Ranibizumab is marketed as LUCENTIS (ranibizumab injection) and was approved by the U.S. Food and Drug Administration (FDA) in June 2006 for the treatment of neovascular (wet) age-related macular degeneration (AMD).

Ranibizumab has also received regulatory approval for the treatment of neovascular AMD in more than 100 countries worldwide. Ranibizumab binds to the receptor binding site of active forms of vascular endothelium growth factor-A (VEGF-A), including the biologically active, cleaved form of this molecule, VEGF110.

VEGF is a secreted homodimeric protein that is a potent vascular endothelial cell mitogen (1). VEGF stimulates vascular endothelial cell growth, functions as a survival factor for newly formed vessels, and induces vascular permeability. VEGF expression is up-regulated by hypoxia as well as by a number of other stimuli. Although other angiogenic factors have been identified, VEGF is perhaps the most potent and specific, with a well-defined role in normal and pathologic angiogenesis. Inappropriate overexpression of VEGF has been hypothesized as playing a key role in the growth of solid tumors and in vascular retinopathies such as AMD (Fig. 17.1).

VEGF-A has been shown to cause neovascularization and leakage in models of ocular angiogenesis and is thought to contribute to the progression of AMD. Inhibition of inappropriate VEGF activity is a so-called antiangiogenic approach to the treatment of these diseases. The binding of ranibizumab to VEGF-A prevents the interaction of VEGF-A with its receptors (VEGFR1 and VEGFR2) on the surface of endothelial cells, reducing endothelial cell proliferation, vascular leakage, and new blood vessel formation. Ranibizumab, an anti-human VEGF, affinity-matured Fab, has been developed by Genentech as a therapeutic agent for treating ocular vascular disease by intravitreal (ITV) injection.

Human VEGF exists as at least six isoforms (VEGF121, VEGF145, VEGF165, VEGF183, VEGF189, and VEGF206) that evolve from alternative splicing of mRNA of a single gene (1). VEGF165, the most abundant isoform, is a basic, heparin binding, dimeric glycoprotein with a molecular mass of approximately 45,000 Da (1). Two VEGF receptor tyrosine kinases, VEGFR1 and VEGFR2, have been identified (2,3,4,5,6,7). VEGFR1 has the highest affinity for VEGF, with a Kd of approximately 10 to 20 pM (8), and VEGFR2 has a somewhat lower affinity for VEGF with a Kd of approximately 75 to 125 pM (9,10,11).

VEGF has numerous biologic functions, including regulation of VEGF gene expression under hypoxic conditions (1), mitogenic activity for micro- and macrovascular endothelial cells (4,5,6,7,12,13,14), and induction of expression of plasminogen activators and collagenase (15). Of particular significance to wet AMD are the angiogenic properties of VEGF, which have been demonstrated in a variety of in vivo models, including the chick chorioallantoic membrane (15,16), rabbit cornea (17), and rabbit bone (17). VEGF also functions as a survival factor for newly formed endothelial cells (16,18). Consistent with prosurvival activity, VEGF stimulates expression of the antiapoptotic proteins Bcl-2 and A1 in human endothelial cells (19). VEGF has been shown to induce vascular leakage in guinea pig skin (19). Dvorak et al. (20,21) suggested that an increase in microvascular permeability is a crucial step in angiogenesis associated with tumors and wound healing. According to this hypothesis, a core function of VEGF in the angiogenic process is the induction of plasma protein leakage. This outcome would result in the formation of an extravascular fibrin gel, which serves as a substrate for endothelial cells. This proposal may have substantial bearing for AMD as it is well known that permeability of the choroidal neovascularization (CNV) membrane results in the transduction of serum components beneath and into the retina, creating serous macular detachment, macular edema, and vision loss.

VEGF is expressed in an assortment of cells in the normal human retina. Colocalization of VEGF mRNA and protein is detected in the ganglion cell, inner nuclear and outer plexiform layers, the walls of the blood vessels, and photoreceptors (22). Retinal pigment epithelium (RPE), Müller cells, pericytes, vascular endothelium, and ganglion cells all manufacture VEGF (23,24).

Studies have documented the immunohistochemical localization of VEGF in surgically resected CNV membranes from AMD patients. Kvanta et al. (20) demonstrated the presence of VEGF mRNA and protein in RPE cells and fibroblast-like cells. Lopez et al. (19) noted that the RPE cells that were strongly immunoreactive for VEGF were present primarily in the highly vascularized regions of CNV membranes, whereas the RPE cells found in fibrotic regions of CNV membranes showed little VEGF reactivity. Kliffen et al. (25) also demonstrated increased VEGF expression in RPE cells and choroidal blood vessels in maculae from patients with wet AMD compared with controls.

An increase in VEGF expression has been noted in experimental models of CNV in rats and in nonhuman primates (26). In addition, transgenic mice with increased VEGF expression in photoreceptors (27) or RPE (28) developed neovascularization reminiscent of CNV seen in
humans with neovascular AMD. This further supports the participation of VEGF in ocular neovascularization. These lines of evidence suggest that VEGF is a practical target for therapeutic intervention in neovascular AMD.