Systemic RANK-Fc protein therapy is efficacious against primary osteosarcoma growth in a murine model via activity against osteoclasts

Investigation of the activity of a novel tropolone in osteosarcoma

Osteosarcoma (OS) is a primary malignant bone tumor characterized by frequent metastasis, rapid disease progression, and a high rate of mortality. Treatment options for OS have remained largely unchanged for decades, consisting primarily of cytotoxic chemotherapy and surgery, thus necessitating the urgent need for novel therapies. Tropolones are naturally occurring seven-membered non-benzenoid aromatic compounds that possess antiproliferative effects in a wide array of cancer cell types. MO-OH-Nap is an α-substituted tropolone that has activity as an iron chelator. Here, we demonstrate that MO-OH-Nap activates all three arms of the unfolded protein response (UPR) pathway and induces apoptosis in a panel of human OS cell lines. Co-incubation with ferric chloride or ammonium ferrous sulfate completely prevents the induction of apoptotic and UPR markers in MO-OH-Nap-treated OS cells. MO-OH-Nap upregulates transferrin receptor 1 (TFR1) protein levels, as well as TFR1, divalent metal transporter 1 (DMT1), iron-regulatory proteins (IRP1, IRP2), ferroportin (FPN), and zinc transporter 14 (ZIP14) transcript levels, demonstrating the impact of MO-OH-Nap on iron-homeostasis pathways in OS cells. Furthermore, MO-OH-Nap treatment restricts the migration and invasion of OS cells in vitro. Lastly, metabolomic profiling of MO-OH-Nap-treated OS cells revealed distinct changes in purine and pyrimidine metabolism. Collectively, we demonstrate that MO-OH-Nap-induced cytotoxic effects in OS cells are dependent on the tropolone's ability to alter cellular iron availability and that this agent exploits key metabolic pathways. These studies support further evaluation of MO-OH-Nap as a novel treatment for OS.

Cracking the code: Deciphering the role of the tumor microenvironment in osteosarcoma metastasis

Osteosarcoma (OS) is the most common malignant bone tumor in children and adolescents. It is characterized by a rapid progression, poor prognosis, and early pulmonary metastasis. Over the past 30 years, approximately 85% of patients with osteosarcoma have experienced metastasis. The five-year survival of patients with lung metastasis during the early stages of treatment is less than 20%. The tumor microenvironment (TME) not only provides conditions for tumor cell growth but also releases a variety of substances that can promote the metastasis of tumor cells to other tissues and organs. Currently, there is limited research on the role of the TME in osteosarcoma metastasis. Therefore, to explore methods for regulating osteosarcoma metastasis, further investigations must be conducted from the perspective of the TME. This will help to identify new potential biomarkers for predicting osteosarcoma metastasis and assist in the discovery of new drugs that target regulatory mechanisms for clinical diagnosis and treatment. This paper reviews the research progress on the mechanism of osteosarcoma metastasis based on TME theory, which will provide guidance for the clinical treatment of osteosarcoma.

Osteosarcoma and Metastasis

Osteosarcoma is the most common primary bone malignancy in adolescents. Its high propensity to metastasize is the leading cause for treatment failure and poor prognosis. Although the research of osteosarcoma has greatly expanded in the past decades, the knowledge and new therapy strategies targeting metastatic progression remain sparse. The prognosis of patients with metastasis is still unsatisfactory. There is resonating urgency for a thorough and deeper understanding of molecular mechanisms underlying osteosarcoma to develop innovative therapies targeting metastasis. Toward the goal of elaborating the characteristics and biological behavior of metastatic osteosarcoma, it is essential to combine the diverse investigations that are performed at molecular, cellular, and animal levels from basic research to clinical translation spanning chemical, physical sciences, and biology. This review focuses on the metastatic process, regulatory networks involving key molecules and signaling pathways, the role of microenvironment, osteoclast, angiogenesis, metabolism, immunity, and noncoding RNAs in osteosarcoma metastasis. The aim of this review is to provide an overview of current research advances, with the hope to discovery druggable targets and promising therapy strategies for osteosarcoma metastasis and thus to overcome this clinical impasse.

