A review of treatments for macular degeneration: a synopsis of currently approved treatments and ongoing clinical trials

Engrafted Neural Progenitor Cells Express a Tissue-Restricted Reporter Gene Associated with Differentiated Retinal Photoreceptor Cells

Neural progenitor cells (NPCs) have shown ability to repair injured CNS, and might provide precursors to retinal neurons. NPCs were isolated from the brains of 14 day murine embryos of transgenic mice that express β-galactosidase (β-gal) on the arrestin promoter, which specifically directs expression to retinal photoreceptor cells. NPCs were transferred to adult, syngeneic mice via inoculation into the anterior chamber of the eye, the peritoneal cavity, or the brain. At 14 weeks postgrafting, tissues were collected and examined to determine if differentiated NPC progeny were present in retina based on histochemical detection of β-gal. Four of six anterior chamber-inoculated recipients showed Bluo-gal-stained cells in retina, indicating the presence of transferred NPCs or their progeny. Because the progenitor cells do not express β-gal, positive staining indicates differentiation leading to activation of the arrestin promoter. Two recipients inoculated by the intraperitoneal route also exhibited Bluo-gal staining in retina. The NPCs did not express β-gal if inoculated into brain, but survived and dispersed. Most recipients, regardless of inoculation route, were PCR positive for β-gal DNA in extraocular tissues, but no Bluo-gal staining was found outside of the retina. Injury to the retina promoted, but was not required, for progenitor cell engraftment. β-Gal-positive cells were concentrated in the outer layers of the retina. In summary, a reporter gene specifically expressed in differentiated retinal photoreceptor cells due to the activity of the arrestin promoter was expressed in recipient mouse retina following transfer of NPCs prepared from the β-gal transgenic mice. The presence of β-gal DNA, but not Bluo-gal staining, in spleen and other tissues revealed that the cells also migrated elsewhere and took up residence in other organs, but did not undergo differentiation that led to β-gal expression.

Drug delivery to the posterior segment of the eye for pharmacologic therapy

Treatment of diseases of the posterior segment of the eye, such as age-related macular degeneration, cytomegalovirus retinitis, diabetic retinopathy, posterior uveitis and retinitis pigmentosa, requires novel drug delivery systems that can overcome the many barriers for efficacious delivery of therapeutic drug concentrations. This challenge has prompted the development of biodegradable and nonbiodegradable sustained-release systems for injection or transplantation into the vitreous as well as drug-loaded nanoparticles, microspheres and liposomes. These drug delivery systems utilize topical, systemic, subconjunctival, intravitreal, transscleral and iontophoretic routes of administration. The focus of research has been the development of methods that will increase the efficacy of spatiotemporal drug application, resulting in more successful therapy for patients with posterior segment diseases. This article summarizes recent advances in the research and development of drug delivery methods of the posterior chamber of the eye, with an emphasis on the use of implantable devices as well as micro- and nanoparticles.

Effect of sorafenib on experimental choroidal neovascularization in the rat

PURPOSE

This study was designed to evaluate the effect of sorafenib administration on laser-induced experimental choroidal neovascularization (CNV) in rats.

METHODS

A total of 36 rats were divided into three groups. Sorafenib, an oral, multitargeted receptor tyrosine kinase inhibitor, was administered at a dose of 10 mg/kg per day (n = 12) or 30 mg/kg per day (n = 12) for 1 day before the laser induction of CNV. Rats continued to receive the drug for 14 days. Fluorescein angiograms were analysed for CNV dye leakage and the thickness of CNV was assessed by histology. The levels of vascular endothelial growth factor, platelet-derived growth factor, monocyte chemoattractant protein-1 and intercellular adhesion molecule-1 mRNA were measured by the use of real-time quantitative reverse-transcription polymerase chain reaction.

