Examining the metastatic niche: targeting the microenvironment

The relationship between the Hippo signaling pathway and bone metastasis of breast cancer

Bone is the most common site of metastasis from breast cancer, which is the most prevalent cancer affecting women globally. Bone metastasis from breast cancer severely affects the quality of life of patients and increases mortality. The molecular mechanisms of metastasis, colonization, and proliferation of breast cancer cells in bone are complex and involve the interaction between breast cancer cells and the bone microenvironment. However, the precise mechanism is not clear at present. In recent years, the Hippo signaling pathway has attracted much attention due to its important role in regulating the expression of major effector molecules during tumor development. In particular, studies have found that the mutation and aberrant expression of the core components of the Hippo signaling pathway affect breast cancer cell migration and invasion, indicating that this pathway plays a role in bone metastasis, although the molecular mechanism of this pathway in breast cancer metastasis has not been fully elucidated. In this review, we discuss the function of the Hippo signaling pathway, introducing its role in breast cancer metastasis, especially bone metastasis of breast cancer, so as to lay a solid theoretical foundation for further research and for the development of effective targeted therapeutic agents.

The Roles of Exosomes in Metastasis of Sarcoma: From Biomarkers to Therapeutic Targets

Sarcoma is a heterogeneous group of mesenchymal neoplasms with a high rate of lung metastasis. The cellular mechanisms responsible for sarcoma metastasis remain poorly understood. Furthermore, there are limited efficacious therapeutic strategies for treating metastatic sarcoma. Improved diagnostic and therapeutic modalities are of increasing importance for the treatment of sarcoma due to their high mortality in the advanced stages of the disease. Recent evidence demonstrates that the exosome, a type of extracellular vesicle released by virtually all cells in the body, is an important facilitator of intercellular communication between the cells and the surrounding environment. The exosome is gaining significant attention among the medical research community, but there is little knowledge about how the exosome affects sarcoma metastasis. In this review, we summarize the multifaceted roles of sarcoma-derived exosomes in promoting the process of metastasis via the formation of pre-metastatic niche (PMN), the regulation of immunity, angiogenesis, vascular permeability, and the migration of sarcoma cells. We also highlight the potential of exosomes as innovative diagnostic and prognostic biomarkers as well as therapeutic targets in sarcoma metastasis.

Adjuvant therapy options in renal cell carcinoma - targeting the metastatic cascade

Localized renal cell carcinoma (RCC) is primarily managed with nephrectomy, which is performed with curative intent. However, disease recurs in ~20% of patients. Treatment with adjuvant therapies is used after surgery with the intention of curing additional patients by disrupting the establishment, maturation or survival of micrometastases, processes collectively referred to as the metastatic cascade. Immune checkpoint inhibitors and vascular endothelial growth factor receptor (VEGFR)-targeting tyrosine kinase inhibitors (TKIs) have shown efficacy in the treatment of metastatic RCC, increasing the interest in the utility of these agents in the adjuvant setting. Pembrolizumab, an inhibitor of the immune checkpoint PD1, is now approved by the FDA and is under review by European regulatory agencies for the adjuvant treatment of patients with localized resected clear cell RCC based on the results of the KEYNOTE-564 trial. However, the optimal use of immunotherapy and VEGFR-targeting TKIs for adjuvant treatment of RCC is not completely understood. These agents disrupt the metastatic cascade at multiple steps, providing biological rationale for further investigating the applications of these therapeutics in the adjuvant setting. Clinical trials to evaluate adjuvant therapeutics in RCC are ongoing, and clinical considerations must guide the practical use of immunotherapy and TKI agents for the adjuvant treatment of localized resected RCC.

Current and Emerging Approaches for Spine Tumor Treatment

Spine tumors represent a significant social and medical problem, affecting the quality of life of thousands of patients and imposing a burden on healthcare systems worldwide. Encompassing a wide range of diseases, spine tumors require prompt multidisciplinary treatment strategies, being mainly approached through chemotherapy, radiotherapy, and surgical interventions, either alone or in various combinations. However, these conventional tactics exhibit a series of drawbacks (e.g., multidrug resistance, tumor recurrence, systemic adverse effects, invasiveness, formation of large bone defects) which limit their application and efficacy. Therefore, recent research focused on finding better treatment alternatives by utilizing modern technologies to overcome the challenges associated with conventional treatments. In this context, the present paper aims to describe the types of spine tumors and the most common current treatment alternatives, further detailing the recent developments in anticancer nanoformulations, personalized implants, and enhanced surgical techniques.

Establishment and Image based evaluation of a New Preclinical Rat Model of Osteoblastic Bone Metastases

Bone remodeling is disrupted in the presence of metastases and can present as osteolytic, osteoblastic or a mixture of the two. Established rat models of osteolytic and mixed metastases have been identified changes in structural and tissue-level properties of bone. The aim of this work was to establish a preclinical rat model of osteoblastic metastases and characterize bone quality changes through image-based evaluation. Female athymic rats (n = 22) were inoculated with human breast cancer cells ZR-75-1 and tumor development tracked over 3-4 months with bioluminescence and in-vivo µCT imaging. Bone tissue-level stereological features were quantified on ex-vivo µCT imaging. Histopathology verified the presence of osteoblastic bone. Bone mineral density distribution was assessed via backscattered electron microscopy. Newly formed osteoblastic bone was associated with reduced mineral content and increased heterogeneity leading to an overall degraded bone quality. Characterizing changes in osteoblastic bone properties is relevant to pre-clinical therapeutic testing and treatment planning.

