Anti-angiogenic therapy as a cancer treatment paradigm

The inhibition of angiogenesis is an emerging therapeutic strategy for cancer treatment. In contrast to conventional therapies, anti-angiogenic therapies primarily target tumor-associated endothelial cells which serve as a lifeline for tumor growth, progression and metastasis. By blocking the supply of essential nutrients and the removal of metabolites, anti-angiogenic therapies aim to delay both primary and metastastic tumor growth while overcoming the inherent cytotoxicities of classical chemotherapies. Indeed, tumor-related angiogenesis is a multi-step process initiated by a cascade of proangiogenic factors secreted from both the tumor and host tissues. These intricate processes involve a close interaction of tumor and associated endothelial cells as well as an intimate communication between proliferating endothelial cells, stromal cells and extracellular matrix components. Inhibition of these proangiogenic mechanisms has become a major challenge for the development of anti-cancer treatment modalities. In this regard, anti-angiogenic therapies embody a potentially powerful adjunct to traditional cancer therapies. In this review, we provide an overview of traditional anti-cancer drugs and discuss the fundamentals of anti-angiogenic therapies. While presenting the salient features of the anti-angiogenic agents targeting the individual phases of angiogenesis, we highlight the potential for specific agent development as novel anti-angiogenic therapeutics. Finally, we present and summarize emerging angiogenesis inhibitors.     

Tumour angiogenesis: a novel therapeutic target in patients with malignant disease

Angiogenesis refers to the formation of new blood vessels from an existing vasculature and is recognised as a necessary requirement for most tumours to grow beyond 1-2 mm in diameter. Factors established as playing a role in angiogenesis may be divided into two principal groups: (a) those that stimulate endothelial cell proliferation and/or elongation, migration and vascular morphogenesis including vascular endothelial growth factor (VEGF), basic fibroblast growth factor (bFGF), platelet derived endothelial cell growth factor (PD-ECGF) and the tie and tek receptors, and (b) proteases and their receptors involved in the breakdown of basement membranes and the extracellular matrix (ECM) including the matrix metalloproteinases (MMPs), cathepsins and those involved in the plasmin cascade. Angiogenesis has been identified as a potential target for development of anticancer agents. The discovery of a range of naturally-occurring factors which negatively regulate angiogenesis, including the thrombospondins, angiostatin and endostatin, and the tissue inhibitors of MMPs (TIMPs), has given added impetus to this approach. Synthetic anti-angiogenic compounds have been developed, including TNP-470, carboxyamidotriazole, VEGF-tyrosine kinase inhibitors and MMP inhibitors (MMPI) which, like the naturally-occurring anti-angiogenic factors, inhibit angiogenesis in vitro and in vivo, and tumour development, growth and metastasis in vivo. Anti-angiogenic agents also enhance the antitumour activity of many conventional cytotoxic chemotherapeutic agents. Such combinations may have a particular role as adjuvant therapies following surgical resection of primary tumours. Unlike tumour cells, tumour associated endothelial cells do not develop resistance to anti-angiogenic agents. Furthermore, anti-angiogenic agents are generally cytostatic rather than cytotoxic. As such, these agents are, in general, likely to be administered over long periods of time. Therefore, as well as having proven antitumour efficacy, an anti-angiogenic compound will need to be well-tolerated if it is to become established in the clinical management of patients with malignant disease.     

Anti-VEGF strategies - from antibodies to tyrosine kinase inhibitors: background and clinical development in human cancer

Tumour angiogenesis (formation of new blood vessels supporting tumour growth and metastasis) is a result of complex interactions between the tumour and the surrounding microenvironment. Targeting tumours with anti-angiogenic therapy remains an exciting area of preclinical and clinical studies. Although many significant advances have been achieved and the clinical use of anti-angiogenic drugs is now well recognized in many solid malignancies, these therapies fall short of their anticipated clinical benefits and leave many unanswered questions like exact mechanism of action, patients' selection and monitoring response to anti-angiogenic drugs. Tumour angiogenesis is controlled by complex signaling cascades and ongoing research into molecular mechanisms of tumour angiogenesis not only helps to understand its basic mechanisms but hopefully will identify new therapeutic targets. In 2012, both monoclonal antibodies and small molecule tyrosine kinase inhibitors remain the two major clinically useful therapeutic options that interfere with tumour angiogenesis in many solid malignancies.     

