New approaches

An improved understanding of retinal neovascularisation is helping to explain the current limitations of anti-vascular endothelial growth factor (anti-VEGF) therapy and guiding the development of new therapeutic approaches, Patricia A D’Amore PhD told a symposium on neovascular age-related macular degeneration (AMD) during the 16th EURETINA Congress in Copenhagen, Denmark. Dr D’Amore, Director of Research, Schepens Eye Research Institute of Massachusetts Eye and Ear, USA, gave delegates an updated look at the pathophysiology of the disease, incorporating new information regarding therapeutic targets and how they fit into current and future treatment strategies. “The initiation of neovascularisation is thought to be caused by three separate events, all of which take place in and around the retinal pigment epithelium (RPE),” explained Dr D’Amore. The first event is the disruption and dedifferentiation of the RPE. Drusen are the driving factor here, growing under the RPE and causing both injury to the RPE and subsequent dedifferentiation of the tissue, she said. The next stage is an hypoxia-induced VEGF expression by RPE cells, and likely also by macrophages. In silico modelling has indicated that 63μm is the approximate drusen threshold size for oxygenation, said Dr D’Amore. “Drusen above this well-known size generate a region of RPE hypoxia, leading to the expression of VEGF. Also, the outer nuclear layer is thinner above high drusen, indicating that photoreceptor death is occurring.” The third event is inflammation that occurs secondary to drusen accumulation. Whether inflammation is caused by the simple presence of drusen, or is something that develops later on is not yet known, she said. Regardless, inflammasomes, intracellular protein structures whose sole purpose is to activate pro-inflammatory cytokines, are detected in both geographic atrophy and neovascular AMD. These factors drive a chronic pathologic process and are responsible for persistent VEGF production. Neovascularisation is the end-stage result. So much for the initiation of neovascularisation but what about maintenance of these new vessels? Bruch’s membrane is a physical and biochemical barrier to vessels from the choriocapillaris. A break in this membrane allows blood vessels to grow, and represents the transition to neovascular AMD. “The reason that anti-VEGF treatment generally has a temporary effect is because the underlying disease process is not influenced by VEGF neutralisation. Despite anti-VEGF treatment, the RPE injury is not corrected, breaks remain in Bruch’s membrane, inflammation remains uninterrupted, the RPE remains hypoxic and VEGF production continues. Treating with anti-VEGF is like putting a band-aid on the problem,” explained Dr D’Amore.

It is suspected that if we can prevent the pericytes from contacting the endothelial cells, they will remain vulnerable to anti-VEGF treatment because they will not mature

NON-RESPONDERS But why are there eyes that seem to be anti-VEGF non-responders? A subset of neovascular vessels are ‘mature’. Stable vessels have associated pericytes or smooth muscle cells and a complete basement membrane, which means that they have become differentiated vessels. These mature, stable vessels are maintained via interaction between endothelial cells and pericytes. The endothelial cells generate platelet-derived growth factor (PDGF), which induces the migration of pericyte precursors. Pericyte coverage leads to inhibition of endothelial cell proliferation and production of basement membrane, which make neovascular vessels less vulnerable to anti-VEGF-induced disruption. Dr D’Amore showed colourised optical coherence tomography-angiography images of large neovascular membranes at baseline and at weeks 4, 6 and 8 during anti-VEGF treatment. “We can see from these images that, although much of the neovascularisation disappears from view, a large component of the vasculature remains, despite the presence of VEGF neutralisation,” she said. “It is suspected that if we can prevent the pericytes from contacting the endothelial cells, they will remain vulnerable to anti-VEGF treatment because they will not mature,” said Dr D’Amore. Pericytes also promote endothelial cell survival through chemical signalling and physical interactions, including production of VEGF by pericytes. Platelet-derived growth factor B (PDGF-B) is the target of a molecule called pegpleranib, developed by Ophthotech Corp. as Fovista®. Patricia A D’Amore: 
patricia_damore@meei.harvard.edu