Metastatic cancers are often resistant to conventional chemotherapy and radiotherapy, reflecting an unmet need for novel therapeutic approaches to inhibit metastasis. Growing evidence highlights the importance of targeting not only the tumour but also its ‘normal’ microenvironment – the milieu which surrounds the tumour, including stromal fibroblasts, infiltrating immune cells, blood vessels, signalling molecules, extracellular matrix and tissue oxygen. The microenvironment varies between different organs, presumably explaining the propensity of certain cancers to spread to particular sites. This review summarises current and emerging therapies targeting the metastatic microenvironment in human tumours. The role of the tumour microenvironment in the establishment and support of metastatic disease is increasingly evident. Novel therapeutic strategies targeting the microenvironment encompassing the complex interactions between tumour cells and surrounds now need to be incorporated into clinical trials with appropriate biomarker endpoints.
Cancer metastases account for over 90% of mortality in cancer patients.1 As such research into the causes of metastases has the potential to yield novel therapeutic targets. Recent research into the oncogenic role of the tumour microenvironment has begun to reveal new insights, triggering the evolution of a paradigm shift in the way we understand and treat metastatic cancers.
Metastasis occurs when a cancer cell undergoes changes allowing extravasation and colonisation at a distant location to form one or more secondary tumour clones. We envisage metastasis as a complex and multistep process – the metastatic cascade – involving loss of tumour cellular adhesion, local tissue invasion, extravasation and survival in the circulation, movement into new tissue and eventually colonisation of a distant site.2-4 In theory, any step(s) in the metastatic cascade could be targeted, however certain steps can occur prior to the clinical manifestation of the metastatic disease prior to clinical presentation of overt metastatic disease, potentially rendering palliative treatments targeting the later steps ineffective.5 Targeting the site of metastatic colonisation may thus prove to be an important strategy in preventing the emergence and/or progression of metastatic disease.
The tumour microenvironment encompasses the milieu surrounding a tumour cell, including stromal fibroblasts, blood and lymphatic vessels, infiltrating immune cells, signalling molecules, tissue oxygen and components of the extracellular matrix (figure 1).6 The tumour microenvironment varies between different organs,3 which may explain the tendency of certain primary cancers to metastasise to particular sites.6 Cancer cells can communicate with and alter their microenvironment, establishing molecular changes that encourage the metastatic colonisation of cancer cells. For example, in a study that grafted cancer cells into mice with either normal fibroblasts, or cancer-associated fibroblasts, cancer proliferation was only apparent in mice with the cancer-associated fibroblasts.7 Therapeutic targeting of the tumour microenvironment is thus an increasingly attractive strategy to prevent or treat metastatic cancer. Furthermore, the genetic stability of the microenvironment compared to the metastatic tumour itself makes it a favourable target for minimising resistance.6 What still remains unclear however, is how the tumour microenvironment – ‘normal’ as this is widely assumed to be – may be both selectively, yet tolerably, targeted.
Cells in the microenvironment may communicate both positive and negative signals to the tumour cells, resulting in establishment of a pro- or anti-tumour microenvironment respectively. Proteins derived from exosomes released by melanoma cells for example, can enhance the metastatic microenvironment via promotion of pro-inflammatory and pro-angiogenic properties of bone-marrow derived progenitor cells, providing a metastatic niche.8 Similar events can lead to upregulation of factors in sentinel lymph nodes that potentiate extracellular matrix production and migration of melanoma cells to these.9Further to this, it has been suggested that ‘re-education’ of stromal cells to provide an anti-tumour environment may be a more effective strategy than non-selectively ablating tumour-associated stromal cells in controlling metastatic disease.6 In this review we describe some current and emerging approaches to targeting the metastatic microenvironment for several cancers (table 1).
