Preclinical approaches to study the biology and treatment of brain metastases

Brain metastases from breast cancer using magnetic resonance imaging: A systematic review

Despite improvements in imaging and treatment approaches, brain metastases (BMs) continue to be the primary cause of mortality and morbidity in about 20% of adult cancer patients. This research aimed to review the magnetic resonance imaging (MRI) and clinical characteristics of BMs resulting from breast cancer (BC). A systematic review of original research articles published from January 2000 to June 2023. We selected studies that reported MRI findings of BMs in BC patients. We excluded reviews, case reports, books/book chapters, animal studies and irrelevant records. We identified 24 studies that included 1580 BC patients with BMs. T1-weighted (T1-w) (pre- and postcontrast), T2-weighted (T2-w), fluid-attenuated inversion recovery (FLAIR) and T2*-weighted (T2*-w) was used to measure the lesion size, shape and area. In other studies, advanced structural techniques including diffusion-weighted imaging (DWI), diffusion tensor imaging (DTI) and susceptibility-weighted imaging (SWI) were used to more precisely and sensitively evaluate the pathological area. Furthermore, functional and metabolic techniques like functional MRI (fMRI), magnetic resonance spectroscopy (MRS) and perfusion-weighted imaging (PWI) have also been utilised. The MRI findings of BMs varied depending on the MRI technique, the BC subtype, the lesion size and shape, the presence of haemorrhage or necrosis and the comparison with other brain tumours. Some MRI findings were associated with prognosis, recurrence or cognitive impairment in BC patients with BMs. MRI detects, characterises and monitors BMs from BC. Findings vary by MRI technique, BC subtype, lesion characteristics and comparison with other brain tumours. More research should validate emerging MRI techniques, determine the clinical implications of findings and explore the underlying mechanisms and biology of BMs from BC. MRI is a valuable tool for diagnosis, targeted therapy and studying BC metastasis.

Brain metastasis-associated fibroblasts secrete fucosylated PVR/CD155 that induces breast cancer invasion

Brain metastasis cancer-associated fibroblasts (bmCAFs) are emerging as crucial players in the development of breast cancer brain metastasis (BCBM), but our understanding of the underlying molecular mechanisms is limited. In this study, we aim to elucidate the pathological contributions of fucosylation (the post-translational modification of proteins by the dietary sugar L-fucose) to tumor-stromal interactions that drive the development of BCBM. Here, we report that patient-derived bmCAFs secrete high levels of polio virus receptor (PVR), which enhance the invasive capacity of BC cells. Mechanistically, we find that HIF1α transcriptionally upregulates fucosyltransferase 11, which fucosylates PVR, triggering its secretion from bmCAFs. Global phosphoproteomic analysis of BC cells followed by functional verification identifies cell-cell junction and actin cytoskeletal signaling as modulated by bmCAF-secreted, -fucosylated PVR. Our findings delineate a hypoxia- and fucosylation-regulated mechanism by which bmCAFs contribute to the invasiveness of BCBM in the brain.

ATM-Inhibitor AZD1390 Is a Radiosensitizer for Breast Cancer CNS Metastasis

PURPOSE

Limited effective treatments are currently available for central nervous system (CNS) metastasis (CM). This is largely driven by the inability of current therapeutics to penetrate the blood brain barrier (BBB) and the lack of preclinical models for testing new therapies. Here we study the efficacy of AZD1390, a BBB penetrating ataxia-telangiectasia mutated inhibitor, as a radiosensitizer for breast cancer CM treatment.

EXPERIMENTAL DESIGN

Three patient-derived xenograft (PDX) tumors including 2 HER2+ and 1 triple-negative breast cancer harboring DNA damage response (DDR) gene mutations, were implanted subcutaneously in the flank of mice to assess tumor growth inhibition by AZD1390 combined with radiation. Animal survival was further assessed by implanting the best responding PDX model orthotopically in the brain.

RESULTS

Pretreatment with AZD1390 followed by radiation therapy inhibited growth of PDX tumors implanted in the flank, and improved survival in orthotopic models with average survival of 222 days compared with 123 days in controls. Administration of AZD1390 posttreatment for 21 days had no further benefits. While the combination therapy resulted in sustained tumor inhibition, sporadic regrowth was observed in some mice 50 to 100 days posttreatment in all models. Gene expression comparing these tumors with complete responders demonstrated changes in upregulation of oncogenic proteins, which are potential drivers of tumor growth after treatment.

CONCLUSIONS

Our results demonstrate that AZD1390 effectively sensitizes breast cancer CM to radiation therapy in DDR mutant tumors. This study demonstrates the potential of using AZD1390 as a novel therapeutic agent for patients with breast cancer CM.

Insights into the Molecular Mechanisms Mediating Extravasation in Brain Metastasis of Breast Cancer, Melanoma, and Lung Cancer

Brain metastasis is an incurable end-stage of systemic cancer associated with poor prognosis, and its incidence is increasing. Brain metastasis occurs through a multi-step cascade where cancer cells spread from the primary tumor site to the brain. The extravasation of tumor cells through the blood-brain barrier (BBB) is a critical step in brain metastasis. During extravasation, circulating cancer cells roll along the brain endothelium (BE), adhere to it, then induce alterations in the endothelial barrier to transmigrate through the BBB and enter the brain. Rolling and adhesion are generally mediated by selectins and adhesion molecules induced by inflammatory mediators, while alterations in the endothelial barrier are mediated by proteolytic enzymes, including matrix metalloproteinase, and the transmigration step mediated by factors, including chemokines. However, the molecular mechanisms mediating extravasation are not yet fully understood. A better understanding of these mechanisms is essential as it may serve as the basis for the development of therapeutic strategies for the prevention or treatment of brain metastases. In this review, we summarize the molecular events that occur during the extravasation of cancer cells through the blood-brain barrier in three types of cancer most likely to develop brain metastasis: breast cancer, melanoma, and lung cancer. Common molecular mechanisms driving extravasation in these different tumors are discussed.

