Age-related macular degeneration: etiology, pathogenesis, and therapeutic strategies

Age-related macular degeneration is the principal cause of registered legal blindness among those aged over 65 in the United States, western Europe, Australia, and Japan. Despite intensive research, the precise etiology of molecular events that underlie age-related macular degeneration is poorly understood. However, investigations on parallel fronts are addressing this prevalent public health problem. Sophisticated biochemical and biophysical techniques have refined our understanding of the pathobiology of drusen, geographic atrophy, and retinal pigment epithelial detachments. Epidemiological identification of risk factors has facilitated an intelligent search for underlying mechanisms and fueled clinical investigation of behavior modification. Gene searches have not only brought us to the cusp of identifying the culpable gene loci in age-related macular degeneration, but also localized genes responsible for other macular dystrophies. Recent and ongoing investigations, often cued by tumor biology, have revealed an important role for various growth factors, particularly in the neovascular form of the condition. Transgenic and knockout studies have provided important mechanistic insights into the development of choroidal neovascularization, the principal cause of vision loss in age-related macular degeneration. This in turn has culminated in preclinical and clinical trials of directed molecular interventions.

VEGF164 is proinflammatory in the diabetic retina

PURPOSE

The objectives of this study were to characterize the differential potency of two major VEGF isoforms, VEGF(120) and VEGF(164), for inducing leukocyte stasis (leukostasis) within the retinal vasculature and blood-retinal barrier (BRB) breakdown and to determine whether endogenous VEGF(164) mediates retinal leukostasis and BRB breakdown in early and established diabetes.

METHODS

Retinal leukostasis and BRB breakdown were simultaneously quantified by combining concanavalin A lectin (ConA) perfusion labeling with a fluorophotometric dextran leakage assay. CD45 immunohistochemistry was performed to confirm that ConA-stained cells within the vasculature were leukocytes. Retinal leukostasis and BRB breakdown were compared in nondiabetic rats receiving intravitreous injections of VEGF(120) or VEGF(164). Retinal intercellular adhesion molecule (ICAM)-1 and VEGF protein levels were studied by Western blot and ELISA, respectively. An anti-VEGF(164(165)) aptamer (EYE001) was administered by intravitreous injection to 2-week and 3-month diabetic rats, and the effect on retinal leukostasis and BRB breakdown was quantified.

RESULTS

Compared with VEGF(120), VEGF(164) more potently increased retinal ICAM-1 levels (2.2-fold), leukostasis (1.9-fold), and BRB breakdown (2.1-fold, P < 0.01 for all), despite negligible differences in vitreoretinal VEGF levels at the time of evaluation (P > 0.05). Retinal leukostasis and leakage increased with the duration of diabetes (P < 0.01) and correlated closely (P < 0.01, r = 0.889). The isoform-specific blockade of endogenous VEGF(164) with EYE001 resulted in a significant suppression of retinal leukostasis and BRB breakdown in both early (72.4% and 82.6%, respectively) and established (48.5% and 55.0%, respectively) diabetes (P < 0.01).

CONCLUSIONS

On an equimolar basis, VEGF(164) is at least twice as potent as VEGF(120) at inducing ICAM-1-mediated retinal leukostasis and BRB breakdown in vivo. The inhibition of diabetic retinal leukostasis and BRB breakdown with EYE001 in early and established diabetes indicates that VEGF(164) is an important isoform in the pathogenesis of early diabetic retinopathy.

Endothelial-directed hepatic regeneration after partial hepatectomy

OBJECTIVE

To determine the role of the microvascular endothelium in the regulation of regenerating liver mass after partial hepatectomy.

SUMMARY BACKGROUND DATA

Angiogenesis is critical for both pathologic and physiologic processes. The ability of certain tissues, such as the liver, kidney, and spleen, to regenerate after injury is poorly understood. The liver will regenerate to its normal mass within 8 days of surgical excision. Because the authors have previously shown that the endothelial cell regulates tumor mass, we hypothesized that normal adult organ mass is also controlled by the endothelial cell.

METHODS

Two-thirds partial hepatectomy was performed in 7- to 8-week-old C57 BL/6 mice, followed by systemic treatment with either the angiogenesis stimulator basic fibroblast growth factor (bFGF) (1 microg/g/d intraperitoneal) or the angiogenesis inhibitor TNP-470 (30 mg/kg/qod subcutaneous). Groups of three mice were then euthanized on postoperative days 2, 4, 6, and 8, and the livers were weighed and analyzed by immunohistochemistry.

RESULTS

bFGF accelerated hepatic regeneration by 42%, 19%, 16%, and 16% on postoperative days 2, 4, 6, and 8, respectively. TNP-470 inhibited hepatic regeneration by 46%, 74%, 67%, and 64% on postoperative days 2, 4, 6, and 8, respectively. Immunohistochemistry revealed that bFGF and TNP-470 primarily affected the endothelial compartment. Specifically, bFGF increased endothelial proliferation and decreased endothelial apoptosis. TNP-470, in contrast, inhibited endothelial cell proliferation. The cessation of the regenerative process correlated with a decrease in endothelial proliferation and an increase in endothelial apoptosis.

