Inhibition of angiogenesis by adenovirus-mediated sFlt-1 expression in a rat model of corneal neovascularization

Ocular Therapeutics and Molecular Delivery Strategies for Neovascular Age-Related Macular Degeneration (nAMD)

Age-related macular degeneration (AMD) is the leading cause of vision loss in geriatric population. Intravitreal (IVT) injections are popular clinical option. Biologics and small molecules offer efficacy but relatively shorter half-life after intravitreal injections. To address these challenges, numerous technologies and therapies are under development. Most of these strategies aim to reduce the frequency of injections, thereby increasing patient compliance and reducing patient-associated burden. Unlike IVT frequent injections, molecular therapies such as cell therapy and gene therapy offer restoration ability hence gained a lot of traction. The recent approval of ocular gene therapy for inherited disease offers new hope in this direction. However, until such breakthrough therapies are available to the majority of patients, antibody therapeutics will be on the shelf, continuing to provide therapeutic benefits. The present review aims to highlight the status of pre-clinical and clinical studies of neovascular AMD treatment modalities including Anti-VEGF therapy, upcoming bispecific antibodies, small molecules, port delivery systems, photodynamic therapy, radiation therapy, gene therapy, cell therapy, and combination therapies.

Gene therapy in the anterior eye segment

This review provides comprehensive information about the advances in gene therapy in the anterior segment of the eye including cornea, conjunctiva, lacrimal gland, and trabecular meshwork. We discuss gene delivery systems including viral and non-viral vectors as well as gene editing techniques, mainly CRISPR-Cas9, and epigenetic treatments including antisense and siRNA therapeutics. We also provide a detailed analysis of various anterior segment diseases where gene therapy has been tested with corresponding outcomes. Disease conditions include corneal and conjunctival fibrosis and scarring, corneal epithelial wound healing, corneal graft survival, corneal neovascularization, genetic corneal dystrophies, herpetic keratitis, glaucoma, dry eye disease, and other ocular surface diseases. Although most of the analyzed results on the use and validity of gene therapy at the ocular surface have been obtained in vitro or using animal models, we also discuss the available human studies. Gene therapy approaches are currently considered very promising as emerging future treatments of various diseases, and this field is rapidly expanding.

Adeno-Associated Virus Mediated Gene Therapy for Corneal Diseases

According to the World Health Organization, corneal diseases are the fourth leading cause of blindness worldwide accounting for 5.1% of all ocular deficiencies. Current therapies for corneal diseases, which include eye drops, oral medications, corrective surgeries, and corneal transplantation are largely inadequate, have undesirable side effects including blindness, and can require life-long applications. Adeno-associated virus (AAV) mediated gene therapy is an optimistic strategy that involves the delivery of genetic material to target human diseases through gene augmentation, gene deletion, and/or gene editing. With two therapies already approved by the United States Food and Drug Administration and 200 ongoing clinical trials, recombinant AAV (rAAV) has emerged as the in vivo viral vector-of-choice to deliver genetic material to target human diseases. Likewise, the relative ease of applications through targeted delivery and its compartmental nature makes the cornea an enticing tissue for AAV mediated gene therapy applications. This current review seeks to summarize the development of AAV gene therapy, highlight preclinical efficacy studies, and discuss potential applications and challenges of this technology for targeting corneal diseases.

Limiting angiogenesis to modulate scar formation

Angiogenesis, the process of new blood vessel formation from existing blood vessels, is a key aspect of virtually every repair process. During wound healing an extensive, but immature and leaky vascular plexus forms which is subsequently reduced by regression of non-functional vessels. More recent studies indicate that uncontrolled vessel growth or impaired vessel regression as a consequence of an excessive inflammatory response can impair wound healing, resulting in scarring and dysfunction. However, in order to elucidate targetable factors to promote functional tissue regeneration we need to understand the molecular and cellular underpinnings of physiological angiogenesis, ranging from induction to resolution of blood vessels. Especially for avascular tissues (e.g. cornea, tendon, ligament, cartilage, etc.), limiting rather than boosting vessel growth during wound repair potentially is beneficial to restore full tissue function and may result in favourable long-term healing outcomes.

Prevention of Corneal Neovascularization by Adenovirus Encoding Human Vascular Endothelial Growth Factor Soluble Receptor (s-VEGFR1) in Lacrimal Gland

Purpose

The aims of this study were (1) to determine the efficacy of adenovirus vector serotype 5 (Ad) encoding human soluble VEGF receptor 1 (s-VEGFR1) gene transfer to the lacrimal gland (LG); (2) to investigate whether expression of s-VEGFR1 prevents corneal neovascularization (CNV) induced by alkali burns; and (3) to evaluate the safety of the procedure.

Methods

AdVEGFR1 vectors (25 μL, 1 × 1010 pfu/mL) were injected in the right LGs of rats and were compared with AdNull vector (25 μL, 1 × 1010 pfu/mL) or 25 μL of saline (Control) before cornea alkali burns with 1 M NaOH. After 7 days, CNV was documented at the slit lamp. Tear secretion was measured with phenol red threads. The animals were tested for s-VEGFR1 mRNA and protein in the LG by quantitative (q)PCR and immunohistochemistry staining, respectively. qPCR was used to compare the mRNA levels of IL-1β, IL-6, and TNF-α in the LG and ipsilateral trigeminal ganglion (TG).

Results

Ad-VEGFR1 transfected 83% (10/12) of the rats. VEGFR1 was present in LG acinar cells. CNV was prevented in 9 of 12 animals in the Ad-VEGFR1 group, compared with the Ad-Null (3:10) and Control groups (1:10) (P = 0.0317). The tear secretion and cytokine mRNA levels in the LG and TG were similar in all three groups (P > 0.05).

Conclusions

Adenoviral vector gene transfer was safe for LG structure and function. The LG as the target tissue showed local expression of human s-VEGFR1, and CNV was prevented in most of the eyes exposed to alkali burns.

