Local controlled release of simvastatin and PDGF from core/shell microspheres promotes bone regeneration in vivo

Simvastatin is demonstrated to be a potent stimulator for bone formation. However, a high dosage of simvastatin is required to induce bone regeneration in systematic administration, which may result in various side effects. Here we achieved the local administration of simvastatin through fabricating core/shell microspheres. Meanwhile, the addition of platelet derived growth factor (PDGF) encapsulated in core/shell microspheres was explored for a better outcome of bone regeneration. The microspheres composed of poly-L-lactide (PLLA) core and alginate shell were formulated via coaxial electrohydrodynamic atomization (CEHDA). The simvastatin and PDGF were incorporated in following models: (1) simvastatin was loaded in the core layer alone (core-simvastatin/shell, M1); (2) simvastatin was encapsulated in the core layer and PDGF was loaded into the shell layer (core-simvastatin/shell-PDGF, M2). Well-formed microspheres with distinct core–shell structures were successfully formulated. In vitro, simvastatin in the core layer displayed a near-zero-order release pattern within 40 days. In contrast, the release profile of PDGF in the shell layer was characterized with a strong initial burst release and being almost depleted in as early as 9 days. The 3(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay showed good biocompatibility with PLLA–alginate microspheres. In in vivo experiments, both the M1 and M2 groups showed greater new bone formation, with significantly increased bone mineral density (BMD), the ratio of mineralized bone volume of the defect tissue volume (BV/TV) and trabecula thickness (Tb.Th) of neogenesis bone, while little newly formed bone was found in sham surgery controls and blank microsphere groups. More importantly, the enhancement of bone regeneration was significantly greater in the M2 group. Thus, we suggested that the core/shell microspheres could be a promising delivery system for simvastatin combined with PDGF to improve bone regeneration.