growth factors, while retaining all the advantages that a nanomatrix provides.  We believe this approach will lead to reproducible, exogenous growth factor-free fusion in a rat spinal arthrodesis model.

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Project Outcomes and Measurables:

This project resulted in the development of several DBM-based composite scaffolds utilizing novel materials intended for use in the setting of spine fusion.  One iteration, a 3D printed hydroxyapatite-DBM composite scaffold, was developed to provide increased structural integrityand osteoconductivity, and preliminary data obtained through this seed funding was instrumental to a successful NIH R01 application ($1.5M over 4 years), which is ongoing.  That work has shown that de novo bone (spicule) forms around DBM particles present in the struts of these scaffolds only when local hydroxyapatite is present (Fig 1), suggesting that high concentrations of mineral ions in close proximity to DBM material may mineralize the DBM, possibly in a cell-independent fashion (manuscript under review).  

A.   MicroCT image showing osteointegration of the HA-DBM scaffold with a native transverse process. B. histological section showing a re-mineralized DBM particle within a scaffold strut. C. Synchrotron microCT 3D rendering of a bone spicule (pink) that has grown around a DBM particle (in a scaffold strut) in the presence of high local concentrations of Ca and P, which were present in the form of hydroxyapatite crystals (white).

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Another iteration of DBM-based composite scaffolds being developed from the original LSRS seed funding incorporates supramolecular peptide amphiphile (PA) nanofibers with DBM.  This work, which is done in conjunction with the lab of Professor Samuel Stupp, also of the Simpson Querrey Institute at Northwestern University, has,shed light on ways they could improve the composite through modifications in post-processing (ie, "annealing" the nanofibers within the composite).  The group has found that this canlead to improved interactions, allowing the PA nanofibers to even more robustly potentiate growth factor-mediated osteoinductivity.  This work has also shown that DBM formulation plays an important role in success of the composite, where DBM fibers may be superior to DBM particles within the composites.

Figure demonstrating the superiority of the DBM/PA composite over a commercially available DBM putty product. PA nanofibers act as a binding agent to hold the DBM fibers in place, while simultaneously binding the growth factors within the DBM and protecting them from degradation

Figure demonstrating the superiority of the DBM/PA composite over a commercially available DBM putty product. PA nanofibers act as a binding agent to hold the DBM fibers in place, while simultaneously binding the growth factors within the DBM and protecting them from degradation

The Hsu laboratory has now leveraged the LSRS seed funding to obtain large-scale funding from the Musculoskeletal Transplant Foundation ($300,000), where the Hsu and Stupp groups are working to optimize the DBM/PA composites for maximal osteoinductivity (e.g. by modifying PA properties) using 3D stem cell culture studies, to characterize PA/DBM tissue fate and degradation rate in the rat spine fusion model, and to correlate degradation with pre-clinical efficacy for spine fusion.  That study also includes an extensive comparative efficacy study to evaluate the efficacy of the PA/DBM composite against INFUSE. 

In more recent work, the Hsu group is investigating the utility of combining the soft PA materials with the 3D-printed DBM-based materials for an integrated approach to promoting bone regeneration in certain surgical settings. 

The results of the work funded by the LSRS were presented in the following forums (selected): 

1. ORS 2016 – Development of a Ceramic-Demineralized Bone Matrix Biomaterial Ink for 3D-printing of Hyperelastic Bone Composite Scaffolds for Spinal Fusion

2. LSRS 2016 – 3D-Printed Hyperelastic Hydroxyapatite-DBM Composite Scaffolds for Spine Fusion

3. SPIE 2016 – Tomographic Characterization of a 3D-Printed Bone Graft Substitute for Spinal Fusion 

4. Global Spine congress 2017 – 3D-printing Hyperelastic Bone Composite Scaffolds for Spinal Fusion using a Novel Ceramic-demineralized Bone Matrix Biomaterial Ink

5. Cervical Spine Research Society 2017 Annual Meeting: A novel 3D-printable bioactive ink made from human allograft for spinal arthrodesis

6. AAOS 2018 Annual Meeting: 3D-printable Hyperelastic “Bone” Composite as a Novel Synthetic Bone Grafting Biomaterial for Spinal Fusion

7. ASBMR 2018 Annual Meeting: Composition of Hyperelastic “Bone” Composite Scaffolds Affects De Novo Bone Formation

8. Global Spine Congress 2018: Optimizing the architecture and geometry of a 3D-printable hyperelastic “bone” composite scaffold designed for spinal fusion

9. NASS 2018 Annual Meeting: Architectural and Geometric Considerations in the Development of a 3D Printed Hyperelastic “Bone” Composite Scaffold as a Bone Graft Substitute for Spinal Arthrodesis