Current Pharmaceutical Design - Volume 15, Issue 4, 2009

Angiogenesis is a complex process of development of new vessels from pre-existing vasculature. In human pathology, neovascularization is associated with excessive or insufficient vessel growth that results in initiation and development of a variety of diseases. The major pharmacological approach for the investigation of excessive angiogenesis is directed to designing new pharmaceutics to inhibit pathological vessel growth. In this context, blocking of the vascularization process in cancer progression gets the most attention. This type of biomedical research resulted in FDA approval of an anti-VEGF therapy in metastatic colorectal cancer by the humanized monoclonal antibody, bevacizumab, which is also in clinical trials for other types of tumors such as malignant gliomas. Surface receptors that are expressed on endothelial cells are also considered as a potential target for angiostatic therapy. Much attention is devoted to growth factor receptors, as well as certain integrins. In these cases, the most advanced clinical studies ongoing are with VEGFRs and αvβ3 integrin, although FGFRs and α5β1, α1β1 and α2β1 integrins are also in consideration. Targeting of intracellular signaling molecules in angiogenesis also has a long history of investigation. The pharmacological blocking of pro-angiogenic factors is mainly achieved by using humanized monoclonal antibodies or low molecular weight, synthetic compounds. Use of these compounds in therapy is usually associated with balancing of effectiveness and side effects, as well as their half-life time in the blood of patients. In the case of brain tumors, penetration of the blood brain barrier by these anti-angiogenesis agents is also very important. Analogical strategy is applied for developing pharmaceutics that will be useful in the promotion of angiogenesis. This treatment is essential for human diseases, in which insufficient angiogenesis is an important factor for progression of pathology. In this context, the most investigated are heart diseases, which require restoration of vasculature. The review articles included to this issue of Current Pharmaceutical Design, summarize recent information from the field of pharmacology of angiogenesis that may be useful for readers working in basic biomedical science and for clinicians to update information about the trends in modern cancer and heart disease therapies. In the first paper [1], the author summarizes the major approaches that were investigated during a history of angiogenesis research. Targeting vessel growth in developing solid cancers was initially proposed by Judah Folkman in the early seventies of XX century. As a surgeon he observed an increased vascularization ratio of tumor in comparison with adjacent non pathologically affected tissue. Following his ideas, angiogenesis generated a lot of attention in biomedical laboratories around the word, and each year reveals new knowledge about this process including explanation of a basic mechanism, as well as the discovery of new pharmaceutical compounds that positively or negatively regulate neovascularization. The past almost four decades of angiogenesis research definitely proved its importance for tumor growth, and showed that other diseases such as autoimmune are strongly affected by this process. In this paper the author presents all major endogenous regulators of angiogenesis with special attention to the therapy of cancer.

In a landmark publication of 1971, Folkman proposed antiangiogenesis as a potential target in cancer biology [1]. Over the past 30 years most research on tumor angiogenesis has been aimed at inhibiting the process of tumorinduced vessel formation. The first angiogenesis inhibitor, bevacizumab, was approved by the Food and Drug Administration in 2004 for the treatment of metastatic carcinoma of the colon-rectum. Antiangiogenesis remains a dynamic and evolving field in oncology. New therapeutic targets continue to emerge followed by the rapid development of new therapeutic agents to be investigated in clinical trials.

