Fracture non-union can be as high as 20% in certain clinical scenarios and has a high associated socioeconomic burden. Boron has been shown to regulate the Wnt/β-catenin pathway in other bodily processes. However, this pathway is also critical for bone healing. Here we aim to demonstrate that the local delivery of boric acid can accelerate bone healing, as well as to elucidate how boric acid, via the regulationtheWnt/β-catenin pathway, impacts theosteogenic response of bone-derived osteoclasts and osteoblasts during each phase of bone repair.

Bilateral femoral cortical defects were created in 32 skeletally mature C57 mice. On the experimental side, boric acid (8mg/kg concentration) was injected locally at the defect site whereas on the control side, saline was used. Mice were euthanized at 7, 14, and 28 days. MicroCT was used to quantify bone regeneration at the defect. Histological staining for ALP and TRAP was used to quantify osteoblast and osteoclast activity respectively. Immunohistochemical antibodies, β-catenin and CD34 were used to quantify active β-catenin levels and angiogenesis respectively. Sclerostin and GSK3β were also quantified and are both inhibitors of the wnt signaling pathway via degradation and inactivation of β-catenin.

The boron group exhibited higher bone volume and trabecular thickness at the defect site by 28 days on microCT. ALP activity was significantly higher in boron group at 7 days whereas boron had no effect on TRAP activity. Additionally, CD34 staining revealed increased angiogenesis at 14 days in boron treated groups. β-catenin activity on immunohistochemistry was significantly higher in the boron group at 7 days, GSK3β was significantly higher in the boron group at 14 days and Sclerostin was significantly higher in the boron group at 28 days.

Boron appears to increase osteoblast activity at the earlier phases of healing. The corresponding early increase in β-catenin along with ALP likely supports that boron increases osteoblast activity via the wnt/β-catenin pathway. Increased angiogenesis at 14 days could be a separate mechanism increasing bone formation independent of wnt/β-catenin activation. Neither GSK3β or Sclerostin levels correlated with β-catenin activity therefore boron likely increases β-catenin through a mechanism independent of both GSK3β and Sclerostin. The addition of this inexpensive and widely available ion could potentially become a non-invasive, cost-effective treatment modality to augment fracture healing and decrease non-union rates in high risk patients.

Bone regeneration includes a well-orchestrated series of biological events of bone induction and conduction. Among them, the Wnt/β-catenin signaling pathway is critical for bone regeneration. Being involved in several developmental processes, Wnt/β-catenin signaling must be safely targeted. There are currently only few specific therapeutic agents which are FDA-approved and already entered clinical trials. A published work has shown that Tideglusib, a selective and irreversible small molecule non-ATP-competitive glycogen synthase kinase 3-β(GSK-3β) inhibitor currently in trial for Alzheimer's patients, can promote tooth growth and repair cavities. [1]Despite some differences, they are some similarities between bone and tooth formation and we hypothesise that this new drug could represent a new avenue to stimulate bone healing.

In this work, we locally delivered Tideglusib (GSK3β inhibitor) in the repair of femoral cortical window defects and investigated bone regeneration. A biodegradable FDA-approved collagen sponge was soaked in GSK-3βinhibitor solution or vehicle only (DMSO) and was implanted in 1 × 2 mm unicortical defects created in femora of 35 adult wild-type male mice. Bone defect repair on control and experimental (GSK-3βinhibitor) groups was assessed after 1 week (n=22), 2 weeks (n=24) and 4 weeks (n=24) with microCT and histological analysis foralkaline phosphatase (ALP, osteoblast activity), tartrate resistant acid phosphatase (TRAP, osteoclasts), and immunohistochemistry to confirm the activation of the Wnt/β-catenin pathway.

Our results showed that Tideglusib significantly enhanced cortical bone bridging (20.6 ±2.3) when compared with the control (12.7 ±1.9, p=0.001). Activity of GSK-3β was effectively downregulated at day 7 and 14 resulting in a higher accumulation of active β-catenin at day 14 in experimental group (2.5±0.3) compared to the control (1.1±0.2, p=0.03). Furthermore, the onset of ALP activity appears earlier in the experimental group (day 14, 1.79±0.28), a level of activity never reached at any end-point by the control defects. At 4 weeks treatment, we observed a significant drop in ALP in the experimental group (0.47±0.05) compared to the control (1.01±0.19, p=0.02) and a decrease in osteoclast (experimental=1.32±0.36, control=2.23±0.67, p=0.04).

Local downregulation of GSK-3β by tideglusib during bone defect repair resulted in significant increase in amount of new bone formation. The early upregulation of osteoblast activity is one explanation of bone healing augmentation. This is likely the effect of upregulation of β-catenin following pharmaceutical inhibition of GSK-3β since β-catenin activation is known to positively regulate osteoblasts, once committed to the osteoblast lineage. As a GSK-3β inhibitor, Tideglusib demonstrates a different mechanism of action compared with other GSK-3β antagonists as treatment was started immediately upon injury and did not interfere with precursor cells recruitment and commitment. This indicates that tideglusib could be used at the fracture site during the initial intraoperative internal fixation without the need for further surgery. This safe and FDA-approved drug could be used in prevention of non-union in patients presenting with high risk for fracture-healing complications.