Anti-Angiogenesis Drug Discovery and Development

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Antiangiogenesis was proposed as a cancer therapy over 20 years ago and the list of compounds reported to possess antiangiogenic activity is extensive. Inhibitors are grouped as specific and non-specific, depending on whether they inhibit proliferation and/or migration of endothelial cells only or are also cytotoxic for tumor cells. Antiangiogenic therapy is applicable to a wide variety of solid and hematological tumors and there is evidence that tumors do not develop resistance to its effects due to the low mutagenic potential of endothelial cells. It is too early to predict whether antiangiogenesis will be of benefit in hematological malignancies. Strategies that target both the stromal and tumor compartments, such as combining traditional cytotoxic chemotherapy with antiangiogenic agents, may indeed have an impact on drug resistance and improve the therapeutic response. Conventional chemotherapeuticals used at very low doses, strikingly and reversibly impact on certain endothelial cell functions without nonspecific cytotoxic or necrotic damage. The use of low doses in “metronomic” chemotherapy (namely, very frequent or continuous low-dose chemotherapy) as antiangiogenic targeting strategy seem particularly effective against drug-resistant tumors, especially when combined with a secondary antiangiogenic drug. Further studies are needed to secure the comprehensive understanding and to elucidate the molecular basis of the use of these new therapeutic approaches in the treatment of hematological malignancies that actually are suboptimally treated with conventional cytotoxic therapy.

Angiogenesis, the formation of new blood vessels from the pre-existing vasculature, is one main mechanism of vascularisation during the embryonic development, growth, regeneration, wound healing and some physiological processes such as formation of the corpus luteum and endometrium. On the other hand, angiogenesis is also involved in a number of pathological processes such as tumor growth, and metastasis. The tumour angiogesis process is believed to be dependent on an ‘angiogenic switch’ formed and a cascade of biologic events as a consequence of the “cross-talk” between tumor cells and several components of local microenvironment including endothelial cells, macrophages, mast cells and stromal components. In this context, a growing body of evidences supports the role of ligands/RAGE axis in angiogenesis. Upon RAGE engagement, profound effects are reported not only in endothelial cells but also in endothelial progenitor cells and many immune cells located at tumor microenvironment, thus turning on the angiogenic switch. The present review to highlight the mechanisms by which RAGE engagement promotes many changes, in both tumor and host cells, leading to the formation of new blood vessels. Finally, some potential pharmacological approaches to circumvent the deleterious effects of RAGE engagement are also discussed; mainly focused on those directed to inhibit or destroy RAGE ligands, such as the use amadorins, inhibitors of AGE formation and cross-link breakers; as well as those approaches targeting RAGE activation, either by blocking RAGE binding, or down-regulating its expression.

The roots of Panax ginseng C.A. Meyer, better known as ginseng, have been the most recognized herbal medicine for thousands of years in China, Korea and Japan. The major active constituents of ginseng are ginsenosides, a diverse group of steroidal saponins. More than 40 types of ginsenosides have been identified; in recent years, new ginsenosides such as ginsenoside Rp1 and Rg5 are being synthesized and characterized. Many studies have demonstrated that most of the pharmacological and therapeutic actions of ginseng in the treatment of cancers, diabetes and cardiovascular disorders, or in modulation of central nervous system and endocrine functions, are attributed to ginsenosides. This review focused on two commercially available ginsenosides - Rg3 (Rg3 Shenyi Jiaonang from China) and it metabolites Rh2 (GOOD LIFE ginsenoside Rh2 capsule from Taiwan), and their effects on the inhibition and prevention of cancers. Existing evidence shows that prolonged administration of red ginseng extracts significantly reduces the incidence of cancer. Animal studies also demonstrate that Rg3 can lower the lung cancer risk, while Rh2 exhibits a tendency of similar benefit. Biological investigations indicate that Rg3 and Rh2 inhibit tumor growth in vitro and in vivo through induction of cell death, inhibition of cell proliferation, invasion and metastasis. We also observed the anti-cancer properties of Rg3 which are related to anti-angiogenesis. Rg3 abolished in vitro tubulogenesis of endothelial cells, in vivo neo-vessels formation in the Matrigel plug model and ex vivo endothelial sprouting of rat aortic fragments. Moreover, DNA microarray analysis further elucidates that Rg3 can modulate the expression of some genes which are related to chromatin remodeling, cytoskeleton, apoptosis, protein folding and tumor suppression. On the other hand, multi-drug resistance is another major concern in cancer chemotherapy. Rg3 can compete with anticancer drugs for binding to Pglycoprotein thereby reducing the drug efflux. To this end, both of Rg3 and Rh2 exhibit promising anti-cancer and angiosuppressive abilities. However, in order to make “ginsenoside” a safe and effective modern medicine, a deeper understanding of its molecular mechanisms, animal studies and clinical evaluation are essential.

