Bisphosphonates Inhibit Osteoblast Proliferation

Objectives: Bisphosphonates are used to treat conditions causing loss of bone mass (eg. osteoporosis) and pathological processes involving bone turnover (eg. bone metastasis). They inhibit osteoclast activity and reduce the rate of bone breakdown although their mechanism of action is unknown. Their chemical structure permits integration into bone, allowing bisphosphonates an indefinite biological half-life and activity dependent upon liberation from bone. When these bisphosphonates are released from bone, their high local concentration can cause osteonecrosis. This can lead to bisphosphonate-related osteonecrosis of the jaw (BRONJ) in patients who received intravenous bisphosphonates years prior to a tooth extraction. Osteonecrosis resulting from the inhibition of osteoclasts is counterintuitive and suggests other cells may respond to bisphosphonates. The purpose of this study was to examine the effects of bisphosphonates on periodontal cells.
Methods: Gingival (GF) and periodontal ligament (PDL) fibroblasts and rat osteosarcoma cells (UMR-106) were treated for 24 hours with various concentrations of bisphosphonates (zoledronate, clodronate and pyrophosphate). Cell survival was assessed fluorometrically using Calcein-AM. Cell proliferation was assessed fluorometrically after 9 days using Cyquant™.
Results: Concentrations of up to 20 μM of all three bisphosphonates did not reduce cell survival of GF, PDL, or UMR cells. Zoledronate concentrations of 2-5 μM slightly reduced GF and PDL cells proliferation, while concentrations of 10 μM and above significantly reduced proliferation. In contrast, zoledronate concentrations of 2 μM and above dramatically reduced UMR cell proliferation.
Conclusions: Based on these results, zoledronate inhibits stroma cell proliferation with osteoblast-like UMR cells displaying the greatest sensitivity. This study supports the likelihood bisphosphonates can have detrimental effects on bone formation, in addition to their previously reported negative effects on osteoclast activity. This data sheds new light on a potential mechanism underlying BRONJ.