Synergistic microenvironments for non-union bone defects

Long bone fractures involve damage to the surrounding tissues and vascular networks and, as a result, the natural bone healing capacity is lost and non-union defects are formed. The prevalence of this clinical problem is 2.5% for bone fractures but this rate increases up to 15% and 45% when there is collateral soft tissue and vascular injuries respectively. Current treatments comprise bone autografts, bone substitutes and the use of growth factors (GFs), specially bone morphogenetic proteins (BMP) with limited success and significant drawbacks.

Thus, there is an unmet clinical need to develop new therapeutic approaches for bone regeneration and vascularisation in non-union bone defects. The role of GFs in bone regeneration is broadly recognised but the delivery of these factors to enhance tissue healing, while maintaining their activity, has not been successful. Soluble administration or controlled delivery (including hydrogels and scaffolds) have failed to meet the need due to the breakdown and clearance of GFs from tissue sites. More importantly, however, catastrophic collateral risks have been reported due to the high dose used. This unsatisfactory clinical translation of growth factors demands robust, safe and effective systems that control delivery.

Human bone morphogenetic protein-2 (BMP-2) is a powerful growth factor that is essential in tissue morphogenesis and is involved in a myriad of cellular processes, including cell recruitment, cell differentiation, and angiogenesis. The use of recombinant BMP-2 (rhBMP-2) has been generalised to promote bone growth in a broad range of clinical applications (spine, oral-maxillofacial and trauma). Current clinical delivery involves the incorporation of the protein in a collagen sponge carrier at a concentration of 1.5 mg/cm3. However, serious clinical complications have been reported that even led the FDA to issue a Public Health Notification of life threatening complications associated with rhBMP-2 (respiratory, neurological, inflammatory).7 Reports since then include observations of uncontrolled bone formation and carcinogenic risks associated to the high doses used. Notwithstanding collateral risks and controversy between health agencies, clinical researchers and industry, the use of rhBMP-2 continues to increase in clinical practice.8

Here we present a simple and robust materials-based strategy to induce bone regeneration in non-union defects. It is based on engineered constructs that tune the interaction between GFs and their receptors and it allows very small rhBMP-2 doses to be used (< 0.1 mg/cm3) while maintaining cell activation. This will clearly make treatments cheaper, safer and more effective. It is important to note that our approach does not tune GF delivery per se, but rather it tunes the effective presentation of growth factors to cell receptors, and hence prevents collateral risks associated with the delivery of soluble rhBMP-2 at the regenerating site. The final strategy involves the implant of a cell-free 3D construct that incorporates synthetic-biodegradable polymers, extracellular matrix proteins and rhBMP-2. The system is designed to recruit stem cells in vivo (no pre-loading the construct with stem cells needed) and to promote bone regeneration in non-union defects, as well as aiding in revascularisation of the new tissue.

The overarching objective of this proposal is to develop a new therapeutic approach to promote bone regeneration in non-union defects. The system is based on the interactions between functional materials and fibronectin (FN) that will tune the effective presentation of rhBMP-2. These interactions promotes the self-organization of FN at the material interface into physiological-like fibrils, that we have called material-driven fibronectin fibrillogenesis and involves conformational changes in the FN molecule, upon adsorption, and the exposure of both cell adhesion and growth factor (GF) - binding domains. This material-based platform will synergistically present binding sites for both integrins and GF receptors to provide retention, delivery and presentation of rhBMP-2 within artificial matrices. More importantly, it will allow much lower rhBMP-2 doses to be used, which makes the system robust in terms of safety and effectiveness and, simultaneously, economically more competitive than current commercially available products. The final construct will be based on a structural - biodegradable 3D material coated with a functional material able to induce the required fibronectin fibrillogenesis. The technology is applicable across many structural materials (polymers ceramics and metals) and it allows combinatorial topologies in terms of porosity of the 3D structural system and even the presence of nanofeatures within the walls of the structural material that could further enhance the synergistic (integrin - GF) signalling provided by the functional coating. The 3D constructs will be investigated for their ability to drive mesenchymal stem cell differentiation in vitro towards the osteogenic lineage. Finally, the system will be investigated in vivo as an acellular system able to recruit stem cells and osteoblasts from the surrounding tissue and promote tissue healing and bone regeneration in non-union bone defects.

The main motivation for our research is the improvement of health by translating high quality research into deployable innovative solution that will offer new therapies. The transformative potential of stem cell and regenerative medicine research towards new clinical solutions has been widely recognised. This is particularly significant in the treatment of non-union bone defects. Moreover, the associated economic impact from regenerative medicine to companies that develop therapies and related infrastructure will be extended on several industrial sectors with a global market value which is expected to reach nearly £6.5 billion by the end of 2013. [Technology and Innovation Futures: UK Growth Opportunities for the 2020s, Government Office for Science, 2010]. Our work will help overcome major challenges in this field by developing simple engineering technologies that will contribute to deliver future therapies. These are precisely the key developments expected by the UK Government that will enable business to turn the knowledge generated into new therapies. [Taking Stock of Regenerative Medicine in the United Kingdom, July 2011].
The following impacts are expected from this project:
- To advance knowledge, understanding and technology,
- Enhancing quality of life, health and well-being,
- Contributing to the economic competitiveness of the United Kingdom,
- Provision of trained researchers with a set of multidisciplinary skills