The strategies currently used to halt the development and progression of atherosclerosis and the restenotic neointimal lesions are suboptimal. Understanding the mechanisms of angiogenic growth within the neointima of atherosclerotic and restenotic lesions may lead to the development of new therapies designed to halt the progression of atherosclerosis and inhibit restenosis after percutaneous coronary intervention. Intra-plaque hemorrhage as a result of premature neointimal vasa vasorum represents a critical event in the induction of instability in atherosclerotic coronary lesions. The removal of abnormal neovascularization by molecular therapies may interfere with intra-plaque hemorrhage and hence may halt the progression of atherosclerosis. The growth factors, the angiogenic enzymes, the chemokines, the endothelial specific receptors, and the adhesion molecules, which are involved in the expansion of vasa vasorum, constitute potential targets for such therapies. The vascular endothelial growth factor (VEGF) has an important role in the induction and the maintenance of new blood vessel formation. Thus, the local administration of soluble VEGF receptor, anti-VEGF antibodies, or antibodies that bind to the VEGF receptor, will block VEGF binding to its receptors and thereby block its angiogenic effects. Either stent- or catheterbased therapy may be a viable treatment option for delivering drugs, such as the VEGF-specific antibody (bevacizumab). By inhibiting intralesion angiogenesis, and thus the thickening of neointima, by catheter-based therapy, we may halt the development of high risk unstable plaques and restenosis after coronary interventions. In addition, a combined therapeutic approach with balloon-based strategies with antiangiogenic, antithrombotic, and prohealing agents, and with stents targeting plaques at multiple steps would be more effective.

The regulation of angiogenesis, both its increase or decrease, could be an important therapeutic target for a number of diseases. The role of angiogenesis in atherosclerosis and other cardiovascular diseases is currently controversial. Indeed, angiogenic therapy has been widely regarded as an attractive approach to treat ischemic heart disease, but a variety of studies also suggest that neovascularization contributes to the growth of atherosclerotic lesions and is a key factor in plaque destabilization leading to rupture. Recent studies have shown that the phosphatidylinositol 3-kinase (PI3K) family of enzymes plays an important role in the regulation of tumor growth and angiogenesis. PI3Ks and their downstream effector PKB/Akt can be activated by a variety of extracellular signals and regulate a number of cellular processes including cell proliferation, survival, protein synthesis, tumor growth and angiogenesis. Several lines of evidence indicate that inhibition of PI3K suppresses angiogenesis and tumor growth. Nonselective PI3K inhibitors, wortmannin and LY294002, are commonly used to inhibit cancer cell proliferation and tumor growth, and sensitize tumor cells to the treatment with chemotherapeutic drugs and radiation. This review will dissect the role of the PI3Ks pathway in different animal models, and will describe possible future directions for the development of novel therapeutic strategies to modulate the angiogenetic process by regulating PI3K signaling.

