EyeNet Magazine


 
News in Review
A Look at Today's Ideas and Trends
By Linda Roach, Contributing Writer
Edited by Brian A. Francis, MD
 
 

• Enzymes Implicated in Angiogenesis
• Surgery May Restore the Natural Lens’ Elasticity
• Antibacterial Contact Lens Entering Clinical Trials
• Vaccine Introduced for Herpes Zoster

Enzymes Implicated in Angiogenesis

With one acronym for an AMD-promoting molecule under their belts, ophthalmologists might have the right to a little self-satisfaction about their knowledge of VEGF’s role in choroidal neovascularization. But the fun is just beginning!

Now come two more acronyms to learn—the names of enzymes that work downstream from VEGF, forming a molecular on-off switch for angiogenesis. PI3K (vessel growth) and PLC-gamma (regression) tip the vascularization process back and forth by competing for the same lipid substrate on the walls of retinal endothelial cells, according to researchers at the Schepens Eye Research Institute. They reported on their discovery in The EMBO Journal.1

“The current drugs for age-related macular degeneration are especially good at preventing angiogenesis from happening,” said coauthor Andrius Kazlauskas, PhD, a Schepens senior scientist and an associate professor of ophthalmology at Harvard University. “They get in the way of new blood vessels forming, and as a beneficial side effect they cause the immature pathological vessels to regress. But the larger, stable pathological vessels largely remain.

“What we’re working on is ways to make existing pathological vessels go away,” Dr. Kazlauskas said. “We want to activate the intrinsic pathway that instructs the vessels to regress.”

Dr. Kazlauskas and coauthor Eunok Im, PhD, a Schepens research fellow, isolated the mutually antagonistic actions of the two enzymes in a complex series of Petri-dish experiments with bovine retinal endothelial cells. The enzymes’ full names are phosphoinositide 3 kinase (PI3K) and phospholipase C gamma (PLC-gamma).

The molecules were known previously to be angiogenic signaling enzymes and, consequently, to be involved somehow in choroidal neovascularization. However, no one knew they were paired in a biochemical signaling dance. Nor was it known that one of them—even though it is activated by VEGF—actually caused vessels to regress, Dr. Kazlauskas said.

The scientists’ next task will be to better characterize these processes in vivo, with mice. For instance, they want to know why PI3K and PLC-gamma stop their action when a blood vessel is mature. Ultimately, the goal would be to design a new class of molecules that would make unwanted blood vessels disappear, and make others grow where they’re needed, such as in damaged heart muscle.

But it won’t be possible to reverse CNV or diabetic retinopathy by dosing people with PLC-gamma to promote regression, Dr. Kazlauskas said. “PLC-gamma is at the top of a pathway that then branches and does many, many things in the body. If you could give a pill and turn on PLC-gamma globally, I expect that the results would be catastrophic.”

Nor would such drugs replace existing therapies like verteporfin (Visudyne), pegaptanib (Macugen) and ranibizumab (Lucentis), Dr. Kazlauskas said. “I don’t think this is an either-or issue. I anticipate you’re going to need both kinds of tools.”

_______________________________

1 EMBO J 2006;25(10):2075–2082.

Research Report

Surgery May Restore the Natural Lens’ Elasticity

The femtosecond laser, originally used in ophthalmology to make LASIK flaps, might be headed for the cataract surgery suite to address that most intransigent of human refractive problems—presbyopia.

A smattering of research reports have appeared on this topic in the last two years, most of them from European research sites. The latest, expected to be published in an ophthalmic research journal, comes from the same University of Michigan site where early work occurred on what eventually became the IntraLase.

Led by Matthew O’Donnell, PhD, professor and chairman of biomedical engineering at U-M, this group has adapted the femtosecond laser to instead vaporize spots of tissue within the lens that are 10- to 50-micrometers in diameter.

More significantly, they use high-frequency ultrasound to determine how much more elastic the lens is after each laser pulse creates a plasma bubble from lens tissue. In effect, this dynamic, in situ method taps on the bubbles with sound waves, measuring how high they move.

“We use acoustics to push the bubbles, so we have very precise measurements of the elastic modulus of a location within the lens, down to the spatial resolution of the bubbles,” Dr. O’Donnell said. “So we can map the elastic modulus of the lens in many places, with the lens sitting right there in the eye and with minimal disruption. We can measure while we’re blasting away.”

Ultimately, the evolving elasticity would tell the clinician doing the treatment when to stop targeting each section of lens.

So far, the system has not been tried in living animals. That and the other steps of moving this idea from the bench to clinical application likely will require a commercial partner, Dr. O’Donnell said.

But the payoff in basic ophthalmic knowledge already has been intriguing. Examination of several human cadaver eyes found that the elasticity of the crystalline lens varied spatially and temporally among individuals, Dr. O’Donnell said.