Immunotherapy as a Turning Point in the Treatment of Melanoma Brain Metastases

The incidence of tumor metastases in the brain is many times more frequent than primary brain tumors, affecting a very large share of patients suffering from systemic cancer. Advanced malignant melanoma is well known for its ability to invade the brain space and current treatment options, such as surgery and radiation therapy, are not very efficient and cause notable complications and morbidity. The aim of this review is to explore the recent advances and future potential of using immunotherapy in the treatment of melanoma brain metastases. Several FDA approved immunotherapeutic drugs have shown to be able to at least double the overall survival rates in such patients. Clinical trials of varying phases are underway and available results are promising, significantly prolonging survival rates in patients with previously untreated melanoma brain metastases. Nevertheless, not all patients respond to these immunotherapies, facing a high percentage of resistant cases, without yet knowing the mechanisms and causes of resistance behind. Also, at the time of immunotherapy, a small percentage of patients is affected by pseudoprogression, which can be difficult to distinguish from true progression given the similarity of symptoms. Therefore, there is a pressing need for future research about treatment effectiveness in patients with brain metastases from melanoma, including outcomes from the perspective of patients.

Brain Vascular Microenvironments in Cancer Metastasis

Primary tumours, particularly from major solid organs, are able to disseminate into the blood and lymphatic system and spread to distant sites. These secondary metastases to other major organs are the most lethal aspect of cancer, accounting for the majority of cancer deaths. The brain is a frequent site of metastasis, and brain metastases are often fatal due to the critical role of the nervous system and the limited options for treatment, including surgery. This creates a need to further understand the complex cell and molecular biology associated with the establishment of brain metastasis, including the changes to the environment of the brain to enable the arrival and growth of tumour cells. Local changes in the vascular network, immune system and stromal components all have the potential to recruit and foster metastatic tumour cells. This review summarises our current understanding of brain vascular microenvironments, fluid circulation and drainage in the context of brain metastases, as well as commenting on current cutting-edge experimental approaches used to investigate changes in vascular environments and alterations in specialised subsets of blood and lymphatic vessel cells during cancer spread to the brain.

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.

Clinical diagnosis and treatment of breast cancer with brain metastases and establishment of a prognostic model: a 10-year, single-center, real-world study of 559 cases

Background

This study involved a retrospective analysis of 559 metastatic breast cancer (MBC) patients with brain metastasis (BM). We aimed to establish the effectiveness of different preferred treatment methods and factors affecting overall survival following BM diagnosis (BMOS) and explore the feasibility of systemic treatment for MBC patients with BM.

Methods

Univariate and multivariate analyses were used to assess the efficacy of different preferred treatments and other factors associated with BMOS, and a nomogram was then established based on the results of the univariate analysis.

Results

Patients that initially received systemic drug therapy exhibited a clinical benefit rate (CBR) of 43.9% and an intracranial disease control rate (DCR) of 80.6%. The median time between BM diagnosis and the requirement for local intracranial treatment due to worsening disease status was 10.0 months for these patients (95% CI: 7.811–12.189 months). The median follow-up was 28.0 months, and the median BMOS was 16.0 months. Following BM diagnosis, the systemic drug treatment group had a better outcome than the local brain treatment group, with a respective median BMOS of 22.0 and 16.0 months (χ²=7.743, P=0.005). At the time of BM diagnosis, the median BMOS for patients without neurological symptoms diagnosed by regular screen was significantly longer than that of patients with neurological symptoms (18.0 vs. 13.0 months, respectively; χ²=11.371, P=0.001). Based on these analyses, a nomogram was constructed that incorporated disease-free survival (DFS), Karnofsky performance status (KPS), molecular subtype, number of extracranial metastases, BM location, number of BMs, neurological symptoms, and the preferred treatment approach, with a prediction probability (c-index) value of 0.76.

Conclusions

Systemic drug treatment has a beneficial effect on brain lesions, and effective treatment delays the need for local intracranial treatment. Cranial magnetic resonance imaging (MRI) screening can detect asymptomatic BM in MBC patients (particularly those with HER2−positive or triple-negative disease), offering these patients an opportunity to undergo systemic drug therapy, thereby prolonging their survival. To our knowledge, this is a well-fitted nomogram including current treatment and medical examination strategies to predict BMOS probability that offers value as an adjunct for the prognostic evaluation of MBC-BM patients.

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.

Brain metastasis with subtype conversion in a patient with male breast cancer: A case report

RATIONALE

Brain metastasis of male breast cancer is extremely rare, and the pathological changes between the primary tumor and the metastatic brain tumor have not been reported. Herein, we report for the first time a case of male breast cancer with metastasis to the parietal lobe with subtype conversion after metastasis.

PATIENT CONCERNS

we describe a 45-year-old male patient admitted for an incidentally found brain tumor after a motorcycle accident. The patient had been treated for breast cancer 5 years previously. The primary tumor was an invasive ductal carcinoma classified as pT1N1M0 with hormone receptor positivity (estrogen receptor ++, progesterone receptor +++, human epidermal growth factor receptor-type2 (HER2) +) and was treated with surgery, adjuvant chemotherapy, radiation therapy and endocrine therapy (tamoxifen).

DIAGNOSES

Magnetic resonance imaging revealed a well enhanced focal solid tumor in the right parietal lobe (5.0 × 4.2 cm in size), Immunohistochemical staining revealed cerebral metastases of breast cancer with HER2 subtype conversion (estrogen receptor +++, progesterone receptor +++, HER2 -).

INTERVENTIONS

The patient was successfully treated with surgery and whole brain irradiation (3 Gy × 10 fractions).

OUTCOMES

There was no additional complication after the surgery and the patient transferred to oncology department for chemotherapy. 2 years later, he had gamma knife radiosurgery due to the recurred brain lesion and after that he discontinued the treatment and opted for hospice care.