Systemic effects of abnormal bone resorption on muscle, metabolism, and cognition

Skeletal tissue is dynamic, undergoing constant remodeling to maintain musculoskeletal integrity and balance in the human body. Recent evidence shows that apart from maintaining homeostasis in the local microenvironment, the skeleton systemically affects other tissues. Several cancer-associated and noncancer-associated bone disorders can disrupt the physiological homeostasis locally in the bone microenvironment and indirectly contribute to dysregulation of systemic body function. The systemic effects of bone on the regulation of distant organ function have not been widely explored. Recent evidence suggests that bone can interact with skeletal muscle, pancreas, and brain by releasing factors from mineralized bone matrix. Currently available bone-targeting therapies such as bisphosphonates and denosumab inhibit bone resorption, decrease morbidity associated with bone destruction, and improve survival. Bisphosphonates have been a standard treatment for bone metastases, osteoporosis, and cancer treatment-induced bone diseases. The extraskeletal effects of bisphosphonates on inhibition of tumor growth are known. However, our knowledge of the effects of bisphosphonates on muscle weakness, hyperglycemia, and cognitive defects is currently evolving. To be able to identify the molecular link between bone and distant organs during abnormal bone resorption and then treat these abnormalities and prevent their systemic effects could improve survival benefits. The current review highlights the link between bone resorption and its systemic effects on muscle, pancreas, and brain.

Role of Exosomes in Prostate Cancer Metastasis

Prostate cancer remains a life-threatening disease among men worldwide. The majority of PCa-related mortality results from metastatic disease that is characterized by metastasis of prostate tumor cells to various distant organs, such as lung, liver, and bone. Bone metastasis is most common in prostate cancer with osteoblastic and osteolytic lesions. The precise mechanisms underlying PCa metastasis are still being delineated. Intercellular communication is a key feature underlying prostate cancer progression and metastasis. There exists local signaling between prostate cancer cells and cells within the primary tumor microenvironment (TME), in addition to long range signaling wherein tumor cells communicate with sites of future metastases to promote the formation of pre-metastatic niches (PMN) to augment the growth of disseminated tumor cells upon metastasis. Over the last decade, exosomes/ extracellular vesicles have been demonstrated to be involved in such signaling. Exosomes are nanosized extracellular vesicles (EVs), between 30 and 150 nm in thickness, that originate and are released from cells after multivesicular bodies (MVB) fuse with the plasma membrane. These vesicles consist of lipid bilayer membrane enclosing a cargo of biomolecules, including proteins, lipids, RNA, and DNA. Exosomes mediate intercellular communication by transferring their cargo to recipient cells to modulate target cellular functions. In this review, we discuss the contribution of exosomes/extracellular vesicles in prostate cancer progression, in pre-metastatic niche establishment, and in organ-specific metastases. In addition, we briefly discuss the clinical significance of exosomes as biomarkers and therapeutic agents.

The migration of metastatic breast cancer cells is regulated by matrix stiffness via YAP signalling

Matrix stiffness is a driver of breast cancer progression and mechanosensitive transcriptional activator YAP plays an important role in this process. However, the interplay between breast cancer and matrix stiffness, and the significance of this interplay remained largely unknown. Here, we showed an increase in YAP nuclear localization and a higher proliferation rate in both highly metastatic MDA-MB-231 cells and the non-metastatic counterpart MCF-7 cells when they were exposed to the stiff matrix. However, in response to the stiff matrix highly metastatic MDA-MB-231 cells instead of MCF-7 cells exhibited upregulated mobility, which was shown to be YAP-dependent. Consistently, MDA-MB-231 cells exhibited different focal adhesion dynamics from MCF-7 cells in response to matrix stiffness. These results suggested a YAP-dependent mechanism through which matrix stiffness regulates the migratory potential of metastatic breast cancer cells.

Skeletal Biology and Disease Modeling in Zebrafish

Zebrafish are teleosts (bony fish) that share with mammals a common ancestor belonging to the phylum Osteichthyes, from which their endoskeletal systems have been inherited. Indeed, teleosts and mammals have numerous genetically conserved features in terms of skeletal elements, ossification mechanisms, and bone matrix components in common. Yet differences related to bone morphology and function need to be considered when investigating zebrafish in skeletal research. In this review, we focus on zebrafish skeletal architecture with emphasis on the morphology of the vertebral column and associated anatomical structures. We provide an overview of the different ossification types and osseous cells in zebrafish and describe bone matrix composition at the microscopic tissue level with a focus on assessing mineralization. Processes of bone formation also strongly depend on loading in zebrafish, as we elaborate here. Furthermore, we illustrate the high regenerative capacity of zebrafish bones and present some of the technological advantages of using zebrafish as a model. We highlight zebrafish axial and fin skeleton patterning mechanisms, metabolic bone disease such as after immunosuppressive glucocorticoid treatment, as well as osteogenesis imperfecta (OI) and osteopetrosis research in zebrafish. We conclude with a view of why larval zebrafish xenografts are a powerful tool to study bone metastasis. © 2021 American Society for Bone and Mineral Research (ASBMR).

Risk factors for bisphosphonate-associated osteonecrosis of the jaw in the prospective randomized trial of adjuvant bisphosphonates for early-stage breast cancer (SWOG 0307)

PURPOSE

Bisphosphonates reduce bone metastases in postmenopausal women with early-stage breast cancer but carry the risk of bisphosphonate-related osteonecrosis of the jaw (BRONJ). We describe risk factors for BRONJ and compare BRONJ provoked by infection or trauma with spontaneous lesions, which carry a better prognosis.

METHODS

SWOG 0307 randomized women with stage I-III breast cancer to receive zoledronic acid (ZA), clodronate (CL), or ibandronate (IB) for 3 years, implemented BRONJ prevention guidelines, and collected information about dental health and development of BRONJ. All statistical tests were two-sided.

RESULTS

Of 6018 women, 48 developed BRONJ. Infection was present in 21 (43.8%). Median time to BRONJ was 2.1 years for ZA, 2.0 years for IB, and 3.4 years for clodronate (p = 0.04). BRONJ was associated with bisphosphonate type (28/2231 (1.26%) for ZA, 8/2235 (0.36%) for CL, 12/1552 (0.77%) for IB), dental calculus (OR 2.03), gingivitis (OR 2.11), moderate/severe periodontal disease (OR 2.87), and periodontitis > 4 mm (OR 2.20) (p < 0.05). Of 57 lesions, BRONJ occurred spontaneously in 20 (35.1%) and was provoked by dental extraction in 20 (35.1%), periodontal disease in 14 (24.6%), denture trauma in 6 (10.5%), and dental surgery in 2 (3.5%). Spontaneous BRONJ occurred more frequently at the mylohyoid ridge. There were no differences in dental disease, infection, or bisphosphonate type between spontaneous and provoked BRONJ.

