Metronomic therapy is a new type of chemotherapy in which anti-cancer drugs are administered in a lower dose than the maximum tolerated dose repetitively over a long period to treat cancers with fewer side effects. Metronomic therapy is shown to affect both tumor microenvironment and tumor cells to achieve its therapeutic effects. [1] Metronomic therapy is also cost-effective as a lower dose is used compared to conventional chemotherapy. [2] The use of metronomic therapy has been extensively investigated and can be advantageous in selected group of patients. [3] Yet, more clinical trials are necessary to generalize the method. [4]
In conventional chemotherapy, a dose close to the maximum tolerated dose is administered in a bolus manner to achieve cytotoxic effects on tumor cells. [5] However, the side effects are often significant as the cytotoxic agents also kill the fast-dividing cells normally present in the body, such as bone marrow cells and epithelial cells of the gastrointestinal tract. [6] A treatment break is thus required to allow recovery in these normal tissues.
In metronomic therapy, a lower dose, typically varying from one-tenth to one-third of the maximum tolerated dose, is administered frequently to maintain a low concentration of the drugs in the plasma. [3] It is commonly given in oral form, which is more convenient for patients and has a lower cost compared with intravenous form used in conventional chemotherapy. [5] At a low concentration, the drugs primarily act on the tumor microenvironment including the tumor endothelial cells and immune cells. As a lower dose is used, the risk of having severe side effects, such as neutropenia, is lower. [3] [7]
Conventional chemotherapy | Metronomic therapy | |
---|---|---|
Dosage | Close to maximum tolerated dose | Much lower than maximum tolerated dose |
Dosing interval | Less frequent | More frequent |
Route of administration | Various e.g. intravenous, oral | Oral |
Mechanisms of action | Cytotoxicity | Various e.g. anti-angiogenesis, immunomodulation |
Side effects | More significant | Less severe |
Multiple mechanisms of action have been studied in both pre-clinical and clinical settings. Instead of directly killing the tumor cells, the drugs in metronomic therapy suppress tumor growth mainly by inhibiting tumor angiogenesis and modulating the immune response against tumors. [1] There is also emerging evidence that metronomic therapy may also act on tumor cells by inducing tumor dormancy and senescence. [8]
Angiogenesis supports tumor growth by ensuring sufficient oxygen and nutrient supply to the rapidly-proliferating tumor cells. [9] Metronomic therapy can inhibit tumor angiogenesis by multiple mechanisms. It selectively inhibits the proliferation and induces apoptosis of tumor endothelial cells, without disrupting the endothelial cells of normal blood vessels. [8] This is probably mediated by increasing the expression of thrombospondin-1 (TSP-1), which inhibits angiogenesis. [5] Another target of metronomic therapy is the bone marrow-derived circulating endothelial progenitor cells (CEPs), which are involved in tumor angiogenesis. Metronomic therapy was found to decrease the level of CEPs. [1]
Tumor cells develop various means to evade immunosurveillance of the host. [9] The number of CD4+ CD25+ FOXP3+ regulatory T cells (Tregs) was found to increase in various types of tumors, suppressing both specific and innate immune responses. [5] Metronomic therapy can selectively inhibit Tregs and therefore activate CD8+ cytotoxic T cells and CD4+ helper T cells responsible for tumor-specific immunity, as well as natural killer cells (NK cells) involved in innate immunity. [10] [11] This selective inhibition is not observed in conventional chemotherapy which reduces the number of all lymphocyte subsets. In addition, some chemotherapeutic agents including cyclophosphamide, methotrexate, vinblastine, paclitaxel and etoposide can promote maturation and antigen-presenting ability of dendritic cells, which in turn facilitate the T-cell mediated immune response against tumors. [12]
In the breast cancer settings, several drugs that are commonly used in conventional chemotherapy such as methotrexate, cyclophosphamide, vinorelbine and fluoropyrimidines have been tested either as monotherapy or in combination with other therapies including hormonal therapy, targeted therapy and vaccines. [13]
Although there have been many cases that metronomic therapy did not create synergy with other therapies, a number of studies have proven the efficacy of metronomic therapy in treatment of intractable breast cancer. [14] In fact, a recent study presented at American Association for Cancer Research (AACR) congress in 2017 announced the effect of metronomic regimens. [15] The study also encourages further studies about finding appropriate regimen and its optimal dosage.
