AMD Future Perspectives: New promising drugs
João Nascimento, MD
Gama Pinto Ophthalmology Institute - Lisbon, Portugal
Rufino Silva, MD, PhD
Coimbra University Hospital - Coimbra, Portugal
Susana Teixeira, MD
Prof. Dr. Fernando Fonseca Hospital, Amadora, Lisbon, Portugal
As researchers and clinicians are beginning to understand that wet age-related macular degeneration (AMD) is more than simply a vascular disease that includes angiogenic, vascular and inflammatory components, they are exploring new agents with different mechanisms of action addressing multiple targets in this complex pathophysiology.
Some of them are already available in human trials or even approved.
Others are still under laboratory investigation and few is known about them.
We will discuss, in this review, promising emerging therapies for neovascular AMD that aim to improve outcomes, safety and treatment burden through novel mechanisms of action.
For the new promising research components an Internet research was made and preliminary therapeutic strategies and results are presented.
There are two aspects that must be addressed: the first is related with the platform therapy, corresponds to how the product is developed and correlates directly with its internal structure; the second is related to the “targeting of action”, is correlated with the mechanism of action and its effects.
There are three potential therapeutic targets:
1- inhibition of angiogenic proteins production;
2- angiogenic protein neutralization;
3- angiogenic proteins receptors inhibition or endothelial cell apoptosis induction.
Almost every ophthalmic drug either currently available or under development for the treatment of AMD began life as a cancer therapy.
Vascular endothelial growth factor (VEGF) blockers
Lucentis® and Avastin® - Wet AMD Intravitreal
These two drugs emerged from the promising technological platforms of monoclonal antibodies.
Technology platforms of monoclonal antibodies are promising and allow the development of many new drugs, given the ease to develop drugs for mediators or receptors that were previously identified.
Current conditions for these drugs are exposed in the corresponding chapter.
Future prospects about their use in AMD are essentially made by process innovation, with adoption of optimized therapeutic schemes of combined therapy and improvement of intra-eye drug delivery.
Macugen®- Wet AMD Intravitreal
Aptamers are oligonucleotide ligands that are selected for their high-affinity to bind molecular targets.
Pegaptanib sodium (Macugen®; Eyetech Pharmaceuticals/Pfizer) is an RNA Aptamer directed against vascular endothelial growth factor (VEGF)-165, the VEGF isoform primarily responsible for pathological ocular neovascularization and vascular permeability.
“Pegaptanib therefore has the notable distinction of being the first aptamer therapeutic approved for use in humans, paving the way for future aptamer applications.”(1)
Like the use of Lucentis® and Avastin® future prospects with the use of Macugen® are essentially made by process innovation, with adoption of optimized therapeutic schemes of combined therapy.
VEGF Trap-Eye - Wet AMD Intravitreal
The aflibercept, VEGF-Trap, results of a process of bioengineering where extramembrane fragments of receptors 1 and 2 of VEGF are merged to IgG1 FC fragment.
This recombinant fusion protein is a composite decoy receptor based on VEGF receptors VEGFR1 and VEGFR2.
The VEGF Trap (Regeneron Pharmaceuticals, Tarrytown, NY, USA) is an 110kDa soluble recombinant protein with the binding portions of VEGF receptor 1 and 2 fused to the Fc region of human IgG that binds all VEGF isoforms with a very high affinity (about 140 times that one of ranibizumab).
Aflibercept is a fully human soluble fusion protein that binds all forms of VEGF-A along with the related Placental Growth Factor (PlGF)(2).
This high affinity fusion protein is used to block the biological activities of VEGF by preventing it to bind to its receptors.
VEGF-Trap effectively suppresses tumor growth and vascularization in vivo(3).
In phase I, randomized, placebo-controlled trial of VEGF Trap administered intravenously for treatment of choroidal neovascularization, the Clinical Evaluation of Antiangiogenesis in the Retina (CLEAR)-AMD 1 group found a dose-dependent increase in systemic blood pressure with a maximum tolerated dose of 1mg/kg.
This dose resulted in the elimination of about 60% of excess retinal thickness after either single or multiple administrations.
CLEAR IT-1 was a phase I dose escalation study of a single intravitreal injection of various doses of VEGF Trap (0.05, 0.15, 0.5, 1, 2, and 4mg)(4).
At 6 weeks, mean visual acuity gain was 4.8 letters and mean OCT central retinal thickness decreased from 298μm to 208μm across all groups.
Higher doses resulted in gaining more letters.
The potential benefit of VEGF Trap is its longer duration of action compared with single injections of other anti-VEGF agents because of its higher affinity and longer intravitreal half-life.
It seems that VEGF Trap would offer less frequent dosing resulting in fewer injections, lower cost and reduced risk of complications(5).
From August 2007 it’s initiated a phase III global development program for VEGF Trap-Eye in wet AMD.
During the first year of the two phase III trials, the companies (Regeneron Pharmaceuticals, Inc. and Bayer HealthCare AG ) are evaluating VEGF Trap-Eye dosed 0.5 mg every 4 weeks, 2 mg every 4 weeks, or 2 mg every 8 weeks (following three monthly doses) in direct comparison with ranibizumab (Lucentis® Genentech, Inc.) administered 0.5 mg every 4 weeks according to its U.S. label.