Malignant gliomas, the most common subtype of primary brain tumor, are aggressive, highly invasive, and neurologically destructive. First-line treatment of gliomas consists of surgery and radiotherapy, followed by chemotherapy with temozolomide. However, even with this strong regimen, the prognosis of patients with the most malignant variant, glioblastoma multiforme is poor. Because of the lack of effective treatments and the high vascularity that characterizes these tumors, antiangiogenic therapy of gliomas is being studied. This approach is supported by encouraging preclinical data in both in vitro and in vivo models. Clinical studies have shown that these agents do not cause high toxicity; and due to the effect they exert on vessel permeability, patients can avoid the use of corticosteroids and their accompanying adverse. Moreover, in studies of these agents, we have observed improvements in several parameters normally used to measure therapy response. However, whether these parameters are reliable for understanding and measuring the anticancer effect of antiangiogenic molecules is unknown. In addition, resistance to angiogenic therapy is already evident, and in studies performed in animal models, this resistance was associated with the appearance of more invasive phenotypes. These models give us the opportunity to further understand what causes therapy resistance and will allow us to test new combination therapies. Future studies are directed to understand if it is possible to target not only the bulk of the tumor but also the putative tumor niche composed of tumor cells, vessels, and stroma.

Angiogenesis during reactive and pathologic processes is characteristically associated with inflammation. Macrophages and dendritic cells present in the inflammatory infiltrate contribute to the angiogenic process by multiple mechanisms. Macrophages produce a broad array of angiogenic growth factors and cytokines, generate conduits for blood flow through proteolytic mechanisms, and promote the remodeling of arterioles into arteries. They can also inhibit angiogenesis and cause reabsorption of neovessels by inducing endothelial cell death. Dendritic cells can stimulate or inhibit angiogenesis depending on their activation status and subset specificity. Dendritic cells stimulate angiogenesis by secreting angiogenic factors and cytokines, promoting the proangiogenic activity of T lymphocytes, and trans-differentiating into endothelial cells. Inflammatory infiltrates associated with angiogenesis also contain Tie2+, VEGFR2+, and GR1+ myelomonocytic cells which actively regulate the angiogenic process through paracrine mechanisms. In this paper we review our current knowledge of this field and discuss how recent advances have provided the rationale for novel therapeutic approaches against cancer.

Angiogenesis is tightly regulated by opposing mechanisms in mammalian cells and is controlled by the angiogenic switch. Other review articles have described a central role for the PTEN/PI-3 kinase/AKT signaling node in the coordinate control of cell division, tumor growth, apoptosis, invasion and cellular metabolism [1, 2]. In this review, we focus on literature that supports the PTEN/PI-3 kinase/AKT signaling node as a major control point for the angiogenic switch in both the on and off positions. We also discuss the rationale for designing small molecule drugs that target the PTEN/PI-3 kinase/AKT signaling node for therapeutic intervention. Our hypothesis is that, instead of inhibiting one cell surface receptor, such as VEGFR2 with bevacizumab (Avastin®), thereby leaving a significant number of receptors free to pulse angiogenic signals, a more effective strategy may be to regulate signaling through an intercept node where redundant cell surface receptor signals converge to transmit important signaling events within the cell. This therapeutic configuration brings coordinate control over multiple cell surface receptors in concert with a physiologic response which may combine arrest of cell cycle progression with growth inhibition and the induction of genes involved in specialized functions such as movement, which are all required for the complex process of angiogenesis to occur in a temporal-spatial paradigm.

In the last fifteen years different extracellular matrix proteins and cleavage products have been identified. These molecules possess the ability to regulate vascular development, repair and function. However, the concept is still inconsistent and only partially understood. In this review, we will focus on angiogenesis regulation by extracellular matrix processing. Therefore, possible regulatory mechanisms in vascular biology controlled by different cleavage products of basement membrane proteins (e.g. endostatin and tumstatin, endorepellin), their activation by proteases and inhibitors, such as matrix metalloproteases (MMPs), cathepsins, tissue inhibitors of MMPs and cystatin, will be reviewed. Up to now there is only limited knowledge about the situations, under which different ECM cleavage products will be released and produced by proteases. Beside vascular growth and the formation of new blood vessels, it is also important to pay attention to the implication of the mentioned proteins in the vascular repair process. Physical exercise and its angio-regulatory potentials have become in the focus in recent years. We will therefore discuss physical exercise and its effects on the mentioned molecules regarding angiogenic inductions. Until today it remains to be clearly stated, which impact might be achieved by matrix cleavage products with respect to the regulation of vascular progenitor cells and their possible therapeutical role in support of vascular repair mechanisms. Furthermore, the current knowledge of the functional role of ECM in the vascular system is highlighted.