While prostate cancer is usually treated with surgery, metronomic therapy may be useful in castration-resistant prostate cancer (CRPC) which is the stage of prostate cancer that does not respond to medical treatment anymore. [16] Although docetaxel, a drug used as regimen was found to be effective in treatment of CRPC in 2004, [17] using docetaxel caused serious side effects such as neuropathy and fatigue. [18] This is undesirable especially when most patients with prostate cancer are old-aged. The response to newer medications, such as enzalutamide and abiraterone, is also variable. [1] However, recent studies have found out that metronomic chemotherapy using cyclophosphamide was more beneficial with fewer side effects as the therapy uses smaller amount of regimens which cause severe side effects. Since metronomic chemotherapy was proven to be an effective alternative, studies to find about the most beneficial combination of regimens with fewest side effects need to be investigated. [19]
Metronomic therapy was discovered to be effective in treatment of lung cancer as well, especially in metastatic non-small-cell lung carcinoma (NSCLC). [20] There are a variety of drugs used for cancer treatment. Vinorelbine is one of the drugs that are used for cancer treatment. Using vinorelbine as a regimen was shown to be feasible for very elderly patients who tend to have multiple comorbidities which is a condition that a patient has more than two diseases at a time, and require multiple medications [20] but, the effect of metronomic therapy is not only limited to aged patients. Etoposide may also be effective in these frail patients. Although metastatic or recurrent lung cancer is difficult to be completely cured with modern medical technology, metronomic therapy is feasible as a palliative therapy by reducing tumor burden and improving patients' quality of life. Future research is expected to be experimenting effects of combinations with vinorelbine and other treatments. [21]
Metronomic therapy has been investigated for treatment of metastatic ovarian cancer as it is less costly and it improves patients’ quality of lives compared to conventional therapy. [22] It may also be useful in patients who have platinum-resistant ovarian cancer. [1]
From 2012 to 2016, six ovarian cancer patients who could not be treated with conventional therapy were treated with metronomic cyclophosphamide. [23] Although the clinical outcomes do not only depend on metronomic therapy but also previous treatments they had received, the treatment provided the progress of ovarian cancers and one case was found to have nearly complete clinical remission. Despite the encouraging results, there are not many clinical trials using metronomic therapy for ovarian cancer. Most of the findings are from case reports and pre-clinical trials. Due to few clinical trials and lack of information about the applications, it is still risky and questionable to substitute conventional therapy used in treatment of ovarian cancer. [24]
Paediatric cancers have been a challenge due to the expense of treatment. In fact, the cure rate in high-income countries and low-income countries are 80% and 10% respectively. [25] [26] Therefore, lowering the expense of paediatric cancers is a key to improve the quality of life worldwide. [2] Metronomic therapy can be a good way to reduce the expense of cancer treatment. In terms of using metronomic therapy for paediatric cancers, it has been very effective as children have stronger immunity and tend to have fewer comorbidities compared to elderly cancer patients. [27] Despite a few clinical trials, some trials still highlighted the effectiveness of metronomic therapy as well as cost-effectiveness. [28] [29]
The use of metronomic therapy is still of limited use and requires further evaluation. Currently, most of the clinical studies are phase I and II trials. [28] There are only about ten studies which have proceeded to phase III. [4]
The studies are heterogeneous in terms of patient selection, chemotherapeutic agents, dosage and dosing interval. [3] Correspondingly, the clinical outcomes are variable. Multiple reviews pointed out that further studies should be carried out to determine the most effective drugs, dosage and dosing interval according to tumor and patient characteristics. [3] [28] [29]
Different biomarkers for monitoring the patients’ response towards metronomic therapy have been tested, but the results showed that the biomarkers did not correlate well with the treatment response. [29] Without proper biomarkers, it is difficult to determine the optimal metronomic dose for the patients.
Blood biomarkers related to angiogenesis, such as VEGF, TSP-1, circulating endothelial cells (CECs) and CEPs, have been tested. It was reviewed that most studies did not show a significant correlation between the level of these biomarkers and the treatment response, in terms of clinical outcomes such as overall survival and progression-free survival. [30] This is likely due to the complex interplay of factors in angiogenesis. [30]
Besides blood biomarkers, an imaging called dynamic contrast-enhanced MRI (DCE-MRI) has also been used. It assesses the tumor vascularity by measuring blood flow, fractional intravascular volume and other related parameters. However, as it only selects one or two portions of the tumor for measurement, it may not represent the overall vascularity and predict the response. [31]
Biomarkers related to the immunomodulatory effects of metronomic therapy are also under investigation. The most commonly studied one is Tregs. With advances in technologies, not only the number of Tregs but also other properties such as receptor profile and functioning of Tregs can be studied, which may aid in finding more suitable biomarkers in the future. [31]
The term “metronomic therapy” was first used by Douglas Hanahan in 2000. [32] In his commentary on two animal studies testing the effects of metronomic dosing of chemotherapeutic agents on tumor growth, he suggested that metronomic therapy was a potential new modality of chemotherapy with clinical value. [32] [33] [34]
Chemotherapy is a type of cancer treatment that uses one or more anti-cancer drugs as part of a standardized chemotherapy regimen. Chemotherapy may be given with a curative intent or it may aim to prolong life or to reduce symptoms. Chemotherapy is one of the major categories of the medical discipline specifically devoted to pharmacotherapy for cancer, which is called medical oncology.
