Repurposing Metformin to Promote Fracture Callus Maturation via AMPK-Driven Metabolic Activation

Femoral shaft fractures cause prolonged disability, and therapies that accelerate bone repair remain limited. Repurposing clinically approved drugs that target biological bottlenecks in healing is a promising strategy. This study investigated whether systemic metformin administration, an anti-diabetic medication with known metabolic regulatory effects, enhances fracture repair in a rat open femoral shaft fracture model. Histological, immunofluorescent, micro-CT, and biomechanical analyses were performed at 6 weeks post-injury comparing metformin-treated and vehicle-treated animals. Metformin markedly accelerated callus maturation, evidenced by earlier hyaline cartilage ossification, increased collagen I deposition and fiber organization, and reduced collagen II and III expression compared with controls. Micro-CT analysis demonstrated increased tissue mineral density, trabecular thickness, and bone volume fraction along with reduced connectivity density, indicating more advanced structural consolidation of the callus. Although biomechanical parameters were not significantly different at intermediate time point, ultimate load and stiffness trended higher in metformin-treated animals, consistent with structural advancement. Mechanistically, metformin increased p-AMPK expression, elevated mitochondrial markers (NDUFB8, TFAM), and reduced extracellular HMGB1 release, suggesting enhanced metabolic capacity and attenuated inflammatory stress during repair. Importantly, metformin's effects were most pronounced during the cartilage-to-bone transition phase, supporting a role for metabolic activation in promoting endochondral ossification. Together, these findings demonstrate that systemic metformin administration promotes earlier structural consolidation of the fracture callus through coordinated metabolic and inflammatory modulation, supporting the potential repurposing of this safe and inexpensive drug as an adjunct strategy to enhance bone repair.