The Immune Landscape of Osteosarcoma: Implications for Prognosis and Treatment Response

Osteosarcoma (OS) is a high-grade malignant stromal tumor composed of mesenchymal cells producing osteoid and immature bone, with a peak of incidence in the second decade of life. Hence, although relatively rare, the social impact of this neoplasm is particularly relevant. Differently from carcinomas, molecular genetics and the role of the tumor microenvironment in the development and progression of OS are mainly unknown. Indeed, while the tumor microenvironment has been widely studied in other solid tumor types and its contribution to tumor progression has been definitely established, tumor-stroma interaction in OS has been quite neglected for years. Only recently have new insights been gained, also thanks to the availability of new technologies and bioinformatics tools. A better understanding of the cross-talk between the bone microenvironment, including immune and stromal cells, and OS will be key not only for a deeper knowledge of osteosarcoma pathophysiology, but also for the development of novel therapeutic strategies. In this review, we summarize the current knowledge about the tumor microenvironment in OS, mainly focusing on immune cells, discussing their role and implication for disease prognosis and treatment response.

Characterization of Macrophages and Osteoclasts in the Osteosarcoma Tumor Microenvironment at Diagnosis: New Perspective for Osteosarcoma Treatment?

Simple Summary

Due to the great genetic instability of osteosarcoma (OS), a recurrent molecular therapeutic target has not been identified to date. Therefore, characterization of the OS tumor microenvironment (TME) might offer new therapeutic perspectives. The OS2006 trial, originally designed to evaluate the impact of zoledronic acid (ZA, osteoclast-inhibitor) addition to conventional OS-therapies, was ended preliminary due to a negative impact on patient survival. Through retrospective biomarker analysis of the unique biological samples collected during the trial, we demonstrate here that ZA not only acts on harmful osteoclasts but also on protective macrophages, clarifying its detrimental effect. By multiplex immunohistochemistry, applied on additional OS biopsies, an important bipotent macrophage-population (CD168+/CD163+), homogenously distributed throughout OS tumor areas, was identified. These bipotent cells might play a determining role in the evolution of OS and offer a novel therapeutic approach. A clear definition of the macrophage populations present at diagnosis could re-enforce therapeutic decisions.

Abstract

Biological and histopathological techniques identified osteoclasts and macrophages as targets of zoledronic acid (ZA), a therapeutic agent that was detrimental for patients in the French OS2006 trial. Conventional and multiplex immunohistochemistry of microenvironmental and OS cells were performed on biopsies of 124 OS2006 patients and 17 surgical (“OSNew”) biopsies respectively. CSF-1R (common osteoclast/macrophage progenitor) and TRAP (osteoclast activity) levels in serum of 108 patients were correlated to response to chemotherapy and to prognosis. TRAP levels at surgery and at the end of the protocol were significantly lower in ZA+ than ZA− patients (p_adj = 0.0011; 0.0132). For ZA+-patients, an increase in the CSF-1R level between diagnosis and surgery and a high TRAP level in the serum at biopsy were associated with a better response to chemotherapy (p = 0.0091; p = 0.0251). At diagnosis, high CD163+ was associated with good prognosis, while low TRAP activity was associated with better overall survival in ZA− patients only. Multiplex immunohistochemistry demonstrated remarkable bipotent CD68+/CD163+ macrophages, homogeneously distributed throughout OS regions, aside osteoclasts (CD68+/CD163−) mostly residing in osteolytic territories and osteoid-matrix-associated CD68−/CD163+ macrophages. We demonstrate that ZA not only acts on harmful osteoclasts but also on protective macrophages, and hypothesize that the bipotent CD68+/CD163+ macrophages might present novel therapeutic targets.

Triple-threat activity of PEDF in bone tumors: Tumor inhibition, tissue preservation and cardioprotection against doxorubicin

Pigment epithelium-derived factor (PEDF) is known for its osteogenic properties, but its effects against primary and secondary bone tumors have not comprehensively been demonstrated. We show the ubiquitous expression of PEDF in murine embryonic tissue. Continuous administration of PEDF in pregnant mice for five days did not adversely affect foetal health, despite PEDF's known potent antiangiogenic properties. In the case of the devastating childhood bone cancer osteosarcoma, PEDF has direct anticancer activity per se, and protects against the toxicity of doxorubicin in the heart, small intestine and testes. PEDF demonstrated anti-proliferative and pro-apoptotic effects against human prostate and breast cancer cells, tumors which are known to metastasize to bone as the preferred secondary site. Caspase-2 was activated in both tumor cell types by PEDF. In models of prostate and breast cancer in bone, PEDF significantly reduced tumor volumes. When combined with zoledronic acid, continuously-administered PEDF significantly reduced breast tumor volume at the bone, and was able to preserve the quality of bone better than the combination therapy. These multiple positive findings make PEDF an ideal endogenous and safe biological for possible future clinical testing.