PRN dosing will be evaluated during the second year of each study.
Currently phase III clinical trials, VIEW 1 and 2 study will assess its efficacy and safety in patients with neovascular AMD.
The VIEW1 study (in the United States and Canada) and the VIEW2 study (in Europe, Asia Pacific, Japan, and Latin America)(6,7) enrolled 1200 patients.
Bevasiranib (Cand5) - Wet AMD Discontinued
Despite Bevasiranib has been discontinued it’s worth mentioning because it was the first therapy based on the Nobel Prize-winning RNA interference (RNAi) technology to advance to phase III clinical trials.
Bevasiranib was a first-in-class small interfering RNA (siRNA) drug designed to silence the genes that produce vascular endothelial growth factor (VEGF).
“The decision to conclude the clinical program follows a review of preliminary trial data by the Independent Data Monitoring Committee, which found that although Bevasiranib showed activity when used in conjunction with Lucentis® (ranibizumab, Genentech), the trial was unlikely to meet its primary endpoint.”(8)
The trial was COBALT for “Combination of Bevasiranib and Lucentis® Therapies” for AMD.
It was a phase III, randomized, double-blinded, parallel-assignment study of the RNAi drug administered either every 8 or 12 weeks as a maintenance therapy following three injections of Lucentis®(9).
PF-04523655 - Wet AMD Intravitreal
It is a siRNA, 19 nucleotides in length, that inhibits the expression of the hypoxia-inductible gene RTP801.
This stress-response gene mediates the mammalian target of rapamycin (mTOR) pathway.
PF-04523655 has been shown to reduce the volume of choroidal neovascularization in a mouse model.
PF-04523655 has been shown to cause regression of CNV in experimental studies of mice and primates.
Intravitreal small-interfering RNA (siRNA) PF-04523655 (Quark; licensed to Pfizer) used to treat choroidal neovascularization (CNV) secondary to age-related macular degeneration (AMD) seems to be safe and well tolerated in an interim phase I analysis.
“No adverse events were observed up to the 3,000-µg dose”(10).
AGN-745 (Sirna-027) - Wet AMD Development was halted
Allergan has halted development of its siRNA-based wet age-related macular degeneration, Sirna-027 is the first chemically modified short interfering RNA (siRNA) targeting Vascular Endothelial Growth Factor Receptor-1 (VEGFR-1).
VEGFR-1 is found primarily on vascular endothelial cells and is stimulated by both VEGF and placental growth factor (PlGF), resulting in the growth of new blood vessels(11).
By targeting VEGFR-1, Sirna-027 is designed to reduce pathologic angiogenesis mediated by both VEGF and PlGF.
Development was halted for AGN-745 after the drug failed to meet a key efficacy endpoint in a phase II study.
The trial compared the effect of three different monthly doses of AGN-745 with Genentech’s antibody drug Lucentis®, the standard of care for AMD, in treating the subfoveal choroidal neovascularization associated with the disease.
Both drugs were administered via intravitreal injection(12).
Apparently no safety issues were associated with AGN-745, a chemically modified siRNA.
But since the drug did “not meet its efficacy hurdle” — improvement in visual acuity — Allergan opted to halt its development(13).
Anti-platelet derived growth factor
E10030 (Aptamer) - Wet AMD Intravitreal
One of them is E10030 (Ophthotech), an anti-platelet-derived growth factor (anti-PDGF-B) aptamer. E10030 strongly binds to PDGF-B.
PDGF-B plays a key role in recruiting the pericytes that envelop the new vessels and make them more resistant to the anti-VEGF attack.
In combination with anti-VEGF, this new agent could represent a breakthrough therapy.
A phase I study(14) with an intravitreal anti-platelet-derived growth factor (PDGF) aptamer that targets pericytes, was evaluated in combination therapy with ranibizumab (Lucentis®, Genentech) in patients with neovascular age-related macular degeneration (AMD) with promising results regarding safety and efficacy(15).
Anti tyrosine kinase
Approximately 2000 kinases are known, and more than 90 Protein Tyrosine Kinases (PTKs) have been found in the human genome.
They are divided into two classes, receptor and non-receptor PTKs.
Anti receptor kinase (suffix ~nib)
“At present, 58 receptor tyrosine kinases (RTKs) are known, grouped into 20 subfamilies.
They play pivotal roles in diverse cellular activities including growth, differentiation, metabolism, adhesion, motility and death.
RTKs are composed of an extracellular domain, which is able to bind a specific ligand, a transmembrane domain, and an intracellular catalytic domain, which is able to bind and phosphorylate selected substrates.
Binding of a ligand to the extracellular region causes a series of structural rearrangements in the RTK that lead to its enzymatic activation.
In particular, movement of some parts of the kinase domain gives free access to adenosine triphosphate (ATP) and the substrate to the active site.
This triggers a cascade of events through phosphorylation of intracellular proteins that ultimately transmit (“transduce”) the extracellular signal to the nucleus, causing changes in gene expression.
