Passive Entrapment of Tumor Cells Determines Metastatic Dissemination to Spinal Bone and Other Osseous Tissues

The Brain Pre-Metastatic Niche: Biological and Technical Advancements

Metastasis, particularly brain metastasis, continues to puzzle researchers to this day, and exploring its molecular basis promises to break ground in developing new strategies for combatting this deadly cancer. In recent years, the research focus has shifted toward the earliest steps in the formation of metastasis. In this regard, significant progress has been achieved in understanding how the primary tumor affects distant organ sites before the arrival of tumor cells. The term pre-metastatic niche was introduced for this concept and encompasses all influences on sites of future metastases, ranging from immunological modulation and ECM remodeling to the softening of the blood-brain barrier. The mechanisms governing the spread of metastasis to the brain remain elusive. However, we begin to understand these processes by looking at the earliest steps in the formation of metastasis. This review aims to present recent findings on the brain pre-metastatic niche and to discuss existing and emerging methods to further explore the field. We begin by giving an overview of the pre-metastatic and metastatic niches in general before focusing on their manifestations in the brain. To conclude, we reflect on the methods usually employed in this field of research and discuss novel approaches in imaging and sequencing.

EphrinB2-EphB4 Signaling in Neurooncological Disease

EphrinB2-EphB4 signaling is critical during embryogenesis for cardiovascular formation and neuronal guidance. Intriguingly, critical expression patterns have been discovered in cancer pathologies over the last two decades. Multiple connections to tumor migration, growth, angiogenesis, apoptosis, and metastasis have been identified in vitro and in vivo. However, the molecular signaling pathways are manifold and signaling of the EphB4 receptor or the ephrinB2 ligand is cancer type specific. Here we explore the impact of these signaling pathways in neurooncological disease, including glioma, brain metastasis, and spinal bone metastasis. We identify potential downstream pathways that mediate cancer suppression or progression and seek to understand it´s role in antiangiogenic therapy resistance in glioma. Despite the Janus-faced functions of ephrinB2-EphB4 signaling in cancer Eph signaling remains a promising clinical target.

Animal Models of Metastatic Lesions to the Spine: a Focus on Epidural Spinal Cord Compression

Epidural spinal cord compression (ESCC) secondary to spine metastases is one of the most devastating sequelae of primary cancer as it may lead to muscle weakness, paresthesia, pain, and paralysis. Spine metastases occur through a multi-step process that can result in eventual ESCC; however, the lack of a preclinical model to effectively recapitulate each step of this metastatic cascade and the symptom burden of ESCC has limited our understanding of this disease process. In this review, we discuss animal models that best recapitulate ESCC; we start with a broad discussion of commonly used models of bone metastasis and end with a focused discussion of models used to specifically study ESCC. Orthotopic models offer the most authentic recapitulation of metastasis development; however, they rarely result in symptomatic ESCC and are challenging to replicate. Conversely, models that involve injection of tumor cells directly into the bloodstream or bone better mimic the symptoms of ESCC; however, they provide limited insight into the epithelial to mesenchymal transition (EMT) and natural hematogenous spread of tumor cell. Therefore, until an ideal model is created, it is critical to select an animal model that is specifically designed to answer the scientific question of interest.

How to Target Spinal Metastasis in Experimental Research: An Overview of Currently Used Experimental Mouse Models and Future Prospects

The spine is one of the organs that is most affected by metastasis in cancer patients. Since the control of primary tumor is continuously improving, treatment of metastases is becoming one of the major challenges to prevent cancer-related death. Due to the anatomical proximity to the spinal cord, local spread of metastasis can directly cause neurological deficits, severely limiting the patient’s quality of life. To investigate the underlying mechanisms and to develop new therapies, preclinical models are required which represent the complexity of the multistep cascade of metastasis. Current research of metastasis focuses on the formation of the premetastatic niche, tumor cell dormancy and the influence and regulating function of the immune system. To unveil whether these influence the organotropism to the spine, spinal models are irreplaceable. Mouse models are one of the most suitable models in oncologic research. Therefore, this review provides an overview of currently used mouse models of spinal metastasis. Furthermore, it discusses technical aspects clarifying to what extend these models can picture key steps of the metastatic process. Finally, it addresses proposals to develop better mouse models in the future and could serve as both basis and stimulus for researchers and clinicians working in this field.

