Scientists at Schepens Eye Research Institute have found that reducing the levels of vascular endothelial growth factor (VEGF), which is best known as a stimulator of new blood vessel growth, in adult mice causes the death of photoreceptors and Muller glia - cells of the retina that are essential to visual function.
This finding holds implications for the chronic use of promising new anti-VEGF drugs such as Lucentis, which eliminate abnormal and damaging blood vessel growth and leakage in the retina by neutralizing VEGF.
"The take home message of this study is that physicians should be vigilant in monitoring patients undergoing anti-VEGF treatments for any possible signs of these side effects," says Principal Investigator Patricia D'Amore, Senior Scientist at Schepens Eye Research Institute. "Drugs such as Lucentis are very good at reducing the edema (fluids) and eliminating the abnormal blood vessels that characterize wet macular degeneration, but our results suggest that there could be unanticipated side effects."
Scientists have long known that VEGF is essential for normal development of the vascular system and for wound healing. It triggers the formation of new blood vessels that nourish the growing body and heal organs and tissues. VEGF also stimulates--in an apparent misguided attempt to heal perceived damage in the retina--the growth of abnormal blood vessels that leak and damage delicate retinal tissue.
However, a growing body of evidence also indicates that beyond its impact on blood vessel growth, VEGF may play other vital roles in the adult body and eye, so that eliminating the growth factor might lead to unexpected consequences.
Given the popularity and promise of the new anti-VEGF drugs for the treatment of macular degeneration, D'Amore and her team believed that investigating the broader role of this growth factor in the normal adult retina was critical. She and her laboratory mimicked the action of the anti-VEGF drugs by introducing into adult mice a soluble VEGF receptor, known as sFlt1, which binds and neutralizes the VEGF-- in much the same way that Lucentis does in the eye.
After two weeks, the team found no effect on blood vessels of the inner retina, but did find a significant increase in the number of dying cells of the inner and outer nuclear layers which include amacrine cells that participate in transmitting the visual signal; Muller cells that also participate in the visual signal and support the photoreceptors; and, photoreceptors, which are responsible for color and night vision. The team then used electroretinograms to measure visual function and found a significant loss in visual function. Consistent with these observations, they discovered that both photoreceptors and Muller cells express VEGFR2, the major VEGF signaling receptor and they found that neighboring Muller cells express VEGF.
Parallel studies in tissue culture demonstrated that suppressing VEGF in Muller cells led to Muller cell death, indicating an autocrine role for VEGF in Muller cells (i.e. Muller cells both make VEGF and use it for survival). Further, they used cultures of freshly isolated photoreceptors to show that VEGF can act as a protectant for these cells.
"Insight into the complex role of VEGF in the eye and in other parts of the body indicates that increased care should be taken in the long-term use of these drugs and that this new information should be considered in the design of future clinical studies to ensure that these possible side effects are taken into account," says D'Amore.
"Mice eyes differ from human eyes in many ways, so we cannot directly extrapolate these results to humans, but this study is an important heads-up that clinical application of anti-VEGF therapy in the eye needs to proceed with caution," she adds.
From a clinical perspective, Dr. Delia Sang of Ophthalmic Consultants of Boston points out that the use of anti-VEGF therapy in the treatment of patients with wet macular degeneration has revolutionized outcomes in this disease. However, in light of the work of Dr. D'Amore and others, in elucidating possible systemic and ocular side effects of these drugs, "caution must be exercised in identifying patients at increased risk of problems with long-=term VEGF blockade, and potential side effects must be detected early in the assessment of patients who will require repeated dosages of anti-VEGF agents."
The study is also relevant to the drug Avastin, which was initially approved for intravenous use as an anti-angiogenic agent in the treatment of cancer, but is also widely used intravitreally for the treatment of wet AMD because of its similar mode of action and much lower cost.
The next steps in D'Amore's research will include investigating the specific functions of VEGF in the eye.
This research is published in the November 3, 2008 PLoS ONE. Authors of the study include: Magali Saint-Geniez, Arindel S. R. Maharaj, Tony E. Walshe, Budd A. Tucker, Eiichi Sekiyama, Tomoki Kurihara, Diane C. Darland, Michael J. Young, Patricia A. D'Amore
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