CONCLUSION

ZA and worse dental health were associated with increased incidence of BRONJ, with a trend toward additive risk when combined. BRONJ incidence was lower than in similar studies, with prevention strategies likely linked to this.

CLINICAL TRIAL NUMBER

NCT00127205 REGISTRATION DATE: July 2005.

Bone-targeting polymer vesicles for simultaneous imaging and effective malignant bone tumor treatment

We present a bone-targeting polymer vesicle with excellent single photon emission computed tomography/computed tomography (SPECT/CT) imaging capability and high antitumor drug delivery efficiency as an integrated platform for the simultaneous diagnosing and treatment of malignant bone tumors. This polymer vesicle can be self-assembled from poly(ε-caprolactone)₆₇-b-poly[(L-glutamic acid)₆-stat-(L-glutamic acid-alendronic acid)₁₆] (PCL₆₇-b-P[Glu₆-stat-(Glu-ADA)₁₆]), directly in water without the aid of a cosolvent. SPECT/CT dynamically tracked the drug distribution in the bone tumor rabbit models, and the tumor size was significantly reduced from >2.0 cm³ to <0.6 cm³ over 11 days. The pathological analysis demonstrated obvious necrosis and apoptosis of the tumor cells. Overall, this bone-targeting polymer vesicle provides us with a new platform for the combination of real-time diagnosis and therapy of malignant bone tumors.

Trends in Bone Metastasis Modeling

Bone is one of the most common sites for cancer metastasis. Bone tissue is composed by different kinds of cells that coexist in a coordinated balance. Due to the complexity of bone, it is impossible to capture the intricate interactions between cells under either physiological or pathological conditions. Hence, a variety of in vivo and in vitro approaches have been developed. Various models of tumor-bone diseases are routinely used to provide valuable information on the relationship between metastatic cancer cells and the bone tissue. Ideally, when modeling the metastasis of human cancers to bone, models would replicate the intra-tumor heterogeneity, as well as the genetic and phenotypic changes that occur with human cancers; such models would be scalable and reproducible to allow high-throughput investigation. Despite the continuous progress, there is still a lack of solid, amenable, and affordable models that are able to fully recapitulate the biological processes happening in vivo, permitting a correct interpretation of results. In the last decades, researchers have demonstrated that three-dimensional (3D) methods could be an innovative approach that lies between bi-dimensional (2D) models and animal models. Scientific evidence supports that the tumor microenvironment can be better reproduced in a 3D system than a 2D cell culture, and the 3D systems can be scaled up for drug screening in the same way as the 2D systems thanks to the current technologies developed. However, 3D models cannot completely recapitulate the inter- and intra-tumor heterogeneity found in patients. In contrast, ex vivo cultures of fragments of bone preserve key cell-cell and cell-matrix interactions and allow the study of bone cells in their natural 3D environment. Moreover, ex vivo bone organ cultures could be a better model to resemble the human pathogenic metastasis condition and useful tools to predict in vivo response to therapies. The aim of our review is to provide an overview of the current trends in bone metastasis modeling. By showing the existing in vitro and ex vivo systems, we aspire to contribute to broaden the knowledge on bone metastasis models and make these tools more appealing for further translational studies.

Molecular mechanisms and clinical management of cancer bone metastasis

As one of the most common metastatic sites of malignancies, bone has a unique microenvironment that allows metastatic tumor cells to grow and flourish. The fenestrated capillaries in the bone, bone matrix, and bone cells, including osteoblasts and osteoclasts, together maintain the homeostasis of the bone microenvironment. In contrast, tumor-derived factors act on bone components, leading to subsequent bone resorption or excessive bone formation. The various pathways involved also provide multiple targets for therapeutic strategies against bone metastases. In this review, we summarize the current understanding of the mechanism of bone metastases. Based on the general process of bone metastases, we specifically highlight the complex crosstalk between tumor cells and the bone microenvironment and the current management of cancer bone metastases.

The CaSR in Pathogenesis of Breast Cancer: A New Target for Early Stage Bone Metastases

The Ca2+-sensing receptor (CaSR) is a class-C G protein-coupled receptor which plays a pivotal role in calciotropic processes, primarily in regulating parathyroid hormone secretion to maintain systemic calcium homeostasis. Among its non-calciotropic roles, where the CaSR sits at the intersection of myriad processes, it has steadily garnered attention as an oncogene or tumor suppressor in different organs. In maternal breast tissues the CaSR promotes lactation but in breast cancer it acts as an oncoprotein and has been shown to drive the pathogenesis of skeletal metastases from breast cancer. Even though research has made great strides in treating primary breast cancer, there is an unmet need when it comes to treatment of metastatic breast cancer. This review focuses on how the CaSR leads to the pathogenesis of breast cancer by contrasting its role in healthy tissues and tumorigenesis, and by drawing brief parallels with the tissues where it has been implicated as an oncogene. A class of compounds called calcilytics, which are CaSR antagonists, have also been surveyed in the instances where they have been used to target the receptor in cancerous tissues and constitute a proof of principle for repurposing them. Current clinical therapies for treating bone metastases from breast cancer are limited to targeting osteoclasts and a deeper understanding of the CaSR signaling nexus in this context can bolster them or lead to novel therapeutic interventions.

Combination radium-223 therapies in patients with bone metastases from castration-resistant prostate cancer: A review

Chemotherapeutic agents (docetaxel, cabazitaxel), hormonal therapies (abiraterone, enzalutamide) and radium-223 improve survival in patients with bone metastatic castration-resistant prostate cancer (mCRPC). Combinations of radium-223 with these agents or novel drugs have been investigated in order to improve survival and decrease bone-related morbidity. In mCRPC, clinical and preclinical data indicate that radium-223, abiraterone and enzalutamide have a direct effect on prostate cancer cells and bone microenvironment when administered as single agents. Initial results from studies of radium-223 and abiraterone, enzalutamide or docetaxel demonstrated efficacy without any safety concern in pre-treated mCRPC; however, this safety profile changed when radium-based combination therapies were administered in un-pretreated mCRPC. This review underline the biological rationale for combining radium strategies, investigating their effects on bone in terms of control of skeletal-related events and bone disease progression. The aim is to understand the possible reasons why different radium-based combination treatments can led to different clinical outcomes.

