Overview of Emerging Molecular Therapies for Neovascular AMD
LOUIS K. CHANG, MD, PhD
The approval of Macugen in December 2004 by the FDA marked the beginning of the molecular era in the treatment of neovascular AMD.1 Pegaptanib sodium is a selective blocker of VEGF165 and has demonstrated the ability to slow the rate of vision loss, but did not show significant improvement in vision in a majority of patients.
The introduction of Lucentis dramatically changed the treatment paradigm for AMD-related CNV. Randomized controlled clinical trials showed that monthly ranibizumab treatment resulted in stabilization of vision in more than 90% of patients, as well as improvement in vision of a significant nature in about a third of all patients treated.2,3 Avastin, a VEGF-neutralizing antibody with properties very similar to Lucentis, is used for the treatment of several systemic cancers. Off-label use of Avastin for the treatment of CNV has become widespread in the US and worldwide.4 A large-scale comparative clinical trial (the CATT trial) is currently underway to evaluate the relative benefits of these 2 molecules in the management of CNV.
The process by which VEGF is generated and results in angiogenesis is a complex cascade of events (Figure). Each step offers the possibility of therapeutic intervention. Activation of signaling cascades that ultimately result in VEGF production may be triggered by hypoxia, exposure to certain growth factors, or other inciting stimuli. A key step in this cascade involves a molecule known as mTOR, the mammalian target of rapamycin. mTOR is a protein kinase that regulates cell proliferation, motility, survival, and protein synthesis.5 It leads to activation of hypoxia-inducible transcription factors, including HIF1α, which in turn activates transcription of several genes, including those that produce VEGF.6 Several agents are being developed to target this portion of the cascade. Sirolimus (rapamycin, MacuSight/Santen), which targets mTOR1, has demonstrated preclinical evidence of anti-inflammatory, antiangiogenic, and anti-fibrotic activity.7 Phase 1 testing has demonstrated evidence of potential effects in AMD through both subconjunctival and intravitreal delivery. Phase 2 trials are currently under way. Another molecule, Everolimus or RAD001 (Novartis), is a derivative of rapamycin and also acts as an mTOR inhibitor.8 It is currently used in the prevention of allograft transplant rejection and for the treatment of systemic cancers, but research into its ophthalmic application is under way.
Figure. The angiogenesis process and where certain drugs intervene.
REDD1 is another molecule that promotes VEGF production through the same pathway.9 RTP801i-14 (Quark/Pfizer), now known as PF-4523655, is a small interfering RNA that has been developed to inhibit REDD1 and suppress VEGF production, as well as inhibit angiogenesis. Results from a phase 1/2 trial showed that intravitreal PF-4523655 was safe and well tolerated in patients with wet AMD. Preclinical evaluation of another siRNA drug that is specifically targeted toward HIF1α is also under way, with plans for bringing it to clinical phase 1 level by 2010.
|Louis K. Chang, MD, PhD, is a clinical fellow in vitreoretinal surgery at Columbia University, Vitreous-Retina-Macula Consultants of New York, and New York University. The author reports no financial interest in any products mentioned in this article. Dr. Chang can be reached via e-mail at firstname.lastname@example.org.|
After VEGF production, the next rational drug target is the VEGF molecule. In addition to anti-VEGF drugs already in use, an additional drug known as VEGF-Trap-Eye (Regeneron/Bayer) is currently undergoing clinical study for inhibition of VEGF. The VEGF-Trap is a fusion protein that combines features of 2 different VEGF receptor sites, thus allowing a higher binding affinity than the anti-VEGF drugs currently in use.10 This molecule has demonstrated effectiveness in improving visual acuity and reducing CNV size and OCT thickness in a phase 2 clinical trial and is now in phase 3 testing in a direct head-to-head comparison with Lucentis.11 Another approach to VEGF neutralization involves a soluble form of the high affinity VEGF receptor VEGFR1/Flt-1. This soluble receptor sequesters VEGF in the extracellular space, preventing the activation of downstream signalling pathways that result from VEGF binding to the receptors on the cell surface. Preclinical trials have shown that adeno-associated virus serotype 2–mediated intravitreal gene delivery of sFLT01 efficiently inhibits angiogenesis in animal models.12,13
VEGF in the extracellular space binds to VEGF receptors on the surface of the target cell, initiating a signaling cascade that result in endothelial cell activation and angiogenesis. Many potential therapeutic molecules that target this portion of the cascade are currently under investigation. One such molecule is AGN-211745 (Allergan/Merck), a small interfering RNA directed against the VEGFR1 that has undergone initial phase I testing for AMD.14 By blocking VEGFR1 production, endothelial cells would be less responsive or unresponsive to elevated levels of VEGF.
Another potential target at this level of the cascade is a family of transmembrane proteins known as integrins, which have a role in signaling and modulating downstream activities.15 Several agents currently being studied target the integrins as a means of controlling neovascularization in AMD. One drug, JSM6427 ( Jerini), is a potent, highly specific integrin α5β1-antagonist that is currently undergoing phase 1 trials involving single and multiple injections for patients with CNV.16 One advantage of this molecule is that it may also inhibit the effects of growth factors and cytokines other than VEGF that promote angiogenesis, inflammation, and fibrosis. Another integrin antagonist is volociximab (Ophthotech), a high-affinity monoclonal antibody that binds to α5β1 integrin and blocks binding of α5β1 integrin to fibronectin, a critical event in angiogenesis. Volociximab administration has resulted in strong inhibition of rabbit and primate retinal neovascularization and laser-induced choroidal neovascularization in monkeys.17 A phase 1 study of volociximab in combination with ranibizumab in patients with wet AMD is ongoing.