Many RTKs are involved in oncogenesis, either by gene mutation, or chromosome translocation, or simply by over-expression.
In every case, the result is a hyper-active kinase, that confers an aberrant, ligand-independent, non-regulated growth stimulus to the cancer cells.”(16)
From these 20 subfamilies of Receptor Tyrosine Kinases (RTK), seven families are promising field of investigation and only two families of RTK represent now the most promised field of drug development in AMD; fibroblast growth factor receptor (FGFR) family and vascular endothelial growth factor receptor (VEGFR) family.
RTK Class I Epidermal growth factor receptor family, RTK Class II Insulin receptor family RTK Class III Platelet-derived growth factor receptor RTK Class IV Fibroblast growth factor receptor (FGFR) family, RTK Class V Vascular endothelial growth factor receptor (VEGFR) family, RTK Class XII RET receptor family (RET proto-oncogene) RTK Class VIII Eph receptor family.
Fibroblast growth factor receptor (FGFR) family – Class IV
Fibroblast growth factors comprise the largest family of growth factor ligands(17).
The natural alternate splicing of four fibroblast growth factor receptor (FGFR) genes results in the production of over 48 different isoforms of FGFR.
These isoforms vary in their ligand binding properties and kinase domains, however all share a common extracellular region composed of three immunoglobulin (Ig) like domains (D1-D3), and thus belong to the immunoglobulin superfamily(18).
Vascular endothelial growth factor receptor (VEGFR) family – Class V
Vascular endothelial growth factor (VEGF) is one of the main inducers of endothelial cell proliferation and permeability of blood vessels.
Two RTKs bind to VEGF at the cell surface, VEGFR-1 (Flt-1) and VEGFR-2 (KDR/Flk-1)(19).
The VEGF receptors have an extracellular portion consisting of seven Ig-like domains so, like FGFRs, belong to the immunoglobulin superfamily.
VEGFR-2 is the major mediator of endothelial cell proliferation, migration, survival, and permeability.
The function of VEGFR-1 is less well defined, although it is thought to modulate VEGFR-2 signaling.
The therapeutic strategy is the blockade of VEGF effects by inhibition of the tyrosine kinase cascade downstream from the VEGF receptor.
The concept of disrupted signaling appears to be effective in the pharmacological treatment of neovascularization(20).
Intracellular inhibition of the tyrosine kinase cascade
This promising therapeutic strategy is the blockade of VEGF effects by inhibition of the tyrosine kinase cascade downstream from the VEGF receptor; such therapies currently in development include, Vatalanib, TG100801, Pazopanib, AG013958 and AL39324.
Vatalanib - Wet AMD Oral
Oral administration of PTK787 (Vatalanib), a tyrosine kinase inhibitor that blocks phosphorylation of VEGF and PDGF receptors, provides inhibition of retinal neovascularization.
The development of new vessels are prevented while there is no effect on mature retinal vessels in murine(21).
Vatalanib (PTK787 or PTK/ZK) is a small molecule protein kinase inhibitor that orally administrated inhibits angiogenesis.
It is being studied as a possible treatment for several types of cancer.
Vatalanib is being developed by Bayer Schering and Novartis.
It inhibits all known VEGF receptors (VEGFR1, VEGFR2, and VEGFR3) as well as platelet-derived growth factor receptor-beta and c-kit, but is most selective for VEGFR-2.
The “Safety and Efficacy of Oral PTK-787 in Patients With Subfoveal Choroidal Neovascularization Secondary to Age-related Macular Degeneration” (ADVANCE) study evaluate the tolerability and safety of 3 months treatment with PTK-787 tablets given daily(22).
TG101095 - Wet AMD Topical
It is a topical tyrosine kinase inhibitor that specifically targets VEGFR, only tested in animal models.
JAK2 is a signalling kinase that acts downstream from erythropoietin, a glycoprotein hormone, which, along with VEGF, is involved in the pathogenesis of diabetic retinopathy.
The VEGF receptor and JAK2 inhibitor TG101095 dosed topically bid for two weeks significantly reduced CNV area in a laser-induced CNV mouse model(23).
Multi-targeted kinase inhibitors
Multi-targeted kinase inhibitors have been shown to be effective in oncology.
Newly developed small molecule kinase inhibitors (including TG100572 and the prodrug TG100801), which inhibits VEGF, PDGF, and FGF receptors in addition to Src family of kinases (sarcoma proto-oncogenic tyrosine kinases family).
They act in intracellular environment(24).
TG100801 - Wet AMD Topical
TG100801 a prodrug version of TG100572, is administered noninvasively as an eye drop and is designed to suppress VEGF mediated leakage and additionally the kinase targets associated with inflammation, edema, and angiogenesis which are the pathological hallmarks of AMD and of other back of the eye diseases including diabetic macular oedema and proliferative diabetic retinopathy(25).
It is synthesized at TargeGen (TargeGen inc San Diego).
Topical administration of TG100801 suppressed CNV in mice and reduced the retinal oedema induced by retinal vein occlusion in rats, without observable safety issues.
Data have suggested that the delivery of these agents occur by local penetration through sclera rather than by systemic absorption as neither compound was detectable in the plasma(24).
