Vascular Endothelial Growth Factor A in Intraocular Vascular Disease

The vascular beds supplying the retina may sustain injury as a result of underlying disease such as diabetes, and/or the interaction of genetic predisposition, environmental insults, and age. The vascular pathologic features observed in different intraocular vascular diseases can be categorized broadly as proliferation, exemplified by proliferative diabetic retinopathy, leakage such as macular edema secondary to retinal vein occlusion, or a combination of proliferation and leakage, as seen in neovascular age-related macular degeneration (AMD). The World Health Organization has identified diabetic retinopathy and AMD as priority eye diseases for the prevention of vision loss in developed countries. The pathologic transformations of the retinal vasculature seen in intraocular vascular disease are associated with increased expression of vascular endothelial growth factor A (VEGF), a potent endothelial-specific mitogen. Furthermore, in model systems, VEGF alone is sufficient to trigger intraocular neovascularization, and its inhibition is associated with functional and anatomic improvements in the affected eye. Therapeutic interventions with effect on VEGF include intraocular capture and neutralization by engineered antibodies or chimeric receptors, downregulation of its expression with steroids, or alleviation of retinal ischemia, a major stimulus for VEGF expression, with retinal ablation by laser treatment. Data from prospective randomized clinical trials indicate that VEGF inhibition is a potent therapeutic strategy for intraocular vascular disease. These findings are changing clinical practice and are stimuli for further study of the basic mechanisms controlling intraocular angiogenesis.Financial Disclosure(s): Proprietary or commercial disclosure may be found after the references. The vascular beds supplying the retina may sustain injury as a result of underlying disease such as diabetes, and/or the interaction of genetic predisposition, environmental insults, and age. The vascular pathologic features observed in different intraocular vascular diseases can be categorized broadly as proliferation, exemplified by proliferative diabetic retinopathy, leakage such as macular edema secondary to retinal vein occlusion, or a combination of proliferation and leakage, as seen in neovascular age-related macular degeneration (AMD). The World Health Organization has identified diabetic retinopathy and AMD as priority eye diseases for the prevention of vision loss in developed countries. The pathologic transformations of the retinal vasculature seen in intraocular vascular disease are associated with increased expression of vascular endothelial growth factor A (VEGF), a potent endothelial-specific mitogen. Furthermore, in model systems, VEGF alone is sufficient to trigger intraocular neovascularization, and its inhibition is associated with functional and anatomic improvements in the affected eye. Therapeutic interventions with effect on VEGF include intraocular capture and neutralization by engineered antibodies or chimeric receptors, downregulation of its expression with steroids, or alleviation of retinal ischemia, a major stimulus for VEGF expression, with retinal ablation by laser treatment. Data from prospective randomized clinical trials indicate that VEGF inhibition is a potent therapeutic strategy for intraocular vascular disease. These findings are changing clinical practice and are stimuli for further study of the basic mechanisms controlling intraocular angiogenesis. Financial Disclosure(s): Proprietary or commercial disclosure may be found after the references. Age-related macular degeneration (AMD), diabetic retinopathy (DR), and retinal vein occlusion (RVO) are examples of intraocular vascular diseases where a combination of predisposing factors such as age, genotype, environmental insults, and underlying systemic illness interact to damage the vascular beds supplying the retina. The ensuing pathophysiologic response may include atrophy, neovascularization, edema, hemorrhage, and fibrosis. Intraocular vascular diseases are an important health problem and pose a significant therapeutic challenge. The World Health Organization has identified AMD and DR as priority eye diseases with respect to the prevention of blindness and visual impairment in industrialized countries. Intraocular vascular diseases can be categorized broadly into 3 groups based on the observed vascular pathology: retinal, disc, and iris neovascularization are primarily proliferations; macular edema results from abnormal vascular permeability alone, whereas choroidal neovascularization (and its intraretinal subtypes) are a combination of proliferation and permeability. The objectives of this review are to evaluate the role of vascular endothelial growth factor A (VEGF) in the pathogenesis of these 3 categories of intraocular vascular diseases and to provide an overview of current and forthcoming therapeutic interventions. Vascular endothelial growth factor is a potent, diffusible, endothelial-specific mitogen that is released in response to hypoxia