In vivo animal models for studying brain metastasis: value and limitations

Biological profile of breast cancer brain metastasis

Breast cancer is one of the leading causes of death worldwide. The aggressive behaviour of breast tumor results from their metastasis. Notably, the brain tissue is one of the common regions of metastasis, thereby reducing the overall survival of patients. Moreover, the metastatic tumors demonstrate poor response or resistance to therapies. In addition, breast cancer brain metastasis provides the poor prognosis of patients. Therefore, it is of importance to understand the mechanisms in breast cancer brain metastasis. Both cell lines and animal models have been developed for the evaluation of breast cancer brain metastasis. Moreover, different tumor microenvironment components and other factors such as lymphocytes and astrocytes can affect brain metastasis. The breast cancer cells can disrupt the blood-brain barrier (BBB) during their metastasis into brain, developing blood-tumor barrier to enhance carcinogenesis. The breast cancer brain metastasis can be increased by the dysregulation of chemokines, STAT3, Wnt, Notch and PI3K/Akt. On the other hand, the effective therapeutics have been developed for the brain metastasis such as introduction of nanoparticles. Moreover, the disruption of BBB by ultrasound can increase the entrance of bioactive compounds to the brain tissue. In order to improve specificity and selectivity, the nanoparticles for the delivery of therapeutics and crossing over BBB have been developed to suppress breast cancer brain metastasis.

In vitro models: Can they unravel the complexities of cancer cell metastasis?

Metastasis still accounts for the majority of cancer-related deaths despite intense research efforts made worldwide to better understand the determinants involved and discover novel ways to halt it. However, studying the pathogenesis of metastasis in actual patients is indeed challenging which renders the need for the development of relevant experimental models urgent. Traditionally, several in vitro and in vivo models have been developed to study metastasis each of which having its own advantages and limitations. In the present review, we analyzed the current approaches used in cancer biology research to study cancer cell metastasis giving emphasis on the newly developed in vitro systems that take into account factors like the three-dimensional (3D) nature of the tumor, the interaction between cancer cells and the extracellular matrix or other cells present in the tumor microenvironment, and thus, better recapitulate the metastatic process. These approaches, namely 3D bioprinting, 3D tissue models, microfluidics systems, and spheroid generation are currently used separately or in combination depending on the research question and the cancer type in order to better represent the actual in vivo setting.

Current preclinical models of brain metastasis

Brain metastases (BMs) represent the most prevalent intracranial malignancy within the adult. They are identified in up to 20% of patients with solid tumors and this percentage varies between tumor types and age. Due to the selective permeability of the blood-brain barrier, most anticancer drugs can't reach significant concentrations in the brain, representing a major obstacle to the patients' survival. Furthermore, intra- and inter-patient heterogeneity and the unique brain microenvironment add a layer of complexity to the clinical management of BMs. In the perspective of finding new therapeutic approaches and better understanding the molecular mechanisms involved in brain metastasis, the use of appropriate preclinical models is essential. Here, we review current in vivo, in vitro and ex vivo models for the study of brain metastasis while outlining their advantages and limitations.

Preclinical evaluation of targeted therapies for central nervous system metastases

The central nervous system (CNS) represents a site of sanctuary for many metastatic tumors when systemic therapies that control the primary tumor cannot effectively penetrate intracranial lesions. Non-small cell lung cancers (NSCLCs) are the most likely of all neoplasms to metastasize to the brain, with up to 60% of patients developing CNS metastases during the disease process. Targeted therapies such as tyrosine kinase inhibitors (TKIs) have helped reduce lung cancer mortality but vary considerably in their capacity to control CNS metastases. The ability of these therapies to effectively target lesions in the CNS depends on several of their pharmacokinetic properties, including blood-brain barrier permeability, affinity for efflux transporters, and binding affinity for both plasma and brain tissue. Despite the existence of numerous preclinical models with which to characterize these properties, many targeted therapies have not been rigorously tested for CNS penetration during the discovery process, whereas some made it through preclinical testing despite poor brain penetration kinetics. Several TKIs have now been engineered with the characteristics of CNS-penetrant drugs, with clinical trials proving these efforts fruitful. This Review outlines the extent and variability of preclinical evidence for the efficacy of NSCLC-targeted therapies, which have been approved by the US Food and Drug Administration (FDA) or are in development, for treating CNS metastases, and how these data correlate with clinical outcomes.

Microinstrumentation for Brain Organoids

Brain organoids are three-dimensional aggregates of self-organized differentiated stem cells that mimic the structure and function of human brain regions. Organoids bridge the gaps between conventional drug screening models such as planar mammalian cell culture, animal studies, and clinical trials. They can revolutionize the fields of developmental biology, neuroscience, toxicology, and computer engineering. Conventional microinstrumentation for conventional cellular engineering, such as planar microfluidic chips; microelectrode arrays (MEAs); and optical, magnetic, and acoustic techniques, has limitations when applied to three-dimensional (3D) organoids, primarily due to their limits with inherently two-dimensional geometry and interfacing. Hence, there is an urgent need to develop new instrumentation compatible with live cell culture techniques and with scalable 3D formats relevant to organoids. This review discusses conventional planar approaches and emerging 3D microinstrumentation necessary for advanced organoid-machine interfaces. Specifically, this article surveys recently developed microinstrumentation, including 3D printed and curved microfluidics, 3D and fast-scan optical techniques, buckling and self-folding MEAs, 3D interfaces for electrochemical measurements, and 3D spatially controllable magnetic and acoustic technologies relevant to two-way information transfer with brain organoids. This article highlights key challenges that must be addressed for robust organoid culture and reliable 3D spatiotemporal information transfer.

State of the Art Modelling of the Breast Cancer Metastatic Microenvironment: Where Are We?

Metastatic spread of tumour cells to tissues and organs around the body is the most frequent cause of death from breast cancer. This has been modelled mainly using mouse models such as syngeneic mammary cancer or human in mouse xenograft models. These have limitations for modelling human disease progression and cannot easily be used for investigation of drug resistance and novel therapy screening. To complement these approaches, advances are being made in ex vivo and 3D in vitro models, which are becoming progressively better at reliably replicating the tumour microenvironment and will in the future facilitate drug development and screening. These approaches include microfluidics, organ-on-a-chip and use of advanced biomaterials. The relevant tissues to be modelled include those that are frequent and clinically important sites of metastasis such as bone, lung, brain, liver for invasive ductal carcinomas and a distinct set of common metastatic sites for lobular breast cancer. These sites all have challenges to model due to their unique cellular compositions, structure and complexity. The models, particularly in vivo, provide key information on the intricate interactions between cancer cells and the native tissue, and will guide us in producing specific therapies that are helpful in different context of metastasis.

Review of metastasis to meningiomas with case examples

Introduction

A tumor-to-tumor metastasis (TTM) is a rare metastatic process where a primary malignant tumor metastasizes to another tumor, most commonly a benign tumor such as a meningioma. Here, we present two recent cases of tumor-to-meningioma metastases (TMM) from our clinical practice and review of recent literature. The primary cancers were prostate and breast cancer, respectively.

Material and methods

We reviewed the electronic medical records of the two patients and conducted a literature review of TTM, focusing on biological mechanisms related to TMM.

Results

Our first patient, a man with a history of stable prostate cancer, underwent resection of two WHO grade 1 meningiomas, and the largest tumor was found to have TMM. Our second patient, a woman with progressive breast cancer, was operated for a WHO grade II meningioma, and the meningioma harbored breast cancer metastases. TMM is a rare occurrence, but breast cancer is a much more frequent cause than prostate cancer and we reviewed 50 cases. Only 15 of cases of TMM from prostate cancer have been described.

Discussion and conclusion

TMM is a rare phenomenon, but it is important to be aware of this as more and more patients live with cancer and meningiomas have a high prevalence, The possibility of TMM may impact not only both the surgical and oncological treatment but also surveillance of incidental meningiomas.

