Novel angiogenesis inhibitors: Addressing the issue of redundancy in the angiogenic signaling pathway

Abstract

Angiogenesis, the formation of new blood vessels from established vasculature, is a fundamental process in the growth and metastasis of solid tumours. It is a complex, tightly regulated process that requires the coordinated action of antiangiogenic and proangiogenic factors, the balance of which becomes disturbed during tumour development. Vascular endothelial growth factor (VEGF) and its receptor are the key mediators of angiogenesis and targets for multiple pharmacologic agents. Many patients treated with VEGF inhibitors survive for a longer period; however, eventual resistance is associated with progressive disease and death. Multiple approaches to overcome resistance have been investigated with varying success, including the use of agents that target multiple angiogenic factors or co-administration of angiogenesis inhibitors with standard chemotherapy or radiotherapy. It would appear that the future of angiogenic inhibitors lies in the intelligent combination of multiple targeted agents with other angiogenic inhibitors, as well as more conventional therapies to maximise therapeutic effect.

Introduction

Angiogenesis is the formation of new blood vessels by ‘sprouting’ and ‘splitting’ from established vasculature. It is a fundamental process that is required for new organ development and differentiation during embryogenesis, growth and wound healing.¹ Angiogenesis is also vital to the growth and metastasis of solid tumours.[1], [2] Vascularisation is required once a tumour reaches a certain size in order to ensure a supply of nutrients, oxygen, growth factors and proteolytic enzymes to the tumour and for the subsequent development of metastases.³

Tumour angiogenesis is triggered as the tumour core becomes hypoxic; hypoxia is thought to be a major stimulus which, along with other stimuli such as hypoglycaemia and mechanical stress,⁴ results in the angiogenic ‘switch’ and production of proangiogenic factors.³ Resting endothelial cells in the nearby blood vessels are activated by these proangiogenic factors; the subsequent release of proteases degrades the extracellular matrix,⁵ thus allowing endothelial cells to migrate towards the growth factor source, proliferate and eventually differentiate into new vessels.⁶ In tumours, these new vessels lack the typical branching organisation of normal vascular networks and have no defined arterioles, venules and capillaries. Tumour blood vessels are irregularly shaped with uneven diameters and imperfect endothelial cell linings. This abnormal endothelial arrangement results in wide junctions at some locations and stacked layers of cells at others.⁶ The abnormal organisation and ultrastructure of tumour blood vessels results in chaotic blood flow, and vessels are described as ‘leaky’.⁶ This leakiness may in part be attributed to abnormal tumour-associated pericytes.⁶ Pericytes are closely associated with endothelial cells and secrete factors, such as vascular endothelial growth factor (VEGF), that are important in endothelial cell differentiation, survival, stabilisation and maturation.[7], [8], [9] In the presence of a tumour, pericytes demonstrate abnormal morphology contributing to the high vascular permeability.[10], [11]

Angiogenesis is a complex, tightly regulated process. It occurs via the activation and inhibition of a number of diverse, yet inter-related signaling pathways involving many different anti- and proangiogenic factors (Table 1) which ultimately result in the activation or inhibition of endothelial cell growth.[1], [12] Some of these factors are highly specific to endothelial cells, such as VEGF, while others such as metalloproteinases, are much less specific acting on a broader range of cells.¹ In a tumour in which the angiogenic ‘switch’ has been turned on, the finely tuned balance between the anti- and proangiogenic factors favours the production of proangiogenic factors.¹³

The proangiogenic factor VEGF plays a central role in angiogenesis¹⁴ (Fig. 1), as indicated by the dependence of endothelial cell survival in newly formed vessels on VEGF¹⁴ and VEGF-induced alteration of endothelial cell behaviour.¹⁵ VEGF regulates vascular permeability and mediates vasculogenesis, angiogenic remodelling and angiogenic sprouting.⁶ Within the VEGF family there are several biologically active forms (VEGF-A, -B, -C, -D, -E and placental growth factor [PlGF]-1 and PlGF-2)¹⁶ and the biological effects of VEGF are mediated by three receptor tyrosine kinases – VEGF receptor 1 (VEGFR1) and VEGFR2¹⁴ and VEGFR3.¹⁷ VEGFR2 is known to have a key role in stimulating angiogenesis and haematopoiesis, and is the major mediator of the mitogenic, angiogenic and vascular-permeabilising effects of VEGF.¹⁴ VEGFR3 is of particular importance in lymphanogiogenesis, a key step in tumour migration.¹⁶

VEGF is expressed in most tumours, often at significantly increased levels.¹⁸ Expression of VEGF has been associated with tumour growth, angiogenesis and metastasis, and increased VEGF levels have been linked to a poor prognosis in breast cancer,[19], [20] colon cancer[21], [22], [23] and non-small cell lung cancer (NSCLC).²⁴ VEGF expression is stimulated by a number of factors (for example, oncogene expression[14], [25]); however, arguably the most important regulator is hypoxia.[14], [18], [25], [26]

A number of external factors are also involved in stimulating angiogenesis indirectly by inducing VEGF expression; these include epidermal growth factor (EGF), platelet-derived growth factor (PDGF) and cytokines.¹⁸ Indeed, despite the importance of the VEGF/VEGFR axis and its possibly obligatory role in the development of new vasculature in cancers, VEGFR is only one of several types of receptor involved in angiogenesis. The fibroblast growth factor (FGF) receptor (FGFR) and platelet-derived growth factor (PDGF) receptor (PDGFR) families have been shown to play a role in angiogenesis. FGF and PDGF control signaling between pericytes and smooth muscle cells and the recruitment of endothelial cells, providing support and stability to the vessel walls.¹⁷ Furthermore, preclinical data have shown that FGF and PDGF synergistically promote tumour angiogenesis and metastasis.²⁷