LESSONS

Male breast cancer with metastasis to the brain is an extremely rare condition. Although a few similar cases have been reported, subtype conversion in similar cases has not been reported. Therefore, we report this case of a male patient with brain metastasis of invasive ductal carcinoma with HER2 status conversion after metastasis.

Patient-derived xenografts of central nervous system metastasis reveal expansion of aggressive minor clones

BACKGROUND

The dearth of relevant tumor models reflecting the heterogeneity of human central nervous system metastasis (CM) has hindered development of novel therapies.

METHODS

We established 39 CM patient-derived xenograft (PDX) models representing the histological spectrum, and performed phenotypic and multi-omic characterization of PDXs and their original patient tumors. PDX clonal evolution was also reconstructed using allele-specific copy number and somatic variants.

RESULTS

PDXs retained their metastatic potential, with flank-implanted PDXs forming spontaneous metastases in multiple organs, including brain, and CM subsequent to intracardiac injection. PDXs also retained the histological and molecular profiles of the original patient tumors, including retention of genomic aberrations and signaling pathways. Novel modes of clonal evolution involving rapid expansion by a minor clone were identified in 2 PDXs, including CM13, which was highly aggressive in vivo forming multiple spontaneous metastases, including to brain. These PDXs had little molecular resemblance to the patient donor tumor, including reversion to a copy number neutral genome, no shared nonsynonymous mutations, and no correlation by gene expression.

CONCLUSIONS

We generated a diverse and novel repertoire of PDXs that provides a new set of tools to enhance our knowledge of CM biology and improve preclinical testing. Furthermore, our study suggests that minor clone succession may confer tumor aggressiveness and potentiate brain metastasis.

High-resolution MRI demonstrates that more than 90% of small intracranial melanoma metastases develop in close relationship to the leptomeninges

BACKGROUND

Despite classic teaching that intracranial metastases typically arise at the gray-white matter junction, small intracranial melanoma metastases (IMM) are frequently observed at the interface between the cortex and leptomeninges (ie, "corticomeningeal interface"), suggesting possible leptomeningeal origin.

METHODS

MRI brain examinations of melanoma patients treated at a specialist oncology center from July 2015 to June 2017 were retrospectively reviewed. The MRI examination on which IMM were first visible was identified, utilizing 1 mm volumetric postcontrast imaging prior to local therapy. Individual metastases (up to 10 per patient) were assessed for the presence of leptomeningeal contact, as well as their number, size, and morphology. Lesions ≥10 mm in long axis were excluded, in order to examine early metastatic disease.

RESULTS

Seventy-five patients had evidence of IMM. Fifteen patients had only lesion(s) measuring ≥10 mm at diagnosis, leaving 60 patients. One hundred ninety-two individual metastases were examined (median 2 per patient; interquartile range, 1-4), 174 (91%) demonstrating leptomeningeal contact. A nodular morphology was observed in 154 of 192 (82%), 32 (17%) were ovoid but elongated along the cortex, and 6 (3%) were linear. Only 3 patients (5%) also exhibited a "classic" linear leptomeningeal disease appearance.

CONCLUSIONS

Most IMM measuring between 2 and 9 mm in diameter are corticomeningeal nodules. These data raise the hypothesis that deeper parenchymal extension of IMM occurs secondarily. If the leptomeninges provide a preferential site for establishment of IMM, further investigation of the underlying biology of this phenomenon may provide opportunities for novel therapeutic strategies for patients with IMM.

A solitary brain metastasis as the only site of recurrence of HR positive, HER2 negative breast cancer: a case report and review of the literature

Background

Breast cancer is one of the most common causes of brain metastases. However, the presence of isolated central nervous system (CNS) metastatic disease early in the course of disease relapse is a rare event in cases of hormone receptor positive, human epidermal growth factor receptor 2 (HER2) negative breast cancer.

Case presentation

We summarize the clinical course of a pre-menopausal, 39-year old Caucasian female with history of operable, hormone receptor positive, HER2 negative breast cancer who was initially treated with curative-intend therapy but who unfortunately developed solitary metastatic lesion in the left thalamus. A biopsy of the lesion confirmed the presence of hormone receptor positive, HER2 negative metastatic breast cancer. Patient’s CNS metastases continued to progress without any evidence of metastatic disease outside of the central nervous system and she eventually passed away about 5 years after the date of her initial diagnosis and 18 months following the diagnosis with brain metastasis.

Conclusion

Based on our case, although rare, patients with treated, operable, hormone receptor positive, HER2 negative breast cancer can present with solitary brain metastasis as the only sign of disease recurrence.

Multifunctional system for combined chemodynamic–photodynamic therapy employing the endothelin axis based on conjugated polymer nanoparticles

Most nanomedicines that attack tumors by Reactive Oxygen Species (ROS) based on lipid peroxidation mechanisms require external activation to work.

Bevacizumab suppresses the growth of established non-small-cell lung cancer brain metastases in a hematogenous brain metastasis model

Brain metastases are common in patients with non-small-cell lung cancer (NSCLC). The efficacy of bevacizumab, an anti-vascular endothelial growth factor (VEGF) humanized antibody, has been demonstrated in patients with nonsquamous NSCLC. We established a transplantable NSCLC cell line (Nluc-H1915) that stably expresses NanoLuc® reporter and confirmed the correlation between total Nluc activity in tumor and tumor volume in vivo. SCID mice inoculated with these cells through the internal carotid artery formed reproducible brain metastases, in which human VEGF was detected. Next, after metastases were established in the model mice (15-17 days), they were intraperitoneally administered weekly doses of human immunoglobulin G (HuIgG) or bevacizumab. Nluc activity in the brain was significantly lower in bevacizumab-treated mice than in HuIgG-treated mice. Additionally, bevacizumab concentration in the brain was higher in mice with brain metastasis than in normal mice, and bevacizumab was primarily observed in brain metastasis lesions. The microvessel density in brain metastasis was lower in bevacizumab-treated mice than in HuIgG-treated mice. We believe bevacizumab's anti-proliferative effect on brain metastasis is due to anti-angiogenic activity achieved by its penetration into brain metastases; this suggests that a bevacizumab-containing regimen may be a promising treatment option for patients with NSCLC brain metastasis.