The path to leptomeningeal metastasis

The leptomeninges, the cerebrospinal-fluid-filled tissues surrounding the central nervous system, play host to various pathologies including infection, neuroinflammation and malignancy. Spread of systemic cancer into this space, termed leptomeningeal metastasis, occurs in 5-10% of patients with solid tumours and portends a bleak clinical prognosis. Previous, predominantly descriptive, clinical studies have provided few insights. Recent development of preclinical leptomeningeal metastasis models, alongside genomic, transcriptomic and proteomic sequencing efforts, has provided groundwork for mechanistic understanding and identification of long-needed therapeutic targets. Although previously understood as an anatomically isolated compartment, the leptomeninges are increasingly appreciated as a major conduit of communication between the systemic circulation and the central nervous system. Despite the unique nature of the leptomeningeal microenvironment, the general principles of metastasis hold true: cells metastasizing to the leptomeninges must gain access to the new environment, survive within the space and evade the immune system. The study of leptomeningeal metastasis has the potential to uncover novel site-specific metastatic principles and illuminate the physiology of the leptomeningeal space. In this Review, we provide a biology-focused overview of how metastatic cells reach the leptomeninges, thrive in this nutritionally sparse environment and evade the detection of the omnipresent immune system.

Noggin contributes to brain metastatic colonization of lung cancer cells

BACKGROUND

Brain metastasis is a common complication among patients with lung cancer, yet the underlying mechanisms remain unclear. In this study, we aimed to investigate the pathogenesis of brain metastasis in lung cancer.

METHODS

We established highly colonizing metastatic lung cancer cells, A549-M2, through multiple implantations of A549 human lung cancer cells in the carotid artery of athymic nude mice.

RESULTS

Compared to parental cells (M0), M2 cells demonstrated slower growth in culture plates and soft agar, as well as lower motility and higher adhesion, key characteristics of mesenchymal-epithelial transition (MET). Further analysis revealed that M2 cells exhibited decreased expression of epithelial-mesenchymal transition markers, including ZEB1 and Vimentin. M2 cells also demonstrated reduced invasiveness in co-culture systems. RNA sequencing and gene set enrichment analysis confirmed that M2 cells underwent MET. Intriguingly, depletion of Noggin, a BMP antagonist, was observed in M2 cells, and replenishment of Noggin restored suppressed migration and invasion of M2 cells. In addition, Noggin knockdown in control M0 cells promoted cell attachment and suppressed cell migration, suggesting that Noggin reduction during brain colonization causes inhibition of migration and invasion of metastatic lung cancer cells.

CONCLUSIONS

Our results suggest that lung cancer cells undergo MET and lose their motility and invasiveness during brain metastatic colonization, which is dependent on Noggin.

Breast Cancer Brain Metastases: Implementation and Characterization of a Mouse Model Relying on Malignant Cells Inoculation in the Carotid Artery

Breast cancer (BC) brain metastases (BCBM) is a severe condition frequently occurring in the triple-negative subtype. The study of BCBM pathogenesis and treatment has been hampered by the difficulty in establishing a reliable animal model that faithfully recapitulates the preferential formation of brain metastases. The injection of BC cells in the carotid artery of mice has been proposed but the procedure is challenging, with the metastatic pattern being scarcely characterized. In this work, we thoroughly describe an improved procedure, highlighting the tricks and challenges of the process, and providing a characterization of the brain and peripheral metastatic pattern at the cellular and molecular level. Triple-negative BC (4T1) cells were inoculated in the common carotid artery of BALB/c mice. Brains and peripheral organs were harvested at 7-14 days for the histological characterization of the metastases' pattern and the immunofluorescence analysis of specific markers. With our surgical procedure, both mouse death and procedure-associated weight loss were negligible. Brain metastases mostly occurred in the hippocampus, while sparse peripheral lesions were only detected in the lungs. Brain-colonizing BC cells presented proliferative (Ki-67) and epithelial (pan-cytokeratin and tomato lectin) features, which account for metastases' establishment. The presented surgical approach constitutes an important and reliable tool for BCBM studies.

Breast cancer brain metastasis: from etiology to state-of-the-art modeling

Currently, breast carcinoma is the most common form of malignancy and the main cause of cancer mortality in women worldwide. The metastasis of cancer cells from the primary tumor site to other organs in the body, notably the lungs, bones, brain, and liver, is what causes breast cancer to ultimately be fatal. Brain metastases occur in as many as 30% of patients with advanced breast cancer, and the 1-year survival rate of these patients is around 20%. Many researchers have focused on brain metastasis, but due to its complexities, many aspects of this process are still relatively unclear. To develop and test novel therapies for this fatal condition, pre-clinical models are required that can mimic the biological processes involved in breast cancer brain metastasis (BCBM). The application of many breakthroughs in the area of tissue engineering has resulted in the development of scaffold or matrix-based culture methods that more accurately imitate the original extracellular matrix (ECM) of metastatic tumors. Furthermore, specific cell lines are now being used to create three-dimensional (3D) cultures that can be used to model metastasis. These 3D cultures satisfy the requirement for in vitro methodologies that allow for a more accurate investigation of the molecular pathways as well as a more in-depth examination of the effects of the medication being tested. In this review, we talk about the latest advances in modeling BCBM using cell lines, animals, and tissue engineering methods.

The brain-penetrant cell-cycle inhibitor p28 sensitizes brain metastases to DNA-damaging agents

Background

Brain metastases (BMs), the most common tumors of the central nervous system, are life-threatening with a dismal prognosis. The major challenges to developing effective treatments for BMs are the limited abilities of drugs to target tumors and to cross the blood-brain barrier (BBB). We aimed to investigate the efficacy of our therapeutic approach against BMs in mouse models that recapitulate the clinical manifestations of BMs.

Methods

BMs mouse models were constructed by injecting human breast, lung cancer, and melanoma intracardially, which allowed the BBB to remain intact. We investigated the ability of the cell-penetrating peptide p28 to cross the BBB in an in vitro 3D model and in the BMs animal models. The therapeutic effects of p28 in combination with DNA-damaging agents (radiation and temozolomide) on BMs were also evaluated.

Results

p28 crossed the intact BBB more efficiently than the standard chemotherapeutic agent, temozolomide. Upon crossing the BBB, p28 localized preferentially to tumor lesions and enhanced the efficacy of DNA-damaging agents by activating the p53-p21 axis. In the BMs animal models, radiation in combination with p28 significantly reduced the tumor burden of BMs.

Conclusions

The cell-cycle inhibitor p28 can cross the BBB localize to tumor lesions in the brain and enhance the inhibitory effects of DNA-damaging agents on BMs, suggesting the potential therapeutic benefits of this molecule in BMs.

The application of zebrafish patient-derived xenograft tumor models in the development of antitumor agents

The cost of antitumor drug development is enormous, yet the clinical outcomes are less than satisfactory. Therefore, it is of great importance to develop effective drug screening methods that enable accurate, rapid, and high-throughput discovery of lead compounds in the process of preclinical antitumor drug research. An effective solution is to use the patient-derived xenograft (PDX) tumor animal models, which are applicable for the elucidation of tumor pathogenesis and the preclinical testing of novel antitumor compounds. As a promising screening model organism, zebrafish has been widely applied in the construction of the PDX tumor model and the discovery of antineoplastic agents. Herein, we systematically survey the recent cutting-edge advances in zebrafish PDX models (zPDX) for studies of pathogenesis mechanisms and drug screening. In addition, the techniques used in the construction of zPDX are summarized. The advantages and limitations of the zPDX are also discussed in detail. Finally, the prospects of zPDX in drug discovery, translational medicine, and clinical precision medicine treatment are well presented.

Advances in Brain Metastasis Models

To obtain achievements in addressing the clinical challenges of brain metastasis, we need a clear understanding of its biological mechanisms. Brain metastasis research is challenged by many practical scientific barriers. Depending on the purpose of the study, experimental brain metastasis models in vivo can be used. It is now possible to re-create the architecture and physiology of human organs. Human organoids provide unique opportunities for the study of human disease and complement animal models. The translation of experimental findings to clinical application has several barriers in the development of treatment for brain metastasis. A variety of models have provided significant contributions to the knowledge of brain metastasis pathology and remain pivotal tools for examining novel therapeutic strategies.