20-HETE in the regulation of vascular and cardiac function

20-HETE, the ω-hydroxylation product of arachidonic acid catalyzed by enzymes of the cytochrome P450 (CYP) 4A and 4F gene families, is a bioactive lipid mediator with potent effects on the vasculature including stimulation of smooth muscle cell contractility, migration and proliferation as well as activation of endothelial cell dysfunction and inflammation. Clinical studies have shown elevated levels of plasma and urinary 20-HETE in human diseases and conditions such as hypertension, obesity and metabolic syndrome, myocardial infarction, stroke, and chronic kidney diseases. Studies of polymorphic associations also suggest an important role for 20-HETE in hypertension, stroke and myocardial infarction. Animal models of increased 20-HETE production are hypertensive and are more susceptible to cardiovascular injury. The current review summarizes recent findings that focus on the role of 20-HETE in the regulation of vascular and cardiac function and its contribution to the pathology of vascular and cardiac diseases.

Corneal Mesenchymal Stromal Cells Are Directly Antiangiogenic via PEDF and sFLT-1

Purpose

To evaluate the angiogenic properties of corneal derived mesenchymal stromal cells (Co-MSC).

Methods

Co-MSCs were extracted from human cadaver, and wild-type (C57BL/6J) and SERPINF1^−/− mice corneas. The MSC secretome was collected in a serum-free medium. Human umbilical vein endothelial cell (HUVEC) tube formation and fibrin gel bead assay (FIBA) sprout formation were used to assess the angiogenic properties of Co-MSC secretome. Complete corneal epithelial debridement was used to induce corneal neovascularization in wild-type mice. Co-MSCs embedded in fibrin gel was applied over the debrided cornea to evaluate the angiogenic effects of Co-MSCs in vivo. Immunoprecipitation was used to remove soluble fms-like tyrosine kinase-1 (sFLT-1) and pigment epithelium-derived factor (PEDF, SERPINF1 gene) from the Co-MSC secretome.

Results

Co-MSC secretome significantly inhibited HUVECs tube and sprout formation. Co-MSCs from different donors consistently contained high levels of antiangiogenic factors including sFLT-1 and PEDF; and low levels of the angiogenic factor VEGF-A. In vivo, application of Co-MSCs to mouse corneas after injury prevented the development of corneal neovascularization. Removing PEDF or sFLT-1 from the secretome significantly diminished the antiangiogenic effects of Co-MSCs. Co-MSCs isolated from SERPINF1^−/− mice had significantly reduced antiangiogenic effects compared to SERPINF1^+/+ (wild-type) Co-MSCs.

Conclusions

These results illustrate the direct antiangiogenic properties of Co-MSCs, the importance of sFLT-1 and PEDF, and their potential clinical application for preventing pathologic corneal neovascularization.

Gene therapy for age-related macular degeneration

INTRODUCTION

In neovascular age related macular degeneration (nAMD), gene therapy to chronically express anti-vascular endothelial growth factor (VEGF) proteins could ameliorate the treatment burden of chronic intravitreal therapy and improve limited visual outcomes associated with 'real world' undertreatment. Areas covered: In this review, the authors assess the evolution of gene therapy for AMD. Adeno-associated virus (AAV) vectors can transduce retinal pigment epithelium; one such early application was a phase I trial of AAV2-delivered pigment epithelium derived factor gene in advanced nAMD. Subsequently, gene therapy for AMD shifted to the investigation of soluble fms-like tyrosine kinase-1 (sFLT-1), an endogenously expressed VEGF inhibitor, binding and neutralizing VEGF-A. After some disappointing results, research has centered on novel vectors, including optimized AAV2, AAV8 and lentivirus, as well as genes encoding other anti-angiogenic proteins, including ranibizumab, aflibercept, angiostatin and endostatin. Also, gene therapy targeting the complement system is being investigated for geographic atrophy due to non-neovascular AMD. Expert opinion: The success of gene therapy for AMD will depend on the selection of the most appropriate therapeutic protein and its level of chronic expression. Future investigations will center on optimizing vector, promoter and delivery methods, and evaluating the risks of the chronic expression of anti-angiogenic or anti-complement proteins.

Phase 2a Randomized Clinical Trial: Safety and Post Hoc Analysis of Subretinal rAAV.sFLT-1 for Wet Age-related Macular Degeneration

BACKGROUND

We present the results of a Phase 2a randomized controlled trial investigating the safety, and secondary endpoints of subretinal rAAV.sFLT-1 gene therapy in patients with active wet age-related macular degeneration (wAMD).

METHODS

All patients (n=32), (ClinicalTrials.gov; NCT01494805), received ranibizumab injections at baseline and week 4, and thereafter according to prespecified criteria. Patients in the gene therapy group (n=21) received rAAV.sFLT-1 (1×10¹¹vg). All patients were assessed every 4weeks to the week 52 primary endpoint.

FINDINGS

Ocular adverse events (AEs) in the rAAV.sFLT-1 group were mainly procedure related and self-resolved. All 11 phakic patients in the rAAV.sFLT-1 group showed progression of cataract following vitrectomy. No systemic safety signals were observed and none of the serious AEs were associated with rAAV.sFLT-1. AAV2 capsid was not detected and rAAV.sFLT-1 DNA was detected transiently in the tears of 13 patients. ELISPOT analysis did not identify any notable changes in T-cell response. In the rAAV.sFLT-1 group 12 patients had neutralizing antibodies (nAb) to AAV2. There was no change in sFLT-1 levels in bodily fluids. In the rAAV.sFLT-1 group, Best Corrected Visual Acuity (BCVA) improved by a median of 1.0 (IQR: -3.0 to 9.0) Early Treatment Diabetic Retinopathy Study (ETDRS) letters from baseline compared to a median of -5.0 (IQR: -17.5 to 1.0) ETDRS letters change in the control group. Twelve (57%) patients in the rAAV.sFLT-1 group maintained or improved vision compared to 4 (36%) in the control group. The median number of ranibizumab retreatments was 2.0 (IQR: 1.0 to 6.0) for the gene therapy group compared to 4.0 (IQR: 3.5 to 4.0) for the control group. Interpretation rAAV.sFLT-1 combined with the option for co-treatment appears to be a safe and promising approach to the treatment of wAMD.

