Ординатура / Офтальмология / Английские материалы / Diabetes and Ocular Disease Past, Present, and Future Therapies 2nd edition_Scott, Flynn, Smiddy_2009

394Diabetes and Ocular Disease

57.Philip S, Fleming AD, Goatman KA, et al. The efficacy of automated disease/no disease grading for diabetic retinopathy in a systematic screening programme. Br J Ophthalmol. 2007;91:1512–1517.

58.Chaum E, Karnowski TP, Govindasamy VP, et al. Automated diagnosis of retinopathy by content-based image retrieval. Retina. 2008;28:1463–1477.

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past seven years, some of these approaches have already been evaluated successfully in initial clinical investigations while others are currently in phase II and III multicenter randomized clinical trials. This chapter describes the rationale behind these modalities, the available supporting experimental data and the developmental status of these strategies, and speculates on their future clinical implications.

HISTORICAL PERSPECTIVES

Diabetic retinopathy is a complex, multifactorial process that occurs as a result of the similarly complex systemic disease diabetes mellitus. Numerous mechanisms explaining the clinical manifestations of diabetic retinopathy have been investigated, including evaluation of the polyol pathway [4], advanced glycation end products [5], oxidative stress [6], protein kinase C signaling [7], cell–matrix and cell–cell interactions [8], retinal blood flow abnormalities [9], and the role of protein factors with angiogenic, inflammatory, and vasopermeability characteristics. A comprehensive discussion of each of these areas is beyond the scope of this chapter, although inhibition of any of these pathways might at least partially ameliorate the ocular complications of diabetes.

Diabetic retinopathy is the prototypical example of a group of disorders known as ischemic retinopathies that are characterized by areas of poor retinal perfusion and the development of intraocular angiogenesis and retinal edema. Recent

Table 20.1. Disorders Associated with Intraocular Neovascularization

investigations have expanded our understanding of the angiogenic factors and molecular mechanisms that mediate vessel growth and excessive retinal permeability. As a result, numerous targets for the pharmacologic inhibition of diabetic retinopathy have become apparent. Agents directed against some of these targets have already been evaluated in controlled clinical trials, and many are now being investigated in late preclinical and phase I to III clinical trials. The development of growth factor inhibitors serves as a useful paradigm for the discussion of future therapies for diabetic retinopathy and is the principle focus of this chapter.

Table 20.1 presents a partial list of the ischemic retinopathies and several other disorders associated with intraocular neovascularization. These conditions share numerous clinical features (Fig. 20.1).

A B

Perfusion

Neovascularization

Vitreous hemorrhage

Nonperfusion

C D

Lipid

Hemorrhage

Figure 20.1. Clinical features shared by diabetic retinopathy and other ischemic retinopathies. The ischemic retinopathies share numerous clinical features. Neovascularization is often preceded by the development of areas of nonperfusion as demonstrated by the fluorescein angiogram of a patient with diabetes in (A). Retinal neovascularization often occurs at the border of perfused and nonperfused zones (B). These vessels are fragile and often bleed resulting in vitreous hemorrhage. Neovascularization can also occur at distant sites in the retina or anteriorly at the pupillary margin and the anterior chamber angle. (C) is an iris angiogram of a diabetic patient with iris neovascularization at both the pupillary margin and the anterior chamber angle. The retinal vessels often exhibit increased vascular permeability with transudation of serum components and deposition of lipid (D). (Source: A, C and D courtesy Wilmer Ophthalmological Institute, also Eye Complications of Diabetes for the Atlas of Clinical Endocrinology, volume 2, entitled Diabetes, edited by C. Ronald Kahn, M.D. Panel B is Early Treatment Diabetic Retinopathy Study standard photograph 7 from the modified Airlie House symposium [167].)

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Neovascularization is often preceded by, and spatially associated with, retinal capillary nonperfusion (Fig. 20.1A) [10,11]. Retinal neovascularization commonly arises at the border of perfused and nonperfused zones and is universally associated with increased vessel permeability (Fig. 20.1B and 20.1D) [12,13]. The extent of capillary nonperfusion is correlated with the risk of neovascularization [14], and the risk of iris neovascularization (Fig. 20.1C) is increased following cataract surgery in patients with diabetic retinopathy, presumably due to removal of the lens and its barrier function [15]. Nearly six decades ago, it was recognized that the clinical attributes shared by intraocular neovascular disorders suggested a common mechanism for the development of the neovascular and permeability complications in conditions such as diabetic retinopathy [16].

