Inhibition of choroidal neovascularization by intravenous injection of adenoviral vectors expressing secretable endostatin

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.

Intraocular expression of endostatin reduces VEGF-induced retinal vascular permeability, neovascularization, and retinal detachment

Endostatin, a proteolytic fragment of collagen XVIII, is an endogenous inhibitor of tumor angiogenesis that also inhibits choroidal neovascularization. In this study, we assessed the effects of increased intraocular expression of endostatin on vascular endothelial growth factor (VEGF)-induced changes in the retina. After subretinal injection of a pair of gutless adenoviral vectors (AGV) designed to provide tamoxifen-inducible expression of endostatin, diffuse endostatin immunoreactivity was induced thoroughout the retina by administration of tamoxifen. Induction of endostatin in double transgenic mice with doxycycline-induced expression of VEGF in the retina resulted in significant suppression of leakage of intravascular [3H]mannitol into the retina. The ability of endostatin to reduce VEGF-induced retinal vascular permeability was confirmed by using [3H]mannitol leakage and two other parameters, fluorescein leakage and retinal thickness, after subretinal injection of a bovine immunodeficiency lentiviral vector coding for endostatin (BIV-vectored endostatin, or BIVendostatin). Subretinal injection of BIVendostatin resulted in more discrete, less intense staining for endostatin in the retina than that seen with the inducible AGV system, which suggested lower levels and allowed visualization of sites where endostatin was concentrated. Endostatin staining outlined retinal blood vessels, which suggested endostatin binding to a component of vessel walls. More prolonged or higher level expression of VEGF in the retina resulted in neovascularization and retinal detachment, both of which were also significantly reduced by BIVendostatin. These data suggest that endostatin may be an endogenous inhibitor of vasopermeability as well as neovascularization. In patients with diabetic retinopathy, endostatin gene transfer may provide a way to decrease the risk of three causes of visual loss: macular edema, neovascularization, and retinal detachment.

Periocular injection of an adenoviral vector encoding pigment epithelium-derived factor inhibits choroidal neovascularization

Gene transfer provides an exciting new approach for the treatment of retinal and choroidal diseases. Two areas of concern are the potential for vector-related toxicity and uncertainties associated with prolonged transgene expression. One way to address these concerns for transfer of genes encoding secreted proteins is to transduce cells on the outside of the eye, provided the gene product can gain access to the eye and have the desired effect. In this study, we investigated the feasibility of this approach. Periocular injection of an adenoviral vector encoding beta-galactosidase (AdLacZ.10) resulted in LacZ-stained cells throughout the orbit and around the eye. Compared to periocular injection of 5 x 10(9) particles of control vector, periocular injection of 5 x 10(9) or 1 x 10(9) particles of an adenoviral vector expressing pigment epithelium-derived factor (PEDF) regulated by a CMV promoter (AdPEDF.11) resulted in significantly elevated intraocular levels of PEDF and suppression of choroidal neovascularization. Periocularly injected recombinant PEDF was also found to diffuse through the sclera into the eye. Although similar experiments are needed in an animal with a human-sized eye, these data suggest that periocular gene transfer deserves consideration for the treatment of choroidal diseases.

Lentivirus-mediated expression of angiostatin efficiently inhibits neovascularization in a murine proliferative retinopathy model

Ischemic retinal diseases, such as diabetic retinopathy, retinopathy of prematurity, and age-related macular degeneration, are a major cause of blindness worldwide. Angiostatin is an internal peptide fragment of plasminogen that inhibits endothelial proliferation in vitro and tumor growth in vivo. We now demonstrate that HIV vector encoding angiostatin (HIV-angiostatin) can inhibit retinal neovascularization in a mouse model of proliferative retinopathy. Intravitreal injections of HIV-angiostatin led to stable expression of the angiostatin gene in retinal tissue. Retinal neovascularization was histologically quantitated by a masked protocol. Retinal neovascularization in the eye injected with HIV-angiostatin was reduced in 90% (9/10; P=0.025) of animals, compared with the eye injected with phosphate-buffered saline. Reduction of histologically evident neovascular nuclei per 6-microm section averaged 68%, with maximal inhibitory effects of 87%. Neovascularization was not reduced in the eyes injected with HIV vector encoding enhanced green fluorescent protein. This is the first report that HIV-angiostatin can reduce neovascular cell nuclei in a murine proliferative retinopathy model. These data suggest that the anti-angiogenic activity of angiostatin has therapeutic potential for the treatment of retinal neovascularization.