Tissue-Engineered Models of the Human Brain: State-of-the-Art Analysis and Challenges

Neurological disorders affect billions of people across the world, making the discovery of effective treatments an important challenge. The evaluation of drug efficacy is further complicated because of the lack of in vitro models able to reproduce the complexity of the human brain structure and functions. Some limitations of 2D preclinical models of the human brain have been overcome by the use of 3D cultures such as cell spheroids, organoids and organs-on-chip. However, one of the most promising approaches for mimicking not only cell structure, but also brain architecture, is currently represented by tissue-engineered brain models. Both conventional (particularly electrospinning and salt leaching) and unconventional (particularly bioprinting) techniques have been exploited, making use of natural polymers or combinations between natural and synthetic polymers. Moreover, the use of induced pluripotent stem cells (iPSCs) has allowed the co-culture of different human brain cells (neurons, astrocytes, oligodendrocytes, microglia), helping towards approaching the central nervous system complexity. In this review article, we explain the importance of in vitro brain modeling, and present the main in vitro brain models developed to date, with a special focus on the most recent advancements in tissue-engineered brain models making use of iPSCs. Finally, we critically discuss achievements, main challenges and future perspectives.

Insights into the Steps of Breast Cancer-Brain Metastases Development: Tumor Cell Interactions with the Blood-Brain Barrier

Brain metastases (BM) represent a growing problem for breast cancer (BC) patients. Recent studies have demonstrated a strong impact of the BC molecular subtype on the incidence of BM development. This study explores the interaction between BC cells of different molecular subtypes and the blood–brain barrier (BBB). We compared the ability of BC cells of different molecular subtypes to overcome several steps (adhesion to the brain endothelium, disruption of the BBB, and invasion through the endothelial layer) during cerebral metastases formation, in vitro as well as in vivo. Further, the impact of these cells on the BBB was deciphered at the molecular level by transcriptome analysis of the triple-negative (TNBC) cells themselves as well as of hBMECs after cocultivation with BC cell secretomes. Compared to luminal BC cells, TNBC cells have a greater ability to influence the BBB in vitro and consequently develop BM in vivo. The brain-seeking subline and parental TNBC cells behaved similarly in terms of adhesion, whereas the first showed a stronger impact on the brain endothelium integrity and increased invasive ability. The comparative transcriptome revealed potential brain-metastatic-specific key regulators involved in the aforementioned processes, e.g., the angiogenesis-related factors TNXIP and CXCL1. In addition, the transcriptomes of the two TNBC cell lines strongly differed in certain angiogenesis-associated factors and in several genes related to cell migration and invasion. Based on the present study, we hypothesize that the tumor cell’s ability to disrupt the BBB via angiogenesis activation, together with increased cellular motility, is required for BC cells to overcome the BBB and develop brain metastases.

Novel human melanoma brain metastasis models in athymic nude fox1nu mice: Site-specific metastasis patterns reflecting their clinical origin

BACKGROUND

Malignant melanomas frequently metastasize to the brain, but metastases in the cerebellum are underrepresented compared with metastases in the cerebrum.

METHODS

We established animal models by injecting intracardially in athymic nude fox1nu mice two human melanoma cell lines, originating from a cerebral metastasis (HM19) and a cerebellar metastasis (HM86).

RESULTS

Using magnetic resonance imaging (MRI), metastases were first detected after a mean of 34.5 days. Mean survival time was 59.6 days for the mice in the HM86 group and significantly shorter (43.7 days) for HM19-injected animals (p < 0.001). In the HM86 group, the first detectable metastasis was located in the cerebellum in 15/55 (29%) mice compared with none in the HM19 group (p < 0.001). At sacrifice, cerebellar metastases were found in 34/55 (63%) HM86-injected mice compared with 1/53 (2%) in the HM19-injected (p < 0.001) mice. At that time, all mice in both groups had detectable metastases in the cerebrum. Comparing macroscopic and histologic appearances of the brain metastases with their clinical counterparts, the cell line-based tumors had kept their original morphologic characteristics.

CONCLUSIONS

The present work demonstrates that human brain-metastatic melanoma cells injected intracardially in mice had retained inherent characteristics also in reproducing interaction with subtle microenvironmental brain tissue compartment-specific features. The models offer new possibilities for investigating tumor- and host-associated factors involved in determining tissue specificity of brain metastasis.

Preclinical Solid Tumor Models to Study Novel Therapeutics in Brain Metastases

Metastases are the most common malignancy of the adult central nervous system and are becoming an increasingly troubling problem in oncology largely due to the lack of successful therapeutic options. The limited selection of treatments is a result of the currently poor understanding of the biological mechanisms of metastatic development, which in turn is difficult to achieve because of limited preclinical models that can accurately represent the clinical progression of metastasis. Described in this article are in vitro and in vivo model systems that are used to enhance the understanding of metastasis and to identify new therapies for the treatment of brain metastasis. © 2021 Wiley Periodicals LLC.

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.

Gain of HIF1 Activity and Loss of miRNA let-7d Promote Breast Cancer Metastasis to the Brain via the PDGF/PDGFR Axis

Early detection and adjuvant therapies have significantly improved survival of patients with breast cancer over the past three decades. In contrast, management of metastatic disease remains unresolved. Brain metastasis is a late complication frequently observed among patients with metastatic breast cancer, whose poor prognosis calls for novel and more effective therapies. Here, we report that active hypoxia inducible factor-1 (HIF1) signaling and loss of the miRNA let-7d concur to promote brain metastasis in a recently established model of spontaneous breast cancer metastasis from the primary site to the brain (4T1-BM₂), and additionally in murine and human experimental models of breast cancer brain metastasis (D2A1-BM₂ and MDA231-BrM₂). Active HIF1 and let-7d loss upregulated expression of platelet-derived growth factor (PDGF) B/A in murine and human brain metastatic cells, respectively, while either individual silencing of HIF1α and PDGF-A/B or let-7d overexpression suppressed brain metastasis formation in the tested models. Let-7d silencing upregulated HIF1α expression and HIF1 activity, indicating a regulatory hierarchy of the system. The clinical relevance of the identified targets was supported by human gene expression data analyses. Treatment of mice with nilotinib, a kinase inhibitor impinging on PDGF receptor (PDGFR) signaling, prevented formation of spontaneous brain metastases in the 4T1-BM₂ model and reduced growth of established brain metastases in mouse and human models. These results identify active HIF1 signaling and let-7d loss as coordinated events promoting breast cancer brain metastasis through increased expression of PDGF-A/B. Moreover, they identify PDGFR inhibition as a potentially actionable therapeutic strategy for patients with brain metastatis. SIGNIFICANCE: These findings show that loss of miRNA let-7d and active HIF1 signaling promotes breast cancer brain metastasis via PDGF and that pharmacologic inhibition of PDGFR suppresses brain metastasis, suggesting novel therapeutic opportunities. GRAPHICAL ABSTRACT: http://cancerres.aacrjournals.org/content/canres/81/3/594/F1.large.jpg.See related article by Thies et al., p. 606.

Anti-EGFR VHH-armed death receptor ligand-engineered allogeneic stem cells have therapeutic efficacy in diverse brain metastatic breast cancers

Basal-like breast cancer (BLBC) shows brain metastatic (BM) capability and overexpresses EGFR and death-receptors 4/5 (DR4/5); however, the anatomical location of BM prohibits efficient drug-delivery to these targetable markers. In this study, we developed BLBC-BM mouse models featuring different patterns of BMs and explored the versatility of estem cell (SC)-mediated bi-functional EGFR and DR4/5-targeted treatment in these models. Most BLBC lines demonstrated a high sensitivity to EGFR and DR4/5 bi-targeting therapeutic protein, E_VDR_L [anti-EGFR VHH (E_V) fused to DR ligand (DR_L)]. Functional analyses using inhibitors and CRISPR-Cas9 knockouts revealed that the E_V domain facilitated in augmenting DR4/5-DR_L binding and enhancing DR_L-induced apoptosis. E_VDR_L secreting stem cells alleviated tumor-burden and significantly increased survival in mouse models of residual-tumor after macrometastasis resection, perivascular niche micrometastasis, and leptomeningeal metastasis. This study reports mechanism based simultaneous targeting of EGFR and DR4/5 in BLBC and defines a new treatment paradigm for treatment of BM.