First, forget the idea of a lens that has uniform elasticity and that hardens evenly with aging. “The lens modulus isn’t the same for everyone. We’ve measured it in the cadaver eyes, and we know that it varies from patient to patient,” he said. “The elasticity also changes from the center to the periphery. The center is over an order of magnitude harder than the periphery. And the changes aren’t just the absolute modulus—the spatial distribution of the elasticity changes over time.”

Some other findings include:

  • In a newborn, the lens is almost liquid, with the nucleus softer than the cortex.
  • By age 6, the nucleus has the consistency of gelatin that has been refrigerated for about 30 minutes.
  • The nucleus doesn’t become stiffer than the cortex until around age 35.
  • Measured in pascals (Pa), the modulus of an infant’s crystalline lens is about 200 Pa. Lenses from people older than 60 years were nearly three orders of magnitude stiffer, between 60,000 and 100,000 Pa.

Dr. O’Donnell, who begins a new job this fall as dean of engineering at the University of Washington, said he hopes to continue the research there. Engineering and ophthalmology have a lot to learn from each other, he said.



Further Reading

Erpelding, T. N. et al. Bubble-based acoustic radiation force for monitoring intraocular lens elasticity. Proc IEEE Ultrason Symp 2004;1:732–735.

Hollman, K. W. et al. Ultrasonic determination of laser manipulated changes in the elastic property of the intraocular lens. Proc IEEE Ultrason Symp 2003;1:656–659.

Krueger, R. R. et al. First safety study of femtosecond laser photodisruption in animal lenses: tissue morphology and cataractogenesis. J Cataract Refract Surg 2005;31(12):2386–2394.



In the Pipeline

Antibacterial Contact Lens Entering Clinical Trials

The recent reports of Fusarium keratitis served as a reminder to the ophthalmic community that—even with the newest materials—contact lens wearers still bear a certain risk of infection. And that’s why a small Australian company and the Institute for Eye Research could have the product of the future.

Biosignal Ltd., of Eveleigh, New South Wales, and the IER are beginning a 200-person trial this fall of silicon hydrogel contact lenses coated with a synthetic furanone, a type of antibacterial molecule produced by the ocean seaweed Delicia pulchra. The molecule, dihydropyrrol-2-one, is chemically bonded to the lens via a plasma reaction.

“Even with the advent of new high-oxygen-permeable silicon hydrogel lenses, the rate of corneal infection remains at one in 500 if you sleep in lenses to one in 2,500 if you wear lenses on a daily wear basis,” said Mark Willcox, PhD, chief scientific officer of the Institute for Eye Research at the University of New South Wales. It was at this university that Biosignal’s founders first characterized the furanone’s properties.

The clinical trial is based on research showing that furanones prevent bacteria from building up into slimy biofilms. They do so by jamming the bacteria’s cell-to-cell signaling systems. (Biofilms were implicated as one cause of microkeratome-related diffuse lamellar keratitis.)

A pilot study of the furanone-coated lenses in 10 patients showed no safety problems, Dr. Willcox said. In the larger trial, subjects will wear the disposable lenses continuously and replace them monthly for six months.

But would furanones protect against fungal infections such as Fusarium? Said Dr. Willcox: “At present we do not know the efficacy of these lenses against fungal contamination, but we have shown that the ‘free’ compounds themselves can block fungal growth.”

In the Clinic

Vaccine Introduced for Herpes Zoster

A new vaccine approved by the FDA in May could reduce both the incidence and morbidity of one troubling eye infection —herpes zoster. “The vaccine is a huge advance, and its approval should mean that we will see less and less of herpes zoster infections,” said John B. Kerrison, MD, practitioner at the Charleston Neuroscience Institute, Retina and Macular Diseases, Charleston, S.C.

There are close to one million episodes of herpes zoster, or shingles, annually in the United States. Fifteen percent of the population will experience shingles during their lifetime.1 Herpes zoster primarily affects the elderly, and the vaccine is approved only for people over 60. However, according to infectious disease experts, it should be approved for other populations soon.

Ophthalmic complications of herpes zoster represent only 10 to 25 percent of cases, and are usually limited to keratitis and conjunctivitis. These infections often resolve when the initial infection is treated, according to Dr. Kerrison.

In rare cases, however, herpes zoster infections can involve the retina or optic nerve—usually in immunocompromised individuals. In these cases, the consequences can be devastating. Without treatment, blindness can result.

Herpes zoster is treated with antiviral drugs such as acyclovir, valacyclovir and famciclovir. The most common complication of herpes zoster is postherpetic neuralgia, a chronic condition that can cause significant pain and dysesthesia, as well as reduced quality of life.

In a clinical trial of the new vaccine with more than 38,000 adults over age 60, the vaccine reduced the incidence of herpes zoster by more than 51 percent, the burden of illness by 61 percent and the incidence of postherpetic neuralgia by over 65 percent.2 Ophthalmic complications were not an endpoint of the study.

Barbara Boughton

1 Insinga, R. P. et al. J Gen Intern Med 2005;20:748–753.
2 Oxman, M. N. et al. N Engl J Med 2005;352:2271–2284.

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