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Metastatic breast cancer is a stage of breast cancer where the disease has spread to distant sites. It is also referred to as metastases, advanced breast cancer, secondary tumours, secondaries or stage 4 breast cancer.
It is a complication of primary breast cancer, usually occurring several years after the primary breast cancer. But sometimes it is diagnosed at the same time as the primary breast cancer or, rarely, before the primary breast cancer has been diagnosed.
Metastatic breast cancer cells frequently differ from the preceding primary breast cancer in properties such as receptor status, have often developed resistance to several lines of previous treatment and acquired special properties that permit them to metastasize to distant sites, making them especially dangerous. Metastatic breast cancer can be treated, sometimes for many years, but it cannot be cured. Distant metastases are the cause of about 90% of deaths due to breast cancer.
Breast cancer primarily metastasizes to the bone, lungs, regional lymph nodes, liver and brain, with the most common site being the bone. Lymph node metastasis into the sentinel node and few surrounding nodes is regarded as a treatable local event and not metastatic breast cancer, both when occurring at primary presentation or later.
Typical environmental barriers in a metastatic event include physical (a basement membrane), chemical (reactive oxygen species or ROS, hypoxia and low pH) and biological (immune surveillance, inhibitory cytokines and regulatory extra-cellular matrix (ECM) peptides) components. Organ-specific anatomic considerations also influence metastasis; these include blood-flow patterns from the primary tumor and the homing ability of cancer cells to certain tissues. The targeting by cancer cells of specific organs is probably regulated by chemo-attractant factors and adhesion molecules produced by the target organ, along with cell-surface receptors expressed by the tumor cells.
The symptoms produced by metastatic breast cancer vary by the location of the metastases. For instance:
Roughly 70% of all patients living with advanced breast cancer have bone metastases. Very often bone metastases can be successfully managed for a long time.
Brain metastasis is observed in 10% of breast cancer patients with metastatic properties. Many of the breast cancer therapies (like targeted antibodies) fail to penetrate the blood–brain barrier ), hence allowing for tumor recurrence in the central nervous system.
The main steps involved in the metastatic cascade of a cancer cell are:
The potential of a tumor cell to metastasize depends on its microenvironment, or “niche” interactions with local factors promoting tumor-cell growth, survival, angiogenesis, invasion and metastasis. This is explained by the seed and soil hypothesis.
Cell-cell and cell-ECM matrix adhesion, motility, and localised proteolysis are mediated mainly by matrix metalloproteases (MMPs). Degradation of the extracellular matrix begins the process of metastasis. The cell develops structures called invadopodia, which are highly concentrated in several proteases and have a highly dynamic actin cytoskeleton.
Most of these processes require a delicate balance between the functions of matrix metalloproteases (MMPs) or metalloprotease-disintegrins (ADAMs) and natural tissue inhibitors of metalloproteases (TIMPs). Regulated proteolysis is an important mechanism to maintain homeostasis. There is increased expression of protease systems in cancer cells, to equip them with the tools necessary to degrade the extracellular matrix and release growth factors or transmembrane receptors. MMP-2 is upregulated in the bone, and increased levels of MMP-1 and MMP-19 are observed in the brain. This in turn, upregulates the signaling pathways necessary to provide increased cell adhesion, cell motility, cell migration, invasion, cancer- cell proliferation and survival.
ECM-tumor cell interactions play a critical role in each of the events of the metastatic cascade. Interactions of the breast cancer cells with integrins, fibronectin, laminins, collagens, hyaluronan and proteoglycans can contribute to the metastatic process. Some of these proteins are discussed here in relation to breast-cancer metastasis.
