Metastatic tumor cells are one of the major players in bone metastases. In essence, the occurrence of bone metastases is a coordinated process in which malignant tumor cells leave the primary site to spread to bone and survive in the bone microenvironment (41). The metastases of malignant tumors generally include tumor cells leaving the primary site, entering the blood circulation and ectopic colonization. In particular, for bone metastases, circulating tumor cells reside and become dormant in the normal vascular of the bone marrow long before clinically detectable metastases develop. Over time they proliferate and regulate the function of bone resorption (osteoclasts) and bone formation (osteoblasts) cells, leading to the development of bone metastases (42). Tumor cells show a tendency to metastasize more easily under certain conditions. For example, the tumor has gene mutation, epithelial-mesenchymal transformation, and metabolic changes. The gene mutation of malignant tumor cells plays a key role in bone metastasis. In a study by Huang et al. on the mechanism of bone metastasis in lung cancer, 425 and 422 genomic alterations were detected in primary and metastatic lesions respectively (43). There were significant differences in tumor mutational burden between primary lung adenocarcinoma and matched bone metastases (43). This indicates that tumor mutational burden may play a role in bone metastasis of lung cancer. Arnold et al. mentioned in their study that the number of somatic mutations in the site of bone metastasis was statistically significantly higher than that in the site of primary or soft tissue metastasis (44). Bartels et al. concluded through their study that mutations in ESR1 are associated with estrogen receptor expression as well as high proliferative activity, and affect bone metastases in a part of estrogen receptor positive breast cancers (45). However, the current researches on the influence of gene mutation on the occurrence of bone metastases are mostly reflected in the research level of epidemiological data statistics, and there are few in-depth researches on the mechanism. In the future, related research needs to be further in-depth. In addition to some reported gene mutations, epithelial mesenchymal transformation (EMT) in tumor cells may also be an important factor promoting the occurrence of bone metastases. EMT refers to the differentiation and transformation process of epithelial cells into mesenchymal cells, which is believed to be related to tumor progression including tumor metastasis (46, 47). Several studies have been published on the pathogenesis of the relationship between epithelial mesenchymal transformation and bone metastases. Liu et al. pointed out in their study that Notch3 was associated with EMT and overexpressed in bone metastases of NSCLC, and inhibition of Notch3 expression could reduce the invasion ability of NSCLC cells in vitro ( 48). Epithelial mesenchymal transformation may also be associated with metastasis of malignant tumors such as breast cancer and prostate cancer. Horas et al. confirmed that the deficiency of vitamin D receptor(VDR) in human breast cancer cells can promote can promote EMT and the spread of cancer cells (49). In the study of the pathogenesis of bone metastases, EMT is often mentioned (50, 51). Therefore, EMT can be used as a key breakthrough in future research on the treatment of bone metastases. In addition, metabolic changes of tumor cells are also considered to be a key factor in the development of tumor metastasis (52). Studies have shown that different tumor stem cells adapt to unique metabolic characteristics for organ metastasis (53). Thysell et al. analyzed the metabolism of bone metastasis in prostate cancer and identified metabolites such as cholesterol that might be associated with prostate cancer metastasis (54). In addition to the mechanisms mentioned above, cancer stem cells (CSCs), a new concept proposed in recent years, are also believed to be closely related to bone metastasis of tumors (55). Based on existing studies, we believe that in bone metastases of malignant tumors, the tumor cells should usually be changed compared to the primary site. Such changes may be at the genetic level, at the metabolic level, or at the cellular phenotype level. However, the specific changes of bone metastases in different malignancies may be different, so specific studies are needed. At present, there is still a relative lack of research on mutant genes or altered metabolic functions. After the relevant epidemiological data are revealed, more mechanism studies should be conducted to identify the target of bone metastasis and design corresponding interventions.

