Novel Protamine-Based Polyelectrolyte Carrier Enhances Low-Dose rhBMP-2 in Posterolateral Spinal Fusion.

STUDY DESIGN: A rodent posterolateral spinal fusion model. OBJECTIVE: This study evaluated a protamine-based polyelectrolyte complex (PEC) developed to use heparin in enhancing the biological activity of low-dose recombinant human bone morphogenetic protein-2 (rhBMP-2) in spinal fusion. SUMMARY OF BACKGROUND DATA: rhBMP-2 is commonly regarded as the most potent bone-inducing molecule. However, poor pharmacokinetics and short in vivo half-life means that large amounts of the bioactive growth factor are required for consistent clinical outcomes. This has been associated with a number of adverse tissue reactions including seroma and heterotopic ossification. Glycosaminoglycans including heparin are known to stabilize rhBMP-2 bioactivity. Previous studies with poly-L-lysine (PLL) and heparin-based PEC carriers amplified the therapeutic efficacy of low-dose BMP-2. However, questions remained on the eventual clinical applicability of relatively cytotoxic PLL. In the present study, a protamine-based PEC carrier was designed to further enhance the safety and efficacy of BMP-2 by delivering lower dose within the therapeutic window. METHODS: A polyelectrolyte shell was deposited on the surface of alginate microbead templates using the polycation (protamine)/polyanion (heparin) layer-by-layer polyelectrolyte self-assembly protocol. rhBMP-2 was loaded onto the outermost layer via heparin affinity binding. Loading and release of rhBMP-2 were evaluated in vitro. The bone-inductive ability of 20-fold reduction of rhBMP-2 with the different carrier vehicle was evaluated using a posterolateral spinal fusion model in rats. RESULTS: In vitro uptake and release analysis, protamine-based PEC showed higher uptake and significantly enhanced control release than PLL-based PEC (P < 0.05). In vivo implantation with protamine-based and PLL-based PEC showed better fusion performances than absorbable collagen sponge-delivered same dose of rhBMP-2, and negative control group through manual palpation, micro-computed tomography, and histological analyses. CONCLUSION: Solid posterolateral spinal fusion was achieved with 20-fold reduction of rhBMP-2 when delivered using protamine-based PEC carrier in the rat posterolateral spinal fusion model. LEVEL OF EVIDENCE: N/A.