Immunomodulatory Drugs in Melanoma Brain Metastases

Brain metastases are about ten times more frequent than a brain primary tumor, being present in 20-40% of adults with systemic cancer. Together with lung cancer and breast cancer, skin cancers such as melanoma are top primary tumors which metastasizes to the brain. Advanced melanoma is well known for its propensity to metastasize to the brain, with 80% of patients presenting brain metastasis at the autopsy. However, current therapies are not very efficient and brain metastases are in most of the cases lethal. Treatment of melanoma brain metastases with surgery and/or radiation therapy results in a median overall survival of only about four months after diagnosis. New immunotherapies such as targeted or immunomodulatory drugs, many in clinical trials, have shown promise, with some immunomodulatory drugs being able to at least double the overall survival rates for patients with melanoma brain metastases. This review focuses on the recent advances and future potential of using immunotherapy, such as the newly developed immunomodulatory drugs, for melanoma brain metastases therapy. Immunomodulatory drugs bring a great promise as new tools for melanoma treatment in particular and for the treatment of other types of malignancies in general.

Brain metastasization of breast cancer

Central nervous system metastases have been reported in 15-25% of breast cancer patients, and the incidence is increasing. Moreover, the survival of these patients is generally poor, with reports of a 1-year survival rate of 20%. Therefore, a better knowledge about the determinants of brain metastasization is essential for the improvement of the clinical outcomes. Here, we summarize the current data about the metastatic cascade, ranging from the output of cancer cells from the primary tumour to their colonization in the brain, which involves the epithelial-mesenchymal transition, invasion of mammary tissue, intravasation into circulation, and homing into and extravasation towards the brain. The phenotypic change in malignant cells, and the importance of the microenvironment in the formation of brain metastases are also inspected. Finally, the importance of genetic and epigenetic changes, and the recently disclosed effects of microRNAs in brain metastasization of breast cancer are highlighted.

Treatment of experimental human breast cancer and lung cancer brain metastases in mice by macitentan, a dual antagonist of endothelin receptors, combined with paclitaxel

BACKGROUND

We recently demonstrated that brain endothelial cells and astrocytes protect cancer cells from chemotherapy through an endothelin-dependent signaling mechanism. Here, we evaluated the efficacy of macitentan, a dual endothelin receptor (ETAR and ETBR) antagonist, in the treatment of experimental breast and lung cancer brain metastases.

METHODS

The effect of macitentan on astrocyte- and brain endothelial cell-mediated chemoprotective properties was measured in cytotoxic assays. We compared survival of mice bearing established MDA-MB-231 breast cancer or PC-14 non-small cell lung cancer (NSCLC) brain metastases that were treated with vehicle, macitentan, paclitaxel, or macitentan plus paclitaxel. Cell division, apoptosis, tumor vasculature, and expression of survival-related proteins were assessed by immunofluorescent microscopy.

RESULTS

Cancer cells and tumor-associated endothelial cells expressed activated forms of AKT and MAPK in vehicle- and paclitaxel-treated groups in both metastasis models, but these proteins were downregulated in metastases of mice that received macitentan. The survival-related proteins Bcl2L1, Gsta5, and Twist1 that localized to cancer cells and tumor-associated endothelial cells in vehicle- and paclitaxel-treated tumors were suppressed by macitentan. Macitentan or paclitaxel alone had no effect on survival. However, when macitentan was combined with paclitaxel, we noted a significant reduction in cancer cell division and marked apoptosis of both cancer cells and tumor-associated endothelial cells. Moreover, macitentan plus paclitaxel therapy significantly increased overall survival by producing complete responses in 35 of 35 mice harboring brain metastases.

CONCLUSIONS

Dual antagonism of ETAR and ETBR signaling sensitizes experimental brain metastases to paclitaxel and may represent a new therapeutic option for patients with brain metastases.

Incipient Melanoma Brain Metastases Instigate Astrogliosis and Neuroinflammation

Malignant melanoma is the deadliest of skin cancers. Melanoma frequently metastasizes to the brain, resulting in dismal survival. Nevertheless, mechanisms that govern early metastatic growth and the interactions of disseminated metastatic cells with the brain microenvironment are largely unknown. To study the hallmarks of brain metastatic niche formation, we established a transplantable model of spontaneous melanoma brain metastasis in immunocompetent mice and developed molecular tools for quantitative detection of brain micrometastases. Here we demonstrate that micrometastases are associated with instigation of astrogliosis, neuroinflammation, and hyperpermeability of the blood-brain barrier. Furthermore, we show a functional role for astrocytes in facilitating initial growth of melanoma cells. Our findings suggest that astrogliosis, physiologically instigated as a brain tissue damage response, is hijacked by tumor cells to support metastatic growth. Studying spontaneous melanoma brain metastasis in a clinically relevant setting is the key to developing therapeutic approaches that may prevent brain metastatic relapse. Cancer Res; 76(15); 4359-71. ©2016 AACR.

A Decade of Experience in Developing Preclinical Models of Advanced- or Early-Stage Spontaneous Metastasis to Study Antiangiogenic Drugs, Metronomic Chemotherapy, and the Tumor Microenvironment

