Nanoparticle Strategies for Bone Metastasis Immunotherapy: Targeting, Immune Reprogramming and Combination Therapy

Bone metastases remain one of the most clinically devastating complications of advanced cancer, particularly in breast, prostate, and lung malignancies, where they drive pain, fractures, hypercalcemia, and progressive functional decline. Their management is further complicated by a highly immunosuppressive bone microenvironment characterized by osteoclast-driven bone destruction, myeloid cell dominance, impaired antigen presentation, and weak effector T-cell infiltration, all of which limit the activity of conventional immunotherapies. In this setting, nanoparticles are emerging not merely as passive drug carriers but as programmable platforms capable of reshaping the metastatic niche. This review discusses how bone-targeted and immune-responsive nanocarriers can improve therapeutic precision through hydroxyapatite-binding ligands, dual-targeting strategies, stealth coatings, enzyme- and pH-responsive release systems, and externally guided platforms. We further examine how these systems modulate key immune compartments within bone metastases, including reprogramming tumor-associated macrophages and myeloid-derived suppressor cells, restoring cytotoxic T-cell activity, enhancing dendritic-cell activation, and enabling in situ vaccination through photothermal or photodynamic immunogenic cell death. Particular attention is given to the delivery of checkpoint inhibitors, cytokines, siRNA/miRNA, mRNA, and clustered regularly interspaced short palindromic repeats (CRISPR)-based payloads, as well as to the rational combination of these with chemotherapy, bone-modifying agents, and radiotherapy. Finally, we highlight major translational barriers, including lesion heterogeneity, limited penetration into mineralized tissue, off-target immune effects, manufacturing complexity, and the continued lack of bone-specific preclinical and clinical validation. Collectively, immunomodulatory nanoparticles represent a promising strategy to convert bone metastases from immune-refractory sites into more therapeutically responsive lesions.