IFITM3 promotes bone metastasis of prostate cancer cells by mediating activation of the TGF-β signaling pathway

Advanced-stage prostate cancer (PCa) is often diagnosed with bone metastasis, for which there are limited therapies. Transforming growth factor β (TGF-β) is known to induce epithelial–mesenchymal transition (EMT), and abundance of TGF-β in the bone matrix is one of the important growth factors contributing to bone metastasis. TGF-β is reported as a key mediator of bone metastasis, but the underlying mechanism has not been elucidated. It was found in our study that Interferon-inducible Transmembrane Protein 3 (IFITM3) played a key role in the regulation of malignant tumor cell proliferation, invasion, and bone migration by binding to Smad4, thus activating the TGF-β-Smads Signaling Pathway. Lentivirus-mediated short hairpin RNA (shRNA) knockdown of IFITM3 inhibited cell proliferation and colony formation, induced apoptosis and inhibited migration by reversing EMT and downregulating the expression of metastasis-related molecules including FGFs and PTHrP. Microarray analysis showed that IFITM3 knockdown could alter the MAPK pathway associated with TGF-β-Smads signaling. By knocking down and overexpressing IFITM3, we demonstrated that IFITM3 expression level had an effect on MAPK pathway activation, and this change was more pronounced upon exogenous TGF-β stimulation. These results suggest that IFITM3 played an oncogenic role in PCa progression and bone metastasis via a novel TGF-β-Smads-MAPK pathway.

Metastasis inhibition in breast cancer by targeting cancer cell extravasation

The spread of cells from primary tumors toward distant tissues and organs, also known as metastasis, is responsible for most cancer-associated deaths. The metastasis cascade comprises a series of events, characterized by the displacement of tumor cells (TCs) from the primary tumor to distant organs by traveling through the bloodstream, and their subsequent colonization. The first step in metastasis involves loss of cell-cell and cell-matrix adhesions, increased invasiveness and migratory abilities, leading to intravasation of TCs into the blood or lymphatic vessels. Stationary TCs must undergo the process of epithelial-mesenchymal transition in order to achieve this migratory and invasive phenotype. Circulating tumor cells that have survived in the circulation and left the blood or lymphatic vessels will reach distant sites where they may stay dormant for many years or grow to form secondary tumors. To do this, cells need to go through the mesenchymal-epithelial transition to revert the phenotype in order to regain epithelial cell-to-cell junctions, grow and become a clinically relevant and detectable tumor mass. This work will review the main steps of the metastatic cascade and describe some strategies to inhibit metastasis by reducing cancer cell extravasation presenting recent studies in the context of breast cancer.

Tumor cell heterogeneity and resistance; report from the 2018 Coffey-Holden Prostate Cancer Academy Meeting

INTRODUCTION

The 2018 Coffey-Holden Prostate Cancer Academy (CHPCA) Meeting, "Tumor Cell Heterogeneity and Resistance," was held in Los Angeles, California from June 21 to 24, 2018.

METHODS

The CHPCA Meeting is a unique, discussion-oriented scientific conference convened annually by the Prostate Cancer Foundation (PCF), which focuses on the most critical topics in need of further study to advance the treatment of lethal prostate cancer. The 6th Annual CHPCA Meeting was attended by 70 investigators and concentrated on prostate cancer heterogeneity and treatment resistance.

RESULTS

The meeting focused on topics including: recognition of tumor heterogeneity, molecular drivers of heterogeneity, the role of the tumor microenvironment, the role of heterogeneity in disease progression, metastasis and treatment resistance, clinical trials designed to target resistance and tumor heterogeneity, and immunotherapeutic approaches to target and overcome tumor heterogeneity.

DISCUSSION

This review article summarizes the presentations and discussions from the 2018 CHPCA Meeting in order to share this knowledge with the scientific community and encourage new studies that will lead to improved treatments and outcomes for men with prostate cancer.

Drosophila intestinal stem and progenitor cells are major sources and regulators of homeostatic niche signals

Epithelial homeostasis requires the precise balance of epithelial stem/progenitor proliferation and differentiation. While many signaling pathways that regulate epithelial stem cells have been identified, it is probable that other regulators remain unidentified. Here, we use gene-expression profiling by targeted DamID to identify the stem/progenitor-specific transcription and signaling factors in the Drosophila midgut. Many signaling pathway components, including ligands of most major pathways, exhibit stem/progenitor-specific expression and have regulatory regions bound by both intrinsic and extrinsic transcription factors. In addition to previously identified stem/progenitor-derived ligands, we show that both the insulin-like factor Ilp6 and TNF ligand eiger are specifically expressed in the stem/progenitors and regulate normal tissue homeostasis. We propose that intestinal stem cells not only integrate multiple signals but also contribute to and regulate the homeostatic signaling microenvironmental niche through the expression of autocrine and paracrine factors.

Modulating Bone Marrow Hematopoietic Lineage Potential to Prevent Bone Metastasis in Breast Cancer

The presence of disseminated tumor cells in breast cancer patient bone marrow aspirates predicts decreased recurrence-free survival. Although it is appreciated that physiologic, pathologic, and therapeutic conditions impact hematopoiesis, it remains unclear whether targeting hematopoiesis presents opportunities for limiting bone metastasis. Using preclinical breast cancer models, we discovered that marrow from mice treated with the bisphosphonate zoledronic acid (ZA) are metastasis-suppressive. Specifically, ZA modulated hematopoietic myeloid/osteoclast progenitor cell (M/OCP) lineage potential to activate metastasis-suppressive activity. Granulocyte-colony stimulating factor (G-CSF) promoted ZA resistance by redirecting M/OCP differentiation. We identified M/OCP and bone marrow transcriptional programs associated with metastasis suppression and ZA resistance. Analysis of patient blood samples taken at randomization revealed that women with high-plasma G-CSF experienced significantly worse outcome with adjuvant ZA than those with lower G-CSF levels. Our findings support discovery of therapeutic strategies to direct M/OCP lineage potential and biomarkers that stratify responses in patients at risk of recurrence.Significance: Bone marrow myeloid/osteoclast progenitor cell lineage potential has a profound impact on breast cancer bone metastasis and can be modulated by G-CSF and bone-targeting agents. Cancer Res; 78(18); 5300-14. ©2018 AACR.

