Roles of Osteoclasts and Macrophages in Medication-Related Osteonecrosis of the Jaw Pathophysiology

Abstract

Medication-related osteonecrosis of the jaw (MRONJ) is a serious side effect of antiresorptive drugs including bisphosphonates (BPs) and denosumab (anti-RANKL antibody) prescribed for osteoporosis and cancer treatments. MRONJ often occurs after tooth extraction as the presence of necrotic bone and impaired soft and hard tissue healing in oral cavity resulting in pain and affecting patients’ quality of life. Although the risk of MRONJ is low, the numbers of patients taking these drugs are high (> 10 million worldwide) and still increasing making this a growing clinical problem. Importantly, the underlying MRONJ pathophysiology have remained elusive, hence, the effective curative treatments have not been established. In this thesis, the effects of anti-RANKL antibody on several cell types which may potentially contribute to MRONJ pathophysiology were investigated. First, osteoclasts, a bone resorbing cell, were studied. Utilizing tooth extraction and anti-RANKL antibody injections in nude mice, absence of osteoclasts and necrotic bone accumulation were observed in the MRONJ mouse model. I further showed that the replenishment of bone resorptive function by the local delivery of iRANK cells, the engineered cells whose osteoclast differentiation can be controlled by CID drug and is independent of RANKL, decreased necrotic bone area, but did not improve other MRONJ features. This suggests that other mechanisms besides osteoclast inhibition may play a role in MRONJ pathophysiology. Next, osteoblasts, a bone forming cell, were studied. Interestingly, anti-RANKL antibody did not have any significant effect on osteogenic differentiation of osteoblasts via RANKL autocrine/paracrine loop in vitro suggesting that it has no direct effect on mineralization. Lastly, macrophages, an immune cell involved in inflammation and wound healing, were studied. The effects of the presence and the absence of RANKL on macrophage polarization were determined. I showed that RANKL has only slight effects on macrophage polarization in vitro; it upregulated several genes similarly to M1 macrophages but in much lower level. However, the increased M1/M2 ratio was observed in the MRONJ mouse model. All these findings support bone remodeling inhibition and inflammation as potential mechanisms of MRONJ as well as paving the way for engineered osteoclasts cell therapy for MRONJ treatment.