and upon binding to the VEGF receptor 2 (VEGFR-2), expressed by the vascular endothelium, elicits angiogenesis and vascular hyperpermeability (Fig 1; see the text and Fig A in Appendix 1, available at http://aaojournal.org, for detailed discussion on the molecular biology of VEGF). There are 3 main reasons to identify VEGF as an important factor in the pathogenesis of intraocular vascular diseases: (1) pathologic transformations of the retinal vasculature—from permeability to remodeling to neovascularization—are associated with increased expression of VEGF; (2) VEGF alone is sufficient to trigger intraocular neovascularization; and (3) inhibition of VEGF is associated with functional and anatomic improvements in the affected eye. All vascularized intraocular tissues express VEGF.1Kim I. Ryan A.M. Rohan R. et al.Constitutive expression of VEGF, VEGFR-1, and VEGFR-2 in normal eyes.Invest Ophthalmol Vis Sci. 1999; 40: 2115-2121PubMed Google Scholar Vascular endothelial growth factor is synthesized in vitro and in situ by human retinal pigment epithelium (RPE) cells and is the only endothelial mitogen produced by cultured human retinal pigment epithelium in response to hypoxia.2Shima D.T. Adamis A.P. Ferrara N. et al.Hypoxic induction of endothelial cell growth factors in retinal cells: identification and characterization of vascular endothelial growth factor (VEGF) as the mitogen.Mol Med. 1995; 1: 182-193Crossref PubMed Google Scholar Increased expression of VEGF in response to hypoxia also has been observed in other types of retinal cells, including pericytes, endothelial cells, and Müller cells.3Aiello L.P. Northrup J.M. Keyt B.A. et al.Hypoxic regulation of vascular endothelial growth factor in retinal cells.Arch Ophthalmol. 1995; 113: 1538-1544Crossref PubMed Scopus (556) Google Scholar In humans, the vitreous levels of VEGF were found to be increased significantly in patients with proliferative DR.4Adamis A.P. Miller J.W. Bernal M.T. et al.Increased vascular endothelial growth factor levels in the vitreous of eyes with proliferative diabetic retinopathy.Am J Ophthalmol. 1994; 118: 445-450Abstract Full Text PDF PubMed Scopus (1225) Google Scholar In nonhuman primate models of intraocular neovascularization, VEGF levels were reported to be elevated in ischemic retinas,5Shima D.T. Gougos A. Miller J.W. et al.Cloning and mRNA expression of vascular endothelial growth factor in ischemic retinas of Macaca fascicularis.Invest Ophthalmol Vis Sci. 1996; 37: 1334-1340PubMed Google Scholar and its expression was found to increase synchronously and proportionally with the severity of the ensuing neovascularization.6Miller J.W. Adamis A.P. Shima D.T. et al.Vascular endothelial growth factor/vascular permeability factor is temporally and spatially correlated with ocular angiogenesis in a primate model.Am J Pathol. 1994; 145: 574-584PubMed Google Scholar Intravitreal injections of recombinant human VEGF165 were sufficient to produce intraocular neovascularization and leakage in a nonhuman primate.7Tolentino M.J. Miller J.W. Gragoudas E.S. et al.Vascular endothelial growth factor is sufficient to produce iris neovascularization and neovascular glaucoma in a nonhuman primate.Arch Ophthalmol. 1996; 114: 964-970Crossref PubMed Scopus (300) Google Scholar Furthermore, the development of intraocular neovascularization is inhibited by intravitreal administration of neutralizing anti-VEGF monoclonal antibody8Adamis A.P. Shima D.T. Tolentino M.J. et al.Inhibition of vascular endothelial growth factor prevents retinal ischemia-associated iris neovascularization in a nonhuman primate.Arch Ophthalmol. 1996; 114: 66-71Crossref PubMed Scopus (588) Google Scholar or antigen binding fragment.9Krzystolik M.G. Afshari M.A. Adamis A.P. et al.Prevention of experimental choroidal neovascularization with intravitreal anti-vascular endothelial growth factor antibody fragment.Arch Ophthalmol. 2002; 120: 338-346Crossref PubMed Scopus (563) Google Scholar Taken together, these findings identify VEGF as a key factor in the pathogenesis of intraocular vascular disease. Intraocular neovascularization develops in response to retinal ischemia and includes neovascularization of the retina, neovascularization of the optic disc, or neovascularization of the iris. The neovasculature is prone to bleeding and leakage, and the ensuing tissue alterations and fibrosis disrupt the structure and function of the affected intraocular tissues.10Miller J.W. Vascular endothelial growth factor and ocular neovascularization.Am J Pathol. 1997; 151: 13-23PubMed Google Scholar Neovascularization is a main pathologic characteristic of proliferative DR and of retinopathy of prematurity, sickle cell disease, and radiation. Neovascularization and neovascular glaucoma also are frequent complications of ischemic central retinal vein occlusion.11McIntosh R.L. Rogers S.L. Lim L. et al.Natural history of central retinal vein occlusion: an evidence-based systematic review.Ophthalmology. 