The clinical circumstance of treating spontaneous metastatic disease, after resection of primary tumors, whether advanced/overt or microscopic in nature, is seldom modeled in mice and may be a major factor in explaining the frequent discordance between preclinical and clinical therapeutic outcomes where the trend is "overprediction" of positive results in preclinical mouse model studies. To evaluate this hypothesis, a research program was initiated a decade ago to develop multiple models of metastasis in mice, using variants of human tumor cell lines selected in vivo for enhanced spontaneous metastatic aggressiveness after surgical resection of established orthotopic primary tumors. These models have included breast, renal, and colorectal carcinomas; ovarian cancer (but without prior surgery); and malignant melanoma. They have been used primarily for experimental therapeutic investigations involving various antiangiogenic drugs alone or with chemotherapy, especially "metronomic" low-dose chemotherapy. The various translational studies undertaken have revealed a number of clinically relevant findings. These include the following: (i) the potential of metronomic chemotherapy, especially when combined with a vascular endothelial growth factor pathway targeting drug to successfully treat advanced metastatic disease; (ii) the development of relapsed spontaneous brain metastases in mice with melanoma or breast cancer whose systemic metastatic disease is successfully controlled for a period with a given therapy; (iii) foreshadowing the failure of adjuvant antiangiogenic drug-based phase III trials; (iv) recapitulating the failure of oral antiangiogenic tyrosine kinase inhibitors plus standard chemotherapy in contrast to the modest successes of antiangiogenic antibodies plus chemotherapy in metastatic breast cancer; and (v) revealing "vessel co-option" and absence of angiogenesis as a determinant of intrinsic resistance or minimal responsiveness to antiangiogenic therapy in lung metastases. Developing similar models of metastatic disease but involving mouse tumors grown in syngeneic immunocompetent mice may also prove useful for future translational studies of immune therapy-based treatments.

Intertumor heterogeneity in vascularity and invasiveness of artificial melanoma brain metastases

Background

Patients diagnosed with melanoma brain metastases have few treatment options and poor prognosis, and improved treatment strategies for these patients require detailed understanding of the underlying pathobiology. In this investigation we studied the vascularity and invasiveness of artificial brain metastases established from four human melanoma cell lines.

Methods

A-07, D-12, R-18, and U-25 cells transfected with GFP were injected intracerebrally and intra-arterially in nude mice. Moribund mice were killed and autopsied, and the brain was evaluated by fluorescence imaging or by histological examination. Expression and secretion of factors involved in angiogenesis and invasion were assessed by quantitative PCR, ELISA, and immunohistochemistry.

Results

The melanoma cells grew preferentially in the meninges and ventricles after intracerebral and intra-arterial injection. Intertumor heterogeneity in the aggressiveness of meningeal tumors reflected differences in angiogenic activity and expression of vascular endothelial growth factor A (VEGF-A) and interleukin 8 (IL-8). In contrast, growth and invasion of the brain parenchyma relied primarily on vascular co-option. The cell lines showed different patterns of invasion from meninges to the scull and from meninges to the brain parenchyma, and these differences were associated with differences in expression of the matrix metalloproteinases MMP-2 and MMP-9. Furthermore, the melanoma cells produced multiple brain lesions after intracerebral implantation by using the meningeal linings of the brain as transport routes.

Conclusions

The melanoma cell lines showed different growth patterns in the brain, and these differences were associated with differences in expression of the angiogenic factors VEGF-A and IL-8 and the matrix metalloproteinases MMP-2 and MMP-9.

Control of the blood-brain barrier function in cancer cell metastasis

Cerebral metastases are the most common brain neoplasms seen clinically in the adults and comprise more than half of all brain tumours. Actual treatment options for brain metastases that include surgical resection, radiotherapy and chemotherapy are rarely curative, although palliative treatment improves survival and life quality of patients carrying brain-metastatic tumours. Chemotherapy in particular has also shown limited or no activity in brain metastasis of most tumour types. Many chemotherapeutic agents used systemically do not cross the blood-brain barrier (BBB), whereas others may transiently weaken the BBB and allow extravasation of tumour cells from the circulation into the brain parenchyma. Increasing evidence points out that the interaction between the BBB and tumour cells plays a key role for implantation and growth of brain metastases in the central nervous system. The BBB, as the tightest endothelial barrier, prevents both early detection and treatment by creating a privileged microenvironment. Therefore, as observed in several in vivo studies, precise targetting the BBB by a specific transient opening of the structure making it permeable for therapeutic compounds, might potentially help to overcome this difficult clinical problem. Moreover, a better understanding of the molecular features of the BBB, its interrelation with metastatic tumour cells and the elucidation of cellular mechanisms responsible for establishing cerebral metastasis must be clearly outlined in order to promote treatment modalities that particularly involve chemotherapy. This in turn would substantially expand the survival and quality of life of patients with brain metastasis, and potentially increase the remission rate. Therefore, the focus of this review is to summarise the current knowledge on the role and function of the BBB in cancer metastasis.

MicroRNA-200 family members and ZEB2 are associated with brain metastasis in gastric adenocarcinoma

Although the incidence of brain metastasis in gastric cancer is relatively low, its prevalence may increase with improved therapy and longer survival tumors. The molecular mechanisms underlying brain metastases are not well understood. To gain insight into the mechanism of brain metastasis, we studied differences in microRNA (miRNA) expression levels in 8 cases of matched primary gastric adenocarcinoma and brain metastatic adenocarcinoma using the Illumina microRNA microarray chip. We identified 6 upregulated and 2 downregulated miRNAs in all 8 cases simultaneously. Interestingly, 2 out of 8 miRNAs (hsa-miR‑141-3p and hsa-miR-200b-3p) belonged to the miR-200 family. Online microRNA database searching revealed that ZEB2 is the top-ranked target gene for hsa-miR141-3p and hsa-miR-200b-3p, prompting us to focus ZEB2 expression in brain metastatic adenocarcinoma. We confirmed that ZEB2 expression was markedly downregulated in some brain metastatic samples. In addition, decreased ZEB2 expression was noted by western blot analysis of 2 metastatic gastric adenocarcinoma cell types that were derived by in vivo selection following intracardiac injection of gastric cancer cell lines. In conclusion, we demonstrate that expression of miRNA-200 family members and ZEB2 are associated with brain metastases of gastric adenocarcinoma, not only in matched patient samples, but also in metastatic cell lines that were derived by in vivo selection.