The activation of VEGF receptors and many of the downstream signaling molecules is dependent on tyrosine kinases. Thus, inhibition of tyrosine kinase activity would be expected to prevent the anticipated sequelae of the angiogenic process: blood vessel growth and leakage. A number of molecules that inhibit tyrosine kinases are being investigated. One such molecule is pazopanib (GlaxoSmithKline), a kinase inhibitor that targets multiple VEGF family members. Pazopanib blocks several receptor tyrosine kinases including VEGF receptors 1, 2, and 3 and PDGFR, c-Kit and fibroblast growth factor receptor 1 and inhibits CNV in a murine model.18 Many of these other receptor tyrosine kinases activate pericytes and endothelial cells that populate choroidal neovascular membranes. By blocking multiple receptors, it is hoped that multi-kinase inhibitors may not only halt new vessel development but also induce regression of CNV itself. Phase 1 trials are underway to evaluate pazopanib, which is delivered by topical application. Other candidate receptor tyrosine kinase inhibitors include the topical drug TG100801 (TargeGen), the oral drug vatalanib (Novartis), and AG013958 (Pfizer) and AL39324 (Alcon), which are delivered by periocular injection. While topical medications are the least invasive method of ocular drug delivery, oral medications may have a role in the concurrent treatment of bilateral exudative AMD.
Despite its central role in angiogenesis, the VEGF pathway is not unique in stimulating and sustaining neovascularization in AMD. Therefore, other approaches to controlling CNV are being explored. Fosbretabulin (combrestatin A4 phosphate, Oxigene) is a novel anti-vascular agent that targets endothelial cells of abnormal vascular structures. The biologically active metabolite CA4 binds to tubulin and inhibits microtubule assembly, leading to occlusion of lumen of actively proliferating blood vessels.19 It is currently undergoing phase 2 testing with intravenous administration in patients with the polypoidal variant of AMD.
Sphingosine-1-phosphate (S1P) is an extracellular signaling and regulatory molecule implicated as one of the earliest responses to stress, promoting cellular proliferation, migration and activating survival pathways. Several lines of evidence suggest that S1P and its receptors play a major regulatory role in the neovascularization, fibrosis and inflammation related to AMD.20 Sonepcizumab (LT1009, LPath) is a humanized monoclonal antibody against S1P that binds S1P with high specificity at physiologic concentrations.21 A phase 1 multicenter, dose-escalation study of LT1009 administered as a single intravitreal injection in patients with wet AMD is under way.
Pigment epithelium derived factor (PEDF) is a naturally occurring inhibitor of angiogenesis.22 It is normally produced in ocular tissues and regulates normal blood vessel growth. PEDF levels have been found to be significantly decreased in eyes with AMD.23 Ad-PEDF.11D (GenVec) is an intravitreal or periocular injected transgene that uses a viral vector to deliver the PEDF gene, resulting in the local production of PEDF in the treated eye.24 A phase 1, dose-escalation study of this agent in eyes with CNV secondary to AMD showed no significant adverse effects.
Platelet-derived growth factor (PDGF) is responsible for the recruitment, growth, and survival of pericytes and serves to regulate vascular maturation. E10030 (Ophthotech) is a pegylated aptamer that binds and inhibits PDGF and has been shown to inhibit or strip pericytes in preclinical models.25 In a phase 1 clinical study in combination with ranibizumab, E10030 was well tolerated and demonstrated significant regression of neovascular lesions. As mentioned previously, some receptor tyrosine kinase inhibitors under investigation for treatment of AMD also inhibit activation of PDGF receptor 1 in addition to VEGF receptors.
Another therapeutic target for wet AMD is the complement system — part of the innate immune system that includes a series of endogenous proteins that act to inhibit excessive activation and protect host cells. Genetic and histopathologic studies support a role for abnormal activation of the complement system in different stages of AMD, including dry AMD.26-28 Thus, drugs targeting the complement system may find utility in the treatment of both wet and dry AMD. ARC1905 (Ophthotech) is an aptamer directed against complement factor C5 that is undergoing phase 1 testing, with intravitreal delivery in combination with ranibizumab in patients with wet AMD. Another agent, POT-4 (Potentia), a small molecule derivative of Compstatin, is directed against complement factor C3.POT-4 has completed phase 1 testing in patients with wet AMD with an excellent safety profile. A unique feature of POT-4 is that it persists as a long-lasting gel deposit after intravitreal injection. The study demonstrated that significant levels of drug are maintained in the vitreous cavity for up to 6 months following a single injection.
Elucidation of molecular pathways involved in angiogenesis has resulted in the development of a number of promising candidate drugs that may more effectively inhibit the VEGF pathway and/or disrupt VEGF-independent pathways involved in CNV. Continued efforts in drug development will hopefully make gains in the treatment of currently unaddressed aspects of neovascular AMD, including prevention of CNV formation, formation and persistence of fibrotic scars and atrophic degeneration of the RPE. RP
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