2010; 117: 1113-1123 e15Abstract Full Text Full Text PDF PubMed Scopus (315) Google Scholar A succession of capillary nonperfusion, inner retina ischemia, and neovascularization is a common theme in the pathogenesis of these disorders (Fig 2). In DR, there is evidence for increased expression of intercellular adhesion molecule 1 and CD18, followed by leukocyte stasis, endothelial cell injury, and blood–retinal barrier (BRB) breakdown.12Joussen A.M. Poulaki V. Le M.L. et al.A central role for inflammation in the pathogenesis of diabetic retinopathy.FASEB J. 2004; 18: 1450-1452Crossref PubMed Scopus (1002) Google Scholar The microvascular lesions in DR include thickening of the capillary basement membrane, loss of pericytes and vascular smooth muscle cells, microaneurysms, and capillary occlusions and acellularity.13Curtis T.M. Gardiner T.A. Stitt A.W. Microvascular lesions of diabetic retinopathy: clues towards understanding pathogenesis?.Eye (Lond). 2009; 23: 1496-1508Crossref PubMed Scopus (270) Google Scholar As the disease progresses, neovascularization develops in response to inner retinal ischemia. Capillary nonperfusion leading to retinal ischemia and neovascularization also may follow radiation damage of the endothelium of the retinal capillaries.14Archer D.B. Doyne Lecture Responses of retinal and choroidal vessels to ionising radiation.Eye (Lond). 1993; 7: 1-13Crossref PubMed Scopus (58) Google Scholar In retinopathy of prematurity, intraocular neovascularization develops in response to a state of relative hypoxia created by the inability of the retinal vasculature to meet the metabolic demands of the developing retina.15Penn J.S. Madan A. Caldwell R.B. et al.Vascular endothelial growth factor in eye disease.Prog Retin Eye Res. 2008; 27: 331-371Crossref PubMed Scopus (553) Google Scholar In sickle cell disease, clusters of rigid and malformed erythrocytes cause vaso-occlusions, ischemia, and neovascularization in the peripheral retina in the form of fragile formations named sea fans for their resemblance of Gorgonia flabellum.16Goldberg M.F. Retinal neovascularization in sickle cell retinopathy.Trans Sect Ophthalmol Am Acad Ophthalmol Otolaryngol. 1977; 83: OP409-OP431PubMed Google Scholar Abundant experimental and clinical evidence indicates that the proliferative response to retinal ischemia is driven by VEGF. Vascular endothelial growth factor RNA and protein have been localized in the retinal tissues affected by neovascularization.17Pe'er J. Shweiki D. Itin A. et al.Hypoxia-induced expression of vascular endothelial growth factor by retinal cells is a common factor in neovascularizing ocular diseases.Lab Invest. 1995; 72: 638-645PubMed Google Scholar The intraocular levels of VEGF are increased in patients with proliferative DR, retinopathy of prematurity, RVO, and neovascular glaucoma,4Adamis A.P. Miller J.W. Bernal M.T. et al.Increased vascular endothelial growth factor levels in the vitreous of eyes with proliferative diabetic retinopathy.Am J Ophthalmol. 1994; 118: 445-450Abstract Full Text PDF PubMed Scopus (1225) Google Scholar, 18Sato T. Kusaka S. Shimojo H. Fujikado T. Vitreous levels of erythropoietin and vascular endothelial growth factor in eyes with retinopathy of prematurity.Ophthalmology. 2009; 116: 1599-1603Abstract Full Text Full Text PDF PubMed Scopus (135) Google Scholar, 19Boyd S.R. Zachary I. Chakravarthy U. et al.Correlation of increased vascular endothelial growth factor with neovascularization and permeability in ischemic central vein occlusion.Arch Ophthalmol. 2002; 120: 1644-1650Crossref PubMed Scopus (196) Google Scholar whereas patients with pronounced proliferative pathologic features are more likely to have increased intraocular levels of VEGF compared with patients without neovascular disease.20Aiello L.P. Avery R.L. Arrigg P.G. et al.Vascular endothelial growth factor in ocular fluid of patients with diabetic retinopathy and other retinal disorders.N Engl J Med. 1994; 331: 1480-1487Crossref PubMed Scopus (3474) Google Scholar Although the pathogenesis of intraocular vascular disease undoubtedly is complex, inhibition of VEGF alone is sufficient to induce regression of the neovascular lesions in an animal model.21Aiello L.P. Pierce E.A. Foley E.D. et al.Suppression of retinal neovascularization in vivo by inhibition of vascular endothelial growth factor (VEGF) using soluble VEGF-receptor chimeric proteins.Proc Natl Acad Sci U S A. 1995; 92: 10457-10461Crossref PubMed Scopus (1179) Google Scholar Taken together, these findings identify VEGF as a major mediator of the intraocular proliferative response to retinal ischemia. Vascular leakage as a result of increased transmural hydrostatic pressure, vascular injury, or a combination of both is a common pathogenic mechanism leading to accumulation of excess extracellular fluid manifested as edema. The retina is particularly vulnerable to edematous changes because there are no lymphatic vessels that can siphon the excess fluid and protect the integrity of its complex, layered structure.13Curtis T.M. Gardiner T.A. Stitt A.W. Microvascular lesions of diabetic retinopathy: clues towards understanding pathogenesis?.Eye (Lond). 2009; 23: 1496-1508Crossref PubMed Scopus (270) Google Scholar Macular edema (ME) is a serious vision-limiting complication of RVO, DR, and uveitis22Johnson M.W. Etiology and treatment of macular edema.Am J Ophthalmol. 