Osteoclast inhibitors to prevent bone metastases in men with high-risk, non-metastatic prostate cancer: A systematic review and meta-analysis

Background

In advanced prostate cancer, osteoclast inhibitors prevent and palliate skeletal related events associated with bone metastases. However, it is uncertain whether they play a disease-modifying role earlier in the course of the disease.

Methods

Medline, EMBASE, Cochrane Central Register of Controlled Trials and Cochrane Database of Systematic Reviews and ASCO conference proceedings were searched for randomized controlled trials that compared osteoclast inhibitors with placebo and/or standard of care (SOC) in patients with high-risk, non-metastatic prostate cancer. The primary outcome measure was incidence of new bone metastases; secondary outcomes included overall survival (OS), prostate cancer specific survival, mortality unrelated to prostate cancer, toxicity and health related quality of life outcomes. Results are presented as relative risk (RR) with 95% confidence intervals (CI).

Results

Six randomized controlled trials (5947 participants) were included, five evaluating bisphosphonates and one denosumab. Overall, there was no difference in incidence of bone metastases between participants treated with osteoclast inhibitors versus placebo/SOC (RR 1.09, 95%CI 0.84–1.41, p = 0.51) however significant heterogeneity was observed between studies. The denosumab trial was the largest and only positive trial amongst the included studies (RR 0.83, 95%CI 0.73–0.95, p = 0.007). No significant difference was observed in OS (RR 0.99 95% CI 0.89–1.10, p = 0.84) nor prostate cancer specific survival (RR 1.12 95%CI 0.93–1.36, p = 0.24). Most studies reported increased rates of osteonecrosis of the jaw (5% or less) and hypocalcemia (2% or less) with osteoclast inhibitors.

Conclusions

While there is limited evidence that bisphosphonates alter the natural history of high-risk, non-metastatic prostate cancer, denosumab delays onset of bone metastases in this patient population. Neither class of osteoclast inhibitor demonstrated an impact on survival outcomes. Future trials with better defined patient selection and a robust definition for high risk disease is critical.

Drug delivery systems functionalized with bone mineral seeking agents for bone targeted therapeutics

The systemic administration of drugs to treat bone diseases is often associated with poor uptake of the drug in the targeted tissue, potential systemic toxicity and suboptimal efficacy. In order to overcome these limitations, many micro- and nano-sized drug carriers have been developed for the treatment of bone pathologies that exhibit specific affinity for bone. Drug carriers can be functionalized with bone mineral seekers (BMS), creating a targeted drug delivery system (DDS) which is able to bind to bone and release therapeutics directly at the site of interest. This class of advanced DDS is of tremendous interest due to their strong affinity to bone, with great expectation to treat life-threatening bone disorders such as osteomyelitis, osteosarcoma or even osteoporosis. In this review, we first explain the mechanisms behind the affinity of several well-known BMS to bone, and then we present several effective approaches allowing the incorporation BMS into advanced DDS. Finally, we report the therapeutic applications of BMS based DDS under development or already established. Understanding the mechanisms behind the biological activity of recently developed BMS and their integration into advanced therapeutic delivery systems are essential prerequisites for further development of bone-targeting therapies with optimal efficacy.

Collagen fibril organization within rat vertebral bone modified with metastatic involvement

Metastatic involvement diminishes the mechanical integrity of vertebral bone, however its specific impact on the structural characteristics of a primary constituent of bone tissue, the collagen-I fibril matrix, has not been adequately characterized. Female athymic rats were inoculated with HeLa or Ace-1 cancer cells lines producing osteolytic or mixed (osteolytic & osteoblastic) metastases respectively. A maximum of 21days was allowed between inoculation and rat sacrifice for vertebrae extraction. Linear polarization-in, polarization-out (PIPO) second harmonic generation (SHG) and transmission electron microscopy (TEM) imaging was utilized to assess the impact of metastatic involvement on collagen fibril organization. Increased observations of deviations in the typical plywood motif or a parallel packing structure and an increased average measured susceptibility ratio (related to relative degree of in-plane vs. out-plane fibrils in the analyzed tissue area) in bone adjacent to metastatic involvement was indicative of change in fibrilar organization compared to healthy controls. In particular, collagen-I fibrils in tumour-induced osteoblastic bone growth showed no adherence to the plywood motif or parallel packing structure seen in healthy lamellar bone, exhibiting a much higher susceptibility ratio and degree of fibril disorder. Negative correlations were established between measured susceptibility ratios and the hardness and modulus of metastatic bone tissue assessed in a previous study. Characterizing modifications in tissue level properties is key in defining bone quality in the presence of metastatic disease and their potential impact on material behaviour.

Soluble bone-derived osteopontin promotes migration and stem-like behavior of breast cancer cells

Breast cancer is a leading cause of cancer death in women, with the majority of these deaths caused by metastasis to distant organs. The most common site of breast cancer metastasis is the bone, which has been shown to provide a rich microenvironment that supports the migration and growth of breast cancer cells. Additionally, growing evidence suggests that breast cancer cells that do successfully metastasize have a stem-like phenotype including high activity of aldehyde dehydrogenase (ALDH) and/or a CD44⁺CD24^- phenotype. In the current study, we tested the hypothesis that these ALDH^hiCD44⁺CD24^- breast cancer cells interact with factors in the bone secondary organ microenvironment to facilitate metastasis. Specifically, we focused on bone-derived osteopontin and its ability to promote the migration and stem-like phenotype of breast cancer cells. Our results indicate that bone-derived osteopontin promotes the migration, tumorsphere-forming ability and colony-forming ability of whole population and ALDH^hiCD44⁺CD24^- breast cancer cells in bone marrow-conditioned media (an ex vivo representation of the bone microenvironment) (p≤0.05). We also demonstrate that CD44 and RGD-dependent cell surface integrins facilitate this functional response to bone-derived osteopontin (p≤0.05), potentially through activation of WNK-1 and PRAS40-related pathways. Our findings suggest that soluble bone-derived osteopontin enhances the ability of breast cancer cells to migrate to the bone and maintain a stem-like phenotype within the bone microenvironment, and this may contribute to the establishment and growth of bone metastases.