Brain Metastasis Cell Lines Panel: A Public Resource of Organotropic Cell Lines

Spread of cancer to the brain remains an unmet clinical need in spite of the increasing number of cases among patients with lung, breast cancer, and melanoma most notably. Although research on brain metastasis was considered a minor aspect in the past due to its untreatable nature and invariable lethality, nowadays, limited but encouraging examples have questioned this statement, making it more attractive for basic and clinical researchers. Evidences of its own biological identity (i.e., specific microenvironment) and particular therapeutic requirements (i.e., presence of blood-brain barrier, blood-tumor barrier, molecular differences with the primary tumor) are thought to be critical aspects that must be functionally exploited using preclinical models. We present the coordinated effort of 19 laboratories to compile comprehensive information related to brain metastasis experimental models. Each laboratory has provided details on the cancer cell lines they have generated or characterized as being capable of forming metastatic colonies in the brain, as well as principle methodologies of brain metastasis research. The Brain Metastasis Cell Lines Panel (BrMPanel) represents the first of its class and includes information about the cell line, how tropism to the brain was established, and the behavior of each model in vivo. These and other aspects described are intended to assist investigators in choosing the most suitable cell line for research on brain metastasis. The main goal of this effort is to facilitate research on this unmet clinical need, to improve models through a collaborative environment, and to promote the exchange of information on these valuable resources.

[10]-Gingerol improves doxorubicin anticancer activity and decreases its side effects in triple negative breast cancer models

PURPOSE

Although doxorubicin is widely used to treat cancer, severe side effects limit its clinical use. Combination of standard chemotherapy with natural products can increase the efficacy and attenuate the side effects of current therapies. Here we studied the anticancer effects of a combined regimen comprising doxorubicin and [10]-gingerol against triple-negative breast cancer, which does not respond to hormonal or targeted therapies.

METHODS

Cytotoxicity was evaluated by MTT assay, cell cycle progression and apoptosis were analyzed by flow cytometry and signaling pathways were analyzed by Western blotting in human and murine triple negative breast cancer cell systems. The anticancer/antimetastatic and toxic effects of the combined regimen was evaluated using syngeneic and xenograft orthotopic models.

RESULTS

The combination of doxorubicin and [10]-gingerol significantly increased the number of apoptotic cells, compared to each compound alone. In 4T1Br4 cells, the combined regimen was the only condition able to increase the levels of active caspase 3 and γH2AX and to decrease the level of Cdk-6 cyclin. In vivo, doxorubicin (3 mg/Kg, D3) and [10]-gingerol (10 mg/Kg, G10) resulted in a significant reduction in the volume of primary tumors and a decrease in the number of circulating tumor cells (CTCs). Interestingly, only the combined regimen led to decreased tumor burdens to distant organs (i.e., metastasis) and reduced chemotherapy-induced weight loss and hepatotoxicity in tumor-bearing animals. Likewise, in a xenograft model, only the combined regimen was effective in significantly reducing the primary tumor volume and the prevalence of CTCs.

CONCLUSIONS

Our data indicate that [10]-gingerol has potential to be used as a neoadjuvant or in combined therapy with doxorubicin, to improve its anticancer activity.

The effect of combining Endostar with radiotherapy on blood vessels, tumor-associated macrophages, and T cells in brain metastases of Lewis lung cancer

Background

Combining Endostar (ES) with radiotherapy (RT) has shown a promising therapeutic effect on non-small cell lung carcinoma with brain metastases (BMs) in clinical practice. However, the specific mechanism is not yet fully understood. The present study aimed to investigate the effects of ES on blood vessels, tumor-associated macrophages (TAMs), and T cells in a tumor microenvironment treated with RT.

Methods

BM models were established by stereotactic and intracarotid injection of luciferase-Lewis lung cancer (LLC) cells into female C57BL mice. The animals were randomly divided into 4 groups: normal saline (NS), ES, RT, and ES plus radiotherapy (ES + RT) groups. Tumor size was determined with the IVIS imaging system. Tumor specimens were stained with CD34 and α-SMA to investigate tumor vascular changes. The proportions of TAMs, CD4⁺ T cells, and CD8⁺ T cells in tumor tissues were determined by flow cytometry and immunofluorescence. The expressions of hypoxia-inducible factor 1α (HIF-1α) and CXCR4 were deduced using western blotting and immunohistochemistry (IHC).

Results

ES + RT significantly suppressed tumor growth compared to the other 3 groups. RT decreased M1 and increased M2 in microglial cells and bone marrow-derived macrophages (BMDMs) relative to NS, while ES had the opposite effect. The ratio of CD8⁺T/CD4⁺T was increased in the ES + RT group compared to the other 3 groups. Tumor vascular maturity (α-SMA⁺/CD34⁺) was increased while HIF-1α was significantly suppressed in the ES + RT group. CXCR4 expression, which is involved in TAM recruitment, increased following RT, whereas, ES attenuated its expression.

Conclusions

Our findings suggest that ES can promote the normalization of tumor blood vessels and increase the anti-tumor immune-related immune cells infiltrating the tumor following RT treatment.

Actein Inhibits Tumor Growth and Metastasis in HER2-Positive Breast Tumor Bearing Mice via Suppressing AKT/mTOR and Ras/Raf/MAPK Signaling Pathways

HER2-positive breast cancer accounts for 15–20% in breast cancer and 50% of the metastatic HER2-positive breast cancer patients died of central nervous system progression. The present study investigated the effects of actein (a natural cycloartane triterpene) on cells adhesion, migration, proliferation and matrix degradation, and its underlying mechanism in HER2-positive breast cancer cells. The in vivo effect of actein on tumor growth and metastasis in MDA-MB-361 tumor-bearing mice as well as the anti-brain metastasis in tail vein injection mice model were also investigated. Our results showed that actein inhibited HER2-positive breast cancer cells viability, proliferation and migration. Actein also induced MDA-MB-361 cells G1 phase arrest and inhibited the expressions of cyclins and cyclin-dependent kinases. For intracellular mechanisms, actein inhibited the expressions of molecules in AKT/mTOR and Ras/Raf/MAPK signaling pathways. Furthermore, actein (15 mg/kg) was shown to exhibit anti-tumor and anti-metastatic activities in MDA-MB-361 breast tumor-bearing mice, and reduced brain metastasis in tail vein injection mice model. All these findings strongly suggested that actein is a potential anti-metastatic agent for HER2-positive breast cancer.

Cancer stem cell and mesenchymal cell cooperative actions in metastasis progression and hormone resistance in prostate cancer: Potential role of androgen and gonadotropin‑releasing hormone receptors (Review)

Prostate cancer (PCa) is the leading cause of male cancer‑associated mortality worldwide. Mortality is associated with metastasis and hormone resistance. Cellular, genetic and molecular mechanisms underlying metastatic progression and hormone resistance are poorly understood. Studies have investigated the local effects of gonadotropin‑releasing hormone (GnRH) analogs (used for androgen deprivation treatments) and the presence of the GnRH receptor (GnRH‑R) on PCa cells. Furthermore, cell subpopulations with stem‑like properties, or cancer stem cells, have been isolated and characterized using a cell culture system derived from explants of human prostate tumors. In addition, the development of preclinical orthotopic models of human PCa in a nonobese diabetic/severe combined immunodeficiency mouse model of compromised immunity has enabled the establishment of a reproducible system of metastatic progression in vivo. There is increasing evidence that metastasis is a complex process involving the cooperative actions of different cancer cell subpopulations, in which cancer stem‑like cells would be responsible for the final step of colonizing premetastatic niches. It has been hypothesized that PCa cells with stemness and mesenchymal signatures act cooperatively in metastatic progression and the inhibition of stemness genes, and that overexpression of androgen receptor (AR) and GnRH‑R decreases the rate the metastasis and sensitizes tumors to hormone therapy. The aim of the present review is to analyze the evidence regarding this cooperative process and the possible influence of stem‑like cell phenotypes, AR and GnRH‑R in metastatic progression and hormone resistance. These aspects may represent an important contribution in the understanding of the mechanisms underlying metastasis and hormone resistance in PCa, and potential routes to blocking these processes, enabling the development of novel therapies that would be particularly relevant for patients with metastatic and castration‑resistant PCa.

