Will The Development Of Angiogenesis Inhibitors Help To Cure Cancer?

Will the Development of Angiogenesis Inhibitors help to Cure Cancer? Maria Knöbel

ABSTRACT All cells (including cancer cells) require blood vessels to deliver them oxygen and metabolites as well as to remove waste products, otherwise they become hypoxic, inducing apoptosis. Metastasis of cancer cells is also dependent on angiogenesis. Thus, this dependence of tumour growth on angiogenesis has inspired a search for anti-angiogenic drugs to treat cancer. To date a number of angiogenesis inhibitors (AIs) have already been shown to have promising anti-tumour activity and even marketed, while many more are to follow; each sabotaging cancer's pathway at a different crossing. VEGF-specific antibodies (e.g. Bevacizumab) and tyrosine kinase inhibitors (TKIs) among others all showed survival benefit either as mono-therapy or in conjunction with standard chemotherapy. But will the future of cancer treatment be in the form of AIs? Hopes are that AIs will provide a safe and comparatively non-toxic therapy. This paper aims to examine current information and trials conducted using angiogenesis inhibitors and to evaluate their potential in the treatment of cancer.

INTRODUCTION Decades ago, pathologists recognized that cancer cells grew preferentially around blood vessels. Even then they noticed how cancer cells never grew beyond 1-2mm in diameter without blood vessels, or else were seen to be dying (Folkman and Hochberg 1973). Angiogenesis, the growth of new blood vessels from existing vessels, allows cancer to receive oxygen and metabolites as well as to remove waste products, escaping hypoxia and apoptosis via the p53 mechanism (Papetti and Hernan 2002). In addition, metastasis of cancer cells is dependent on angiogenesis. Tumor cells penetrate proliferating capillaries that have fragmented basement membranes and are leaky (Zetter 1998). Once they have access to the vasculature they can exit at the target organ and must once again induce angiogenesis to proliferate (Nicolson 1988). In this way, angiogenesis is required for the beginning and completion of metastasis. This dependence of tumour growth on angiogenesis has inspired a search for anti-angiogenic drugs to treat cancer. The dynamic process of angiogenesis can be observed during physiological processes such as embryonic development, implantation of placenta, and very importantly, wound healing (Pettet et al. 1996). Angiogenesis may be stimulated by genetic changes (such as oncogene activation of tumour-suppressor mutation) (Ferrara et al. 2005) or by local imbalances such as hypoxia, glucose deprivation, and oxidative and mechanical stress (Bergers and Benjamin 2003). Physiological angiogenesis is tightly regulated by pro-angiogenic and anti-angiogenic growth factors such as endogenous inhibitors (Bergers and Benjamin 2003). Angiogenesis will take place when the delicate balance of these regulators tips to the side of pro-angiogenesis regulators (see figure 1). These are mainly vascular endothelial growth factor-A (VEGF) platelet-derived growth factor (PDGF), fibroblast growth factors (FGF), and epidermal growth factor (EGF), most of which are released into the microenvironment by malignant and inflammatory stromal cells in response to stimuli as previously described (Evans et al. 2001). When tumour cells secrete VEGF they induce tyrosine kinase activity in nearby endothelial cells causing them to proliferate and form vessels (Plate et al. 1993). Table 1 depicts a timeline of key events in the development of angiogenesis inhibitors. The first angiogenic inhibitor described was thrombospondin-1, which regulates endothelial-cell proliferation and motility (Volpert et al. 1995). Now many inhibitory molecules, such as statins, are being derived in attempts to tip this balance of angiogenesis regulators backwards, to either normalize the vasculature or destroy it. The Angiogenesis Foundation estimates that there are 30 natural angiogenesis inhibitors in the body, and over 300 have been developed to date. There are different strategies being used currently in attempts to manipulate these angiogenesis inhibitors to treat cancer. In the conventional sense, angiogenesis inhibitors were sought to deprive the tumour of oxygen and thereby destroying it. However, an alternative hypothesis described by Jain in 2005 is to normalize the vasculature to create a 'normalization window' in order to deliver drugs appropriately to the area, and targeting the cancer thus. The purpose of this paper is to examine current information and trials conducted using angiogenesis inhibitors and evaluate their potential in treatment of cancer. However, some miscellaneous inhibitors such as Thalidomide and Coxibs will not be discussed in this paper in order to provide a better treatise of the primary agents today.

