Send to printer »

Feature Articles : Apr 1, 2007 (Vol. 27, No. 7)

Combating Ocular Disease

Cellular Proliferation Strategies Are Rewriting the Current Paradigm for Macular Degeneration
  • Elizabeth Lipp

Macular degeneration, be it wet or dry, gets a lot of airtime in a variety of healthcare forums. And no wonder—at this time there is no cure, although there are treatments to stem the damage. As our population ages and gains weight, the incidence of macular degeneration and diabetes-related retinopathy will continue to rise.

Fortunately, drug development in the field of ocular diseases has experienced something tantamount to a revolution, particularly with regard to angiogenesis modalities and antiangiogenic treatments for back-of-the-eye neovascular pathologies. At IBC Life Sciences’ “Ocular Antiangiogenesis Conference,” to be held later this month in Cambridge, MA, some of these options will be explored in detail.

Age-related macular degeneration is “the leading cause of blindness in the western world,” notes Lucian V. Del Priore, M.D., Ph.D., professor in the department of ophthalmology, college of physicians and surgeons, Columbia University.

Working Backward as Well as Forward

Patients with AMD lose vision by way of two distinct mechanisms. Nonexudative, or dry, AMD is more common (approximately 80–90% of all AMD cases) and involves atrophy of the outer retina and the accumulation drusen, which are localized deposits of extracellular material concentrated in the macula. Exudative, or wet, AMD is characterized by neovascular growth from the outer blood supply into Bruch’s membrane, the subretinal space, and sometimes the retina itself.

“If you look back over the last two years, managing wet AMD has improved with the introduction of anti-VEGF drugs,” Dr. Del Priore says. “But many patients continue to lose sight with wet AMD and there are still no effective treatments for advanced wet AMD or any stages of dry AMD.”

Furthermore, attempts at macular reconstruction using cellular transplantation have not been effective in reversing vision loss. Dr. Del Priore’s group is investigating the role that aging of human Bruch’s membrane plays in the development of AMD. “What we’ve discovered in the last 10 years is that aging of Bruch’s membrane plays an important role in the development of disease in AMD. While patients show the symptoms of AMD in their 50s, the earliest signs of changes in Bruch’s membrane actually happen about 10 to 20 years earlier,” Dr. Del Priore says. “The membrane is made predominantly of collagen, and is 1–2 microns thick in youth, but it thickens to 4–8 microns as the patient starts to age.”

The consequence of the aging changes in Bruch’s membrane is that they interfere with the survival rate of cellular transplants. “So we are working at both ends of the spectrum,” says Dr. Del Priore. “We are looking to see how Bruch’s membrane disease may initiate AMD, and how we can prevent AMD before it starts, and trying to find a way to restore the vision after it has been lost. Thus the Bruch’s membrane study has a dual purpose—understanding how age-related disease within Bruch’s membrane will affect the behavior of cells already there and how it affects the transplant at the advanced state.”

Anti-VEGF

Many studies have shown that vascular endothelial growth factor (VEGF) is a key mediator in the formation of new blood vessels and in choroidal neovascularization (CNV) observed in wet age-related macular degeneration (AMD). Human VEGF delivered to the murine retina via adenovirus rapidly generates CNV, allowing use of this model for the screening of anti-VEGF therapies that may be useful in the treatment of AMD.

Rajendra Kumar-Singh, Ph.D., associate professor of ophthalmology at the Tufts University School of Medicine, and his team, including Siobhan Cashman, assistant research professor of ophthalmology, have developed adenovirus vectors expressing short hairpin RNAs (shRNAs) targeting human VEGF that can prevent formation of CNV in a mouse model of AMD.

“Our lab is looking at RNA interference-based therapies for CNV and AMD,” says Dr. Kumar-Singh. “We developed several shRNAs expressed from recombinant adenovirus. These shRNAs were expressed in human retinal pigment epithelium cells in the presence of adenovirus vectors over-expressing VEGF and the amount of VEGF attenuation evaluated. Adenovirus vectors expressing VEGF were subsequently injected into the subretinal space of mice and induction of CNV measured in the presence of adenovirus vectors expressing shRNA targeting VEGF.”

Dr. Kumar-Singh identified potent shRNA sequences able to silence VEGF in human RPE cells. When expressed from adenovirus backbones, these shRNA constructs silenced VEGF by 94%. Adenovirus vectors expressing high levels of VEGF could induce CNV in mice within five days. Co-injection of VEGF-expressing viruses into mice with shRNA targeting VEGF led to a substantial reduction in CNV. The results show that shRNA targeting VEGF from adenovirus vectors allows potent attenuation of VEGF and prevents CNV.