Peripheral artery disease is characterized by reduced blood flow to the lower limb, resulting in chronic ischemia in these muscles, which can lead to eventual amputation of the affected limb. Stimulation of angiogenesis in the ischemic region would be of therapeutic benefit; however, attempts to increase angiogenesis through delivery of vascular endothelial growth factor (VEGF) largely have been unsuccessful. Recent studies have shown that VEGF signaling through its receptors, VEGFR1 and VEGFR2, is much more complex than previously appreciated. This review will examine current research into the function of VEGFR1 and -2 signaling pathways, and evidence of cross-talk between these two receptors. The potential impact of endothelial cell co-stimulation via other growth factors/cell surface receptors (such as angiopoietins and ephrins) on angiogenesis also will be discussed. Evidence suggesting deficiencies in VEGF pathway signaling in individuals with chronic ischemia and diabetes will be discussed. Numerous pro-angiogenic therapies for ischemia have been employed. The successes and limitations of these therapies will be illustrated, emphasizing more recent angiogenesis therapies that focus on activating co-ordinated patterns of pro-angiogenic genes as the most promising direction in the treatment of ischemic muscle tissue in peripheral artery disease.

Neuroblastic Tumors is a group of pediatric cancers with a high incidence in the preschool age. Neuroblastic Tumors are classified in several histological categories but neuroblastoma, composed by undifferentiated cell without stromal tissue is the most aggressive disseminated tumor occurring in patients over 18 months of age. Neuroblastoma attracted the attention of scientists since the middle century because metastatic tumor in infants very often regressed while in older patients was very aggressive. In 1973 Biedler and coworkers established cell cultures from human metastatic neuroblastoma tissues and opened a new era in biological and pharmacological study of this cancer. Afterwards several human cell lines were established and transplanted in mice giving the opportunity to study the effect of drug both in vitro and in vivo. Meantime patients were carefully managed and risk of relapse was assessed by age, extension of disease, biological and genomic factors. Furthermore, specimens of primary tumors were accurately stored and biological repositories go a long way towards studying biological and molecular aspects of neuroblastoma. Although the great advances in tumor biology, patients with metastatic stage 4 disease still have the worst prognosis. From several years the Children's Oncology Group (COG) and Societe Internationale Oncologie Pediatrique Europeenne Neuroblastoma (SIOPEN) have reduced therapy for children with localized disease in which surgery is the most effective approach to improve patient survival. Risk of patients with localized disease is increased by the presence of MYCN gene amplification in tumor cells and these patients receive chemotherapy. Conversely, high-dose therapy and retinoic acid administered after bone marrow ablation are used for patients with advanced disease. Unfortunately, about 6% of patients with localized tumors and more than 50% of patients with disseminated aggressive tumor have a disease progression and died. So, new therapeutic approaches are urgently necessary. Meanwhile, COG and SIOPEN groups are working to define new genetic/biological markers in order to perform a therapy more precise in children with neuroblastoma. Information on the MYCN copy number, the most important oncogene associated with tumor progression in neuroblastoma, has gained central importance in decision- making process concerning patients' treatment. The absence or presence of MYCN amplification represents frequently the only criterion which prompts clinicians to choose between a ‘wait and see strategy’ or an aggressive cytotoxic treatment.

Immunotherapy is an attractive option for patients with high risk neuroblastoma due to their poor long-term survival rates after conventional treatment. Neuroblastoma cells are derived from the embryonic neural crest and therefore express tumor antigens not widely seen in normal cells, making them potential targets for immunologic attack. There is already considerable experience with monoclonal antibodies that target these tumor associated antigens, and in this review we focus on more exploratory approaches, using tumor vaccines and adoptive transfer of tumor-directed T cells.