VCAM-1 targeted alpha-particle therapy for early brain metastases

BACKGROUND

Brain metastases (BM) develop frequently in patients with breast cancer. Despite the use of external beam radiotherapy (EBRT), the average overall survival is short (6 months from diagnosis). The therapeutic challenge is to deliver molecularly targeted therapy at an early stage when relatively few metastatic tumor cells have invaded the brain. Vascular cell adhesion molecule 1 (VCAM-1), overexpressed by nearby endothelial cells during the early stages of BM development, is a promising target. The aim of this study was to investigate the therapeutic value of targeted alpha-particle radiotherapy, combining lead-212 (212Pb) with an anti-VCAM-1 antibody (212Pb-αVCAM-1).

METHODS

Human breast carcinoma cells that metastasize to the brain, MDA-231-Br-GFP, were injected into the left cardiac ventricle of nude mice. Twenty-one days after injection, 212Pb-αVCAM-1 uptake in early BM was determined in a biodistribution study and systemic/brain toxicity was evaluated. Therapeutic efficacy was assessed using MR imaging and histology. Overall survival after 212Pb-αVCAM-1 treatment was compared with that observed after standard EBRT.

RESULTS

212Pb-αVCAM-1 was taken up into early BM with a tumor/healthy brain dose deposition ratio of 6 (5.52e108 and 0.92e108) disintegrations per gram of BM and healthy tissue, respectively. MRI analyses showed a statistically significant reduction in metastatic burden after 212Pb-αVCAM-1 treatment compared with EBRT (P < 0.001), translating to an increase in overall survival of 29% at 40 days post prescription (P < 0.01). No major toxicity was observed.

CONCLUSIONS

The present investigation demonstrates that 212Pb-αVCAM-1 specifically accumulates at sites of early BM causing tumor growth inhibition.

Novel Breast Cancer Brain Metastasis Patient-Derived Orthotopic Xenograft Model for Preclinical Studies

The vast majority of mortality in breast cancer results from distant metastasis. Brain metastases occur in as many as 30% of patients with advanced breast cancer, and the 1-year survival rate of these patients is around 20%. Pre-clinical animal models that reliably reflect the biology of breast cancer brain metastasis are needed to develop and test new treatments for this deadly condition. The patient-derived xenograft (PDX) model maintains many features of a donor tumor, such as intra-tumor heterogeneity, and permits the testing of individualized treatments. However, the establishment of orthotopic PDXs of brain metastasis is procedurally difficult. We have developed a method for generating such PDXs with high tumor engraftment and growth rates. Here, we describe this method and identify variables that affect its outcomes. We also compare the brain-orthotopic PDXs with ectopic PDXs grown in mammary pads of mice, and show that the responsiveness of PDXs to chemotherapeutic reagents can be dramatically affected by the site that they are in.

Supportive roles of brain macrophages in CNS metastases and assessment of new approaches targeting their functions

Metastases to the central nervous system (CNS) occur frequently in adults and their frequency increases with the prolonged survival of cancer patients. Patients with CNS metastases have short survival, and modern therapeutics, while effective for extra-cranial cancers, do not reduce metastatic burden. Tumor cells attract and reprogram stromal cells, including tumor-associated macrophages that support cancer growth by promoting tissue remodeling, invasion, immunosuppression and metastasis. Specific roles of brain resident and infiltrating macrophages in creating a pre-metastatic niche for CNS invading cancer cells are less known. There are populations of CNS resident innate immune cells such as: parenchymal microglia and non-parenchymal, CNS border-associated macrophages that colonize CNS in early development and sustain its homeostasis. In this study we summarize available data on potential roles of different brain macrophages in most common brain metastases. We hypothesize that metastatic cancer cells exploit CNS macrophages and their cytoprotective mechanisms to create a pre-metastatic niche and facilitate metastatic growth. We assess current pharmacological strategies to manipulate functions of brain macrophages and hypothesize on their potential use in a therapy of CNS metastases. We conclude that the current data strongly support a notion that microglia, as well as non-parenchymal macrophages and peripheral infiltrating macrophages, are involved in multiple stages of CNS metastases. Understanding their contribution will lead to development of new therapeutic strategies.

Evaluating Magnetic Resonance Spectroscopy as a Tool for Monitoring Therapeutic Response of Whole Brain Radiotherapy in a Mouse Model for Breast-to-Brain Metastasis

Brain metastases are the most common intracranial tumor in adults and are associated with poor patient prognosis and median survival of only a few months. Treatment options for brain metastasis patients remain limited and largely depend on surgical resection, radio- and/or chemotherapy. The development and pre-clinical testing of novel therapeutic strategies require reliable experimental models and diagnostic tools that closely mimic technologies that are used in the clinic and reflect histopathological and biochemical changes that distinguish tumor progression from therapeutic response. In this study, we sought to test the applicability of magnetic resonance (MR) spectroscopy in combination with MR imaging to closely monitor therapeutic efficacy in a breast-to-brain metastasis model. Given the importance of radiotherapy as the standard of care for the majority of brain metastases patients, we chose to monitor the post-irradiation response by magnetic resonance spectroscopy (MRS) in combination with MR imaging (MRI) using a 7 Tesla small animal scanner. Radiation was applied as whole brain radiotherapy (WBRT) using the image-guided Small Animal Radiation Research Platform (SARRP). Here we describe alterations in different metabolites, including creatine and N-acetylaspartate, that are characteristic for brain metastases progression and lactate, which indicates hypoxia, while choline levels remained stable. Radiotherapy resulted in normalization of metabolite levels indicating tumor stasis or regression in response to treatment. Our data indicate that the use of MR spectroscopy in addition to MRI represents a valuable tool to closely monitor not only volumetrical but also metabolic changes during tumor progression and to evaluate therapeutic efficacy of intervention strategies. Adapting the analytical technology in brain metastasis models to those used in clinical settings will increase the translational significance of experimental evaluation and thus contribute to the advancement of pre-clinical assessment of novel therapeutic strategies to improve treatment options for brain metastases patients.

Neoadjuvant neratinib promotes ferroptosis and inhibits brain metastasis in a novel syngeneic model of spontaneous HER2+ve breast cancer metastasis

Background

Human epidermal growth factor receptor-2 (HER2)-targeted therapies prolong survival in HER2-positive breast cancer patients. Benefit stems primarily from improved control of systemic disease, but up to 50% of patients progress to incurable brain metastases due to acquired resistance and/or limited permeability of inhibitors across the blood-brain barrier. Neratinib, a potent irreversible pan-tyrosine kinase inhibitor, prolongs disease-free survival in the extended adjuvant setting, and several trials evaluating its efficacy alone or combination with other inhibitors in early and advanced HER2-positive breast cancer patients are ongoing. However, its efficacy as a first-line therapy against HER2-positive breast cancer brain metastasis has not been fully explored, in part due to the lack of relevant pre-clinical models that faithfully recapitulate this disease. Here, we describe the development and characterisation of a novel syngeneic model of spontaneous HER2-positive breast cancer brain metastasis (TBCP-1) and its use to evaluate the efficacy and mechanism of action of neratinib.

Methods

TBCP-1 cells were derived from a spontaneous BALB/C mouse mammary tumour and characterised for hormone receptors and HER2 expression by flow cytometry, immunoblotting and immunohistochemistry. Neratinib was evaluated in vitro and in vivo in the metastatic and neoadjuvant setting. Its mechanism of action was examined by transcriptomic profiling, function inhibition assays and immunoblotting.

Results

TBCP-1 cells naturally express high levels of HER2 but lack expression of hormone receptors. TBCP-1 tumours maintain a HER2-positive phenotype in vivo and give rise to a high incidence of spontaneous and experimental metastases in the brain and other organs. Cell proliferation/viability in vitro is inhibited by neratinib and by other HER2 inhibitors, but not by anti-oestrogens, indicating phenotypic and functional similarities to human HER2-positive breast cancer. Mechanistically, neratinib promotes a non-apoptotic form of cell death termed ferroptosis. Importantly, metastasis assays demonstrate that neratinib potently inhibits tumour growth and metastasis, including to the brain, and prolongs survival, particularly when used as a neoadjuvant therapy.