“This shows a lot of promise,” says Dr. Kumar-Singh. “Currently, the only treatment for wet AMD is intraocular injection of anti-VEGF molecules every four to six weeks. The current goal is to try to reduce theses injections to once a year, but the long-term goal is to completely prevent AMD.”

Delivery of Anti-VEGF and More

Another company looking closely at the problem of administration is GenVec (www.genvec.com). GenVec is using its adenovector technology to deliver the PEDF gene to ocular cells, where it triggers production of the therapeutic protein PEDF (pigment epithelium-derived factor). This biologically active protein is a normal regulator of vascular development. A deficiency of PEDF is present in many patients with wet AMD or diabetic retinopathy.

GenVec delivers AdPEDF to the site of disease utilizing a modified adenoviral vector delivery system. AdPEDF provides the ocular cells with the machinery necessary to produce PEDF.

“In addition to being an anti-VEGF approach, PEDF also inhibits/antagonizes other angiogenic factors such as platelet-derived growth factor and fibroblastic growth factor,” notes Lisa Wei, director of preclinical sciences. PEDF also is a neurotrophic factor, and therefore it can block vision loss by stopping blood vessel growth characteristic of wet AMD while simultaneously protecting the eye’s vision-sensing cells, the neurons.

GenVec has taken AdPEDF through Phase I testing. Wei will report on the safety and feasibility of intravitreal injection of AdPEDF in patients with wet age-related macular degeneration in both those with best corrected vision of 20/200 or worse and in patients with best corrected vision of 20/40 to 20/320 in the study eye. “We found that AdPEDF is safe and generally well-tolerated at all dose levels tested in the initial portion of the Phase I study, which included 28 patients,” Wei explains. “Intravitreous delivery with adenovectors appears safe and feasible in both populations.”

Reapplication of Known Efficacious Compounds

Another problem pharma seeks to address is the treatment sustainability bottleneck. Currently, the treatments available consist of injections of Lucentis (ranibizumab injection) or off-label use of Avastin (bevacizumab). “It’s not surprising that given the antiproliferative properties of many anticancer drugs they would be explored for inhibiting cell proliferation and neovascularization that comes with wet AMD,” says Richard M. Soll, vp of research and development at TargeGen (www.targen.com).

MacuSight (www.macusight.com) is reapplying known efficacious compounds for use in the eye. Its proprietary formulation of sirolimus (rapamycin) for serious ocular diseases is being advanced. Currently, rapamycin is used as an immunosuppressant to prevent organ rejection in transplant patients, as well as in drug-coated stents for patients with coronary artery disease.

However, according to David A. Weber, president and CEO of MacuSight, there are some potentially rewarding alternative applications of the drug. “Sirolimus is a broad-acting small molecule that provides critical efficacy advantages for minimally invasive local delivery to the eye,” he says. “Studies have shown that sirolimus inhibits the activity of all forms of VEGF as well as other pro-angiogenic factors and has been shown to inhibit proliferation, inflammation, and fibrosis—so really, it makes sense that this compound could be an attractive therapy option for AMD and other ocular diseases.”

MacuSight says that its sirolimus product candidate has already yielded promising results across several animal models of choroidal neovascularization and retinal angiogenesis and has also demonstrated that it exerts a direct inhibitory effect on VEGF-induced microvascular hyperpermeability. “And because of its broad mechanisms of action, this compound could provide a unique opportunity to mimic combination therapy using a single compound,” Weber says.

New discoveries are often made when research in several labs converge. “In 2005, a number of important papers were published that specifically linked complement factor H polymorphisms to age-related macular degeneration,” says Pascal Deschatelets, Ph.D., cofounder and COO of Potentia Pharmaceuticals (www.potentiapharma.com)“We had already been working on developing complement inhibitors to treat AMD, but these studies indicated that we were on the right track.”

Nobody knows what triggers macular degeneration. “One thing we do know is that the main complication of late-stage AMD is angiogenesis—so anti-angiogenic approaches similar to those being developed to treat cancer have provided the first real breakthrough in the treatment of wet AMD,” notes Dr. Deschatelets. “We want to act earlier in the progression of the disease, hopefully interfering before any vision is lost. We focus on that development.”

For Potentia, the confirmation that complement activation plays an important role in the pathogenesis of AMD led it to looking at complement inhibition as a therapeutic approach to AMD.

