The lack of access to circulating oxygen, growth factors, and nutrients in early tumor development limits tumor growth because tumors do not have their own blood supply. Consequently, solid tumors initially are composed of only a small population of transformed cells whose growth is controlled by a balance between apoptosis and tumor cell proliferation. To overcome this problem, tumors grow toward pre-existing nearby blood vessels. Tumor cells may then infiltrate these blood vessels regionally and form vessels consisting of normal endothelial cells mixed with infiltrative tumor cells called a “mosaic vessel” (29). Since this process mainly serves the tumor periphery, further tumor expansion leads to increasing central hypoxia. This initial phase of limited tumor growth may persist for months or even years.

The next step in the process of angiogenesis is called the angiogenic switch, because the tumor switches to its angiogenic phenotype. It is a phase of rapid tumor growth and involves a multitude of peptide angiogenic factors that are produced in response to tumor hypoxia. These include the VEGF, the acidic and basic fibroblast growth factors (aFGF, bFGF), and platelet-derived endothelial cell growth factor (PD-ECGF) (30). Local angiogenesis inhibitors like thrombospondin-1, endostatin, angiostatin, or antiangiogenic antithrombin III can also be found in tumor tissue.

The angiogenic switch occurs when the tumors produce angiogenic factors in excess of local angiogenesis inhibitors. The angiogenic growth factors diffuse toward nearby pre-existing blood vessels and bind to receptors located on endothelial cells like receptors to VEGF (VEGF-R1/Flt-1, VEGF-R2/KDR/Flk-1, VEGF-R3/KDR, Flt-1, VEGF-R2, VEGF-R3/Flt-4, VEGF-R4/neuropilin-1) (31). The binding of ligands to their receptors leads to activation of endothelial cells by receptor dimerization and activation of various signal transduction pathways, like phosphorylation of tyrosine kinases, protein kinases, and mitogen-activated protein kinases (32,33,34,35,36,37,38). Once endothelial cells become activated, the original vessels undergo characteristic morphological changes and form mother vessels, which are characterized by basement membrane degradation, a thinned endothelial cell lining, increased endothelial number, decreased pericyte numbers, and pericyte detachment (39). These vessels have an enlarged diameter and are hyperpermeable compared to normal microvessels (40). Consequently, the earliest histopathological features of angiogenesis are microvascular dilatation, hyperpermeability, edema, and extravascular fibrin deposition. This transient process only lasts for a few days and in the next
step, mother vessels undergo at least four divergent morphological transformations (40,41,42,43).

The first mechanism for tumor vascularization is sprouting angiogenesis, which involves the proliferation and migration of endothelial cells from pre-existing blood vessels and the organization of tubular vascular structures (44). This is the most important and best understood process, and it requires the focal dissolution of the basement membrane of surrounding mother vessels. This is achieved by a number of proteolytic enzymes, like matrix metalloproteinases and plasminogen activator, which enable endothelial cells to exit the vessel abluminally (45).

Activated angiogenic endothelial cells proliferate rapidly and migrate into the extracellular matrix toward the angiogenic stimulus (46,47). Important factors in this step of angiogenesis are adhesion molecules known as integrins, such as the αvβ3 and αvβ5 integrin, which facilitate migration and vascular survival (48,49,50,51). Collagenases such as matrix metalloproteinases are secreted at the sprouting tips of growing vessels by endothelial cells and are responsible for the degradation of the extracellular matrix and facilitate cell invasion (52,53).

Finally, a lumen within an endothelial cell tubule has to be formed to allow for the supply of nutrients and oxygen to the tumor via circulation. This requires interactions between the extracellular matrix and cell-associated surface proteins, such as galactin-2, platelet endothelial cell adhesion molecule-1 (PECAM-1), and VE-cadherin (54,55,56). The newly formed vessels are stabilized through the recruitment of smooth muscle cells and pericytes. In this process, the angiopoietin family plays a major role, like angiopoietin-1 (Ang-1), which binds to the Tie-2 receptor on angiogenic endothelium (49).

Besides sprouting angiogenesis, various other processes may occur. Mother vessels may retain their large diameter and evolve into medium-sized arteries and veins by acquiring a smooth muscle and internal elastica. This process takes from a few days to several months. Alternatively, the endothelium of a mother vessel may form smaller separate well-differentiated vessel channels by projecting cytoplasmic structures into the lumen, which form transluminal bridges. This takes from several days to 3 weeks.

Another process is called intussusception, which involves focal invagination of connective tissue pillars from within the mother vessel and takes from several days to several weeks (41,42). During intussusception, two opposite endothelial cell membranes get in contact with each other and interendothelial junctions develop. The gap between the two newly formed vessels is filled by mesenchymal cells composed of fibroblasts and pericytes to form a pillar or an interstitial or intervascular tissue structure, and extracellular matrix proteins, such as collagen or fibrin, accumulate within the pillar. Intussusception generates vessels more rapidly than sprouting. The molecular mechanism of intussusception is not fully understood, but an increasing rate of blood flow plays a major role, as well as shear stress, which can be sensed by endothelial cells and transduced inside the cell by PECAM-1, resulting in increased expression of angiogenic factors and adhesion molecules.

Antiangiogenic drugs are classified according to their main mechanism of action. True angiogenesis inhibitors do not destroy pre-existing blood vessels within a tumor, but only stop the formation of neovasculature. The expected effect of true angiogenesis inhibitors is thus disease stabilization rather than tumor regression. Contrary to that, vascular targeting agents also destroy the pre-existing tumor vasculature. Finally, nonselective antiangiogenic agents, which are basically conventional chemotherapeutic agents, show cytotoxic, antiproliferative, or anti-invasive effects on multiple cell types, including angiogenic endothelial cells. Especially at low concentrations, several conventional cytotoxic chemotherapeutic drugs have shown antiangiogenic effects. A multitude of antiangiogenic drugs targeting different steps in the angiogenic cascade have been or are currently being studied in clinical trials, and only a short overview of the most important ones are presented here.