S1P Modulation via S1P Lyase Inhibition Enhances Bone Regeneration in a Murine Critical-size Defect Model of Atrophic...

Atrophic nonunion is a challenging complication of fracture healing with few noninvasive therapeutic options. Given that sphingosine-1-phosphate (S1P) regulates bone remodeling and angiogenesis, systemic S1P modulation may offer a regenerative strategy; however, its efficacy in nonunion has not been established.

We therefore asked whether S1P lyase inhibition (1) enhances osteogenesis and consolidation in a murine critical-size bone defect model of atrophic nonunion, (2) modulates bone remodeling (osteoblast differentiation and osteoclast activity), and (3) augments cellular proliferation and endothelial activity, and (4) whether quantitative micro-CT corroborates histologically observed improvements.

In a validated murine critical-size bone defect model of atrophic nonunion, 12-week-old male and female C57BL/6J mice were treated by continuous systemic administration of the S1P lyase inhibitor 4-deoxypyridoxine (DOP) for 5 weeks, while a reference group remained untreated after nonunion induction. Bone regeneration was assessed by histology and immunohistochemistry to evaluate osteoblast differentiation, osteoclast activity, cellular proliferation, and angiogenesis (n = 7 per group) and by high-resolution micro-CT to quantify newly formed bone within the defect region (n = 6 per group).

DOP treatment resulted in increased mineralized tissue and defect bridging on overview histology (aniline blue staining) compared with untreated controls (mean ± SD histologic bone formation score [pixels] DOP 80,000 ± 20,000 versus controls 52,000 ± 22,000, mean difference 28,000 [95% confidence interval (CI) 4200 to 53,000]; p = 0.03). Osteoblast differentiation was higher in DOP-treated animals (RUNX2 6200 ± 3200 versus 2200 ± 1400, mean difference 4000 [95% CI 1200 to 6900]; p = 0.01 and osteocalcin 60,000 ± 24,000 versus 28,000 ± 12,000, mean difference 32,000 [95% CI 9400 to 54,000]; p = 0.009), while osteoclast activity was also higher, indicating more active bone remodeling (TRAP 43,000 ± 15,400 versus 23,000 ± 7400, mean difference 20,000 [95% CI 6600 to 35,000]; p = 0.008). Cellular proliferation (PCNA 22,000 ± 7500 versus 14,000 ± 5700, mean difference 8000 [95% CI 470 to 16,000]; p = 0.04) and endothelial marker expression (PECAM-1 23,000 ± 6100 versus 13,700 ± 5500, mean difference 9300 [95% CI 2900 to 16,400]; p = 0.009) were both higher in the DOP group than in controls. Quantitative micro-CT confirmed greater newly formed bone volume within the defect region in DOP-treated mice compared with untreated controls (bone volume [%] 20.6 ± 8.2 versus 8.8 ± 3.3, mean difference 11.8 [95% CI 4.5 to 19.0]; p = 0.004).

These results suggest that pharmacologic modulation of the S1P axis could represent a novel, noninvasive adjunct to surgical management of nonunions and delayed unions.

Key unanswered questions include optimal dosing, treatment duration, and safety of systemic S1P modulation beyond murine models. Addressing these questions in translational large-animal studies and early clinical trials will be essential to advance this approach toward clinical application.