Conclusions

The TBCP-1 model recapitulates the spontaneous spread of HER2-positive breast cancer to the brain seen in patients and provides a unique tool to identify novel therapeutics and biomarkers. Neratinib-induced ferroptosis provides new opportunities for therapeutic intervention. Further evaluation of neratinib neoadjuvant therapy is warranted.

Electronic supplementary material

The online version of this article (10.1186/s13058-019-1177-1) contains supplementary material, which is available to authorized users.

In Vivo Murine Models of Brain Metastasis

Metastases are the most common tumor type to affect the adult central nervous system. In vivo modeling of brain metastases provides insight into the mechanisms of metastatic development as well as a clinically relevant therapeutic screening platform. Here we describe the development of a novel mouse model of brain metastasis from a primary lung cancer utilizing primary patient samples. These models provide an accurate representation of different stages of the clinical progression of the disease.

A Blazing Landscape: Neuroinflammation Shapes Brain Metastasis

Brain metastases are more common than primary CNS tumors and confer grave prognosis on patients, as existing treatments have very limited efficacy. The tumor microenvironment has a central role in facilitating tumorigenesis and metastasis. In recent years, there has been much progress in our understanding of the functional role of the brain metastatic microenvironment. In this review, we discuss the latest advances in brain metastasis research, with special emphasis on the role of the brain microenvironment and neuroinflammation, integrating insights from comparable findings in neuropathologies and primary CNS tumors. In addition, we overview findings on the formation of a hospitable metastatic niche and point out the major gaps in knowledge toward developing new therapeutics that will cotarget the stromal compartment in an effort to improve the treatment and prevention of brain metastases.

Long-term characterization of activated microglia/macrophages facilitating the development of experimental brain metastasis through intravital microscopic imaging

Background

Microglia/macrophages (M/Ms) with multiple functions derived from distinct activation states are key surveillants maintaining brain homeostasis. However, their activation status and role during the brain metastasis of malignant tumors have been poorly characterized.

Methods

Heterozygous CX3CR1-GFP transgenic mice were used to visualize the dynamic changes of M/Ms during the development of experimental brain metastasis through long-term intravital imaging equipped with redesigned bilateral cranial windows. The occurrence of experimental brain metastasis was evaluated after M/Ms were depleted with PLX3397, a CSF-1R inhibitor. The possible mediators of M/Ms in facilitating the brain metastasis were determined using reverse transcription-PCR, immunofluorescence, correlational analysis, and MMP inhibition.

Results

Here, we showed that M/Ms were persistently activated and facilitated the formation of melanoma brain metastasis in vivo. We observed that M/Ms gradually and massively accumulated in the metastasis, with a 2.89-fold increase. To precisely depict the dynamic changes in the activation state of M/Ms, we defined the branching parameter to quantify their morphological alterations. The quantitative data showed that the extent of activation of M/Ms in metastatic foci was enhanced, with a 2.27-fold increase from day 1 to day 21. Along with the activation, the M/Ms increased their moving velocity (4.15-fold) and established a rapid, confined, and discontinuous motility behavior. The occurrence of melanoma brain metastasis was significantly hindered under M/M elimination, indicating the key role of M/Ms in the experimental brain metastasis. Interestingly, we found that M/Ms highly expressed matrix metalloproteinase 3 (MMP3), which were strongly correlated with M/M activation and the decrease of tight junction protein zonula occludens-1 (ZO-1). An MMP inhibitor moderately decreased the occurrence of melanoma brain metastasis, suggesting that MMP3 secreted by M/Ms may facilitate melanoma cell growth.

Conclusions

Our results indicated that the activated M/Ms were essential in the development of melanoma brain metastasis, suggesting that M/Ms are a potential therapeutic target for tumor brain metastasis.

Electronic supplementary material

The online version of this article (10.1186/s12974-018-1389-9) contains supplementary material, which is available to authorized users.

Method of establishing breast cancer brain metastases affects brain uptake and efficacy of targeted, therapeutic nanoparticles

HER2-targeted therapies effectively control systemic disease, but their efficacy against brain metastases is hindered by their low penetration of the blood-brain and blood-tumor barriers (BBB and BTB). We investigate brain uptake and antitumor efficacy of transferrin receptor (TfR)-targeted, therapeutic nanoparticles designed to transcytose the BBB/BTB in three murine models. Two known models involving intracranial (IC) or intracardiac (ICD) injection of human breast cancer cells were employed, as was a third model developed here involving intravenous (IV) injection of the cells to form whole-body tumors that eventually metastasize to the brain. We show the method of establishing brain metastases significantly affects therapeutic BBB/BTB penetration. Free drug accumulates and delays growth in IC- and ICD-formed brain tumors, while non-targeted nanoparticles show uptake and inhibition only in IC-established metastases. TfR-targeted nanoparticles accumulate and significantly delay growth in all three models, suggesting the IV model maintains a more intact BBB/BTB than the other models.

Species-dependent extracranial manifestations of a brain seeking breast cancer cell line

PURPOSE

Metastatic brain tumors pose a severe problem in the treatment of patients with breast carcinoma. Preclinical models have been shown to play an important role in unraveling the underlying mechanisms behind the metastatic process and evaluation of new therapeutic approaches. As the size of the rat brain allows improved in vivo imaging, we attempted to establish a rat model for breast cancer brain metastasis that allows follow-up by 7 tesla (7T) preclinical Magnetic Resonance Imaging (MRI).

PROCEDURES

Green fluorescent protein-transduced (eGFP) MDA-MB-231br breast cancer cells were labeled with micron-sized particles of iron oxide (MPIOs) and intracardially injected in the left ventricle of female nude rats and mice. 7T preclinical MRI was performed to show the initial distribution of MPIO-labeled cancer cells and to visualize metastasis in the brain. Occurrence of potential metastasis outside the brain was evaluated by 2-deoxy-2-[18F]fluoro-D-glucose ([18F]FDG) positron emission tomography (PET)/computed tomography (CT) and potential bone lesions were assessed using [18F]sodium fluoride ([18F]NaF) PET/CT.

RESULTS

The first signs of brain metastasis development were visible as hyperintensities on T2-weighted (T2w) MR images acquired 3 weeks after intracardiac injection in rats and mice. Early formation of unexpected bone metastasis in rats was clinically observed and assessed using PET/CT. Almost no bone metastasis development was observed in mice after PET/CT evaluation.

CONCLUSIONS

Our results suggest that the metastatic propensity of the MDA-MB-231br/eGFP cancer cell line outside the brain is species-dependent. Because of early and abundant formation of bone metastasis with the MDA-MB-231br/eGFP cancer cell line, this rat model is currently not suitable for investigating brain metastasis as a single disease model nor for evaluation of novel brain metastasis treatment strategies.

Clinical trial design for systemic agents in patients with brain metastases from solid tumours: a guideline by the Response Assessment in Neuro-Oncology Brain Metastases working group

Patients with active CNS disease are often excluded from clinical trials, and data regarding the CNS efficacy of systemic agents are usually obtained late in the drug development process or not at all. In this guideline from the Response Assessment in Neuro-Oncology Brain Metastases (RANO-BM) working group, we provide detailed recommendations on when patients with brain metastases from solid tumours should be included or excluded in clinical trials of systemic agents. We also discuss the limitations of retrospective studies in determining the CNS efficacy of systemic drugs. Inclusion of patients with brain metastases early on in the clinical development of a drug or a regimen is needed to generate appropriate CNS efficacy or non-efficacy signals. We consider how to optimally incorporate or exclude such patients in systemic therapy trials depending on the likelihood of CNS activity of the agent by considering three scenarios: drugs that are considered very unlikely to have CNS antitumour activity or efficacy; drugs that are considered very likely to have CNS activity or efficacy; and drugs with minimal baseline information on CNS activity or efficacy. We also address trial design issues unique to patients with brain metastases, including the selection of appropriate CNS endpoints in systemic therapy trials.