ADAM9 promotes lung cancer metastases to brain by a plasminogen activator-based pathway

The transmembrane cell adhesion protein ADAM9 has been implicated in cancer cell migration and lung cancer metastasis to the brain, but the underpinning mechanisms are unclear and clinical support has been lacking. Here, we demonstrate that ADAM9 enhances the ability of tissue plasminogen activator (tPA) to cleave and stimulate the function of the promigratory protein CDCP1 to promote lung metastasis. Blocking this mechanism of cancer cell migration prolonged survival in tumor-bearing mice and cooperated with dexamethasone and dasatinib (a dual Src/Abl kinase inhibitor) treatment to enhance cytotoxic treatment. In clinical specimens, high levels of ADAM9 and CDCP1 correlated with poor prognosis and high risk of mortality in patients with lung cancer. Moreover, ADAM9 levels in brain metastases derived from lung tumors were relatively higher than the levels observed in primary lung tumors. Our results show how ADAM9 regulates lung cancer metastasis to the brain by facilitating the tPA-mediated cleavage of CDCP1, with potential implications to target this network as a strategy to prevent or treat brain metastatic disease.

Role of the endothelin axis in astrocyte- and endothelial cell-mediated chemoprotection of cancer cells

BACKGROUND

Recent evidence suggests that astrocytes protect cancer cells from chemotherapy by stimulating upregulation of anti-apoptotic genes in those cells. We investigated the possibility that activation of the endothelin axis orchestrates survival gene expression and chemoprotection in MDA-MB-231 breast cancer cells and H226 lung cancer cells.

METHODS

Cancer cells, murine astrocytes, and murine fibroblasts were grown in isolation, and expression of endothelin (ET) peptides and ET receptors (ETAR and ETBR) compared with expression on cancer cells and astrocytes (or cancer cells and fibroblasts) that were co-incubated for 48 hours. Type-specific endothelin receptor antagonists were used to evaluate the contribution of ETAR and ETBR to astrocyte-induced activation of the protein kinase B (AKT)/mitogen-activated protein kinase (MAPK) signal transduction pathways, anti-apoptotic gene expression, and chemoprotection of cancer cells. We also investigated the chemoprotective potential of brain endothelial cells and microglial cells.

RESULTS

Gap junction signaling between MDA-MB-231 cancer cells and astrocytes stimulates upregulation of interleukin 6 (IL-6) and IL-8 expression in cancer cells, which increases ET-1 production from astrocytes and ET receptor expression on cancer cells. ET-1 signals for activation of AKT/MAPK and upregulation of survival proteins that protect cancer cells from taxol. Brain endothelial cell-mediated chemoprotection of cancer cells also involves endothelin signaling. Dual antagonism of ETAR and ETBR is required to abolish astrocyte- and endothelial cell-mediated chemoprotection.

CONCLUSIONS

Bidirectional signaling between astrocytes and cancer cells involves upregulation and activation of the endothelin axis, which protects cancer cells from cytotoxicity induced by chemotherapeutic drugs.

Multimodal imaging enables early detection and characterization of changes in tumor permeability of brain metastases

Our goal was to develop strategies to quantify the accumulation of model therapeutics in small brain metastases using multimodal imaging, in order to enhance the potential for successful treatment. Human melanoma cells were injected into the left cardiac ventricle of immunodeficient mice. Bioluminescent, MR and PET imaging were applied to evaluate the limits of detection and potential for contrast agent extravasation in small brain metastases. A pharmacokinetic model was applied to estimate vascular permeability. Bioluminescent imaging after injecting d-luciferin (molecular weight (MW) 320 D) suggested that tumor cell extravasation had already occurred at week 1, which was confirmed by histology. 7T T1w MRI at week 4 was able to detect non-leaky 100 μm sized lesions and leaky tumors with diameters down to 200 μm after contrast injection at week 5. PET imaging showed that (18)F-FLT (MW 244 Da) accumulated in the brain at week 4. Gadolinium-based MRI tracers (MW 559 Da and 2.066 kDa) extravasated after 5 weeks (tumor diameter 600 μm), and the lower MW agent cleared more rapidly from the tumor (mean apparent permeabilities 2.27 × 10(-5)cm/s versus 1.12 × 10(-5)cm/s). PET imaging further demonstrated tumor permeability to (64)Cu-BSA (MW 65.55 kDa) at week 6 (tumor diameter 700 μm). In conclusion, high field T1w MRI without contrast may improve the detection limit of small brain metastases, allowing for earlier diagnosis of patients, although the smallest lesions detected with T1w MRI were permeable only to d-luciferin and the amphipathic small molecule (18)F-FLT. Different-sized MR and PET contrast agents demonstrated the gradual increase in leakiness of the blood tumor barrier during metastatic progression, which could guide clinicians in choosing tailored treatment strategies.

αB-crystallin: a novel regulator of breast cancer metastasis to the brain

PURPOSE

Basal-like breast tumors are typically (ER/PR/HER2) triple-negative and are associated with a high incidence of brain metastases and poor clinical outcomes. The molecular chaperone αB-crystallin is predominantly expressed in triple-negative breast cancer (TNBC) and contributes to an aggressive tumor phenotype in preclinical models. We investigated the potential role of αB-crystallin in brain metastasis in TNBCs.

EXPERIMENTAL DESIGN

αB-crystallin expression in primary breast carcinomas and brain metastases was analyzed by immunohistochemistry among patients with breast cancer with brain metastases. αB-crystallin was overexpressed or silenced in two different TNBC cell lines. The effects on cell adhesion to human brain microvascular endothelial cells (HBMEC) or extracellular matrix proteins, transendothelial migration, and transmigration across a HBMEC/astrocyte coculture blood-brain barrier (BBB) model were examined. In addition, the effects of overexpressing or silencing αB-crystallin on brain metastasis in vivo were investigated using orthotopic TNBC models.

