Therapeutic approach of primary bone tumours by bisphosphonates

Bone tumours can be dissociated in two main categories: i) primary bone tumours (benign or malignant) including mainly osteosarcoma and other sarcomas.ii)and giant cell tumour and bone metastases originate from others cancer (Breast, prostate, kidney cancer, etc). These tumours are able to destroy or/and induce a new calcified matrix. However, the first step of bone tumour development is associated with an induction of bone resorption and the establishment of a vicious cycle between the osteoclasts and the tumour growth. Indeed, bone resorption contributes to the pathogenesis of bone tumour by the release of cytokines (IL6, TNFα) which govern the bone tumour's development and which are trapped into the bone matrix. Bisphosphonates (BPs) are chemical compounds of P-C-P structure with a high affinity for bone hydroxyapatite crystals. Thus, they have been used as a carrier for radio nucleotides to develop novel approaches of bone imaging. BPs exert also indirect anti-tumour activities in vivo. Indeed, BPs directly interfere with the bone microenvironment and target osteoclasts, endothelial cells and immune cells (tumour-associated macrophages, γ9δ2 T cells). BPs induce tumour cell death in vitro and same activity is suspected in vivo. The present review summarizes the mechanisms of actions of BPs as well as their clinical interests in bone primary tumours.

Farnesyl diphosphate synthase is involved in the resistance to zoledronic acid of osteosarcoma cells

We recently demonstrated original anti-tumor effects of zoledronic acid (Zol) on osteosarcoma cell lines independently of their p53 and Rb status. The present study investigated the potential Zol-resistance acquired by osteosarcoma cells after prolonged treatment. After 12 weeks of culture in the presence of 1 μm Zol, the effects of high doses of Zol (10–100 μm) were compared between the untreated rat (OSRGA, ROS) and human (MG63, SAOS2) osteosarcoma cells and Zol-pretreated cells in terms of cell proliferation, cell cycle analysis, migration assay and cytoskeleton organization. Long-term treatment with 1 μm Zol reduced the sensitivity of osteosarcoma cells to high concentrations of Zol. Furthermore, the Zol-resistant cells were sensitive to conventional anti-cancer agents demonstrating that this resistance process is independent of the multidrug resistance phenotype. However, as similar experiments performed in the presence of clodronate and pamidronate evidenced that this drug resistance was restricted to the nitrogen-containing bisphosphonates, we then hypothesized that this resistance could be associated with a differential expression of farnesyl diphos-phate synthase (FPPS) also observed in human osteosarcoma samples. The transfection of Zol-resistant cells with FPPS siRNA strongly increased their sensitivity to Zol. This study demonstrates for the first time the induction of metabolic resistance after prolonged Zol treatment of osteosarcoma cells confirming the therapeutic potential of Zol for the treatment of bone malignant pathologies, but points out the importance of the treatment regimen may be important in terms of duration and dose to avoid the development of drug metabolic resistance.

Zoledronic acid activates the DNA S-phase checkpoint and induces osteosarcoma cell death characterized by apoptosis-inducing factor and endonuclease-G translocation independently of p53 and retinoblastoma status

The molecular mechanisms responsible for the cellular effects of the nitrogen-containing bisphosphonate zoledronic acid (Zol) were assessed on several osteosarcoma cell lines differing in their p53 and retinoblastoma (Rb) status. Zol inhibited cell proliferation and increased atypical apoptosis. The Zol effects on proliferation were due to cell cycle arrest in S and G2/M phases subsequent to the activation of the intra-S DNA damage checkpoint with an increase in P-ATR, P-chk1, Wee1, and P-cdc2 levels and a decrease in cdc25c, regardless of the p53 and Rb status. In addition, the atypic apoptosis induced by Zol was independent of caspase activation, and it was characterized by nuclear alterations, increased Bax expression, and reduced Bcl-2 level. Furthermore, mitochondrial permeability was up-regulated by Zol independently of p53 in association with the translocation of apoptosis-inducing factor (AIF) and endonuclease-G (EndoG). Zol also disturbed cytoskeletal organization and cell junctions and inhibited cell migration and phosphorylation of focal adhesion kinases. The main difficulty encountered in treating cancer relates to mutations in key genes such as p53, Rb, or proteins affecting caspase signaling carried by many tumor cells. We have demonstrated for the first time that zoledronic acid activated the DNA damage S-phase checkpoint and the mitochondrial pathway via AIF and EndoG translocation, and it inhibited cell proliferation and induced cell death, bypassing these potentials mutations. Therefore, zoledronic acid may be considered as an effective therapeutic agent in clinical trials of osteosarcoma in which mutation for p53 and Rb very often occur, and where current treatment with traditional chemotherapeutic agents is ineffective.

Enhanced tumor regression and tissue repair when zoledronic acid is combined with ifosfamide in rat osteosarcoma

The efficacy of zoledronic acid (ZOL), with or without the anticancer drug ifosfamide (IFO), was tested on primary bone tumor growth using a rat-transplantable model of osteosarcoma. The effects on bone remodeling and tumor growth were analyzed by radiography, micro-computed tomography (micro-CT), and histological staining. The in vitro effects of ZOL were studied by proliferation, apoptosis, and cell cycle analyses on the osteosarcoma cells OSRGA compared to rat primary osteoblasts. Treatment with ZOL was effective in preventing the formation of osteolytic lesions that developed in bone sites and in reducing the local tumor growth, as compared to the untreated rats. The combination of ZOL and IFO was more effective than each agent alone in preventing tumor recurrence, improving tissue repair, and increasing bone formation as revealed by the analysis of trabecular architecture. In vitro studies demonstrated that ZOL was more potent against the OSRGA cell line than osteoblasts (with a half-maximal inhibitory effect on proliferation seen at 0.2 and 20 microM, respectively), the ZOL-induced inhibition of OSRGA proliferation being due to cell cycle arrest in S-phase. No effect on OSRGA apoptosis could be observed in vitro, as assessed by Hoechst staining and caspase-1 and -3 activation. In situ cell death was determined by TUNEL staining on tumor tissue sections. No significant difference in TUNEL-positive cells could be observed between ZOL-treated and -untreated rats. This is the first report of the anti-bone resorption and antitumoral activities of zoledronic acid in a rat model of osteosarcoma, and its beneficial association with an antitumoral chemotherapeutic drug in preventing tumor recurrence.