Osteosarcomas are the most frequent primary bone sarcomas, affecting mainly children, adolescents, and young adults, and with a second peak of incidence in elderly individuals. The current therapeutic management, a combined regimen of poly-chemotherapy and surgery, still remains largely insufficient, as patient survival has not improved in recent decades. Osteosarcomas are very heterogeneous tumors, both at the intra- and inter-tumor level, with no identified driver mutation. Consequently, efforts to improve treatments using targeted therapies have faced this lack of specific osteosarcoma targets. Nevertheless, these tumors are inextricably linked to their local microenvironment, composed of bone, stromal, vascular and immune cells and the osteosarcoma microenvironment is now considered to be essential and supportive for growth and dissemination. This review describes the different actors of the osteosarcoma microenvironment and gives an overview of the past, current, and future strategies of therapy targeting this complex ecosystem, with a focus on the role of extracellular vesicles and on the emergence of multi-kinase inhibitors.

Osteosarcoma is the most frequent malignant primary bone tumor and a main cause of cancer-related death in children and adolescents. Although long-term survival in localized osteosarcoma has improved to about 60% during the 1960s and 1970s, long-term survival in both localized and metastatic osteosarcoma has stagnated in the past several decades. Thus, current conventional therapy consists of multi-agent chemotherapy, surgery and radiation, which is not fully adequate for osteosarcoma treatment. Innovative drugs and approaches are needed to further improve outcome in osteosarcoma patients. This review describes the current management of osteosarcoma as well as potential new therapies.

Osteosarcoma is the most frequent primary bone tumor that develops mainly in the young, the median age of diagnosis being 18 years. Despite improvement in osteosarcoma treatment, survival rate is only 30% at 5 years for patients with pulmonary metastases at diagnosis. This warrants exploration of new therapeutic options, and among them, osteoprotegerin (OPG), a naturally occurring protein that inhibits bone resorption, is very promising in blocking the vicious cycle between bone resorption and tumor proliferation that takes place during tumor development in bone site. As OPG binds and inhibits the activity of tumor necrosis factor–related apoptosis-inducing ligand, the truncated form of murine OPG 1-194 was used. The cDNA encoding OPG was administered by gene transfer using replication-defective adenoviral vector or was associated with an amphiphilic polymer in two models of rodent osteosarcoma. In both models, OPG gene transfer was effective in preventing the formation of osteolytic lesions associated with osteosarcoma development, in reducing the tumor incidence and the local tumor growth, leading to a 4-fold augmentation of mice survival 28 days postimplantation. On the contrary, OPG did not prevent the development of pulmonary metastasis alone, suggesting that bone environment is necessary for OPG therapeutic efficacy. Because OPG has no direct activity on osteosarcoma cells in vitro (cell binding, cell proliferation, apoptosis, or cell cycle distribution), we show that OPG exerts indirect inhibitory effect on tumor progression through the inhibition of RANKL whose production is enhanced in bone tumor environment, leading to osteolysis inhibition as reflected by osteoclast number decrease. [Cancer Res 2007;67(15):7308–18]

Osteosarcomas are the most prevalent malignant primary bone tumors in children. Despite intensive efforts to improve both chemotherapeutics and surgical management, 40% of all osteosarcoma patients succumb to the disease. Specifically, the clinical outcome for metastatic osteosarcoma remains poor; less than 30% of patients who present metastases will survive five years after initial diagnosis. Treating metastatic osteosarcoma thus remains a challenge. One of the main characteristics of osteosarcomas is their ability to deregulate bone remodelling. The invasion of bone tissue by tumor cells indeed affects the balance between bone resorption and bone formation. This deregulation induces the release of cytokines or growth factors initially trapped in the bone matrix, such as transforming growth factor-β (TGF-β), which in turn promote tumor progression. Over the past years, there has been considerable interest in the TGF-β pathway within the cancer research community. This review discusses the involvement of the TGF-β signalling pathway in osteosarcoma development and in their metastatic progression.

Osteosarcoma and Ewing sarcoma are the most common malignant primary bone tumors mainly occurring in children, adolescents and young adults. Current standard therapy includes multidrug chemotherapy and/or radiation specifically for Ewing sarcoma, associated with tumor resection. However, patient survival has not evolved for the past decade and remains closely related to the response of tumor cells to chemotherapy, reaching around 75% at 5 years for patients with localized forms of osteosarcoma or Ewing sarcoma but less than 30% in metastatic diseases and patients resistant to initial chemotherapy. Despite Ewing sarcoma being characterized by specific EWSR1-ETS gene fusions resulting in oncogenic transcription factors, currently, no targeted therapy could be implemented. It seems even more difficult to develop a targeted therapeutic strategy in osteosarcoma which is characterized by high complexity and heterogeneity in genomic alterations. Nevertheless, the common point between these different bone tumors is their ability to deregulate bone homeostasis and remodeling and divert them to their benefit. Therefore, targeting different actors of the bone tumor microenvironment has been hypothesized to develop new therapeutic strategies. In this context, it is well known that the Wnt/β-catenin signaling pathway plays a key role in cancer development, including osteosarcoma and Ewing sarcoma as well as in bone remodeling. Moreover, recent studies highlight the implication of the Wnt/β-catenin pathway in angiogenesis and immuno-surveillance, two key mechanisms involved in metastatic dissemination. This review focuses on the role played by this signaling pathway in the development of primary bone tumors and the modulation of their specific microenvironment.

Background: Osteosarcoma is the most common type of primary bone tumor. The use of aggressive chemotherapy has drastically improved the prognosis of the patients with non-metastatic osteosarcomas, however the prognosis of the patients with metastasis is still very poor. Then, new and more effective treatments for curing osteosarcoma, such as immunotherapy are needed. Tumor-infiltrating lymphocytes (TIL) have been involved in the control of tumor development and already assessed with success for the treatment of several cancers including melanoma. While TIL represent a fascinating therapeutic approach in numerous malignant pathologies, there is few report concerning adult bone-associated tumors including osteosarcoma. Methods: Human TIL were isolated and characterized (phenotype, lytic activity) from twenty-seven patients with bone-associated tumors (osteosarcoma, Ewing's sarcoma, giant cell tumor, chondrosarcoma, plasmocytoma and bone metastases). Similar experiments were performed using rat osteosarcoma model. Results: While TIL with a main CD4⁺ profile were easily isolated from most of the tumor samples, only TIL extracted from osteosarcoma were cytotoxic against allogeneic tumor cells. In all cases, TIL lytic activity was significantly higher compared to autologous peripheral blood leukocytes. Similar data were observed in rat osteosarcoma model where TIL were characterized by a main CD4⁺ profile and high lytic activity against allogeneic and autologous tumor cells. Moreover, rat TIL expansion was not accompanied by refractoriness to further activation stimulus mainly by tumor antigens. Conclusion: These results demonstrated that TIL therapy could be a very efficient strategy for the treatment of adult osteosarcoma.