CONCLUSIONS

The systemic administration of angiogenesis agents modulates the regeneration of hepatic mass primarily by affecting endothelial cell proliferation or apoptosis. Endothelial cell apoptosis is associated with the cessation of the regenerative process in control mice. These results suggest that the endothelial cell is one of the key mediators of regenerating adult tissue mass in this partial hepatectomy model.

Characterization of retinal leukostasis and hemodynamics in insulin resistance and diabetes: role of oxidants and protein kinase-C activation

Increases in leukostasis/monocyte adhesion to the capillary endothelium (leukostasis) and decreases in retinal blood flow may be causally associated and are implicated in the pathogenesis of diabetic retinopathy. In this study, we demonstrate that increases in leukostasis are observed in insulin-resistant states without diabetes, whereas decreases in retinal blood flow require diabetes and hyperglycemia. Microimpaction studies using beads mimicking retinal capillary obstruction by leukocytes did not affect retinal blood flow. In diabetic rats, treatment with the antioxidant alpha-lipoic acid normalized the amount of leukostasis but not retinal blood flow. In contrast, treatment with D-alpha-tocopherol and protein kinase-C beta-isoform inhibition (LY333531) prevented the increases in leukostasis and decreases in retinal blood flow in diabetic rats. Serum hydroxyperoxide, a marker of oxidative stress, was increased in diabetic rats, but normalized by treatment with antioxidants alpha-lipoic acid and D-alpha-tocopherol and, surprisingly, PKC beta-isoform inhibition. These findings suggest that leukostasis is associated with endothelial dysfunction, insulin resistance, and oxidative stress but is not related to retinal blood flow and is not sufficient to cause diabetic-like retinopathy. Moreover, treatment with PKC beta inhibition is effective to normalize diabetes or hyperglycemia-induced PKC beta-isoform activation and oxidative stress.

Expression and function of receptors for advanced glycation end products in bovine corneal endothelial cells

PURPOSE

The corneal endothelium is a target of the aging process. This study was undertaken to reveal the relationship between corneal endothelial cell (CEC) death and the accumulation of advanced glycation end products (AGEs), by investigating the possible mechanism of accumulation of AGE in CECs and its effects on CEC death.

METHODS

First, the in vivo expression of the receptor was investigated for AGE (RAGE) and galectin-3, both receptors for AGE, at both the mRNA and protein levels. Second, AGEs were added to the culture media of the cultured CECs, and the uptake of AGEs, the generation of reactive oxygen species, and the induction of apoptosis were investigated.

RESULTS

Immunohistochemistry and RT-PCR demonstrated that both RAGE and galectin-3 were expressed in bovine CECs. After administration of AGE-modified bovine serum albumin to the culture medium, uptake of AGE was observed in the cytoplasm of the cultured bovine CECs. In addition, with increasing concentration of AGEs, the generation of reactive oxygen and the number of apoptotic cells also increased.

CONCLUSIONS

These results show that the accumulation of AGEs in CECs induced apoptosis, in part, by increasing cellular oxidative stress. The accumulation of AGEs in the CECs of elderly patients may be involved in the loss of CECs during the aging process.

Sustained inhibition of corneal neovascularization by genetic ablation of CCR5

PURPOSE

To determine whether genetic ablation of the CC chemokine receptor CCR5 (involved in leukocyte and endothelial chemotaxis) inhibits the development of corneal neovascularization.

METHODS

Wild-type C57BL/6J mice and species-specific counterparts with targeted homozygous disruption of the CCR5 gene underwent chemical and mechanical denudation of corneal and limbal epithelium. Corneas were harvested 2 and 4 weeks after injury. Neovascularization was quantified by CD31 immunostaining. Expression of VEGF protein was quantified by ELISA.

RESULTS

The mean percentages of neovascularized corneal area in control mice and CCR5-deficient mice 2 weeks after denudation were 58.3% and 38.5% (P = 0.05), respectively. At 4 weeks after denudation, the corresponding percentages were 67.6% and 44.0% (P = 0.028). In CCR5-deficient mice, VEGF protein levels were reduced 51.1% at 2 weeks (P = 0.05) after injury and 37.3% at 4 weeks (P = 0.03).

CONCLUSIONS

CCR5-deficient mice showed a persistent 34% to 35% inhibition of corneal neovascularization for up to 4 weeks. This inhibition correlates with reduced expression of VEGF. These data implicate CCR5 as one essential component in the development of corneal neovascularization.

