Targeting nano-regulator based on metal–organic frameworks for enhanced immunotherapy of bone metastatic prostate cancer

Abstract Bone metastasis is the main cause of death in patients with prostate cancer (PCa), but there lacks effective treatment method. Immunotherapy shows new hopes for bone metastatic PCa patients, while the efficacy is still unsatisfactory and limited by the unique immunosuppressive microenvironment in metastatic bone site. Here, we developed a bone-targeted nano-delivery system as a nano-regulator to enhance the immunotherapy of bone metastatic PCa. The nanosystem was assembled via coordination between phytic acid (PA) and Fe 3+ to form nano-sized metal–organic framework (MOF), through which mitoxantrone (MTO) was encapsulated. At cellular level, the nanosystem showed selective cytotoxicity towards RM-1 PCa cells over immune cells, and could induce tumor cells immunogenic cell death (ICD) to improve the immunogenicity of the tumor. Moreover, the nanosystem was able to induce ubiquitination of TGFβ receptor (TβR) on immune cells to promote its degradation, thus serving as a nano-regulator to block the functions of TGF-β, an abundant cytokine that has a systematically immunosuppressive effect in the tumor microenvironment. Upon intravenous injection, the nanoparticle showed pro-longed blood circulation and targeting accumulation into bone metastatic site, and imposed robust anti-tumor effect in combination with αCTLA-4. In addition, bone destruction was significantly alleviated after treatment to reduce the skeletal-related events. Overall, this work provides a biocompatible nanomedicine to restore immune sensitivity of bone metastatic tumor for enhanced immunotherapy by blocking TGF-β signaling pathway.