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Opportunities for Therapeutic Intervention:
Molecular Biology of Diabetic Retinopathy

Jointly sponsored by The Dulaney Foundation and
Diabetic Microvascular Complications Today.
Release Date: January 2006. Expiration Date: January 31, 2007.
This continuing medical education activity is supported by
an educational grant from Eli Lilly and Company.
By Lloyd Paul Aiello, MD, PHD
STATEMENT OF NEED

The risk of death among people with diabetes is about two times that of those without the disease. The annual cost in the United States alone is more than $132 billion dollars.
Diabetic retinopathy develops in virtually 100% of patients with type 1 diabetes and 60% of those with type 2. Within 10 years of diabetes onet, 20% of type 1 diabetic patients and 14% of type 2 diabetics develop diabetic macular edema. It is the leading cause of moderate vision loss in diabetes.

TARGET AUDIENCE

This activity is designed for ophthalmologists, specifically retinal specialists and others who treat diabetic retinopathy.

LEARNING OBJECTIVES

After successful completion of this program the participants should be able to:
• describe the molecular biology underlying diabetic retinopathy;
• identify therapeutic strategies for intervention; and
• discuss the DRCR.net initiatives.

METHOD OF INSTRUCTION

Participants should read the learning objectives and CME program in their entirety. After reviewing the material, they must complete the self-assessment test, which consists of a series of multiple-choice questions.
Participants have a choice of completing this activity online by visiting www.DiabeticMCToday.com; getting real-time results at www.CMEToday.net; or by using the print forms following this activity.

ACCREDITATION

This activity has been planned and implemented in accordance with the essentials and standards of the ACCME through the joint sponsorship of The Dulaney Foundation and Diabetic Microvascular Complications Today.

DISCLOSURE

In accordance with the disclosure policies of The Dulaney Foundation and to conform with ACCME and FDA guidelines, all program faculty are required to disclose to the activity participants: 1) the existence of any financial interest or other relationships with the manufacturers of any commercial products/devices, or providers of commercial services that relate to the content of their presentation/material or the commercial contributors of this activity; and 2) identification of a commercial product/device that is unlabeled for use or an investigational use of a product/device not yet approved.

FACULTY DISCLOSURE DECLARATIONS

Lloyd Paul Aiello, MD, PhD, serves as a consultant to and receives honoraria from Eli Lilly and Company and Eyetech.

FACULTY CREDENTIALS

Dr. Aiello is associate professor of ophthalmology at Harvard Medical School, director of the Beetham Eye Institute and head of the section on eye research at Joslin Diabetes Center, Boston.

INTRODUCTION

Diabetic retinopathy exists across a spectrum of stages and severity, and macular edema can occur at any point across the spectrum. Even before diabetic retinopathy is evident, biochemical changes begin.1
Hyperglycemia plays a role with regard to cellular dysfunction and damage, with a host of complex processes that come into play. Hyperglycemia is the principal underlying cause of the complications of diabetes.

PKC

Protein kinase C (PKC) activation appears to be a key player in many of these pathways.2 PKC inhibition may help ameliorate hyperglycemia-induced cellular dysfunction The PKC enzyme modulates molecules in the body and the beta isoform is primarily involved in the complications of diabetes and other vascular issues.3 In the state of hyperglycemia diaclyglycerol is synthesized, which in turn activates PKC. Therefore, the fundamental problem is that PKC is activated in the diabetic condition.
The level of PKC activity then correlates to the severity of diabetic retinopathy. PKC-beta activation plays a key role in mediating early retinal changes in diabetes. Even when PKC beta was overexpressed in nondiabetic mice, retinal abnormalities were seen.4

VEGF

Growth factor mediation – particularly vascular endothelial growth factor (VEGF) – is part of retinopathy. Proliferative diabetic retinopathy appears to be driven by ischemia in the retina. Growth factors in the eye lead to neovascularization.