Strain-specific metastatic phenotypes in pheochromocytoma allograft mice

Somatostatin receptor-targeting endoradiotherapy offers potential for treating metastatic pheochromocytomas and paragangliomas, an approach likely to benefit from combination radiosensitization therapy. To provide reliable preclinical in vivo models of metastatic disease, this study characterized the metastatic spread of luciferase-expressing mouse pheochromocytoma (MPC) cells in mouse strains with different immunologic conditions. Bioluminescence imaging showed that, in contrast to subcutaneous non-metastatic engraftment of luciferase-expressing MPC cells in NMRI-nude mice, intravenous cell injection provided only suboptimal metastatic spread in both NMRI-nude mice and hairless SCID (SHO) mice. Treatment of NMRI-nude mice with anti-Asialo GM1 serum enhanced metastatic spread due to substantial depletion of natural killer (NK) cells. However, reproducible metastatic spread was only observed in NK cell-defective SCID/beige mice and in hairless immunocompetent SKH1 mice bearing disseminated or liver metastases, respectively. Liquid chromatography tandem mass spectrometry of urine samples showed that subcutaneous and metastasized tumor models exhibit comparable renal monoamine excretion profiles characterized by increasing urinary dopamine, 3-methoxytyramine, norepinephrine and normetanephrine. Metastases-related epinephrine and metanephrine were only detectable in SCID/beige mice. Positron emission tomography and immunohistochemistry revealed that all metastases maintained somatostatin receptor-specific radiotracer uptake and immunoreactivity, respectively. In conclusion, we demonstrate that intravenous injection of luciferase-expressing MPC cells into SCID/beige and SKH1 mice provides reproducible and clinically relevant spread of catecholamine-producing and somatostatin receptor-positive metastases. These standardized preclinical models allow for precise monitoring of disease progression and should facilitate further investigations on theranostic approaches against metastatic pheochromocytomas and paragangliomas.

Identification of brain metastasis genes and therapeutic evaluation of histone deacetylase inhibitors in a clinically relevant model of breast cancer brain metastasis

Breast cancer brain metastases remain largely incurable. Although several mouse models have been developed to investigate the genes and mechanisms regulating breast cancer brain metastasis, these models often lack clinical relevance since they require the use of immunocompromised mice and/or are poorly metastatic to brain from the mammary gland. We describe the development and characterisation of an aggressive brain metastatic variant of the 4T1 syngeneic model (4T1Br4) that spontaneously metastasises to multiple organs, but is selectively more metastatic to the brain from the mammary gland than parental 4T1 tumours. As seen by immunohistochemistry, 4T1Br4 tumours and brain metastases display a triple-negative phenotype, consistent with the high propensity of this breast cancer subtype to spread to brain. In vitro assays indicate that 4T1Br4 cells have an enhanced ability to adhere to or migrate across a brain-derived endothelial monolayer and greater invasive response to brain-derived soluble factors compared to 4T1 cells. These properties are likely to contribute to the brain selectivity of 4T1Br4 tumours. Expression profiling and gene set enrichment analyses demonstrate the clinical relevance of the 4T1Br4 model at the transcriptomic level. Pathway analyses implicate tumour-intrinsic immune regulation and vascular interactions in successful brain colonisation, revealing potential therapeutic targets. Evaluation of two histone deacetylase inhibitors, SB939 and 1179.4b, shows partial efficacy against 4T1Br4 metastasis to brain and other sites in vivo, and potent radio-sensitising properties in vitro. The 4T1Br4 model provides a clinically relevant tool for mechanistic studies and to evaluate novel therapies against brain metastasis.

This article has an associated First Person interview with Soo-Hyun Kim, joint first author of the paper.

Summary: The authors introduce a new syngeneic mouse model of spontaneous breast cancer brain metastasis, demonstrate its phenotypic, functional and transcriptomic relevance to human TNBC brain metastasis, and test novel therapies.

Cranial irradiation increases tumor growth in experimental breast cancer brain metastasis

Whole-brain radiotherapy is the standard of care for patients with breast cancer with multiple brain metastases and, although this treatment has been essential in the management of existing brain tumors, there are many known negative consequences associated with the irradiation of normal brain tissue. In our study, we used in vivo magnetic resonance imaging analysis to investigate the influence of radiotherapy-induced damage of healthy brain on the arrest and growth of metastatic breast cancer cells in a mouse model of breast cancer brain metastasis. We observed that irradiated, but otherwise healthy, neural tissue had an increased propensity to support metastatic growth compared with never-irradiated controls. The elucidation of the impact of irradiation on normal neural tissue could have implications in clinical patient management, particularly in patients with residual systemic disease or with residual radio-resistant brain cancer.

Hacking macrophage-associated immunosuppression for regulating glioblastoma angiogenesis

Glioblastoma (GBM) is the most lethal primary adult brain tumor and its pathology is hallmarked by distorted neovascularization, diffuse tumor-associated macrophage infiltration, and potent immunosuppression. Reconstituting organotypic tumor angiogenesis models with biomimetic cell heterogeneity and interactions, pro-/anti-inflammatory milieu and extracellular matrix (ECM) mechanics is critical for preclinical anti-angiogenic therapeutic screening. However, current in vitro systems do not accurately mirror in vivo human brain tumor microenvironment. Here, we engineered a three-dimensional (3D), microfluidic angiogenesis model with controllable and biomimetic immunosuppressive conditions, immune-vascular and cell-matrix interactions. We demonstrate in vitro, GL261 and CT-2A GBM-like tumors steer macrophage polarization towards a M2-like phenotype for fostering an immunosuppressive and proangiogenic niche, which is consistent with human brain tumors. We distinguished that GBM and M2-like immunosuppressive macrophages promote angiogenesis, while M1-like pro-inflammatory macrophages suppress angiogenesis, which we coin "inflammation-driven angiogenesis." We observed soluble immunosuppressive cytokines, predominantly TGF-β1, and surface integrin (α_vβ₃) endothelial-macrophage interactions are required in inflammation-driven angiogenesis. We demonstrated tuning cell-adhesion receptors using an integrin (α_vβ₃)-specific collagen hydrogel regulated inflammation-driven angiogenesis through Src-PI3K-YAP signaling, highlighting the importance of altered cell-ECM interactions in inflammation. To validate the preclinical applications of our 3D organoid model and mechanistic findings of inflammation-driven angiogenesis, we screened a novel dual integrin (α_vβ₃) and cytokine receptor (TGFβ-R1) blockade that suppresses GBM tumor neovascularization by simultaneously targeting macrophage-associated immunosuppression, endothelial-macrophage interactions, and altered ECM. Hence, we provide an interactive and controllable GBM tumor microenvironment and highlight the importance of macrophage-associated immunosuppression in GBM angiogenesis, paving a new direction of screening novel anti-angiogenic therapies.

Emerging tumor spheroids technologies for 3D in vitro cancer modeling

Cancer is a leading cause of mortality and morbidity worldwide. Around 90% of deaths are caused by metastasis and just 10% by primary tumor. The advancement of treatment approaches is not at the same rhythm of the disease; making cancer a focal target of biomedical research. To enhance the understanding and prompts the therapeutic delivery; concepts of tissue engineering are applied in the development of in vitro models that can bridge between 2D cell culture and animal models, mimicking tissue microenvironment. Tumor spheroid represents highly suitable 3D organoid-like framework elucidating the intra and inter cellular signaling of cancer, like that formed in physiological niche. However, spheroids are of limited value in studying critical biological phenomenon such as tumor-stroma interactions involving extra cellular matrix or immune system. Therefore, a compelling need of tailoring spheroid technologies with physiologically relevant biomaterials or in silico models, is ever emerging. The diagnostic and prognostic role of spheroids rearrangements within biomaterials or microfluidic channel is indicative of patient management; particularly for the decision of targeted therapy. Fragmented information on available in vitro spheroid models and lack of critical analysis on transformation aspects of these strategies; pushes the urge to comprehensively overview the recent technological advancements (e.g. bioprinting, micro-fluidic technologies or use of biomaterials to attain the third dimension) in the shed of translationable cancer research. In present article, relationships between current models and their possible exploitation in clinical success is explored with the highlight of existing challenges in defining therapeutic targets and screening of drug efficacy.