Regulators of colon cancer angiogenesis

Folkman’s discovery that the growth and spread of tumours depend on angiogenesis has created new avenues of research designed to better understand cancer biology and facilitate the development of new therapeutic strategies. The survival and metastasis of tumours depend on a shift in the normal balance among myriad endogenous angiogenic and anti-angiogenic factors in favour of increased angiogenesis. Several growth factors that regulate angiogenesis in colon cancer have been identified, including the pro-angiogenic factors vascular endothelial growth factor (VEGF), platelet-derived endothelial cell growth factor (PD-ECGF) and the anti-angiogenic factor, thrombospondin. A thorough understanding of the roles that these factors play in the angiogenic process has led to the development of agents intended to inhibit tumour angiogenesis. However, the complexity and redundancy of the angiogenic process continue to present substantial challenges to the development of anticancer therapies.     

Tumor vasculature directed drug targeting: applying new technologies and knowledge to the development of clinically relevant therapies

Recognition of the dependence of solid tumor growth on the formation of new blood vessels has ignited an enormous research effort aimed at the development of new therapeutic strategies for cancer. Besides direct application of drugs inhibiting endothelial cell function during angiogenesis, tumor vasculature directed drug-targeting strategies have been investigated for this purpose. In animal models of disease, proof of principle regarding the potential of selective interference with tumor blood flow as a powerful tumor therapy has been generated to its full extent. The challenge for the coming years will be to develop these strategies into clinically applicable ones. New insights into the molecular mechanisms prevailing in the endothelium during angiogenesis and into the mechanism(s) of action of drugs with anti-angiogenic activities, as well as new techniques to identify useful tumor endothelium specific target epitopes have in recent years been exploited to meet this challenge. This review summarizes vasculature directed therapeutic strategies proven to be successful in pre-clinical models and new (drug targeting) technologies enabling the development of more effective therapeutics for the treatment of cancer.     

Evading tumor evasion: Current concepts and perspectives of anti-angiogenic cancer therapy

Within three decades, anti-angiogenic therapy has rapidly evolved into an integral component of current standard anti-cancer treatment. Anti-angiogenic therapy has fulfilled a number of its earlier proposed promises. The universality of this approach is demonstrated by the broad spectrum of malignant and benign tumor entities, as well as non-neoplastic diseases, that are currently treated with anti-angiogenic agents. In contrast to tumor cell targeting therapies, the development of acquired drug resistance (e.g., via mutations in growth factor receptor signaling genes) has not been described yet for the principal target of anti-angiogenic therapy—the tumor endothelium. Moreover, the tumor endothelium has emerged as a critical target of conventional cancer therapies, such as chemotherapy and radiotherapy. The presumption that tumor growth and metastasis are angiogenesis-dependent implies that the number of potential targets of an anti-cancer therapy could be reduced to those that stimulate the angiogenesis process. Therefore, the set of endogenous angiogenesis stimulants might constitute an “Achilles heel” of cancer. Direct targeting of tumor endothelium via, e.g., endogenous angiogenesis inhibitors poses another promising but clinically less explored therapeutic strategy. Indeed, the majority of current anti-angiogenic approaches block the activity of a single or at most a few pro-angiogenic proteins secreted by tumor cells or the tumor stroma. Based on our systems biology work on the angiogenic switch, we predicted that the redundancy of angiogenic signals might limit the efficacy of anti-angiogenic monotherapies. In support of this hypothesis, emerging experimental evidence suggests that tumors may become refractory or even evade the inhibition of a single pro-angiogenic pathway via compensatory upregulation of alternative angiogenic factors. Here, we discuss current concepts and propose novel strategies to overcome tumor evasion of anti-angiogenic therapy. We believe that early detection of tumors, prediction of tumor evasive mechanisms and rational design of anti-angiogenic combinations will direct anti-angiogenic therapy towards its ultimate goal—the conversion of cancer to a dormant, chronic, manageable disease.     

Vascular endothelial growth factor and anti-angiogenic peptides as therapeutic and investigational molecules

The fundamental importance of angiogenesis in health and disease makes both inhibition and stimulation of blood vessel formation a major therapeutic goal. Vascular endothelial growth factor (VEGF) is the single most important angiogenic factor in disease-associated neovascularization and is both a major focus for the development of anti-angiogenic medicines for cancer and a candidate pro-angiogenic cytokine for the treatment of ischemic heart disease. The development of VEGF-specific or vascular-specific peptides is an emerging approach to the design of therapeutic molecules targeted against pathophysiological angiogenesis. This article highlights recent progress toward the development of anti-angiogenic and vascular-specific peptides with a focus on peptidic VEGF receptor antagonists.     