VEGF is a 46 kDa homodimeric glycoprotein that was first identified in vascularized tumors. VEGF is produced by many ocular cell types and is induced by hypoxia. It binds to high-affinity receptors on retinal endothelial cells and stimulates retinal endothelial cell growth. VEGF promotes vascular permeability and induces disassociation of tight junction components.

Retinal VEGF levels are elevated in diabetes and VEGF can cause diabetic retinopathy-like changes in the eye.5-7 Data shows that VEGF plays a key role in mediating retinal neovascularization and vascular permeability under ischemic retinal conditions, such as diabetic retinopathy.

If an agent was able to block VEGF, we might be able to prevent diabetic complications in the eye. VEGF inhibition in mice and primate models was shown to be successful.8,9

ANTI-VEGF THERAPIES

Current anti-VEGF approaches that are being studied include pegaptanib sodium, ranibizumab, VEGF trap and bevacizumab.

• Pegaptanib sodium (Macugen, Eyetech) acts as an intravitreal aptamer VEGF inhibitor. Its potential indication is diabetic macular edema (DME) treatment and it is in phase 3 trials. It has been Food and Drug Administration (FDA) approved for the treatment of neovascular or wet age-related macular degeneration.

• Ranibizumab (Lucentis, Genentech) acts as an intravitreal humanized anti-VEGF antibody fragment. Its potential indication is DME and studies are planned. Genentech will soon seek FDA approval for the indication of the treatment of neovascular or wet age-related macular degeneration.

• The VEGF Trap (Regeneron) is an intravitreal VEGF receptor construct that is being studied for the potential treatment of DME.

• Bevacizumab (Avastin, Genentech) is an intravitreal anti-VEGF antibody that is being studied in DME treatment, off-label. Currently it is approved as first-line treatment for patients with colorectal cancer that has spread to other parts of the body. Mechanism of VEGF action is shown in Figure 1.

PKC-BETA INHIBITION

Another therapeutic target option would be to block PKC-beta activation along the pathway. A PKC-beta inhibitor should be highly PKC selective, highly beta isoform selective and orally bioavailable. Eli Lilly and Company’s ruboxistaurin, currently in development, is all of these things. In clinical studies it has been shown to reduce retinal neovascularization, reduce diabetes-induced permeability and normalize retinal blood flow.
A ruboxisaurin phase 2/3 trial studied 252 total patients in the diabetic retinopathy arm and 426 total patients in the DME arm. In the diabetic retinopathy arm, the overall goal of the trial was to see if the agent slowed the progression of nonproliferative diabetic retinopathy or prevented laser treatment. Diabetic retinopathy severity was 47b to <53e (moderate to severe nonproliferative diabetic retinopathy [NPDR]). Patients had no prior panretinal photocoagulation but could have prior focal treatment and could have macular edema at baseline (Table 1).

In the DME study the outcome was the slowing or reversal of the progression of macular edema or to prevent laser treatment. The diabetic retinopathy severity was 20 to ≤47 a (mild to moderate NPDR). Patients did not have prior panretinal photocoagulation or focal treatment and did have macular edema at baseline. The Cox proportional hazard model is as shown in Figure 2.

The primary study endpoint in the PKC-DRS was the progression of diabetic retinopathy, meaning a 3+ step change in the ETDRS retinopathy person severity scale (patients with two eyes <level 61) or 2+ step change in the ETDRS retinopathy eye severity scale (patients with only one eye <level 61) or panretinal photocoagulation. The Cox proportional hazard model for moderate visual loss is shown in Figure 3.

The PKC-beta inhibition study showed that ruboxistaurin decreased the development of macular edema threatening the center of the macula and the occurrence of visual loss. Ruboxistaurin did not prevent the progression of diabetic retinopathy nor the combined outcome of DME progression or the application of laser photocoagulation. Two large, high-power, single-dose trials evaluating the impact of ruboxistaurin on these endpoints are underway (Figure 4).

