Calcium-binding PLGA microparticles releasing rhBMP-2 for enhancement of bone healing with minimal ectopic bone formation in spine fusion

Abstract

We designed a delivery vehicle that can maintain the local concentration of recombinant human bone morphogenetic protein-2 (rhBMP-2) only around the bone surgery site for the enhancement of bone healing without excessive bone formation at non-targeted sites. The microparticles were fabricated by encapsulation of rhBMP-2 into a poly(lactic-co-glycolic acid) (PLGA) core and by coating with poly(butyl methacrylate-co-methacryloyloxyethyl phosphate) (PBMP), which contains both calcium-binding phosphomonoester groups and PLGA-binding butyl groups. The PBMP-coated PLGA microparticles (PLGA/PBMP) successfully encapsulated rhBMP-2 without compromising its bioactivity and demonstrated encapsulation efficiencies and release kinetics similar to uncoated PLGA microparticles. Notably, the PBMP coating conferred a significantly higher bone-binding affinity compared to PLGA alone. This enhanced binding led to a higher local concentration of rhBMP-2, which in turn significantly upregulated osteogenic markers in preosteoblast MC3T3-E1 cells. We then evaluated the bone growth and ectopic bone formation after treatment with rhBMP-2-encapsulating PLGA/PBMP (BMP-2-PLGA/PBMP) in a rat model of spine fusion surgery using autologous iliac bone. The results revealed that the sustained release of rhBMP-2 from PLGA/PBMP could gradually stimulate bone growth, while free rhBMP-2 boosted bone regeneration only during the first few weeks. More importantly, unlike free rhBMP-2 which showed a high incidence of ectopic bone formation (>60%), BMP-2-PLGA/PBMP showed almost no incidence of ectopic bone formation (≤10%), indicating site-specific delivery. Collectively, our study demonstrates that the PLGA/PBMP microparticle system has great potential for enhancing bone regeneration by enabling localized delivery of rhBMP-2 with minimal side effects.