Pre-metastatic niches: organ-specific homes for metastases

It is well established that organs of future metastasis are not passive receivers of circulating tumour cells, but are instead selectively and actively modified by the primary tumour before metastatic spread has even occurred. Sowing the 'seeds' of metastasis requires the action of tumour-secreted factors and tumour-shed extracellular vesicles that enable the 'soil' at distant metastatic sites to encourage the outgrowth of incoming cancer cells. In this Review, we summarize the main processes and new mechanisms involved in the formation of the pre-metastatic niche.

Exosomes: The Messengers of Health and Disease

Exosomes are small vesicles comprised of a lipid bilayer containing various proteins, RNAs and bioactive lipids. They act as intercellular messengers that give the ability to communicate between both cells of the same type and other cell types. They are released by healthy cells, both constitutively and upon cell activation and play an important role in immune system function. Exosomes are essential for healthy physiological conditions, however under pathological circumstances, they act to potentiate cellular stress and damage. This review explores the characteristics, biogenesis, role(s) in the pathogenesis of diseases and role(s) in progression of cancer of these nano-sized messages-in-a-vesicle: exosomes.

Implementation of Nanoparticles in Cancer Therapy

Cancer is a wide group of diseases and generally characterized by uncontrolled proliferation of cells whose metabolic activities are disrupted. Conventionally, chemotherapy, radiotherapy, and surgery are used in the treatment of cancer. However, in theory, even a single cancer cell may trigger recurrence. Therefore, these treatments cannot provide high survival rate for deadly types. Identification of alternative methods in treatment of cancers is inevitable because of adverse effects of conventional methods. In the last few decades, nanotechnology developed by scientists working in different disciplines—physics, chemistry, and biology—offers great opportunities. It is providing elimination of both circulating tumor cells and solid cancer cells by targeting cancer cells. In this chapter, inadequate parts of conventional treatment methods, nanoparticle types used in new treatment methods of cancer, and targeting methods of nanoparticles are summarized; furthermore, recommendations of future are provided.

Parathyroid Hormone-Related Protein, Its Regulation of Cartilage and Bone Development, and Role in Treating Bone Diseases

Although parathyroid hormone-related protein (PTHrP) was discovered as a cancer-derived hormone, it has been revealed as an important paracrine/autocrine regulator in many tissues, where its effects are context dependent. Thus its location and action in the vasculature explained decades-long observations that injection of PTH into animals rapidly lowered blood pressure by producing vasodilatation. Its roles have been specified in development and maturity in cartilage and bone as a crucial regulator of endochondral bone formation and bone remodeling, respectively. Although it shares actions with parathyroid hormone (PTH) through the use of their common receptor, PTHR1, PTHrP has other actions mediated by regions within the molecule beyond the amino-terminal sequence that resembles PTH, including the ability to promote placental transfer of calcium from mother to fetus. A striking feature of the physiology of PTHrP is that it possesses structural features that equip it to be transported in and out of the nucleus, and makes use of a specific nuclear import mechanism to do so. Evidence from mouse genetic experiments shows that PTHrP generated locally in bone is essential for normal bone remodeling. Whereas the main physiological function of PTH is the hormonal regulation of calcium metabolism, locally generated PTHrP is the important physiological mediator of bone remodeling postnatally. Thus the use of intermittent injection of PTH as an anabolic therapy for bone appears to be a pharmacological application of the physiological function of PTHrP. There is much current interest in the possibility of developing PTHrP analogs that might enhance the therapeutic anabolic effects.

The unresolved role of systemic factors in bone metastasis

Systemic factors including cytokines, cell-free nucleic acids, microvesicles, and platelets are appreciated as important regulators of adenocarcinoma progression. Research findings using pre-clinical mouse models have revealed that many such systemically acting factors are either secreted by or responsive to peripheral tumors and impact bone and bone marrow (collectively referred to as the bone microenvironment) to initiate processes that ultimately govern disease progression, even in the absence of detectable bone metastases. In some cases, cancer-driven modulation of the bone microenvironment involves mobilization of bone marrow hematopoietic and mesenchymal cells into the circulation that are subsequently recruited into peripheral tissues and tumors. In other cases, systemic factors alter bone marrow cell (BMC) differentiation and/or gene expression to render the BMCs pro-tumorigenic even prior to their mobilization into the circulation. Given their effect on the bone microenvironment, it stands to reason that such systemic factors might also influence metastases in the bone; however, this hypothesis remains to be comprehensively tested. Here, we briefly review what is known, and not known, about systemic factors that regulate the bone microenvironment and thereby influence bone metastases. We also pose a number of currently unanswered questions in this active area of research. A better understanding of systemic processes that influence bone metastasis should aid discovery of therapeutic approaches that aim to eradicate or reduce disease burden in the bone, which is the cause of significant patient mortality and morbidity and is currently incurable.

Skeletal metastases and impact of anticancer and bone-targeted agents in patients with castration-resistant prostate cancer

Incidence of bone metastases is very high in advanced prostate cancer patients. Bone metastases likely have a significant impact on functional status and quality of life, not only related to pain, but also to the relevant risk of skeletal-related events. A better understanding of mechanisms associated with bone metastatic disease secondary to prostate cancer and more specifically to the cross-talk between tumor cells and bone microenvironment in metastatic progression represented the background for the development of new effective bone-targeted therapies. Furthermore, a better knowledge of biological mechanisms driving disease progression led to significant advances in the treatment of castration-resistant prostate cancer, with the development and approval of new effective drugs. Aim of this review is to outline the physiopathology of bone metastases in prostate cancer and summarize the main results of clinical trials conducted with different drugs to control morbidity induced by skeletal metastases and bone disease progression. For each agent, therapeutic effect on bone metastases has been measured in terms of pain control and/or incidence of skeletal-related events, usually defined as a composite endpoint, including the need for local treatment (radiation therapy or surgery), spinal cord compression, pathological bone fractures. In details, data obtained with chemotherapy (mitoxantrone, docetaxel, cabazitaxel), new generation hormonal agents (abiraterone, enzalutamide), radium-223, bone-targeted agents (zoledronic acid, denosumab) and with several experimental agents (cabozantinib, dasatinib, anti-endothelin and other agents) in patients with castration-resistant prostate cancer are reviewed.

