The Potential of Angiostatic Approaches to Cancer Therapy

The Potential of Angiostatic Approaches to Cancer Therapy

Introduction

Angiogenesis is the growth of new capillaries which occurs in adults in response to any physiological stimuli in the body like when a wound is healing or one is doing exercise. It is in this line that Angiogenesis is considered as a promising target in the treatment of cancer. Most of the angiogenesis inhibitors approved for cancer treatment act indirectly in the endothelial cells. Angiogenesis either neutralizes angiogenic growth factor from the circulation or block the signaling pathways activated by these growth factors. Angiostatic compounds are one of the groups of angiogenesis inhibitors. A large number of angiostatic molecules have been used of which many have been proved valuable in clinical conventional chemotherapy in reducing tumor loads and prolonging survival (Ferrara and Kerbel, 2005). These compounds have the direct effect on the endothelium, affecting cellular regulatory pathways independently of the tumor cells. Angiogenesis is therefore considered a process of new capillary out growth from pre-existing blood vessels. It is one of the most important alterations done in cell physiology that collectively dictate malignant growth (Weinberg and Hanaham, 2000). Angiogenesis occurs in a coordinated series of steps which can be divided into: destabilization, a proliferation and maturation phase. Inhibition of angiogenesis can prevent diseases that are associated with excessive vessel growth such as cancer, diabetes and arthritis (Griffioen and Molema, 2000).

In a study conducted by the Istituto Clinico Humanita, Rozzano, Italy, the mitogen-activated protein kinase c-jun N-terminal kinase has some controversial role in the process of angiogenesis. The study revealed some eveidence supporting JNK as both positive and negative regulator of blood vessel growth. In the study phosphorylated JNK was observed in cultured endothelial cells and when observed, the levels were constant regardless of the composition of the extracellular matrix composition. When sp600125 was used, inhibition of JNK tremendously reduced endothelial cell proliferation and migration in vitro (Silvio, 2008). A downstream target of JNK, that is, JNK inhibition and siRNA knockdown of c-jun substantially reduced protein levels of the transcription factor Egr-1. Egr is a regulator of genes which is involved in proliferation and migration. Another reduced production was Matrix metalloproteinase-2(MMP-2). This was attributed to the fact that the sprout culture was treated with sp600125. Both MMP-2 and membrane type -1(MT1)-MMP mRNA in endothelial cells were reduced by silencing of c-jun as activators of proteolysis. These findings provided evidence that JNK and its target downstream of c-jun regulate angiogenesis positively via the activation of endothelial cell proliferation, migration and proteolysis (Silvio, 2008)

A negative regulator of angiogenesis, thrombospondin-1, plays a new role of I IBD-associated angiogenesis. It was also observed that a recombinant thrombospondin-1 may is likely to inhibit pathological angiogenesis. This may offer a new therapeutic approach to intestinal inflammation.

Angiostatic have also proved to be effective therapeutics for neovascular eye diseases hence reducing the rate of severe vision loss (Rosenfeld et al, 2006). However the treatments using single angiostatics have yet to demonstrate complete inhibition of the neurovascular growth in the clinic and hence have been used to delay tumor growth (McCarty et al, 2003)

The growth of the new blood vessels is considered as a fundamental biological process which is essential for the survival of living organisms. These mechanisms have evolved to facilitate the growth of the new blood vessels. In Vivo, angiogenesis is initiated by the combined activation of multiple pathways. These mechanisms may ultimately limit the therapeutic potential of antiangiogenic monotherapies; this is due to the blockage of single pathway that in turn induces the compensation by other proangeogenic pathways (Mizukani et al, 2006). During the angiogenic process, the stimulation of the endothelial cell proliferation and migration by multiple growth factors takes place first (Carmeliet, 2005). Following this, the dividing endothelial cells  mediate the controlled  degradation of the extracellular matrix (Stevenson et al, 2001), the navigation of the extracellular  through receptors and cell to cell adhesion molecules, organinizing the formation of the central lumen, then they mature into functional vessels. It has been hypothesized the combination of antiangiogenic treatments may yield higher efficacy that the monotherapy. The combined action of the three classes of angiostatic compounds, where each is targeting the different aspect of angiostatic process.