Fibronectin is an extracellular glycoprotein that can bind to integrins and other ECM components like collagen, fibrin and heparan sulphate proteoglycans(HSPGs). Several different integrins bind to fibronectin. Fibronectin-integrin interactions are important in tumor cell migration, invasion, metastasis and cell proliferation by signaling through integrins. Integrin-mediated tumor cell adhesion to ECM proteins can trigger signal transduction and cause upregulation of gene expression, increased tyrosine phosphorylytion of the focal adhesion kinase, and activation and nuclear translocation of mitogen-activated protein (MAP) kinases.
Heparanase cleaves heparin sulfate chains of HSPGs, which have an extensive network with several proteins on the cell surface and ECM. The basic HSPG structure consists of a protein core to which several linear heparin sulfate (HS) chains are covalently O-linked; this acts as an assembly of different ECM proteins, including fibronectin, laminins, interstitial collagens, heparin-binding growth factors, chemokines and lipoproteins.HSPGs are prominent components of blood vessels. Binding to HS stabilizes FGFs and vascular endothelial growth factors (VEGFs) and prevents them from inactivation. HS chains function as low-affinity co-receptors which promote dimerization of FGFs, aids in the sequestration of the GFs and causes activation of the signaling tyrosine kinase receptors even under low circulating concentrations of growth factors. Heparanase expressed by cancer cells participates in angiogenesis and neovascularization by degrading the polysaccharide scaffold of the endothelial BM, thereby releasing angiogenic growth factors from the ECM.
The ECM protein tenascin C (TNC) is up-regulated in metastatic breast cancer. TNC is an adhesion-modulating extracellular matrix glycoprotein. It is highly expressed in tumor stroma and stimulates tumor-cell proliferation. It is hypothesised that TNC stimulates invasion via up-regulation of MMP-1 expression through activation of the MAPK pathway. MMP-1 (interstitial collagenase) cleaves collagen type I, II, III, VII and X. Therefore, tenascin C over-expression can significantly alter collagen in the ECM and influence tumor cell migration in cartilaginous tissues.
Endoglin is a cell-surface disulfide-linked homodimeric glycoprotein which binds to integrins and other RGD ligands and is a co-receptor for TGF-beta. Brain-metastatic breast-tumor cells express endoglin in large amounts. Endoglin-overexpressing cells develop large numbers of invadopodia; endoglin is localized in these structures. Endoglin expression in tumor cells contributes to metastasis by upregulating MMP-1 and MMP-19. MMP-19 cleaves components of the basal lamina such as collagen type IV, laminin 5, nidogen (entactin) and other ECM proteins such as tenascin, aggrecan and fibronectin. Therefore, endoglin over-expression alters the proteolytic balance of the cells to greater matrix degradation and increased invasive properties of breast cancer.
The primary extracellular matrix components and cell-surface receptors which aid in metastasis are discussed here:
Integrin αvβ3 (a cell-surface adhesion molecule) is important for tumor attachment, cell-to-cell communication between the breast tumor cells and the environment in bone, osteoclast bone resorption and angiogenesis. Integrin-mediated adhesion between cancer cells and osteoclasts in bone metastases induces phosphorylation of extracellular signal-regulated kinases (ERK1/2) in osteoclasts, which in turn induces osteoclast differentiation and survival.
Metastatic breast-cancer cells excrete lysophosphatidic acid (LPA) which binds to receptors on tumor cells, inducing cell proliferation and release of cytokines(IL-6 and IL-8, potent bone resorptive agents) and stimulating bone resorption. After the breast-cancer cells have left the primary tumor, they interact with the bone microenvironment and secrete osteolytic factors capable of osteoclast formation and bone resorption. Apart from the breast tumor cells, the resident stromal cells also contribute to tumor survival. Growth factors such as epidermal growth factor (EGF), fibroblast growth factor (FGF) and transforming growth factor beta (TGF-β) are implicated in the development and progression of metastatic breast cancer.