THE GROWTH FACTOR HYPOTHESIS OF NEOVASCULARIZATION

On the basis of these observations, Dr. I.C. Michaelson proposed the growth factor hypothesis of intraocular neovascularization in 1948 [16]. This theory was later refined by his student Ashton and others [16,17]. In essence, the hypothesis states that ischemia of the retina induces a factor or factors capable of stimulating the growth of new vessels (Fig. 20.2). These factors must meet several criteria in order to account completely for the classic clinical observations (Table 20.2). The factors should be freely diffusible within the eye to account for neovascularization of retinal tissue both adjacent to, and distant from, areas of nonperfusion, including neovascularization of the iris and anterior chamber angle. The factors should also be endothelial mitogens capable of inducing proliferation of

Table 20.2. Expected Attributes of a Major Growth Factor Mediator of Neovascularization in Diabetic Retinopathy

NVI Aqueous

Lens

Vitreous

VEGF121,165

Ischemia/hypoxia

NVE Estrogen

bFGF GH

IGF-1

NVD

Trauma

Cell death

VEGF189,206, Pericytes, Endothelial cells,

Retinal pigment epithelium,

Müller cells, Glial cells

Figure 20.2. Schematic representation of the growth factor hypothesis of neovascularization. Growth factors such as Vascular endothelial growth factor (VEGF) produced by numerous retinal cells act locally within the retina or are free to diffuse through the vitreous down concentration gradients represented by the arrow width in the figure. The larger VEGF isoforms

(VEGF189,206) tend to be nondiffusible and act locally, while the shorter isoforms (VEGF121,165) are freely diffusible within the eye. Because of their potential for diffusion, the growth factors

can therefore elicit neovascularization at distant sites in the retina or on the iris and within the anterior chamber angle, where they are eventually cleared through the trabecular meshwork. Other factors such as basic fibroblast growth factor (bFGF), growth hormone (GH) and insulin like growth factor 1 (IGF-1) probably act as synergistic or mediating factors, respectively. Basic FGF release is increased by trauma and cell death. (Source: Adapted from Aiello [55], with permission.)

new vessels, their expression should be induced by retinal hypoxia, and retinal endothelial cells should possess receptors for these molecules to permit cellular responses. Intraocular concentrations that progressively decline more anteriorly within the eye would account for diffusion of a retinal-produced factor towards the trabecular meshwork for clearance and for neovascularization arising at the iris and anterior chamber angle. Finally, concentrations of a postulated contributory growth factor would be expected to increase during or just prior to periods of active intraocular neovascularization and to diminish when neovascularization becomes quiescent due to either natural progression of the disease or successful therapy.

The process of growth factor stimulation of intraocular neovascularization can be broken down into a series of stages presented schematically in Figure 20.3. Diabetes mellitus induces vascular damage to the retina through a variety of mechanisms resulting in vascular nonperfusion and retinal ischemia (Fig. 20.3A). These changes stimulate expression and secretion of the growth factors from a variety of retinal cells (Fig. 20.3B). The growth factors diffuse within the retina

Metalloproteinases

Figure 20.3. Schematic representation of the basic stages of growth factor induction of intraocular neovascularization. (A) Diabetes results in retinal damage by a diverse array of mechanisms, eventually leading to capillary nonperfusion and retinal ischemia. (B) The damaged retina induces the production of growth factors (light blue) such as Vascular endothelial growth factor (VEGF, partially as a result of) retinal ischemia. The factors are free to act within the retina or to diffuse into the vitreous. (C) The growth factors bind to high affinity receptors (orange) on retinal endothelial cells (green). (D) The receptor binding induces a series of intracellular reactions (black arrows), producing an intracellular signal transduction cascade, that ultimately results in endothelial cell proliferation via a complex mechanism. This cascade likely involves numerous mediators such as the integrins and metalloproteases. (Source: Adapted from Aiello [53], with permission.)

and eye, eventually binding to high-affinity receptors on retinal endothelial cells (Fig. 20.3C). Receptor binding induces a series of intracellular biochemical reactions that transmit the signals for cell replication and increased permeability (Fig. 20.3D). Each of these steps is a potential target site for a therapeutic intervention. Once the intracellular signal is transmitted, the regulation of cell proliferation involves numerous other molecules such as the integrins [8,18], angiostatin [19], endostatin [20], and metalloendoproteases [21–23], all of which may be exploited as targets in the development of therapeutic modalities for diabetic retinopathy.

