Introduction: Treatment of large bone losses resulting from high-energy trauma, non-union, tumor resection, or maxillofacial diseases, remains nowadays a challenge for the surgeon. Growth factors are currently used in clinics. On the material side, the only approved carrier to date for BMP delivery in clinics remain a collagen sponge, into which the BMP-2 solution is soaked at a very high concentration (1.5 mg/mL, total dose from few mg to 12 mg), leading to its rapid clearance from the implantation site and poorly controlled delivery. Recently, severe concerns have been raised against the approved products [1] in view of the side effects. Thus, there is a clear need for optimizing the spatio-temporal delivery of BMPs from new carrier materials. Recently, our group has developed a polyelectrolyte thin coating made of biopolymers, which can be used as a carrier for BMP-2 [2] and is osteoinductive in vivo [3]. In this study, our aim was to repair a critical size bone defect using a polyelectrolyte film coating of a clinically used implant.
Materials and Methods: A hollow polymeric tube made of PLGA was taken as supporting scaffold. The polyectrolyte film made by self-assembly of Poly(L-lysine) and hyaluronan was prepared using an automated-dipping machine, then crosslinked at controlled degree and finally post-loaded with BMP-2 at tunable amounts. The amounts of BMP-2 loaded in the film were precisely quantified by fluorescence spectrometry. The film-coated implant was assessed for its osteoinductive properties in vitro using ALP assay on BMP-2 responsive myoblasts. The effect of the crosslinking extent and BMP-2 doses were tested. Film-coated implants were implanted in a critical size bone defect [4] and followed 8 weeks post-surgery using X ray radiography (every two weeks) and microcomputed tomography (after 8 weeks). X-ray scores were obtained in a blind manner, which enabled to quantify the repair kinetics and the plateau value of the score. Histological staining was also made in order to characterize the newly formed bone tissue. Finally, the retrieved implants were analyzed at high resolution at ESRF to obtain tomographic information.
Results and Discussion: In this study, we show that a critical size bone defect can be repaired thanks an adaptive film loaded with tunable doses of BMP-2, which was coated on a hollow polymeric (PLGA) tube as supporting scaffold. It was possible to obtain a large range of bone formation, from the lack of bone maturation to full bone repair by adjusting the dose of BMP-2 in the coating. The kinetics of bone regeneration also depended on the BMP-2 dose and could be as fast as 6 days for the highest doses of BMP-2. The threshold to induced full bone repair was found to be between 6 and 8 µg. High resolution computed tomography performed at ESRF highlighted that the BMP-2 influenced the formation of a thick cortical bone lining outside of the PLGA tube, whereas trabecular bone formed inside the hollow tube independently on the BMP-2 dose.
Conclusions: The development of a hybrid device made of PLGA coated with a biopolymeric film that can deliver locally BMP-2 may provide a simple low cost therapeutic strategy that can be adapted to various clinical situations of bone loss in large defects. As the polyelectrolyte films can be dried, sterilized and stored off the shelf for at least several months, this new technology may be translated to industrial-scale implant production.
European Research Council, ERC consolidator Grant, FP7, GA259370 (BIOMIM); European Research Council, ERC Proof of Concept, PF7, GA334966; Association Gueules Cassées, contract n°322013
References:
[1] Carragee EJ et al., Spine J. 2011, 11:471-491
[2] Crouzier, T. et al, Small 2009, 5:598-608.
[3] Guillot, R., Biomaterials 2013, 34:5737-5746.
[4] Zara JN. Tissue Eng Part A 2011, 17:1389-1399