The role of osteoblasts/osteoclasts in bone metastases has been studied for a long time, and many drugs are gradually being completed in clinical trials. During the occurrence and development of bone metastases, many pathological changes are related to abnormal regulation of osteoblasts and osteoclasts. Inhibition of osteoblasts and abnormal activation of osteoclasts are often the key mechanisms of osteolytic metastases. Osteoblasts and osteoclasts are the direct “executors” of bone destruction in bone metastases, and their regulation may be related to a variety of cells and factors, such as tumor cells, immune cells and inflammatory factors (56–58). For osteoblasts and osteoclasts themselves, Wnt/β-catenin pathway, RANK-RANKL pathway and other pathways closely related to osteogenesis/osteoclast process are the focus of research (59, 60). Wnt signaling pathway may play an important role in bone metastasis of malignant tumors (61). The Wnt pathway and the role of osteoblasts have attracted much attention since bone metastases of prostate cancer are often manifested as osteoblastic lesions. Dai et al. showed in their study that prostate cancer can promote osteoblast differentiation through classical and non-classical Wnt signaling pathways and stimulate BMP-dependent and BMP-independent osteoblast differentiation (62). However, there are some different studies. Aufderklamm et al. have shown that DKK-1, an inhibitor of the Wnt pathway, mediated osteoblast inhibition contributes to prostate cancer progression (63). The RANK/RANKL signaling pathway has also received attention in bone metastases. This pathway mainly affects the function of osteoclasts in the local microenvironment of bone metastases. It has been suggested that the RANK/RANKL signaling pathway is involved in the castration-insensitive prostate cancer (64). SREs can be prevented with the RANKL inhibitor Denosumab (65). Interestingly, RANKL connects bone to the immune system, while RANK-RANKL is a regulator of osteoclast development, lymph node development, bone metabolism, and T cell/dendritic cell communication (66). This suggests that the regulation of the RANK/RANKL signaling pathway does not only affect osteoclasts. Not only the above common pathways, but also the effects of other factors on osteoblasts/osteoclasts have been extensively studied. For example, osteoblasts may be negatively regulated by cancer cells and appear apoptosis (67). The main mechanisms of interaction and regulation of osteoblasts/osteoclasts with tumor cells in osteolytic bone metastases are summarized in Figure 2 . In the future, some more detailed cell interactions on osteoblasts/osteoclasts in bone metastases should be further investigated, for example, the regulation of osteoblasts/osteoclasts by exosomes produced by bone metastatic tumor cells. RANK/RANKL and Wnt/β-catenin pathways are both downstream signaling pathways. In bone metastatic cancer, which signaling pathway changes may trigger the changes of the above downstream pathways is a more valuable research direction.

Different from cells, cytokines are a class of small molecules that regulate cell function with a wide range of effects. Common cytokines include interleukin(IL), tumor necrosis factor(TNF), and so on (68). In the past few decades, cytokines and cytokine receptors have been extensively studied as targets for cancer treatment (69). In the pathogenesis of metastatic tumor, cytokines may be secreted by tumor cells and immune cells, and the target may include tumor cells, immune cells, osteoblasts/osteoclasts, etc. There are many types of interleukin, which is closely related to inflammation and tumor growth, etc. At present, certain studies have been conducted in bone metastases. Claudia et al. reported in their review that IL-1B is important in the inflammatory process, and influences the growth of bone metastases in breast cancer, including angiogenesis, etc. (10). IL-6 overexpression is also associated with bone metastases (70). Interleukin is also produced by osteoclasts to regulate tumor cells. The study of He et al. showed that lung cancer cells induced osteoclasts to secrete IL-19 to act on IL20RB on the surface of lung cancer cells, thus promoting the proliferation and bone metastasis of lung cancer cells (71). Tumor necrosis factor also plays an important role in the development of bone metastases. Hamaguchi et al. found that inhibition of TNF-α has a novel role in reducing or preventing bone metastasis in breast cancer models (72). Interferon has been less studied in bone metastases. Chemokines are a class of cytokines secreted by cells, which can induce the directed migration of nearby cells (73). Chemokines play an important role in metastatic tumors because they have an important effect on cell migration, colonization and other processes. Chemokine/chemokine receptor CXCL12/CXCR4 pathway and CCR3/CCL7 pathway can be used as mediators in the process of bone metastasis and may affect the colonization of tumor cells in bone (74, 75). According to current studies, the interleukin family and chemokine family related pathways may be relatively important in the influence of bone metastases. The design of relevant targeted drugs for these two pathways may be an important idea to delay the progression of bone metastases or prevent the appearance of bone metastases.