Suppression of Fas-FasL-induced endothelial cell apoptosis prevents diabetic blood-retinal barrier breakdown in a model of streptozotocin-induced diabetes

Diabetic macular edema, resulting from increased microvascular permeability, is the most prevalent cause of vision loss in diabetes. The mechanisms underlying this complication remain poorly understood. In the current study, diabetic vascular permeability (blood-retinal barrier breakdown) is demonstrated to result from a leukocyte-mediated Fas-FasL-dependent apoptosis of the retinal vasculature. Following the onset of streptozotocin-induced diabetes, FasL expression was increased in rat neutrophils (P<0.005) and was accompanied by a simultaneous increase in Fas expression in the retinal vasculature. Static adhesion assays demonstrated that neutrophils from diabetic, but not control, rats induced endothelial cell apoptosis in vitro (P<0.005). The latter was inhibited via an antibody-based FasL blockade (P<0.005). In vivo, the inhibition of FasL potently reduced retinal vascular endothelial cell injury, apoptosis, and blood-retinal barrier breakdown (P<0.0001) but did not diminish leukocyte adhesion to the diabetic retinal vasculature. Taken together, these data are the first to identify leukocyte-mediated Fas-FasL-dependent retinal endothelial cell apoptosis as a major cause of blood-retinal barrier breakdown in early diabetes. These data imply that the targeting of the Fas-FasL pathway may prove beneficial in the treatment of diabetic retinopathy.

VEGF-dependent conjunctivalization of the corneal surface

PURPOSE

To investigate the mechanisms governing corneal neovascularization and the appearance of goblet cells in a murine model of limbal insufficiency.

METHODS

The spatial and time-dependent relationship between corneal neovascularization and goblet cell density was analyzed in corneal flatmounts. Immunohistochemical detection of the vascular endothelial growth factor (VEGF) receptor Flt-1 (VEGFR1) was performed in paraffin-embedded sections. A transgenic mouse that expresses the reporter gene lacZ targeted to the Flt-1 locus through homologous recombination was used to analyze corneal expression of Flt-1. The presence of soluble and membranous goblet cell Flt-1 mRNA and protein content was assessed with Northern and Western blot analyses, respectively. Finally, systemic adenoviral expression of a soluble Flt-1/Fc construct was used to study the effect of inhibition of VEGF bioactivity on the appearance of goblet cells and neovascularization.

RESULTS

Corneal neovascularization preceded the appearance of goblet cells, although both processes overlapped temporally. Flt-1 was abundant in the conjunctiva-like epithelium covering the cornea, as well as in the goblet cells, invading leukocytes, and vasculature. A similar expression pattern was observed in the transgenic mice expressing the lacZ gene downstream from the Flt-1 promoter. Isolated human and rat goblet cells in culture expressed Flt-1 mRNA and protein, as did freshly isolated human conjunctiva. The systemic inhibition of VEGF bioactivity potently suppressed both corneal neovascularization (8.3% +/- 8.1% vs. 41.1% +/- 15.3% corneal area; P < 0.001) and corneal goblet cell density (1.6% +/- 2.5% vs. 12.2% +/- 2.4% corneal area; P < 0.001).

CONCLUSIONS

Two important features of corneal conjunctivalization, the appearance of goblet cells and neovascularization, are regulated by VEGF. Both processes are probably mediated, in part, through the Flt-1 receptor. Taken together, these data indicate that an anti-VEGF therapeutic approach may limit the visual loss associated with conjunctivalization of the corneal surface.

Controlled delivery of the anti-VEGF aptamer EYE001 with poly(lactic-co-glycolic)acid microspheres

PURPOSE

To develop a controlled-drug delivery system for the long-term inhibition of vascular endothelial growth factor (VEGF) and its mediated responses.

METHODS

Poly(lactic-co-glycolic)acid (PLGA) microspheres containing anti-VEGF RNA aptamer (EYE001) formulations in the solid-state were developed by an oil-in-oil solvent evaporation process. In vitro experiments were performed to characterize the release profiles. Stability and bioactivity of the released drug were assayed by monitoring the RNA aptamer's ability to inhibit VEGF-induced cell proliferation in human umbilical vein endothelial cells (HUVECs). Cell proliferation experiments were conducted with aptamer aliquots collected after short-, mid-, and long-term release time points. To investigate the feasibility of this polymer device as a potential transscleral delivery device, an in vitro apparatus was developed to assess polymer hydration and degradation through rabbit sclera and subsequent delivery through it.

RESULTS

PLGA microspheres were able to deliver EYE001 in a sustained manner, with an average rate of 2 micro g/d over a period of 20 days. Solid-state stabilization of the aptamer with disaccharide trehalose before lyophilization and encapsulation in PLGA rendered the drug more stable after release. Cell proliferation experiments demonstrated that the bioactivity of the aptamer was preserved after release, as indicated by inhibition of endothelial cell proliferation after incubation with VEGF. Microspheres packed into a sealed chamber and placed onto the "orbital" part of a rabbit sclera for a period of 6 days became hydrated and started to degrade, as shown by scanning electron microscopy (SEM). As a result, the aptamer was delivered from the microspheres through the sclera, as determined spectrophotometrically.