Periocular gene transfer of sFlt-1 suppresses ocular neovascularization and vascular endothelial growth factor-induced breakdown of the blood-retinal barrier

Vascular endothelial growth factor (VEGF) is a critical stimulus for both retinal and choroidal neovascularization, and for diabetic macular edema. We used mouse models for these diseases to explore the potential of gene transfer of soluble VEGF receptor-1 (sFlt-1) as a treatment. Intravitreous or periocular injection of an adenoviral vector encoding sFlt-1 (AdsFlt-1.10) markedly suppressed choroidal neovascularization at rupture sites in Bruch's membrane. Periocular injection of AdsFlt-1.10 also caused significant reduction in VEGF-induced breakdown of the blood-retinal barrier, but failed to significantly inhibit ischemia-induced retinal neovascularization. Periocular delivery of an adenoviral vector encoding pigment epithelium-derived factor (PEDF), another secreted protein, resulted in high levels of PEDF in the retinal pigmented epithelium and choroid, but not in the retina. This may explain why periocular injection of AdsFlt-1.10 inhibited choroidal, but not retinal neovascularization. Periocular delivery offers potential advantages over other routes of delivery and the demonstration that sFlt-1 enters the eye from the periocular space in sufficient levels to achieve efficacy in treating choroidal neovascularization and retinal vascular permeability is a novel finding that has important clinical implications. These data suggest that periocular gene transfer of sFlt-1 should be considered for treatment of choroidal neovascularization and diabetic macular edema.

New functional roles for non-collagenous domains of basement membrane collagens

Collagens IV, XV and XVIII are major components of various basement membranes. In addition to the collagen-specific triple helix, these collagens are characterized by the presence of several non-collagenous domains. It is clear now that these ubiquitous collagen molecules are involved in more subtle and sophisticated functions than just the molecular architecture of basement membranes, particularly in the context of extracellular matrix degradation. Degradation of the basement membrane collagens occurs during numerous physiological and pathological processes such as embryonic development or tumorigenesis and generates collagen fragments. These fragments are involved in the regulation of functions differing from those of their original intact molecules. The non-collagenous C-terminal fragment NC1 of collagen IV, XV and XVIII have been recently highlighted in the literature because of their potential in reducing angiogenesis and tumorigenesis, but it is clear that their biological functions are not limited to these processes. Proteolytic release of soluble NC1 fragments stimulates migration, proliferation, apoptosis or survival of different cell types and suppresses various morphogenetic events.

Inhibition of retinal neovascularization by intraocular viral-mediated delivery of anti-angiogenic agents

Neovascularization characterizes diabetic retinopathy and choroidal neovascularization associated with age-related macular degeneration, the most common causes of severe visual loss in the developed world. Gene transfer to the eye using adeno-associated viral (AAV) vectors is a promising new treatment for inherited and acquired ocular diseases. We used an AAV vector with rapid onset and high levels of gene expression in the retina to deliver three anti-angiogenic factors (pigment epithelium-derived factor, tissue inhibitor of metalloproteinase-3, and endostatin) to the eyes of mice in a mouse model of retinopathy of prematurity. All three vectors inhibited ischemia-induced neovascularization.

Blockade of nitric-oxide synthase reduces choroidal neovascularization

Nitric oxide (NO) promotes retinal and choroidal neovascularization, although different isoforms of nitric-oxide synthetase (NOS) are critical in each. Deficiency of endothelial NOS (eNOS) suppresses retinal but not choroidal neovascularization, whereas deficiency of neuronal NOS (nNOS) or inducible NOS (iNOS) suppresses choroidal, but not retinal neovascularization. In this study, we investigated the effect of N(G)-monomethyl-L-arginine (L-NMMA), a nonspecific NOS inhibitor, in three models of ocular neovascularization. Oral administration of L-NMMA caused significant inhibition of choroidal neovascularization in mice with laser-induced rupture of Bruch's membrane and significantly inhibited subretinal neovascularization in transgenic mice with expression of vascular endothelial growth factor (VEGF) in photoreceptors (rho/VEGF mice) but did not inhibit retinal neovascularization in mice with ischemic retinopathy. By extensive mating among mice deficient in NOS isoforms, triple homozygous mutant mice deficient in all three NOS isoforms were produced. These mice had marked suppression of choroidal neovascularization at sites of rupture of Bruch's membrane and near-complete suppression of subretinal neovascularization in rho/VEGF mice but showed no difference in ischemia-induced retinal neovascularization compared with wild-type mice. These data indicate that NO is an important stimulator of choroidal neovascularization and that reduction of NO by pharmacologic or genetic means is a good treatment strategy. However, the situation is more complex for ischemia-induced retinal neovascularization for which NO produced in endothelial cells by eNOS is stimulatory, but NO produced in other retinal cells by iNOS and/or nNOS is inhibitory. Selective inhibitors of eNOS may be needed for treatment of retinal neovascularization.