Therefore, TG100801 may offer equal efficacy to injectable agents, while offering the convenience and potential safety advantages due to a non-invasive route of delivery and eye penetration.
Currently a multicentric, open-label, randomized, phase II study is evaluating the effects of 30 days of dosing with two dose levels of TG100801, instilled twice a day, on central retinal/lesion thickness, as measured by optical coherence tomography (OCT).
The safety of TG100801 in patients with AMD will also be evaluated in this trial(26).
Pazopanib (GW786034) - Wet AMD Topical
Pazopanib (GW786034), by GlaxoSmithKline, is a second-generation multi-targeted tyrosine kinase inhibitor against all VEGF receptors (VEGFR-1, VEGFR-2, VEGFR-3) PDGFR-a, PDGFRβ, and c-kit that blocks tumour growth & inhibits angiogenesis.
An early phase trial is evaluating the safety, and pharmacokinetics of Pazopanib eye drops in patients with neovascular AMD(27).
AG013958: Wet AMD Subtenon
AGO013958 (Pfizer Inc.) is a subtenon injectable Tyrosine kinase inhibitor.
A phase I/II, randomised, masked, single and multiple dose, sequential dose-escalation study of the safety and efficacy of AG-013958 in subjects with subfoveal choroidal neovascularization associated with age-related macular degeneration has been completed(28).
Sirolimus - Wet AMD Subconjunctival and intravitreal
Sirolimus (MacuSight inc.), also known as rapamycin, is an immunosuppressant drug used to prevent rejection in organ transplantation.
It was originally developed as an antifungal agent and has potent immunosuppressive and antiproliferative properties.
Sirolimus inhibits the mammalian target of rapamycin (mTOR).
The mammalian target of rapamycin is a protein kinase that regulates cell growth and metabolism in response to changes in the environment.
Sirolimus administered via subconjunctival injections was as effective as sirolimus administered via intravitreal injection.
A phase II, randomized, multicentric study in wet AMD is now taking place; (EMERALD) is currently recruiting patients with wet AMD for a phase II study of an ocular sirolimus formulation in combination with Lucentis®.
This is a randomized, multicentric study and is taking place throughout the United States(29).
Anti integrins therapy
JSM6427 inihibitor of integrin α5β1 - Wet AMD Intravitreal
Intravitreal JSM6427 (Jerini Inc) is a potent and selective inhibitor of integrin α5β1.
Animal studies have shown an inhibition of choroidal neovascularization (CNV).
This suggests that JSM6427 may provide a new approach for the treatment of age-related macular degeneration in humans.
Integrins are transmembrane receptors composed of α and β subunits that mediate binding to extracellular matrix or other cellular receptors.
Blocking angiogenesis through inhibition of integrin-mediated signaling has the potential to inhibit the cellular responses to growth factors as well as to cytokines and other inflammatory mediators(30).
Volociximab: α5β1 Antagonist - Wet AMD Intravitreal
Volociximab (Ophthotech) is a high-affinity chimeric monoclonal antibody (Mab) that inhibits the functional activity of alpha-5 beta-1 integrin found on the endothelial cells involved in the formation of blood vessels.
“Volociximab binds to α5β1 integrin and blocks the binding of α5β1 integrin to fibronectin, thereby inhibiting a pivotal interaction required for angiogenesis.
Volociximab administration has resulted in strong inhibition of rabbit and primate retinal neovascularization.
In monkeys with laser-induced choroidal neovascularization (CNV), volociximab significantly inhibited CNV proliferation and reduced the degree of lesion formation.
In a rabbit model, volociximab administered either intravenously or intravitreally prior to the onset of neovascularization significantly reduced angiogenesis as compared to control.
Similar anti-angiogenic efficacy with volociximab has also been shown in multiple preclinical models of tumor angiogenesis.”(31)
A phase I open-label, multicenter study of volociximab is currently on going.
The objectives of this study are to evaluate the safety, tolerability, and pharmacokinetic profile of volociximab intravitreous injection in subjects with subfoveal choroidal neovascularization secondary to age-related macular degeneration (AMD)(32).
Vascular disrupting agents
Zybrestat (OXIGENE Inc.) is a synthetic prodrug, Combretastatin A4 Phosphate that is converted to Combretastatin inside endothelial cell. It was originally derived from the root bark of the South African Bushwillow tree (Combretum caffrum).
The mechanism of action is a vascular disrupting agent (VDA) by a dual action: tubulin depolymerizing agent and cell junction disruption.
These actions upset the physical structure of the existing blood vessels.
VE-cadherin disrupts the VE-cadherin/b-catenin complex interfering with cell-cell contact and induces loss of cell-cell contact that increases vascular permeability, leading to increased interstitial pressure and additional loss of blood flow.
Tubulin depolymerization acts at the colchicines-binding site of the b-subunit of endothelial tubulin, inducing disruption of the endothelial cytoskeleton that results in shape changes.
Normally flat, the endothelial cells become more spherical, and this decreases the size of the blood vessel lumen, causing decreased blood flow and thrombosis.
