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Summary for the Clinician
•Corneal lymphangiogenesis may also be involved in the pathogenesis of chronic dry eye disease.
•Corneal lymphatic vessels can be visualized in vivo in animal corneas using confocal and multiphoton microscopy.
Novel Anti(lymph)Angiogenic Treatment Options at the Cornea
Antiangiogenic therapeutic approaches at the cornea can broadly be divided into two categories:
1.Angiostatic/antiangiogenic, i.e. to stop the outgrowths of newly outgrowing immature vessels (classical antiangiogenic approach)
2.Angioregressive/angioocclusive (meaning regression of already established mature pathologic vessels, especially important, e.g. in prevascularized high-risk eyes).
Mature, pericyte-covered vessels do not depend on VEGF and other angiogenic growth factors so much, so we need additional physical approaches to close these mature vessels. In contrast for immature vessels, anti-VEGFs are a potent treatment approach.
As outlined above, lymphatic vessels seem to be the prime mediators of corneal graft rejection [10]. That means that in the transplant context, it would most likely be sufficient to block lymphangiogenesis to promote graft survival. Specific inhibitors of lymphangiogenesis are desirable, since these would probably not interfere with corneal wound healing. We and others have recently described several new pharmacological approaches to (relatively) specifically target inflammatory corneal lymphangiogenesis. These approaches include
(a)Blocking peptides against integrins expressed on lymphatic vascular endothelial cells. Blocking peptides against integrin alpha 5, being expressed on lymphatic vascular endothelium in vivo, at certain concentrations relatively specifically block lymphwithout affecting hemangiogenesis [25].
(b)Blocking antibodies against the lymphatic vascular endothelial-specific VEGF receptor 3 also have been shown to relatively specifically block inflammatory lymphwithout affecting hemangiogenesis [26].
None of these approaches has yet entered clinical trials for use in patients, but they are promising tools for a (relative) specific blockade of lymphangiogenesis in the context of corneal transplantation to promote graft survival in the near future [10].
Current Treatment Options for Immature Corneal (Blood and Lymphatic) Vessels
The drugs available so far to inhibit active corneal (lymph)angiogenesis in the cornea include topical steroids/Cyclosporine A, anti VEGFs (such as Bevacizumab, Ranibizumab, VEGF Trap, and Pegaptanib) as well as GS101/Aganirsen (i.e., strategies
5 Antiangiogenic Treatment Options in the Cornea |
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targeting IRS-1 signaling). These will be discussed below. Most of these compounds not only inhibit hembut also lymphangiogenesis.
Steroids
Steroids have long been and still are the mainstay of topical anti-inflammatory treatment in the context of corneal transplantation. Steroids are partly antiangiogenic and as we could recently show also antilymphangiogenic [27]. That effect is mediated not only by inhibiting inflammatory cells influx, but also by direct effects on vascular endothelial cells. The antihemand antilymphangiogenic potency of steroids significantly varies, with Prednisolone and Dexamethasone being the most potent antilymphangiogenic drugs. That implies that in the transplant context, Prednisolone should be given. Nonetheless, steroids are only partly antiangiogenic and have severe long-term side effects. Therefore, more specific antiangiogenic drugs are needed to achieve more safety and efficacy. In fact, a combined antiinflammatory and antiangiogenic treatment strategy (e.g., by combining steroids and modern anti-VEGFs) did achieve the best antiangiogenic effect in models of experimental corneal neovascularization (Fig. 5.4).
Anti-VEGFs (Bevazicumab, Ranibuzumab, Pegaptanib, VEGF Trap)
Anti-VEGF pharmacological strategies using Bevacizumab (Avastin®), Ranibizumab (Lucentis®), Pegaptanib (Macugen®), or VEGF Trap® potently inhibit experimental corneal neovascularization. Avastin eye drops, VEGF Trap, and Lucentis eye drops in addition also have been shown to inhibit corneal lymphangiogenesis in animal models [10, 28]. Based on these experimental data, that treatment approach has already been translated into clinical application (Fig. 5.5).
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Blood vessels Lymphatic vessels
Fig. 5.4 Antihemand antilymphangiogenic potency of topically applied steroids (From Ref. [27]). There is a significant difference in the antihemand antilymphangiogenic potency between dexamethasone and prednisolone on one side and fluorometholones on the other side. In the transplant setting therefore, the first two drugs should be applied to effectively block lymphangiogenesis
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Fig. 5.4 |
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Lymphangiogenesis |
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vessels(%) |
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100 |
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lymphaticby |
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covered |
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Hemangiogenesis |
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Patients being treated with Avastin eye drops off-label for pathologic corneal neovascularization which were unresponsive to conventional therapy, show a significant but variable response to additional Avastin eye drops when neovascularization was analyzed morphometrically on standardized digitized slit-lamp pictures [29]. We use Avastin in 5 mg/ml dilution 5×/day. Eye drops were well tolerated. Nonetheless, there is concern about wound healing delay and neurotoxic effects by targeting VEGF at the ocular surface. Numerous published case reports, small case series, and also one controlled trial support the concept of Avastin eye drops or subconjunctival applications being relatively effective and relatively safe. Therefore, prospective randomized studies are necessary to define indications, safety, and effectivity of that approach in inhibiting corneal neovascularization.
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Fig. 5.5 Avastin eye drops significantly inhibit progressive corneal neovascularization (From Ref. [29]). There is a significant decrease of corneal neovessels comparing prior to (a) and after additional treatment for 4 weeks with Avastin eye drops (b). The respective image analysis is shown in panels (c) and (d)
For the other two off-label available anti-VEGF agents, there is much less known about their effect on human corneal neovascularization. Initial clinical data suggest that Lucentis eye drops – perhaps because of smaller size and better penetration into the cornea – have an even stronger antiangiogenic effect at the cornea. Preclinical data show both of them being active against corneal neovascularization, with Ranibizumab – as said above – also being active against corneal lymphangiogenesis.
Anti-IRS 1-Strategies (Antisense Oligonucleotides Against IRS 1)
Insulin receptor substrate (IRS) 1 has recently been shown to be a key mediator of inflammatory angiogenesis in several in vivo models. Based on that, an antisense oligonucleotide significantly blocking IRS-1 signaling was developed and tested preclinically and clinically. Phase II trial data from a prospective multicenter randomized European trial recently showed that anti-IRS-1 eye drops (Aganirsen) applied twice daily significantly inhibit progressive corneal neovascularization and are well-tolerated. Animal experimental data show that this compound not only blocks visible hembut also invisible lymphangiogenesis. Based on the positive phase II data, a randomized phase III trial is currently being conducted in Europe. Anti-IRS-1 treatment strategies may evolve as an effective and safe treatment approach to topically target progressive corneal neovascularization in the future (Fig. 5.6).