Immunotherapy in the Battle Against Bone Metastases: Mechanisms and Emerging Treatments

Bone metastases are a prevalent complication in advanced cancers, particularly in breast, prostate, and lung cancers, and are associated with severe skeletal-related events (SREs), including fractures, spinal cord compression, and debilitating pain. Conventional bone-targeted treatments like bisphosphonates and RANKL inhibitors (denosumab) reduce osteoclast-mediated bone resorption but do not directly impact tumor progression within the bone. This review focuses on examining the growing potential of immunotherapy in targeting the unique challenges posed by bone metastases. Even though immune checkpoint inhibitors (ICIs) have significantly changed cancer treatment, their impact on bone metastases appears limited because of the bone microenvironment's immunosuppressive traits, which include high levels of transforming growth factor-beta (TGFβ) and the immune-suppressing cells, such as regulatory T cells (Tregs) and myeloid-derived suppressor cells (MDSCs). This review underscores the investigation of combined therapeutic approaches that might ease these difficulties, such as the synergy of immune checkpoint inhibitors with agents aimed at bones (denosumab, bisphosphonates), chemotherapy, and radiotherapy, as well as the combination of immune checkpoint inhibitors with different immunotherapeutic methods, including CAR T-cell therapy. This review provides a comprehensive analysis of preclinical studies and clinical trials that show the synergistic potential of these combination approaches, which aim to both enhance immune responses and mitigate bone destruction. By offering an in-depth exploration of how these strategies can be tailored to the bone microenvironment, this review underscores the need for personalized treatment approaches. The findings emphasize the urgent need for further research into overcoming immune evasion in bone metastases, with the goal of improving patient survival and quality of life.