2009; 147: 11-21 e1Abstract Full Text Full Text PDF PubMed Scopus (158) Google Scholar and is considered to be clinically significant if it involves or threatens to spread to the center of the macula. The pathogenesis of diabetic ME is not completely elucidated. The extravasation of fluid is attributed to both an increase in the retina blood flow and to BRB breakdown.22Johnson M.W. Etiology and treatment of macular edema.Am J Ophthalmol. 2009; 147: 11-21 e1Abstract Full Text Full Text PDF PubMed Scopus (158) Google Scholar The BRB integrity is maintained by tight junctions between the retinal vascular endothelial cells (inner BRB) and the RPE cells (outer BRB), as well as by a scarcity of intraendothelial cell vesicles.23Vinores S.A. Derevjanik N.L. Ozaki H. et al.Cellular mechanisms of blood-retinal barrier dysfunction in macular edema.Doc Ophthalmol. 1999; 97: 217-228Crossref PubMed Google Scholar Vascular injury and RPE dysfunction lead to BRB breakdown accompanied by leakage.24Omri S. Behar-Cohen F. de Kozak Y. et al.Microglia/macrophages migrate through retinal epithelium barrier by a transcellular route in diabetic retinopathy: role of PKCζ in the Goto Kakizaki rat model.Am J Pathol. 2011; 179: 942-953Abstract Full Text Full Text PDF PubMed Scopus (154) Google Scholar Mechanisms contributing to the microvascular damage include the direct toxic effects of hyperglycemia, sustained alterations in cell signaling pathways including VEGF, and chronic leukocyte-mediated injury (Fig 3).12Joussen A.M. Poulaki V. Le M.L. et al.A central role for inflammation in the pathogenesis of diabetic retinopathy.FASEB J. 2004; 18: 1450-1452Crossref PubMed Scopus (1002) Google Scholar, 13Curtis T.M. Gardiner T.A. Stitt A.W. Microvascular lesions of diabetic retinopathy: clues towards understanding pathogenesis?.Eye (Lond). 2009; 23: 1496-1508Crossref PubMed Scopus (270) Google Scholar, 22Johnson M.W. Etiology and treatment of macular edema.Am J Ophthalmol. 2009; 147: 11-21 e1Abstract Full Text Full Text PDF PubMed Scopus (158) Google Scholar, 25Sheetz M.J. King G.L. Molecular understanding of hyperglycemia's adverse effects for diabetic complications.JAMA. 2002; 288: 2579-2588Crossref PubMed Scopus (833) Google Scholar In an experimental model, increased expression of intercellular adhesion molecule 1, which is critical for the adhesion of leucocytes to the vascular endothelium, retinal leukostasis, and breakdown of the BRB, is observed as early as 1 week after the induction of diabetes.26Miyamoto K. Khosrof S. Bursell S.E. et al.Prevention of leukostasis and vascular leakage in streptozotocin-induced diabetic retinopathy via intercellular adhesion molecule-1 inhibition.Proc Natl Acad Sci U S A. 1999; 96: 10836-10841Crossref PubMed Scopus (687) Google Scholar In RVO, the flow of blood in the retinal veins may be obstructed by thrombus formation at the level of the branch or central retinal vein. Increased intravascular pressure in the venous bed distal to the site of thrombus formation leads to transudation of fluid in the extracellular space (Figure 2, Figure 3). The formation of edema is accelerated further by the release of VEGF and other vascular hyperpermeability agents, such as the inflammatory cytokine interleukin 6, in response to hypoxia.27Noma H. Funatsu H. Yamasaki M. et al.Pathogenesis of macular edema with branch retinal vein occlusion and intraocular levels of vascular endothelial growth factor and interleukin-6.Am J Ophthalmol. 2005; 140: 256-261Abstract Full Text Full Text PDF PubMed Scopus (305) Google Scholar Cystoid macular edema, characterized by the accumulation of fluid into cyst-like spaces, is the most frequent complication of uveitis. It is believed that inflammatory mediators (arachidonic acid metabolites, interleukins, tumor necrosis factor-α) induce a breakdown of the BRB, which leads to leakage and cystoid macular edema.22Johnson M.W. Etiology and treatment of macular edema.Am J Ophthalmol. 2009; 147: 11-21 e1Abstract Full Text Full Text PDF PubMed Scopus (158) Google Scholar A key role of VEGF in intraocular vascular leakage is supported first by the intrinsic biologic potencies of VEGF; second, by the temporal and direct relationship between vascular hyperpermeability and VEGF expression; and third, by the fact that inhibition of VEGF alone is sufficient to reduce leakage. Induction of microvascular hyperpermeability in pre-existing vessels is one of the main biologic roles of VEGF, although the effect is short acting and therefore may be of consequence only in acute conditions.28Collins P.D. Connolly D.T. Williams T.J. Characterization of the increase in vascular permeability induced by vascular permeability factor in vivo.Br J Pharmacol. 1993; 109: 195-199Crossref PubMed Scopus (152) Google Scholar Recent evidence indicates that significant and sustained vascular leakage in response to VEGF results from induction of growth of structurally abnormal vessels.29Nagy J.A. Feng D. Vasile E. et al.Permeability properties of tumor surrogate blood vessels induced by VEGF-A.Lab Invest. 