CONCLUSIONS

The loading of aptamer-containing microspheres into a device and placing it on the orbital surface of the sclera was assessed and shown to be feasible. RNA aptamer EYE001 encapsulated in PLGA was delivered over a period of 20 days with retained activity. This method represents a promising approach for the transscleral delivery of drugs and the treatment of choroidal and retinal diseases.

The inflammatory milieu associated with conjunctivalized cornea and its alteration with IL-1 RA gene therapy

PURPOSE

This study was designed to gain an insight into the inflammatory milieu into which a donor limbal graft is routinely introduced. The objective of this study was to modulate this environment by gene therapy with the anti-inflammatory cytokine interleukin-1 receptor antagonist (IL-1 RA).

METHODS

In a mouse model, the ocular surface cytokine environment associated with a conjunctivalized cornea was assessed 4 weeks after injury. Total corneal epithelial and limbal debridement was performed with a combination of alkali and scrape injury. The cytokines and adhesion molecules measured included IL-1alpha, IL-1beta, IL-6, VEGF, intercellular adhesion molecule (ICAM)-1, and vascular adhesion molecule (VCAM)-1, by real-time PCR or ELISA. Injured corneas were transfected with IL-1 RA by injection of naked plasmid vector pIRES-EGFP-IL-1 RA immediately after injury. Corneas transfected with pIRES-EGFP served as the control. Expression of corneal IL-1 RA after transfection with pIRES-EGFP-IL1-RA was assessed over a 2-week period by real-time PCR and Western blot analysis. In addition, limbal stem cell grafts transfected with IL-1 RA were assessed for leukocyte influx.

RESULTS

Conjunctivalized corneas showed increased expression of IL-1alpha, IL-1beta, IL-1 RA, IL-6, VEGF, ICAM-1, and VCAM-1, compared with normal cornea. Transfection-efficiency experiments indicated that corneal expression of IL-1 RA peaked between 12 and 24 hours and lasted up to 2 weeks after the initial transfection. IL-1 RA corneal gene therapy resulted in a downregulation of IL-1beta and VCAM-1 expression at 4 weeks after injury, whereas downregulation of IL-6 was evident only at 1 week after injury. Corneal neovascularization was also reduced. In addition, corneal limbal stem cell grafts transfected with IL-1 RA showed a decreased leukocyte influx compared with control grafts.

CONCLUSIONS

Transfection of a cornea with IL-1 RA immediately after epithelial injury selectively altered the cytokine profile of the resultant conjunctivalized cornea and suppressed corneal neovascularization. Transfection of corneal limbal donor tissue with IL-1 RA before engraftment can reduce leukocyte influx into the graft. The findings demonstrate the feasibility of using transient cytokine gene expression, either in donor or recipient corneal tissue, to alter the ocular surface environment beneficially.

Angiostatin inhibits and regresses corneal neovascularization

OBJECTIVE

To determine the ability of angiostatin and the angiostatin-producing low-metastatic (LM) clone of Lewis lung carcinoma (LLC) to inhibit and regress corneal neovascularization, as compared with the non-angiostatin-producing high-metastatic (HM) clone.

METHODS

Three groups of C57BL6/J mice underwent chemical and mechanical denudation of corneal and limbal epithelium. One group remained tumor free while the other 2 were implanted with LLC cells (either the HM or LM clones) subcutaneously the day before, 2 weeks after, or 4 weeks after denudation. Corneas were harvested 2 weeks after tumor implantation (at 2, 4, and 6 weeks after denudation for tumor-free mice). Neovascularization was quantified by CD31 immunostaining. In a second experiment, recombinant angiostatin was delivered continuously for 2 weeks via an osmotic pump in mice with established corneal neovascularization.

RESULTS

The mean percentages of neovascularized corneal area in mice 2 weeks after LM-LLC implantation were 4.6%, 3.7%, and 37.0%, at 2, 4, and 6 weeks after scraping, respectively. In contrast, in the mice implanted with HM-LLC, the corresponding values were 45.4% (P =.01), 90.1% (P =.03), and 80.3% (P =.005). For tumor-free mice, the corresponding values were 62.0% (P =.003), 68.9% (P =.03), and 59.3% (P =.06). Mice implanted with angiostatin pumps had a 37.7% neovascularized corneal area 2 weeks after implantation and 4 weeks after scraping while mice implanted with sham pumps had 60.5% (P =.007).

CONCLUSION

Angiostatin inhibits and regresses corneal neovascularization induced by mechanical and alkali corneal injury.

CLINICAL RELEVANCE

This appears to be the first evidence of biologically induced regression of corneal neovascularization, and the first direct demonstration of angiostatin-induced regression of neovascularization in any tissue.