RESULTS

Sorafenib-treated rats had significantly less fluorescence leakage as compared with vehicle-treated rats (P < 0.05), The CNV thickness in sorafenib-treated rats was significantly reduced as compared with vehicle-treated rats in a dose-dependent manner (P = 0.00163 for 10 mg/kg and P < 0.00001 for 30 mg/kg). After sorafenib (30 mg/kg) administration, expression of the vascular endothelial growth factor and intercellular adhesion molecule-1 genes was significantly decreased (P < 0.05 and P = 0.00802).

CONCLUSION

These results suggest that sorafenib may be potentially beneficial for the treatment of CNV in human; further studies on this subject are warranted.

The role of complement in ocular pathology

Functionally active complement system and complement regulatory proteins are present in the normal human and rodent eye. Complement activation and its regulation by ocular complement regulatory proteins contribute to the pathology of various ocular diseases including keratitis, uveitis and age-related macular degeneration. Furthermore, a strong relationship between age-related macular degeneration and polymorphism in the genes of certain complement components/complement regulatory proteins is now well established. Recombinant forms of the naturally occurring complement regulatory proteins have been exploited in the animal models for treatment of these ocular diseases. It is hoped that in the future recombinant complement regulatory proteins will be used as novel therapeutic agents in the clinic for the treatment of keratitis, uveitis, and age-related macular degeneration.

Receptor tyrosine kinase inhibitors AG013764 and AG013711 reduce choroidal neovascularization in rat eye

Age-related macular degeneration (AMD) is the major cause of blindness for people over 60. In the "wet" form of AMD compounds targeting growth factor signaling pathways such as VEGF have been a major focus for therapeutic interventions. In a previously developed rat model of CNV, we utilized two receptor tyrosine kinase inhibitors (RTKi) to block VEGFR-1, VEGFR-2 and PDGFR signaling following the establishment of CNV. AAV-VEGF(165) was injected into the subretinal space of rats at postnatal days 15-17. Six weeks later, a suspension of RTK inhibitors, AG013764 or AG013711, was injected intraperitoneally (IP, twice daily) or intravitreally (every five days) over a two week period. FITC-dextran whole-mounts of RPE-choroid-sclera were prepared after the animals were sacrificed. CNV area was quantified using Neurolucida to measure the hyperfluorescence on FITC-dextran whole-mounts. Histology and immunohistochemistry were performed as described previously. VEGF expression in control and treated eyes was confirmed by immunohistochemistry and histological sections indicated recovery of retinal morphology and CNV reduction in treated eyes. In the animals IP injected with AG013764 or AG013711 the mean CNV level was reduced by 25 to 33% compared to control, but this effect did not achieve statistical significance. Intravitreal injections of AG013764 or AG013711 reduced the level of CNV by approximately 60% compared to control (p<0.005 or p<0.05, respectively). These data show that two RTK inhibitors, AG013764 or AG013711, delivered intravitreally, significantly reduce blood vessel proliferation in this AAV-VEGF(165) model of CNV.

A neuroprosthesis for restoring sight

Macular degeneration (MD) and retinitis pigmentosa (RP), two diseases that cause degeneration of retinal photoreceptor cells, are the leading causes of blindness in the United States. Anatomical studies have shown that other retinal neuronal cells (bipolar cells, ganglion cells) are preserved in these diseases and they are capable of eliciting visual percepts when electrically stimulated. We describe the design of a prototype 16-electrode retinal prosthesis, and the physiological and clinical results on six blind patients with RP who had the device implanted. The US Department of Energy artificial retina program is described. The goal of the program is construction of a 1000-electrode retinal neuroprosthesis with the potential of enabling blind patients to read large print and ambulate with ease.