Canine sarcomas as a surrogate for the human disease

Pet dogs are becoming increasingly recognized as a population with the potential to inform medical research through their treatment for a variety of maladies by veterinary health professionals. This is the basis of the One Health initiative, supporting the idea of collaboration between human and animal health researchers and clinicians to study spontaneous disease processes and treatment in animals to inform human health. Cancer is a major health burden in pet dogs, accounting for approximately 30% of deaths across breeds. As such, pet dogs with cancer are becoming increasingly recognized as a resource for studying the pharmacology and therapeutic potential of anticancer drugs and therapies under development. This was recently highlighted by a National Academy of Medicine Workshop on Comparative Oncology that took place in mid-2015 (http://www.nap.edu/21830). One component of cancer burden in dogs is their significantly higher incidence of sarcomas as compared to humans. This increased incidence led to canine osteosarcoma being an important component in the development of surgical approaches for osteosarcoma in children. Included in this review of sarcomas in dogs is a description of the incidence, pathology, molecular characteristics and previous translational therapeutic studies associated with these tumors. An understanding of the patho-physiological and molecular characteristics of these naturally occurring canine sarcomas holds great promise for effective incorporation into drug development schemas, for evaluation of target modulation or other pharmacodynamic measures associated with therapeutic response. These data could serve to supplement other preclinical data and bolster clinical investigations in tumor types for which there is a paucity of human patients for clinical trials.

Immune infiltration and PD-L1 expression in the tumor microenvironment are prognostic in osteosarcoma

Osteosarcoma patient survival has remained stagnant for 30 years. Novel therapeutic approaches are needed to improve outcomes. We examined the expression of Programmed Death Ligand 1 (PD-L1) and defined the tumor immune microenvironment to assess the prognostic utility in osteosarcoma. PD-L1 expression in osteosarcoma was examined in two patient cohorts using immunohistochemistry (IHC) (n = 48, n = 59) and expression was validated using quantitative real time PCR (n = 21) and western blotting (n = 9). IHC was used to determine the presence of tumor infiltrating lymphocytes and antigen-presenting cells (APCs) in the tumor. Expression of PD-L1 was correlated with immune cell infiltration and event-free-survival (EFS). The 25% of primary osteosarcoma tumors that express PD-L1 were more likely to contain cells that express PD-1 than PD-L1 negative tumors (91.7% vs 47.2%, p = 0.002). Expression of PD-L1 was significantly associated with the presence of T cells, dendritic cells, and natural killer cells. Although all immune cell types examined were present in osteosarcoma samples, only infiltration by dendritic cells (28.3% vs. 83.9%, p = 0.001) and macrophages (45.5% vs. 84.4%, p = 0.031) were associated with worse five-year-EFS. PD-L1 expression was significantly associated with poorer five-year-EFS (25.0%. vs. 69.4%, p = 0.014). Further studies in osteosarcoma are needed to determine if targeting the PD-L1:PD-1 axis improves survival.

Rank ligand as a target in musculoskeletal neoplasms

Receptor activator of nuclear factor-κB ligand (RANKL) is a tumor necrosis factor (TNF) family member, which signals through the osteoclast surface RANK. As such, RANKL is required for osteoclast differentiation and function, namely bone resorption. There is now growing evidence that RANKL is a therapeutic target for musculoskeletal neoplasms, namely giant cell tumor of bone (GCTB) and osteosarcoma.

RANK and RANK ligand expression in primary human osteosarcoma

Receptor activator of nuclear factor kappa-B ligand (RANKL) is an essential mediator of osteoclast formation, function and survival. In patients with solid tumor metastasis to the bone, targeting the bone microenvironment by inhibition of RANKL using denosumab, a fully human monoclonal antibody (mAb) specific to RANKL, has been demonstrated to prevent tumor-induced osteolysis and subsequent skeletal complications. Recently, a prominent functional role for the RANKL pathway has emerged in the primary bone tumor giant cell tumor of bone (GCTB). Expression of both RANKL and RANK is extremely high in GCTB tumors and denosumab treatment was associated with tumor regression and reduced tumor-associated bone lysis in GCTB patients. In order to address the potential role of the RANKL pathway in another primary bone tumor, this study assessed human RANKL and RANK expression in human primary osteosarcoma (OS) using specific mAbs, validated and optimized for immunohistochemistry (IHC) or flow cytometry.