FUNDING

National Health and Medical Research Council of Australia (AP1010405), Lions Eye Institute, Perth Australia, Avalanche Biotechnologies, Menlo Pk, CA, USA.

Preliminary in vitro and in vivo assessment of a new targeted inhibitor for choroidal neovascularization in age-related macular degeneration

Choroidal neovascularization (CNV) in age-related macular degeneration usually causes blindness. We established a novel targeted inhibitor for CNV in age-related macular degeneration. The inhibitor CR2-sFlt 1 comprises a CR2-targeting fragment and an anti-vascular endothelial growth factor (VEGF) domain (sFlt 1). The targeting of CR2-sFlt 1 was studied using the transwell assay in vitro and frozen sections in vivo using green fluorescent labeling. Transwell assay results showed that CR2-sFlt 1 migrated to the interface of complement activation products and was present in the retinal tissue of the CR2-sFlt 1-treated CNV mice. Treatment effects were assessed by investigating the VEGF concentration in retinal pigmented epithelial cell medium and the thickness of the CNV complex in the mice treated with CR2-sFlt 1. CR2-sFlt 1 significantly reduced the VEGF secretion from retinal pigmented epithelial cells in vitro and retarded CNV progress in a mouse model. Expression analysis of VEGF and VEGFRs after CR2-sFlt 1 intervention indicated the existence of feedback mechanisms in exogenous CR2-sFlt 1, endogenous VEGF, and VEGFR interaction. In summary, we demonstrated for the first time that using CR2-sFlt 1 could inhibit CNV with clear targeting and high selectivity.

Gene therapy in keratoconus

Keratoconus (KC) is the most common ectasia of the cornea and is a common reason for corneal transplant. Therapeutic strategies that can arrest the progression of this disease and modify the underlying pathogenesis are getting more and more popularity among scientists. Cumulating data represent strong evidence of a genetic role in the pathogenesis of KC. Different loci have been identified, and certain mutations have also been mapped for this disease. Moreover, Biophysical properties of the cornea create an appropriate candidate of this tissue for gene therapy. Immune privilege, transparency and ex vivo stability are among these properties. Recent advantage in vectors, besides the ability to modulate the corneal milieu for accepting the target gene for a longer period and fruitful translation, make a big hope for stupendous results reasonable.

Clinical correlates of common corneal neovascular diseases: a literature review

A large subset of corneal pathologies involves the formation of new blood and lymph vessels (neovascularization), leading to compromised visual acuity. This article aims to review the clinical causes and presentations of corneal neovascularization (CNV) by examining the mechanisms behind common CNV-related corneal pathologies, with a particular focus on herpes simplex stromal keratitis, contact lenses-induced keratitis and CNV secondary to keratoplasty. Moreover, we reviewed CNV in the context of different types of corneal transplantation and keratoprosthesis, and summarized the most relevant treatments available so far.

Peroxynitrite upregulates angiogenic factors VEGF-A, BFGF, and HIF-1α in human corneal limbal epithelial cells

PURPOSE

Corneal neovascularization (NV) is a sight-threatening condition often associated with infection, inflammation, prolonged contact lens use, corneal burns, and acute corneal graft rejection. Macrophages recruited to the cornea release nitric oxide (NO) and superoxide anion (O2(-)), which react together to form the highly toxic molecule peroxynitrite (ONOO(-)). The role of ONOO(-) in upregulating multiple angiogenic factors in cultured human corneal limbal epithelial (HCLE) cells was investigated.

METHODS

Human corneal limbal epithelial cells were incubated with 500 μM of ONOO(-) donor for various times. VEGF-A, BFGF, and hypoxic-inducible factor-alpha (HIF-1α) were investigated via Western blot and RT-PCR was performed for VEGF. Functional assays using human umbilical vein endothelial cells (HUVEC) used conditioned media from ONOO(-)-exposed HCLE cells. Secreted VEGF from conditioned media was detected and analyzed using ELISA.

RESULTS

Increased angiogenic factors were observed as early as 4 hours after HCLE exposure to ONOO(-). HIF-1 expression was seen at 4, 6, and 8 hours post-ONOO(-) exposure (P < 0.05). BFGF expression was elevated at 4 hours and peaked at 8 hours after treatment with ONOO(-) (P < 0.005). Increased VEGF-A gene expression was observed at 6 and 8 hours post-ONOO(-) treatment. Functional assays using conditioned media showed increased HUVEC migration and tube formation.

CONCLUSIONS

Exposure to elevated extracellular concentrations of ONOO(-) results in upregulation of angiogenic factors in HCLE cells. It is possible that, in the setting of inflammation or infection, that exposure to ONOO(-) could be one contributor to the complex initiators of corneal NV. Validation in vivo would identify an additional potential control point for corneal NV.

Microvascular complications and diabetic retinopathy: recent advances and future implications

Retinal microvascular alterations have been observed during diabetic retinopathy (DR) due to the retinal susceptibility towards subtle pathological alterations. Therefore, retinal microvascular pathology is essential to understand the nature of retinal degenerations during DR. In this review, the role of retinal microvasculature complications during progression of DR, along with recent efforts to normalize such alterations for better therapeutic outcome, will be underlined. In addition, current therapeutics and future directions for advancement of standard treatment for DR patients will be discussed.