In addition to common cytokines some enzymes can also promote the disease progression of bone metastases by influencing immunity and bone formation. Matrix metalloproteinases (MMPs) is a type of enzyme containing zinc, which can decompose extracellular matrix (76). Since MMPs is closely related to the synthesis of bone matrix and the regulation of osteoblasts/osteoclasts, it is believed that MMPs may promote the onset of bone metastases. Pego et al. mentioned in their review that MMPs, especially MMP-9, played an important role in bone metastasis of prostate cancer (77). MMP-9 is also significant in the occurrence and development of other bone metastases, such as breast cancer bone metastases, and may be a therapeutic target for bone metastases (78). In addition, MMPs such as MMP-13 also play a role in promoting bone metastasis of malignant tumors (79). In addition to MMPs, the role of Cyclooxygenase-2 (COX-2) in bone metastases is also attracting increasing attention. COX-2 is a key rate-limiting enzyme in the synthesis of prostaglandin E2 (PGE2), which is closely related to inflammation, tumor growth, angiogenesis and other aspects (80). Studies have shown that COX-2 can increase the proportion of osteoclast and is one of the key genes in breast cancer bone metastasis (81). Karavitis et al. mentioned that COX-2 and PGE2 can regulate bone metastasis by influencing immunity (82). In addition, enzymes such as Indoleamine 2, 3-dioxygenase 1 (IDO1) have also been found to be associated with bone metastases (83). In the future, more enzymes with the potential function of promoting tumor bone metastasis can be identified through RNA sequencing and proteomics. As a special catalyst, enzymes often correspond to certain characteristics of substrates and products, as well as related chemical reactions, which may provide conditions for targeted therapy of bone metastases.

The immune cells in the body include specific immune cells and non-specific immune cells. The specific immune cells include T cells, B cells and so on, and their mechanism of action is often highly specific. Non-specific immune cells include monocytes/macrophages, dendritic cells, etc., which usually exhibit low specificity and are responsible for assisting specific immune cells in some cases. In cancer patients, it is generally believed that local immunity plays a potential role in promoting the occurrence, development and metastasis of tumors. The relevant immune cells may “migrate” to the tumor tissue and “protect” it instead. Interestingly, bone is actually an important immune organ in the body, because bone marrow is an important site of white blood cell production (84). So there has to be a special environment for immune activation to be suppressed. The relationship between bone marrow and metastatic tumors began to be studied earlier, and many cells were found to be related to immunosuppression, such as myeloid-derived suppressor cells (MDSCs) and Mesenchymal stem cells (MSCs) (85, 86). However, the mechanisms related to immune microenvironment are. For example, regarding the Irf7 pathway, existing studies have shown that its role in bone metastasis of breast cancer and prostate cancer seems to be suppressive (87, 88). The immunosuppressive mechanism of bone metastases with different primary lesions should be studied independently. Regulatory T cells (Tregs) play an important role in specific immunity (89). CD4+CD25+ Tregs are an important group of T cells in bone marrow and may be highly related to immunosuppression (90). The study of Tan et al. indicated that the RANKL-RANK pathway may affect the content of Tregs, thus affecting local immunity (91). Tregs can secrete anti-inflammatory cytokines such as IL-10, TGF-β and IL-35, and act on such as CD8+T cells to achieve immune suppression (92, 93). These related cytokines may play a key role in circulating tumor cell dormancy in bone metastases or in tumor cell proliferation in metastatic sites. CD8+T cells are also regulated by immature myeloid cells and osteoblasts (94). In non-specific immunity, macrophages, especially tumor-associated macrophages(TAMs), have a great influence on the pathogenesis of metastatic tumors (95). The role of macrophages is diverse, and under different circumstances they will polarize into different subtypes, mainly including M1 type and M2 type (96). Their main effects on tumor cells are almost opposite, with M1-type macrophages often showing killing effect on tumor, while M2-type macrophages often showing promoting effect on tumor (97). TAMs in malignant tumor usually exhibit an M2-like appearance (98). Macrophages are often regulated by cytokines and other factors, which may promote the occurrence of bone metastases (99). According to the results of current studies, the representative role of different types of immune cells in bone metastases is shown in Figure 3 . In future studies, we believe that in terms of the immune regulation of bone metastases, how to correctly find the immune cells that promote tumor bone metastases and make them defunction, apoptosis or transform into normal immune cells is the key to the research.