2006; 86: 767-780Abstract Full Text Full Text PDF PubMed Scopus (100) Google Scholar Vascular endothelial growth factor also has been shown to induce changes in the phosphorylation state of the tight junction proteins occludin and zonula occluden 1.30Antonetti D.A. Barber A.J. Hollinger L.A. et al.Vascular endothelial growth factor induces rapid phosphorylation of tight junction proteins occludin and zonula occluden 1 A potential mechanism for vascular permeability in diabetic retinopathy and tumors.J Biol Chem. 1999; 274: 23463-23467Abstract Full Text Full Text PDF PubMed Scopus (570) Google Scholar Elevated levels of VEGF mRNA in the retina have been reported to be associated with breakdown of the BRB and increased vascular permeability, leading to the development and progression of ME.17Pe'er J. Shweiki D. Itin A. et al.Hypoxia-induced expression of vascular endothelial growth factor by retinal cells is a common factor in neovascularizing ocular diseases.Lab Invest. 1995; 72: 638-645PubMed Google Scholar Retinal VEGF mRNA expression is elevated in an animal model of laser-induced RVO,6Miller J.W. Adamis A.P. Shima D.T. et al.Vascular endothelial growth factor/vascular permeability factor is temporally and spatially correlated with ocular angiogenesis in a primate model.Am J Pathol. 1994; 145: 574-584PubMed Google Scholar and the concentration of VEGF in the vitreous correlates with the severity of the disease in patients with diabetic ME and RVO.31Shimada H. Akaza E. Yuzawa M. Kawashima M. Concentration gradient of vascular endothelial growth factor in the vitreous of eyes with diabetic macular edema.Invest Ophthalmol Vis Sci. 2009; 50: 2953-2955Crossref PubMed Scopus (45) Google Scholar, 32Noma H. Minamoto A. Funatsu H. et al.Intravitreal levels of vascular endothelial growth factor and interleukin-6 are correlated with macular edema in branch retinal vein occlusion.Graefes Arch Clin Exp Ophthalmol. 2006; 244: 309-315Crossref PubMed Scopus (249) Google Scholar Increased VEGF levels have been reported in the aqueous humor and serum of patients with quiescent uveitis.33Paroli M.P. Teodori C. D'Alessandro M. et al.Increased vascular endothelial growth factor levels in aqueous humor and serum of patients with quiescent uveitis.Eur J Ophthalmol. 2007; 17: 938-942Crossref PubMed Scopus (34) Google Scholar In rodent and primate models, intraocular administration of VEGF-neutralizing chimeric proteins and antibodies prevented the development of neovascularization after induction of RVO, providing evidence for a pathogenetic role for VEGF.8Adamis A.P. Shima D.T. Tolentino M.J. et al.Inhibition of vascular endothelial growth factor prevents retinal ischemia-associated iris neovascularization in a nonhuman primate.Arch Ophthalmol. 1996; 114: 66-71Crossref PubMed Scopus (588) Google Scholar, 9Krzystolik M.G. Afshari M.A. Adamis A.P. et al.Prevention of experimental choroidal neovascularization with intravitreal anti-vascular endothelial growth factor antibody fragment.Arch Ophthalmol. 2002; 120: 338-346Crossref PubMed Scopus (563) Google Scholar, 21Aiello L.P. Pierce E.A. Foley E.D. et al.Suppression of retinal neovascularization in vivo by inhibition of vascular endothelial growth factor (VEGF) using soluble VEGF-receptor chimeric proteins.Proc Natl Acad Sci U S A. 1995; 92: 10457-10461Crossref PubMed Scopus (1179) Google Scholar It should be noted, however, that there are no primary animal models of ME. Most experimental data are derived from RVO models, where ischemia may be the main driver of VEGF expression. The reduction of leakage observed with VEGF blockade is attributed in part to the induction of a more stable microvascular phenotype characterized by increased pericyte coverage.34Ferrara N. Damico L. Shams N. et al.Development of ranibizumab, an anti-vascular endothelial growth factor antigen binding fragment, as therapy for neovascular age-related macular degeneration.Retina. 2006; 26: 859-870Crossref PubMed Scopus (710) Google Scholar The intraocular vascular diseases discussed so far affect primarily the vascular bed supplying the inner retina. The outer retina is supplied by the choroidal vasculature (Fig 2). Choroidal neovascularization (CNV) is a serious vision-limiting complication in which the integrity of the Bruch's membrane is compromised and leaky tortuous vessels sprout from the choriocapillaris into the subretinal pigment epithelium, the subretinal spaces, or both. The abnormal vessels and edema disrupt the precise alignment of the photoreceptor cells and the RPE (Fig 2), whereas the coagulation and fibrosis that occur after hemorrhages may lead to permanent scarring. Disease presenting with CNV include neovascular AMD,35Green W.R. Histopathology of age-related macular degeneration.Mol Vis. 1999; 5: 27PubMed Google Scholar, 36Ferris 3rd, F.L. Fine S.L. Hyman L. Age-related macular degeneration and blindness due to neovascular maculopathy.Arch Ophthalmol. 1984; 102: 1640-1642Crossref PubMed Scopus (928) Google Scholar, 37Hotchkiss M.L. Fine S.L. Pathologic myopia and choroidal neovascularization.Am J Ophthalmol. 1981; 91: 177-183Abstract Full Text PDF PubMed Google Scholar, 38Watzke R.C. Packer A.J. Folk J.C. et al.Punctate inner choroidopathy.Am J Ophthalmol. 