RESULTS

In a cohort of women with breast cancer brain metastasis, αB-crystallin expression in primary breast carcinomas was associated with poor overall survival and poor survival after brain metastasis, even among patients with TNBC. Stable overexpression of αB-crystallin in TNBC cells enhanced adhesion to HBMECs, transendothelial migration, and BBB transmigration in vitro, whereas silencing αB-crystallin inhibited these events. αB-crystallin promoted adhesion of TNBC cells to HBMECs, at least in part, through an α3β1 integrin-dependent mechanism. αB-crystallin overexpression promoted brain metastasis, whereas silencing αB-crystallin inhibited brain metastasis in orthotopic TNBC models.

CONCLUSION

αB-crystallin is a novel regulator of brain metastasis in TNBC and represents a potential biomarker and drug target for this aggressive disease.

Ultrasonography-guided intracardiac injection: an improvement for quantitative brain colonization assays

Brain metastasis is a frequent occurrence in patients with cancer, with devastating consequences. The current animal models for brain metastasis are highly variable, leading to a need for improved in vivo models that recapitulate the clinical disease. Herein, we describe an experimental brain metastasis model that uses ultrasonographic guidance to perform intracardiac injections. This method is easy to perform, giving consistent and quantitative results. Demonstrating the utility of this method, we have assessed a variety of metastatic cell lines for their ability to develop into brain metastases. Those cell lines that were competent at brain colonization could be detected in the brain vasculature 4 hours after intracardiac injection, and a few adherent cells persisted until colonization occurred. In contrast, those cell lines that were deficient in brain colonization were infrequently found 4 hours after introduction into the arterial circulation and were not detected at later time points. All of these cells were capable of brain colonization after intraparenchymal injection. We propose that adherence to the brain vasculature may be the key limiting step that determines the ability of a cancer cell to form brain metastases successfully. Identifying brain endothelium-specific adhesion molecules may enable development of screening modalities to detect brain-colonizing cancer cells and therapies to prevent these metastatic cells from seeding the brain.

The biology of brain metastasis

BACKGROUND

It is estimated that at least 200 000 cases of brain metastases occur each year in the US, which is 10 times the number of patients diagnosed with primary brain tumors. Brain metastasis is associated with poor prognosis, neurological deterioration, diminished quality of life, and extremely short survival. Favorable interactions between tumor cells and cerebral microvascular endothelial cells encourage tumor growth in the central nervous system, while tumor cell interactions with astrocytes protect brain metastases from the cytotoxic effects of chemotherapy.

CONTENT

We review the pathogenesis of brain metastasis and emphasize the contributions of microvascular endothelial cells and astrocytes to disease progression and therapeutic resistance. Animal models used to study brain metastasis are also discussed.

SUMMARY

Brain metastasis has many unmet clinical needs. There are few clinically relevant tumor models and no targeted therapies specific for brain metastases, and the mean survival for untreated patients is 5 weeks. Improved clinical outcomes are dependent on an enhanced understanding of the metastasis-initiating population of cells and the identification of microenvironmental factors that encourage disease progression in the central nervous system.

Combined targeting of HER2 and VEGFR2 for effective treatment of HER2-amplified breast cancer brain metastases

Brain metastases are a serious obstacle in the treatment of patients with human epidermal growth factor receptor-2 (HER2)-amplified breast cancer. Although extracranial disease is controlled with HER2 inhibitors in the majority of patients, brain metastases often develop. Because these brain metastases do not respond to therapy, they are frequently the reason for treatment failure. We developed a mouse model of HER2-amplified breast cancer brain metastasis using an orthotopic xenograft of BT474 cells. As seen in patients, the HER2 inhibitors trastuzumab and lapatinib controlled tumor progression in the breast but failed to contain tumor growth in the brain. We observed that the combination of a HER2 inhibitor with an anti-VEGF receptor-2 (VEGFR2) antibody significantly slows tumor growth in the brain, resulting in a striking survival benefit. This benefit appears largely due to an enhanced antiangiogenic effect: Combination therapy reduced both the total and functional microvascular density in the brain xenografts. In addition, the combination therapy led to a marked increase in necrosis of the brain lesions. Moreover, we observed even better antitumor activity after combining both trastuzumab and lapatinib with the anti-VEGFR2 antibody. This triple-drug combination prolonged the median overall survival fivefold compared with the control-treated group and twofold compared with either two-drug regimen. These findings support the clinical development of this three-drug regimen for the treatment of HER2-amplified breast cancer brain metastases.

Combining cetuximab with killer lymphocytes synergistically inhibits human cholangiocarcinoma cells in vitro

AIM

We explored the possibility of combining adoptive immunotherapy with cytokine-activated killer (CAK) cells and the epidermal growth factor receptor monoclonal antibody, cetuximab, as a treatment for cholangiocarcinoma.

MATERIALS AND METHODS

CAK cells were cultured with a high-dose of interleukin-2 and anti-CD3 monoclonal antibodies. This cell population contained both activated CD16+/CD56+ (NK) cells and CD3+/NKG2D(high+) T-cells. The effect of CAK cells and cetuximab, alone and in combination, on the viability of human cholangiocarcinoma cells was evaluated.

RESULTS

Culture of CAK cells alone, but not cetuximab alone, exhibited modest cytotoxicity toward cholangiocarcinoma cells. However, combining CAK cells with cetuximab significantly enhanced cytotoxicity. This enhancement was inhibited by the addition of excess human immunoglobulins, suggesting that antibody-dependent cytotoxicity, mediated by activated NK cells in the CAK cell culture was involved in this mechanism.

CONCLUSION

Cetuximab may be used to enhance CAK cell therapeutic activity in patients with cholangiocarcinoma, by potentiating antibody-dependent cellular cytotoxicity.

