The activity of zoledronic acid on neuroblastoma bone metastasis involves inhibition of osteoclasts and tumor cell survival and proliferation

Metastasis to the bone is seen in 56% of patients with neuroblastoma and contributes to morbidity and mortality. Using a murine model of bone invasion, we have reported previously that neuroblastoma cells invade the bone by activating osteoclasts. Here, we investigated the antitumoral and antiosteolytic activities of zoledronic acid, a bisphosphonate inhibitor of osteoclasts, in combination with cytotoxic chemotherapy in our model. We first show that zoledronic acid given at the same time (early prevention) or 2 weeks after tumor cell injection (late prevention) significantly prevented the formation of severe osteolytic lesions. It also prevented formation of these lesions when given 4 weeks after tumor cell injection (intervention) when combined with chemotherapy including cyclophosphamide and topotecan. The combination of zoledronic acid + cyclophosphamide/topotecan also significantly improved survival (P < 0.001). In mice treated with zoledronic acid, we observed a marked inhibition of osteoclasts inside the bone associated with a decrease in tumor cell proliferation and increase in tumor cell apoptosis. In vitro, zoledronic acid inhibited neuroblastoma cell proliferation and induced apoptosis, and these effects were significantly enhanced by the addition of 4-hydroxyperoxycyclophosphamide (4-HC). The proapoptotic effect of zoledronic acid and zoledronic acid in combination with 4-HC on tumor cells was associated with an increase in caspase-3 activity and a decrease in phosphorylated BcI-2, Bcl-2, and Bcl-XL expression. Zoledronic acid inhibited the association of Ras with the plasma membrane and activation of c-Raf, Akt, and extracellular signal-regulated kinase 1/2. The data indicate that zoledronic acid, in addition to inhibiting osteoclasts, is active against tumor cells and suggest that zoledronic acid in combination with cytotoxic chemotherapy may be effective in children with neuroblastoma that has metastasized to the bone.