Table 1. Milestones in the Development of Angiogenesis Inhibitors Year Event 1800s Tumour growth is noted to be restricted by unavailability of vasculature. 1971 J. Folkman proposes tumour growth is angiogenesis dependent. 1983 Senger et al. find tumour vessels exhibited a greater permeability than control vessels. 1985 Acidic and basic Fibroblast growth factors, potent angiogenic proteins, are isolated. (Thomas et al. 1985; Esch et al. 1985) Angiogenin determined to be potent angiogenic stimulator. (Fett et al. 1985) 1987 Tumor necrosis factor-? determined to play a role in angiogenesis by multiple independent teams. 1989 Ferrara et al, and Connolly et al. sequence VEGF/VPF. 1991 Placental growth factor (PIGF) is isolated by Maglione et al. 1992 VEGF expression is linked to hypoxia by Shweiki et al. 1993 Kim et al. first report suppression of tumour growth in vivo using monoclonal antibodies against VEGF. After which many laboratories demonstrated tumour inhibition using anti-VEGF monoclonal antibodies (Warren et al. 1995; Melnyk et al. 1996; Borgstrom et al. 1996). 1995 Thrombospondin-1 described as angiogenic inhibitor (Volpert et al. 1995) 1996 Endostatin demonstrated as endogenous inhibitor of angiogenesis and tumour growth. (O'Reilly et al. 1996) Angiopoietin, a ligand of TIE2 receptor is cloned and demonstrated to be critical in angiogenesis in vivo. (Suri et al. 1996) 2004 Bevacizumab (Avastin) becomes the first anti-VEGF monoclonal antibody approved for marketing by the U.S. FDA. VPF, vascular permeability factor; VEGF, vascular endothelial growth factor; FDA, Food and Drugs Administration.

THE ANGIOGENIC SWITCH AND ITS MOLECULAR COMPONENTS Most human tumours arise without angiogenic activity, and may even exist in situ without neovascularization for years before it switches to an angiogenic phenotype (Hanahan et al. 1996). Hypoxic tumour cells will continue to trigger p53 dependent apoptosis until a point in time when some of these cells acquire the ability to provoke neoangiogenesis: this is the angiogenic switch. Tumours are believed to produce large amounts of VEGF long before the angiogenic switch occurs; however, these are efficiently sequestered by the surrounding extracellular matrix (ECM) and therefore unable to stimulate angiogenesis. This is the reason why the angiogenic switch is accompanied by an increase in matrix metalloproteinases (MMPs) to digest the ECM and release VEGF. After Folkman first proposed that tumour growth is angiogenesis dependent in 1971, it was not until 1982 that the first angiogenic protein, basic fibroblast growth factor (bFGF), was isolated, followed shortly by acidic FGF (aFGF) (Folkman 1971; Shing et al. 1984). FGFs stimulate endothelial cell proliferation and migration in vitro, and are among the most potent angiogenic proteins in vivo. Although many different cells synthesize bFGF, it is believed that tumours recruit macrophages to secrete bFGF, or attract mast cells which may sequester bFGF due to its high heparin content (for which bFGF has great affinity) (Schulze-Osterhoff et al. 1990; Folkman et al. 1988). However, bFGF and other positive regulators of angiogenesis may operate through VEGF or be VEGF-dependent. bFGF acts synergistically and induces the expression of VEGF (Pepper et al. 1992). The action of transforming growth factor-? (TGF-?) has also been shown to be dependent on VEGF presence (Ramsauer 2007).

Figure 1. a) A scale representing the angiogenic balance and its players. b) The 'angiogenic switch' is characterised by an increase in pro-angiogenic factors leading to excess vessel development. c) The original goal of anti-angiogenesis first described by Folkman, to deprive the tumours of blood supply. (Nakamuro and Matsumoto 2005; Hamano and Kalluri 2005; two-photon images of vessels from Jain 2005)

CLASSES AND ACTIONS OF ANGIOGENESIS INHIBITORS Angiogenic inhibitors may act directly, indirectly, or through a miscellaneous mechanism (some of which are unknown). Direct agents will inhibit the ability of tumour endothelial cells to proliferate, migrate, or form new vessels by preventing them from responding to pro-angiogenic stimuli (Abdollahi et al. 2003) Indirect inhibitors decrease the expression of angiogenic factors (such as VEGF or bFGF) in the tumour or neutralize pro-angiogenic proteins in circulation; in effect interfering with downstream signalling pathways. However, since tumours have been shown to switch production of angiogenic factors over the course of their growth (Yoshiji et al. 1997) resistance to these inhibitors may be developed simply by producing a different pro-angiogenic factor. At present it is still not known what the optimal treatment strategy is, however a study by Abdollahi et al. (2003) found that combined therapy of direct (endostatin) and indirect inhibitors (VEGFR2 receptor tyrosine kinase inhibitor) [continued...]