In an experiment done by the Scripps research institute in North torrey; and edited by Judah Folkman of Harvard Medical school Boston (2006). They demonstrated the synergistic effects of combining angiostatic molecules that target distinct aspects of angiogenic process which later results in to the complete inhibition of neovascular growth which is associated with development, ischemic retinotherapy, and tumor growth, with little or no effect on normal, mature tissue vasculature. The tumor vascular obliteration was used when combined with angiostatic therapy; the result was that it is associated with reduced tumor mass and increased survival in a rat gliosarcoma model, whereas individual monotherapies were in effective. It was found that treatment with a combination angiostatic therapy reduced compensatory-up regulation.

Source: Adopted from the experiment done by the Scripps research institute in North torrey; and edited by Judah Folkman of Harvard Medical school Boston (2006).

The figure above shows the combination of angiostatic therapy tumor, the disease tumor size and increases survival. In this experiment the vessels of rays were used. Section A: shows that the vessels were absent in tumors of animals treated in three days with triple combination therapy; this is shown in the lower diagram. The tumor vasculature is found to be normal in control PBS treated tumors; the upper diagram. In section B of the diagram, treated tumors are found to be highly proliferative as shown in the upper diagram as indicated by Ki-67 staining, whereas no proliferation is seen in the avascular triple combination-treated tumors as shown in the lower diagram. In section C; massive infiltrates of mononuclear cell infiltration are observed in the avascular tumor region after treatment with triple combination in the lower diagram. In the upper diagram, PBS controls small areas of mononuclear cell infiltration is seen within large areas of normal tumor growth. In section D, it was found that after six days of triple combination treatment empty cavities presented in the form of star, areas of mononuclear infiltrate shown by arrows, and the smaller areas reduced by vasculature are observed in the tumor implantation areas as represented by letter T. it is also found that the normal brain vasculature in the adjacent regions were not affected; as shown by letter N. Section represented by letter E shows that PBS treated tumors have extensive highly vascular tumors. The region represented by letter F shows that triple combination significantly increases survival. It is also worth notice that the treatments were initiated six days after the tumor implantation through the use of constant, local, convention enhanced delivery to the central tumor.

In a recent research done by the Harvard Medical School, when neovascularization was used in vivo, solid tumors were produced. When a small fragment of anaplastic Brown-pearce carcinoma implanted directly on the iris in susceptible rabbits, they vascularized. Another observation was a characteristic growth pattern that consisted prevascular, vascular and late phase. These terminated when the eye was destroyed after two weeks. The exponential increase of volume was realized to concide with vascularization of the implant. Contrary to that, inplants placed in anterior chamber, slightly a distance from the iris was not vascularized. They remained arrested at a small size after the initial growth in spheroids. Even though they look dormant as far as expansion is concerned, the vascular tumors contained viable and active tumor cells mitotically. This is because when they were reimplanted in iris, vascularization occurred followed by rapid and invasive growth. This observation clearly indicated that neovascularization is a condition for the malignat growth of solid tumor. However, when a small mass of tumor cells is stopped from causing new vessel ingrowth from the surrounding host tissues, population dormancy resulted. Therefore the data suggests that when you specifically block a tumor- induced angiogenesis, you may effectively control neoplastic growth.

Cancer is often referred to as a single condition, the fact is that it consists of more that one hundred different diseases. One of the characteristics of these diseases is uncontrolled growth and spread of abnormal cells. Characteristically, cancer can arise in many sites and behave differently depending on its organ of origin. It is important to note that cancer which originates from one part of the body takes its characteristics with it even if it spreads to other parts of the body. Cancer as a disease can affect almost every organ of the body in the human body. The organs of the body are made of cell which divide and multiply as the body needs them. When these cells continue to multiply at a time when the body does not need them, they result into a growth called tumor. The tumors can be either malignant or benign; where benign is non-cancerous while malignant is cancerous. The cells within the malignant tumor have the ability to invade the neighboring tissues and organs and thus they spread the disease. The mediators of tumor angiogenesis are now being elucidated, and angiostatic agents have been developed. The effect of treatment with angiostatic agents TNP-470 in an experimental model results in a significant reduction of tumor growth rate, reduced micro vascular counts and reduced fraction of viable tumor cells compared to controls. TNP-470 as single therapy has an objective tumor static