Evasion of apoptosis, the cell's intrinsic death program, is a hallmark of human cancers including neuroblastoma. Also, failure to undergo apoptosis may cause treatment resistance, since the cytotoxic activity of anticancer therapies commonly used in the clinic, e.g. chemotherapy, γ-irradiation or immunotherapy, is predominantly mediated by triggering apoptosis in tumor cells. Therefore, a better understanding of the signaling pathways and molecules that govern apoptosis in neuroblastoma cells is expected to open new avenues for the design of molecular targeted therapies for neuroblastoma.

Histone deacetylases (HDACs) are an emerging class of novel anti-cancer drug targets. Recently, studies in adult cancers and in neuroblastoma have shown that individual HDAC family members are aberrantly expressed in tumors and correlate with disease stage and prognosis. In neuroblastoma, knockdown of individual HDAC family members causes distinct phenotypes ranging from differentiation to apoptosis. HDACs are involved in controlling MYCN function and are upregulated in chemotherapy-resistant neuroblastoma cells. Treatment with unselective pan-HDAC inhibitors causes cell cycle arrest, differentiation, apoptosis, and inhibition of clonogenic growth of neuroblastoma cells, and restores susceptibility to chemotherapy treatment. The molecular mechanisms mediating the anti-cancer effects of HDAC inhibitors on neuroblastoma cells are incompletely understood and involve targeting of aberrant epigenetic repression of tumor suppressor genes, activation of developmental differentiation pathways, as well as changing the acetylation level and function of non-histone proteins. In neuroblastoma mouse models, unselective HDAC inhibitors demonstrate antitumoral effects. First phase I clinical trials in children with refractory cancers using HDAC inhibitors depsipeptide and the recently approved vorinostat are underway. This review summarizes our current knowledge about classical HDAC family members as novel drug targets for neuroblastoma therapy and discusses the potential role of next generation, selective HDAC inhibitors.

Neuroblastoma is an extracranial solid tumor which occurs in infants and young children and accounts for 8% of pediatric cancers. It origins from neural crest cells of the sympathetic nervous system. Disease-free survival ranges from 95% for localized tumors to 30% for metastatic disease in children over 1 year of age and patients' outcome depends on dissemination and tissue histology. Despite the most recent therapies, the overall survival for high risk patients is still low and the outcome is invariably fatal. Improvement of neuroblastoma treatment is one of the highest priorities in pediatric oncology and a major challenge to clinicians and researchers. Understanding the biology and genetics of pediatric malignancies will be the key to identify molecular targets for innovative treatments as well as to individual management of disease. The success of human genome project and recent advances in technology have provided new tools to investigate cancer cells and to discover new tumor-associated genes. High-throughput efforts include array-based comparative genomic hybridization, single-nucleotide polymorphism arrays and expression microarrays. Here we present an overview on the most recent advances in wide-genome analysis of neuroblastoma. We also focus on the potential clinical application of genome and transcriptome information to the diagnosis, prognosis and neuroblastoma therapy.

Neuroblastoma arises from precursor cells of the sympathetic nervous system and presently accounts for 15% of all childhood cancer deaths. These tumors display remarkable heterogeneity in clinical behavior, ranging from spontaneous regression to rapid progression and resistance to therapy. The clinical behavior of these tumors is associated with many factors, including patient age, histopathology and genetic abnormalities such as MYCN amplification. More recently, the dysregulation of some miRNAs, including the miR-17-5p-92 cluster and miR-34a, has been implicated in the pathobiology of neuroblastoma. MiR-17-5p-92 family members act in an oncogenic manner while miR-34a has tumor suppressor functions. The evidence for the contribution of miRNAs in the aggressive neuroblastoma phenotype is reviewed in this article, along with exciting possibilities for miRNA mediated therapeutics.