Preclinical Modeling and Therapeutic Avenues for Cancer Metastasis to the Central Nervous System

Metastasis is the dissemination of cells from the primary tumor to other locations within the body, and continues to be the predominant cause of death among cancer patients. Metastatic progression within the adult central nervous system is 10 times more frequent than primary brain tumors. Metastases affecting the brain parenchyma and leptomeninges are associated with grave prognosis, and even after successful control of the primary tumor the median survival is a dismal 2-3 months with treatment options typically limited to palliative care. Current treatment options for brain metastases (BM) and disseminated brain tumors are scarce, and the improvement of novel targeted therapies requires a broader understanding of the biological complexity that characterizes metastatic progression. In this review, we provide insight into patterns of BM progression and leptomeningeal spread, outlining the development of clinically relevant in vivo models and their contribution to the discovery of innovative cancer therapies. In vivo models paired with manipulation of in vitro methods have expanded the tools available for investigators to develop agents that can be used to prevent or treat metastatic disease. The knowledge gained from the use of such models can ultimately lead to the prevention of metastatic dissemination and can extend patient survival by transforming a uniformly fatal systemic disease into a locally controlled and eminently more treatable one.

Mouse Models of Brain Metastasis for Unravelling Tumour Progression

Secondary tumours in the brain account for 40 % of triple negative breast cancer patients, and the percentage may be higher at the time of autopsy. The use of in vivo models allow us to recapitulate the molecular mechanisms potentially used by circulating breast tumour cells to proliferate within the brain.Metastasis is a multistep process that depends on the success of several stages including cell evasion from the primary tumour, distribution and survival within the blood stream and cerebral microvasculature, penetration of the blood-brain barrier and proliferation within the brain microenvironment. Cellular adhesion molecules are key proteins involved in all of the steps in the metastatic process. Our group has developed two different in vivo models to encompass both seeding and colonisation stages of the metastatic process: (1) haematogenous dissemination of tumour cells by direct injection into the left ventricle of the heart, and (2) direct implantation of the tumour cells into the mouse brain.This chapter describes, in detail, the practical implementation of the intracerebral model, which can be used to analyse tumour proliferation within a specific area of the central nervous system and tumour-host cell interactions. We also describe the use of immunohistochemistry techniques to identify, at the molecular scale, tumour-host cell interactions, which may open new windows for brain metastasis therapy.

Quality Assessment of Stereotactic Radiosurgery of a Melanoma Brain Metastases Model Using a Mouselike Phantom and the Small Animal Radiation Research Platform

PURPOSE

To establish a novel preclinical model for stereotactic radiosurgery (SRS) with combined mouselike phantom quality assurance in the setting of brain metastases.

METHODS AND MATERIALS

C57B6 mice underwent intracranial injection of B16-F10 melanoma cells. T1-weighted postcontrast magnetic resonance imaging (MRI) was performed on day 11 after injection. The MRI images were fused with cone beam computed tomography (CBCT) images using the Small Animal Radiation Research Platform (SARRP). The gross tumor volume (GTV) was contoured using the MRI. A single sagittal arc using the 3 × 3 mm² collimator was used to deliver 18 Gy prescribed to the isocenter. MRI was performed 7 days after radiation treatment, and the dose delivered to the mice was confirmed using 2 mouselike anthropomorphic phantoms: 1 in the axial orientation and the other in the sagittal orientation. The SARRP output was measured using a PTW Farmer type ionization chamber as per the American Association of Physicists in Medicine Task Group report 61, and the H-D curve was generated up to a maximum dose of 30 Gy. Irradiated films were analyzed based on optical density distribution and H-D curve.

RESULTS

The tumor volume on day 11, before intervention, was 2.48 ± 1.37 mm³ in the no-SRS arm versus 3.75 ± 1.19 mm³ in the SRS arm (NS). In the SRS arm, GTV maximum dose (Dmax) and mean dose were 2048 ± 207 and 1785 ± 14 cGy. Using the mouselike phantoms, the radiochromic film showed close precision in comparison with projected isodose lines, with a Dmax of 1903.4 and 1972.7 cGy, the axial and sagittal phantoms, respectively. Tumor volume 7 days after treatment was 7.34 ± 8.24 mm³ in the SRS arm and 60.20 ± 40.4 mm³ in the no-SRS arm (P=.009). No mice in the control group survived more than 22 days after implantation, with a median overall survival (mOS) of 19 days; mOS was not reached in the SRS group, with 1 death noted.

CONCLUSIONS

Single-fraction SRS of 18 Gy delivered in a single arc can be delivered accurately with MRI T1-weighted postcontrast-based treatment planning. The mouse like phantom allows for verification of dose delivery and accuracy.

Maintaining unperturbed cerebral blood flow is key in the study of brain metastasis and its interactions with stress and inflammatory responses

Blood-borne brain metastases are associated with poor prognosis, but little is known about the interplay between cerebral blood flow, surgical stress responses, and the metastatic process. The intra-carotid inoculation approach, traditionally used in animal studies, involves permanent occlusion of the common carotid artery (CCA). Herein we introduced a novel intra-carotid inoculation approach that avoids CCA ligation, namely - assisted external carotid artery inoculation (aECAi) - and compared it to the traditional approach in C57/BL6 mice, assessing cerebral blood flow; particle distribution; blood-brain barrier (BBB) integrity; stress, inflammatory and immune responses; and brain tumor retention and growth. Doppler flowmetry and two-photon imaging confirmed that only in the traditional approach regional and capillary cerebral blood flux were significantly reduced. Corticosterone and plasma IL-6 levels were higher in the traditional approach, splenic numbers of NK, CD3+, granulocytes, and dendritic cells were lower, and many of these indices were more profoundly affected by surgical stress in the traditional approach. BBB integrity was unaffected. Administration of spherical beads indicated that CCA ligation significantly limited brain distribution of injected particles, and inoculation of D122-LLC syngeneic tumor cells resulted in 10-fold lower brain tumor-cell retention in the traditional approach. Last, while most of the injected tumor cells were arrested in extra-cranial head areas, our method improved targeting of brain-tissue by 7-fold. This head versus brain distribution difference, commonly overlooked, cannot be detected using in vivo bioluminescent imaging. Overall, it is crucial to maintain unperturbed cerebral blood flow while studying brain metastasis and interactions with stress and inflammatory responses.

In vivo magnetic resonance imaging investigating the development of experimental brain metastases due to triple negative breast cancer

Triple negative breast cancer (TNBC), when associated with poor outcome, is aggressive in nature with a high incidence of brain metastasis and the shortest median overall patient survival after brain metastasis development compared to all other breast cancer subtypes. As therapies that control primary cancer and extracranial metastatic sites improve, the incidence of brain metastases is increasing and the management of patients with breast cancer brain metastases continues to be a significant clinical challenge. Mouse models have been developed to permit in depth evaluation of breast cancer metastasis to the brain. In this study, we compare the efficiency and metastatic potential of two experimental mouse models of TNBC. Longitudinal MRI analysis and end point histology were used to quantify initial cell arrest as well as the number and volume of metastases that developed in mouse brain over time. We showed significant differences in MRI appearance, tumor progression and model efficiency between the syngeneic 4T1-BR5 model and the xenogeneic 231-BR model. Since TNBC does not respond to many standard breast cancer treatments and TNBC brain metastases lack effective targeted therapies, these preclinical TNBC models represent invaluable tools for the assessment of novel systemic therapeutic approaches. Further pursuits of therapeutics designed to bypass the blood tumor barrier and permit access to the brain parenchyma and metastatic cells within the brain will be paramount in the fight to control and treat lethal metastatic cancer.

Advances in decoding breast cancer brain metastasis

The past decade has witnessed impressive advances in cancer treatment ushered in by targeted and immunotherapies. However, with significantly prolonged survival, upon recurrence, more patients become inflicted by brain metastasis, which is mostly refractory to all currently available therapeutic regimens. Historically, brain metastasis is an understudied area in cancer research, partly due to the dearth of appropriate experimental models that closely simulate the special biological features of metastasis in the unique brain environment and to the sophistication of techniques required to perform in-depth studies of the extremely complex and challenging brain metastasis. Yet, with increasing clinical demand for more effective treatment options, brain metastasis research has rapidly advanced in recent years. The present review spotlights the recent major progresses in basic and translational studies of brain metastasis with focuses on new animal models, novel imaging technologies, omics "big data" resources, and some new and exciting biological insights on brain metastasis.