Angiogenesis in the treatment of cancer

Angiogenesis is considered to be playing a very important role in the growth and progression of cancer. It is found that the regulation of tumor angiogenesis depends on the balance of angiogenic factor and the antiangiogenic factors which are selected by both tumor cells and host infiltration cells. A number of studies have indicated that the assessment of angiogenic activity by either micro vessel density or expression of angiogenic factors in cancer can provide information independent of the conventional clinicopathological factors such as tumor staging (Stupack, 2004). Different studies have also suggested that the assessment of tumor angiogenesis may be used in the prediction cancer response to chemotherapy or radio therapy. One of the most important clinical implications of tumor angiogenesis can be said to be the development of novel strategy of anticancer therapy targeting tumor vessels instead of cancer cells. The major reason for Antiangiogenic therapy is to inhibit the growth of tumor. From the current evidences, it is suggested that antoangiogenic works best in combination with conventional cytotoxic chemotherapy. Recently, a monoclonal antibody against vascular endothelial growth factor, which is one of the most potent angiogenic factors, has been approved for clinical use in colorectal cancer patients after a clinical trial confirmed that combining the antibody with standard chemotherapy regimen could prolong patient survival.

Angiogenesis is required for solid tumors to grow beyond approximately 1-2mm. Highly vascularized tumor is associated with poor clinical prognosis, this is not only because of the potential for exponential tumor growth but also because of the increased access capacity to the capillaries (Williams et al, 2001). The methods to inhibit the angiogenic sprouting provide a unique opportunity to arrest tumor growth and prevent the formation of metastasis, either alone or in combination with conventional therapies. The combination of angiogenic inhibitors with radiation (Mauceri et al, 1998), gene therapy (Wilezynska et al, 2001) or chemotherapy (Techer et al, 1992) has been shown to be successful. The stimulation of angiogenesis by malignant cells has been shown to play a very important role in the process of tumor proliferation and metastasis (Alvarez et al, 1999). The inhibition of angiogenesis is therefore a promising way to curb the tumor growth and prevent metastasis. The approach of using antiangiogenic combination therapy provides the best evaluation of the agents which proves to be beneficial. The results show that the combination of antiangiogenic agents with chemotherapeutic agents alone will produce a greater effect than combinations of antiangiogenic agents alone. Studies also indicate that combining antiangiogenic compound anginex with conventional chemotherapy allows the dosage of the chemotherapeutic to be reduced while maintaining the ability to effect tumor growth and reduction (Cao, 2004).

The identification of specific makers of tumor endothelium is considered to be an important field of research that may reveal the novel therapheutic targets based on angiogenesis inhibition. The neutralization of angiogenic factor; Vascular endothelial growth factor (VEGF) has been found to prolong life expectancy of patients with colon, breast and lung cancer. This approach of neutralization has demonstrated proof of concept for clinical application of antiangiogenesis compounds, targeting VGEF is a way of treating tumor cells which proves difficult due to different resistance that can be acquired by virtue of plasticity of those cells (Griffioen, 2007). A better strategy for angiogenesis inhibition is therefore considered as a pivotal determinant of the biology of the endothelial cells, this in turn lead to growth inhibition or cell death without tumor cell interference. This approach takes an advantage of the inherent attraction of angiogenesis inhibition i.e. the endothelial cell compartment which is generally stable and therefore it is unlikely to mutate into drug resistant variants hence it is considered to be superior target compared with tumor cell.

Most of the deaths caused by prostrate cancer are due to metastases that are resistant to therapy (Greenlee et al 2000). The pathogenesis of the metastasis of cancer consists of a series of sequential and selective steps which include: tumor cell proliferation, angiogenesis, detachment, inversion, extravasations, survival in the circulation, adhesion to endothelial cells and growth in distant organs (Fidler, 1990). Many reports have demonstrated that in prostrate cancer and other types of tumor, the metastatic potential of tumor cells directly correlates with the expression level of several angiogenic genes including vascular endothelial growth factor (VEGF) (Balbey et al, 1999). VEGF has been shown to induce the proliferation of the endothelial cells, to increase vascular permeability, to increase endothelial cells survival and to induce the production of plasminogen activator by the cells (Senger et al, 1983). Recent studies have demonstrated that pleiotropic transcription factor NF-kB regulates the expression of multiple genes in several types of cells. Importantly, NF-kB has been shown to be constitutively activated in prostrate cancer cells (Mukaida et al, 1994). It plays an important role in the regulation of cell proliferation (Higgins et al). In the present study, highly malignant human prostrate cancer constitutively expressed high levels of NF-kB activity. The result demonstrated that the suppression of NF-kB activity in human prostate cancer cells by inhibitory proteins inhibited their tumorigenic and metastatic properties in nude mice by suppressing angiogenesis and inversion. The result therefore provided a direct evidence for the involvement of NF-kB in the regulation of angiogenesis and metasis of prostrate cells. The progressive growth and metastasis of prostrate cancer are angiogenesis-dependent. Tumor angiogenesis is mediated, by the secretion of angiogenic factors by tumor cells and host cells (Folkman 1992). The expression vascular endothelial growth factor by prostrate cancer cell has been shown to directly correlate with malignant potential (Ferrer et al, 1997). The proteomic screen as performed by Dimberg and others as well as the previously published genomic screens of tumor epithelium (Velculescu et al, 2000) may help to pinpoint more markers for direct targeting of tumor epithelial.

