Bisphosphonate-related Osteonecrosis of the Jaws in a Spontaneously Diabetic Torii Rat Model

Statement of the problem: Bisphosphonates (BPs) are a widely used class of drugs that are effective in the treatment and prevention of hypercalcemia of malignancy and skeletal events associated with multiple myeloma, bone metastasis from cancer, and osteoporosis. However, reports of BP-related osteonecrosis of the jaws (BRONJ) have generated a great concern regarding the safety of these drugs. Some reports in the literature have documented a few animal models with risk factors contributing to the development of BRONJ. Diabetes mellitus is one of the systemic risk factors for this condition. We evaluated the side effects of BPs on extraction socket healing in spontaneously diabetic Torii (SDT) rats, which is an established model of non-obese type 2 diabetes mellitus.

Materials and methods: We used female Sprague–Dawley rats and SDT rats for this study. Zoledronic acid (ZOL)-treated groups of SD or SDT rats (SD/ZOL or SDT/ZOL) were intravenously administered a ZOL (35 μg/mL) bolus every 2 weeks. After treatment initiation, the maxillary left molars were extracted at 22 weeks. The rats were euthanized 2, 4, or 8 weeks after tooth extraction and the maxillary tissues were collected (n=6). The imaging of the tibiae was performed by micro focus X-ray computed tomography. Calcein was administered prior to the sacrifice to label active formation sites. Histological examination, tartarate-resistant acid phosphatase (TRAP) detection, and bone labeling were performed for these specimens. Before the rats were euthanized, their blood samples were collected and examined for serum chemistry.

Results: The blood glucose level of the SDT rats reached ≥700 mg/dL by 25 weeks of age. The levels of cancellous bone volume (BV/TV) and trabecular number (Tb.N) of the tibiae were significantly increased by ZOL treatment in SDT rats. The level of trabecular separation (Tb.Sp) of was significantly attenuated by ZOL treatment in SDT rats. Bone exposure to the oral cavity had been continued at the tooth extraction site of the SDT/ZOL group (6/6; 100%) until 8 weeks after tooth extraction. Necrotic bone areas with empty lacunae, microbial colonies, and less inflammation were observed on detailed histological observations. The characteristic finding of the SDT/ZOL group was lesser invasion by inflammatory cells. The number of TRAP-positive osteoclasts was lower in the SDT/ZOL group than in the SDT/control group. Two clear calcein-labeled lines were recognizable in the newly formed bone. The distance between the two calcein-labeled lines was narrower in the SDT/ZOL group than in the SDT/control group. The levels of CTX and TRACP-5b in serum chemistry were attenuated by ZOL treatment in the SDT and SD rats. The TRACP-5b level was significantly decreased in the SDT group compared with that in the SD group.

Conclusion: The study demonstrated the development of BRONJ-like lesions after tooth extraction in ZOL-treated SDT rats. These results suggest that low bone turnover with less invasion of inflammatory cells plays a significant role in the development of BRONJ under these circumstances.

Referenscs:

1)    Sonis ST, Watkins BA, et al: Bony changes in the jaws of rats treated with zoledronic acid and dexamethasone before dental extractions mimic bisphosphonate-related osteonecrosis in cancer patients. Oral Oncol 2009, 45, 164-172.

2)    Hokugo A, Christensen R, et al. Increased prevalence of bisphosphonate-related osteonecrosis of the jaw with vitamin D deficiency in rats. J Bone Miner Res 2010, 25, 1337-1349.