Vascular endothelial growth factor receptors: molecular mechanisms of activation and therapeutic potentials

Angiogenesis-associated eye diseases are among the most common cause of blindness in the United States and worldwide. Recent advances in the development of angiogenesis-based therapies for treatment of angiogenesis-associated diseases have provided new hope in a wide variety of human diseases ranging from eye diseases to cancer. One group of growth factor receptors critically implicated in angiogenesis is vascular endothelial growth factor receptors (VEGFR), a subfamily of receptor tyrosine kinases (RTKs). VEGFR-1 and VEGFR-2 are closely related receptor tyrosine kinases and have both common and specific ligands. VEGFR-1 is a kinase-impaired RTK and its kinase activity is suppressed by a single amino acid substitution in its kinase domain and by its carboxyl terminus. VEGFR-2 is highly active kinase, stimulates a variety of signaling pathways and broad biological responses in endothelial cells. The mechanisms that govern VEGFR-2 activation, its ability to recruit signaling proteins and to undergo downregulation are highly regulated by phosphorylation activation loop tyrosines and its carboxyl terminus. Despite their differential potentials to undergo tyrosine phosphorylation and kinase activation, both VEGFR-1 and VEGFR-2 are required for normal embryonic development and pathological angiogenesis. VEGFR-1 regulates angiogenesis by mechanisms that involve ligand trapping, receptor homodimerization and heterodimerization. This review highlights recent insights into the mechanism of activation of VEGFR-1 and VEGFR-2, and focuses on the signaling pathways employed by VEGFR-1 and VEGFR-2 that regulate angiogenesis and their therapeutic potentials in angiogenesis-associated diseases.

Reliability and Consistency of Visual Acuity and Contrast Sensitivity Measures in Advanced Eye Disease

PURPOSE

The purpose of this article is to establish reliability of standard visual function measures in patients with severe vision loss.

METHODS

Legally blind subjects were enrolled in a repeated-measures study to determine the reproducibility of a battery of psychophysical vision measures. Tests reported here include ETDRS visual acuity (VA) under regular and dim illumination and Pelli-Robson letter contrast sensitivity (CS) under regular, dim, and glare conditions. Two to five test repetitions were performed at monthly intervals. Subject groups included retinitis pigmentosa (RP) (n = 29), macular disease (MD) (n = 17), optic nerve disease (ON) (n = 4), diabetic retinopathy (DR) (n = 5), and other retinal diseases (OR) (n = 10).

RESULTS

For most subject groups, VA measures had mean 95% coefficient of repeatability (CR.95) of 0.20 (group means between 0.13 and 0.36) log units, and CS measures had mean CR.95 of 0.35 (group means between 0.19 and 0.48) log units.

CONCLUSIONS

The established reliability of VA and CS tests in subjects with severe visual impairments will allow investigators to determine whether a significant change in vision has occurred during future clinical trials to reverse, halt, or slow vision loss. The criterion for change needs to be larger than for normally sighted controls, i.e., approximately two to three lines for VA and between eight to 11 letters for CS.

Effect of exogenous administration of nerve growth factor in the retina of rats with inherited retinitis pigmentosa

NGF is implicated in retinal damage regression. To study whether this is a direct effect or an effect mediated by NGF on other endogenous biological mediators, we investigated the effect of exogenous administration of NGF in RCS rats affected by retinitis pigmentosa. We found that NGF administration exerts a rescue effect on photoreceptors in this animal model. NGF injection enhances brain-derived neurotrophic factor, beta-fibroblast growth factor, transforming growth factor-beta, vascular endothelial factor and neuropeptide-Y. This suggests that NGF has an effect on RCS rat retina, probably also through the stimulation of other biological mediators produced and released in the retina.

Discovery and validation of drug targets for tumour angiogenesis

The formation of blood vessels is a key process in the progression of solid tumours, providing the means for tumour growth and metastasis. A number of drugs are currently being developed to exploit inhibition of angiogenesis in the therapy of cancer. An even greater number of genes that are regulated in models of in vitro angiogenesis have been identified. These genes present potential drug targets for the development of novel, more efficient, drugs that will enable the judicious design of drug cocktails that may be able to account for the many different cancer pathologies and their drug resistance properties. Dealing with the validation of hundreds of potential angiogenesis drug targets requires the utilisation of experimental technology platforms that enable concomitant and dynamic target selection filtering and validation. Such platforms should act as a funnel-like medium-to-low throughput processes that enable the sequential short-listing of hundreds of candidates culminating in the selection of only a small number of well-validated targets that are manageable by drug screening regimes.