The corneal epithelial stem cell

The aim of this paper was to develop a GFP-expressing transgenic mouse model for the keratoepithelioplasty and to use this to follow the outcome of this form of graft, when placed on an inflamed corneal surface. Further aims were to characterize both the graft and the epithelial surface of the mouse and rat cornea using putative stem cell markers (P63 and Telomerase) and marker of cell differentiation (14-3-3 sigma). Keratepithelioplasty was carried out using a GFP transgenic mouse cornea as donor tissue. Fluorescent epithelial outgrowth from each keratepithelioplasty was scored and quantified. Donor corneal graft tissue was obtained from the paracentral region or the anatomical limbal region of murine corneas. Paracentral donor grafts (n = 20) consistently demonstrated a significant increase in proliferative potential compared to grafts obtained from the anatomical limbal region of the mouse cornea (n = 25) (P = 0.000, Mann-Whitney U). Correspondingly, P63 expression was maximal in the paracentral region of the mouse cornea, in keeping with the demonstrated increased proliferative potential of donor grafts harvested from this region of the cornea. The murine corneal epithelium demonstrated decreased rather than increased cellular layers at the limbal region, in contrast to that of the rat or human epithelium. In addition, as a general finding in all species tested, there was an apparent increase noted in P63 expression in basal corneal epithelial cells in regions that had increased cellular layers (limbus in humans and rats and the paracentral corneal region in the mouse). Epithelium, which had migrated from donor grafts onto recipient corneas, retained P63 expression for the period of time examined (up to 3 days postengraftment). In addition, the conjunctival surface of an injured conjunctivalized displayed an abnormal pattern of P63 expression. Telomerase expression was widespread throughout many layers of both the murine and rat corneal epithelium. In the mouse and rat corneal epithelium P63 expression was maximal in areas of increased proliferative potential. Its expression, however, was not confined to stem cells alone. Migrating cells from transplanted keratoepithelial grafts retained P63 expression at least in the early stages post-transplantation. Finally, damaged conjunctivalized corneas displayed an abnormal P63 expression pattern when compared to either normal conjunctiva or normal cornea.

Role of syndecan-1 in leukocyte-endothelial interactions in the ocular vasculature

PURPOSE

Leukocyte endothelial interactions are a key feature of ocular angiogenesis but also play a role in nonproliferative vascular alterations as are found in early diabetes or uveitis. The adhesion of leukocytes to endothelial cells during inflammation is a multistep process that involves leukocyte rolling, adhesion, and extravasation mediated by selectins, cell adhesion molecules (CAMs), integrins, and chemokines. Heparan sulfate (HS) is known to bind to and modify the function of these molecules under physiological conditions. In this study, the role of the HS proteoglycan syndecan-1 in mediating leukocyte-endothelial interactions in the ocular vasculature was investigated.

METHODS

Mice carrying a deletion in the gene encoding the cell surface HS proteoglycan syndecan-1 (sdc1) were used to study the interactions of leukocytes and endothelial cells in vivo, using a perfusion technique with FITC-coupled ConA and intravital microscopy.

RESULTS

In a retina perfusion model, Sdc1(-/-) mice showed increased leukocyte adhesion that was largely attributable to the leukocytes. Intravital microscopy studies revealed a dramatic increase in adhesion after tumor necrosis factor (TNF)-alpha treatment of sdc1(-/-) mice compared with similarly treated wild-type mice. The higher degree of leukocyte adhesion may account for the increase in inflammation-mediated corneal angiogenesis observed in sdc1(-/-) mice.

CONCLUSIONS

The results indicate a role for syndecan-1 as a negative regulator of leukocyte-mediated inflammatory responses. Thus, syndecan-1 could have use as a target for prevention of pathologic leukocyte-endothelial interactions in angiogenesis and inflammation.

Acute intensive insulin therapy exacerbates diabetic blood-retinal barrier breakdown via hypoxia-inducible factor-1alpha and VEGF

Acute intensive insulin therapy is an independent risk factor for diabetic retinopathy. Here we demonstrate that acute intensive insulin therapy markedly increases VEGF mRNA and protein levels in the retinae of diabetic rats. Retinal nuclear extracts from insulin-treated rats contain higher hypoxia-inducible factor-1alpha (HIF-1alpha) levels and demonstrate increased HIF-1alpha-dependent binding to hypoxia-responsive elements in the VEGF promoter. Blood-retinal barrier breakdown is markedly increased with acute intensive insulin therapy but can be reversed by treating animals with a fusion protein containing a soluble form of the VEGF receptor Flt; a control fusion protein has no such protective effect. The insulin-induced retinal HIF-1alpha and VEGF increases and the related blood-retinal barrier breakdown are suppressed by inhibitors of p38 mitogen-activated protein kinase (MAPK) and phosphatidylinositol (PI) 3-kinase, but not inhibitors of p42/p44 MAPK or protein kinase C. Taken together, these findings indicate that acute intensive insulin therapy produces a transient worsening of diabetic blood-retinal barrier breakdown via an HIF-1alpha-mediated increase in retinal VEGF expression. Insulin-induced VEGF expression requires p38 MAPK and PI 3-kinase, whereas hyperglycemia-induced VEGF expression is HIF-1alpha-independent and requires PKC and p42/p44 MAPK. To our knowledge, these data are the first to identify a specific mechanism for the transient worsening of diabetic retinopathy, specifically blood-retinal barrier breakdown, that follows the institution of intensive insulin therapy.