1984; 98: 572-584Abstract Full Text PDF PubMed Google Scholar, 39Prasad A.G. Van Gelder R.N. Presumed ocular histoplasmosis syndrome.Curr Opin Ophthalmol. 2005; 16: 364-368Crossref PubMed Scopus (29) Google Scholar pathologic myopia, punctate inner choroidopathy, and presumed ocular histoplasmosis syndrome. In AMD, neovascularization also can be intraretinal (retinal angiomatous proliferation), manifesting as both vascular proliferation and leakage (Fig 2). Age-related macular degeneration is a disease of the outer retina characterized initially by the gradual accumulation of basal deposits and drusen.35Green W.R. Histopathology of age-related macular degeneration.Mol Vis. 1999; 5: 27PubMed Google Scholar In most patients, the process is slow and leads to atrophic changes known as geographic atrophy, whereas in others the disease progresses to CNV (neovascular [wet] AMD).35Green W.R. Histopathology of age-related macular degeneration.Mol Vis. 1999; 5: 27PubMed Google Scholar The vast majority of patients with AMD and legal blindness have the neovascular form of AMD.36Ferris 3rd, F.L. Fine S.L. Hyman L. Age-related macular degeneration and blindness due to neovascular maculopathy.Arch Ophthalmol. 1984; 102: 1640-1642Crossref PubMed Scopus (928) Google Scholar Also, CNV is a serious vision-threatening complication of pathologic myopia, especially in younger (<50 years) individuals.37Hotchkiss M.L. Fine S.L. Pathologic myopia and choroidal neovascularization.Am J Ophthalmol. 1981; 91: 177-183Abstract Full Text PDF PubMed Google Scholar In punctate inner choroidopathy, CNV may develop from scars after the resolution of small, fluorescein-leaking lesions of the inner choroid and pigment epithelium.38Watzke R.C. Packer A.J. Folk J.C. et al.Punctate inner choroidopathy.Am J Ophthalmol. 1984; 98: 572-584Abstract Full Text PDF PubMed Google Scholar Presumed ocular histoplasmosis syndrome may result from Histoplasma capsulatum exposure and is characterized by maculopathy, chorioretinal, and peripapillary scars.39Prasad A.G. Van Gelder R.N. Presumed ocular histoplasmosis syndrome.Curr Opin Ophthalmol. 2005; 16: 364-368Crossref PubMed Scopus (29) Google Scholar Genotype and expression analyses, gene targeting, and experimental inhibition studies have provided strong evidence that VEGF is a key factor in the pathogenesis of intraocular disease with proliferation and leakage. Vascular endothelial growth factor gene promoter polymorphisms were found to be associated with neovascular AMD.40Churchill A.J. Carter J.G. Lovell H.C. et al.VEGF polymorphisms are associated with neovascular age-related macular degeneration.Hum Mol Genet. 2006; 15: 2955-2961Crossref PubMed Scopus (162) Google Scholar Experimental in vivo models have provided evidence for a temporal relationship between VEGF expression and the development of CNV,41Wang F. Rendahl K.G. Manning W.C. et al.AAV-mediated expression of vascular endothelial growth factor induces choroidal neovascularization in rat.Invest Ophthalmol Vis Sci. 2003; 44: 781-790Crossref PubMed Scopus (85) Google Scholar whereas VEGF has been detected in choroidal neovascular membranes obtained from human subjects with AMD.42Lopez P.F. Sippy B.D. 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Afshari M.A. Adamis A.P. et al.Prevention of experimental choroidal neovascularization with intravitreal anti-vascular endothelial growth factor antibody fragment.Arch Ophthalmol. 2002; 120: 338-346Crossref PubMed Scopus (563) Google Scholar Several molecules have been used for intraocular capture and neutralization of VEGF. Pegaptanib sodium injection (Macugen; OSI Eyetech, Inc., Cedar Knolls, NJ) is a 28-base RNA aptamer covalently linked to 2 branched 20-kD polyethylene glycol moieties that binds with high affinity and specificity to VEGF165, the most abundant isoform of VEGF.44Gragoudas E.S. Adamis A.P. Cunningham Jr, E.T. et al.Pegaptanib for neovascular age-related macular degeneration.N Engl J Med. 2004; 351: 2805-2816Crossref PubMed Scopus (2161) Google Scholar Two related molecules—ranibizumab (rhuFab V2, Lucentis; Genentech, Inc., South San Francisco, CA), a humanized monoclonal antibody fragment with molecular weight of 48 kDa, and bevacizumab, a recombinant humanized monoclonal antibody with molecular weight of 149 kDa (rhuMAb VEGF, Avastin; Genentech, Inc.)—were engineered to bind with high affinity and to neutralize all biologically active isoforms of VEGF.34Ferrara N. Damico L. Shams N. et al.Development of ranibizumab, an anti-vascular endothelial growth factor antigen binding fragment, as therapy for neovascular age-related macular degeneration.Retina. 