More than a decade ago people observed that angiogenic responses could be inhibited by antioxidants or enhanced by exogenous reactive oxygen molecules such as hydrogen peroxide. Accumulating experimental evidence suggests that redox modulation of the function of thiol-containing proteins, such as the protein tyrosine phosphatases, by reactive oxygen species has a pivotal role in modulating intracellular signalings. NADPH oxidase is the sole enzymatic system identified so far that has a dedicated function of generating reactive oxygen molecules. It is not surprising that this enzyme has been implicated in modulating angiogenic responses as shown in both in vitro and in vivo studies. NADPH oxidase may exert proangiogenic effects by inducing growth factor release from parenchymal cells and promoting angiogenic activities of vascular endothelial and mural cells. Here we review the redox signaling mechanisms that are involved in modulating angiogenesis, and different NADPH oxidase isoforms that may have a role in such a process. Unlike growth factor receptor-mediated angiogenic signalings, a feature of NADPH oxidase-mediated redox signaling is that, by inhibiting protein tyrosine phosphatases, NADPH oxidase may represent an intracellular signal amplifier for multiple receptor tyrosine kinase-mediated pathways involved in angiogenesis. Pharmacological inhibitors of NADPH oxidase have been used to suppress angiogenesis in different models. Encouragingly, drug targeting of NADPH oxidase has been shown to be effective in reducing pathological angiogenesis such as retinopathy. In summary, NADPH oxidase may be a new drug target for antiangiogenic therapies, yet we are still facing great challenges in achieving specific inhibition of different NADPH oxidase isoforms by drugs.

Thymidine phosphorylase (TP) is an enzyme that acts as a proangiogenic factor. Overexpression of the enzyme was observed in various solid tumors and several chronic inflammatory diseases, where it was correlated with level of angiogenesis and elevated progression of the disease. TP inhibitors are compounds designed to interfere with the processes connected with catalytic function of this enzyme including TP-mediated angiogenesis and the degradation of important antiviral and anticancer drugs. This chapter comprehensively reviews the hitherto known TP inhibitors with further insight into their potential applications in the clinical practices.

Since the discovery of the principles of angiogenesis and the drugs with antiangiogenic properties, the science has evolved greatly in understanding the pathophysiology of several diseases like cancer and the development of drugs that act in the cascade of mechanisms related to angiogenesis. Due to the systemic action of antiangiogenic therapy, patients may experience several side effects, among them those related to the nervous system. Several neurological complications have been described in patients treated with antiangiogenic therapy. The neurological side effect profile of the new agents is largely unknown and may include central (reversible posterior leukoencephalopathy, strokes, encephalopathy, seizures) and peripheral neurotoxicity (autonomic, sensory or sensorimotor neuropathy) depending on the specific agent. The peripheral neurotoxicity of relatively older agents such as bortezomib and thalidomide is well described and health care professionals dealing with patients treated with such medications need to be aware of these complications. Further research is necessary to understand the mechanisms and foster prevention and treatment of these neurological complications.

The angiogenesis phenomenon is one of the most important parameters in developing the inflammatory reactions. Angiogenesis is a biological event mediated by several cell types and mediators, such as various types of macrophages, endothelial cells, fibroblasts growth factors, cytokines and chemokines that is essential not only in physiological processes, such as reproduction, embryonic development, and tissue repair, but also in numerous pathological conditions including autoimmune disorders and other inflammatory diseases. Autoimmune diseases are the consequence of an immune response against self-antigens, due to genetic or environmental factors which are categorized to organ- and/or nonorgan- specific and are characterized by expanding the chronic inflammatory reactions. In rheumatoid arthritis that is characterized by joints chronic inflammation, angiogenesis process play an important role in the maintenance and progression of this disease, and/or in multiple sclerosis which is an inflammatory demyelinating disorder of the central nervous system with vascular features, angiogenesis can represent a notable role in expanding the lesions contributing to disease progression. The suppression of angiogenesis by blocking the function of angiogenic mediators, or by the use of angiostatic compounds could be useful in inhibiting the inflammatory reactions. An anti-inflammatory targeting in autoimmune diseases including the inhibition of angiogenic mediator’s production, such as D-penicillamine, gold salts, sulfasalazine, methotrexat can decrease the production of angiogenic mediators in inflammatory cells. In this chapter, we will review the correlation between the angiogenesis phenomenon and immunopathology of autoimmune disease with particular focus on efficacy of antiangiogenesis drugs for inhibiting the disease progression.

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