Prevention of experimental choroidal neovascularization with intravitreal anti-vascular endothelial growth factor antibody fragment

OBJECTIVE

To evaluate the safety and efficacy of intravitreal injections of an antigen-binding fragment of a recombinant humanized monoclonal antibody directed toward vascular endothelial growth factor (rhuFab VEGF) in a monkey model of choroidal neovascularization (CNV).

METHODS

In phase 1 of the study, each animal received intravitreal injections, 500 microg per eye, of rhuFab VEGF in one eye (prevention eye), while the contralateral eye received rhuFab VEGF vehicle (control eye) at 2-week intervals. On day 21, laser photocoagulation was performed to induce CNV. In phase 2, the vehicle-treated eye was crossed over and both eyes received 500 microg of rhuFab VEGF beginning 21 days following laser-induced injury at days 42 and 56. The eyes were monitored by ophthalmic examinations, color photographs, and fluorescein angiography.

RESULTS

rhuFab VEGF did not cause any ocular hemorrhages. All eyes treated with rhuFab VEGF developed acute anterior chamber inflammation within 24 hours of the first injection that resolved within 1 week, and this inflammation was less severe with subsequent injections. The incidence of CNV, defined angiographically, was significantly lower in the prevention eyes than the control eyes (P<.001). Subsequent treatments were associated with less leakage in eyes with established CNV that were crossed over from the control eyes to the treatment eyes (P =.001).

CONCLUSIONS

Intravitreal rhuFab VEGF injections prevented formation of clinically significant CNV in cynomolgus monkeys and decreased leakage of already formed CNV with no significant toxic effects.

CLINICAL RELEVANCE

This study provides the nonclinical proof of principle for ongoing clinical studies of intravitreally injected rhuFab VEGF in patients with neovascular age-related macular degeneration.

Nonsteroidal anti-inflammatory drugs prevent early diabetic retinopathy via TNF-alpha suppression

Leukocyte adhesion to the diabetic retinal vasculature results in blood-retinal barrier breakdown, capillary nonperfusion, and endothelial cell injury and death. Intercellular adhesion molecule-1 (ICAM-1) and the leukocyte integrin CD18 are required for these processes. Diabetes was induced in Long Evans rats, resulting in a two- to threefold increase in retinal leukocyte adhesion. Following one week of diabetes, neutrophil CD11a, CD11b, and CD18 expression was increased significantly, as were retinal ICAM-1 levels. Animals were treated with aspirin, a cyclooxygenase 2 (COX-2) inhibitor (meloxicam), or a soluble tumor necrosis factor alpha (TNF-alpha) receptor/Fc construct (TNFR-Fc, etanercept). High-dose aspirin, etanercept, and high-dose meloxicam each reduced leukocyte adhesion and suppressed blood-retinal barrier breakdown. High-dose aspirin also reduced the expression of CD11a, CD11b, and CD18, whereas meloxicam and etanercept did not. High-dose aspirin, etanercept, and high-dose meloxicam each reduced retinal ICAM-1 expression. Aspirin and meloxicam both lowered retinal TNF-alpha levels. Notably, aspirin, meloxicam, and etanercept did not change retinal vascular endothelial growth factor levels. High-dose aspirin, etanercept and high-dose meloxicam, each suppressed the retinal expression of eNOS and the DNA-binding capacity of retinal nuclear factor-kappaB. High-dose aspirin also suppressed Erk kinase activity, which is involved in CD18 up-regulation. Taken together, these data identify COX-2 and TNF-alpha as operative in the early signature pathologies of diabetic retinopathy, a newly recognized inflammatory disease.

Mechanisms of age-related macular degeneration

AMD is a poorly understood disease at this time. Since it is the leading cause of blindness in the elderly in the developed world, there is a pressing need for better treatment. Therefore, there is extensive ongoing research in both pathogenesis and therapy of AMD. Epidemiological studies have shown significant risk associated with increasing age and cigarette smoking, future studies may identify environmental risk factors though at present studies have been inconclusive. Genetic studies may identify subgroups of disease and thus help provide a selective approach to treatment. The vascular model of AMD may provide better understanding of the blood flow and post capillary resistance and help in early and newer intervention in the disease. Vascular endothelial growth factor has been extensively studied in choroidal neovascularization. It has been demonstrated in human and animal models of CNV and VEGF antagonists are currently in clinical trial. Extensive work is ongoing to prevent and treat CNV with antiangiogenic agents.