Prostate cancer, the most common non-skin cancer in Australian men, preferentially metastasises to bone, often resulting in abnormal bone formation (osteosclerosis), fractures, pain and neural compression.10 Newly diagnosed prostate cancers tend to be dependent on stimulation via the androgen receptor for survival and proliferation. Medical prostate cancer therapy therefore first involves targeting the cancer cells via androgen deprivation therapy such as orchiectomy or pharmacological castration, for example using gonadotropin-releasing hormone agonists or antagonists.11 Although androgen deprivation therapy tends to be initially effective, most prostate cancers relapse within a few months or years, progressing to castrate-resistant prostate cancer. Chemotherapy (for example, using docetaxel or cabazitaxel) remains a key treatment for castrate-resistant prostate cancer.12 However, treatment-induced damage to the tumour microenvironment may swiftly promote resistance to standard chemotherapy,13 for example, by inducing tumour cell epithelial-mesenchymal transition,14 with associated changes in gene expression profile.
Targeting cells in the bone microenvironment is also useful in the treatment of metastatic castrate-resistant prostate cancer. Osteoblasts in bone express the protein RANK-L, which is responsible for the activation of osteoclasts. In prostate cancer, RANK-L is upregulated in osteoblasts, osteocytes and fibroblasts in bone in response to tumour-secreted growth factors, leading to release of growth factors from bone matrix that may promote the development of skeletal metastases.15 Denosumab, a monoclonal antibody against RANK-L, appeared to inhibit castrate-resistant prostate cancer metastasis to bone in a phase III trial.15 Radium-223, an alpha-emitting calcium mimetic radioisotope that selectively binds the hydroxyapatite moiety of the osteoblastic microenvironment, also unexpectedly prolongs survival compared to placebo, consistent with a changing paradigm of how direct modulation of the metastatic microenvironment may impact on disease natural history.16
Immune cells in the microenvironment are important potential therapeutic targets. The host immune response can induce tumour suppression, but this response is generally ineffective due to local secretion of immunosuppressive agents.17 For example, T-lymphocytes express the membrane protein CTLA-4, which when activated, downregulates activity of the immune system. Ipilimumab, a monoclonal antibody to CTLA-4, promotes T-cell activation, allowing the immune system to destroy cancer cells.17,18 Preliminary studies of ipilimumab have demonstrated anti-tumour activity in patients with metastatic castrate-resistant prostate cancer.18,19 Immune cells are similarly targeted by sipuleucel-T therapy, an ex vivo autologous cellular immunotherapy derived from a patient’s own peripheral leukocytes.20 The extracted leukocytes are incubated with a fusion protein consisting of a prostate cancer antigen recombinantly linked to granulocyte-macrophage colony stimulating factor. Following this ex vivo activation, the blood product is reinfused into the patient to elicit an immune response against the prostate cancer cells.20 Intriguingly, despite relatively modest tumourolytic activity and prostate specific antigen (PSA) response rates, sipuleucel-T treatment has been associated with clear improvements in overall survival, and is the first successful vaccination-based approach for prostate cancer.20
Breast cancer is the most common cancer in Australian women, excluding non-melanoma skin cancer, and is the second leading cause of cancer-related death in Australian women. Three main receptors are used to classify breast cancers: the oestrogen receptor (ER); the progesterone receptor (PR); and human epidermal growth factor receptor (EGFR) type 2 (HER2).21 The pattern of metastatic spread in breast cancer is dependent on the receptors present on the cancer cells,22 with bone being the most common site of metastases.23
ER- and PR-positive cancer cells are typically dependent on hormonal signalling for growth in metastatic sites ab initio, and adjuvant hormonal therapies are effective in reducing the frequency of micrometastatic spread of these breast cancers.24 ER-positive lobular carcinomas, which do not express the epithelial-stromal adhesion factor E-cadherin, metastasise more often to serosal surfaces (for example, pleura and peritoneum) than do invasive ductal cancers. 25
The survival benefits of adjuvant cytotoxic therapy, particularly in ER-negative premenopausal patients with lymph node metastases,26 have been associated with indicators of stromal toxicity such as neutropaenia,27 consistent with a therapeutic role for paracrine loop disruption. The small but significant survival benefit associated with post-mastectomy radiotherapy, suggests further that the cytotoxic modulation of the local peritumoral microenvironment may improve not only local recurrence rates, but also frequencies of distant relapse.28
