Background/Purpose: Various types of orthopedic procedures, including spinal fusion and repair of bone defects due to trauma, infection or metastatic disease, require formation of new bone. Adenosine, generated from the catabolism of adenine nucleotides, modulates cell function by interacting with specific cell-surface receptors (A₁R, A_2AR, A_2BR, A₃R). We have previously reported that A₁R receptor blockade and A_2AR stimulation inhibit osteoclast (OC) differentiation but only A_2BR stimulation affects osteoblast (OB) differentiation or function. We determined whether A₁R blockade, A_2AR stimulation or enhancing adenosine concentrations via blockade of purine transport into cells via ent1 with dipyridamole regulates bone formation in a murine calvarial model. 

Methods: Male C57Bl/6 mice were anesthetized, a 3mm trephine defect was formed and covered with a collagen scaffold soaked in saline or 1mM adenosine receptor agonists/antagonists. Animals received appropriate treatment daily until sacrifice. At 0, 2, 4, 6 and 8 weeks calvarias were harvested and prepared for microCT and histology. XenoLight Rediject Bone Probe 680 was injected intravenously at different time points

Results: 8 weeks after surgery microCT examination of mouse calvaria demonstrate that an A₁R antagonist (DPCPX), A_2AR agonist (CGS21680M) or dipyridamole markedly enhances bone regeneration (77±0.2%, 60±2% and 79±2% bone regeneration, respectively, vs. 32±4% in control, p<0.001, n=5 mice per condition) whereas an A₃R agonist (IB-MECA) had no effect (32±3% regeneration, n=5). In DPCPX-, CGS21680- and dipyridamole-treated mice there is increased immunostaining for osteoblast or bone formation markers (Alkaline Phosphatase, Osteocalcin and Osteonectin) in the bony defects (Alkaline Phosphatase positive cells/hpf increased from 15±1 for control to 22±1 for DPCPX, 21±1 for CGS21680 and 24±1 for Dipyridamole, p<0.001), and diminished inmunostaining for macrophages (CD163, TNfa) and osteoclasts (RANKL, RANK) in treated defects when compared to control. TRAP staining revealed fewer OCs in DPCPX-, CGS21680- and Dipyridamole-treated defects (14±1, 17±1 and 16±1 OC/hpf respectively vs. 24±1 Osteoclast/hpf for control, p<0.001) 8 weeks after defect formation. In vivo imaging with XenoLight Rediject Bone Probe 680 (a marker of bone formation) reveals a strong fluorescent signal in treated animals (DPCPX, CGS21680 and Dipyridamole) compared to control as soon as one week after bone defect formation and lasting for at least 7 weeks.

Conclusion: Inhibition of OC formation via A_2AR stimulation, A₁R blockade or increasing local adenosine concentration stimulates new bone formation and represents a novel approach to stimulating bone regeneration.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/adenosine-receptors-stimulate-bone-regeneration-by-targeting-osteoclasts/