Our results demonstrate RANKL expression was observed in the tumor element in 68% of human OS using IHC. However, the staining intensity was relatively low and only 37% (29/79) of samples exhibited≥10% RANKL positive tumor cells. RANK expression was not observed in OS tumor cells. In contrast, RANK expression was clearly observed in other cells within OS samples, including the myeloid osteoclast precursor compartment, osteoclasts and in giant osteoclast cells. The intensity and frequency of RANKL and RANK staining in OS samples were substantially less than that observed in GCTB samples. The observation that RANKL is expressed in OS cells themselves suggests that these tumors may mediate an osteoclastic response, and anti-RANKL therapy may potentially be protective against bone pathologies in OS. However, the absence of RANK expression in primary human OS cells suggests that any autocrine RANKL/RANK signaling in human OS tumor cells is not operative, and anti-RANKL therapy would not directly affect the tumor.

Receptor activator of nuclear factor κB expression is a prognostic factor in human osteosarcoma

Receptor activator of nuclear factor κB (RANK), a member of the tumour necrosis factor family, is activated by its ligand and regulates the differentiation of osteoclasts and dendritic cells. Local growth of osteosarcoma involves destruction of the host bone by osteoclasts and proteolytic mechanisms. Although the prognosis of patients with osteosarcoma has been improved by advances in chemotherapy over the last four decades, the issues of non-responders, and the lack of effective prognostic markers have remained. The present study aimed to investigate the prognostic and predictive value of RANK expression in human osteosarcoma. The expression of RANK was immunohistochemically evaluated in biopsies of 43 patients (mean age 25.4 years) with high-grade osteosarcoma, and was found to be correlated with histological response to chemotherapy, disease-free status and overall survival. RANK expression was detected in eight of the 43 osteosarcoma specimens (18%), whereas the remaining specimens were negative for RANK. A statistically significant correlation was detected between RANK expression and the overall survival of patients. A total of 7/8 patients with RANK-expressing tumours succumbed to the disease (88% mortality rate amongst patients with RANK-positive tumours vs. 37% with RANK-negative tumours; P<0.05). No significant difference was found when comparing RANK expression status with response to chemotherapy; 50% of RANK-positive patients exhibited a poor response to chemotherapy, compared with 66% in the RANK negative group. In addition, the appearance of metastases was not correlated with RANK expression status (38% metastases in RANK-positive tumours vs. 34% in RANK-negative tumours). In conclusion, the results of the present study suggested that RANK expression is likely to be of prognostic, but not of predictive, value.

RANKL inhibitors induce osteonecrosis of the jaw in mice with periapical disease

Antiresorptive medications are essential in treating diseases of pathologic osteoclastic bone resorption, including bone cancer and osteoporosis. Bisphosphonates (BPs) are the most commonly used antiresorptives in clinical practice. Although inhibition of bone resorption is important in regulating unwanted malignant and metabolic osteolysis, BP treatment is associated with potential side effects, including osteonecrosis of the jaws (ONJ). Recently, non-BP antiresorptive medications targeting osteoclastic function and differentiation, such as denosumab, have entered the clinical arena. Denosumab treatment results in a similar rate of ONJ as BPs. Animal models of ONJ, using high-dose BP treatment in combination with tooth extraction or dental disease, provide valuable tools and insight in exploring ONJ pathophysiology. However, the ability of other antiresorptives to induce ONJ-like lesions in animal models has not been explored. Such studies would be beneficial in providing support for the role of osteoclast inhibition in ONJ pathogenesis versus a direct BP effect on oral tissues. Here, we tested the ability of the receptor activator of NF-κB ligand (RANKL) inhibitors RANK-Fc (composed of the extracellular domain of RANK fused to the fragment crystallizable [Fc] portion of immunoglobulin G [IgG]) and OPG-Fc (composed of the RANKL-binding domains of osteoprotegerin [OPG] linked to the Fc portion of IgG) to induce ONJ in mice in the presence of periapical disease, but in the absence of dental extractions. We demonstrate radiographic evidence of ONJ in RANK-Fc-treated and OPG-Fc-treated mice, including inhibition of bone loss, increased bone density, lamina dura thickening, and periosteal bone deposition. These findings closely resembled the radiographic appearance of an ONJ patient on denosumab treatment. Histologic examination revealed that RANK-Fc treatment and OPG-Fc treatment resulted in absence of osteoclasts, periosteal bone formation, empty osteocytic lacunae, osteonecrosis, and bone exposure. In conclusion, we have successfully induced ONJ in mice with periapical disease, using potent osteoclast inhibitors other than BPs. Our findings, coupled with ONJ animal models using high-dose BPs, suggest that osteoclast inhibition is pivotal to the pathogenesis of ONJ.

