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The bone targeted miRNA delivery system PCA/miR-34a for miR-34a was developed for the gene therapy to bone-associated metastatic breast cancer. PCA/miR-34a showed specific bone-targeting ability and enhanced anti-tumor performance along with minor side effects, which provides a good strategy for the gene therapy of bone metastatic cancer in the clinic. [Display omitted] Bone metastatic cancer is the most common occurrence in breast cancer, and the treatment is also facing great challenges. MicroRNA-34a (miRNA-34a) is a promising anti-cancer miRNA for gene therapy to bone metastatic cancer patients. However, the lack of specificity to bone and low accumulation at the site of bone tumor remains the major challenge when used bone-associated tumor. To solve this problem, a bone-targeted vector for delivery of miR-34a to bone metastatic breast cancer was constructed by using the commonly used gene vector branched polyethylenimine 25 k (BPEI 25 k) as the skeleton and linking with alendronate (ALN) moieties for bone targeting group. The constructed gene delivery system PCA/miR-34a can efficiently prevent miR-34a from degradation during blood circulation and enhance the specific bone delivery and distribution. PCA/miR-34a nanoparticles can be uptake into tumor cells through clathrin and caveolae-mediated endocytosis, and directly regulate the expression of oncogenes, thus promoting tumor cell apoptosis and relieving bone tissue erosion. The results of experiments in vitro and in vivo confirmed that the constructed bone-targeted miRNA delivery system PCA/miR-34a can enhance the anti-tumor efficacy in bone metastatic cancer, and provide a potential strategy for gene therapy in bone metastatic cancer. [ABSTRACT FROM AUTHOR]