Ephrin-B2-EphB4 communication mediates tumor-endothelial cell interactions during hematogenous spread to spinal bone in a melanoma metastasis model

Metastases account for the majority of cancer deaths. Bone represents one of the most common sites of distant metastases, and spinal bone metastasis is the most common source of neurological morbidity in cancer patients. During metastatic seeding of cancer cells, endothelial-tumor cell interactions govern extravasation to the bone and potentially represent one of the first points of action for antimetastatic treatment. The ephrin-B2-EphB4 pathway controls cellular interactions by inducing repulsive or adhesive properties, depending on forward or reverse signaling. Here, we report that in an in vivo metastatic melanoma model, ephrin-B2-mediated activation of EphB4 induces tumor cell repulsion from bone endothelium, translating in reduced spinal bone metastatic loci and improved neurological function. Selective ephrin-B2 depletion in endothelial cells or EphB4 inhibition increases bone metastasis and shortens the time window to hind-limb locomotion deficit from spinal cord compression. EphB4 overexpression in melanoma cells ameliorates the metastatic phenotype and improves neurological outcome. Timely harvesting of bone tissue after tumor cell injection and intravital bone microscopy revealed less tumor cells attached to ephrin-B2-positive endothelial cells. These results suggest that ephrin-B2-EphB4 communication influences bone metastasis formation by altering melanoma cell repulsion/adhesion to bone endothelial cells, and represents a molecular target for therapeutic intervention.

Role of mTOR and VEGFR Inhibition in Prevention of Metastatic Tumor Growth in the Spine

Objective: Spinal metastatic disease remains a major problem of oncological diseases. Patients affected may suffer pain, spinal instability, and severe neurological deficits. Today, palliative surgery and radiotherapy are the mainstays of therapy. In contrast, preventive treatment strategies or treatment concepts for an early stage are lacking. Here, we have used a syngeneic, experimental spine metastases model in the mouse to test the efficacy of mTOR inhibition and anti-angiogenesis on the formation and progression of spinal melanoma metastases.

Methods: We used our previously established syngeneic spinal metastases mouse model by injecting luciferin-transfected B16 melanoma cells into the common carotid artery. Following injection, mice were treated with everolimus, an inhibitor of the mammalian target of rapamycin (mTOR) complex, axitinib, a tyrosine kinase inhibitor, that blocks vascular endothelial growth factor receptors (VEGFR) 1-3, as well as placebo. Animals were followed-up daily by neurological assessment and by repeat in vivo bioluminescence imaging. With occurrence of neurological deficits, a spinal MRI was performed, and mice were sacrificed. The whole spine was dissected free and analyzed by immunohistochemical techniques.

Results: Overall survival was 23 days in the control group, significantly prolonged to 30 days (p = 0.04) in the everolimus group, and to 28 days (p = 0.04) in the axitinib group. While 78% of mice in the placebo group developed symptomatic metastatic epidural spinal cord compression, only 50% did so in the treatment groups. The mean time to manifestation of paralysis was 22 days in the control group, 26 days (p = 0.10) in the everolimus group, and 27 days (p = 0.06) in the axitinib group. Screening for spinal metastases by bioluminescence imaging on two different time points showed a decrease in metastatic tumor formation in the treatment groups compared to the controls. Immunohistochemical analysis confirmed the bioactivity of the two compounds: The Ki67 proliferation labeling index was reduced in the everolimus group and numbers of CD31 positive endothelial cells were reduced in the axitinib group.

Conclusion: Both, the mTOR inhibitor everolimus as well as antiangiogenetic effects by the VEGFR inhibitor axitinib showed potential to prevent and retard formation of symptomatic spinal metastases. However, the therapeutic efficacy was only mild in this experimental model.