Corneal gene therapy: basic science and translational perspective

Corneal blindness is the third leading cause of blindness worldwide. Gene therapy is an emerging technology for corneal blindness due to the accessibility and immune-privileged nature of the cornea, ease of vector administration and visual monitoring, and ability to perform frequent noninvasive corneal assessment. Vision restoration by gene therapy is contingent upon vector and mode of therapeutic gene introduction into targeted cells/tissues. Numerous efficacious vectors, delivery techniques, and approaches have evolved in the last decade for developing gene-based interventions for corneal diseases. Maximizing the potential benefits of gene therapy requires efficient and sustained therapeutic gene expression in target cells, low toxicity, and a high safety profile. This review describes the basic science associated with many gene therapy vectors and the present progress of gene therapy carried out for various ocular surface disorders and diseases.

Ocular neovascularization

Retinal and choroidal vascular diseases constitute the most common causes of moderate and severe vision loss in developed countries. They can be divided into retinal vascular diseases, in which there is leakage and/or neovascularization (NV) from retinal vessels, and subretinal NV, in which new vessels grow into the normally avascular outer retina and subretinal space. The first category of diseases includes diabetic retinopathy, retinal vein occlusions, and retinopathy of prematurity, and the second category includes neovascular age-related macular degeneration (AMD), ocular histoplasmosis, pathologic myopia, and other related diseases. Retinal hypoxia is a key feature of the first category of diseases resulting in elevated levels of hypoxia-inducible factor-1 (HIF-1) which stimulates expression of vascular endothelial growth factor (VEGF), platelet-derived growth factor-B (PDGF-B), placental growth factor, stromal-derived growth factor-1 and their receptors, as well as other hypoxia-regulated gene products such as angiopoietin-2. Although hypoxia has not been demonstrated as part of the second category of diseases, HIF-1 is elevated and thus the same group of hypoxia-regulated gene products plays a role. Clinical trials have shown that VEGF antagonists provide major benefits for patients with subretinal NV due to AMD and even greater benefits are seen by combining antagonists of VEGF and PDGF-B. It is likely that addition of antagonists of other agents listed above will be tested in the future. Other appealing strategies are to directly target HIF-1 or to use gene transfer to express endogenous or engineered anti-angiogenic proteins. While substantial progress has been made, the future looks even brighter for patients with retinal and choroidal vascular diseases.

AAV5-mediated sFLT01 gene therapy arrests retinal lesions in Ccl2(-/-)/Cx3cr1(-/-) mice

To test the effects of adeno-associated virus encoding sFLT01 (AAV5.sFLT01) on the retinal lesions in Ccl2(-/-)/Cx3cr1(-/-) mice, a model for age-related macular degeneration (AMD), AAV5.sFLT01 was injected into the subretinal space of the right eyes and the left eyes served as controls. Histology found no retinal toxicity due to the treatment after 3 months. The treated eyes showed lesion arrest compared with lesion progression in the left eyes by fundus monitoring monthly and histological evaluation 3 months after treatment. Retinal ultrastructure showed fewer lipofuscin and better preserved photoreceptors after the treatment. A2E, a major component of lipofuscin, was lower in the treated eyes than in the control eyes. Molecular analysis showed that AAV5.sFLT01 lowered retinal extracellular signal-regulated kinase (ERK) phosphorylation and inducible nitric oxide synthetase expression, which suggested the involvement of reactive nitrogen species in the retinal lesions of Ccl2(-/-)/Cx3cr1(-/-). We concluded that local delivery of AAV5.sFLT01 can stabilize retinal lesions in Ccl2(-/-)/Cx3cr1(-/-) mice. The findings provide further support for the potential beneficial effects of sFLT01 gene therapy for age-related macular degeneration.

Matrix revolution: molecular mechanism for inflammatory corneal neovascularization and restoration of corneal avascularity by epithelial stem cell transplantation

Corneal neovascularization (CNV) associated with severe limbal stem cell (LSC) deficiency remains a challenging ocular surface disease in that corneal inflammation may persist and progress, and the condition will not improve without LSC transplantation. A prominent feature after successful LSC transplantation is the suppression of corneal inflammation and CNV, which is generally attributed to the endogenous anti-angiogenic/anti-inflammatory factors secreted by corneal epithelial cells. In addition, corneal epithelial basement membrane (EBM) plays a unique role in the regulation of angiogenesis; several potent anti-angiogenic factors are derived from the matrix component of EBM, such as endostatin (from collagen XVIII) and restin (from collagen XV). Also, angio-inhibitory thrombospondin and tissue inhibitor of metalloproteinase-3 are deposited in EBM. Moreover, the heparan sulphate proteoglycan in EBM can bind and sequester VEGF and FGF-2 from activation. Recently, cultivated corneal epithelial transplantation (CCET) and cultivated oral mucosal epithelial transplantation (COMET) have emerged as promising techniques for the treatment of LSC deficiency. When human limbo-corneal epithelial (HLE) cells are cultivated on cryopreserved amniotic membrane, production of endostatin, restin, and IL-1ra is enhanced. This highlights the significance of delicate epithelial-matrix interactions in the generation of anti-angiogenic/anti-inflammatory factors by HLE cells, and this may, in part, explain the rapid restoration of corneal avascularity following CCET. In addition, whether epithelial stem cells can persist after transplantation is the key for CCET and COMET. Emerging evidence of long-term survival of cultivated epithelial cells after transplantation suggest that epithelial stem cells can be isolated and cultivated in vitro, and can re-establish the epithelial phenotype in vivo. Taken together, the merits of enhanced anti-angiogenic activity and the preservation of corneal epithelial stem cells encourage further application of this tissue engineering technique for ocular surface reconstruction.

Emerging techniques to treat corneal neovascularisation

Neovascularisation is a major cause of visual loss in a number of ophthalmic diseases. This review aims to outline the basic regulators of vessel growth in corneal neovascularisation. An understanding of the underlying principles of physiological and pathophysiological vascular development helps to appreciate current approaches to prevent or treat corneal neovascularisation. Options for future interventions will be discussed in the light of recent evidence provided by animal models of corneal neovascularisation.