Denosumab and giant cell tumour of bone-a review and future management considerations

Giant cell tumour of bone (gctb) is one type of giant-cell-rich bone lesion characterized by the presence of numerous multinucleated osteoclast-type giant cells. Giant cells are known to express rankl (receptor activator of nuclear factor κB ligand) and are responsible for the aggressive osteolytic nature of the tumour. No available treatment option is definitively effective in curing this disease, especially in surgically unsalvageable cases. In recent years, several studies of denosumab in patients with advanced or unresectable gctb have shown objective changes in tumour composition, reduced bony destruction, and clinical benefit. Denosumab is a fully human monoclonal antibody that targets and binds with high affinity and specificity to rankl. Several large phase iii studies have shown that denosumab is more effective than bisphosphonates in reducing skeletal morbidity arising from a wide range of tumours and that it can delay bone metastasis. The relevant articles are reviewed here. The controversies related to the future use of denosumab in the treatment of gctb are discussed.

Children's Oncology Group's 2013 blueprint for research: bone tumors

In the US, approximately 650 children are diagnosed with osteosarcoma and Ewing sarcoma (ES) each year. Five-year survival ranges from 65% to 75% for localized disease and <30% for patients with metastases. Recent findings include interval-compressed five drug chemotherapy improves survival with localized ES. In osteosarcoma a large international trial investigating the addition of ifosfamide/etoposide or interferon to standard therapy has completed accrual. For ES an ongoing trial explores the addition of cyclophosphamide/topotecan to interval-compressed chemotherapy. Trials planned by the Children's Oncology Group will investigate new target(s) including IGF-1R and mTOR in ES, and RANKL and GD2 in osteosarcoma.

Differential expression of the RANKL/RANK/OPG system is associated with bone metastasis in human non-small cell lung cancer

Background

Human non-small cell lung cancer (NSCLC) patients exhibit a high propensity to develop skeletal metastasis, resulting in excessive osteolytic activity. The RANKL/RANK/OPG system, which plays a pivotal role in bone remodeling by regulating osteoclast formation and activity, is of potential interest in this context.

Materials and Methods

Reverse transcriptase polymerase chain reaction, western blotting, and immunohistochemical analysis were used to examine the expression of RANKL, RANK, and OPG in human NSCLC cell lines with different metastatic potentials, as well as in 52 primary NSCLC samples and 75 NSCLC bone metastasis samples. In primary NSCLC patients, the expression of these proteins was correlated with clinicopathological parameters. Recombinant human RANKL and transfected RANKL cDNA were added to the PAa cell line to evaluate the promoter action of RANKL during the process of metastasis in vitro and in vivo.

Results

Up-regulated RANKL, RANK, and OPG expression and increased RANKL:OPG ratio were detected in NSCLC cell lines and in tumor tissues with bone metastasis, and were correlated with higher metastatic potential. The metastatic potential of NSCLC in vitro and in vivo, including migration and invasion ability, was significantly enhanced by recombinant human RANKL and the transfection of RANKL cDNA, and was impaired after OPG was added. The increased expression of RANKL and OPG correlated with tumor stage, lymph node metastasis, and distant metastasis.

Conclusions

Differential expression of RANKL, RANK, and OPG is associated with the metastatic potential of human NSCLC to skeleton, raising the possibility that the RANKL/RANK/OPG system could be a therapeutic target for the treatment of metastatic NSCLC patients.

Bone sarcomas: from biology to targeted therapies

Primary malignant bone tumours, osteosarcomas, and Ewing sarcomas are rare diseases which occur mainly in adolescents and young adults. With the current therapies, some patients remain very difficult to treat, such as tumour with poor histological response to preoperative CT (or large initial tumour volume for Ewing sarcomas not operated), patients with multiple metastases at or those who relapsed. In order to develop new therapies against these rare tumours, we need to unveil the key driving factors and molecular abnormalities behind the malignant characteristics and to broaden our understanding of the phenomena sustaining the metastatic phenotype and treatment resistance in these tumours. In this paper, starting with the biology of these tumours, we will discuss potential therapeutic targets aimed at increasing local tumour control, limiting metastatic spread, and finally improving patient survival.