Chemotherapy combined with adjuvant anti-HER2 humanised monoclonal antibody, significantly improves clinical outcomes and survival for patients with HER2-positive metastatic breast cancer.29,30 Metastatic disease often progresses and resistance to HER2 antibody can be encountered over time; alternative therapies and targets are thus being explored. Recent studies have used chemotherapy with adjuvant combinations of HER2 antibodies (trastuzumab and pertuzumab) that target different regions of the receptor and found clinical efficacy.31 Dual inhibitors of EGFR, HER2 and HER4 (such as lapatinib, afatinib and neratinib) are also being trialled for metastatic breast cancer.32
Approximately 10% to 15% of women with metastatic breast cancer will develop brain metastases, patients with HER2-positive or triple-negative breast cancer being more susceptible. Conventional systemic therapies are limited in this context related to blood-brain barrier (BRB) permeability. Novel approaches are currently being explored including BRB permeable cytotoxic agents, physical disruption of the BRB, and HER2-directed therapies.33
Bone metastases in breast cancer tend to result in abnormal bone resorption (osteolysis), predisposing to pathological fractures, in contrast to the bone formation (osteosclerosis) noted above for observed for metastatic prostate cancer. Bisphosphonates or denosumab are used concurrently to reduce metastatic bone complications and/or to counteract the osteoporotic effects of long-term aromatase-inhibitory hormonal therapy.34 Interestingly, the bone metastases microenvironment may induce HER2 expression, regulated by the RANK-L protein discussed above, with potential for stimulating breast cancer stem cells and bone micrometastasis.35,36
Hypoxic conditions, such as may be found in bone metastases, can lead to production of vascular endothelial growth factor (VEGF), and tumour-related vessel growth. However, despite this plausible rationale for targeted therapy, adjuvant use of bevacizumab (VEGF antibody) with chemotherapy for breast cancer metastases does not increase overall survival,37 is associated with increased adverse outcomes, and is not indicated for metastatic breast cancer. This lack of preventative adjuvant efficacy is also true of small-molecule kinase-inhibitory anti-angiogenic drugs such as sorafenib and sunitinib.38,39
There is also evidence for reduced rates of breast cancer incidence and post-primary recurrence, associated with non-pharmacological modulation of the normal metabolic microenvironment by interventions such as weight loss and / or physical activity.40,41These interventions appear to work via several physiological microenvironment-related processes, including altering the circulating hormone levels, and regulating various growth factor and/or apoptosis-related signalling pathways.42
Colorectal cancer is the second most commonly diagnosed cancer in Australia, and often metastasises to multiple sites, including the liver.43 Bevacizumab is of clinical value when used together with chemotherapy in the palliative, but not adjuvant, setting for metastatic colorectal cancer.44,45 Similarly, antibodies to the epidermal growth factor receptor (EGFR) pathway, such as cetuximab or panitumumab, are effective as tumourolytic drugs for cancers that lack constitutively activating KRAS mutations (wild-type KRAS tumours).46,47 However, EGFR overexpression or amplification in colorectal cancer biopsies is not consistently predictive of cetuximab efficacy.48,49 Several gene polymorphisms have been identified as positive outcome predictors in patients with wild-type KRAS tumours that are not cetuximab-responsive.49,50 Early onset of skin toxicity (within two weeks of starting treatment) predicts a better response to cetuximab in colorectal cancer patients.51 Although the mechanism for the severe skin toxicity has not been clearly identified, cytokine regulatory changes in skin (and tumour) microenvironment related to EGFR inhibition have been suggested.52
Regorafenib, an oral small molecule multi-kinase inhibitor, can inhibit a wide range of stromal, angiogenic and tumour-related receptor tyrosine kinases that may be important for the metastatic cascade, including promotion of new tumour vessels and lymphatic vessel formation. Recent studies show clinical benefits (prolonged survival) of regorafenib for patients with progressive metastatic colorectal cancer who have been treated previously with standard cytotoxic agents and targeted therapies.53 Preclinical observations discussed above, suggested that regorafenib regulates processes important in the tumour microenvironment, although the mechanisms involved clinically are not fully understood.