Overview of Glutamatergic Dysregulation in Central Pathologies

As the major excitatory neurotransmitter in the mammalian central nervous system, glutamate plays a key role in many central pathologies, including gliomas, psychiatric, neurodevelopmental, and neurodegenerative disorders. Post-mortem and serological studies have implicated glutamatergic dysregulation in these pathologies, and pharmacological modulation of glutamate receptors and transporters has provided further validation for the involvement of glutamate. Furthermore, efforts from genetic, in vitro, and animal studies are actively elucidating the specific glutamatergic mechanisms that contribute to the aetiology of central pathologies. However, details regarding specific mechanisms remain sparse and progress in effectively modulating glutamate to alleviate symptoms or inhibit disease states has been relatively slow. In this report, we review what is currently known about glutamate signalling in central pathologies. We also discuss glutamate's mediating role in comorbidities, specifically cancer-induced bone pain and depression.

Differential remodeling of extracellular matrices by breast cancer initiating cells

Cancer initiating cells (CICs) have been the focus of recent anti-cancer therapies, exhibiting strong invasion capability via potentially enhanced ability to remodel extracellular matrices (ECM). We have identified CICs in a human breast cancer cell line, MX-1, and developed a xenograft model in SCID mice. We investigated the CICs' matrix-remodeling effects using Second Harmonic Generation (SHG) microscopy to identify potential phenotypic signatures of the CIC-rich tumors. The isolated CICs exhibit higher proliferation, drug efflux and drug resistant properties in vitro; were more tumorigenic than non-CICs, resulting in more and larger tumors in the xenograft model. The CIC-rich tumors have less collagen in the tumor interior than in the CIC-poor tumors supporting the idea that the CICs can remodel the collagen more effectively. The collagen fibers were preferentially aligned perpendicular to the CIC-rich tumor boundary while parallel to the CIC-poor tumor boundary suggesting more invasive behavior of the CIC-rich tumors. These findings would provide potential translational values in quantifying and monitoring CIC-rich tumors in future anti-cancer therapies. CIC-rich tumors remodel the collagen matrix more than CIC-poor tumors.

Metastatic castration-resistant prostate cancer: time for innovation

Androgen deprivation is the mainstay of advanced prostate cancer treatment. Despite initial responses, almost all patients progress to castration-resistant prostate cancer (CRPC). The understanding of the biology of CRPC and the evidence that CRPC still remains driven by androgen receptor signaling led to the discovery of new therapeutic targets. In the last few years, large Phase III trials showed improvements in survival and outcomes and led to the approval of a CYP17 inhibitor (abiraterone), an androgen receptor antagonist (enzalutamide), the taxane cabazitaxel, an α-emitter (radium-223), the bone resorption-targeting drug denosumab and an immunotherapy (sipuleucel-T). This article describes the molecular mechanisms underlying castration resistance, discusses recent and ongoing trials and offers some insights into identifying the best sequence of new drugs.

Metabolism and pharmacokinetics of radium-223 in prostate cancer

INTRODUCTION

Prostate cancer metastatic to bone is a cause of significant morbidity and mortality. Bone pain and other skeletal events negatively impact the quality of life in patients who might otherwise be functioning well. As such, there has been intense interest in the development of strategies and pharmaceuticals to address this problem.

AREAS COVERED

The authors reviewed the current literature for articles relevant to metastatic prostate cancer, clinical radiopharmaceuticals, castrate-resistant prostate cancer and development of Radium-223 . The authors have referenced primary literature, clinical trials and relevant review articles that summarize the history, development and current utilization of radiopharmaceuticals for management of bone metastases from prostate cancer.

EXPERT OPINION

Radium-223 is the first radiopharmaceutical with an overall survival benefit approved for the treatment of patients with castration-resistant prostate cancer, symptomatic bone metastasis and no known visceral metastatic disease. The additional benefit of clinically significant improved overall survival should lead to exploration of whether this agent can be used earlier in the treatment algorithm or combined with chemotherapy or androgen deprivation therapy. An individualized approach needs to be tailored to each patient based on their overall symptoms, disease burden, hematologic profile and goals of care.

Tumor cell p38 MAPK: A trigger of cancer bone osteolysis

Osteolytic bone destruction is a hallmark of bone-metastatic cancers. Current therapy is unable to completely cure or prevent this disease in patients. The p38 mitogen-activated protein kinase (MAPK) affects a diverse range of intracellular responses with well-known roles in development, cell-cycle and differentiation, inflammation, apoptosis, senescence, and tumorigenesis. This article is an overview of the contribution of tumor cell-expressed p38 MAPK to the regulation of osteoclastogenesis, osteoblastogenesis, and osteolyticbone lesions.

In vitro microenvironments to study breast cancer bone colonisation

Bone metastasis occurs frequently in patients with advanced breast cancer and is a major cause of morbidity and mortality in these patients. In order to advance current therapies, the mechanisms leading to the formation of bone metastases and their pathophysiology have to be better understood. Several in vitro models have been developed for systematic studies of interactions between breast cancer cells and the bone microenvironment. Such models can provide insights into the molecular basis of bone metastatic colonisation and also may provide a useful platform to design more physiologically relevant drug testing assays. This review describes different in vitro approaches and discusses their advantages and disadvantages.