Targeting angiogenesis for the treatment of prostate cancer

INTRODUCTION: While multiple therapies exist that prolong the lives of men with advanced prostate cancer, none are curative. This had led to a search to uncover novel targets for prostate cancer therapy, distinct from those of traditional hormonal approaches, chemotherapies, immunotherapies and bone-targeting approaches. The process of tumor angiogenesis is one target that is being exploited for therapeutic gain. AREAS COVERED: The most promising anti-angiogenic approaches for treatment of prostate cancer, focusing on clinical development of selected agents. These include VEGF-directed therapies, tyrosine kinase inhibitors, tumor-vascular disrupting agents, immunomodulatory drugs and miscellaneous anti-angiogenic agents. While none of these drugs have yet entered the market for the treatment of prostate cancer, several are now being tested in Phase III registrational trials. EXPERT OPINION: The development of anti-angiogenic agents for prostate cancer has met with several challenges. This includes discordance between traditional prostate-specific antigen responses and clinical responses, which have clouded clinical trial design and interpretation, potential inadequate exposure to anti-angiogenic therapies with premature discontinuation of study drugs and the development of resistance to anti-angiogenic monotherapies. These barriers will hopefully be overcome with the advent of more potent agents, the use of dual angiogenesis inhibition and the design of more informative clinical trials.     

Matrix metalloproteinases as therapeutic targets in cancer

Degradation of extracellular matrix is crucial for malignant tumor growth, invasion, metastasis and angiogenesis. Matrix metalloproteinases (MMPs) are a family of zinc-dependent neutral endopeptidases collectively capable of degrading essentially all matrix components. Elevated levels of distinct MMPs can be detected in tumor tissue or serum of patients with advanced cancer, and their role as prognostic indicators in cancer has been widely examined. In addition, therapeutic intervention in tumor growth and invasion based on inhibition of MMP activity is under intensive investigation and several MMP inhibitors (MMPIs) are in clinical cancer trials. Even though results of the first clinical trials in advanced cancer have been mostly disappointing, there are also positive results. Recent observations show, that certain MMPs limit tumor growth. Therefore, identification of proper MMPs for therapeutic intervention with array-based molecular classifications of tumors and targeting these with more specific MMPIs in combination with conventional chemotherapy is expected to provide a feasible approach for cancer therapy. MMPIs represent a totally different therapeutic modality from proven anti-cancer drugs and thus traditional approaches to evaluate drug efficiency cannot be used without modification. In this review, we discuss the current view on the feasibility of MMPs as targets for therapeutic intervention in cancer.     

Receptor tyrosine kinase inhibitors as anti-angiogenic agents

Receptor tyrosine kinases (RTKs) and their ligands have been implicated in angiogenesis and current data suggest that they are potential cancer therapeutic targets. Split-kinase domain RTKs, including receptors for vascular endothelial growth factor receptor, platelet-derived growth factor and fibroblast growth factor, have important roles in tumor angiogenesis. RTK inhibitors targeting tumor microvasculature have shown promising results in preclinical studies, and use of these compounds as single agents, or in combination with other cytotoxic therapies, including radiation, is an area of active clinical investigation. Preclinical and clinical data from representative anti-angiogenic RTK inhibitors will be discussed in this review.     

Angiogenesis inhibitors in cancer therapy: mechanistic perspective on classification and treatment rationales

Angiogenesis, a process of new blood vessel formation, is a prerequisite for tumour growth to supply the proliferating tumour with oxygen and nutrients. The angiogenic process may contribute to tumour progression, invasion and metastasis, and is generally accepted as an indicator of tumour prognosis. Therefore, targeting tumour angiogenesis has become of high clinical relevance. The current review aimed to highlight mechanistic details of anti-angiogenic therapies and how they relate to classification and treatment rationales. Angiogenesis inhibitors are classified into either direct inhibitors that target endothelial cells in the growing vasculature or indirect inhibitors that prevent the expression or block the activity of angiogenesis inducers. The latter class extends to include targeted therapy against oncogenes, conventional chemotherapeutic agents and drugs targeting other cells of the tumour micro-environment. Angiogenesis inhibitors may be used as either monotherapy or in combination with other anticancer drugs. In this context, many preclinical and clinical studies revealed higher therapeutic effectiveness of the combined treatments compared with individual treatments. The proper understanding of synergistic treatment modalities of angiogenesis inhibitors as well as their wide range of cellular targets could provide effective tools for future therapies of many types of cancer.     