It can be seen from the above description that the pathogenesis of bone metastases is very complex. As the main function of bone metastases, different types of cells are widely affected by immune, metabolic and tumor microenvironments. In recent years, the role of some connective tissue cells in bone metastases, such as fibroblasts and endothelial cells, in bone metastases has received increasing attention. Similar to macrophages, fibroblasts in malignant tumor tissues are known as cancer-associated fibroblasts (CAFs) and they often show potential tumor-promoting effects (100). CAFs may promote tumor metastasis (101). Li et al. mentioned in their study that CAFs played a key role in bone metastasis of breast cancer cells by influencing tumor microenvironment and other aspects (102). Mukaida et al. fully described the possible effects of CAFs on tumor bone metastasis, including the function of tumor cells and immune cells through the secretion of cytokines by CAFs (103). The relevant content is illustrated in Figure 4 . In addition to CAFs, the role of endothelial cells in metastases has also been emphasized. Zhang et al. indicated that bone-derived endothelial cells (BDECs) may be involved in pathologic bone lysis in the pathogenesis of bone metastases (104). Wang et al. proposed that tumor cell-vertebral bone marrow endothelial cell interactions promote spinal metastasis in NSCLC (105). In fact, whether in the primary lesion of malignant tumor or the metastasis of bone metastases, tumor cells are only part of the tumor, and the influence of non-tumor cells on the occurrence and development of bone metastases should be paid more attention. Regulation of these cells may have a positive significance in reducing the incidence of bone metastases, delaying the occurrence time of bone metastases, and alleviating the symptoms of bone metastases.

The surgical treatment of bone metastases has been paid more attention due to the progress of epidemiological research. In recent years, more emphasis has been placed on the surgical treatment of bone metastases. Surgical treatment of bone metastases often includes pain relief, quality of life improvement and SREs treatment, and may also be used as a means to create conditions for radiotherapy. Because of the complex local pathology, excision may have some positive significance. However, it is important to note that not all patients are candidates for surgery. Whether or not a patient should be treated surgically depends on a number of factors, including systemic conditions, primary tumor status, number and location of metastases, expected survival time, and financial status of the patient. Prior to surgical treatment of bone metastases, it is important to conduct examinations. Some patient-specific scores are important in assessing whether a patient with bone metastases is ready for surgery. For example, for patients with spinal metastases, the Tokuhashi score is a commonly used method to determine whether a patient should be operated on (106). At the same time, the New England Spinal Metastasis Score (NESMS) score had relatively good clinical accuracy in predicting complications after spinal metastasis surgery (107). For patients with limb metastases, Katagiri score might be important references (108, 109). With the development of treatment methods, the surgical methods of spinal metastasis and limb metastasis are gradually diversified. Both open surgery and minimally invasive surgery are used in bone metastases ( Table 2 ), and their adaptations have been recognized based on epidemiological studies (115). In particular, the development of new techniques has led to advances in bone metastases surgery. For example, the application of 3D printing technology in joint prostheses enables patients to achieve better motor function and improve the quality of life of patients (116). However, it should be noted that there are potential complications, including intraoperative and postoperative complications, such as spinal cord and vascular injury, failure of internal fixation, local tumor recurrence and so on, no matter what kind of surgery. Bone metastatic tumor surgery has been used as an important treatment method for many patients, but it is different from general orthopaedic trauma surgery, orthopaedic joint surgery and other conventional operations, it is often difficult to operate, high risk, and so far there is a lack of appropriate procedure standards. Therefore, surgery for bone metastases needs to be conducted by an experienced orthopaedic surgeon who carefully evaluates each patient and follows the principle of “personalization.” More epidemiological studies should be carried out.

The deepening of scientific research also has a certain impact on the concept of chemotherapy for bone metastases. Surgical treatment of bone metastases usually has limited effects. As mentioned earlier, circulating tumor cells may metastasize before they are detected. Subsequently, these circulating tumor cells may form micrometastases, which are the main cause of tumor recurrence and a major factor affecting survival. Therefore, it is necessary to supplement the corresponding medical treatment. The specific chemotherapy regimen for different bone metastases is different, which is related to the pathological type of the primary lesion.