Vascular endothelial growth factor gene regulation and action in diabetic retinopathy

It is conceivable that VEGF inhibition may prevent edema formation at the early stages of diabetic retinopathy. Once the retina is irreversibly ischemic or new vessels have formed, however, antagonizing VEGF may lead to retinal necrosis due to chronic ischemia. An alternative approach would be the induction of neovascular maturation. Once the new vessels become mature, retina ischemia resolves. There would be no edema, hemorrhage, or retinal detachment. Acute administration of an angiogenic molecule called angiopoietin-1 protects vasculature from leaking [103]. Angiopoietins bind to the endothelial cell-specific receptor Tie 2 and play an important role is vascular development, especially vessel maturation. The proposed mechanisms include recruiting pericytes and organizing vascular matrix [103]. Since VEGF is constitutively expressed at low levels in normal eyes [46], it may contribute to the maintenance of vascular integrity. Thus, oversuppression of VEGF expression may be harmful to the retinal vasculature. Inhibiting VEGF action may need to be delivered in a tightly regulated manner such that complete inhibition may be avoided both to maintain basal levels and to provide rapid reversal of inhibition when acute angiogenic responses are desired [72]. VEGF is involved in normal angiogenic processes in adults such as cardiac collateral circulation, wound healing and menstrual cycle [27]. Local drug delivery seems to be more appealing than systemic administration to avoid the side effects. Some VEGF antagonists, such as VEGF receptor chimeric protein and the VEGF neutralizing antibodies are large molecules with poor diffusion into tissues. Repetitive invasive procedures such as intravitreal injection seem to be impractical due to potential complications of retinal detachment and bacterial infection. Recent progress on transscleral delivery of bioactive proteins and DNAs to the choroid and retina provides promising future on local delivery of therapeutic agents [12,13].

Transscleral drug delivery to the retina and choroid

Rapid advances in the molecular pathogenesis of retinal and choroidal disorders have highlighted the urgent need for innovative drug delivery modalities to the loci of pathology. In vitro and in vivo studies suggest that the transscleral route may offer a means to achieve the goal of sustained, targeted drug delivery to the posterior segment. Potentially therapeutic concentrations of macromolecules with retention of bioactivity can be attained in the choroid and retina via minimally invasive transscleral delivery.

Pathologic features of vascular endothelial growth factor-induced retinopathy in the nonhuman primate

PURPOSE

Vascular endothelial growth factor (VEGF) is a potent ischemia-upregulated angiogenic protein that has been implicated in diabetic retinopathy. Intravitreal VEGF injections have not previously been shown to produce preretinal neovascularization. The purpose of this study was to further characterize the angiopathic changes that occur after intravitreal injections in a nonhuman primate and determine if preretinal neovascularization develops.

DESIGN

Experimental animal study.

METHODS

Vascular endothelial growth factor 165 was injected into the eyes of normal cynomolgus monkeys at regular intervals. As a control, normal eyes were injected with phosphate buffered saline. Color photography and fluorescein angiography were performed at regular intervals. The retinas were incubated for adenosine diphosphatase (ADPase) activity to visualize retinal vessels. The retinas were flat-embedded and areas of potential preretinal neovascularization were identified en bloc and serially sectioned.

RESULTS

Areas of capillary nonperfusion and vessel dilation and tortuousity were seen by angiography. In serial sections, the nonperfused areas were found to be associated with endothelial cell hyperplasia in vessel lumens. Preretinal neovascularization originating only from superficial veins and venules was observed throughout peripheral retina, but was not seen in the posterior pole. Lacunae-like veins were subdivided by the process of intussusception and endothelial cell bridging. Arterioles demonstrated endothelial cell hyperplasia and microaneurysms.

CONCLUSION

Intraocular injections of VEGF were sufficient to produce preretinal neovascularization in the nonhuman primate. Most vasculopathic structures were associated with endothelial cell hyperplasia. These results demonstrate that VEGF alone can produce many features of both nonproliferative and proliferative diabetic retinopathy including the previously undescribed development of preretinal neovascularization. This well-characterized VEGF-induced primate model of retinal neovascularization may be useful as a means of testing new treatments for retinal neovascularization.

Retinal vascular endothelial growth factor induces intercellular adhesion molecule-1 and endothelial nitric oxide synthase expression and initiates early diabetic retinal leukocyte adhesion in vivo