It seems that the cytoskeleton of newly formed cells is sensitive to CA-4-P, whereas the cytoskeleton of mature cells is not.
This appears to underlie the selective shutdown of neovascular vessels compared to that of normal vessels.
Currently Zybrestat has been tested intravenously-administered in clinical studies in patients with forms of macular degeneration.
The topical administration is being tested in animal studies.
A phase II study in patients with polypoidal choroidal vasculopathy (PCV) is initiated(33).
Current therapies active against wet AMD appear to have limited benefits in patients with PCV, and OXiGENE (Oxigene inc. San Francisco) believes the abnormal vasculature in the retina and choroid that contributes to PCV patients loss of vision may be susceptible to treatment with Zybrestat.
Nicotine has a potent angiogenic effect.
It has two distinct but interdependent pathways for angiogenesis; nAChRs are involved in the native angiogenic response, and this pathway is distinct from those triggered by VEGF or FGF.
Nicotine induces morphological changes in endothelial cells identical to those induced by VEGF with increase in endothelial cell proliferation and reduction of apoptosis what leads to increase in capillary network formation(34).
Antagonists of nAChR abolish the proangiogenic effect of nicotine nAChR and VEGF: Two distinct but interdependent angiogenesis pathways(35).
Neutralization of VEGF resulted in a significant but not complete inhibition of nAChR-mediated network formation.
Nicotinic acetylcholine receptor antagonists
Non-selective cholinergic agonists such as nicotine have been shown to induce angiogenesis, enhancing tumor progression.
Moreover, α7 AChR (nicotinic acetylcholine receptor) selective antagonists such as α-bungarotoxin and methyllycaconitine as well as the non-specific antagonist mecamylamine have been shown to inhibit endothelial cell proliferation and ultimately blood vessel formation.
Such pharmacologic properties can lead to the discovery of new specific cholinergic antagonists as anti-AMD therapies.
Conversely, the pro-angiogenic effect of specific agonists can be used to treat diseases that respond to revascularization such as diabetic ischemia and atherosclerosis, as well as to accelerate wound healing(36).
ATG003: nAChR antagonist - Wet AMD Topical
ATG003 (Mecamylamine) topical delivery in animal models significantly inhibits laser induced CNV in a mouse model of AMD.
Currently a phase IIa study and phase IIb study is terminated, 330 subjects are enrolled with dose ranging: 0.3% and 1% topical solutions and the data is not published(37).
Phase IIb study, with 60 subjects, was design to study the safety of 1% topical mecamylamine bid for 48 weeks in patients receiving maintenance injections of Lucentis® or Avastin®; this study is a double-masked, randomized, placebo-controlled study.
The study is ongoing, but not recruiting participants(38). In this phase the patients receive either a dose of ATG003 or a placebo, while continuing the Lucentis® or Avastin® treatment.
All patients will be treated for up to 48 weeks, during which time they will be monitored to assess the drug’s safety, tolerability and efficacy.
Pigment epithelium-derived factor (PEDF) is one of the most potent antiangiogenic proteins.
It inhibits VEGF-induced proliferation, migration of endothelial cells, reduces VEGF-induced hypermeability and causes vessel regression in established neovascularization(39,40).
PEDF is also a neurotrophic factor(41) and also inhibits/antagonizes other angiogenic factors such as platelet-derived growth factor and fibroblastic growth factor. GenVec (GenVec inc, Gaithersburg, Maryland) delivers AdPEDF utilizing a modified adenoviral vector delivery system.
In phase I studies testing the safety and feasibility of intravitreal injection AdPEDF was safe and generally well-tolerated at all dose levels tested in 28 patients(42,43).
Other phase I studies(44,45) in 22 patients with less severe wet AMD find similar results; there were no dose-limiting toxicities or drug-related severe adverse events.
Further studies for AdPEDF in patients with wet AMD are under way.
Lentivirus: Wet and Dry AMD
The LentiVector (Oxford BioMedica plc, Oxford, England) has been shown to efficiently and effectively deliver genes to the specialized, nondividing cells of the retina.
The system is based on the lentivirus equine infectious anemia virus (EIAV).
Applications for the gene vector system include gene therapy, trangenesis, stem cell manipulation, somatic disease models, target validation and gene discovery.
For AMD, RetinoStat aims to preserve and improve the vision of patients with wet AMD through antiangiogenesis by delivering endostatin and angiostatin genes directly to the retina(46,47).
Complement activation has been implicated in a number of acute and chronic conditions.
There is strong evidence that AMD is an inflammatory disease; Aberrant activation of the complement system is implicated in the wet and dry forms of AMD(48,49).
Patients with AMD demonstrate elevated systemic inflammatory biomarkers of inflammation (CRP, IL-6 and homocysteine).
Histopathologic analyses of human AMD neovascular complex specimens demonstrate inflammatory infiltrates.
Recent studies implicated local inflammation and activation of the complement cascade in the formation of drusen(50).
All this, strong support for the complement-mediated disease in wet and dry forms of AMD.
POT- 4 inhibits C3 - Dry AMD Intravitreal
POT-4 developed by Potentia Pharmaceutical (Potencia Pharmaceutical, inc., Louiseville) affects the body’s “complement” system. Inflammation plays a role in developing macular degeneration.