With the progress of research on the pathogenesis of bone metastases and primary bone tumors, the concept of “precision therapy” has been gradually formed. Some malignancies may be hormone-related, so some hormone-targeted therapies have been developed, such as Tamoxifen (estrogen inhibitor), Darolutamide (androgen receptor inhibitor), etc. (117, 118) A more widely known type of targeted therapy is targeting specific proteins or signaling pathways, such as Bevacizumab (VEGF inhibitor) (119), Trastuzumab (HER2 inhibitor) (120), Imatinib (tyrosine kinase inhibitor) (121), Olaparib (PARP inhibitor) (122), etc. Other organ-specific drugs such as I¹³¹ also act as targeted therapies (123). The basic principle of tumor targeted therapy is to design drugs or antibodies for molecules that may be abnormally expressed or have abnormal functions in certain malignant tumors according to epidemiological and pathogenesis studies, so as to interfere with tumor growth and promote tumor killing. Targeted therapy drugs usually cause less damage to normal human tissue than conventional chemotherapy drugs. The combination of targeted therapy with conventional chemotherapy often produces better effects (124). Now, targeted therapy is starting to be used in bone metastases. Tokito et al. showed that bevacizumab may enhance the antitumor activity of chemotherapy against bone metastases and reduce the incidence of SREs (125). A HER2-overexpressed Salivary carcinoma reported by Bergamini et al. developed bone metastases and the treatment plan included trastuzumab (126). It is worth noting that bone-targeting drugs are more widely used in bone metastases. In patients with bone metastases, targeted drugs targeting the primary tumor are often used as a means of comprehensive therapy. In addition to targeting the primary tumor, the more commonly used targeted therapy for bone metastases is “bone-modulatory drugs” for bone lesions, which can be regarded as a type of bone targeting. As mentioned in the previous part, osteoclasts and osteoblasts play an important role in the occurrence of bone metastases. Although the therapeutic effect on tumor is limited, bone targeting drugs can regulate osteoblasts/osteoclasts to inhibit bone destruction and delay the occurrence of SREs, which will greatly improve the quality of life of patients. Some commonly used bone-targeting drugs such as bisphosphonates (BPs) can promote osteoclast apoptosis (127). Denosumab inhibits osteoclast differentiation and activity as a RANK/RANKL inhibitor to delay bone metastasis (128). Some common bone-regulating drugs that inhibit bone destruction in bone metastases are shown in Table 3 . The positive effect of bone-targeting drugs in bone metastases confirms the necessary for their use in patients.

However, in practice, targeted drugs are not targeted to tumor cells, which may limit the efficiency of their application to some extent. In recent years, combined with published pathogenesis and clinical studies, more targeted therapies are being developed. Among them, nanotechnology as an emerging means of targeted therapy has attracted wide attention. Nanomaterials can be targeted by a variety of relevant chemical modifications, and properly designed nanomaterials often show high safety and degradability (139). It is a common idea to combine nanotechnology with traditional bone targeting drugs to prepare nanoparticles for the treatment of bone metastases. For example, He et al. have designed a nanoparticle DSP-Zn@PEG-ALN targeted to focal bone via the alendronate molecule, which has great potential for improving the efficacy of chemotherapy for bone metastatic breast cancer (140). Qiao et al. highlighted the importance of therapeutic nanomedicine and osteocyte-targeted therapy in the treatment of early bone metastases (141). More representative studies related to nanomaterials with bone targeting drugs for the treatment of bone metastases are presented in Table 4 . Also, Tamura et al. mentioned in their review that extracellular vesicles may play an important role in tumor bone metastasis, especially in influencing the local tumor microenvironment (148). Ge et al. designed a multifunctional scaffold called CePO4/CS/GO scaffold that promotes bone formation while killing tumors for the treatment of breast cancer with bone metastases (149). These studies are closely related to the pathogenesis of bone metastases. The relevant signaling pathways here have been mentioned in related pathogenesis studies. In future studies, targeting immune cells, osteoblasts or other stromal cells may be an important direction for the innovation of targeted therapy for bone metastases. Until now, the main methods to target nanomaterials to cells have been through specific ligands on the cell surface, through essential substances for cell metabolism, or through the preparation of biomimetic nanoparticles (nanoparticles coated with natural cell membranes, etc.). These methods can be used as reference for the design of nanomaterials for the treatment of bone metastases. The development of new targeted therapies must be strictly dependent on the pathogenesis of bone metastases. Therefore, the development of clinical diagnosis and treatment and the study of pathogenesis are complementary.