Lung cancer is the leading cause of cancer death in Australian men and women. Palliative systemic therapy for lung cancer includes anti-mitotic compounds such as cisplatin, carboplatin or taxanes.54 Molecular drug targets with demonstrated efficacy include the EGFR pathway (erlotinib/gefitinib), and the VEGF pathway.
Hypoxic conditions in the cancer microenvironment can increase the risk of metastasis,55making hypoxia and its metabolic responses potential therapeutic targets. Hypoxia stabilises the hypoxia-induced factor (HIF) family of proteins, which bind to HIF-response elements, upregulating proteins involved in angiogenesis (in particular, VEGF) and tumour invasion.55,56 HIF-1 is overexpressed in 60% of non-small cell lung cancers, and is predictive of a poor overall survival.57 Targeting HIF family proteins could potentially reduce production of downstream proteins such as VEGF, important in tumour growth and the metastatic cascade. A study investigating the effects of PX-478, a small molecule inhibitor of HIF-1 protein synthesis, found reduced tumour volume and metastatic spread in a mouse model of small-cell lung cancer.58 Phase I clinical trials have found that oral PX-478 is well tolerated, and further trials are now needed to evaluate its effectiveness in a clinical setting.59
Hypoxia can also induce tumour resistance to radiotherapy via the action of antioxidants which repair the radiation-induced DNA strand breaks.60,61 This is relevant to management of cerebral metastases, which often involves whole brain radiotherapy. Efaproxiral, a drug that allosterically modifies haemoglobin by reducing its oxygen binding affinity, may facilitate the release of oxygen to tissues, mitigating radiotherapy resistance in hypoxic tumours.62 However, phase III clinical trials investigating whole brain radiotherapy and adjuvant efaproxiral for the treatment of brain metastases from lung cancer have not, to date, found an improvement in overall survival compared to treatment with whole brain radiotherapy alone.63
Cutaneous melanoma is the fourth most common cancer in Australia, accounting for 3% of all skin cancers, but 75% of skin cancer-related deaths. Melanoma cells can spread throughout the body at early stages and the prognosis in metastatic disease remains poor. Recent studies targeting the BRAF/MEK pathway via inhibitors of the mutated BRAF kinase in patients with metastatic cutaneous melanoma have been associated with dramatic responses, and modestly improved overall survival, including those with brain metastases.64,65 Immunotherapy with the T-cell-activating CTLA4 antagonist ipilimumab, causes fewer responses but does seem to lead to greater survival gains,66 emphasising the importance of ‘off-target’ effects (in this case, immune regulation) in modulating tumour natural history.
Uveal melanoma (affecting the inner vascular pigmented layer of the eye) is the most common primary eye tumour in adults,67 and as with cutaneous melanoma, metastatic disease is associated with a poor prognosis. Intriguingly, liver metastases are seen in 95% of uveal melanoma patients with metastatic disease,68 suggesting possible therapeutic targets within the liver microenvironment. Immunotherapy using ipilimumab is currently being trialled for patients with metastatic uveal melanoma.
Metastatic disease in both cutaneous and uveal melanoma shows poor responses to conventional chemotherapeutics such as dacarbazine,69 indicating a need for new treatment strategies targeting the tumour-host interface.
National Foundation for Medical Research and Innovation.