Gene therapy in the cornea: 2005--present

Successful restoration of vision in human patients with gene therapy affirmed its promise to cure ocular diseases and disorders. The efficacy of gene therapy is contingent upon vector and mode of therapeutic DNA introduction into targeted cells/tissues. The cornea is an ideal tissue for gene therapy due to its ease of access and relative immune-privilege. Considerable progress has been made in the field of corneal gene therapy in last 5 years. Several new gene transfer vectors, techniques and approaches have evolved. Although corneal gene therapy is still in its early stages of development, the potential of gene-based interventions to treat corneal abnormalities has begun to surface. Identification of next generation viral and nanoparticle vectors, characterization of delivered gene levels, localization, and duration in the cornea, and significant success in controlling corneal disorders, particularly fibrosis and angiogenesis, in experimental animal disease models, with no major side effects have propelled gene therapy a step closer toward establishing gene-based therapies for corneal blindness. Recently, researchers have assessed the delivery of therapeutic genes for corneal diseases and disorders due to trauma, infections, chemical, mechanical, and surgical injury, and/or abnormal wound healing. This review provides an update on the developments in gene therapy for corneal diseases and discusses the barriers that hinder its utilization for delivering genes in the cornea.

Microvascular modifications in diabetic retinopathy

Patients struggling with diabetes are at elevated risks for several sight-threatening diseases, including proliferative diabetic retinopathy (DR). DR manifests in two stages: first, the retinal microvasculature is compromised and capillary degeneration occurs; subsequently, an over-compensatory angiogenic response is initiated. Early changes in the retinal microcirculation include disruptions in blood flow, thickening of basement membrane, eventual loss of mural cells, and the genesis of acellular capillaries. Endothelial apoptosis and capillary dropout lead to a hypoxic inner retina, alterations in growth factors, and upregulation of inflammatory mediators. With disease progression, pathologic angiogenesis generates abnormal preretinal microvessels. Current therapies, which include panretinal photocoagulation and vitrectomy, have remained unaltered for several decades. With several exciting preclinical advances, emergent technologies and innovative cellular targets may offer newfound hope for developing "next-generation" interventional or preventive clinical approaches that will significantly advance current standards of care and clinical outcomes.

Equine infectious anemia viral vector-mediated codelivery of endostatin and angiostatin driven by retinal pigmented epithelium-specific VMD2 promoter inhibits choroidal neovascularization

Equine infectious anemia virus (EIAV) is a nonprimate lentivirus that does not cause human disease. Subretinal injection into mice of a recombinant EIAV lentiviral vector in which lacZ is driven by a CMV promoter (EIAV CMV LacZ) resulted in rapid and strong expression of LacZ in retinal pigmented epithelial (RPE) cells and some other cells including ganglion cells, resulting in the presence of 5-bromo-4-chloro-3-indolyl-beta-D-galactopyranoside within the optic nerve. Substitution of the RPE-specific promoter from the vitelliform macular dystrophy (VMD2) gene for the CMV promoter resulted in prolonged (at least 1 year) expression of LacZ that was restricted to RPE cells, albeit reduced 6- to 10-fold compared with the CMV promoter. Similarly, the amount of FLAG-tagged endostatin detected in eyes injected with the EIAV VMD2 Endo(FLAG) vector was similar to that seen in eyes injected with a vector that expressed both endostatin and angiostatin [EIAV VMD2 Endo(FLAG)/Angio]; expression was approximately 6-fold lower than with identical vectors in which the CMV promoter drove expression. Compared with murine eyes treated with a control EIAV vector, subretinal injection of EIAV vectors expressing murine endostatin alone or in combination with angiostatin driven by either the CMV or VMD2 promoter caused significant suppression of choroidal neovascularization (NV) at laser-induced rupture sites in Bruch's membrane. These data support proceeding toward clinical studies with EIAV-based gene therapy for choroidal NV, using the VMD2 promoter to selectively drive expression of a combination of endostatin and angiostatin in RPE cells.

Corneal transparency: genesis, maintenance and dysfunction

Optimal vision is contingent upon transparency of the cornea. Corneal neovascularization, trauma and, surgical procedures such as photorefractive keratectomy and graft rejection after penetrating keratoplasty can lead to corneal opacification. In this article we identify the underlying basis of corneal transparency and factors that compromise the integrity of the cornea. With evidence from work on animal models and clinical studies, we explore the molecular mechanisms of both corneal avascularity and its dysfunction. We also seek to review therapeutic regimens that can safely salvage and restore corneal transparency.

Ocular gene therapy: current progress and future prospects

As gene therapy begins to produce its first clinical successes, interest in ocular gene transfer has grown owing to the favorable safety and efficacy characteristics of the eye as a target organ for drug delivery. Important advances also include the availability of viral and non-viral vectors that are able to efficiently transduce various ocular cell types, the use of intraocular delivery routes and the development of transcriptional regulatory elements that allow sustained levels of gene transfer in small and large animal models after a single administration. Here, we review recent progress in the field of ocular gene therapy. The first experiments in humans with severe inherited forms of blindness seem to confirm the good safety and efficacy profiles observed in animal models and suggest that gene transfer has the potential to become a valuable therapeutic strategy for otherwise untreatable blinding diseases.

IFN-gamma acts as anti-angiogenic cytokine in the human cornea by regulating the expression of VEGF-A and sVEGF-R1

Inflammatory processes within the cornea are known to be associated with corneal neovascularization (CN). We examined the effects of inflammatory mediators on the expression of angiogenic factors by corneal cells. TNF-alpha and IL-1 induced VEGF-A secretion by corneal fibroblasts (HCRF) and this was inhibited significantly by IFN-gamma. Constitutively secreted VEGF-A by corneal epithelial cells (HCE) was not affected by these cytokines. Moreover, sVEGF-R1(sFlt-1) secretion by HCRF was stimulated significantly by IFN-gamma. JAK-STAT pathway inhibitor reversed the effects of IFN-gamma on VEGF-A and sFlt-1 secretion by HCRF. RT-PCR analysis showed that IFN-gamma influences the expression of VEGF-A and sFlt-1 by affecting their mRNA level. IFN-gamma inhibited TGF-beta induced VEGF-A secretion but not sVEGF-R1 secretion. This is the first report demonstrating the inhibitory and stimulatory effects of IFN-gamma on VEGF-A and sFlt-1 secretion, respectively. Our results suggest that IFN-gamma acts as an anti-angiogenic cytokine in the human cornea.

