There is evidence to suggest that both age-related macular degeneration (AMD) and diabetic macular edema could be pro-inflammatory conditions. It was reported that anti-tumor necrosis factor (TNF) monoclonal antibody Infliximab, when given systemically, regressed and resolved, at least, partially choroidal neovascularization (CNV) secondary to age-related macular degeneration (AMD) (1). It was found that the plasma level of IL-6 concentration and the state of the posterior vitreous detachment (PVD) correlated significantly with the severity of macular edema suggesting that increased plasma interleukin-6 (IL-6) concentration can predict the development of macular edema (2). It was observed that the plasma levels of IL-6 concentration correlated significantly with the severity of macular edema compared to other factors such as vascular endothelial growth factor (VEGF), transforming growth factor (TGF)-β1, tumor necrosis factor (TNF)-α, and lipoprotein(a) in plasma, and serum level of von Willebrand factor and thrombomodulin (2). The concentrations of VEGF and IL-6 in undiluted aqueous specimens were significantly correlated with the severity of macular edema as well as with the aqueous protein concentration.  Aqueous levels of VEGF and IL-6 were significantly higher than their respective plasma levels, the aqueous level of VEGF was significantly correlated with that of IL-6 and the status of the posterior vitreous significantly correlated with the severity of macular edema, suggesting that both VEGF and IL-6 are produced together in the intraocular tissues, and are involved in the pathogenesis of macular edema (2). Th1/Th2 ratios were significantly associated with macular edema (3), emphasizing the involvement of inflammatory mediators in the pathogenesis of diabetic macular edema and confirming that AMD, diabetic macular edema and diabetic retinopathy are low-grade inflammatory conditions. 

    A recent concluded that anti-TNF-α antibody could form an effective therapy for diabetic macular edema (4). But, caution need to be exercised with regard to the use of infliximab for diabetic macular edema. In patients with refractory diabetic macular edema or choroidal neovascularization secondary to age-related macular degeneration, intravitreal administration of infliximab (Remicade) did not produce any change in cystoid macular edema on optical coherence tomography, and showed a decrease in maximal combined responses, from 7% to 24% from baseline. All patients declined on microperimetry and panuveitis and vitritis and development of systemic antibodies against infliximab was noted, suggesting that intravitreal infliximab is immunogenic and probably retinotoxic (5).



Since TNF-α and IL-6 and other pro-inflammatory cytokines have a role in AMD, diabetic macular edema and diabetic retinopathy, inhibition of their production or action could be of benefit in them. Furthermore, presence of increased VEGF levels in AMD, macular edema and diabetic retinopathy led to the use of anti-VEGF therapy in these conditions. Figurska and Stankiewicz (6) showed that regular monthly administration of intravitreal ranibizumab (anti-VEGF antibody fragment) to patients with AMD preserves visual acuity though it led only to a chance of stabilization and improvement of the topical state, and regular monthly treatment is necessary even for smallest signs of deterioration. I propose that employing endogenous inhibitors of inflammation-resolving molecules such as lipoxins, resolvins, protectins and maresins could be of benefit in AMD, diabetic macular edema and diabetic retinopathy.



Arachidonic acid (AA), the ω-6 polyunsaturated fatty acid (PUFA), not only forms precursor to pro-inflammatory compounds such as prostaglandins (PGs), leukotrienes (LTs) and thromboxanes (TXs) but also gives rise to lipoxins (LXs-LXA4 and LXB4) that are anti-inflammatory molecules and inhibit NF-κB activation (7). Lipoxins signal macrophages to clear the debris of inflammation. The proinflammatory cytokines IFN-γ and IL-1β induce the expression of lipoxins to promote the resolution of inflammation. Patients with diabetes mellitus have low AA levels in their plasma phospholipid fraction (7) that could lead to reduced formation of lipoxins. 

    Similar to lipoxins, an analogous class of anti-inflammatory compounds, the resolvins, are derived from ω-3 EPA and DHA (eicosapentaenoic acid and docosahexaenoic acid respectively). Another analogous class, the epi-lipoxins, is formed by non-enzymatic peroxidation. They are produced by the COX-2 pathway especially in the presence of aspirin. Resolvins reduce inflammation and promote resolution of the inflammation. 

DHA is converted to 17R-resolvins by a similar aspirin-triggered COX2 mechanism to the resolvins. In the absence of aspirin, COX2 in human microvascular endothelial cells converts DHA to 13-S-hydroxy-DHA that generates either 7S,(8)-epoxy or a 4S,(5)-epoxy intermediate, which are converted by 5-lipoxygenase to form resolvins. The former produces the aspirin-triggered resolvins D1 and D2, while the latter produces the aspirin-triggered resolvins D3 and D4. All contain a 17R hydroxyl group. Thus, there are parallels in the biosynthesis of resolvins and protectins from EPA and DHA with that of epi-lipoxins or aspirin-triggered lipoxins from AA.



15-lipoxygenase generates 17S-hydroxy-DHA that is converted to 7S-hydroperoxy,17S-hydroxy-DHA by a 5-lipoxygenase, and thence via an epoxy intermediate to epimeric resolvin D1 and resolvin D2. Lipoxygenase-generated intermediate from 17S-hydroxy-DHA, i.e. 4Shydroperoxy, 17S-hydroxy-DHA, is transformed via an epoxide to resolvins D3 and D4. Both 22:5(n-3) and 22:5(n-6) are good substrates for 15-lipoxygenase, and the latter gives 17S-hydroxy-22:5(n-6) and 10,17S-dihydroxy-22:5(n-6) products that are potent anti-inflammatory agents. In response to aspirin treatment, new docosatrienes termed ‘protectins’ are formed in brain, possibly retina, murine inflammatory exudates and lung, in peripheral human blood and in other cell types. 