Eight complement proteins are associated with AMD and POT-4 affects C3.
The problem is that inflammation is useful in fighting infections.
If the complement system is completely shut down, there is an increase risk for the development of bacterial infection.
Potentia has completed a phase I trial for POT-4 (called ASap) in patients with wet AMD.
The trial was designed to determine the safety and tolerability of an intravitreal injection of POT-4, as well as its stability and depot-forming properties.
In the study investigators observed only minimal and mild local adverse events related to the injection with no serious adverse events related to the drug itself(53,54).
JPE1375 inhibits C5 - Dry AMD Intravitreal
JPE-1375 (Jerini AG, Berlin Germany) is a small molecule peptidomimetic antagonist targeting the complement pathway, a highly validated target for dry AMD.
It targets C5aR, the receptor for complement factor C5a, which is a key component involved in the activation of inflammatory cells(55).
JPE-1375 has shown significant efficacy in multiple preclinical models.
ARC1905 inhibits C5 - Dry/Wet AMD Intravitreal
ARC 1905 (Ophthotech Corp. Princeton, NJ) is a PEGylated, stabilized aptamer targeting complement factor C5.
ARC1905 inhibits activation of the downstream proinflammatory complement cascade (including generation of C5a and the membrane attack complex).
“Ophthotech’s anti-C5 aptamer, ARC1905, is a potent and selective inhibitor of factor C5 of the complement cascade.
Inhibition of the complement cascade at the level of C5 prevents the formation of the key terminal fragments responsible for tissue pathology, C5a and the membrane attack complex (MAC: C5b-9).
The C5a fragment is pro-inflammatory, while the membrane attack complex initiates cell lysis and releases proangiogenic molecules (eg. PDGF and VEGF).
Histopathologic specimens of human dry AMD lesions strongly stain for C5 and MAC at the key sites of pathology.
ARC1905 spares the formation of upstream complement components such as C3b, which are important in host defense mechanisms.
By inhibiting C5-mediated inflammatory and MAC activities, therapeutic benefit may be achieved in both dry and wet AMD while sparing the immunoprotective functions of the complement system.
A phase I open-label, multicenter study of ARC1905, in combination with an anti-VEGF agent (Lucentis®), in patients with wet AMD is ongoing.
In addition, a study investigating ARC1905 in patients with dry AMD will be initiated in Q2 2009.”(56)
Eculizumab: Inhibits C5 - Dry AMD Intravitreal
Eculizumab (Soliris, Alexion Pharmaceuticals) is a humanized monoclonal antibody derived from a murine antihuman C5 antibody.
Eculizumab specifically inhibits the terminal complement protein C5, thereby preventing its cleavage to C5a and C5b during complement activation.
The strategic blockade of the complement cascade at C5 prevents the release of the downstream anaphylatoxin C5a and prevents the formation of the cytolytic membrane attack complex (MAC).
Eculizumab is FDA-approved for the intravenous treatment of another complement-mediated disease known as paroxysmal nocturnal hemoglobinuria.
Currently a phase II study with eculizumab for the treatment of patients with dry AMD, known as the Complement Inhibition with Eculizumab for the Treatment of Non-Exudative Age-related Macular Degeneration (COMPLETE) Study.(57)
Patients with GA or high-risk drusen are being randomized 2:1 to receive intravenous infusions of eculizumab or placebo.
Strontium-90 beta radiation -Wet AMD
CABERNET is a multicenter, randomized, controlled phase III study that enrolls 450 subjects at 45 clinical centers worldwide.
In this research, patients receive either the standard injection of Lucentis® (ranibizumab) or the radiation plus Lucentis®(58).
A tiny source of radiation is placed inside the eye near the macula, held there for about 4 minutes and then removed.
The radiation destroys the abnormal blood vessels and prevents the growth of blood vessels to stop the progression of wet macular degeneration vision loss.
The system treats neovascularization of retinal tissue by means of a focal, directional delivery of radiation to the target tissues in the retina.
Using standard vitreoretinal surgical techniques, the sealed radiation source is placed temporarily over the retinal lesion by means of a handheld medical device.
If epiretinal brachytherapy proves successful, it can reduce the number of injections needed to just two injections over a period of 12 months.
TheraSightTM ocular brachytherapy System: Wet AMD
A trial sponsored by Theragenics Corporation® will investigate the safety and ability of using the TheraSightTM Ocular Brachytherapy System to treat wet AMD.
A radioactive button mounted on an applicator wand is positioned behind the eye and held in place touching the outer surface of the eye for 5 to 20 minutes.
A study will take place at 6 clinical sites and will compare 3 different dosages or amounts of radiation, so all participants will receive treatment(59).
Enrollment is still underway for this clinical trial.
Glatiramer Acetate: T helper 2 inducer -Dry AMD Subcutaneous
Another proposed new treatment of dry AMD is a subcutaneous injection of glatiramer acetate (Copaxone, Teva Pharmaceutical Industries).
Glatiramer acetate has been shown to reduce cognitive decline, eliminate plaque formation, and induce neuron survival and neurogenesis in a mouse model for Alzheimer’s disease (AD).