Extracellular tropomyosin: a novel common pathway target for anti-angiogenic therapy

Angiogenesis is characterized by the development of new vasculature from pre-existing vessels and plays a central role in physiological processes such as embryogenesis, wound healing and female reproductive function, as well as pathophysiologic events including cancer, rheumatoid arthritis and diabetic retinopathy. The growth and metastasis of tumors is critically dependent upon angiogenesis. Although targeting angiogenesis as a therapeutic strategy has to date met with limited success in the clinic, the recent FDA approval of the anti-VEGF antibody Avastin has validated the use of anti-angiogenic therapeutic strategies for cancer treatment. We have recently identified several plasma proteins having anti-angiogenic properties, including Histidine-Proline-Rich Glycoprotein (HPRG) and activated high-molecular-weight kininogen (HKa). Both of these proteins are able to induce apoptosis in endothelial cells in vitro and can inhibit angiogenesis in vivo. Recent studies from our laboratories have also identified a novel cell-surface binding protein for HKa that mediates its anti-angiogenic activity. This protein, tropomyosin, is normally found inside the cell and is associated with the actin cytoskeleton, where it plays a critical role in stabilizing actin filaments in a variety of cell types. However, in angiogenic endothelial cells, tropomyosin appears to have extracellular localization. Previous studies have also suggested the involvement of tropomyosin in the anti-angiogenic activity of endostatin, and our recent work indicates that tropomyosin may mediate the antiangiogenic activity of HPRG as well. In this review, we summarize data describing extracellular tropomyosin as a novel receptor for multiple anti-angiogenic proteins. Extracellular tropomyosin may therefore represent a previously undescribed central target for the development of anti-angiogenic therapy.     

Angiogenesis,anti-angiogenesis,and tumor suppression

Recent studies have revealed that a number of gene products such as vascular endothelial growth factor and its receptors are deeply involved in the process of angiogenesis. Most of these genes were characterized not only by the biochemical/molecular biological approach but also by the genetical approach including the use of gene-targeted mice. Furthermore, some of these genes are strongly correlated with the formation of pathological blood vessels such as tumor angiogenesis, suggesting that these gene products are good candidates for screening anti-angiogenic materials. Inhibitors against VEGF and its receptors have been most extensively studied and developed, but inhibitors to other factors such as MMPs and angiopoietins may also be useful for developing anti-angiogenic materials. In addition to these molecules, unidentified gene products could be specifically involved in certain types of pathological angiogenesis.     

Molecular and cellular regulators of cancer angiogenesis

Current cancer chemotherapeutic drugs have limited efficacy due to the fact that tumour cells are a rapidly changing target characterised by genomic instability. Unlike tumour cells, activated endothelial cells (ECs) required for angiogenesis, a process indisputably crucial to tumour growth and metastasis, were originally considered to be ideal therapeutic targets free of drug resistance. Additionally, unlike preclinical studies in mice using inhibitors targeting the powerful EC mitogen--vascular endothelial growth factor (VEGF)--overall survival benefit with anti-VEGF therapy used as monotherapy has yet to be demonstrated in phase III clinical trials. In contrast, VEGF-specific antibodies combined with current chemotherapy have resulted in improved outcomes in certain previously untreated cancers. This has led some researchers to hypothesize that combined treatments targeting other angiogenic molecules besides VEGF, and/or targeting not only ECs but other angiogenic non-EC types, may offer alternative but effective therapeutic options for eradicating malignant tumours. A rational approach to effective anti-angiogenic combination therapy will, however, require further understanding of the molecular and cellular mechanisms which undergird tumour vascularisation. Recent studies involving judicious use of powerful new genetic approaches have provided unprecedented insights into how different molecular and cellular mechanisms cooperate to build, branch and mature the growing vessel network so pivotal to tumour growth and survival. This review covers our current understanding of how the various key players--the tumour cells, stromal cells, endothelial cells and pericytes, and bone-marrow-derived haematopoietic and putative endothelial progenitors interact via their cell-derived pro- or anti-angiogenic factors to regulate tumour angiogenesis.     