CANDIDATE MEDIATORS OF INTRAOCULAR

NEOVASCULARIZATION IN DIABETIC RETINOPATHY

Numerous growth factors have been evaluated as possible mediators of intraocular neovascularization. Some of those that have received the most extensive investigation with regard to diabetic retinopathy are listed in Table 20.3. These include basic fibroblast growth factor (bFGF), growth hormone (GH), insulin-like growth

* Synergistic with regard to the action of other growth factors. Therapies combining inhibitors of multiple factors would be expected to have increased effectiveness over single agents in most cases.

Watanabe D, Suzuma K, Matsui S, et al. Erythropoietin as a retinal angiogenic factor in proliferative diabetic retinopathy. N Engl J Med. 2005;353:782–792.

‡ Zhang SX, Wang JJ, Gao G, Parke K, Ma JX. Pigment epithelium-derived factor downregulates vascular endothelial growth factor (VEGF) expression and inhibits VEGF–VEGF receptor 2 binding in diabetic retinopathy. J Mol Endocrinol. August 2006;37(1):1–12.

** Tsujikawa A, Qin W, QaumT, et al. Suppression of diabetic retinopathy with angiopoietin-1. Am J Pathol. 2002;160:1683–1693.

†† Oh H, Takagi H, Suzuma K, Otani A, Matsumura M, Honda Y. Hypoxia and vascular endothelial growth factor selectively up-regulate angiopoietin-2 in bovine microvascular endothelial cells. J Biol Chem. 1999;274:15732–15739.

‡‡ Majka S, McGuire PG, Das A. Regulation of matrix metalloproteinase expression by tumor necrosis factor in a murine model of retinal neovascularization. Invest Ophthalmol Vis Sci. January 2002;43(1):260–266.

*** Ottino P, Finley J, Rojo E, et al. Hypoxia activates matrix metalloproteinase expression and the VEGF system in monkey choroid-retinal endothelial cells: involvement of cytosolic phospholipase A2 activity. Mol Vis. May 17, 2004;10:341–350.

†† Arjamaa O, Nikinmaa M. Oxygen-dependent diseases in the retina: role of hypoxia-inducible factors. Exp Eye Res. September 2006;83(3):473–483.

factor 1 (IGF-1), and vascular endothelial growth factor (VEGF). There are three main actions by which growth factors could influence the development of diabetic retinopathy: (1) as a primary stimulator of angiogenesis, (2) as permissive agents allowing other primary stimulators to induce neovascularization but not primarily stimulating the neovascularization themselves, and/or (3) in a synergistic fashion to increase the angiogenic ability of other factors. The current understanding of the relative role of each growth factor is indicated in Table 20.3.

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Basic fibroblast growth factor (bFGF) is tightly associated with the extracellular matrix [24,25] and induces endothelial cell proliferation, migration, and vasculogenesis; however, bFGF is not secreted from cells by classical mechanisms [26–28]. Basic FGF has been demonstrated in the retina [29] but no causal relationship with neovascularization has been identified [30]. Studies using transgenic mice have demonstrated that bFGF is neither necessary nor sufficient to induce retinal neovascularization [31]; however, bFGF is synergistic in its mitogenic activity with VEGF [32–34] and probably acts primarily as a potentiating factor in diabetic retinopathy.

Growth hormone (GH) and its biological mediator insulin-like growth factor 1 (IGF-1 [35] have been studied for many years as possible mediators of diabetic retinopathy [36,37], leading to a brief period during which hypophysectomy was employed as a treatment for diabetic retinopathy [38]. Although GH/IGF-1 reduction was modestly correlated with regression of proliferative diabetic retinopathy, this treatment was also associated with extensive morbidity in diabetic patients and was abandoned with the advent of laser photocoagulation. Studies using an inhibitor of GH secretion and transgenic mice expressing a GH antagonist suggest that GH plays a permissive role in ischemia-induced retinopathy rather than acting as the principal stimulating factor [39].