MMP-2 is the main metalloprotease secreted by breast-cancer cells or induced in the adjacent bone stroma; it plays an important role in the degradation of the extracellular matrix essential for metastasis. Tumor cells use MMP-2 secreted by bone marrow fibroblasts (BMFs). MMP-2 is stored in an inactive conformation in association with the cell surface (or extracellular matrix) of BMFs. Inactive MMP-2 present on the surface of BMFs is displaced by breast-cancer cells. Cancer cells can then use the proteinase to facilitate tissue invasion, which requires the degradation of connective tissue associated with vascular basement membranes and interstitial connective tissue.
The brain is a unique organ for metastasis, since the breast-tumor cells have to pass the blood–brain barrier (BBB) to form micrometastases.
CD44 (a cell-surface transmembrane glycoprotein) is a receptor for hyaluronic acid, involved in cell adhesion by binding to specific extracellular matrix components. A proposed mechanism for the function of CD44 is to regulate the adhesion of circulating cancer cells in the brain to the endothelium at the secondary site with the help of a hyaluronate matrix ligand or by its cytoplasmic attachments to actin-associated proteins of the merlin/ezrin/radixin/moesin family.
Cell-surface sialylation has been implicated in cell–to-cell interactions, and over-expression of a brain sialyltransferase in breast-cancer cells is a mechanism highlighting the role of cell-surface glycosylation in organ-specific metastatic interactions. Breast-cancer metastasis to the brain involves mediators of extravasation through non-fenestrated capillaries, complemented by specific enhancers of BBB-crossing and brain colonization.
The "seed and soil" hypothesis states that specific organs harbor metastases from one type of cancer by stimulating their growth better than other types of cancer. This interaction is dynamic and reciprocal, since cancer cells modify the environment they encounter.
In the detection of bone metastases, skeletal scintigraphy (bone scan) is very sensitive and is recommended as the first imaging study in asymptomatic individuals with suspected breast-cancer metastases. X-ray radiography is recommended if there is abnormal radionuclide uptake from the bone scan and in assessing the risk of pathological fractures, and is recommended as the initial imaging study in patients with bone pain. MRI or the combination PET-CT may be considered for cases of abnormal radionuclide uptake on bone scan, when radiography does not give an acceptably clear result.
Metastasis is a complex and interconnected multi-step process. Each step in the process is a potential target for therapies to prevent or reduce metastasis. Those steps which have a good clinical window are the best targets for therapy. Each event in metastasis is highly regulated and requires a synergistic activation of different ECM proteins, growth factors and so on. Although the occasional patient with metastatic breast cancer benefits from surgical resection of an isolated metastasis and most patients receive radiotherapy (often for palliation alone) during the course of their disease, the treatment of metastatic breast carcinoma typically involves the use of systemic therapy.
Chemotherapy is one of the most important components of therapy for metastatic breast cancer. Therapy of choice is determined by previous treatments and tumor characteristics. Observation of metastases provides direct feedback on the effectiveness of the treatment, often a number of chemotherapy agents are tried sequentially to determine one that works.
The taxanes are very active in metastatic breast cancer, and abraxane is approved for patients with metastatic breast cancer who either relapsed within six months of adjuvant chemotherapy or failed to respond to combination chemotherapy. This has a higher response rate than solvent-based paclitaxel (15% vs 8%). Abraxane can also deliver a 49% higher dose of medication than solvent-based paclitaxel; however, the side effects are severe and include chemotherapy-induced peripheral neuropathy. Combination chemotherapy is often used in patients with metastatic breast cancer. Vinorelbine is also active in metastatic breast cancer. Eribulin was approved in the US in Nov 2010.
A groundbreaking drug, Kadcyla, was approved in February 2013. This drug targets only cancerous cells and is compared to a "smart-bomb" for cancer. It works by only releasing its toxic payload when triggered by a protein found in the cancerous cells in HER2+ breast cancer. It has extremely low side effects using this target therapy method.
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For estrogen-receptor-positive metastatic breast carcinoma the first line of therapy is often tamoxifen or another anti-estrogen drug unless there are liver metastases, significant lung involvement, rapidly progressive disease or severe symptoms requiring immediate palliation.