In addition, electrophilic fatty acids could also be generated during inflammation by non-enzymatic reactions that have the ability to suppress inflammation. DHA and docosapentaenoic acid (DPA) of the ω-3 series generate electrophilic oxo-derivatives (EFOX) by a COX-2-catalyzed mechanism in activated macrophages whose formation is enhanced by aspirin. EFOX activate Nrf2-dependent anti-oxidant gene expression, act as peroxisome proliferator-activated receptor-γ (PPAR-γ) agonists and inhibit pro-inflammatory cytokine and nitric oxide production. Thus, lipoxins, resolvins, protectins and EFOX are potent anti-inflammatory compounds generated from AA, EPA and DHA that suppress the production of TNF-α, IL-6 and other pro-inflammatory cytokines (7,  8). 



PUFAs are present in large amounts in retina. EPA inhibits the abnormal gap junctional intercellular communication (GJIC) induced by hypoxia/reoxygenation (H/R) via suppressing tyrosine kinase (TK) activation and protected against VEGF-induced reduction in GJIC and phosphorylation of connexin (9), attenuated VEGF induced proliferation of endothelial cells and improved hypoxia-reoxygenation-induced endothelial dysfunction through inhibition of tyrosine kinase activation (10), a mechanism by which PUFAs protect against the development and progression of diabetic retinopathy. Aspirin-triggered lipoxin stable analog 15-epi-16-(parafluoro)-phenoxy-lipoxin A4 is a potent inhibitor of VEGF-induced angiogenesis and lipoxins prevented hypoxia-induced proliferative retinopathy in experimental animals (11). Thus, lipoxins, resolvins and protectins could prevent the development and progression of diabetic retinopathy, age-related macular degeneration (AMD) and retinopathy of premature that are also characterized by proliferative retinopathy (12, 13). I suggest that intravitreal administration of lipoxins, resolvins and protectins (especially lipoxin A4) or their synthetic analogues by themselves or in combination with anti-TNF antibody could be of significant benefit in AMD, diabetic retinopathy and diabetic macular edema. Lipoxins, resolvins and protectins being naturally occurring anti-inflammatory lipid molecules are likely to be less toxic, and might act on the retinal cells rapidly and specifically. Our preliminary studies are in support of the role of lipoxins in AMD, macular edema and diabetic retinopathy.



References:

1.    Markomichelakis NN, Theodossiadis PG, Sfikakis PP. Regression of neovascular age-related macular degeneration following infliximab therapy. Am J Ophthalmol 2005; 139: 537-540.

2.    Funatsu H, Yamashita H, Noma H, Mimura T, Yamashita T, Hori S. Increased levels of vascular endothelial growth factor and interleukin-6 in the aqueous humor of diabetics with macular edema. Am J Ophthalmol 2002; 133: 70-77.

3.    Itoi K, Nakamura K, Oku H, Ishizaki E, Sugiyama T, Ueki M, Maeno T, Sato B, Ikeda T. Relationship between diabetic macular edema and peripheral Th1/Th2 balance. Ophthalmologica 2008; 222: 249-253.

4.    Sfikakis PP, Grigoropoulos V, Emfietzoglou I, Theodossiadis G, Tentolouris N, Delicha E, Katsiari C, Alexiadou K, Hatziagelaki E, Theodossiadis PG. Infliximab for diabetic macular edema refractory to laser photocoagulation: a randomized, double-blind, placebo-controlled, crossover, 32-week study. Diabetes Care 2010; 33: 1523-1528.

5.    Giganti M, Beer PM, Lemanski N, Hartman C, Schartman J, Falk N. Adverse events after intravitreal infliximab (Remicade). Retina 2010; 30: 71-80.

6.    Figurska M, Stankiewicz A. Effectiveness of ranibizumab intravitreal injections for exudative age-related macular degeneration treatment: 12-month outcomes. Med Sci Monit 2011; 17: CR485-CR490.

7.    Das UN. Molecular Basis of Health and Disease. Springer, New York, 2011. 

8.    Serhan CN. Novel chemical mediators in the resolution of inflammation: resolvins and protectins. Anesthesiol Clin 2006; 24: 341-364.

9.    Zhang YW, Yao XS, Murota S, Morita I. Inhibitory effects of eicosapentaenoic acid (EPA) on the hypoxia/reoxygenation-induced tyrosine kinase activation in cultured human umbilical vein endothelial cells. Prostaglandins Leukot Essent Fatty Acids 2002; 67: 253-261.

10.    Yang SP, Morita I, Murota SI. Eicosapentaenoic acid attenuates vascular endothelial growth factor-induced proliferation via inhibiting Flk-1 receptor expression in bovine carotid artery endothelial cells. J Cell Physiol 1998; 176: 342-349.

11.    Fierro IM, Kutok JL, Serhan CN. Novel lipid mediator regulators of endothelial cell proliferation and migration: aspirin-triggered-15Rlipoxin A(4) and lipoxin A(4). J Pharmacol Exp Ther 2002; 300: 385-392.

12.    Connor KM, SanGiovanni JP, Lofqvist, et al. Increased dietary intake of ω-3 polyunsaturated fatty acids reduces pathological retinal angiogenesis. Nat Med 2007; 7: 868-873.

13.    Das UN. Polyunsaturated fatty acids in pathological retinal angiogenesis. Current Nutr        Food Sci 2009; 5: 94-111.



 

Keywords: Macular Degeneration - prevention & control, Lipoxins - therapeutic use, Macular Edema - prevention & control, Diabetic Retinopathy - prevention & control, Antibodies - therapeutic use