The role of osteoclasts and tumour-associated macrophages in osteosarcoma metastasis

Osteosarcoma (OS) is the most common primary bone tumour in the paediatric age group. Treatment-refractory pulmonary metastasis continues to be the major complication of OS, reducing the 5-year survival rate for these patients to 10-20%. The mechanisms underlying the metastatic process in OS are still unclear, but undoubtedly, a greater understanding of the factors and interactions involved in its regulation will open new and much needed opportunities for therapeutic intervention. Recent published data have identified a new role for bone-specific macrophages (osteoclasts) and tumour-associated macrophages (TAMs), in OS metastasis. In this review we discuss the contribution of TAMs and osteoclasts in the establishment and maintenance of secondary metastatic lesions, and their novel role in the prevention of metastatic disease in a primary bone cancer such as osteosarcoma.

Denosumab--a powerful RANKL inhibitor to stop lytic metastases and other bone loss actions by osteoclasts

Denosumab is a perfect example on the targeted anticancer therapy. The inhibition of RANKL activity suppressed the osteoclasts' resorptive function and so prevented skeletal related events. This effect is useful not only against bone metastases, but also in the treatment of other diseases caused by bone loss. In different solid tumors with bone metastasis the quality of life also improved, although the overall survival usually showed no change. On the market the main competitors for denosumab are still the bisphosphonates (questions of costs and reimbursement are not discussed) and some potential new agents e.g. Src kinases (as dasatinib, saracatinib, bosutinib), cathepsin K inhibitors, (e.g. odanacatib), and new selective estrogen receptor modulators (e.g. bazedoxifene, lasofoxifene). Nevertheless, today denosumab is one of the most powerful agents in bone-saving area.

Denosumab in patients with cancer-a surgical strike against the osteoclast

Elucidation of the molecular pathways underlying bone turnover has revealed potential therapeutic targets, including receptor activator of nuclear factor-κB ligand (RANKL), which is a mediator of osteoclast formation, function and survival. Denosumab is a fully human monoclonal antibody that binds to and inhibits RANKL. This agent has been developed for use in patients with early-stage and advanced-stage cancer, as well as for the treatment of osteoporosis, and can prevent bone loss and reduce fragility fractures in both types of disease. In the bone metastasis setting, several large phase III studies have shown that denosumab is more effective than bisphosphonates, namely zoledronic acid, in reducing skeletal morbidity arising from a wide range of tumors. In addition, a remarkable activity of denosumab has been demonstrated in giant-cell tumors of the bone. Subsequent studies of denosumab have demonstrated that it can delay bone metastasis in patients with castration-resistant prostate cancer; adjuvant studies in patients with breast cancer are in progress. This Review critically explores the emerging role of denosumab in maintaining bone health in the oncology setting, and discusses the factors that are likely to influence the choice between bisphosphonates and denosumab in clinical practice.

[The role of RANK/RANKL/osteoprotegerin (OPG) triad in cancer-induced bone diseases: physiopathology and clinical implications]

Bone homeostasis is maintained by the remodelling of bone which depends on a balance between osteoclast-mediated bone resorption and osteoblast-mediated bone formation. Malignant bone lesions are very common in patients with cancer; whether they result from a tumor in bone (giant cell tumour of bone, osteosarcoma, multiple myeloma...) or they are bony metastases from advanced cancers of which the most osteotropic are breast and prostate cancer. Malignant cells within the bone disrupt the normal bone remodelling process, leading to increased bone destruction and occurence of pathological fractures. Receptor activator of NF-kB (RANK) and its ligand (RANKL) play a pivotal role in the regulation of bone remodelling; by binding to RANK, RANKL stimulates osteoclastogenesis and bone resorption, whereas its cognate decoy receptor osteoprotegerin (OPG) blocks this process by interacting with RANKL. Tumour cells produce different factors that manipulate the RANK/RANKL/OPG pathway in order to stimulate bone destruction. Furthermore, pending on the tumour type, RANKL plays a role in the migration, invasion and proliferation of malignant cells within the bone, while OPG increases survival of tumour cells. Inhibition of RANK/RANKL system may therefore offer new therapeutic perspectives for the treatment of primitive and secondary bone cancers.