Inhibition of corneal neovascularization by recombinant adenovirus-mediated sFlk-1 expression

The interaction of vascular endothelial growth factor (VEGF) and its receptors (Flt-1, Flk-1/KDR) is correlated with neovascularization in the eyes. Therefore, blocking the binding of VEGF and the corresponding receptor has become critical for inhibiting corneal neovascularization. In this study, we have expressed the cDNA for sFlk-1 under the control of cytomegalovirus immediate-early promoter (CMV) from an E1/partial E3 deleted replication defective recombinant adenovirus, and Ad.sflk-1 expression was determined by Western blotting. We have shown that conditioned media from Ad.sflk-1-infected ARPE-19 cells significantly reduced VEGF-induced human umbilical vein endothelial cells (HUVEC) and murine endothelial cells (SVEC) proliferation in vitro compared with the control vector. In vivo, adenoviral vectors expressing green fluorescent protein alone (Ad.GFP) were utilized to monitor gene transfer to the cornea. Moreover, in the models of corneal neovascularization, the injection of Ad.sflk-1 (10(8)PFU) into the anterior chamber could significantly inhibit angiogenic changes compared with Ad.null-injected and vehicle-injected models. Immunohistochemical analysis showed that corneal endothelial cells and corneal stroma of cauterized rat eyes were efficiently transduced and expressed sFlk-1. These results not only support that adenoviral vectors are capable of high-level transgene expression but also demonstrate that Ad.sflk-1 gene therapy might be a feasible approach for inhibiting the development of corneal neovascularization.

Corneal gene therapy

Gene therapy to the cornea can potentially correct inherited and acquired diseases of the cornea. Factors that facilitate corneal gene delivery are the accessibility and transparency of the cornea, its stability ex vivo and the immune privilege of the eye. Initial corneal gene delivery studies characterized the relationship between intraocular modes of administration and location of reporter gene expression. The challenge of achieving effective topical gene transfer, presumably due to tear flow, blinking and low penetration of the vector through epithlelial tight junctions left no alternative but invasive administration to the anterior chamber and corneal stroma. DNA vaccination, RNA interference and gene transfer of cytokines, growth factors and enzymes modulated the corneal microenvironment. Positive results were obtained in preclinical studies for prevention and treatment of corneal graft rejection, neovascularization, haze and herpetic stromal keratitis. These studies, corneal gene delivery systems and modes of administration, and considerations regarding the choice of animal species used are the focus of this review. Opportunities in the field of corneal gene therapy lie in expanding the array of corneal diseases investigated and in the implementation of recent designs of safer vectors with reduced immunogenicity and longer duration of gene expression.

Inhibition of VEGF expression and corneal neovascularization by siRNA targeting cytochrome P450 4B1

Injury to the cornea leads to formation of mediators that initiate and amplify inflammatory responses and neovascularization. Among these are lipid mediators generated by a cytochrome P450 (CYP) enzyme identified as CYP4B1. Increased corneal CYP4B1 expression increases limbal angiogenic activity through the production of 12-hydroxyeicosatrienoic acid (12-HETrE), a potent inflammatory and angiogenic eicosanoid. We used siRNA duplexes targeting CYP4B1 to substantiate the link between CYP4B1 expression, 12-HETrE production and angiogenesis in a model of suture-induced corneal neovascularization. Intrastromal sutures induced a time-dependent neovascular response which was significantly attenuated by CYP4B1-specific siRNAs but not by nonspecific siRNA. CYP4B1 mRNA was reduced by 60% and 12-HETrE's levels were barely detected in corneal homogenates from eyes treated with the CYP4B1-specific siRNA. The decreased neovascular response in CYP4B1 siRNA-treated eyes was associated with a 75% reduction in corneal VEGF mRNA levels. Transfection of rabbit corneal epithelial cells with CYP4B1 cDNA induced VEGF expression. Conversely, treatment with CYP4B1 siRNA or addition of a CYP4B1 inhibitor significantly decreased VEGF mRNA levels; addition of 12-HETrE potently increased them. The results strongly implicate the corneal CYP4B1 as a component of the inflammatory and neovascular cascade initiated by injury and further suggest that CYP4B1-12-HETrE is a proximal regulator of VEGF expression.

Inhibition of corneal neovascularization with endostatin delivered by adeno-associated viral (AAV) vector in a mouse corneal injury model

The use of a recombinant adeno-associated viral (rAAV) vector carrying endostatin gene as an anti-angiogenesis strategy to treat corneal neovascularization in a mouse model was evaluated. Subconjunctival injection of recombinant endostatin-AAV was used to examine the inhibition of corneal neovascularization induced by silver nitrate cauterization in mice. The results showed that gene expression in corneal tissue was observed as early as 4 days after gene transfer and stably lasted for over 8 months with minimal immune reaction. Subconjunctival injection of a high-titer rAAV-endostatin successfully inhibited neovascularization. Immunohistchemistry staining of CD 31 and endostatin showed that the treatment significantly inhibits angiogenesis in cornea. We concluded that the rAAV was capable of directly delivering genes to the ocular surface epithelium by way of subconjunctival injection and was able to deliver sustained, high levels of gene expression in vivo to inhibit angiogenesis.

Unique Homologous siRNA Blocks Hypoxia-Induced VEGF Upregulation in Human Corneal Cells and Inhibits and Regresses Murine Corneal Neovascularization

PURPOSE

To evaluate the results of amniotic membrane transplantation (AMT) for ocular surface reconstruction in chemical and thermal injuries.

