Abstract
Advanced-stage cancers often metastasize to bone, and is the major cause of cancer-related morbidity and mortality. Due to poor biodistribution of intravenously administered anticancer drugs within the bone, chemotherapy is not optimally effective in treating bone metastasis. Additionally, overexpression of receptor activator of nuclear factor κB ligand (RANKL) in the bone microenvironment drives the vicious, destructive cycle of progression of bone metastasis and bone resorption. We hypothesized that the combination treatment - with docetaxel (TXT), an anticancer drug encapsulated in sustained release biodegradable nanoparticles (TXT-NPs) that are designed to localize in bone marrow, and denosumab monoclonal antibody (DNmb), which binds to RANKL - could be more effective than either treatment alone. We tested our hypothesis in intraosseous prostate cancer (PC-3) cell-induced osteolytic mouse model of bone metastasis with treatments given intravenously. The results demonstrated better efficacy with TXT-NPs than with TXT-CrEL or saline control in inhibiting progression of metastasis and improving survival. TXT-NPs showed ~3-fold higher drug levels in metastasized bone tissue at 1 wk post-administration than TXT-CrEL, thus explaining their efficacy. However, the combination treatment (TXT-NPs + DNmb) given simultaneously was significantly more effective in inhibiting metastatic progression; it caused early tumor regression and improved survival, and caused no body weight loss or tumor relapse, even when the treatment was discontinued, whereas TXT-NPs or DNmb alone treatments showed tumor relapse after an initial regression. Micro-CT analysis of the bone from the combination treatment showed no bone loss and normal bone mineral content, bone density, and bone volume fraction, whereas TXT-NPs or DNmb alone treatments showed bone loss. Confirming the above results, histochemical analysis of the bone from the combination treatment demonstrated normal bone morphology, and osteoblast and osteoclast cell activities. In conclusion, TXT-NPs and DNmb in combination, because of their complementary roles in breaking the cross talk between cancer cells and bone cells, was significantly effective in treating bone metastasis.