Metastatic dissemination patterns of different primary tumors to the spine and other bones

Metastatic spine disease (MSD) is a severe event in cancer patients. Experimental data indicate that bone metastasis is mostly mediated by blood flow-dependent, passive arrest of circulating tumor cells to the bone metastatic niche (BMN). Here, we have set out to test these experimental observations in a clinical, human setting to improve our understanding of MSD. 507 patients, treated on spinal metastases in our institution from 2005 to  2015 were retrospectively evaluated. We identified 259 patients with accessible staging reports of the skeleton before and at initial diagnosis of MSD. Data analysis comprised localizations of bone metastases, underlying malignancy and time to development of MSD. Dissemination pattern of bone metastasis was correlated with red bone marrow (RBM) content of the respective bone as a measure of blood flow. Spinal metastases occurred most frequently in lung cancer (21%), prostate cancer (19%), and breast cancer (12%). At the diagnosis of MSD, majority of patients have multiple extra-spinal bone metastases (2/3). The distribution of metastases to extra-spinal bones and to the spine is mostly proportional to the RBM content of the involved bone. Corresponding to the high RBM content, thoracic spine, pelvic bones and ribs represent a predilection site for bone metastasis. We confirm a distinct preference of cancer types to metastasize to bones. When it comes to bone metastases all primaries show uniform distribution pattern, which supports the hypothesis of a predominantly blood flow-dependent distribution of tumor cells and passive arrest to the BMN rather than a spine-specific homing mechanism.

Uncoupling Traditional Functionalities of Metastasis: The Parting of Ways with Real-Time Assays

The experimental evaluation of metastasis overly focuses on the gain of migratory and invasive properties, while disregarding the contributions of cellular plasticity, extra-cellular matrix heterogeneity, niche interactions, and tissue architecture. Traditional cell-based assays often restrict the inclusion of these processes and warrant the implementation of approaches that provide an enhanced spatiotemporal resolution of the metastatic cascade. Time lapse imaging represents such an underutilized approach in cancer biology, especially in the context of disease progression. The inclusion of time lapse microscopy and microfluidic devices in routine assays has recently discerned several nuances of the metastatic cascade. Our review emphasizes that a complete comprehension of metastasis in view of evolving ideologies necessitates (i) the use of appropriate, context-specific assays and understanding their inherent limitations; (ii) cautious derivation of inferences to avoid erroneous/overestimated clinical extrapolations; (iii) corroboration between multiple assay outputs to gauge metastatic potential; and (iv) the development of protocols with improved in situ implications. We further believe that the adoption of improved quantitative approaches in these assays can generate predictive algorithms that may expedite therapeutic strategies targeting metastasis via the development of disease relevant model systems. Such approaches could potentiate the restructuring of the cancer metastasis paradigm through an emphasis on the development of next-generation real-time assays.

Bone metastasis risk factors in breast cancer

Bone is the single most frequent site for bone metastasis in breast cancer patients. Patients with bone-only metastasis have a fairly good prognosis when compared with patients with visceral disease. Nevertheless, cancer-induced bone disease carries an important risk of developing skeletal related events that impact quality of life (QoL). It is therefore particularly important to stratify patients according to their risk of developing bone metastasis. In this context, several risk factors have been studied, including demographic, clinicopathological, genetic, and metabolic factors. Most of them show conflicting or non-definitive associations and are not validated for clinical use. Nonetheless, tumour intrinsic subtype is widely accepted as a major risk factor for bone metastasis development and luminal breast cancer carries an increased risk for bone disease. Other factors such as gene signatures, expression of specific cytokines (such as bone sialoprotein and bone morphogenetic protein 7) or components of the extracellular matrix (like bone crosslinked C-telopeptide) might also influence the development of bone metastasis. Knowledge of risk factors related with bone disease is of paramount importance as it might be a prediction tool for triggering the use of targeted agents and allow for better patient selection for future clinical trials.