INTRAVITREAL STEROIDS

The potential of intravitreal steroid treatment is that it can reduce retinal edema, but there is a time limited effect. The visual acuity response of this method of treatment does not always commensurate with reduction in retinal edema. Intravitreal steroids come with a high risk of cataract, intraocular pressure elevation and endophthalmitis, and often it is an off-label use.
Intravitreal steroids are being investigated as part of the Diabetic Retinopathy Clinical Research Network. The DRCR.net is dedicated to multicenter clinical research of diabetic retinopathy, macular edema and associated disorders. (To read more about DRCR.net, see page 29 of the November/December 2005 issue of Diabetic Microvascular Complications Today.)

Triamcinolone acetonide is being studied in a phase 3, multicenter, randomized clinical trial. The treatment is delivered as a preservative-free, pH balanced solution, in a single-dose prefilled syringe. Patients are randomized to one of three treatment groups: standard of care (conventional treatment including modified ETDRS photocoagulation), intravitreal injection of 1 mg triamcinolone acetonide or intravitreal injection of 4 mg triamcinolone acetonide.

The injection volume is always 0.05 mL and follow-up is 3 years. The first patient was enrolled in July 2004 and more than 417 patients have been enrolled to date. The goal is 690 patients total. I recently presented this information at the 2005 American Academy of Opthalmology Annual meeting in Chicago.10

CONCLUSION

Diabetes is a systemic disease. Diabetic retinopathy is the leading cause of blindness in working age adults. Diabetic neuropathy is the leading cause of nontraumatic amputations and diabetic nephropathy is the leading cause of end-stage renal disease. Also, with regard to the macrovascular picture, patients with diabetes have a two- to fourfold increase in cardiovascular mortality and stroke. 

1. Early Treatment Diabetic Retinpathy Study Research Group. Fundus photographic risk factors for profession of diabetic retinopathy. ETDRS report No 12. Ophthalmology. 1991;98:823-833.
2. Sheetz MJ, King GL. Molecular understanding of hyperglycemia’s adverse effects for diabetic complications. JAMA. 2002;288:2579-2588.
3. Ishii, Koya D, King GL. Protein kinase C activation and its role in the development of vascular complications in diabetes mellitus. J Mol Med. 1998;76:21-31.
4. Wakasaki H, Koya D, Schoen FJ, et al. Targeted overexpression of protein kinase C beta2 isoform in myocardium causes cardiomyopathy. Proc Natl Acad Sci U S A. 1997;94:9320-9325.
5. Aiello, Avery RL, Arrigg PG, et al. Vascular endothelial growth factor in ocular fluid of patients with diabetic retinopathy an other retinal disorders. N Engl J Med. 1994;331:1480-1487.
6. Adams AP, Miller JW, Bernal MT, et al. Increased vascular endothelial growth factor levels in the vitreous of eyes with proliferative diabetic retinopathy. Am J Ophthalmol.. 1994;118:445-450.
7. Qaum T, Xu O, Joussen AM, et al. VEGF-initiated blood retinal barrier breakdown in early diabetes. Invest Opthalmol Vis Sci. 2001;42:2408-2413.
8. Tolentino MJ, Miller JW, Gragoudases, et al. Intravitreous injections of VEGF produce retinal ischemia and microangiopathy in adult primates. Ophthalmology. 1996;103:1820-1828.
9. Tolentino MJ, McLeod DS, Taomoto M, et al. Pathologic features of vascular endothelial growth factor-induced retinopathy in tghe nonhuman primate. Am J Ophthalmol. 2002;133:373-385.
10. Aiello LP. Molecular biology of diabetic retinopathy: Opportunites for therapeutic intervention. CME Symposium presented during the the 2005 American Academy of Ophthalmology meeting. October 15, 2005. Chicago.
For a downloadable pdf of this article, including Tables and Figures, click here.