Radiotherapy is used in the treatment of metastatic breast cancer. The most common reasons for a patient with metastatic breast carcinoma to be treated with radiotherapy are:
Some patients with metastatic breast cancer opt to try alternative therapies such as vitamin therapy, homeopathic treatments, a macrobiotic diet, chiropractic or acupuncture. There is no evidence that any of these therapies are effective; they may be harmful, either because patients pass up effective conventional therapies such as chemotherapy or anti-estrogen therapy in favor of alternative treatments, or because the treatments themselves are harmful (as in the case of apricot-pit therapy—which exposes the patient to cyanide—or in chiropractic, which can be dangerous to patients with cancer metastatic to the spinal bones or spinal cord. A macrobiotic diet is neither effective nor safe as it could hypothetically induce weight loss due to severe dietary restriction. There is limited evidence that acupuncture might relive pain in cancer patients, but data so far is insufficient to recommend its use outside of clinical trials.
Treatment of metastatic breast cancer is currently an active area of research. Several medications are in development or in phase I/II trials. Typically new medications and treatments are first tested in metastatic cancer before trials in primary cancer are attempted.
Another area of research is finding combination treatments which provide higher efficacy with reduced toxicity and side effects.
Scheduling of drug treatments and combination treatment can have substantial impact on treatment efficacy.
Nanomedicine is being studied, and there are several developments involving the targeting of cancer cells using nanoprobes. Some instances where nanoprobes are used to target specific tumor cells (based on the organ to which they have metastasized) are:
Clinically symptomatic CNS metastases are reported in 10-15% of patients with metastatic breast cancer; in large autopsy studies, up to 40% of woman who died of metastatic breast cancer were reported to have at least one brain metastasis. CNS metastases are often viewed by patients and doctors as a late complication of metastatic breast cancer for which few effective treatments exist. In most cases, CNS involvement occurs after metastatic dissemination to the bones, liver and/or lungs has already occurred; for that reason, many patients already have refractory, terminal breast cancer by the time they are diagnosed with brain metastases. The diagnosis of brain metastases from breast cancer relies mainly on patient-reported symptoms and neuroimaging.
Symptoms of brain metastases from breast cancer are:
Of all brain-metastatic patients, those with a controlled extra-cranial tumor, age less than 65 years and a favorable general performance (Karnofsky performance status ≥70) fare best; older patients with a Karnofsky performance status <70 do poorly. Effective treatments for brain metastases from breast cancer exist, although symptomatic therapy alone may be chosen for those with poor performance status. Corticosteroids are crucial to the treatment of brain metastases from any source (including the breast), and are effective in reducing peri-tumoral edema and providing symptomatic relief.Chemotherapy has not been found to be effective in the treatment of brain metastases from breast cancer, due to the inability of most chemotheraputic agents to penetrate the blood–brain barrier. Whole-brain radiation may provide a median survival of 4 to 5 months, which can be further extended by months with stereotactic surgery. Several non-randomized studies have suggested that stereotactic surgery may provide a nearly equivalent outcome, compared with surgery followed by whole brain-irradiation. Surgery tends to reduce symptoms quickly and prolong life, with an improved quality of life. Multiple metastases (up to three) may be removed surgically with a risk similar to that of a single lesion, providing similar benefits. Adjuvant radiotherapy follows surgical resection; this combined approach has been shown to prolong median survival up to 12 months, depending on the factors noted above. There is evidence that surgery may be useful in select patients with recurrent brain metastases. Mean survival from diagnosis of a brain metastasis varies between studies, ranging from 2 to 16 months (depending on involvement of the CNS, the extent of the extra-cranial metastatic disease, and the treatment applied). The mean 1-year survival is estimated at 20%. Improvements in the treatment of brain metastases are clearly needed.
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