The systemic delivery of an oncolytic adenovirus expressing decorin inhibits bone metastasis in a mouse model of human prostate cancer

In an effort to develop a new therapy for prostate cancer bone metastases, we have created Ad.dcn, a recombinant oncolytic adenovirus carrying the human decorin gene. Infection of PC-3 and DU-145, the human prostate tumor cells, with Ad.dcn or a non-replicating adenovirus Ad(E1-).dcn resulted in decorin expression; Ad.dcn produced high viral titers and cytotoxicity in human prostate tumor cells. Adenoviral-mediated decorin expression inhibited Met, the Wnt/β-catenin signaling axis, vascular endothelial growth factor A, reduced mitochondrial DNA levels, and inhibited tumor cell migration. To examine the anti-tumor response of Ad.dcn, PC-3-luc cells were inoculated in the left heart ventricle to establish bone metastases in nude mice. Ad.dcn, in conjunction with control replicating and non-replicating vectors were injected via tail vein. The real-time monitoring of mice, once a week, by bioluminescence imaging and X-ray radiography showed that Ad.dcn produced significant inhibition of skeletal metastases. Analyses of the mice at the terminal time point indicated a significant reduction in the tumor burden, osteoclast number, serum TRACP 5b levels, osteocalcin levels, hypercalcemia, inhibition of cancer cachexia, and an increase in the animal survival. Based on these studies, we believe that Ad.dcn can be developed as a potential new therapy for prostate cancer bone metastasis.

Downregulation of CXCR4 by SDF-KDEL in SBC-5 cells inhibits their migration in vitro and organ metastasis in vivo

Metastasis is the principal cause of morbidity and mortality in cancer patients. The master genes that govern organ-selective metastasis remain elusive. We compared the expression levels of C-X-C chemokine receptor type 4 (CXCR4) in the human small cell lung cancer (SCLC) cells, SBC-5 and SBC-3, by flow cytometric analysis and found that CXCR4 was expressed at markedly higher levels in the SBC-5 cells which can produce multiple organ metastasis, particularly bone metastasis compared to the SBC-3 cells. Stromal-derived-factor-1 (SDF-1)-CXCR4 has been shown to regulate cell migration and metastasis in a various types of cancer; however, the roles of SDF-1-CXCR4 in the organ-selective metastasis of SCLC in vivo remain to be elucidated. Thus, in this study, we constructed a phenotype of SBC-5 cells in which CXCR4 was knocked out using the intrakine strategy and found that the downregulation of CXCR4 inhibited cell migration and invasion, but did not affect cell proliferation or apoptosis in vitro. In in vivo experiments, the knockout of CXCR4 suppressed the development of metastastic lesions in the lungs, liver and bone, but did not decrease metastasis to the kidneys. Our data demonstrate that CXCR4 is a candidate gene involved in the development of metastastic lesions in specific organs, such as the lungs, bone and liver, which can secrete high concentrations of SDF-1, the sole ligand of CXCR4. Thus, CXCR4 may prove to be a promising target for the prevention and effective treatment of metastastic lesions due to SCLC.

The molecular signature of the stroma response in prostate cancer-induced osteoblastic bone metastasis highlights expansion of hematopoietic and prostate epithelial stem cell niches

The reciprocal interaction between cancer cells and the tissue-specific stroma is critical for primary and metastatic tumor growth progression. Prostate cancer cells colonize preferentially bone (osteotropism), where they alter the physiological balance between osteoblast-mediated bone formation and osteoclast-mediated bone resorption, and elicit prevalently an osteoblastic response (osteoinduction). The molecular cues provided by osteoblasts for the survival and growth of bone metastatic prostate cancer cells are largely unknown. We exploited the sufficient divergence between human and mouse RNA sequences together with redefinition of highly species-specific gene arrays by computer-aided and experimental exclusion of cross-hybridizing oligonucleotide probes. This strategy allowed the dissection of the stroma (mouse) from the cancer cell (human) transcriptome in bone metastasis xenograft models of human osteoinductive prostate cancer cells (VCaP and C4-2B). As a result, we generated the osteoblastic bone metastasis-associated stroma transcriptome (OB-BMST). Subtraction of genes shared by inflammation, wound healing and desmoplastic responses, and by the tissue type-independent stroma responses to a variety of non-osteotropic and osteotropic primary cancers generated a curated gene signature ("Core" OB-BMST) putatively representing the bone marrow/bone-specific stroma response to prostate cancer-induced, osteoblastic bone metastasis. The expression pattern of three representative Core OB-BMST genes (PTN, EPHA3 and FSCN1) seems to confirm the bone specificity of this response. A robust induction of genes involved in osteogenesis and angiogenesis dominates both the OB-BMST and Core OB-BMST. This translates in an amplification of hematopoietic and, remarkably, prostate epithelial stem cell niche components that may function as a self-reinforcing bone metastatic niche providing a growth support specific for osteoinductive prostate cancer cells. The induction of this combinatorial stem cell niche is a novel mechanism that may also explain cancer cell osteotropism and local interference with hematopoiesis (myelophthisis). Accordingly, these stem cell niche components may represent innovative therapeutic targets and/or serum biomarkers in osteoblastic bone metastasis.

Castration-induced bone loss triggers growth of disseminated prostate cancer cells in bone

Up to 90% of patients with castrate-resistant prostate cancer develop bone metastases, and the majority of these men have received androgen deprivation therapy known to cause bone loss. Whether this treatment-induced change to the bone microenvironment affects disseminated tumour cells, potentially stimulating development of bone metastasis, remains to be determined. The objective of this study was to use an in vivo model mimicking androgen ablation to establish the effects of this intervention on disseminated prostate cancer cells in bone. We mimicked the effects of androgen deprivation on bone metastasis by castrating 12-week-old BALB/c nude mice that had disseminated, hormone-insensitive PC3 prostate cancer cells present in the long bones. Castration caused increased bone resorption and loss of bone volume, compared with sham operation. In addition, castration triggered growth of disseminated PC3 cells to form bone metastasis in 70% of animals. In contrast, only 10% of sham-operated animals had detectable long bone tumours. Weekly administration of 100 μg/kg zoledronic acid (ZOL) prevented castration-induced tumour growth in bone and increased bone volume, but did not eliminate the disseminated tumour cells. ZOL had no effect on tumour growth in the sham-operated animals, despite causing a significant increase in bone volume. This is the first demonstration that, in a model of prostate cancer bone metastasis, mimicking androgen ablation results in growth of disseminated tumour cells in bone through osteoclast-mediated mechanisms. We provide the first biological evidence supporting the administration of ZOL to prostate cancer patients at the time of androgen ablation to prevent subsequent relapse in bone.