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

BACKGROUND

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.

QUESTIONS/PURPOSES: 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.

METHODS

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).

RESULTS

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).

CONCLUSION

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

CLINICAL RELEVANCE: 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.