Angiogenesis in glioblastoma multiforme: navigating the maze

Blood vessel formation is a fundamental process that occurs during both normal and pathologic periods of tissue growth. In aggressive malignancies such as glioblastoma multiforme (GBM), vascularization is often excessive and facilitates tumor progression. In an attempt to maintain tumors in a state of quiescence, multiple anti-angiogenic agents have been developed. Although several angiogenesis inhibitors have produced enhanced clinical benefits in GBM, many of these pharmacologic agents result in transitory initial response phases followed by evasive tumor resistance. Thus, a significant need exists for the discovery of novel and effective anti-angiogenic therapies. The development of new molecular-targeted therapeutic strategies is often complicated by the complexity of angiogenic signal transduction. Due to the labyrinthine nature of these signaling pathways, increased production of other angiogenic factors may compensate for the inhibition of key vascular targets like vascular endothelial growth factor (VEGF). Such compensatory mechanisms facilitate vascularization and allow tumor growth to proceed even in the presence of anti-angiogenic agents. This review presents the challenges of targeting the intricate vascular network of GBM and discusses the clinical implications for recent advancements in targeted anti-angiogenic drug therapy.     

OSU-A9 inhibits angiogenesis in human umbilical vein endothelial cells via disrupting Akt–NF-κB and MAPK signaling pathways

Since the introduction of angiogenesis as a useful target for cancer therapy, few agents have been approved for clinical use due to the rapid development of resistance. This problem can be minimized by simultaneous targeting of multiple angiogenesis signaling pathways, a potential strategy in cancer management known as polypharmacology. The current study aimed at exploring the anti-angiogenic activity of OSU-A9, an indole-3-carbinol-derived pleotropic agent that targets mainly Akt–nuclear factor-kappa B (NF-κB) signaling which regulates many key players of angiogenesis such as vascular endothelial growth factor (VEGF) and matrix metalloproteinases (MMPs). Human umbilical vein endothelial cells (HUVECs) were used to study the in vitro anti-angiogenic effect of OSU-A9 on several key steps of angiogenesis. Results showed that OSU-A9 effectively inhibited cell proliferation and induced apoptosis and cell cycle arrest in HUVECs. Besides, OSU-A9 inhibited angiogenesis as evidenced by abrogation of migration/invasion and Matrigel tube formation in HUVECs and attenuation of the in vivo neovascularization in the chicken chorioallantoic membrane assay. Mechanistically, Western blot, RT-PCR and ELISA analyses showed the ability of OSU-A9 to inhibit MMP-2 production and VEGF expression induced by hypoxia or phorbol-12-myristyl-13-acetate. Furthermore, dual inhibition of Akt–NF-κB and mitogen-activated protein kinase (MAPK) signaling, the key regulators of angiogenesis, was observed. Together, the current study highlights evidences for the promising anti-angiogenic activity of OSU-A9, at least in part through the inhibition of Akt–NF-κB and MAPK signaling and their consequent inhibition of VEGF and MMP-2. These findings support OSU-A9's clinical promise as a component of anticancer therapy.     

Targeting endothelial metabolism for anti-angiogenesis therapy: A pharmacological perspective

Current anti-angiogenic therapies in malignant and ocular diseases target growth factor signaling in order to attenuate excessive vascular growth. Although initial responses are promising, overall therapeutic success is limited due to insufficient efficiency, tumor refractoriness and resistance. Emerging evidence suggests that diverse growth factor signaling pathways in endothelial cells (ECs) converge onto cellular metabolism, creating an attractive target for novel alternative anti-angiogenic therapies. Recent studies show that ECs rely on glycolysis for ATP and biomass synthesis, necessary for proliferation and migration, key processes of angiogenesis. In addition, fatty acid β-oxidation (FAO) is essential for de novo nucleotide synthesis during EC proliferation. Initial proof-of-evidence has been given that administration of pharmacological inhibitors of those metabolic pathways can be used to inhibit pathological angiogenesis in vivo. Deciphering the role of other metabolic pathways and exploring the therapeutic potential of blocking these pathways await further investigation.     

VEGF-VEGFR Signals in Health and Disease

Vascular endothelial growth factor (VEGF)-VEGF receptor (VEGFR) system has been shown to play central roles not only in physiological angiogenesis, but also in pathological angiogenesis in diseases such as cancer. Based on these findings, a variety of anti-angiogenic drugs, including anti-VEGF antibodies and VEGFR/multi-receptor kinase inhibitors have been developed and approved for the clinical use. While the clinical efficacy of these drugs has been clearly demonstrated in cancer patients, they have not been shown to be effective in curing cancer, suggesting that further improvement in their design is necessary. Abnormal expression of an endogenous VEGF-inhibitor sFlt-1 has been shown to be involved in a variety of diseases, such as preeclampsia and aged macular degeneration. In addition, various factors modulating angiogenic processes have been recently isolated. Given this complexity then, extensive studies on the interrelationship between VEGF signals and other angiogenesis-regulatory systems will be important for developing future strategies to suppress diseases with an angiogenic component.