“Numerous other studies confirmed the initial correlation. And if you look at the literature, the correlation was expanded to include a number of other complement proteins such as the complement component C2 and complement factor B. It’s reasonable to conclude that excessive complement activation is a crucial factor in the initiation and progression of AMD.”

Reasonable enough of a conclusion that the first few complement inhibitors for this indication are currently making their way through the drug pipeline. Potentia is among this first wave with POT-4, a peptide inhibitor of complement component C3 derived from Compstatin. “POT-4 is able to interfere with all three major pathways of complement activation,” says Dr. Deschatelets.

Ultimately, Potentia hopes to treat AMD in its entirety, not just one specific aspect of it. “Our hope is to target the disease before it gets to the angiogenesis stage,” says Dr. Deschatelets. “In 2007, we are looking to launch the first clinical trial using complement inhibition to treat AMD.”

Topical Delivery

One method of drug delivery for AMD and proliferative diabetic retinopathy that has proven elusive is a topical application. TargeGen’s Soll will describe TG100801, a topically applied multitargeted VEGFR/Src kinase inhibitor. “This agent and other VEGFR/Src kinase inhibtors in our portfolio exhibit efficacy in vivo after topical application in models of angiogenesis and permeability. In addition, they have the potential to address the underlying inflammation associated with these diseases,” Soll says.

And the delivery of the drug is potentially key. “Working with a small molecule enables you to look at other options besides injection,” Soll says. “Kinase inhibitors may not be suitable for systemic administration, particularly from the safety perspective. Injectable approaches still have limitations.

“Historically, topical delivery was accepted as problematic. For the most part, it was thought you couldn’t get a drop to the back of the eye, or at least achieve the sufficient concentrations you’d need for efficacy.”

Until now, that is. “Preclinical data looks particularly good,” says Soll. “With that mode of delivery, a validated pathway, a comprehensive mode of action, and a little systemic exposure we feel pretty good about this compound. And we have second-generation agents that are VEGFR/Src inhibitors based on different chemistries as well as agents that address the JAK STAT pathway that is clearly up-regulated in a number of ocular disorders. Thus, these multitargeted kinase inhibitors have the potential to take us beyond VEGF. It’s a different approach. Blended kinase inhibitors give us the opportunity to address many facets of these complicated diseases.”

CoMentis (www.comentis.com), formerly Athenagen, is also investigating the topical application option. Its compound, ATG003 (mecamylamine ophthalmic solution), successfully completed a Phase I trial in January. This novel anti-angiogenic drug candidate is currently in clinical development for neovascular age-related macular degeneration.

ATG003 acts by inhibiting the angiogenic pathway mediated by the nicotinic acetylcholine receptors found on endothelial cells (EC-NAChR), which were discovered by Stanford scientists in the late 1990s. Inhibiting this pathway also inhibits the synthesis and cellular responses mediated by growth factors such as VEGF and bFGF. Cigarette smoking is the major independent risk factor in the pathogenesis of neovascular AMD as well as the transition from dry AMD to neovascular AMD.

ATG003 significantly inhibits laser-induced choroidal neovasculaization in a mouse model of AMD as well as enhanced vascular permeability in the same species. Topical delivery of ATG003 leads to high levels of the active drug in the retina choroid (back of the eye) in animal models with little reaching the systemic circulation.

“We now know that both the intrinsic physiochemical property of the molecule as well as the formulation determine the extent of drug penetration to the back of the eye but the fact that little of the drug reaches the systemic circulation was totally unexpected and could not have been predicted from what is known about ocular drug delivery,” says Ken M. Kengatharan, vp, preclinical research and development at CoMentis.

Kinase Inhibitors

Alcon (www.alconlabs.com) is also investigating kinase inhibitors. “We will be presenting preclinical data on the compound, AL 39324, a novel receptor tyrosine kinase inhibitor,” says senior scientist Xiaolin Gu. “It is an active anti-angiogenic compound that provides robust efficacy in a variety of ocular modles.”

Alcon’s lead preclinical candidate demonstrated promise for treating wet AMD and other ocular diseases, such as diabetic macular edema. “It has been shown to inhibit angiogenesis from the beginning of the insult and has shown 100% prevention in some models,” Gu reports.

Preclinical data show that in the 14-day model of choroidal neovascularization, when dosing is begun on the seventh day, substantial regression in new blood vessel growth is achieved. “If translated to man, this unique effect may provide a treatment advantage for patients with posterior segment neovascularization,” notes Gu.

Alcon is actively pursuing local delivery options for the drug. “During our preclinical work, we have primarily used intravitreal administration,” says Gu, “but Alcon has proprietary modes of local delivery and is exploring a variety of options.”