Drusen formation in age-related macular degeneration (AMD) shares some similarities with Alzheimer’s disease (AD), which is associated with amyloid deposits.
Aggregated beta-amyloid induces microglia to become cytotoxic and block neurogenesis.
This medication, increases the proportion of T helper 2 lymphocytes (these T cells are anti-inflammatory in nature).
It seems that these glatiramer–acetate-specific T helper 2 cells would produce cytokines such as interleukin (IL)-4 and reduce amyloid-induced retinal microglial cytotoxicity in AMD(60).
Copaxone® is administered as a subcutaneous injection.
Two double blind, randomized clinical trials at the New York Eye & Ear Infirmary and the Kaplan Medical Center, Rehovot, Israel, have been initiated in 2006 and 2007 respectively, and are enrolling up to 60 patients combined.
The primary outcome tested in these trials is the reduction in the total area of drusen(61,62). Results have not been published yet.
Prevent injury (anti-oxidants)
OT-551 - Dry AMD Topical
OT-551 is administered topically, developed by Othera Pharmaceuticals, Inc. (Exton, PA)(63).
This eyedrop contains a small molecule that downregulates the overexpression of the protein complex nuclear factor (NF)- B.
NF- B is a transcription factor that is highly activated when oxidative stress, inflammation, and angiogenesis occurs.
In preclinical models, OT-551 has demonstrated anti-oxidative, anti-inflammatory and anti-angiogenic activity.
These results support the development in diseases such as AMD and cataract.
OT-551 is the first eye drop to be tested in a clinical trial as a treatment for dry AMD.
There are 2 phase II, 2-year trials ongoing for patients with GA secondary to AMD(64,65).
Fenretinide (Compound ST-602) - Dry AMD Oral
Fenretinide, or (N-[4-hydroxyphenylretinamide), is an oral compound that decreases serum retinol by binding to retinol-binding protein, and promotes renal clearance of retinol.
This in turn decreases the bioavailability of retinol for the retinal pigment epithelium (RPE) and photoreceptors.
A2E (N-retinylidene-N-retinylethanolamine), a retinoid byproduct, is a major fluorophore in lipofuscin and a significant source of RPE cytotoxicity(66).
It is hypothesized that by reducing toxic retinoid byproducts of visual cycling, there will be a slowing of GA progression.
Sirion Therapeutics, Inc. (Tampa, FL), is sponsoring a phase II trial to assess the benefit of fenretinide in the treatment of GA(67).
The study group is ongoing.
Patients have been randomized to 1 of 2 doses (100 mg or 300 mg) or placebo, and they are being followed for 2 years.
Another potential treatment for AMD is replacing damaged and unhealthy RPE by healthy tissue.
In a study series of ten patients (four had AMD), human neural retinal progenitor cell layers and RPE were transplanted.
All four patients with AMD had vision of 20/200 or worse and experienced improved visual acuity, none improved to better than 20/200.
There was no graft rejection during a follow-up time of up to six years(68).
Soluble (s)Flt-1 is a naturally occurring protein antagonist of VEGF formed by alternative splicing of the pre-mRNA for the full length VEGFR-1(69).
The angiostatic activity of sFlt-1 results from inhibition of VEGF.
It is not clear if sFlt-1 has a role in normal eyes, but several studies show the evidence of the effect of overexpression of sFlt-1 in ocular neovascularization models(70-73).
Recombinant sFlt-1 chimeric proteins - Wet AMD Intravitreal
Inhibition of VEGF by intravitreal injections of recombinant sFlt-1 chimeric proteins and antisense oligodeoxynucleotides have been shown to prevent retinal neovascularization in mouse models(74).
AdsFlt-1 - Wet AMD Intravitreal
Intraocular injection of AdsFlt-1 suppressed retinal and choroidal neovascularization(72,73,75).
Periocular injection of AdsFlt-1 resulted in transduction of episcleral cells, penetration of the sclera and high levels of AdsFlt-1 in the choroid, which markedly suppressed CNV(70).
Long-term suppression of CNV was achieved with intraocular injection of AAVsFlt-1 in mice and monkeys(75).
Ciliary neurotrophic factor - Dry AMD Implant
Ciliary neurotrophic factor (CNTF) is being investigated as a treatment for dry AMD because it has a potent neuroprotective action; it has been shown to inhibit photoreceptor apoptosis in an animal model of retinal degeneration.
CNTF has been shown to slow photoreceptor degeneration in animal models of retinal degenerations and thus may be effective in protecting photoreceptors in AMD(76).
A phase II trial utilizes an encapsulated cell technology (ECT) to deliver CNTF to the retina.
The implant is a small capsule that contains human retinal pigment epithelium cells.
These cells have been given the ability to make CNTF and release it through the capsule membrane into the surrounding fluid.
In this study, two different CNTF dose levels will be used: a high dose and a low dose, as well as a sham surgery (or placebo) group(77).
The cells can survive for approximately 18 months following implantation into the vitreous cavity with a single scleral suture(78).
The trial, sponsored by Neurotech Pharmaceuticals USA (Lincoln, RI), is ongoing and not recruiting.