METHODS

Retrospective review of case records of patients who had undergone AMT for chemical injuries (January 1998 to May 2001).

RESULTS

Seventy two eyes of 69 patients were studied of which 24 were acute cases (median-2 days, range, 1-20 days) and 48 were chronic cases (median-12.4 months, range, 1.02-95.8 months). Mean age was 22.4 years (SD +/- 13.34 years) and average follow up duration was 7.8 months (SD +/- 7.1). Main clinical findings were symblephara (52.8%), corneal vascularization (51.3%), conjunctivalization (45.8%), Limbal ischemia (45.8%), Limbal stem cell deficiency (55.5%) and epithelial defect (48.6%). 18 cases were due to acid injuries (5 acute, 13 chronic), 52 were due to alkali (18 acute and 34 chronic) and 2 cases were due to thermal burns (1 each acute and chronic). Overall success rate was 87.5% in acute cases and 72.9% in chronic cases. Indication-wise success rates were 94.3% for epithelial defect healing, 88.2% for symptomatic relief, 59.7% for ocular surface reconstruction, and 55% for improving limbal stem cell function. Success was not achieved in any outcome measure in 1/24 (4.2%) in acute group and 6/48 (12.5%) in chronic group.

CONCLUSION

AMT helps in ocular surface reconstruction, promotes rapid epithelial healing and partially restores limbal stem cell function. It can be considered as an effective modality for the ocular surface restoration in chemical and thermal injuries in selected cases. Success rates in acute and chronic cases are comparable.

Seeing the light: New insights into the molecular pathogenesis of retinal diseases

In the past, most treatments for retinal diseases have been empirical. Steroids and/or laser photocoagulation and/or surgery have been tried for almost every condition with little or no understanding of the underlying disease. Over the past several years vision researchers have uncovered molecular components of processes, such as visual transduction and the visual cycle, that are critical for visual function, and identified other molecules that lead to dysfunction and disease processes such as neovascularization and macular edema. It is becoming clear that dysregulation of certain molecules can have major effects on retinal structure and function. Studies in animal models have suggested that inhibiting or augmenting levels of a single molecule can have major effects in complex disease processes. Although several molecules probably contribute to neovascularization and excessive vascular permeability in the eye, blockade of vascular endothelial growth factor (VEGF) has remarkable beneficial effects in animal models that have now been proven to apply to human diseases in clinical trials. Intraocular injection of VEGF antagonists has revolutionized the treatment of choroidal neovascularization (CNV) and macular edema and serves as a model of targeted ocular pharmacotherapy. Significant progress elucidating the molecular pathogenesis of several disease processes in the eye may soon lead to new treatments following the lead of VEGF antagonists. Initial treatments that provide benefit from frequent intraocular injections are likely to be followed by sustained delivery of drugs and/or prolonged protein delivery by gene transfer. The eye has entered the era of molecular therapy.

Soluble vascular endothelial growth factor receptor-1 contributes to the corneal antiangiogenic barrier

PURPOSE

Pathological neovascularisation within the normally avascular cornea is a serious event that can interfere with normal vision. Upregulation of vascular endothelial growth factor (VEGF) has been associated with neovascularisation in the eye, suggesting that maintaining low levels of VEGF is important for corneal avascularity and intact vision. This study aims to determine the expression profile and possible contribution of sVEGFR-1 to the corneal avascular barrier.

DESIGN

Experimental case series investigating VEGF and soluble fms-like tyrosine kinase (sFlt) levels in normal and neovascularised human corneas.

PARTICIPANTS

Four normal human corneas, five human corneas with alkali burns, three human corneas with aniridia, one with ocular cicatricial pemphigoid and two with interstitial keratitis were examined.

METHODS

Western blot and immunohistochemical analyses were performed to determine sFlt and VEGF levels in normal and neovascularised human corneas. Immunoprecipitation was utilised to demonstrate sFlt-VEGF binding.

RESULTS

Normal human corneas strongly express sFlt in the corneal epithelium and weakly in the corneal stroma close to the limbus. VEGF is bound by sFlt in the normal human cornea. Neovascularised human corneas have greatly reduced expression of sFlt and significantly less VEGF bound by sFlt.

CONCLUSIONS

sFlt is highly expressed in the human cornea and normally sequesters VEGF.

Nonvascular VEGF receptor 3 expression by corneal epithelium maintains avascularity and vision

Transparency of the cornea, the window of the eye, is a prerequisite for vision. Angiogenesis into the normally avascular cornea is incompatible with good vision and, therefore, the cornea is one of the few tissues in the human body where avascularity is actively maintained. Here, we provide evidence for a critical mechanism contributing to corneal avascularity. VEGF receptor 3, normally present on lymphatic and proliferating blood vascular endothelium, is strongly constitutively expressed by corneal epithelium and is mechanistically responsible for suppressing inflammatory corneal angiogenesis.

Ocular angiogenesis: the role of growth factors

Ocular angiogenesis, the formation of new vessels from the existing vascular tree, is a major cause of severe vision loss. It can affect different structures in the eye, including the retina, choroid and cornea. During the last decade our knowledge in the mechanisms underlying ocular angiogenesis has increased dramatically. We have witnessed the identification of key molecules. Many are classified as growth factors due to their biological properties, regulating angiogenesis. This knowledge has propelled the development of a new group of therapeutic tools, the antiangiogenic agents. This review gives an update on the role of growth factors in ocular angiogenesis from both a basic and a clinical perspective.

Molecular targets for retinal vascular diseases

The elucidation of the molecular pathogenesis of a disease in animal models provides candidate targets for treatment. As specific antagonists for a target are developed and tested in clinical trials, if benefit is achieved, the candidate becomes a validated target. Validated targets stimulate additional research to identify optimal ways of attacking the target and studies in related disease processes to determine if the molecule is also a target in that context. Vascular endothelial growth factor (VEGF) has been identified as a validated target for several retinal vascular diseases. This has led to a flurry of activity resulting in beneficial treatments for patients and intensification of the search for other targets. This review summarizes preclinical and clinical trial results obtained with VEGF antagonists and describes evidence supporting the candidacy of other molecules currently being tested or soon to be tested for target status.