2006; 26: 859-870Crossref PubMed Scopus (710) Google Scholar Aflibercept (VEGF-TrapR1R2) is a fusion protein containing the second immunoglobulin domain of VEGFR-1 and the third immunoglobulin domain of VEGFR-2, which binds all isoforms of VEGF, VEGF-B, and placental growth factor.45Holash J. Davis S. Papadopoulos N. et al.VEGF-Trap: a VEGF blocker with potent antitumor effects.Proc Natl Acad Sci U S A. 2002; 99: 11393-11398Crossref PubMed Scopus (1509) Google Scholar Data from prospective controlled clinical trials indicate that VEGF inhibition is an effective therapeutic strategy for intraocular vascular disease (see Table A in Appendix 1, available at http://aaojournal.org). In patients with neovascular AMD, VEGF inhibition is associated with preservation of visual acuity and in many cases clinically significant visual acuity gain (≥15 Early Treatment Diabetic Retinopathy letters). Vascular endothelial growth factor inhibition also was found to be more effective than the current standard of care for ME secondary to diabetes or RVO (see Table A in Appendix 1, available at http://aaojournal.org). Intravitreal triamcinolone acetonide downregulates the expression of VEGF and VEGFR-2 while upregulating the expression of the decoy receptor VEGFR-1.46Zhang X. Bao S. Lai D. et al.Intravitreal triamcinolone acetonide inhibits breakdown of the blood-retinal barrier through differential regulation of VEGF-A and its receptors in early diabetic rat retinas.Diabetes. 2008; 57: 1026-1033Crossref PubMed Scopus (141) Google Scholar Similarly, intravitreal dexamethasone has been shown to downregulate the intraocular expression of VEGF and intercellular adhesion molecule 1.47Wang K. Wang Y. Gao L. et al.Dexamethasone inhibits leukocyte accumulation and vascular permeability in retina of streptozotocin-induced diabetic rats via reducing vascular endothelial growth factor and intercellular adhesion molecule-1 expression.Biol Pharm Bull. 2008; 31: 1541-1546Crossref PubMed Scopus (107) Google Scholar Panretinal (scatter) laser photocoagulation and vitrectomy modulate VEGF signaling by diminishing the physiologic stimuli for VEGF expression. It has been proposed that both interventions improve the oxygenation of the inner retina, therefore reducing the hypoxia-driven VEGF expression.48Stefansson E. The therapeutic effects of retinal laser treatment and vitrectomy A theory based on oxygen and vascular physiology.Acta Ophthalmol Scand. 2001; 79: 435-440Crossref PubMed Scopus (247) Google Scholar Vascular endothelial growth factor plays important roles in both the development and the maintenance of the vasculature. For example, activation of the VEGF/VEGFR-2 signaling pathway leads to the production of nitric oxide and prostacyclin I2,49He H. Venema V.J. Gu X. et al.Vascular endothelial growth factor signals endothelial cell production of nitric oxide and prostacyclin through flk-1/KDR activation of c-Src.J Biol Chem. 1999; 274: 25130-25135Abstract Full Text Full Text PDF PubMed Scopus (418) Google Scholar which are critically important in the regulation of the vascular tone and the coagulation cascade. Furthermore, tightly regulated VEGF expression in podocytes is essential for the normal development and function of the glomerular filtration barrier.50Eremina V. Sood M. Haigh J. et al.Glomerular-specific alterations of VEGF-A expression lead to distinct congenital and acquired renal diseases.J Clin Invest. 2003; 111: 707-716Crossref PubMed Scopus (1115) Google Scholar Finally, VEGF also has been identified as the primary angiogenic mediator in the proliferative phase of wound healing.51Nissen N.N. Polverini P.J. Koch A.E. et al.Vascular endothelial growth factor mediates angiogenic activity during the proliferative phase of wound healing.Am J Pathol. 1998; 152: 1445-1452PubMed Google Scholar Based on experience from the use of anti-VEGF therapy in oncology, VEGF inhibition may be associated with several so-called class adverse effects, including hypertension, proteinuria, arterial thromboembolic events, cardiomyopathy, hemorrhage, wound complication, gastrointestinal perforation, and reversible posterior leukoencephalopathy syndrome.52Chen H.X. Cleck J.N. Adverse effects of anticancer agents that target the VEGF pathway.Nat Rev Clin Oncol. 2009; 6: 465-477Crossref PubMed Scopus (452) Google Scholar Although it is unclear if these adverse events are relevant to the use of anti-VEGF therapy in ophthalmology, where the exposure is many fold less, the incidence of systemic adverse events is monitored closely in randomized controlled trials in ophthalmology. Identification of genetic and biologic markers for VEGF toxicity may provide additional clinically relevant criteria for selection of patients for anti-VEGF treatment. The long-term consequences of intraocular VEGF suppression on the retina also have been a subject of interest and debate in the scientific and clinical communities. Interpretation of data from some animal and in vitro studies suggests that VEGF may be a survival factor for retinal neurons.53Nishijima K. Ng Y.S. Zhong L. et al.Vascular endothelial growth factor-A is a survival factor for retinal neurons and a critical neuroprotectant during the adaptive response to ischemic injury.Am J Pathol. 2007; 171: 53-67Abstract Full Text Full Text PDF PubMed Scopus (597) Google Scholar However, data from a large number of patients receiving ranibizumab do not support the concept that long-term (at least through 2 years) intraocular VEGF inhibition has a neurotoxic effect.54Lalwani G.A. Rosenfeld P.J. Fung A.E. et al.A variable-dosing regimen with intravitreal ranibizumab for neovascular age-related macular degeneration: year 2 of the PrONTO Study.Am J Ophthalmol. 