Pharmacological regulation of protein expression from adeno-associated viral vectors in the eye

The control, over time and space, of the levels of therapeutic proteins is crucial for successful retinal gene therapy. We tested the ability of adeno-associated viral vectors (AAV) delivered intraocularly to release a secreted protein (erythropoietin (Epo) used as a marker) in the eye, either constitutively or in a pharmacologically regulated manner using the dimerizer-inducible transcriptional regulatory system. Following delivery of a constitutively expressing vector to the intravitreal or subretinal space of nude rats, Epo protein was detected in both the anterior chamber and vitreous fluids. A dual-vector system inducible by the dimerizer rapamycin and expressing Epo was administered into the subretinal space in an attempt to achieve pharmacologic control of trangene expression in the eye. Before induction with rapamycin, the intraocular Epo level was negligible. However, following a systemic administration of rapamycin, Epo was detected in the anterior chamber, peaking on day 3 and returning to baseline 2-3 weeks after withdrawal of the drug. Peak-induced Epo in the anterior chamber was proportional to the dose of rapamycin and was not detected in serum. Similar results were obtained following subretinal administration of the vectors in one nonhuman primate. The rapamycin inducible system promises to be useful for developing gene therapies for inherited retinal degeneration and ocular neovascularization.

Adeno-associated virus type-2 expression of pigmented epithelium-derived factor or Kringles 1-3 of angiostatin reduce retinal neovascularization

Neovascular diseases of the retina include age-related macular degeneration and diabetic retinopathy, and together they comprise the leading causes of adult-onset blindness in developed countries. Current surgical, pharmaceutical, and laser therapies for age-related macular degeneration (AMD) rarely result in improved vision, do not significantly prevent neovascularization (NV), and often result in at least some vision loss. To address this therapeutic gap, we determined the efficacy of recombinant adeno-associated viral (rAAV) serotype-2-mediated expression of pigment epithelium-derived factor (PEDF) or Kringle domains 1-3 of angiostatin (K1K3) in reducing aberrant vessel formation in a mouse model of ischemia-induced retinal NV. Both PEDF and K1K3 are potent inhibitors of NV when injected directly, hence expression of these therapeutic factors from rAAV may provide long-term protection from neovascular eye disease. rAAV vectors expressing the therapeutic gene were injected into one eye of postnatal day 0 (P0) newborn mouse pups. Retinal NV was induced in P7 mice by exposure to elevated oxygen for 5 days followed by room air for another five days. Retinal NV was quantified by the number of vascular-endothelial-cell nuclei above the inner-limiting membrane in P17 eyes. The number of such vascular endothelial cell nuclei in eyes treated with rAAV-PEDF or rAAV-K1K3 was significantly reduced (both P < 0.0000002) compared with control eyes. Ocular protein levels detected by ELISA correlate well with the reduction in NV and confirm that expression of antineovascular agents from rAAV vectors may be a therapeutically useful treatment of retinal or choroidal neovascular disease.

Gene therapy for retinal and choroidal diseases

The eye is a small compartment separated from the systemic circulation by the blood-ocular barriers, providing advantages for intraocular gene transfer - an approach which is being investigated for several types of retinal and choroidal diseases. A compelling application is gene replacement for homozygous loss-of-function mutations in genes differentially expressed in photoreceptors or retinal pigmented epithelial (RPE) cells that result in retinal degeneration. Considerable progress has been made in this area, including demonstration of return of visual function in RPE65 (-/-) dogs after subretinal injection of adeno-associated viral vectors encoding RPE65, providing groundwork for a clinical trial in patients with Leber's Congenital Amaurosis. Proof of principle has been provided for intraocular gene transfer of ribozymes for dominantly inherited retinal degenerations. Survival factor gene therapy shows promise for treatments that may be used in multiple retinal degenerations. Transduction of intraocular and/or periocular cells with constructs that encode antiangiogenic proteins provides a new approach for sustained local delivery treatment of retinal and choroidal neovascularisation. While considerable investigation remains to work out critical details, there is substantial evidence suggesting that in the near future, gene therapy-based treatments will be an important addition to what is currently offered to patients with retinal and/or choroidal diseases.

Looking into anti-angiogenic gene therapies for disorders of the eye

Age-related macular degeneration (AMD) and proliferative diabetic retinopathy (DR) are the most common causes of visual impairment in the developed world. Because the key factor in AMD and DR is aberrant neovascularization in the retina (DR) or in the choroid (AMD), strategies to inhibit abnormal neovascularization represent a compelling therapeutic approach. Here we review various anti-angiogenic strategies for the treatment of ocular neovascular diseases with special emphasis on gene transfer as a way of achieving high, sustained concentrations of anti-angiogenic proteins in the back of the eye without concomitant systemic toxicity.