While pegaptanib (Macugen) was the first FDA approved anti-VEGF agent, its efficacy was limited with respect to patients gaining significant vision (three lines of visual gain using a standardized visual acuity chart). Subsequent clinical trials have demonstrated that other agents targeting VEGF, such as ranibizumab (Lucentis), bevacizumab (Avastin) and aflibercept (VEGF-Trap) result in significant visual gain in approximately one-third of the patients with wet AMD. (Nguyen Q, et al. IOVS 2011;52:ARVO E-Abstract 3128)^1-3

There is widespread acceptance that the majority of the therapeutic benefit of monotherapy anti-VEGF agents in use today is due to their potent anti-permeability property.⁴ Anti-VEGF monotherapy does not, however, induce disease-modifying regression of the pathologic NV tissue.^1-3 Therefore, stability of the NV complex ensues despite continued treatment, which may limit significant visual gain in a majority of treated patients. Hence, targeted pharmacotherapeutic strategies inducing NV regression coupled with resolution of permeability in wet AMD may result in enhanced visual outcome.

Neovascularization is a normal process involved in embryonic development and tissue repair but becomes pathological when blood vessels proliferate into abnormal tissues (i.e., tumors), the avascular cornea or the subretinal space. The proliferation, invasion and migration of the NV vessels are coordinated by complex molecular mechanisms involving growth factors, cytokines and their associated receptors. The diverse set of signaling pathways sets the stage for a multi-targeted pharmacotherapy approach for the treatment of AMD.

The NV tissue consists of endothelial cells, pericytes and inflammatory cells (i.e., occasional macrophages). Based on our current understanding, the NV process involves the formation of angiogenic sprouts, led by tip cell filopodia, from existing capillaries into the avascular space. VEGF is a master switch responsible for activation, survival and proliferation of EC and has been found in the localized space at the tip of the angiogenic sprout. Following and lagging behind the EC angiogenic sprout formation, the newly formed vessels attract and are coated by pericytes, leading to NV maturation.

Pericytes are derived via differentiation of mural cells. Their presence on capillaries not only leads to NV support and stabilization but promotes endothelial cell survival through chemical signaling and physical interactions including pericyte production of VEGF.⁶ This endothelial survival signaling by integrated pericytes is critical and may explain the resistance of the NV tissue to VEGF withdrawal, i.e., lack of NV regression to monotherapy anti-VEGF treatment. Therefore, stripping of pericytes combined with anti-VEGF therapy may lead to regression of NV vessels and improved visual outcomes.

The coating of the NV endothelial cells by pericytes is initiated by endothelial cell expression of the paracrine platelet-derived growth factor B, which forms the homo dimer PDGF-BB. The EC expressed PDGF-B is retained proximally by heparin sulfate proteoglycan, with the highest concentration in the tip cells of the angiogenic sprout. The recognition of the proximally retained PDGF-BB by the pericyte-bound PDGFR-β receptor initiates the proliferation and migration of the pericytes along the growing neovascularization. The role of PDGF-B/PDGFR-β interaction for pericyte recruitment and capillary maturation has been demonstrated through gene knockout, ectopic delivery of competing PDGF-B, blocking the PDGF-B/PDGFR-β interaction by external agents, or deletion of the PDGF-B heparin sulfate proteoglycans binding motif.⁴ The attachment and maturation of the pericytes lags behind the endothelial cell proliferation at the angiogenic sprout in both time and space.^7,8

The importance of the PDGF-B/PDGFR-β interaction in pericyte recruitment, maturation and resistance to anti-VEGF mediated regression has been demonstrated in animal models. These studies show that blocking the PDGF–B/PDGFR-β interaction leads to lack of pericyte coverage, abnormal/leaking blood vessels, inhibition of NV capillary maturation and neonatal lethality.⁹ Specifically, animal models of corneal and choroidal neovascularization have demonstrated that administration of agents that block the PDGF-B/PDGFR-β interaction leads to pericyte stripping from the pathological neovasculature.¹⁰ Co-administration of anti-VEGF and anti-PDGF agents leads to greater NV inhibition than either agent alone. More importantly, such combination induces NV regression as well.

Established, mature, non-pathological vascularization is unaffected by inhibitors of the PDGF-B/PDGFR-β interaction. A possible hypothesis for the decreased PDGF dependency on pericytes residing on mature host vessels as opposed to proliferating NV tissue may be related to lack of PDGFR-β expression in mature pericytes. Alternatively, the nature of pericyte-EC association differs in mature and pathologically proliferating vessels. In the mature host vascular bed (non-pathologic vascular tissue), Sphingosine-1-phosphate (SP1) mediated N-Cadherin and Gap junction activated TGF-β1 leads to a stabilized pericyte-EC interaction and mural cell differentiation.¹¹ In addition, the pericytes and EC share a common basement membrane with individual pericytes contacting multiple endothelial cells. Conversely, pericytes involved in pathological neovascularization appear to have an abnormal morphology with a looser association to the underlying EC vasculature.¹²

New PDGF Antagonist
A PDGF antagonist, E10030, with high specificity and affinity in the form of a pegylated oligonucleotide (aptamer) has been developed by Ophthotech Corp.13 E10030 is 32-mer DNA-based oligonucleotide containing 2’-methoxy and 2’-fluoro modified bases and a 40-kDa, branched PEG attached to the 5’end. E10030 binds to PDGF with a Kd of ~300 pM and blocks its binding to PDGFR-β. In vitro/in vivo studies using this compound in combination with an anti-VEGF agent have demonstrated its effectiveness at preventing the formation of NV vessels, as well as stripping of pericytes leading to regression of established NV vessels.⁸

A Phase-1 clinical study to investigate the safety and tolerability of intravitreous injections of E10030 in combination with ranibizumab has been completed. This was a first-in-human clinical trial of intravitreous injections of E10030 in a dose-escalation (0.03 mg, 0.3 mg, 1.5 mg and 3.0 mg) scheme. Treatment-naïve patients with predominantly classic and predominantly occult subtypes were enrolled with best corrected visual acuity of 20/63 to 20/200. Mean baseline ETDRS letters read was 49.1.