As a result of these data, somatostatin analogs that are GH release inhibitors have been investigated in human clinical studies for their potential ability to ameliorate diabetic retinopathy. Initial results from a small case series were encouraging with proliferative disease stabilizing or regressing in all patients [40]. In another study of 46 eyes in which the somatostatin analogue octreotride was used to treat patients with severe nonproliferative diabetic retinopathy (NPDR) and early proliferative diabetic retinopathy (PDR), the incidence of disease progression was decreased from 42% to 27%. In addition, only one of the octreotride treated patients required scatter (panretinal) photocoagulation compared to nine of the control patients [41]. Boehm et al. further noted that octreotride significantly reduced the risk of vitreous hemorrhage in 19 patients with severe PDR [42]. However, in a study of 25 patients with PDR who were administered a growth hormone receptor antagonist for a period of 12 weeks, retinopathy progressed in nine (36%) patients and was unchanged in 16 (64%) [43]. On the basis of two phase III multicenter clinical trials that failed to demonstrate significant efficacy of intramuscular injection of octreotride to treat PDR, further development of this compound for diabetic retinopathy was terminated [44,45].

Hepatocyte growth factor (HGF), a protein with mitogenic and motogenic effects on many nonocular cells, is elevated in the vitreous of patients with PDR [46]. Concentrations of HGF are highest in patients with active PDR and are reduced when proliferation is quiescent. The extent of HGF’s role in mediating PDR remains unknown. Angiostatin [19] and endostatin [20] are endogenous inhibitors of angiogenesis known to be involved in tumor suppression. Their significance in diabetic retinopathy is also currently unknown.

The role of vascular endothelial growth factor (VEGF) in the eye has been evaluated extensively for nearly 15 years. Considerable evidence now suggests that VEGF mediates a significant portion of the retinal neovascularization and

excessive vascular permeability associated with PDR. VEGF appears involved in the development of macular edema as well, although it may not be the sole mediator of this condition. VEGF may also play a role in the development and progression of NPDR as discussed below.

VEGF is a highly conserved protein with potent vasopermeability [47] and angiogenic activities [48,49]. Five different forms of VEGF exist in the human [50].

The smaller two isoforms (VEGF121,165) are freely diffusible, whereas the larger molecules (VEGF189,206) are nondiffusible because they are bound to cell surfaces and basement membranes (Fig. 20.2). The general functions of VEGF in ocular

disease have been reviewed extensively [51–54] and will not be described in detail here; however, it is important to note that VEGF possesses all of the attributes predicted for a major mediator of neovascularization in diabetic retinopathy as detailed in Table 20.2. VEGF is an endothelial cell mitogen [55], whose expression is increased up to thiry-fold by hypoxia in various cultured ocular cells [56]. At least two types of high-affinity VEGF receptors exist [55,57,58], and numerous retinal cells express VEGF including pigment epithelial cells [53], pericytes, endothelial cells, glial cells, Müller cells, and ganglion cells [56,59]. Thus, the actions of VEGF within the eye are highly consistent with the classic paradigm for growth factor mediation of diabetic retinopathy.

CLINICAL ASSOCIATIONS OF VEGF IN PROLIFERATIVE

DIABETIC RETINOPATHY

The in vivo evidence associating VEGF with retinal and iris neovascularization in PDR is extensive. Ischemia-induced retinal neovascularization that histologically resembles diabetic retinopathy is observed in neonatal rats [60], cats [61], and mice (Fig. 20.4A and 20.4B) [62,63]. Similar iris neovascularization is observed in the primate [64]. VEGF expression is correlated temporally with neovascularization in these models, increasing just prior to the onset of neovascularization (Fig. 20.4C and 20.4D) [60,61,63–66] and slowly declining as neovascularization regresses.

VEGF concentrations are elevated in the vitreous of patients with PDR as compared with vitreous from those with nonproliferative disease or quiescent proliferative disease or from nondiabetic patients without neovascularization as shown in Figure 20.5 [67–69]. Intravitreal VEGF concentrations are also correlated with diabetic macular edema [70,71] and are elevated when neovascularization is present owing to other ischemic retinal disorders such as central retinal vein occlusion. Neovascular membranes obtained from patients with PDR demonstrate near-universal VEGF expression (Fig. 20.6) [72–78].

VEGF INDUCTION OF DIABETES-LIKE RETINAL PATHOLOGY

VEGF in Proliferative Diabetic Retinopathy. Several findings support the conclusion that VEGF can induce intraocular neovascularization resembling that of PDR. Growth of retinal microvascular endothelial cells in culture is increased by