Bevacizumab, sold under the brand name Avastin among others, is a medication used to treat a number of types of cancers and a specific eye disease. For cancer, it is given by slow injection into a vein (intravenous) and used for colon cancer, lung cancer, glioblastoma, and renal-cell carcinoma. In many of these diseases it is used as a first-line therapy. For age-related macular degeneration it is given by injection into the eye (intravitreal).
Cyclophosphamide (CP), also known as cytophosphane among other names, is a medication used as chemotherapy and to suppress the immune system. As chemotherapy it is used to treat lymphoma, multiple myeloma, leukemia, ovarian cancer, breast cancer, small cell lung cancer, neuroblastoma, and sarcoma. As an immune suppressor it is used in nephrotic syndrome, granulomatosis with polyangiitis, and following organ transplant, among other conditions. It is taken by mouth or injection into a vein.
An angiogenesis inhibitor is a substance that inhibits the growth of new blood vessels (angiogenesis). Some angiogenesis inhibitors are endogenous and a normal part of the body's control and others are obtained exogenously through pharmaceutical drugs or diet.
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Epithelioid sarcoma is a rare soft tissue sarcoma arising from mesenchymal tissue and characterized by epithelioid-like features. It accounts for less than 1% of all soft tissue sarcomas. It was first clearly characterized by F.M. Enzinger in 1970. It commonly presents itself in the distal limbs of young adults as a small, soft mass or a series of bumps. A proximal version has also been described, frequently occurring in the upper extremities. Rare cases have been reported in the pelvis, vulva, penis, and spine.
Follicular dendritic cell sarcoma (FDCS) is an extremely rare neoplasm. While the existence of FDC tumors was predicted by Lennert in 1978, the tumor wasn't fully recognized as its own cancer until 1986 after characterization by Monda et al. It accounts for only 0.4% of soft tissue sarcomas, but has significant recurrent and metastatic potential and is considered an intermediate grade malignancy. The major hurdle in treating FDCS has been misdiagnosis. It is a newly characterized cancer, and because of its similarities in presentation and markers to lymphoma, both Hodgkin and Non-Hodgkin subtypes, diagnosis of FDCS can be difficult. With recent advancements in cancer biology better diagnostic assays and chemotherapeutic agents have been made to more accurately diagnose and treat FDCS.
Cancer can be treated by surgery, chemotherapy, radiation therapy, hormonal therapy, targeted therapy and synthetic lethality, most commonly as a series of separate treatments. The choice of therapy depends upon the location and grade of the tumor and the stage of the disease, as well as the general state of the patient. Cancer genome sequencing helps in determining which cancer the patient exactly has for determining the best therapy for the cancer. A number of experimental cancer treatments are also under development. Under current estimates, two in five people will have cancer at some point in their lifetime.
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Angiokinase inhibitors are a new therapeutic target for the management of cancer. They inhibit tumour angiogenesis, one of the key processes leading to invasion and metastasis of solid tumours, by targeting receptor tyrosine kinases. Examples include nintedanib, afatinib and motesanib.
Chemotherapy-induced peripheral neuropathy (CIPN) is a nerve-damaging side effect of antineoplastic agents in the common cancer treatment, chemotherapy. CIPN afflicts between 30% and 40% of patients undergoing chemotherapy. Antineoplastic agents in chemotherapy are designed to eliminate rapidly dividing cancer cells, but they can also damage healthy structures, including the peripheral nervous system. CIPN involves various symptoms such as tingling, pain, and numbness in the hands and feet. These symptoms can impair activities of daily living, such as typing or dressing, reduce balance, and increase risk of falls and hospitalizations. They can also give cause to reduce or discontinue chemotherapy. Researchers have conducted clinical trials and studies to uncover the various symptoms, causes, pathogenesis, diagnoses, risk factors, and treatments of CIPN.
A cancer biomarker refers to a substance or process that is indicative of the presence of cancer in the body. A biomarker may be a molecule secreted by a tumor or a specific response of the body to the presence of cancer. Genetic, epigenetic, proteomic, glycomic, and imaging biomarkers can be used for cancer diagnosis, prognosis, and epidemiology. Ideally, such biomarkers can be assayed in non-invasively collected biofluids like blood or serum.
mTOR inhibitors are a class of drugs that inhibit the mechanistic target of rapamycin (mTOR), which is a serine/threonine-specific protein kinase that belongs to the family of phosphatidylinositol-3 kinase (PI3K) related kinases (PIKKs). mTOR regulates cellular metabolism, growth, and proliferation by forming and signaling through two protein complexes, mTORC1 and mTORC2. The most established mTOR inhibitors are so-called rapalogs, which have shown tumor responses in clinical trials against various tumor types.
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The tumor microenvironment (TME) is the environment around a tumor, including the surrounding blood vessels, immune cells, fibroblasts, signaling molecules and the extracellular matrix (ECM). The tumor and the surrounding microenvironment are closely related and interact constantly. Tumors can influence the microenvironment by releasing extracellular signals, promoting tumor angiogenesis and inducing peripheral immune tolerance, while the immune cells in the microenvironment can affect the growth and evolution of cancerous cells.
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