2009; 148: 43-58 e1Abstract Full Text Full Text PDF PubMed Scopus (801) Google Scholar Although anti-VEGF therapy has provided significant treatment benefit in multiple indications, not all patients achieve an optimal response. It is likely that to extend or sustain treatment effects, additional targets or mechanisms of action would be required (see Appendix 1, available at http://aaojournal.org, for discussion of other factors implicated in the pathogenesis of intraocular vascular diseases). Several other molecules that provide targeted inhibition of VEGF or inhibition of other pathways involved in its synthesis or regulation currently are being investigated. At this time, aflibercept, a decoy receptor fusion protein of VEGFR-1 and VEGFR-2 that binds all isoforms of VEGF, placental growth factor, and VEGF-B, has been approved by the United States Food and Drug Administration for the treatment of neovascular AMD. Twelve-month data suggest a decreased treatment burden (see the anti-VEGF trials table in Appendix 1, available at http://aaojournal.org). Alternative methods for direct inhibition of VEGF signaling, including siRNAs,55Kaiser P.K. Symons R.C. Shah S.M. et al.RNAi-based treatment for neovascular age-related macular degeneration by Sirna-027.Am J Ophthalmol. 2010; 150: 33-39 e2Abstract Full Text Full Text PDF PubMed Scopus (154) Google Scholar and small molecule inhibitors that target VEGFR,56Takahashi K. Saishin Y. Saishin Y. et al.Suppression and regression of choroidal neovascularization by the multitargeted kinase inhibitor pazopanib.Arch Ophthalmol. 2009; 127: 494-499Crossref PubMed Scopus (71) Google Scholar are in early clinical development. Examples include vatalanib (PTK787), a receptor tyrosine kinase inhibitor that blocks VEGF-R activity, and pazopanib,56Takahashi K. Saishin Y. Saishin Y. et al.Suppression and regression of choroidal neovascularization by the multitargeted kinase inhibitor pazopanib.Arch Ophthalmol. 2009; 127: 494-499Crossref PubMed Scopus (71) Google Scholar a multikinase inhibitor. It should be noted that because tyrosine kinases are ubiquitous and inhibitors are generally nonspecific, toxicities are more likely, making it harder to develop them as a therapeutic. These newer agents may offer potential advantages over existing therapies because they could be administered via routes other than intravitreal injection. One critical issue regarding the applicability of siRNAs for treatment of intraocular vascular disease in humans is that CNV suppression seems to be a generic property of siRNAs, and their use may be associated with unanticipated side effects.57Kleinman M.E. Yamada K. Takeda A. et al.Sequence- and target-independent angiogenesis suppression by siRNA via TLR3.Nature. 2008; 452: 591-597Crossref PubMed Scopus (814) Google Scholar Additional studies also are needed to decipher further the complex pathophysiologic mechanisms of retinal damage in intraocular vascular diseases. For instance, it would be important to understand RPE dysfunction as a mechanism of fluid accumulation. This should lead to the identification of more molecular targets for therapeutic interventions. Data from experimental models suggest that inhibition of the platelet-derived growth factor pathway may augment the therapeutic effect of VEGF inhibition,58Jo N. Mailhos C. Ju M. et al.Inhibition of platelet-derived growth factor B signaling enhances the efficacy of anti-88.4vascular endothelial growth factor therapy in multiple models of ocular neovascularization.Am J Pathol. 2006; 168: 2036-2053Abstract Full Text Full Text PDF PubMed Scopus (289) Google Scholar whereas inhibition of angiopoietin 2 is associated with the inhibition of corneal angiogenesis and retinal neovascularization.59Coxon A. Bready J. Min H. et al.Context-dependent role of angiopoietin-1 inhibition in the suppression of angiogenesis and tumor growth: implications for AMG 386, an angiopoietin-1/2-neutralizing peptibody.Mol Cancer Ther. 2010; 9: 2641-2651Crossref PubMed Scopus (141) Google Scholar There is also new evidence for role of notch signaling in CNV.60Ahmad I. Balasubramanian S. Del Debbio C.B. et al.Regulation of ocular angiogenesis by Notch signaling: implications in neovascular age-related macular degeneration.Invest Ophthalmol Vis Sci. 2011; 52: 2868-2878Crossref PubMed Scopus (42) Google Scholar In summary, the development of anti-VEGF therapies for intraocular vascular disease is an example of how breakthrough basic research could be translated into effective therapeutic interventions. The efficacy and safety of anti-VEGF therapies for the treatment of intraocular vascular diseases is becoming well established, with agents available for clinical use for some indications and in late-phase clinical trials for others. These developments are changing clinical practice and are stimuli for further study of the basic mechanisms controlling intraocular angiogenesis. Download .pdf (.18 MB) Help with pdf files Appendix 1 Download .pdf (.03 MB) Help with pdf files Appendix 2