Roles for endothelin receptor B and BCL2A1 in spontaneous CNS metastasis of melanoma

Metastatic spread of melanoma to the central nervous system (CNS) is a common and devastating manifestation of disease progression, which, despite its clinical importance, remains poorly understood with respect to underlying molecular mechanisms. Using a recently developed preclinical model of spontaneous melanoma CNS metastasis, we have identified alterations in expression of endothelin receptor B (EDNRB) as a potential factor that influences brain metastatic potential. Induced overexpression of this gene mediated enhanced overall metastatic disease, and resulted in an increased incidence of spontaneous CNS metastases. In contrast, the overexpression of other highlighted genes, such as BCL2A1, did not affect the incidence of CNS metastases but nevertheless appears to facilitate intracranial tumor growth. The prometastatic effect in the CNS associated with EDNRB appears to be mediated by the interaction with its ligands resulting in enhanced tumor cell proliferation and thus intracranial melanoma growth. That EDNRB contributes to melanoma metastasis is underscored by the fact that its therapeutic inhibition by the EDNRB-specific inhibitor A192621 translated into improved outcomes when treating mice with either visceral metastases or intracranial tumors. The identification of an influential role of EDNRB in CNS melanoma spontaneous metastasis may provide both a target for therapeutic intervention as well as a potential prognostic marker for patients having an increased predisposition for incidence of CNS melanoma metastases.

Multiorgan metastasis of human HER-2+ breast cancer in Rag2-/-;Il2rg-/- mice and treatment with PI3K inhibitor

In vivo studies of the metastatic process are severely hampered by the fact that most human tumor cell lines derived from highly metastatic tumors fail to consistently metastasize in immunodeficient mice like nude mice. We describe a model system based on a highly immunodeficient double knockout mouse, Rag2^−/−;Il2rg^−/−, which lacks T, B and NK cell activity. In this model human metastatic HER-2⁺ breast cancer cells displayed their full multiorgan metastatic potential, without the need for selections or additional manipulations of the system. Human HER-2⁺ breast cancer cell lines MDA-MB-453 and BT-474 injected into Rag2^−/−;Il2rg^−/− mice faithfully reproduced human cancer dissemination, with multiple metastatic sites that included lungs, bones, brain, liver, ovaries, and others. Multiorgan metastatic spread was obtained both from local tumors, growing orthotopically or subcutaneously, and from cells injected intravenously. The problem of brain recurrencies is acutely felt in HER-2⁺ breast cancer, because monoclonal antibodies against HER-2 penetrate poorly the blood-brain barrier. We studied whether a novel oral small molecule inhibitor of downstream PI3K, selected for its penetration of the blood-brain barrier, could affect multiorgan metastatic spread in Rag2^−/−; Il2rg^−/− mice. NVP-BKM120 effectively controlled metastatic growth in multiple organs, and resulted in a significant proportion of mice free from brain and bone metastases. Human HER-2⁺ human breast cancer cells in Rag2^−/−;Il2rg^−/− mice faithfully reproduced the multiorgan metastatic pattern observed in patients, thus allowing the investigation of metastatic mechanisms and the preclinical study of novel antimetastatic agents.

Challenging the current paradigm of melanoma progression: brain metastasis as isolated first visceral site

Melanoma brain metastasis that develops as the isolated first visceral site challenges the current paradigm of tumor progression in which brain metastasis is regarded as the final stage. Here we test the hypothesis that melanoma patients who develop brain metastasis as the isolated first visceral site have distinct clinicopathological features at the time of primary melanoma diagnosis. Cutaneous melanoma patients enrolled in 2 prospectively collected databases were studied (Cohort 1: 1972-1982, Cohort 2: 2002-2009). Patients who developed brain metastasis as isolated first visceral site were compared with (1) all other patients, (2) patients who developed visceral metastasis: extracranial only or extracranial and brain, and (3) patients who progressed to other isolated visceral sites first. Two hundred seven of 2280 (9.1%) patients developed brain metastasis (median follow-up, 5.2 y). Seventy-four of 207 (35.7%) brain metastasis patients progressed to brain metastasis as the isolated first visceral site. These patients presented with primaries that were thinner and had no mitosis compared with all other visceral metastasis patients (Fisher's combined P = .02, .05, respectively), and there was a significant difference in American Joint Committee on Cancer stage distribution at initial melanoma diagnosis (combined P = .02). Post-visceral metastasis survival, however, was shorter in patients with brain metastasis as isolated first visceral site than in patients with visceral metastasis: extracranial and brain (combined P = .03). Brain metastasis as isolated first visceral site is a distinct clinicopathological entity. Studies are needed to better understand the biological factors driving this phenotype at the time of primary melanoma diagnosis and to determine its clinical implications.

Management of brain metastasis: past lessons, modern management, and future considerations

Brain metastasis is a major challenge for patients, physicians, and the broader health care system, with approximately 170,000 new cases per year. After a diagnosis of brain metastasis, patients have a poor prognosis, but modern management has made significant advances in the past two decades to improve palliative efficacy and patient survival through a multidisciplinary approach. A number of factors must be taken into consideration in the treatment approach, including the number of intracranial lesions, the control of extracranial disease, and the patient's overall health, while weighing the benefits of treatment against the toxicities, both acute and chronic. With quality of life as an emphasis, emerging concepts for modern management of brain metastasis have sought to minimize long-term toxicities. The economic impact of such strategies for patients and the health care system has been demonstrated in some studies, but has not been a consistent area of focus. Each of these strategies, as well as novel therapeutics, has embraced the concept of personalized treatment. This review will discuss the current knowledge of modern multidisciplinary management of brain metastasis and look forward to emerging concepts.

Emerging insights into the molecular biology of brain metastases

One of the foremost challenges in oncology is developing improved therapies for preventing and treating metastases to the brain. Recent research in this area is bringing about a shift in the understanding of brain metastases. Previously, the occurrence and poor outcomes associated with brain metastases have been largely attributed to the exclusion of anticancer drugs from the brain by the blood-brain barrier (BBB). However, studies in multiple tumor types have also demonstrated that brain metastases have significant molecular differences from primary tumors and extracranial metastases. These molecular differences may not only promote the formation of brain metastases, but they may also contribute to these tumors' poor responsiveness to therapies. Such changes may be intrinsic to the cancer cells or driven by unique interactions with the brain microenvironment. An improved understanding of the molecular characteristics of brain metastases that contribute to their aggressive behaviors will facilitate the development of rational, more effective treatments for these tumors.