Mouse models of age-related macular degeneration

Recent advances in genetic technologies have greatly accelerated our ability to find disease-related genes and to generate animal models. The availability of ocular tissues with known genetic diseases are greatly contributing to our understanding of retinal disease processes including age-related macular degeneration (AMD), and panretinal and cone degenerations. While the macula is a highly specialised area of the retina not present in many mammals, the use of animal models such as mouse strains will give basic physiology and visual processing genetics relevant to human AMD. This review aims to provide a framework for better understanding some of the existing animal models and the knowledge that has been derived from their evaluations.

Antiangiogenic peptides and proteins: from experimental tools to clinical drugs

The formation of a 'tumor-associated vasculature', a process referred to as tumor angiogenesis, is a stromal reaction essential for tumor progression. Inhibition of tumor angiogenesis suppresses tumor growth in many experimental models, thereby indicating that tumor-associated vasculature may be a relevant target to inhibit tumor progression. Among the antiangiogenic molecules reported to date many are peptides and proteins. They include cytokines, chemokines, antibodies to vascular growth factors and growth factor receptors, soluble receptors, fragments derived from extracellular matrix proteins and small synthetic peptides. The polypeptide tumor necrosis factor (TNF, Beromun) was the first drug registered for the regional treatment of human cancer, whose mechanisms of action involved selective disruption of the tumor vasculature. More recently, bevacizumab (Avastin), an antibody against vascular endothelial growth factor (VEGF)-A, was approved as the first systemic antiangiogenic drug that had a significant impact on the survival of patients with advanced colorectal cancer, in combination with chemotherapy. Several additional peptides and antibodies with antiangiogenic activity are currently tested in clinical trials for their therapeutic efficacy. Thus, peptides, polypeptides and antibodies are emerging as leading molecules among the plethora of compounds with antiangiogenic activity. In this article, we will review some of these molecules and discuss their mechanism of action and their potential therapeutic use as anticancer agents in humans.

Long-term Evaluation of AAV-Mediated sFlt-1 Gene Therapy for Ocular Neovascularization in Mice and Monkeys

Vascular endothelial growth factor (VEGF) is one of the major mediators of retinal ischemia-associated neovascularization. We have shown here that adeno-associated virus (AAV)-mediated expression of sFlt-1, a soluble form of the Flt-1 VEGF receptor, was maintained for up to 8 and 17 months postinjection in mice and in monkeys, respectively. The expression of sFlt-1 was associated with the long-term (8 months) regression of neovascular vessels in 85% of trVEGF029 eyes. In addition, it resulted in the maintenance of retinal morphology, as the majority of the treated trVEGF029 eyes (75%) retained high numbers of photoreceptors, and in retinal function as measured by electroretinography. AAV-mediated expression of sFlt-1 prevented the development of laser photocoagulation-induced choroidal neovascularization in all treated monkey eyes. There were no clinically or histologically detectable signs of toxicity present in either animal model following AAV.sFlt injection. These results suggest that AAV-mediated secretion gene therapy could be considered for treatment of retinal and choroidal neovascularizations.

Inhibition of corneal neovascularization by a peroxisome proliferator-activated receptor-gamma ligand

PURPOSE

To determine the efficacy of the peroxisome proliferator-activated receptor gamma agonist, pioglitazone, in inhibiting corneal neovascularization.

METHODS

Twenty-six adult male Sprague-Dawley rats were randomly divided into three groups. Each group received intrastromal polymer micropellets containing one of the following: Group 1, no active ingredient (n=10); Group 2, vascular endothelial growth factor (VEGF) (n=7); Group 3, VEGF and pioglitazone (n=9). Neovascularization was evaluated 7 days after pellet implantation. After systemic India ink injection, digital photographs of the eyes were taken. The area and density of neovascularization were measured using imaging software.

RESULTS

Mean area of neovascularization was 0.43+/-0.18 mm2 for Group 1, 2.87+/-0.48 mm2 for Group 2 and 2.10+/-0.22 mm2 for Group 3. Statistical analysis showed significant differences between Groups 1 and 2 and Groups 1 and 3. There was no significant difference between Groups 2 and 3. Mean density of neovascularization was 2.16+/-0.66 for Group 1, 27.14+/-2.93 for Group 2 and 12.02+/-2.24 for Group 3. All comparisons between groups were statistically significant (P<0.01).

CONCLUSIONS

Pioglitazone is effective in decreasing the density of angiogenesis in a VEGF-induced neovascular rat cornea model. There is possibility of even greater effect with higher doses of the drug. Pioglitazone is a promising drug for the treatment of ocular neovascularization.

Ocular neovascularisation and excessive vascular permeability

Diseases complicated by vascular leakage and/or neovascularisation in the eye are responsible for the vast majority of visual morbidity and blindness in developed countries. The molecular signals that control vascular permeability and neovascularisation in the eye are being defined. Members of the vascular endothelial growth factor (VEGF) family are key stimulators that interact with two tyrosine kinase receptors, VEGF receptor (VEGFR)1 and 2; binding to two other receptors that lack tyrosine kinase activity, the neuropilins, is also important. Signalling through the VEGF pathway is modulated by the Tie2 receptor and its binding partners, the angiopoietins. Each of the participants in these two signalling pathways provides a potential target for intervention. Several proteins with antiangiogenic activity balance the stimulators and the outcome is determined by the net balance. Endostatin suppresses vascular permeability as well as ocular neovascularisation, suggesting that vascular leakage may also be regulated by counteracting proteins. Gene transfer provides a useful way to influence these balances. Clinical trials are underway to test whether these mechanisms can be translated into new therapies.