The progression of tumor and its development of distant metastases require the presence of an extensive vasculature (Folkman, 2000). Studies also show that active angiogenesis is a hallmark of most of the malignancies and the inhibition of this process is considered to be one of the best strategies for the treatment of tumors. It should be noted that having a fundamental understanding of the molecular differences between endothelial cells and their normal counterparts, is essential in the development of most specific and effective antiangiogenic therapies for treating cancer. Angiogenesis is not limited to pathogenic conditions and its careful evaluation is to prevent adverse effects associated with impaired normal physiologic angiogenesis. Different models of cell culture have been developed to study angiogenesis but the temporal and complex actions of all factors exerting effects on endothelial cells in vivo may not be accurately reflected in cultured endothelial cells (Beijnun, 2005). The gene-expression analysis of tumor endothelia cells can be problematic mainly because these cells are embedded in the complex tissues and they constitute only a small fraction of the cells present in a tumor.

Tumor is said to escape from immunity and failure of several anti-cancer vaccination and cellular immunotherapy approaches, this situation is said to be due to angiogenesis –mediated suppression of endothelial cell adhesion molecules involved in the in vessel wall interaction. The inhibitors of angiogenesis are: anginex, endostatin and angiostatin and chemopeutic agent stimulate leukocyte-vessel interactions by circumvention o endothelial cell energy in vivo (Cautinho et al, 2007). This it does through regulation of endothelial adhesion molecules in the tumor vessels. Peptide anginex, considered as the new angiostatic designer was the most potent at the overcoming of endothelial energy; the enhanced leukocyte-vessel interactions led to an increase in the number of tumor infiltrating leukocytes (Winter et al, 2008). This is new cancer combination of drug that destroys solid tumors by preventing the growth of the new blood vessels, which provides the essential nutrients to the growing cancer mass. Significant biological results including one case of complete cancer remission have also been shown under the compassionate use conditions. Angiostatin (4.5) molecules are found to have the characteristic of binding receptors that cause endothelial cells to undergo opoptosis (programmed cell death) such that the growth of new blood vessels, that are essential to tumor development is prevented. The growing tumor is thereby starved of the nutrition needed to survive and grow.  A prior clinical study done using low dose angiostatic cocktail to treat patients with advanced tumor showed encouraging results including the fact that some tumor necrosis and decline in tumor serum tumor makers. Cancer patients also exhibited positive clinical results; one with complete remission and another with marked reduction in tumor mass. The other patients demonstrated increased levels of Angiostatin (4.5) molecules (AS4.5) in the blood stream. Angiostatic cocktail also provide significant advantages in terms of known safety and efficacy comparing it with other therapies on the market or in development (Ingber, 2002). It is also considered to be effective against a wide variety of cancers as it attacks genetically normal epithelial cells that normally responds to tumor growth factors and which forms tumor blood supply. It is also possible that it can prevent metastatic spread of solid tumors.

The therapy of angiostatic cocktail is therefore seen as a great promise in treating prostrate, colon, and other solid human cancer. It is also said that it can be used in patients where other treatments such as hormonal blockade or therapy have failed. Angiostatic cocktail can also be used to enhance the efficacy of other therapeutic approaches (Eyetech Study Group 2002).

Conclusion

This discussion has demonstrated in a wider perspective, the daramatic, synagystic angiostatic activity by combining compounds that that inhibit distinct aspects of antistatic process. It is found that the use of triple combination angiostatis therapy results in to nearly complete inhibition of developmental and pathogenical and as well as substantial reduction in tumor associated vascular growth. It is found that the combined angiostatic therapy results into reduced tumor growth hence prolonged lives. The results of the experiment included in this paper provided the proof of the concept targeting multiple angiostatic pathways can increase the effectiveness of antiangiogenic therapy may provide an option for the treatment of neovascular diseases where complete inhibition of neovascularization is desirable. It is also found that the combination of therapies prove to be more effective than the uncombined. It is therefore concluded that angiostatic therapy is a approach in cancer therapy.

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