Rheopheresis - Dry AMD
Rheopheresis is still an unproven therapy for dry macular degeneration.
Rheopheresis is a form of therapeutic plasmapheresis designed to remove species circulating in the blood that are larger than 25 nm (about 500 kilodaltons) using a doublestaged membrane filtration system.
The intended targets include immune complexes, immunoglobulin M, beta 2-macroglobulin, fibrinogen, von Willebrand factor, low density lipoprotein cholesterol, and others(79).
“This procedure has been proposed as a possible treatment to prevent the progression of dry AMD by improving the retinal and choroidal microcirculation.
The largest study performed to assess the effectiveness of rheopheresis in dry AMD is the Multicenter Investigation of Rheopheresis for Age-related macular degeneration (MIRA-1) trial.
Study patients had at least 10 soft drusen within 2 disc diameters from the foveal center and/or GA. Interpretation of the results from the MIRA-1 trial has been controversial.
The sole outcome measure was LogMAR VA. At 1 year, the treated group had a LogMAR VA of 0.02 ±0.213, and the placebo patients had a VA of 0.02 ±0.20 (P=.977).
This may have implied that the treatment was not effective in improving VA.
However, a post hoc analysis showed that a large proportion of the subjects (37% of treated and 29% of placebo) were mistakenly included in the trial and that a number of the subjects did not receive the required number of rheopheresis treatments.
When reanalyzed, the treatment arm of this “modified per protocol” group of subjects did have a statistically significant improvement in visual acuity (treated improved 0.08 ±0.166, placebo decreased 0.01 ±0.164, P=.001).
Furthermore, a larger proportion of treated subjects experienced an adverse event requiring intervention (24.0%) compared to those receiving placebo (5.8%)(80).
The Occulogix (Waltham, MA) phase II study was suspended.“(82,83)
Improvements in ophthalmic drug delivery
“Ophthalmic drugs have traditionally been administered topically, which in general provides therapeutic levels to the anterior chamber of the eye but not to the posterior segment.
Therefore, topical administration of drugs has been largely infeasible for posterior segment diseases such as AMD and diabetic macular oedema.
In contrast, intravitreous injection provides direct delivery to the posterior segment and allows therapeutic levels to be attained.
However, this route of administration can require repeated injections for chronic disorders and is associated with a small risk of complications.
Several alternative strategies for drug delivery have therefore been developed, such as implantable devices that deliver small, highly potent, lipophilic therapeutics intraocularly.
The small size of these implants precludes long-term (>30 days) delivery of large, water-soluble compounds, but they have been used to deliver corticosteroids.
In addition, these implantable devices generally cannot be used to deliver proteins, antibodies and other high-molecular-mass biotherapeutics.
Administration of compounds to the eye by approaches that do not involve injection through the sclera also remain attractive alternatives to intravitreous injection.
High-molecular-mass compounds such as immunoglobulins, and oligonucleotides as large as 24 nucleotides have been found to be capable of diffusing through the sclera when deposited on or within the sclera.”(84)
Nanotecnology and Iontophoresis represent promising drug delivery systems.
“Iontophoresis is a technique that consists of the administration of drugs to the body through tissues using an electric field involving a small potential difference.
The active electrode, which is in contact with the drug, is placed at the site to be treated, and a second electrode, with the purpose to close the electric circuit, is placed at another site of the body.
The electric field facilitates the transport of the drug that should be mainly ionized.”(85)
Wet AMD pipeline: most promising candidates
adPEDF.11: Gene therapy, intravitreal injection, phase I
AGN211745: siRNA, intravitreal injection, phase II
Zybrestat: vascular disrupting agent, topical, animal studies
Sirolimus: Multi-mechanism, Subconjunctival or intravitreal injection or oral, phase II
ATG003: nAChR antagonist, topical , phase II
Avastin® and Lucentis®: anti-VEGF, intravitreal injection, available
Macugen®: anti-VEGF, intravitreal injection, available
VEGF Trap: VEGF receptor decoy, intravitreal injection, phase III
Vatalanib: Tyrosine kinase inhibitor, oral, phase II
Pazopanib: Tyrosine kinase inhibitor, , topical , phase II
TG101095 / TG100801: Tyrosine kinase inhibitor, topical, phase II
AL-39324: Tyrosine kinase inhibitor, intravitreal injection, animal studies
AG013958: Tyrosine kinase inhibitor, subtenon injection, animal studies
JSM6427: integrin antagonist, intravitreal injection, phase I
PF-04523655 (REDD14NP) Wet AMD; phase I (Quark/Pfizer)
Dry AMD pipeline: most promising candidates
Ciliary Neurotrophic Factor: Neuroprotective implant, phase II
Fenretinide: Decreases serum retinol, oral, phase II
OT-551: Anti-oxidant, anti inflammatory, topical, phase II
POT-4: Complement C3 inhibitor, phase II
Glatiramer Acetate: Immunomodulator, subcutaneous, phase II
RPE transplantation: animal studies and phase I
Rheopheresis: Plasmapheresis, phase III
Gene transfer: Intravitreous and subtenon, phase I