Leukocyte adhesion to the diabetic retinal vasculature results in early blood-retinal barrier breakdown, capillary nonperfusion, and endothelial cell injury and death. Previous work has shown that intercellular adhesion molecule-1 (ICAM-1) and CD18 are required for these processes. However the relevant in vivo stimuli for ICAM-1 and CD18 expression in diabetes remain unknown. The current study investigated the causal role of endogenous vascular endothelial growth factor (VEGF) and nitric oxide in initiating these events. Diabetes was induced in Long-Evans rats with streptozotocin, resulting in a two- to threefold increase in retinal leukocyte adhesion. Confirmed diabetic animals were treated with a highly specific VEGF-neutralizing Flt-Fc construct (VEGF TrapA(40)). Retinal ICAM-1 mRNA levels in VEGF TrapA(40)-treated diabetic animals were reduced by 83.5% compared to diabetic controls (n = 5, P < 0.0001). VEGF TrapA(40) also potently suppressed diabetic leukocyte adhesion in retinal arterioles (47%, n = 11, P < 0.0001), venules (36%, n = 11, P < 0.0005), and capillaries (36%, n = 11, P < 0.001). The expression of endothelial nitric oxide synthase (eNOS), a downstream mediator of VEGF activity, was increased in diabetic retina, and was potently suppressed with VEGF TrapA(40) treatment (n = 8, P < 0.005). Further, VEGF TrapA(40) reduced the diabetes-related nitric oxide increases in the retinae of diabetic animals. The inhibition of eNOS with N-omega-nitro-L-arginine methyl ester also potently reduced retinal leukocyte adhesion. Although neutrophil CD11a, CD11b, and CD18 levels were increased in 1-week diabetic animals, VEGF TrapA(40) did not alter the expression of these integrin adhesion molecules. Taken together, these data demonstrate that VEGF induces retinal ICAM-1 and eNOS expression and initiates early diabetic retinal leukocyte adhesion in vivo. The inhibition of VEGF bioactivity may prove useful in the treatment of the early diabetic retinopathy.

In vivo retinal gene expression in early diabetes

PURPOSE

Studies have demonstrated a causal role for specific molecules in the pathogenesis of diabetic retinopathy. Among the implicated mediators are growth factors such as vascular endothelial growth factor (VEGF) as well as adhesion molecules and proliferation- and apoptosis-related genes. However, a coordinated large-scale investigation of gene expression in the diabetic retina has not yet been reported. Here the retinal gene expression profile of diabetic and nondiabetic animals using cDNA microarrays were analyzed and compared.

METHODS

Long-Evans rats were made diabetic with streptozotocin. Retinal gene expression was analyzed over 3 weeks using high-density nylon filter-based cDNA arrays. Genes were sorted into clusters according to their temporal expression profiles. They were also grouped according to their potential pathophysiological significance. The in vivo gene expression profiles of selected genes were verified via RNase protection assay.

RESULTS

The rat GeneFilter contains a total of 5147 genes, of which 1691 are known genes and 3456 are expressed sequence tags (ESTs). On day 3, the expression of 27 known genes was increased by more than twofold. On days 7 and 21, the corresponding numbers were 60 and 12, respectively. A transient upregulation (>2-fold) in expression was seen in 627 of 5147 total genes. A subset of 926 genes exhibited a modest (<2-fold) decrease in expression. No genes showed a greater than twofold decrease in expression. Overall, the identity of the genes that were upregulated suggests that the response of the retina to the diabetic challenge contains an inflammatory component. Moreover, most regulatory activity occurs during the first week of diabetes.

CONCLUSIONS

The development of a rational therapy for diabetic retinopathy will be assisted by detailed knowledge regarding the molecular pathophysiology of the disease. Here, an expression profile of an underlying retinal inflammatory process in early diabetes was extracted. Beyond providing insight into the general nature of the response to a pathogenic challenge, gene expression profiling may also allow the efficient identification of potential drug targets and markers for monitoring the course of disease.

VEGF-initiated blood-retinal barrier breakdown in early diabetes

PURPOSE

The objectives of this study were to (1) determine whether endogenous vascular endothelial growth factor (VEGF) triggers diabetic blood-retinal barrier breakdown, and (2) identify the site as well as phenotype of the hyperpermeable diabetic retinal vessels.

METHODS

Retinal VEGF mRNA levels were quantified in 1-week diabetic rats using the RNase protection assay. VEGF bioactivity was blocked via the systemic administration of a highly specific VEGF-neutralizing soluble Flt/F(c) construct (VEGF TrapA(40)). An inactive IL6 receptor/F(c) construct (IL6R Trap) was used as an isotype control. Blood-retinal barrier breakdown was quantified using the Evans blue technique and was spatially localized with fluorescent microspheres.

RESULTS

Retinal VEGF mRNA levels in 1-week diabetic animals were 3.2-fold higher than in nondiabetic controls (P < 0.0001). Similarly, retinal vascular permeability in 8-day diabetic animals was 1.8-fold higher than in normal nondiabetic controls (P < 0.05). Diabetes-induced blood-retinal barrier breakdown was dose-dependently inhibited with VEGF TrapA(40), with 25 mg/kg producing complete inhibition of the diabetes-induced increases (P < 0.05). Blood-retinal barrier breakdown in diabetic animals treated with solvent alone or IL6R Trap did not differ significantly from untreated diabetic animals (P > 0.05). Spatially, early blood-retinal barrier breakdown was localized to the retinal venules and capillaries of the superficial retinal vasculature.

CONCLUSIONS

Early blood-retinal barrier breakdown in experimental diabetes is VEGF dependent and is restricted, in part, to the venules and capillaries of the superficial inner retinal vasculature. VEGF inhibition should prove a useful therapeutic approach in the treatment of early diabetic blood-retinal barrier breakdown.