Bone metastasis in prostate cancer: emerging therapeutic strategies

Metastatic bone disease (MBD) in advanced-stage cancer increases the risk of intractable bone pain, pathological skeletal fracture, spinal-cord compression and decreased survival. The disease manifestation course during MBD is largely driven by homotypic and heterotypic cellular interactions between invading tumor cells, osteoblasts and osteoclasts. The outcome is a sustained vicious cycle of bone matrix remodeling. Osteoclast-mediated bone degradation and subsequent bone loss are the hallmarks of secondary bone metastases from most solid tumors. An additional complication in prostate cancer is the predominance of osteosclerotic lesions typified by inappropriate bone production. Successful therapeutic strategies for the treatment of osteolytic MBD include the administration of intravenous bisphosphonates or subcutaneous inhibitors of receptor activator of nuclear factor kappa B ligand (RANKL). Inhibitors of SRC and cABL kinases and cathepsin K are under clinical investigation as potential anti-osteolytics. In contrast to the rapid progress being made in the development of anti-osteolytic therapies, the treatment of osteosclerotic MBD remains restricted to palliative radiotherapy for symptomatic solitary lesions and systemic taxane-based chemotherapy for widespread multiple lesions. This Review discusses the complex pathology of bone lesions in metastatic castration-resistant prostate cancer and focuses on new therapeutic strategies and targets that are emerging in preclinical studies.